Catechism of the Locomotive

PREFACE.

Books, like individuals, have their histories, and it seems but proper that in introducing them somewhat of their ancestry should be detailed. The present book originated in this wise: the publishers of the Railroad Gazette procured a copy of the “Katechismus der Einrichtung und des Betriebes der Locomotive,” by Georg Kosak. As no English translation of this excellent little book was known to be in existence, the editors of the above paper determined to translate it and adapt it to the American practice in the construction and management of locomotive steam engines, and republish it in their journal. The translation was therefore made and submitted to the writer for revision and adaptation, according to the original intention. Before the latter was entertained, however, he had commenced writing an elementary treatise on the locomotive. In revising the first part of the translation of Mr. Kosak’s book, it was found that the latter occupied only to a very limited extent the ground which the writer had “staked out” in his own incomplete plan. He therefore concluded to abandon the original intention of “adapting” Mr. Kosak’s work, and determined to rewrite it and make substantially a new book of it. For the “idea,” however, and to some extent its plan, and for much valuable material, the author must acknowledge his indebtedness to Mr. Kosak. In some few cases the language of the translator has been employed, in part or in whole, without quotation marks, but with an acknowledgment in a foot-note. A similar plan has also been pursued in using some other books. This was done to avoid cutting up paragraphs and sentences into fragmentary parts with numerous quotation marks.

The following books have been consulted and used in writing the Catechism of the Locomotive: Heat considered as a Mode of Motion, by Prof. Tyndall; The Conservation of Energy, by Balfour Stewart; Railway Machinery, by D. K. Clark; Treatise on the Locomotive Engine, by Zerah Colburn; Treatise on the Steam Engine, by W. J. M. Rankine; Indicator Experiments on Locomotives, by Professor Bauschinger; Richards’ Steam Indicator, by Charles T. Porter; Die Schule des Locomotivführers, by J. Brosius and R. Koch; Mechanics, by A. Morin; The New Chemistry, by J. P. Cooke, Jr.; Combustion of Coal and the Prevention of Smoke, by C. Wye Williams; A Treatise on Steam Boilers, by Robert Wilson; Reports of the American Railway Master Mechanics’ Association; Link Valve Motion, by William S. Auchincloss, and Emergencies and How to Treat Them, by Dr. Joseph W. Howe.

For the title of the book an apology is perhaps needed, as the word Catechism is associated in nearly all persons’ minds we will trust with early religious and theological instruction, and therefore a Catechism of the Locomotive is very apt to sound more ludicrous than scientific. The title of Mr. Kosak’s book was adopted before it was determined to rewrite it, and it was afterwards not deemed best to change it. To those who are disposed to smile at it, the precedent of Mr. Bourne’s excellent Catechism of the Steam Engine is quoted, and if they will refer to Webster’s Dictionary for the definition of the word “catechism,” they will find that it means “an elementary book containing a summary of principles in any science or art, but appropriately in religion, reduced to the form of questions and answers, and sometimes with notes, explanations and reference to authorities,” which is exactly what the present book is intended to be.

To persons accustomed to books and study, the catechetical form is very apt to seem cumbrous and awkward, but it has some very decided advantages in writing for those who have not acquired studious habits of thought. To such the question asked presents first a distinct image of the subject to be considered, so that the explanation or instruction which follows is much more apt to be understood than it would be if no such question had been asked.

The author is indebted to Mr. D. B. Grant for the use of drawings from which most of the engravings of details of locomotives with which this book is illustrated have been made, and to other locomotive builders, whose engines are illustrated in the full-page plates, for the drawings thereof. He has also received very valuable aid from Mr. Richard H. Buel, Mechanical Engineer; Mr. William Buchanan, Master Mechanic of the Hudson River Railroad; Mr. Frank D. Child, Superintendent of the Hinkley Locomotive Works; and Mr. E. T. Jeffrey, Assistant Superintendent of Machinery of the Illinois Central Railroad.

The object in writing the book was to furnish a clear and easily understood description of the principles, construction and operation of the locomotive engine of the present day, a subject which is not concisely or adequately treated in any one similar book. If the author has succeeded in making what he has written plain to plain people, his aim will be fully accomplished.

No. 73 Broadway, New York.

CONTENTS. — APPENDIX.

INTRODUCTION.

The Catechism of the Locomotive is intended for a large class of readers, among whom are all kinds of railroad officers and employes, consisting of locomotive runners, firemen, and the many different kinds of mechanics employed in railroad shops and in the construction of locomotive and other kinds of railroad machinery and material. Besides these there are many amateur engineers, students, and persons interested directly or indirectly in railroads, and a not inconsiderable class who are always seeking information on all subjects whatsoever. It is evident, therefore, that the only way to adapt the book to all the classes for whom it is intended, was to make it so plain that the “wayfaring man” will have no difficulty in comprehending it. It has therefore been written in as clear language as the writer could command, and the subjects presented are treated as simply and as plainly as his ability enabled him to do, and with the least possible employment of either scientific or practical technicalities. The only deviation from this plan will be found in the use of algebraic symbols to designate arithmetical calculations. This was done to save space, and because it was thought that they could be explained so that even those without any knowledge whatsoever of algebra could easily comprehend them. To such as have no such knowledge the following explanation is given:

Suppose it is necessary to add two numbers, say 1,872 and 468. The calculation, if made arithmetically, would be thus:

This it will be seen occupies the space of several lines of print. If we want to express this calculation algebraically, it can be done by simply writing the two numbers and placing the sign +, called plus, between the two, which indicates that they are to be added together, thus:

To indicate what the sum will be, or what the two added together will amount to, the sign =, called equal to, or the sign of equality, is placed after the two numbers and between them and the sum, thus:

which can be read as follows:

Now the only use of the algebraic signs + and = is that they save time in writing and room in printing, and when persons become accustomed to their use they make plain a number of operations at a single glance, as will be shown hereafter.

In the same way that the sign + means added to, the sign - means less or subtracted from, thus:

which is the same as though it was printed as follows:

The sign × means multiplied by, or is the sign of multiplication. Thus:

that is,

The sign ÷ means divided by, thus:

which means:

The same thing is expressed by putting a line under the dividend and writing the divisor under the line, thus:

These signs are combined in various ways. Thus, supposing we wanted to add 1,872 to 468 and then divide the sum by 117, it would be necessary, in order to represent the arithmetical calculation, to do it as follows:

Algebraically it would be stated thus:

If you wanted to add 124 to the quotient 20 above, the calculation would be as follows:

This operation could be expressed by writing it as follows:

If we wanted to multiply the quotient 20 by 124 we would simply put the sign × instead of + before 124, thus:

The sign of subtraction or division can be used in the same way.

With these explanations it is believed that any one, with nothing more than an ordinary knowledge of the four elementary rules of arithmetic, can understand all the mathematics contained in the following pages. A little explanation may also be needed of the method of representing machinery and other structures by mechanical drawings.

A

B

 

 

C

D

 

 

If we want to represent the outside of any object, say an apple, we make a drawing of it as shown at A. Now if we want to show the inside of the apple, say the seeds and core, we can cut it in half and represent it as shown at C, which is then called a section or sectional view of the apple. If we represent it as it will appear if we are above it and looking down on it as shown at B, it is called a top view or plan.

It is evident, too, that it might be desirable to show the arrangement of the seeds in the apple as they would appear if it was cut through in the other direction, say on the line a b, fig. A, and as is shown at D. There are therefore two kinds of sections; one C, in which the object is supposed to be cut through vertically, and therefore called a vertical section, the other when the object is supposed to be cut through horizontally, and therefore called a horizontal section, as shown at D.

It is also evident that in looking at a locomotive or any other object, the appearance of the engine depends upon our position in relation to it. Thus, if we stand on the side of it, we see that part of the engine, and a drawing which represents the side, is therefore called a side view or side elevation. A drawing which represents a locomotive or other object as it would appear to us if we stood in front of it, is called a front view or front elevation, and a representation of the back part of any object as it would appear to us if we stood behind it is called a back view or back elevation. Plate I is a side view, Plate II a section, Plate III a top view or plan;[1] the vignette in the title page is a front view and fig. 71 a back view of a locomotive. If the drawing is made as the object would appear if it was turned upside down, and we were looking at it from above, then it is called an inverted plan.

It is obvious, too, that it is possible to make a great many different sectional views of nearly any object, especially of a machine. Thus, we could suppose a locomotive cut through vertically and lengthwise, as is shown in Plate II. Such a representation is called a longitudinal section. A locomotive could also be cut through crosswise, as shown in fig. 40, which is called a transverse section. It is of course possible to represent a transverse section of a machine like a locomotive at a great many different points; for example, it could be shown as though it was cut through the smoke-stack as in fig. 40, or through the boiler farther back, as the latter is shown in fig. 42. Usually when a section is shown through a cylindrical object like a smoke-stack or boiler, it is shown through its centre. If, however, this is not apparent from the drawing or engraving, it should be stated at what point it is supposed to be taken, thus the section D of the apple is on the line a b of fig. A, and the section C is on the line c d.

In drawing sections, the parts which are supposed to be cut in two are usually shaded with parallel diagonal lines drawn at equal distances apart, as shown in the sections of the apple at C and D. Sections are also sometimes represented with solid black surfaces, as in Plates II and III, and in the engraving of a pump in fig. 66.

Objects which are behind others which are in front of them are often shown with dotted lines, so as to indicate their position. The seeds of the apple are thus indicated at A.

It is also customary, in drawings of machinery, to take great liberties with the objects represented and to show them with parts removed or broken away, if their construction can thus be made plainer. It should be remembered that the purpose of drawings of this kind is not to give a pictorial representation of the object as it appears to the eye, but to make its construction and mode of operation apparent to the mind. In such drawings therefore all perspective is disregarded. It would lead us too far were we to explain the reasons for this, and therefore readers must accept the assertion without the proof.

PART I. THE STEAM ENGINE.

CATECHISM OF THE LOCOMOTIVE.

Question 1. What is the motive power employed in ordinary steam engines?

Answer. The expansive force of steam.

Question 2. How is this expansive force of steam applied?

Fig. 1.

Scale ³⁄₈ in. = 1 foot.

A. Cylinder.
B. Piston.
R. Piston-Rod.
D. Front Cylinder-Head.
C. Back Cylinder-Head.

Answer. It is applied by admitting it into a cylinder (A, fig. 1) in which a piston, B, is fitted so as to move air-tight from one end of the cylinder to the other. The steam, if admitted at C, will force the piston B to the opposite end[2] of the cylinder. When it has reached that end, if the steam is allowed to escape and a fresh supply is admitted to the cylinder through the opening d, it will move the piston back again. In this way, by alternately admitting steam at one end and exhausting it from the other, the piston receives a reciprocating motion, which is communicated to the outside of the cylinder by a rod, R, which is called the piston-rod, which works air-tight through an opening in one of the cylinder-covers, or cylinder-heads, as they are usually called.

Fig. 2.

Scale ³⁄₈ in. = 1 foot.

A. Cylinder.
B. Piston.
R. Piston-Rod.
c. and d. Steam-ways.
g. Exhaust-Port.
V. Slide-Valve.
H. Exhaust-Cavity.
f.f. Valve-Seat.
I. I. Steam-Chest.
M. Valve-Stem.
r.r. Rocker.
F. F. Fly-Wheel.
s. Rocker-Shaft.
G. Eccentric.
K. Eccentric-Strap.
L. Eccentric-Rod.
E. Connecting-Rod.
P. Crank.
N. Crank-Pin.

Fig. 3.

Fig. 4.

c. and d. Steam-Ports.
g. Exhaust-Port.

Fig. 5.

Scale ³⁄₈ in. = 1 foot.

Question 3. How is this reciprocating motion of the piston converted into rotary motion?

Answer. By connecting the end of the piston-rod R (fig. 2) by another rod, E; called a connecting-rod, with a crank, P, which is attached to a revolving shaft, S. It is apparent that if the piston B is moved in the direction shown by the dart R, a rotary motion will be given to the crank in the direction of the dart N. When, however, the crank reaches the position shown by the dotted lines in fig. 5, it is plain that a force applied to move the piston in either direction will no longer produce a rotary movement of the crank and shaft. The same thing will occur when the crank is in the opposite position, shown by the full lines. These two positions are called the dead-points of the crank.

Question 4. How is the crank of an ordinary steam engine carried past the dead-points?

Answer. Stationary engines are usually provided with a large and heavy wheel, called a fly-wheel (F F, fig. 2) which is attached to the shaft S. This wheel receives a sufficient amount of momentum from the crank, while the latter is moving from one dead-point to the other, to carry it past those points.

Question 5. How is the steam admitted to and exhausted from the cylinder?

Answer. It is admitted through two channels, c, d, fig. 2, called steam-ways, cast in the cylinder. These ways terminate in a smooth flat surface, f f, called the valve-seat. Their openings in the valve-seat are called steam-ports. Between them is another port or cavity, g, called the exhaust-port, which communicates with the open air. The form of these ports is long and narrow, as shown in fig. 4, which represents a plan of them. Over these ports a valve, V, called a slide-valve, usually made of cast iron, with a cavity, H, on its under side, is fitted so that by moving it backwards or forwards it will alternately cover and uncover the two steam-ports. The valve and valve-seat are inclosed in a sort of box, I I, fig. 2, made of cast iron, called a steam-chest, into which steam is admitted from the boiler by a pipe, J. When the valve is in the position represented in fig. 2, the front steam-port is uncovered and the steam is admitted to the front end of the cylinder, and thus forces the piston towards the back end. If, when the piston reaches the back end, the valve be moved into the position shown in fig. 3, the back steam-port will be uncovered and steam will be admitted to that end of the cylinder. At the same time it will be observed that the aperture of the front steam-port c and that of the exhaust-port are both covered by the cavity in the slide-valve, so that the steam which was admitted to the front end of the cylinder can escape through the steam-port c into the exhaust-port, and thus into the open air. In this way, by moving the valve alternately back and forth, steam is simultaneously admitted first to one end and exhausted from the other, and vice versa.

Fig. 6.

G. Eccentric.
K. K. Eccentric-Strap.
L. Eccentric-Rod.

Question 6. How is the slide-valve moved so as to admit and exhaust the steam at the right time?

Answer. This is done by what is called an eccentric, which is a circular disc, G (fig. 6), the axis of which is not in the centre. The outside of the eccentric is embraced by a metal ring, K K, made in two halves, called an eccentric-strap. The eccentric is attached to the shaft by screws or keys, and revolves with it and inside of the eccentric-strap. To the latter is also attached a rod, L, called an eccentric-rod. It is obvious from fig. 6 that if the eccentric revolves inside of the strap it will impart a reciprocating motion to the rod L. The eccentric, G, strap, K, and rod, L, are represented in fig. 2. Before describing their operation, or rather their connection with the valve V, it is necessary to understand that usually the valve-seat is placed on top of the cylinder, in which position it is difficult to connect the eccentric-rod with the valve. For convenience, therefore, what is called a rocker, r r, is placed between the cylinder and the main shaft of the engine. This rocker has two arms attached to a shaft, s, and the two arms have a vibratory motion about it, as indicated by the dotted lines. The eccentric-rod L is attached by a pin to the lower arm of the rocker, and the valve is connected to the upper arm by the rod M, called the valve-stem, or valve-rod. It is obvious that as the shaft S and eccentric G revolve, a reciprocating or vibratory motion will be given to the rocker, which will be communicated to the valve by the valve-stem; and it is only necessary to fix the eccentric in the proper position on the shaft, in relation to the crank and piston, to give the valve the required motion for admitting and exhausting the steam to and from the cylinder at the right time.

PART II. THE FORCES OF AIR AND STEAM.

Question 7. What is meant by the pressure of the air?

Answer. It is the pressure exerted by the weight of the air on every point with which it is in contact. The globe of the earth is surrounded by a layer of air about 50 miles thick, and, like every other substance, the air possesses weight, and hence presses upon every object with which it is in contact.

Question 8. How can it be shown that the air possesses weight?

Answer. By weighing a flask when it is filled with air, and again when the air is exhausted from it. In the latter condition the weight of the flask will be found to be sensibly less than it was when full of air, showing that the air which the flask contained when it was first weighed increased its weight.

Question 9. Why do we not feel this pressure on our bodies?

Answer. Because the air surrounds us on all sides, and presses just as much in one direction as it does in another, so that the pressures in different directions just balance each other, or are in equilibrium; but if you disturb this balance, for example, by sucking the air from a tube closed at one end, it will cling to your tongue; or if you take a thick piece of leather under ordinary conditions it will not adhere to anything, but if it be thoroughly wet and pressed hard against the surface of a smooth stone, so as to force out the air from under it, the stone, as nearly all school-boys know, can be lifted up if a string is attached to the leather; or if the air be sucked out of a tube, one end of which is inserted in a liquid, the latter will be forced up the tube. These phenomena are due to the pressure of the atmosphere in the first case on one side of the person’s tongue, pressing it against the mouth of the tube; in the second, to the same pressure on the top of the leather, causing it to adhere to the stone; and in the last, to the weight of the air pressing on the surface of the liquid, forcing it into the vacuum in the tube.

Fig. 7.

Scale ¹⁄₄.

Question 10. What is the amount of the pressure of the atmosphere, and how is it measured?

Answer. It is usually measured by the pressure on one square inch of surface, which at the earth’s surface is 15 pounds.[3] If, for example, we have a cylinder, A, fig. 7, with an air-tight piston, B, fitted to it whose area is just one square inch, if we exhaust the air through the tube C from the cylinder above the piston, the air will press against the under side of the piston so that, if no power is required to overcome its friction in the cylinder, the pressure of the air will raise a weight of 15 pounds. The pressure of the air varies, however, as you ascend or descend from the surface of the earth, because as you go up on a mountain or in a balloon the layer of air above you becomes thinner, and, therefore, its weight and consequent pressure are diminished; and as you descend, as in a deep mine, the layer is thicker, and its pressure consequently greater.

Question 11. What is steam?

Answer. Steam is water changed by means of heat into a gas. At every temperature there is formed from water, on its surface, vapor of which the clouds are formed at all seasons of the year. This change of water into vapor, or evaporation of water, takes place at low temperatures only on its surface, however. But if we heat water in a vessel to a temperature of 212 degrees Fahrenheit, then the inner particles of the mass of water (lying on the heating surface of the vessel) are changed into steam, and rise to the surface in bubbles, which is the phenomenon we call boiling. It must not be imagined, however, that the visible cloud which escapes from a kettle or the exhaust-pipe of a steam engine is true steam. It is rather small particles of water, into which the steam has condensed through contact with the cold air. True steam is invisible, as we may observe near the mouth of a kettle or the exhaust-pipe of an engine from which we know it is escaping.

Question 12. If water is heated in an open vessel what occurs?

Answer. It continues for some time to increase in temperature, and the evaporation becomes more and more rapid. At length bubbles of vapor break out and reach the surface, and the process of boiling or ebullition has begun. When this takes place the temperature of the water ceases to rise, and it remains stationary until all the water has boiled away, the only difference being that if the supply of heat be very great the process is very rapid, and if the supply of heat be small the process is very slow. The point at which ebullition commences is called the boiling-point.

Question 13. On what does the boiling-point depend?

Answer. Chiefly on the pressure on the surface of the water, but to some extent upon the purity of the water. Thus, boiling, which takes place at 212 degrees under the ordinary atmospheric pressure, in lighter air, as on high mountains, takes place at a much lower temperature than on lowlands, and so water boils in a glass tube from which the air has been exhausted by the warmth of the hand, that is, at 92 degrees.

Question 14. What is the pressure of steam which escapes from boiling water in an open vessel?

Answer. It is exactly equal to the pressure of the atmosphere in which it is boiled. Ordinarily this is 15 lbs., and the boiling-point 212 degrees; but if we go up on a mountain where the atmospheric pressure is only 10 lbs. per square inch, the water will then boil at a temperature of 193.3 degrees, and the steam which escapes will have the same pressure as the atmosphere, or 10 lbs. per square inch. On the other hand, if we could go down into a mine where the atmospheric pressure was 20 lbs. per square inch, the water would not boil until it was heated to 228 degrees, and the pressure of the escaping steam would then be 20 lbs. per square inch.

Question 15. If water is boiled in an enclosed vessel like a covered tea-kettle or a steam boiler, what occurs?

Answer. The steam rises and fills the space above the water, and, if it cannot escape, increases in pressure. The temperature of both the water and the steam rises with the pressure, and will continue to do so as long as the heat is increased, or until the steam can escape or the vessel is exploded. The boiling point also rises as the steam pressure increases.

Question 16. Is there any pressure which corresponds to the temperature of steam and water?

Answer. Yes. There is a fixed pressure for every temperature, when steam is in contact with water, and its pressure cannot be increased or diminished without at the same time heating or cooling the water, and the higher the temperature of the water the greater will be the corresponding steam pressure. Thus water at 212 degrees produces steam with a pressure equal to that of the atmosphere; at 240 degrees the steam will have a pressure of 25 lbs., or 10 lbs. more than the atmospheric pressure; at 281 degrees a pressure of 50 lbs.; and at 328 degrees, 100 lbs. As this relation of pressure to temperature is fixed, if we know the one we can tell the other. This is true, however, only where the steam is in contact with water, when it is called saturated steam. If it is separated from water it may be heated to a higher temperature, and is then called superheated steam.

Question 17. How is the pressure of steam measured?

Answer. In the same way as that of the atmosphere,—that is, by the force exerted on one square inch of surface. Thus if steam is admitted into the cylinder A, fig. 8, under the piston B, whose area is equal to one square inch of surface, supposing, as we did before, that no power is required to overcome its friction in the cylinder, if the steam thus admitted would just balance the atmosphere, its pressure would be equal to 15 lbs. If, besides overcoming the pressure of the atmosphere, it would raise a weight of 15 lbs., then its pressure per square inch would be equal to 30 lbs. When the atmospheric pressure is included with that of the steam, we call it the absolute steam pressure. In ordinary engines, however, the steam must always overcome the pressure of the atmosphere, and therefore the only part of the pressure which is effective is that above, or by which it exceeds, the atmospheric pressure. For example, although the steam admitted under the piston in fig. 8 has an absolute pressure of 30 lbs. per square inch, yet it will only raise a weight of 15 lbs., because it must first overcome the pressure of the air on the other side of the piston. The pressure of the steam used in most stationary and in locomotive engines is, therefore, measured by its pressure above the atmosphere. That is, if steam introduced under the piston in fig. 8 will raise a weight of only 15 lbs., we say it has a pressure of 15 lbs. per square inch; if it will raise 50 lbs., its pressure is said to be 50 lbs. per square inch, and so on. The pressure of the atmosphere is disregarded, and all steam-gauges used on locomotives are graduated in that way. In speaking of steam pressure in future, therefore, unless otherwise specified, we shall mean effective and not absolute pressure.

Fig. 8.

Scale ¹⁄₄.

Question 18. What is meant by the expansion of steam?

Answer. In all gases a repulsion is exerted between the various particles, so that any gas, however small in quantity, will always fill the vessel in which it is held. Steam possesses this same property, and if placed in any vessel the particles in endeavoring to separate from each other will exert a force on all its sides. This force we call the steam pressure. To illustrate this we will suppose that the cylinder A in fig. 8 is half filled with steam of 30 lbs. pressure. If now the supply of steam is shut off, the steam in the cylinder will expand so as to push the piston upward, but with a somewhat diminishing force, the nature of which we will explain hereafter.

Question 19. What is meant by the volume of steam?

Answer. It means the space which the steam occupies.

Question 20. What is the proportion which exists between the volume and the pressure of steam?

Answer. If the temperatures remain the same they are INVERSELY PROPORTIONAL TO EACH OTHER; that is, the one increases in the same proportion as the other diminishes. If we admit steam of 30 lbs. pressure per square inch into the cylinder A, fig. 8, and then cut off the supply by closing the cock C and allow the steam in the cylinder to expand to double its volume by pushing the piston to the end of the cylinder, the steam pressure will then be only 15 lbs.; if it should expand to three times its volume its pressure would be only one-third, or 10 lbs. per square inch. This method for calculating the pressure of steam after it has expanded is correct only for the absolute and not for the effective pressures of steam. In order to ascertain the effective pressures of steam after expansion, it is only necessary to make the calculation with the absolute pressure and deduct the atmospheric pressure from the result. If, after being thus expanded, the piston be pushed down again so as to compress the steam into its original space, its pressure will again be 30 lbs., providing no heat has been lost in any way.

Question 21. With a cylinder of any given stroke[4] how can we determine approximately the pressure of the steam after expansion for any given point of cut-off?[5]

Answer. By multiplying the absolute pressure per square inch of the steam in the cylinder before it is cut off, by the distance from the beginning of the stroke at which it is cut off, and dividing the product by the whole length of the stroke. Thus, if we have a cylinder whose piston has a stroke of 24 inches, if we cut off the steam at 8 inches, and have an ABSOLUTE pressure of 90 lbs. in the cylinder, the calculation is as follows:

If we cut off at 10, 12 and 15 inches, the final pressure would be 37¹⁄₂, 50 and 56¹⁄₄ lbs., respectively. To get the effective pressure deduct the atmospheric pressure from this result.

Question 22. What is the proportion between the volume of steam and that of the water from which it is formed?

Answer. At the pressure of the atmosphere (15 lbs.) each cubic inch of water will make 1,610 cubic inches of steam. At double that pressure, or 30 lbs. absolute pressure, it will make a little more than half as much, or 838 cubic inches; at four times, or 60 lbs. absolute pressure, 437 cubic inches, or a little more than a fourth as much as at the pressure of the atmosphere.

Question 23. Why is it that the quantity of steam at high pressures is somewhat greater than in inverse proportion to the pressure?

Answer. Because the boiling-point of water, as has already been explained, is higher as the pressure increases, and therefore the temperature of the steam produced at such pressure is also higher than at lower pressures; and as all gases are expanded by heat, therefore the volume of steam at the higher pressures is somewhat greater than in inverse proportion to its pressure, on account of being somewhat expanded by its high temperature. To make this plain, if we take a cubic inch of water and convert it into steam of atmospheric pressure, its volume will be 1,610 times that of the water, and its temperature 212 degrees.[6] If we convert this quantity of water into steam with a pressure double that of the atmosphere, the volume of the steam will be 838 times that of the water and its temperature will be 250.4 degrees. If the volume of the steam were exactly inversely proportional to the pressure, the cubic inch of water at double the atmospheric pressure would make only 805 cubic inches of steam; but as the boiling-point at that pressure is 38.4 degrees higher, the steam is expanded 33 cubic inches by the increase of its heat due to the higher boiling-point.

A table in the appendix gives the pressure, temperature and volume of saturated steam up to 300 lbs. absolute pressure.

Question 24. What is meant by the condensation of steam?

Answer. It is the reconversion of steam into water by cooling it, or depriving it of part of its heat. It has been shown that the temperature of water must be raised to a certain point to generate steam of a given pressure. If the process is reversed, and we deprive the steam of a part of its heat, some of the steam is then at once reconverted into water, or condensed, and the pressure of that which remains will be reduced just in proportion as the heat is lost. When the temperature gets below 212 degrees under atmospheric pressure, all the steam will be condensed. As the useful work which steam can do in an engine is due to its pressure, which in turn depends on its temperature, any loss of heat will diminish its effective power. For this reason, all waste of heat from a steam engine should, as far as possible, be prevented.

Question 25. How is the heat of the steam wasted or lost in an ordinary steam engine?

Answer. It is wasted in three ways: first, by conduction; second, by convection; and third, by radiation.

Question 26. What is meant by these three terms?

Answer. 1. By conduction is meant that phenomenon which is manifested when we put one end of a metal bar two or three feet long into the fire and heat it. The heat is then gradually conveyed from one particle of the metal to that next to it until finally the end of the bar farthest from the fire becomes so hot that it cannot be touched. The heat is then said to be conducted through the bar. In the same way the metal of the boiler, pipes, cylinders and other parts of the engine becomes heated on one side, and the heat is thus conveyed to the outside of these parts.

2. The air with which they are surrounded then becomes heated, and being then lighter than the cold air, it rises and is again replaced with air which is not heated. In this way the heat is conveyed away by the air, and this phenomena is therefore called convection.

3. If an iron plate be placed in front of an ordinary grate fire three or four feet from it and exposed to the rays of heat from the fire, it will soon become so hot that you cannot bear your hand on it. If you place your hand between the iron plate and the fire you will find that only the side of your hand which is exposed to the fire will become hot, showing that the air between the plate and the fire is not nearly so hot as the plate soon becomes, and therefore that the heat is not conveyed to the plate by the air between it and the fire, but by the rays from the fire. This phenomenon is called radiation. The same thing occurs from any hot body, as for example a coil of steam pipe for heating a room, a steam boiler or cylinder of an engine.

Question 27. Is there any difference in the conducting and radiating power of different substances?

Answer. Yes, very great. The difference in the conducting power of wood and iron is shown if we place one end of a bar of each in the fire. The wood will be consumed without warming the bar more than a few inches from the fire, whereas the iron will soon become hot two or three feet from the fire. Owing to the difference in the conducting power of cotton and wool, we wear cotton clothing in summer and woolen in winter, because cotton allows the heat of the body to be conducted away from it, whereas woolen cloth prevents to a great degree this loss of heat. For the same reason, the venders of roasted chestnuts on our streets wrap them in a piece of blanket to keep them hot, that is, to keep the heat in; and in summer we wrap ice in the same way to keep it cold, that is, keep the warmth of the air out. The wool, being a very bad conductor of heat, simply prevents the heat from being transferred from the inside to the outside, and vice versa. It is for this reason that steam boilers, pipes and cylinders are nearly always covered with wood, and sometimes with felt.

The difference in the radiating power of various substances can be shown if we take a large thermometer and heat it up to the temperature of boiling water. If this thermometer is hung up in a room having the temperature of melting ice, it will lose heat in two ways,—first by heating the air which surrounds it, that is by convection, and also by radiation. In order to confine ourselves to the latter process, we will suppose that the chamber is a vacuum. If we first cover the bulb of the thermometer with a thin coating of polished silver, and then ascertain how much heat it radiates in a minute, and then coat it with lamp-black, and repeat the same experiment,—that is to say, allow the thermometer at the boiling-point to cool for one minute in a vacuum chamber at the freezing-point,—it will be found that the thermometer loses much more in a minute when coated with lamp-black than it did when coated with silver, showing that much more heat is radiated from a surface covered with lamp-black than from polished silver. Generally it may be stated that polished metals radiate much less heat than surfaces which are not polished.[7] For this reason, as well as for ornament, locomotive and other boilers and cylinders are usually covered with Russia iron or polished brass.

PART III. ON WORK, ENERGY, AND THE MECHANICAL EQUIVALENT OF HEAT.

Question 28. For what purpose are all steam engines used?

Answer. They are used to produce motion, which is opposed by some resistance. Thus, if an engine is employed to raise grain from a railroad car to the top of a warehouse, it must produce motion, which is resisted by the weight of the grain; if it is used to saw wood, it must give motion to the saw, which is resisted by the fibres of the wood; a locomotive engine must produce motion of a train of cars, which is resisted by the air, the friction of the journals and the rolling of the wheels on the track; if the locomotive is employed on a grade or incline, besides the frictional resistance referred to it must overcome that due to its own weight and that of the train, which is gradually lifted as it ascends the incline. In producing motion opposed by some resistance an engine is said to be doing “work.”

Question 29. Can this work be accurately measured?

Answer. Yes; but in order to measure anything we must first establish some accurate standard or unit of measurement. Thus we say a bar of iron is so many inches long, or a road is so many miles long. In like manner we speak of so many seconds, or minutes, or hours, or days, or years, when we speak of time. So it is necessary, in order to estimate or measure “work” in a strictly scientific manner, for us to fix upon some accurate standard or unit. In this country and in Great Britain the unit agreed upon for this purpose is the amount of power required to raise ONE POUND ONE FOOT, and is called a foot-pound. If we raise one pound two feet we do two foot-pounds of work; if three feet, three foot-pounds, and so on. Again, if we raise a weight of two pounds one foot high, we likewise do two foot-pounds of work; or if we raise it two feet high, we do four foot-pounds, and so on. In order to determine the amount of work done, we must MULTIPLY THE MOTION PRODUCED (in feet) BY THE RESISTANCE (in pounds), AND THE RESULT WILL BE THE WORK DONE IN FOOT-POUNDS.

Question 30. How many foot-pounds of work are performed in a pile-driving machine in raising a weight of 1,200 lbs. 24 feet?

Answer. 1,200 × 24 = 28,800 foot-pounds.

Question 31. When this weight is raised, is the force which was exerted in raising it annihilated or lost?

Answer. No; because the weight has the capacity of doing an equal amount of work when it falls, from the momentum[8] it acquires in falling. This power of doing work which it acquires in falling is called energy. Now, although the weight has no motion-producing power when it is raised to the top of the machine, yet obviously such action is then possible which when it rested on the earth was not possible. It has no energy as it hangs there dead and motionless; but energy is possible to it, and we might fairly use the term possible energy to express this power of motion which the weight possesses,[9] and which is therefore called potential energy. As soon as the weight is allowed to fall it acquires a greater velocity the farther it falls, and its potential energy thus becomes and is called actual energy.

Question 32. How do we explain such phenomena as the heating of a car-axle while turning under a car, the heating of brake-blocks when the brakes are applied to car-wheels, the heating of an iron rod by hammering, and of a turning tool when cutting a piece of metal?

Answer. All of these phenomena are due to the fact that the actual energy of motion is converted into heat, as has been repeatedly proved by many able and ingenious investigators and experiments.

Question 33. When the weight of the pile-driver falls, is its energy also converted into heat?

Answer. A part is expended in compressing the material into which the pile is driven and in overcoming the friction of the earth against the pile, each of which efforts develops heat, and another portion is converted into heat by the impact or blow of the falling weight on the head of the pile.

Question 34. Is all energy convertible into heat and heat into energy?

Answer. Yes. Science has demonstrated very clearly that they are mutually convertible.

Question 35. Has it been ascertained how much heat is equivalent to one foot-pound of work?

Answer. Yes; it has been found, from the most carefully-made experiments that the amount of heat which is required to raise the temperature of one pound of liquid water by one degree of Fahrenheit[10] is equivalent to 772 foot-pounds of work. It must be remembered that this is the theoretical equivalent of heat, and that only a very small proportion of this amount of work is ever realized from the heat developed by the combustion of fuel.

Question 36. If, then, heat is convertible into work and work into heat, can the transmutation of the heat of the steam in the cylinder of an engine into work, and the reverse process, be explained?

Fig. 9.

Scale ³⁄₈ in. = 1 foot.

Answer. Yes. Take a cylinder, fig. 9, and, in order to make the conditions of the experiment as simple as possible, imagine it to be placed in a vacuum. Now let saturated steam be admitted under the piston so as to fill the cylinder half full at an absolute pressure of 100 lbs. If we will allow this steam to expand to double its volume and raise the piston without doing any work, and then repeat the experiment with a load of 50 pounds on the piston, whose area is one square inch, it will be found that the temperature of the steam is sensibly less, after lifting the weight, than in the previous experiment, in which it expanded without doing work, showing that part of the heat was abstracted from the steam by doing work, or, in other words, was converted into work. If then, after the steam has expanded and lifted the weight, we press the piston down so that the steam under the piston is compressed to its original volume, we shall find that its temperature is the same as before, as the work done in compressing it is converted into heat. In these experiments it is assumed that there is no friction of the piston, nor loss of heat from radiation or conduction. The same phenomena can be observed in machines used for compressing air, which is heated to so high a temperature when it is compressed that it is necessary to cool the cylinders of such machines by circulating a current of cold water around them.

Question 37. What practical relation is there between the convertibility of heat into work, and the conducting and radiating properties of different substances explained in answer to Question 27?

Answer. The fact that heat is only another form of energy, or “the power of doing work,” indicates that its loss by conduction or radiation lessens that power just as much as or more than the loss or waste of coal would, and therefore every effort should be made to protect the different parts of engines from loss of heat by covering them with substances which conduct or radiate very little heat. Care should also be taken to exclude cold air from circulating in contact with these parts, and excepting for supporting combustion, the nature of which will be explained hereafter, it should be excluded from the heating surface of boilers.

Question 38. What is meant by the term LATENT HEAT OF EVAPORATION?

Answer. By latent heat is meant that heat which apparently disappears when water or other liquids are vaporized. Thus, it is found that if any quantity of water is converted into steam at any pressure, it is necessary not only to heat it to a temperature equivalent to that of the steam, or to the boiling-point, but after it has reached that temperature an additional amount of heat must be added in order to keep up the process of boiling. Notwithstanding this addition of heat to the water, the temperature of the steam produced will not be higher than that of the boiling water, thus showing that a considerable quantity of heat is absorbed, the only effect of which is to change the water into a gas or steam. This apparent disappearance of heat can be shown if we take a pound of boiling water whose temperature is 212 degrees and mix it with a pound of ice-cold water at 32 degrees. The result will be a mixture of two pounds of water of a mean temperature of 122 degrees. If now we convert a pound of water into steam at atmospheric pressure, the steam will heat 6.37 lbs. of ice-cold water up to 122 degrees, showing that a pound of steam at atmospheric pressure contains over six times as much heat as a pound of water of the same temperature as indicated by a thermometer. A similar apparent disappearance of heat occurs when other liquids are evaporated, and when ice or any other solid is converted into a liquid.

Question 39. What is the explanation of these phenomena?

Answer. The exact reasons which will explain them fully are probably not yet clearly understood, but it is at least extremely probable that when any substance is changed from a solid to a liquid, or from a liquid to a gaseous condition, “a large portion of the heat is spent in doing work against the force of cohesion.”[11] The particles of solid bodies, as we know, are so united that it requires more or less force, according to the nature of the substance, to tear them apart. Now we can conceive that the heat is changed into a form of energy, and in that condition resists this attraction of the particles to each other, and that being thus transformed it has lost the capacity of expanding the mercury in the thermometer. A similar effect takes place when a liquid is converted into a gas. In the former condition the particles move freely about each other and have little or no attraction for each other, but when it becomes a gas they have a repulsion from each other. The heat is thus converted into the energy of repulsion, and therefore is in reality no longer in the condition of heat and consequently does not affect the thermometer. We can illustrate this by supposing that by using steam heat is converted into work by raising the weight, or drop as it is called, of a pile-driving machine. When the weight is raised to the top of the guides from which it falls, although, as already explained, the heat is converted into potential energy, yet if we attached a thermometer to the drop we would not find that it was any warmer than before the drop was raised. If it were possible to make an instrument sufficiently sensitive to indicate an instantaneous change of temperature in the weight while falling, we would not find any increase of its temperature at the instant it had acquired its greatest momentum and just before it struck the object under it, although its potential energy would at that instant be converted into actual energy of motion. If, however, the weight should strike an unyielding object, its actual energy would at once be reconverted into heat, which our thermometer would indicate. The phenomenon of what is called latent heat of evaporation seems to be very similar to that described—the heat when the water is changed from a liquid to a gaseous condition is transformed into energy, which, as already stated, has no effect upon the mercury of the thermometer.

Question 40. What is meant by the TOTAL HEAT of steam?

Answer. The “total heat of steam” is a phrase used to denote the sum of the heat required to raise the temperature of water from some given point up to the boiling-point due to a given pressure, and of the heat which disappears in evaporating one pound of water under a given pressure (or latent heat of evaporation.) Thus the latent heat of one pound of steam at atmospheric pressure (14.7 lbs.) is 966.1 units; and 212 units of heat are necessary to raise water from zero to the boiling-point; therefore the total heat counted from zero of steam of atmospheric pressure is 1,178.1 units. At 100 pounds absolute pressure the latent heat is 885.5 and the sensible heat 327.9 degrees; therefore the total heat measured from zero is 1,213.4 units.

PART IV. THE SLIDE-VALVE.

Question 41. What are the essential conditions which a slide-valve must fulfill in governing the admission and exhaust of steam to and from the cylinder of an ordinary engine?

Answer. 1. It must admit steam to one end only of the cylinders at one time. 2. It must allow the steam to escape from one end at least as soon as it is admitted to the other end; and 3. it must cover the steam-ports so as not to permit the steam to escape from the steam-chest into the exhaust-port.

Fig. 10.

Scale ³⁄₁₆ in. = 1 inch.

Question 42. What was the first form of slide-valve used?

Answer. That represented in fig. 10. The smallest movement of this valve either way opens one of the steam-ports for the admission of steam and puts the other in communication with the exhaust-port. By cutting a piece of ordinary writing paper to the form of the section of the valve, and moving it on the line f f, the action of the valve will be clearly shown.

Plate I.

AMERICAN LOCOMOTIVE.
By The Grant Locomotive Works, Paterson, New Jersey.
Scale, ³⁄₈ in. = 1 foot.

Question 43. How was the admission and escape of the steam effected by this valve?

Answer. In order to explain this clearly, a series of diagrams will be necessary. Before referring to them, however, it should be explained first that the motion of an eccentric is exactly the same as that of a small crank. It is in fact a crank with a crank-pin whose diameter is very much enlarged. In the diagrams, figs. 11 to 25, the eccentrics will therefore be represented as small cranks, and most of the other parts by their centre-lines and points only, so as to make the diagrams as simple as possible. The dimensions selected for these illustrations are for the cylinder 16 in. diameter and 24 in. stroke, and a connecting-rod 7 ft. long. The steam-ports are 1¹⁄₄ in., the exhaust-port 2¹⁄₂ in., and the metal or bars between them, which are called bridges, are 1¹⁄₈ in. wide. The eccentric produces a lateral movement of 3 in., which is called its throw. In fig. 11 the piston is at the beginning of the backward stroke. The valve is then in the centre of the valve-face, and the eccentric is consequently at half-throw. The slightest movement of the crank in the direction of the dart N will move the eccentric enough to open the front steam-port to the steam and the back one to the exhaust. In fig. 12 the piston is represented as having moved 4 in. of its stroke; the valve has then partly opened the front steam-port, and the other one is open to the exhaust. In fig. 13 the piston has moved 8 in. of its stroke, and the ports are now wide open, the front one to the steam and the back one to the exhaust. In fig. 14 the piston has moved 12 in., or is at half-stroke, and the valve has then moved as far as it will in that direction. In fig. 15 the piston has moved 16 in. and the valve has begun to return. In fig. 16 the piston has moved 20 in., and the valve has nearly closed the front port to the steam and the other to the exhaust. In fig. 17 the forward stroke is completed, and both ports are closed by the valve. Figs. 18, 19, 20, 21, 22 and 23 represent the piston and the valve on the return stroke in the positions corresponding with those described for the backward stroke.

Fig. 11.

Fig. 12.

Fig. 13.

Fig. 14.

Fig. 15.

Fig. 16.

Fig. 17.

Scale ¹⁄₄ in. = 1 foot.

Fig. 24.

Fig. 23.

Fig. 22.

Fig. 21.

Fig. 20.

Fig. 19.

Fig. 18.

Scale ¹⁄₄ in. = 1 foot.

Question 44. Is there any other method by which the motion of a valve can be represented by a drawing?

Answer. Yes, by what are called motion-curves. It is, however, difficult to explain these clearly, and as they are purely imaginary, it is difficult to understand their nature and purpose. Close attention will therefore be required to the following description:

We will suppose, in the first place, that the line f f, fig. 25, represents the valve-face, c and d the steam and g the exhaust-port, drawn to a larger scale than in the preceding figures. We will now draw the valve H in the position represented in fig. 11, where the piston is at the beginning of the stroke. In order to show the valve in the position represented in fig. 12, where the piston has moved 4 in. of the stroke, we will draw a line 4-20, four inches below and parallel to f f, and extend the lines, representing the edges of the ports c, d and g, downward. On the horizontal line 4-20 we will now draw the edges i, r, i′, r, of the valve in the same position in relation to the port c that it has in fig. 12. We will then draw another horizontal line, 8-16, eight inches below f f, and parallel to it, and on this represent the valve in the position shown in fig. 13. In the same way we will draw lines 12, 16, 20 and 24 in. below f f, and draw the valve on each one respectively in the positions shown in figs. 14, 15, 16 and 17. The distance between the lower line 24-0, and f f, will then represent the stroke of the piston, or 24 in. If now we begin from the edge h of the valve on the line f f, and draw a curve, h i j k l m n, through the same edge of the valve, represented on each of the parallel lines below, the curve will indicate the position of the valve in relation to the steam-port c at each point of the stroke. To illustrate this, suppose we draw lines 1-23, 2-22, and 3-21, one inch apart and parallel to f f, and between it and 4-20. They will then represent the position of the piston after it has moved 1, 2 and 3 in. from the beginning of the stroke, and where they intersect the curved line will be the position of the edge of the valve when the piston has moved 1, 2 and 3 in. of the stroke. The curved line will in fact represent the position of the valve at any point of the stroke between these lines. Other horizontal lines, 5-19, 6-18, etc., can be drawn to represent every inch of the rest of the stroke. The curve line h i j k l m n, or motion-curve as it is called, will then show the exact position of the edge of the valve and of the width of the opening of the steam-port during the whole stroke. From it we see that the valve opens the port c for the admission of steam simultaneously with the movement of the piston, and when the latter has made one inch of its stroke the port c is half open. At 4¹⁄₂ inches of the stroke the port is wide open,[12] and at 19¹⁄₂ inches it begins to be closed, but is not completely closed until the end of the stroke.

Fig. 25.

Scale ³⁄₁₆ in. = 1 inch.

Similar motion-curves, such as h′ i′ j′ k′ l′ m′ n′, (represented in fig. 25) can be drawn to represent the position of the other edges of the valve, and also for the return stroke. The latter are shown in dotted lines. If we follow the curve h′ i′ j′ k′ l′ m′ n′, which represents the position of the edge of the valve h′ which governs the exhaust from the back end of the cylinder, we see that the port d is opened and closed to the exhaust simultaneously with the opening and closing of the port c for the admission of steam to the front end of the cylinder, and that they both remain open until the completion of the stroke.

The width that the ports are opened by the valve is thus ascertainable from these diagrams, for any point of the stroke, and in fact can be seen at a glance. By the aid of such motion-curves, the movement of slide-valves can therefore be analyzed more perfectly than is possible without them.

Question 45. What were some of the disadvantages of valves, like that shown in fig. 10, and which are shown by the motion-curves in fig. 25.

Answer. The free admission of the steam until the completion of the stroke by the piston was hurtful to the machinery, as it co-operated with the momentum of the piston and its connections in producing undue strains in the working parts. The steam then escaped from the cylinder without expansion, so that much of its useful energy was lost. The steam was not allowed to escape from one end of the cylinder until it was admitted at the opposite end, and as the process of exhausting it occupies some time, there was always more or less back pressure until all the exhaust steam was expelled from the cylinder. In practice, the imperfections of the valve-gear frequently delayed the opening of the ports, both for admitting and exhausting steam, until after the commencement of the stroke of the piston.[13]

Question 46. How may some of these evils be overcome?

Answer. By moving the eccentric forward on the axle so that the motion of the valve is advanced to the same extent, and the admission and exhaust of the steam will occur a little before the completion of the stroke of the piston. In this way the steam is admitted into the cylinder so as to act as a cushion to receive the momentum of the piston, and some time is given to the exhaust steam to escape, before the return stroke.

Question 47. What is meant by lead?

Answer. By lead is meant the width of the opening of the steam-ports at the beginning of the stroke of the piston. On the steam side of the valve it is called outside lead; on the exhaust side inside lead. In fig. 26 the opening h of the steam-port is the outside lead and h′ the inside lead.

Fig. 26.

Scale ³⁄₁₆ in. = 1 inch.

Question 48. What is meant by the travel of a valve?

Answer. By the travel we mean the motion of the valve back and forth, or in other words its stroke. If the arms of the rocker are of the same length, the travel of the valve is equal to the throw of the eccentric. For the preceding illustrations we have selected an eccentric with three inches throw, which is the travel of the valve.

Fig. 27.

Scale ³⁄₁₆ in. = 1 inch.

Question 49. How is the steam made to work expansively with a slide-valve?

Answer. By giving the valve what is called lap. That is, by allowing the edges of the valve when it is in the center of the valve-seat to overlap the edges of the steam-ports, as shown in fig. 27. Where this overlap, L L, is on the outside of the valve, it is called outside lap; when on the inside, l l, inside lap. When a valve has lap, those portions of the face[14] h, i, and h′, q, which cover the steam-ports, being wider than the ports, therefore occupy some time in moving over them, during which time the steam is enclosed in the end of the cylinder, as there is then no communication either with the steam-chest or the exhaust-port. This action is shown very clearly by the motion-curves in fig. 26. The valve in this case has ¹⁄₄ inch lead. At 4¹⁄₂ inches of the stroke of the piston the valve has moved as far as it will go in that direction, and the steam-port has its maximum width of opening. From that point the valve will begin to close the steam-port, and at 14¹⁄₂ inches of the stroke the port will be entirely covered, and the steam therefore be cut off. The port will remain closed until the piston has moved 21³⁄₄ inches, when it will be observed from the motion-curve r s t u v w x, that the port c is opened to the exhaust and the steam escapes, or, as it is technically called, the release takes place. From the time the steam is cut off to the time it is released, it works expansively in the cylinder.

Question 50. What relation is there between the amount of lap[15] and the degree of expansion?

Answer. The greater the lap with any given travel, the shorter will be the period of admission of steam, and, consequently, the more time and space for expansion.

Question 51. What is the effect of inside lap?

Answer. It delays the release of the steam. Thus in fig. 26 the valve has ¹⁄₈ in. inside lap. The motion-curve r s t u v w x shows that the release takes place during the back stroke at 21³⁄₄ in. If now there was no inside lap, the dotted line y, x would represent the exhaust edges of the valve, and the release would then occur somewhat earlier, or at 21 in. For this reason no inside lap is usually given to valves for engines which run at high rates of speed, as it allows too little time for the steam to escape. In fact, in some cases, what is called inside clearance is given to the valve; that is, the valve as shown in fig. 29, when it is in the middle of the valve-face, does not entirely cover the steam-ports. The effect of this is just the reverse of that produced by inside lap; that is, it causes the release to occur earlier in the stroke.

Fig. 29.

Scale ³⁄₁₆ in. = 1 inch.

Question 52. With the same outside lap, what is the effect of changing the travel of the valve?

Answer. By increasing the travel, the period of admission is increased and that for expansion lessened; and by reducing it, the admission is lessened, and the degree of expansion is increased. This is shown by the motion-curves in fig. 28, in which the same valve and ports are represented as are shown in fig. 26, but the valve has a travel of 5 instead of 3 inches. The valve also has the same lead. By following the motion-curve h i j k l m n, it will be seen that the steam is thus admitted up to 20¹⁄₂ inches of the stroke of the piston, and the period of expansion, as compared with that in fig. 26, is correspondingly lessened. It will also be seen by comparing fig. 26 with fig. 28 that with the short travel of the valve the ports are not opened so wide as they are when the travel is increased. This evil is practically obviated, however, by making the ports so long that with a comparatively small opening they will still have area sufficient to admit enough steam to fill the cylinders, and it is known that an opening less than the whole area of the steam-ports is sufficient to facilitate the passage of steam into the cylinder.

Fig. 28.

Scale ³⁄₁₆ in. = 1 inch.

Question 53. How is the exhaust affected by lap and lead?

Answer. The steam is released earlier in the stroke in proportion as the amount of outside lap and lead is increased, but the steam-port is also closed to the exhaust, or compression, as it is called, begins earlier with lap and lead than without. Thus, in fig. 25, it will be seen that at the beginning of the stroke both ports are entirely closed; in fig. 26, however, in which the valve has both lap and lead, the port d is nearly wide open at the beginning of the stroke, and by following the motion-curve r s t u v w x, which represents the position of the exhaust edge of the valve, it will be seen that the steam was released from the port c before the piston had completed its stroke, or when it had still nearly 3¹⁄₂ inches to move. In fig. 25 the port c is not opened to the exhaust until the commencement of the stroke, but it remains open to its completion, whereas in fig. 26 it is closed, or compression begins, at 18 inches of the return stroke, as shown by the dotted motion-curve.

Question 54. How does the action of the connecting-rod influence the motion of the valve in relation to the piston?

Answer. By delaying the movement of the crank in the backward stroke of the piston, and accelerating it in the forward stroke. This will be best explained by reference to fig. 14, in which the piston is represented in the center of the cylinder, or the middle of the backward stroke. If now we take a pair of dividers set to a length equal to that of the connecting-rod, and from the center, f, describe an arc of a circle, a b, from the center of the shaft, and through the lower half of the circle which represents the path of the crank-pin, we will find that the point of intersection, a, falls short of the vertical line, c d, and that the crank-pin has not made quite one-quarter of a revolution while the piston was moving through the first half of the backward stroke. By referring to fig. 21, in which the piston is again in the middle of its stroke, but is moving forward, and by describing another arc of a circle, b a, from the center of the shaft and intersecting the path of the crank-pin, it will be seen that the latter has moved more than a quarter revolution, while the piston has made the first half of the forward stroke. Owing to this angularity, as it is called, of the connecting-rod, the crank-pin is behind the piston during its backward stroke and ahead of it during the forward stroke. As the valve is moved by the eccentric, and it in turn by the shaft and crank, any irregularities of the latter are of course communicated to the valve. We therefore find, by referring to fig. 26, that the point of cut-off occurs during the backward stroke at 14¹⁄₂ inches, and during the forward stroke at 12 inches. A similar inequality is observable in the points of release for the front and back strokes. It is not, however, a matter of very great practical importance with stationary engines which run at comparatively slow speeds; but if it is thought desirable, the period of admission and the point of release for both strokes can be equalized, either by giving the valve more lead or lap at one end than the other, or by making the one steam-port wider than the other. The mechanism employed for moving locomotive slide-valves, however, furnishes us with the means of modifying their motion in relation to that of the piston, and of thus equalizing the periods of admission and release for the front and back strokes. The methods of doing this will be more fully explained hereafter.

PART V. THE EXPANSION OF STEAM.

Fig. 30.

Scale ²⁄₃ in. = 1 foot.

Fig. 31.

Scale ¹⁄₄ in. = 1 inch.

Question 55. How can we determine by experiment the pressure of the steam in the cylinder at all points of the stroke?

Answer. By the use of an instrument made for that purpose, called an indicator. Its action can be best explained by supposing that we have a small cylinder and piston, T, fig. 30, (shown on an enlarged scale in fig. 31) attached by a pipe U to one end of the cylinder A, so that when steam is admitted to the latter it will be conducted to the small cylinder T through the pipe U. Over the small piston and attached to it is a spiral spring, s s, fig. 31, which is compressed when the piston rises and extended when it falls. To the top of the piston-rod, V, a pencil, W, is attached. Behind this pencil we will suppose there is a card, a b c d, and that this card is so arranged that we can slide it horizontally and in contact with the pencil point. With only the pressure of the atmosphere above and below the piston T, the spring would be neither compressed nor extended, and the piston would then stand in the position shown in fig. 31. If now we move card horizontally, the pencil will draw a line, g, h, called the atmospheric line. We will now suppose that the tension of the spring is such that a pressure of 10 lbs. per square inch above or below the piston will either extend or compress the spring ¹⁄₄ inch. In other words, every pound of pressure per square inch in the piston will move it ¹⁄₄₀ of an inch. If we could produce a vacuum under the piston, it would be pressed down by the atmosphere above it ¹⁵⁄₄₀, or ³⁄₈ of an inch. If, when it is thus depressed, we again slide the card along in contact with the pencil-point, it will draw another line, e, f, called the vacuum-line. Assuming that we have drawn these two lines, and that the piston and card are in the position shown in figs. 30 and 31, we will then suppose that a reciprocating motion can be given to the card by the lever L, M, N, fig. 30, which is pivoted at M and attached at N to the piston-rod by a short connecting-rod. It is obvious that by connecting the upper end L of the lever with a rod, L c, to the card, the latter will be moved backwards and forwards by the motion of the piston B, and that the motion of the card will be simultaneous with that of the piston B, but of course of shorter stroke. We will assume that the stroke of the card is equal to the length of the atmospheric and vacuum lines g h and e f, fig. 31. If now, the piston being at the beginning of the stroke as shown in fig. 30, we admit steam of 85 lbs. effective pressure per square inch (which is equal to 100 lbs. absolute pressure) into the cylinder A, it will be conveyed through the pipe U to the cylinder T, and will force up the piston ⁸⁵⁄₄₀ or 2¹⁄₈ inches above the atmospheric line, or ¹⁰⁰⁄₄₀ or 2¹⁄₂ inches above the vacuum line, as shown in fig. 32, and the pencil will draw a vertical line, g i, on the card, (represented by a dotted line in fig. 32.) We will suppose that steam is admitted during 8 inches of the stroke, and is then cut off. When the piston B, fig. 30, has moved that distance, which is one-third of its stroke, the card will also have moved one-third of its stroke, and will stand in relation to the pencil in the position represented in fig. 33, and as the absolute steam pressure in the cylinder was maintained at 100 lbs. while the card was moving that distance, the pencil will have drawn a horizontal line, i j. The steam is now cut off and begins to expand, and its pressure is thereby reduced. When the piston of the engine is at half-stroke, the card will also be at half-stroke, and the steam will be expanded from 8 to 12 inches of the stroke. By the rule given in the answer to question 20, its absolute pressure would then be 66²⁄₃ lbs., and the indicator-piston will then be pressed down by the spring, so that the pencil will stand in the position shown in fig. 34, or 66²⁄₃ fortieths of an inch above the atmospheric line. The pencil meanwhile will have drawn the curved line j k. When the piston has moved 16 inches, the steam will be expanded to double its volume and its absolute pressure will therefore be 50 lbs., and consequently the pencil will stand 50 fortieths or 1¹⁄₄ inch above the atmospheric line as shown in fig. 35, and the pencil will have continued the curve j k to l. At 20 inches the steam will have 40 lbs., and at the completion of the stroke 33²⁄₃ lbs. absolute pressure, and the pencil will have completed the curve j k l m n, as shown in figs. 36 and 37. This curve is called the expansion curve, and its form is that which mathematicians call a hyperbolic curve. If the steam is exhausted, the indicator-piston will descend and carry the pencil down to the atmospheric line, and the vertical line n h, fig. 38, will be drawn. On the return stroke, after the steam is exhausted from the main cylinder A, fig. 30, the pencil would draw the atmospheric line g h, fig. 38, thus showing that there is no steam pressure under the piston. Such a diagram is called an indicator diagram.[16] In practice there are a great many influences which modify it, such as condensation, performance of work, imperfection of valve gear, etc., but for the present these are disregarded.

Fig. 32.

Fig. 33.

Fig. 34.

Fig. 35.

Fig. 36.

Fig. 37.

Fig. 38.

Scale ¹⁄₄ in. = 1 inch.

Question 56. How can we ascertain the pressure of the steam for any point of the stroke from such a diagram?

Answer. By measuring the vertical distance by the expansion curve (fig. 38) from the vacuum or the atmospheric line, as for example 8 j, 12 k, 16 l, 20 m. As the indicator spring is extended or compressed one-fortieth of an inch[17] from every pound of pressure per square inch, either above or below the indicator piston, if we construct a scale S, S, fig. 38, with division of one-fortieth of an inch each, one of them will represent one pound of pressure per square inch if measured vertically from the atmospheric or vacuum line. If we sub-divide the vacuum line with the same number of parts as there are inches in the stroke of the piston (see fig. 39) we can draw vertical lines from these points and thus determine the pressure by comparing the length of such lines with the scale S, S. Thus the line 8 j measures 100 fortieths of an inch, thus showing that the absolute steam pressure at 8 inches of the stroke was 100 lbs. per square inch; the line 12 k measures 66²⁄₃ fortieths of an inch, thus showing that at 12 inches of the stroke the steam pressure was 66²⁄₃ lbs. At 16, 20 and 24 inches of the stroke the vertical lines measure 50, 40 and 33²⁄₃ fortieths; and therefore there were that number of pounds of steam pressure when the piston was at the point of the stroke named. Similar measurements could be made from other points, such as 2, 6, 10, or any other number of inches of the stroke. Of course, if we measure from the vacuum line we will have the absolute steam pressure, or the pressure above a vacuum, as it is sometimes called; if we measure from the atmospheric line we will have the effective pressure, or the pressure above the atmosphere.

Fig. 39.

Scale ¹⁄₄ in. = 1 inch.

Question 57. How can we determine the average pressure during the whole stroke of steam which works expansively?

Answer. This can be determined approximately by the following method: In the first place, divide the vacuum line (fig. 39) into any number of equal divisions, say six. From the points of division, 4, 8, 12, 16 and 20, which in this case correspond with the points which represent inches of the stroke, draw perpendicular lines, which will divide the indicator diagram into six divisions. It is obvious that during the time the steam is working full stroke the pressure is uniformly 100 lbs. absolute. While the piston is moving from 8 to 12 inches the pressure falls from 100 to 66²⁄₃ lbs., so that at 10 inches we have very nearly the average pressure during the period named. So from 12 to 16, 16 to 20 and 20 to 24 the average is nearly 57.1, 44.4 and 36.3 lbs., respectively. Now, by adding together the pressures in the middle of each one of a number of equal divisions of the stroke and dividing by the number of divisions, we will obtain approximately the average absolute pressure during the whole stroke. To get the average effective pressure, deduct the atmospheric pressure from the result. The calculation would in the above case be as follows:

A more accurate way of calculating the average or mean pressure, as it is called, when steam is used expansively, and the one which is usually employed, is to divide the length of the piston’s stroke in inches by the number of inches at which the steam is cut off: the quotient is the ratio of expansion. Get the Hyperbolic Logarithm of the ratio of expansion from the table of logarithms in the appendix, add 1 to it, and divide the sum by the ratio of expansion and multiply the quotient by the mean absolute steam pressure in the cylinder during its admission. The result will be the mean absolute pressure during the stroke. To get the mean effective pressure, deduct the atmospheric pressure.

The calculation for the above example would be as follows:

The table of hyperbolic logarithms given in the appendix will be needed in calculating the mean pressure of steam used expansively:

Question 58. What advantages result from using steam expansively?

Answer. There is a very important saving in the amount of steam required to do a given amount of work, and the strains and shocks which are produced by the rapid motion of the piston and other reciprocating and revolving parts of the engine are very much diminished by allowing the steam to expand, and thus become reduced in pressure during the latter part of the stroke.

Question 59. How is steam saved by using it expansively?

Answer. Less steam is required when it is used expansively:

1. Because when steam of a high pressure is introduced into the cylinder, and allowed to expand until its pressure is comparatively low, it escapes at a lower pressure than the average pressure during the whole stroke. If steam of a pressure equal to the average pressure is worked full stroke, it would exert exactly the same force on the piston as the steam of higher pressure did when working expansively, but the pressure in the latter case, when the piston reaches the end of the stroke, or the final pressure, as it is called, would be considerably lower than in the other. The pressure of steam represents energy, or capacity for doing work, and therefore if we allow it to escape with a comparatively high pressure without doing work, it is a waste of energy. To illustrate this, we will take the same conditions which were used in the answer to Question 57, in calculating the average pressure. In that case the mean absolute pressure of the steam was 69.95 pounds per square inch, but the pressure at the end of the stroke, when the steam escaped, was only 33²⁄₃ pounds absolute. If, therefore, steam had been used of the average pressure through the whole stroke, it would have escaped with a pressure of 69.95 pounds, or more than twice that of the expanded steam, and the work done in both cases would have been the same.

2. There is also another incidental advantage in this, because low-pressure steam can be exhausted more quickly from a cylinder than steam of a high pressure, and consequently there is less resistance, or back pressure, as it is called, in the exhausted end of the cylinder to the movement of the piston.

3. The causes which produce the greatest economy when steam is used expansively cannot be fully explained without discussing principles of science more abstruse than it is desirable to introduce here. They can, however, with the aid of the table of the “Properties of Steam,”[18] in the appendix, be illustrated by a few simple calculations, so that the economy of using steam expansively will be apparent.

For the basis of the calculations the same data and dimensions will be employed that were used in the previous illustration; that is, a cylinder of 16 in. diameter and piston with 24 in. stroke and steam of 100 lbs. absolute pressure cut off at 8 in. of the stroke. We will suppose, further, that the steam used is generated from water of a temperature of 60 degrees, and we will then calculate the total number of units of heat in the steam used for each stroke of the piston. The area of a piston 16 in. in diameter is 201 square inches; and as the steam is admitted until the piston moves 8 inches of its stroke, therefore the quantity of steam would be 8 times 201 cubic inches, or

From the table it will be seen that one cubic foot of steam of 100 lbs. pressure weighs .2307 lbs.; therefore the weight of the fraction of a cubic foot given above would be calculated as follows:

= weight of 1608 cubic in. of steam of 100 lbs. absolute pressure.

From the table it will be seen that the total heat above zero of steam of 100 lbs. absolute pressure is 1213.4 degrees. That is, as was explained in answer to Question 40,[19] in order to boil water under a pressure of 100 lbs. per square inch we must first heat water up to 327.9 degrees, and then, to convert it into steam, 885.5 degrees more must be added. It was also explained in the answer to Question 35 that one pound of water heated one degree is the standard of measurement or unit of heat. Now if we have 1 lb. of water with a temperature of zero, evidently it will take 1213.4 units of heat to convert it into steam of 100 lbs. absolute pressure. But as the water from which our steam was generated had a temperature of 60 degrees, we must deduct that much from 1213.4: 1213.4 - 60 = 1153.4 = units of heat in one pound of steam of 100 lbs. absolute pressure generated from water of 60 degrees temperature.

If now one pound of steam has 1153.4 units of heat, the following calculation will give the units of heat in .2146 lbs.: 1153.4 × .2146 = 247.51 = units of heat in .2146 lbs., or 1608 cubic in. of steam of 100 lbs. absolute pressure.

It was shown in answer to Question 57 that the average pressure of steam of 100 lbs. cut off at 8 in. of the stroke was 69.95 lbs. per square inch. Disregarding the small fraction, we will call it 70 lbs. Now if we admit steam of this pressure through the whole stroke of the piston, we will use 4,824 cubic inches. It will be found by a calculation similar to the above, that to generate this quantity of steam of 70 lbs. pressure from water of a temperature of 60 degrees would require 527 units of heat, or more than twice as many as were required to do the same work with steam of 100 lbs. pressure cut off at 8 inches when using it expansively during the rest of the stroke. The actual difference in practice is not so great as this, because the loss of heat from radiation and condensation in the cylinder and other causes is greater when steam of a high pressure is expanded than when lower pressure steam is admitted through the whole stroke. But after allowance is made for all such sources of loss and waste, there is still an enormous gain from using steam expansively.

Question 60. What is meant by wire-drawn steam?

Answer. It is the fall which the pressure of the steam undergoes during its passage from the boiler to the cylinder,[20] and which is due to the contracted opening of the steam pipes or valves.

Question 61. What is the economical effect of reducing the pressure, or of wire-drawing it, by partly closing the valve by which it is admitted to the cylinders?

Answer. By reducing the pressure of steam in this or any other way, it is necessary in doing the same amount of work to admit steam to the cylinder for a longer period, and therefore to reduce the degree of expansion. To illustrate the effect of this, we will estimate the total heat required to exert a pressure of 70 lbs. on the piston described above. It will be assumed that the steam pressure in the boiler is 100 lbs. absolute, and that this is wire-drawn down to 70 lbs. and admitted to the cylinder through the whole stroke. As was shown in the preceding answer, 4,824 cubic inches of steam are required to fill the cylinder. Now 3,376.8 cubic inches of steam of 100 lbs. pressure, if expanded to 70 lbs. pressure, will make 4,824 cubic inches. The total heat required to generate 3,376.8 cubic inches of steam of 100 lbs. absolute pressure from water of 60 degrees is 519.9 units, so that to do the same work by using steam of high pressure cut off at one-third of the stroke, using steam of low boiler pressure full stroke, and using wire-drawn steam full stroke, would, in the example we have selected, require 247.5, 527 and 519.9 units of heat respectively.

Question 62. To what extent can we work steam expansively, with advantage and economy?

Answer. The theoretical economy of using steam increases with the degree of expansion and the pressure. This is shown very clearly in the following table, in the first column of which the number of inches of the piston stroke is given during which steam is admitted to a cylinder 16 in. in diameter and 24 in. stroke. In the second column is given the pressure of the steam, or initial pressure, as it is called, which must be admitted into the cylinder in order to produce a mean pressure of 70 lbs. per square inch when it is cut off at the point indicated in the first column. In the third column is given the total heat which is required to generate the steam required in each case, and in the last column the percentage of saving is given which results from the different degrees of expansion and a mean pressure of 70 lbs. per square inch in each case.

RESULTS OF USING STEAM EXPANSIVELY.

From this table it will be seen that theoretically 22¹⁄₂ per cent. of heat is saved by cutting off at ³⁄₄ of the stroke and using steam of 72.5 lbs. pressure instead of steam of 70 lbs. worked full stroke. Cutting off at half stroke and using steam of 82.7 lbs., 41¹⁄₄ per cent. of heat is saved, and cutting off at quarter stroke with steam of 117.4 lbs. saves 58 per cent. of heat; and at one-twelfth of the stroke, or expanding steam of 241.4 lbs. pressure to twelve times its volume, saves 72¹⁄₂ per cent. of heat.

As stated before, the above is the theoretical advantage of using steam expansively. There are, however, practical difficulties in the way of using some of these high degrees of expansion. It has already been explained that if steam is cut off early in the stroke and the degree of expansion increased, the pressure and consequently the temperature of the steam must also be increased. The danger of explosion is greater with the higher pressures, and stronger and more expensive boilers and machinery are therefore needed. With steam of very high temperature the metal of the cylinders, pistons and valves becomes so much heated that they soften, and then the friction of the one on the other causes them to cut or scratch each other. The high temperature at the same time destroys the oil or other lubricant used in contact with the steam. It is also impossible to admit and cut off steam very early in the stroke with the ordinary mechanical appliances used for moving slide-valves of locomotives. This latter difficulty will be more fully explained hereafter.

PART VI. GENERAL DESCRIPTION OF A LOCOMOTIVE ENGINE.

Question 63. What are the principal parts of an ordinary locomotive engine?

Answer. A boiler for generating steam and a pair of high-pressure steam engines, which are all mounted on a suitable frame and wheels adapted for running on a track consisting of two iron or steel rails.

Question 64. How is the power of high-pressure engines applied to locomotives?

Answer. By connecting the engines with the wheels so as to give the latter a rotary motion.

Question 65. When they revolve what will occur?

Answer. Either they will slip on the track, or the locomotive will move either backward or forward, according to the direction the wheels are turning.

Question 66. What will determine whether the wheels will slip or the locomotive move?

Answer. The friction or adhesion, as it is called, between the wheels and the track. If this adhesion is greater than the resistance opposed to the movement of the locomotive, the latter will overcome the resistance; but if the latter is greater than the friction, the wheels will slip.

Question 67. Upon what does the amount of friction or adhesion of the wheels depend?

Answer. Chiefly on the weight which they bear, but to some extent upon the condition of the rails. Under ordinary circumstances, the adhesion of the wheels of a locomotive is in direct proportion to the weight they carry.

Question 68. Why are two cylinders employed on locomotives?

Answer. Because if only one was used, it would be impossible or very difficult to start the engine, if it should stop on one of the dead points.

Question 69. How is this difficulty overcome by the use of two cylinders?

Answer. By attaching the two cranks to the same shaft or axle, and placing them at right angles to each other, so that when the one is at a dead point the other is in the position where the steam can exert the maximum power on the crank.

Question 70. How are the cranks of an ordinary locomotive made?

Answer. They are cast in one piece with the wheels that drive the locomotive, which are therefore called driving-wheels. In this country the centre portion of such wheels, or wheel-centres as they are called, is always made of cast iron, with tires of wrought iron or steel around the outside, and is fastened to the axles of the locomotive. The shaft of a locomotive engine is called the main driving-axle, and the wheels attached to it the main driving-wheels.

Question 71. How are the cylinders and driving-wheels of a locomotive usually placed?

Answer. The cylinders A, plates I, II and III, are placed at the front end of the locomotive, and the main driving-axle, B, far enough behind them to permit the connecting rods, C, to be attached to pins, D, in the cranks, called crank-pins. In this country these cranks are now universally placed on the outside of the wheels, and therefore the cylinders must be placed far enough apart (as shown in fig. 40 and Plate III) to permit the connecting-rods to be attached to the crank-pins. The cylinders are therefore placed outside the frames, H, H, Plate III, (the latter are inside of the wheels,) and are now nearly always horizontal, although in old engines they are often inclined. Plate I is a side view of an ordinary eight-wheeled American locomotive, Plate II a longitudinal section, Plate III a plan, and fig. 40, a transverse section through the cylinder and smoke-box.

Fig. 40.

Scale ¹⁄₄ in. = 1 foot.

Question 72. What are the smaller wheels, E E, called, and what are they for?

Answer. They are called truck-wheels and carry the weight of the cylinders and other parts of the front end of the locomotive, and serve to guide and steady the machine in a manner which will be more fully explained hereafter.

Question 73. Why are more than one pair of driving wheels necessary for locomotives?

Answer. Because if all the weight which is needed to create the requisite adhesion of the wheels of locomotives to pull heavy loads was placed on one pair of wheels, it would be so excessive as to partly crush and injure the rails. It is therefore distributed, usually on two pairs, but sometimes on three or four or even more pairs.

Question 74. Where is the second pair of driving-wheels usually placed?

Answer. These wheels, F—called the back or trailing driving-wheels—are, in the ordinary type of locomotives used in this country, situated behind the main driving-wheels, far enough back to give the room necessary for the boiler, G, between the two axles, as shown in plates I, II and III.

Question 75. How are the axles, cylinders, etc., held in the right position in relation to each other?

Answer. By longitudinal frames, H, H, H, H, which hold the axles in the proper position, and are bolted to the cylinders, and also fastened to the boiler at I, I, Plate I.

Question 76. How is a locomotive engine made to run either backward or forward?

Answer. By having two eccentrics, J, J, Plate III (also shown in Plate II,) for each cylinder. One of these is fixed or set on the shaft in such a position as to move the valve so that the engine will run in one direction; the other eccentric is set so that the engine will run the reverse way. The ends of the two eccentric rods are attached to what is called a link, L, (Plates II and III,) the object of which is to furnish the means of quickly engaging and disengaging either eccentric rod to or from the rocker, K. The link is operated by a system of levers, consisting of the lifting shaft, M, and arms, N, N, and the reverse lever, O, O, (Plate II). The principles and working of these will be more fully explained hereafter.

Question 77. What are the principal parts or “organs” of a locomotive boiler?

Answer. 1. A fire-place, or, as it is called a fire-box, G, (Plate II,) which is surrounded with water.

2. A cylindrical part, P P, (Plates I and II,) attached to the fire-box at one end and to a chamber, Q, called the smoke-box, at the other.

3. The tubes or flues a a′, (Plate II and fig. 40,) which connect the fire-box with the smoke-box and pass through the cylindrical part of the boiler and are surrounded with water.

4. The smoke-stack or chimney R R.

Question 78. What is each of these parts or organs for and of what do they consist?

Answer. The fire-box G furnishes the room for burning the fuel, and consists of an inner and outer shell made of boiler plate, with the space between the two filled with water; a grate, b b, (Plate II,) formed of cast-iron bars, with spaces between them for admitting air for the combustion of the fuel, which is placed on the top of them; a door, C, called the furnace-door, for supplying the grate with fuel; a receptacle, d d, below the grate, to collect ashes, and therefore called the ash-pan, which is supplied with suitable dampers, n′, n′, for admitting or excluding the air from the fire.

The cylindrical part P P, or waist of the boiler as it is sometimes called, contains the greater part of the water to be heated.

The flues or tubes, as they are generally called, of which a locomotive has from one to two hundred, are usually two inches in diameter, and about eleven feet long. They conduct the smoke and products of combustion from the fire-box to the smoke-box. These tubes are made of small diameter so as to sub-divide the smoke into many small streams and thus expose it to a large radiating surface through which the heat is conducted to the water.

The smoke-stack serves partly for removing into the open air the smoke which passes through the flues, and partly for producing a strong draft of air, which is indispensably necessary for the rapid combustion of the fuel, and also for collecting and extinguishing the sparks from the fire.

Question 79. How is the draft produced in locomotive boilers?

Answer. By conducting the exhaust steam through pipes (e, e, fig. 40) from the cylinders to the smoke-box and allowing it to escape up the smoke-stack from apertures, f, f, (Plate II, fig. 40.) called exhaust nozzles. The strong current of steam thus produced in the smoke-stack produces a vacuum, by which the smoke is sucked into the smoke-box with great power and forced out of the smoke-stack into the open air.

Question 80. How are the water and fuel carried which must be supplied to a locomotive while it is running?

Answer. The water is carried in a tank, which is constructed in the form of the letter U, so as to give room for the stowage of fuel between its two branches or sides. This tank is carried on a set of wheels, and forms a separate vehicle, independent of the locomotive, called a tender, the construction of which will be explained in a future chapter.

Question 81. What are the dimensions of the principal parts of a locomotive?

Answer. There is a great variety in the plan, size and capacity of locomotives, but the type which is more generally used in this country than any other, and which has been selected for the preceding illustrations, and will be described in the succeeding chapters of the Catechism, has four driving and four truck wheels, and weighs in working condition about 60,000 lbs. The following are the dimensions of its principal parts: The driving-wheels are about 5 feet and the truck-wheels from 26 to 30 inches in diameter. The longitudinal distance between the centres of the driving-wheels is usually about 7 feet, and between the centres of the truck-wheels 5 ft. 9 in., and the total distance from the centre of the back driving-wheels to the centre of the front truck-wheels, which is called the wheel-base, is 21 ft. 8 in. The weight on each driving-wheel is usually about 10,000 lbs., and on each truck-wheel about 5,000 lbs. The cylinders are 16 in. in diameter and the piston has 24 in. stroke, and the connecting-rod is 7 ft. long measured between the centres of the pins to which it is attached. The centres of the cylinders are about 6 feet apart, measured across the track. The fire-box inside is 5 feet long and 2 ft. 11 in. wide, and the cylindrical part of the boiler is 4 feet in diameter measured on the outside of the smallest portion. The water spaces around the fire-box are about 3 inches wide. There are 140 tubes, which are 2 in. in diameter measured on their outside, and 11 ft. long. The inside of the smoke-stack is 16 in. in diameter, and it is 14 ft. 3 in. high measured from the top of the rails. The tender carries about 1,800 gallons of water and about 8,000 lbs. of coal. When loaded it weighs about 40,000 lbs., making the total weight of the engine and tender 100,000 lbs.

The following is a list of parts designated by the letters of reference on plates I, II, III and fig. 40.

PART VII. THE LOCOMOTIVE BOILER.

Question 82. How does the quantity of steam generated in locomotive boilers in a given time compare with that generated in the boilers of stationary and marine engines?

Answer. Locomotive engine boilers must produce much more steam in a given time, in proportion to their size, than is required of the boilers of any other class of engines, (excepting perhaps those of steam fire-engines,) because the space which locomotive boilers can occupy and also their weight is limited.

Question 83. How is their steam-generating capacity increased above that of marine and stationary boilers?

Answer. By creating a very strong draft of air through the fire and then passing the smoke and heated air through a great many small tubes, which are surrounded by water. By this means the smoke and hot air are divided into many small streams or currents which are exposed to the inside surface of the tubes to which and to the surrounding water their heat is imparted.

Question 84. How is the action of the exhaust steam in producing a draft in the chimney explained?

Answer. The exhaust steam escapes from the cylinders through one or two contracted openings or exhaust-nozzles (f, Plate II, also shown in fig. 40[21]), which point directly up the centre of the chimney or smoke-stack. The exhaust steam escapes from this orifice with great velocity, and expands as it rises, so that it fills the pipe p and the smoke-stack R R. It thus acts somewhat like a plunger or piston forced violently up the chimney, and pushes up the air above it, and, owing to the friction of the particles of air, carries that which surrounds it along up the stack, from which it all escapes finally into the open air, thus leaving a partial vacuum behind in the smoke-box. The external pressure of the atmosphere then forces in air through any and every opening in the smoke-box, to take the place of that already drawn out or exhausted from it. As the only inlet is through the tubes, to which the gases of combustion have free access from the fire-box, and as the external air can only pass through the fire-grate, and through the burning fuel, to reach the fire-box, there is a constant draft of air through the grate as long as the waste steam escapes from the blast-pipe and up the chimney. It is thus that, within certain limits, the more the steam that is required, the more the steam that is produced; for all the steam used in the engine draws in the air in its final escape, to excite the fire to generate more steam.[22] Sometimes one blast-orifice is used for each cylinder, as shown in plates II and III and fig. 40; in other cases the exhaust steam from each cylinder escapes through the same orifice.

Question 85. How much water is it necessary to evaporate in order to furnish the steam required to run an ordinary train at its usual speed?

Answer. For an ordinary “American” locomotive,[23] weighing 60,000 lbs. and with cylinders of 16 inches diameter and 24 inches stroke, from 6,000 to 12,000 lbs. of water must be evaporated per hour.

Question 86. How much water will a pound of coal evaporate in ordinary practice?

Answer. The quantity of water which is converted into steam by a pound of coal varies very materially with the quality of the coal, and the construction and condition of the boiler; but from 6 to 8 lbs. of water per pound of coal is about the average performance of ordinary locomotives. It is, therefore, necessary to burn from 500 to 2,000 lbs. of coal per hour in order to generate the quantity of steam required by ordinary engines.

Question 87. How large a grate is needed to burn this quantity of coal?

Answer. The maximum rate of combustion may be taken at about 125 lbs. of coal on each square foot of grate surface per hour, so that to burn 2,000 lbs. we need a grate with about 16 square feet of surface.

Question 88. How much heating surface is needed for a given size of grate?

Answer. In common practice about 50 square feet of heating surface are given for each square foot of grate. There are, however, no reasons for the proportions of either grate or heating surface which are given, excepting that it has been found that they work well in practice. It is, however, quite certain that the larger a boiler is, and the greater its heating surface in proportion to the steam it must generate, other things being equal, the more economical will it be in its consumption of fuel, or, in other words, the more water will it evaporate per pound of coal.

Question 89. Why is it necessary to use small tubes or flues in order to have the required amount of heating surface?

Answer. Because there is a great deal more surface in a small tube of a given length, in proportion to the space it occupies, than in a large one. Thus a tube two inches in diameter and eleven feet long has 829 square inches of surface, and one four inches in diameter has 1,658 square inches, or just double the quantity. But the four-inch tube occupies four times as much space as the other, as it is twice as high and twice as wide. Therefore, in proportion to the space it occupies, the tube which is two inches in diameter has twice as much surface as the larger one. If we compare a two-inch with an eight-inch tube, we will find that the former has four times as much surface, in proportion to its size, as the eight-inch tube. As the size and weight of locomotive boilers are limited, it is therefore necessary, in order to get the requisite heating surface in the space to which we are confined, to use tubes of small diameter.

Small tubes also have the advantage that they may be made of thinner material, and yet have the same strength to resist a bursting pressure from within, or a collapsing pressure from without, as larger tubes made of thicker metal. The advantage of thin tubes is, that the heat inside of them is conducted to the water outside more rapidly than it would be through thicker metal, which is important when combustion is as rapid as it is in locomotive boilers.

The reason tubes of smaller diameter than two inches are not ordinarily used is because they are then liable to become stopped up with cinders and pieces of unconsumed fuel.

Question 90. How is the fire-box of a locomotive constructed?

Answer. It usually consists of a rectangular box (G, figs. 41 and 42) about three feet wide[24] and, for the size of engine we have selected as an example, about five or five and a half feet long inside. This box is composed of metal plates, either iron, steel or copper, which, excepting on the front side, are from ⁵⁄₁₆ to ³⁄₈ of an inch thick. This box is called the inside shell of the fire-box, and is surrounded by another shell, A B C D E F, fig. 42, of either iron or steel plates, of about the same thickness as those composing the inside. This is called the outside shell of the fire-box and, as already explained, is so much larger than the inside that there is a space, called the water-space, from 2¹⁄₂ to 4¹⁄₂ inches wide, on all the sides of the fire-box between the inner and outer plates.

The top g, g, of the inside shell, which is called the crown-sheet or crown-plate, is flat, whereas the outside shell is arched, as shown in fig. 42. To the front plate of the inside shell the tubes a a′, a a′ are attached. For this reason its thickness is usually made greater than that of the other plates, and is usually from ³⁄₈ to ³⁄₄ of an inch. The edges of one of the plates at each corner of the fire-box, where they are united together, as shown in figs. 41 and 42, are bent at right angles, and the other is fastened to it with rivets from ⁵⁄₈ to ³⁄₄ of an inch in diameter.

The inside and the outside shells of the fire-box are united to each other by a wrought-iron bar or ring (A F, figs. 41 and 42) which completely surrounds the inner shell and closes the water-space between the two shells. This bar is bent and welded to the proper form to extend around the bottom of the inside fire-box, and it is riveted to both shells. The water in the water-space is in free communication with the rest of the water in the boiler; and thus the flat sides of the respective shells of the fire-box are exposed to the full pressure of the steam, which tends to burst the outside shell and collapse the inside one. These flat sides, by themselves, would be unable to resist the strain upon them, but as the strain upon the respective fire-boxes is in opposite directions, and necessarily equal for equal areas of surface, tie-bolts, n, n, n, n, (figs. 41 and 42,) or, as they are called stay-bolts, which are from ³⁄₄ to 1 inch in diameter, are screwed through the plates at frequent intervals, usually from 3¹⁄₂ to 4¹⁄₂ in. apart, so as to connect the two fire-boxes securely together, the ends of the stay-bolts being also riveted or spread out by hammering so as still further to increase their holding power. These bolts, owing to the expansion and contraction of the boiler and other strains to which they are subjected, very frequently break, and if they are made of solid bars of metal there is no way of discovering with certainty whether they are in good condition or not without taking the boiler to pieces. They should therefore be made of the best quality of wrought iron, brass or copper and should also be made tubular, that is they should have a hole through the centre, so that when they break the water will escape at the fracture into the hole and the leak will thus indicate the defect and danger. The latter is much greater from this cause than is usually supposed, and it is not unusual to find on taking a boiler to pieces that a large number of the stay-bolts are broken.

Fig. 41.

Scale ¹⁄₄ in. = 1 foot.

Fig. 42.

Scale ¹⁄₄ in. = 1 foot.

Question 91. How can the strain on the flat surface of a boiler between the stay-bolts be calculated?

Answer. By MULTIPLYING THE AREA IN INCHES BETWEEN ADJACENT STAY-BOLTS BY THE PRESSURE. The reason for this is, that each stay-bolt must sustain the pressure on a part of the plate to which it is attached. Thus in fig. 43 it is plain that the bolt s must sustain the pressure on one-half of that part of the plate between it and the bolts v, t, w, u, around it, or the pressure on the square a b d c, whose sides are equal to the distance (4 inches) between the centres of the bolts. With a pressure of 100 pounds per square inch, the calculation would therefore be: 4 × 4 × 100 = 1,600 lbs. on each bolt.

Fig. 43. Scale 1¹⁄₂ in. = 1 foot.

Stay-bolts should never be subjected to a strain of more than one-eighth or one-tenth of their breaking strength.

Question 92. How do stay-bolts often fail without breaking?

Answer. By tearing or stripping the thread of the bolt, or that in the plate, but oftener perhaps by the stretching of the plates between the holes. With a heavy pressure, the tendency of the plates between the holes, especially if they are heated very hot, is to “bulge” outward and thus stretch the hole in every direction until it is so large that the bolt is drawn out without much injury to the screw-thread.

Question 93. How is the flat-top or crown-sheet strengthened?

Answer. It is sometimes strengthened with stay-bolts similar to those used for the sides, which pass through the inner and outer shells;[25] but usually the crown-sheet is strengthened by a series of iron bars, (f, f, fig. 41 and 42) called crown-bars, placed on edge, and of considerable depth, which are firmly fastened to it by T-head rivets or bolts. The crown-sheet can therefore only be crushed downwards by bending these bars, which are of great strength. They usually extend crosswise of the length of the fire-box, but are sometimes placed lengthwise. These bars bear on the fire-box only at each end, as shown in fig. 42, and are usually made with a projection, k, k, which rests on the edge of the side plates. Iron rings or washers from ³⁄₄ to 1¹⁄₂ inches thick are interposed between the plate and the bars at the points where the bolts or rivets which secure the rivets pass through. This permits the water to circulate under the bars, and prevents the crown-sheet from being burnt or overheated, as it would be if the water were excluded from the whole under surface of the crown-bars. [26]The crown-bars are also attached to the outer shell and the dome by braces, e, e, l, l.

The opening c, fig. 41, at the back end is for the door through which fuel is supplied to the grate.

Question 94. How are the grates constructed?

Answer. They are generally made of cast-iron bars, and for burning coal are usually arranged so that the fire can be shaken by moving the bars. For burning anthracite coal, the grates are sometimes made of wrought-iron tubes, through which a current of water circulates to prevent them from being overheated.

Question 95. How are cinders and burning coals prevented from falling through the grate upon the road?

Answer. By attaching a sheet-iron receptacle or ash-pan (d′ d′, fig. 41) as it is called, under the grate, which it completely encloses from the outside air. This then serves two purposes, as it is often important when the engine is standing still to prevent any access of air to the fire-box, and therefore the ash-pan is made to fit tightly to the fire-box. Suitable doors, or dampers as they are called, are placed in front and behind, and sometimes on the sides, which can be opened or closed to admit or exclude air as may be needed.

Question 96. How are the tubes or flues of a locomotive arranged?

Answer. They are fastened into accurately drilled holes in the tube sheet (a, a, figs. 41 and 42) which forms the front of the fire-box and in similar holes in a plate (a′, a′, fig. 41,) which forms the front end of the cylindrical part of the boiler. They thus connect the fire-box with the smoke-box. The tubes are arranged so that each tube will have a space of from ⁵⁄₈ to ⁷⁄₈ of an inch between it and those adjoining. The position of the holes for the tubes in relation to each other is determined by describing from the centre of one tube (o, fig. 44) a circle with a radius, o k, equal to the sum of the diameter of a tube and the distance which they are intended to be apart, and then subdividing this circle with the radius into six parts, k, r, s, l, g and p. Each point of subdivision and also the centre, o, of the circle will be the centre of a tube. By drawing them from these centres it will be found that the distances a b, c d between adjoining tubes will be the same between all of them. By describing circles from the centres of the outside tubes and subdividing the circles as before the position of other tubes will be determined around those first laid down. This can, of course, be carried out indefinitely. A difference in the arrangement of the tubes will be observed if, when we subdivide the first circle shown in fig. 44, instead of commencing from the intersection of a vertical line we begin from a horizontal line, h i, as shown in fig. 45. In the former case the tubes are said to be in vertical rows, and in the latter in horizontal rows. It is apparent from the figures and as shown by the arrows that the water can circulate in ascending currents more freely when tubes are arranged in vertical rows than when they are arranged horizontally.

Fig. 44. Scale ¹⁄₄.

Fig. 45. Scale ¹⁄₄.

Question 97. How are the tubes fastened and made water-tight in the tube-sheets?

Fig. 46. Scale ¹⁄₄.

Fig. 47.

Answer. They are inserted into the holes drilled to receive them, and the ends are allowed to project about a quarter of an inch beyond the tube-sheets. A tapered plug, fig. 46, is then driven into the tube, to expand it so that it will fit the hole. A tool is used called a tube-expander, fig. 47, which is what might be called an expanding plug, consisting of a number of sections, a, b, c, d, e, f, g, h, held together by a spring clasp s, which embraces them, as shown in the engraving. This plug when the sections are drawn together is inserted into the mouth of the tube, and the tapered plug p p, is then driven into the opening left in the center of the cluster of sections, which are thus expanded. By this means, the ridge a b c d expands the tube at the inner edge of the tube-sheet, forming a ridge or corrugation, as shown at c c, fig. 48. At the same time the shoulder j k l on the tool expands the outer edge of the tube somewhat as is shown at f f, fig. 48. By repeating this process, and slightly turning the expander each time, the tubes can be made perfectly water-tight. There are other forms of tube-expanders, but the one described, known as Prosser’s expander, is more generally used than any other. In many cases, after the tubes are expanded with the tool described, the outer edge is turned over still more with what is called a thumb-tool, fig. 49, probably from its resemblance in form to a man’s thumb. By placing the curved shoulder a on the end f, fig. 48, of the tube it is turned over, somewhat in the form shown in the engraving, by repeated blows of a hammer on the end of the tool. Copper ferrules, represented by the black shading, a a, are also much used now on the outside of locomotive tubes, and it is said that with them the joints can be kept tight much easier than without. By turning over the outside edge of the tube as shown in fig. 48, it not only protects the copper ferrule, but, as the tubes must act as braces to sustain the pressure of steam in the flat tube-sheets, it gives the joints the requisite strength for resisting such strains.

Fig. 48. Scale ¹⁄₄.

Fig. 49.

Question 98. How can the strain on the cylindrical part of a boiler be calculated?

Answer. By multiplying the diameter in inches by the length in inches and the product by the steam-pressure per square inch. Thus for a boiler 48 inches in diameter and 10 feet long with 100 pounds pressure the calculation would be 48 × 120 × 100 = 576,000 lbs.

Question 99. Why do we multiply the diameter, instead of the circumference, by the length, to get the strain on the cylindrical part?

Answer. The reason for multiplying by the diameter instead of by the circumference is because only a portion of the pressure on the inside surface of the boiler exerts a force to burst the shell at any one point. Thus, supposing the following diagram, fig. 50, to represent a section of a boiler, if we have a force acting on the shell in the direction of the line a b, at the point b, where it is exerted against the shell of the boiler, it would be composed of two forces, one acting in the direction b e, and tending to tear the boiler apart on the line c d, and the other acting in the direction f b, to tear it apart on the line h g. It is so with all pressure inside the boiler, excepting that, say a h, which acts exactly at right angles to the line of rupture c d, it is all composed of two forces, only one of which tends to tear the boiler apart at one point. It is therefore only a part of the pressure on the circumference which tends to burst the boiler at a given place, and that part is equivalent to the pressure on a surface whose width is equal to the diameter and not the circumference.

Fig. 50.

Fig. 51.

This we know is a little difficult for those to understand who are not familiar with the principles of what is called the “resolution of forces,” and we will therefore try to make it clear in another way.

To do this we will suppose that we have a boiler, a b, fig. 51, made in two halves and bolted together at a and b by flanges. It is evident that if we brought a pressure against the inside of the flanges in the direction of the darts c and d, such a pressure would not have a tendency to tear apart the bolts a and b. Some distortion of the boiler might in fact take place, if, for example, we put a jack-screw inside and forced out the flanges as indicated, without subjecting the bolts to a tensile strain. We see therefore that the forces acting in the direction c and d have no tendency to tear apart the bolts at a and b, but it is only the forces such as e, f and g, which act at right angles to a b, that exert a strain on the flanges.

That this force is equivalent to a pressure on a surface with a width equal to the diameter of the boiler is apparent if we suppose that we have a boiler, a b, fig. 52, and that each half, c and d, is nearly filled with some substance, say wood or cement, which is fitted so tight that no steam can get between it and the shell of the boiler. It is apparent now that if we admit steam into the space f, the force exerted on the bolts a and b is that due to the pressure on the surface of the wood or cement exposed to the steam whose width is equal to the diameter of the boiler. It might be said though that if this substance were elastic, like india-rubber, the effect of the steam would be different. If it were elastic, and a pressure on the surfaces f caused it to spread in the direction g and h so as to produce a pressure in those directions, it would, as has already been shown, not exert a force on the bolts a and b to tear them apart, but have a tendency to rupture the boiler at right angles to a b. The sides of the boiler must therefore have a strength sufficient to resist this force which tends to tear them asunder. If the boiler is made of iron ³⁄₈ inch thick there would be a sectional area of 45 square inches on each side, or a total of 90 square inches, to resist this strain, so that each square inch must bear 6,400 lbs. of strain. The correctness of this rule can be demonstrated by the use of mathematics, which would be out of place here. Its practical truth has however been proved by experiment.

Fig. 52.

Question 100. How much strain per square inch is good boiler plate capable of resisting, and how much is it safe to subject it to?

Answer. There is great variation in the tensile strength[27] of rolled iron boiler plate, but that of good plate will average about 50,000 pounds per square inch, if the strain is applied in the direction of the “grain” or the fibres of the iron[28], and about ten per cent. less if the strain is applied crosswise of the grain. It has, however, been found by experiment that when a tensile strain is applied to a bar of iron or other material, it is stretched a certain amount in proportion to the length of the bar and to the degree of strain to which it is subjected. It is found that if this strain does not exceed about one-fifth of that which would break the bar, it will recover its original length, or will contract after being stretched, when the strain is removed. The greatest strain which any material will bear without being permanently stretched is called its limit of elasticity, and so long as this is not exceeded no appreciable permanent elongation or “set” will be given to iron by any number of applications of such strains or loads. If, however, the limit of elasticity is exceeded, the metal will be permanently elongated, and this elongation will be increased by repeated applications of the strain until finally the bar will break. At the same time the character of the metal will be altered by the repeated application of strains greater than its elastic limit, and it will become brittle and less able to resist a sudden strain, and will ultimately break short off. It is therefore unsafe to subject iron, or in fact any other material, to strains greater than its elastic limit. This limit for iron boiler plates may be taken at about one-fifth its breaking, or, as it is called, ultimate strength. It should be remembered, however, in this connection, that it often happens that the steam pressure is not the greatest force the boiler must withstand, as sudden or unequal expansion and contraction are probably more destructive, to locomotive boilers especially, than the pressure of the steam.

Question 101. How are the plates of boilers fastened together?

Answer. With rivets, which are made with a head at one end, and are inserted while they are red-hot into holes drilled or punched in the edges of the plates. After they are in the holes a head is formed on the other end, either with blows from hand hammers, or by a machine constructed for the purpose. In these machines the rivet after it is in the holes is brought between a fixed and a movable die, the head which is made with the rivet being placed against the fixed die, and the movable die is then pressed, either by steam or hydraulic pressure with great force against the other end of the rivet, thus forcing the end of the rivet into the form of the die, which is made of the proper shape and size for the rivet head. The powerful pressure which is thus brought on the rivet causes it to be pressed into all parts of the two holes, thus completely filling them both; whereas with hand riveting, the holes are not nearly so completely filled, as it is impossible with blows of a hammer to subject the rivets to so powerful or uniform a pressure as the machine brings upon them.

Fig. 53.

Question 102. What is the strength of riveted seams compared with that of the solid plate?

Answer. The strength of a riveted seam depends very much upon the arrangement and proportion of the rivets, but with the best design and construction, the seams are always weaker than the solid plates, as it is always necessary to cut away a part of the plate for the rivet holes, which weakens the plate in three ways: 1. By lessening the amount of material to resist the strains. 2. By weakening that left between the holes. 3. By disturbing the uniformity of the distribution of the strains. The first cause of weakness is obvious from an inspection of an ordinary seam, riveted with a single row of rivets, fig. 53. In this we have two plates 7¹⁄₂ inches wide and ³⁄₈ thick fastened with four rivets ¹¹⁄₁₆ inches in diameter and 1⁷⁄₈ inches from centre to centre. The section of the plate calculated with decimals[29] would therefore be .375 × 7.5 = 2.81 square inches. A piece ¹¹⁄₁₆ inch wide and ³⁄₈ inch thick would be removed to form each hole, or a sectional area for the whole plate of .375 × .6875 × 4 = 1.03 square inches, so that the section of the plate would be reduced through the holes 2.81 - 1.03 = 1.78 square inches. In other words, on the dotted line a b it will have only about 63 per cent. of the sectional area of the solid plate.

The second cause of the reduction of strength is owing to the injury sustained by the plates during the process of drilling and punching. The knowledge existing regarding this subject is not very accurate, although numerous experiments have been made to determine the exact amount of weakening caused by punching plates. It is, however, certain that in many cases the strength of the metal left between the holes of boiler plates is reduced from 10 to 30 per cent. by the process of punching. It is probable, however, that soft ductile metal is injured less than that which is harder and more brittle. Some kinds of steel plates are especially liable to injury from punching. It is also probable that the condition of the punch, and the proportions of the die used with it, have much to do with its effect upon the metal

The third cause of weakness is owing to the fact that if one or more holes are made in a plate of any material, and it is then subjected to a tensile strain, the strain, instead of being equally distributed through the section left between the holes, will be greatest in that part of the metal nearest them. This can be illustrated by taking a band of india-rubber, fig. 54, and cutting a round hole in it to represent a rivet hole. If we draw two parallel lines, A B, across the band and then stretch it, the lines, instead of remaining parallel when the band is stretched, will separate most next to the hole, as shown in fig. 55, indicating that the fibres of the rubber nearest the hole are strained most. A similar effect takes place when a plate of iron is stretched, so that a fracture is liable to begin next to the hole, after which the plate will be broken as it were in detail.

Fig. 54.

Fig. 55.

 

 

Question 103. How may a boiler seam like that shown in fig. 53 break?

Answer. It may break in three different ways:

1. By the plate tearing between the rivet holes on the line a b.

2. By the rivets shearing off.

3. By the plate in front of the rivets crushing, as shown in fig. 56.

Fig. 56.

Question 104. How can the strength of a boiler seam be calculated at each of these three points?

Answer. The strength through the rivet holes is calculated by TAKING THE AREA IN SQUARE INCHES OF THE METAL WHICH IS LEFT BETWEEN THE RIVET HOLES, AND MULTIPLYING IT BY THE ULTIMATE STRENGTH OF THE METAL AFTER THE HOLES ARE MADE. Thus, in fig. 53, the area of each of the plates between the rivet holes is 1.78 square inches. As already stated, good boiler plate will break at a strain of about 50,000 pounds in the direction of its fibres,[30] but the strength of the metal left between punched holes is probably 20 per cent. and that between drilled holes 10 per cent. less than that of the solid plate. We must, therefore, in calculating the strength of a punched seam, take the ultimate strength of the metal between the holes at only 40,000 pounds per square inch. The calculation for the strength through the holes would therefore be: 1.78 × 40,000 = 71,200 pounds.

It has also been found by experiment that the strength of rivets to resist shearing is about the same as that of good boiler plate to resist tearing apart, or 50,000 lbs. per square inch. The strength of the rivets, therefore, is calculated by MULTIPLYING THE AREA IN SQUARE INCHES OF ONE RIVET BY THE NUMBER OF RIVETS, AND THE PRODUCT BY THE STRENGTH OF THE METAL TO RESIST SHEARING. The calculation for fig. 53 would therefore be:

or a little more than the strength of the plates through the holes.

The resistance offered by a plate to the crushing strain of a rivet has been found also by experiment to be about 90,000 pounds per square inch. It can be proved that the area which resists the crushing strain of a rivet in a plate, fig. 53, IS MEASURED BY MULTIPLYING THE DIAMETER OF THE RIVET BY THE THICKNESS OF THE PLATE. The calculation for the strength of this part of the seam will therefore be: diameter of hole = .6875 × .375 × 4 × 90,000 = 92,812.

The strength of the solid plate would be EQUAL TO ITS SECTIONAL AREA MULTIPLIED BY 50,000 POUNDS, or 7.5 × .375 × 50,000 = 140,625 pounds. The ultimate strength of our seam would then be as follows:

It will thus be seen that the strength of the weakest part of the above seam, fastened with a single row of rivets in punched holes, is very little more than half (50.6 per cent.) of that of the plates. It will be noticed that the weakest part of the seam is the plates between the holes.

Fig. 57.

Fig. 58.

Fig. 59.

Fig. 60.

Question 105. How can the strength of such a single-riveted boiler seam be increased?

Answer. The most obvious way of increasing the strength of such a seam is to place, or, as it is called, space, the rivets further apart, which would leave more metal between the holes, and thus strengthen the seam at its weakest part. But if this is done, it is said that there is difficulty in keeping the seam water-tight, as the plates are then liable to spring apart between the rivets. Another way of increasing its strength is to drill the rivet holes. As already stated, the difference in the strength of the metal left between drilled and punched holes has been shown to be from 10 to 20 per cent. There is also another advantage in drilling the holes for rivets. In punching them, it is necessary to punch each plate separately, and even with the utmost care and skill it is impossible to get the holes to match perfectly. Some of them will overlap each other, as shown in fig. 57, so that when the rivet is set, it will assume somewhat the form shown in fig. 58. There is then danger that those rivets which fill the holes that match each other will be subjected to an undue strain. If, for example, we have five rivet holes, as shown in fig. 59, and only the centre ones correspond with each other, then the rivets in all the other holes will assume somewhat the form shown in fig. 58, and therefore the centre rivet c, in fig. 59, which fits the holes accurately, must take the strain of the other four until they draw up “to a bearing.” Under such circumstances, which are not unusual, there will be great danger either of shearing off the rivet c, or of starting a fracture in the plates, as indicated by the irregular line a b, between the adjoining rivets. It is also obvious that a rivet like the one in fig. 58 will not hold the plates together so well as one which fits more perfectly, as shown in section in fig. 53, and therefore there is more danger of leakage between the plates from badly fitted rivets than from those which fill the holes more perfectly; consequently rivets which fit imperfectly must be placed nearer together than those which are well fitted. It is true that rivets which are set with a riveting machine fill any inaccuracies of the holes more perfectly than those which are set by hand. But even if they are made to fill the holes as shown in fig. 60 they are still not so strong to resist shearing nor so efficient in holding the plates together as they would be if the holes conformed more perfectly to each other. In drilling the holes, the second plate can be drilled from the holes in the first, so that the holes in each will correspond with each other perfectly. The rivets will therefore fit more accurately, and consequently can be spaced further apart, and still keep the plates tight, and thus have more material between the holes, which is the weakest part of the seam. It has been shown that a rivet ¹¹⁄₁₆ inch in diameter has a resistance to shearing of 18,560 pounds. There is therefore no advantage in spacing such rivets further apart than 1¹³⁄₁₆ from centre to centre, because the metal left between drilled holes that distance apart would be slightly stronger than the rivets. If therefore the rivets are placed further apart, their diameter must be increased. There is, however, a limit beyond which the diameters of rivets cannot be increased with advantage, because if we increase their diameters, their sectional area to resist shearing is increased in proportion to the square of the diameter, whereas the section of metal in the plate to resist crushing is increased only in proportion to the diameter. This will be apparent if we compare a rivet ¹⁄₂ inch with one 1 inch in diameter. The first has a sectional area of .1963 inch, the other .7854 inch, or four times that of the first one. Now the area which resists the crushing strain of the rivets is increased only in proportion to their diameters, or is twice as much for the one as for the other. If, therefore, we increase the diameters of the rivets, we very soon reach a point at which the plate has less strength to resist crushing than the rivet has to resist shearing. The diameter of rivet which will give just the same resistance to both strains varies with the thickness of the plates; with ³⁄₈ inch plates a ⁷⁄₈ rivet will have a resistance to shearing of 30,065 pounds and the plate in front of it a resistance to crushing of 29,530 pounds. A ⁷⁄₈ rivet is, therefore, the largest size which can be used to advantage in ³⁄₈ plates. If now we were to space such rivets so far apart that the metal left between the holes would have a strength just equal to that of the rivets, we would have the strongest possible seam that can be made with a single row of rivets. This distance would be 1³⁄₄ inches between the edges of the rivets, or 2⁵⁄₈ from center to center, as shown in fig. 61. The following table will show the strength of such a seam composed of four rivets, and two plates 10¹⁄₂ inches wide,[31] with drilled holes:

Fig. 61.

From this it is seen that the strength of the seam with drilled plates is 60 per cent. of that of the solid plates, or it is about 18¹⁄₂ per cent. stronger than that made with plates having punched holes and the rivets nearer together. It should be noted that a great part of the superiority of the seams made with drilled holes is due to the superior accuracy of the work done in that way, which makes it possible to use larger rivets spaced further apart. It is probable that with the use of some recently designed machines, intended to produce greater accuracy in punching rivet holes, part of the above advantage may be realized with that kind of work. The greatest distance that rivets may be spaced apart without incurring danger of leakage between the plates must, however, be determined more by practical than theoretical considerations. It is certain, however, that rivets may be spaced much further apart than they are in ordinary practice, and the seams still be kept tight, if the work is done with sufficient accuracy and care.

Question 106. What other methods are there of making boiler seams which are stronger than those which have been described?

Answer. In this country two rows of rivets are used and also what is called a “welt,” or covering-strip, the latter with both single and double-riveted seams.

Fig. 62.

Question 107. How are the rivets arranged when two rows are used?

Answer. They are often placed just behind each other as shown in fig. 62, which is called chain-riveting. Such an arrangement of rivets obviously adds nothing to the strength of the seam, because its weakest part, as has already been shown, is the section of the plates through the rivet-holes, and this is not at all strengthened by adding another row of rivets, because the plates are just as liable to break through the rivet-holes with two rows of rivets as they would be with one.

Fig. 63.

A much better arrangement is to place them alternately in the two rows, as shown in fig. 63. Rivets arranged in that way are said to be staggered, or placed zigzag. The method of laying off the holes and proportioning such a seam in order to get the most strength is, first, to determine the greatest distance which can be allowed between the rivet-holes, and yet keep the seams water-tight. Supposing this is 1⁵⁄₁₆ inches,[32] with plates ³⁄₈ inch thick, the diameter of rivet whose sectional area will give an equal amount of strength must then be calculated. From the preceding data it will be found that for drilled plates the resistance of the portion left between such holes will be 22,148 pounds, and a rivet ³⁄₄ inch in diameter will have a resistance of 22,085 pounds. On the line a b, fig. 63, which is to be the first row of rivets, we will draw one hole, c. From the center of this hole we will describe the arc of a circle, d e, with a radius, c d, equal to the sum of the distance between the holes, 1³⁄₈ inches, and one-half the diameter of the hole, or ³⁄₈ inch, making the radius 1³⁄₄ inches. From d, the intersection of this arc with the line of rivets, a b, with the same radius, we will step off the distance d f will then be the centre of the second rivet-hole on the line a, b. The rivet can therefore be drawn, and from its centre, with the same radius employed before, another arc, d g, should be drawn. If now we draw a rivet-hole (h) between the two arcs and touching each of them, we will have all three of the holes so arranged that the metal between the holes c, f, will be just equal to that between them and the hole h. In other words, the strength of the plates on the line c f is just the same as on the line c h f. Therefore the strength of the plates on the straight line a b is just the same as on the zigzag line c h f, etc. The strength of this seam would therefore be as follows:

The weakest portion of this seam, it will be seen, has a strength of 70 per cent. of the plate, or is 38 per cent. stronger than a single-riveted seam with punched holes. If we were to use ⁷⁄₈ inch rivets and make the spaces between them 1³⁄₄ inches, the strength of a similar seam would be as follows:

This seam would then have a strength of 72 per cent. of the solid plates, or be 42¹⁄₄ per cent. stronger than the ordinary riveted seam with punched holes. It is important to observe that an increase of strength results from the use of larger rivets spaced farther apart than is usual in ordinary practice, and that this is possible only with the best and most accurate workmanship.

Fig. 64.

Question 108. What is the form of construction of boiler seams made with a welt or covering-strip?

Answer. The plates (a, b, fig. 64) are lapped over each other as for an ordinary seam. Another plate, c, about nine inches wide, is then placed on the inside of the seam and bent so as to conform to the lap of the two plates. The rivets r, whether a double or single row, pass through all three plates, and two more rows of rivets are put next to the edges of the covering plate, c. It is plain that the strength of the seam, r, is increased up to a certain point by an amount just equal to that of the rivets in the edges of the covering plate. If, however, these are placed too close together, the plates a and b will be weaker through the outside rows of rivets than the seam is through either of the outside ones and the middle one taken together. If, for example, we take a single-riveted seam, like that shown in fig. 53, whose strength is only a little more than half that of the solid plate, and should add to it a covering plate, as shown in fig. 64, and then space the rivets in the edges of the covering plate the same distance apart as in the middle seam, then obviously the plates would be just as liable to break through the outer rows of holes as through the center row before the covering plate was added. If, however, the holes in the two outside plates are spaced at say twice the distance apart, or 3³⁄₄ inches, then the only way the seam can break through the outer rows of holes is by shearing the rivets, because the plates between the holes are then stronger than the rivets. But before these rivets can be sheared, the centre seam must give way. Thus the strength of such a seam is equal to THE SUM OF THE STRENGTH AT THE WEAKEST POINTS OF THE MIDDLE AND THE OUTSIDE SEAMS. The strength of the plates between the holes of the outside rows of rivets must, however, be as great as the sum referred to, otherwise the seam will be the weakest at that point, and the failure will occur there. The rivets in the outside rows should be spaced at least twice as far apart as those in the middle seam. The number of rivets to resist shearing will then be 50 per cent. greater, so that the strength of a seam like that shown in fig. 53, with a covering plate added, will be as follows:

An ordinary single-riveted seam with punched holes, with a welt or covering plate added, would thus have a strength equal to 65.3 per cent. of the solid plate, or be 29 per cent. stronger than the seam without the covering plate. It is probable, however, that the injury to the plate from punching the outside rows of holes which are further apart is not so great as it is when they are punched nearer together and to the edge, so that the strength is somewhat greater than our estimate.

The relative strength of the different forms of seams described in percentage of the strength of the solid plate is then as follows:

Question 109. Are there any other forms of boiler seams used?

Answer. In Europe what are called butt-joints are used to some extent. In these the ends of the two plates abut against each other, with a covering strip on one or both sides. This form of joint is, however, not used in this country, and therefore its peculiarities will not be described.

Question 110. How are the seams of boilers made water-tight?

Answer. By what is called caulking. That is, by the use of a blunt instrument somewhat resembling a chisel, the end of which is placed against one or both of the edges of the plates c, d, fig. 53, which are then riveted down by blows of a hammer, somewhat as the joints of a ship are made tight. Before the edges, which are called the caulking edges, of the plates are made tight in this way, they are cut or trimmed off with a chisel. In this process the plate under the edge is often injured seriously by the carelessness of workmen, who sometimes allow the chisel to cut a groove in the plate under the edge, thus weakening it at a point where the greatest strength is needed. There is also danger of forcing the plates apart in the manner shown in fig. 65, if it is done carelessly and with a heavy hammer.

Fig. 65.

Question 111. How are the flat ends of the boiler strengthened?

Answer. By braces, u, u, r, r, fig. 41, which are fastened to the end plates and to the outer shell of the boiler. They are fastened at one end to ⟂ shaped pieces called crow-feet, which are riveted to the end plates of the boiler. At the other end they are made with a broad foot, which is riveted to the outer shell of the boiler. Especial attention should be given to the form, proportion and arrangement of these braces when a boiler is constructed, and they should be frequently examined while the engine is in use, as they are liable to be neglected or carelessly constructed and to become weakened or broken by corrosion, or the constant strain to which they are subjected. Great ignorance is often displayed in the design and proportions of these braces, especially in their attachments to the shell of the boiler.

Question 112. How should the braces for strengthening the ends of a boiler be proportioned?

Answer. Every part should be made equally strong. If, for example, the brace itself is made of a bar of round iron one inch in diameter, its sectional area would be .7854 square inch. The iron used for these braces should be of such strength that a force of not less than 50,000 lbs. per square inch of transverse section should be required to tear it apart lengthwise. A bar of the size referred to would therefore require not less than 39,270 pounds to pull it apart. All the other parts should be capable of resisting an equal strain. It is, for example, not unusual to find a brace of the size we have described, and even of larger diameter, fastened to a crow-foot which is attached to the boiler plate with two rivets ⁵⁄₈ inches in diameter. A similar fastening for the other end of the brace is also often used. The sectional area of these two rivets is considerably less than that of the brace, and at the same time the strain is brought upon them at such an angle as to have a tendency to “snap” them off. For this reason, and also because a rivet is apt to be deteriorated in strength by the hammering, the rivets should always have a sectional area very nearly or quite double that of the bar which forms the brace. The metal around the eyes through which the pins are inserted should also be carefully proportioned, and the transverse section at any one point should always be at least 1¹⁄₂ times greater than that of the bar. The area of the pins used for attaching the bars to the crow-feet should always be a little more than half that of the bar. That is, an inch bar should have a pin not less than ³⁄₄ inch in diameter. When flat braces are used, it is not unusual to find a bar 3 inches wide with a hole an inch in diameter punched or drilled so near the end that a pin is sure either to pull out the end of the bar or else to break it crosswise at the hole with a much less strain than the brace itself would resist. The ends of braces should always be enlarged enough to give them sufficient strength to resist as much strain as the bar itself. Although these precautions may appear unimportant, and unfortunately are often so regarded, yet it is upon just such details as these that the lives and the safety of every locomotive runner, fireman and others near them are constantly dependent.

Question 113. How much water is usually carried in a locomotive boiler?

Answer. There must always be enough water in the boiler to cover completely all the parts which are exposed to the fire, otherwise they will be heated to so high a temperature as to be very much weakened or permanently injured. In order to be sure that all the heating-surface will at all times be covered with water, it is usually carried so that its surface will be from 4 to 8 inches above the crown-sheet.

Question 114. How much space should there be over the water for steam?

Answer. No exact rule can be given to determine this. It may, however, generally be assumed that the more steam space the better. In order to increase the steam room, locomotive boilers are very generally made in this country with what is called a wagon-top, C, fig. 41, that is, the outside shell of the boiler over the fire-box is elevated from 4 to 12 or even 18 inches above the cylindrical part.

Question 115. What is a steam-dome and for what purpose is it intended?

Answer. A steam-dome, X, fig. 41, is a cylindrical chamber made of boiler-plate and attached to the top of the boiler. Its object is to increase the steam room and to furnish a reservoir which is elevated considerably above the surface of the water, from which the supply of steam to be used in the cylinders can be drawn. The reason for drawing the steam from a point considerably above the water is that during ebullition more or less spray or particles of water are thrown up and mixed with the steam. When this is the case, steam is said to be wet, and when there is little or no unevaporated water mixed with it it is said to be dry. It is found by experience that wet steam is much less efficient than that which is dry. There is also danger that the cylinders, pistons or other parts of the machinery may be injured if much water is carried over from the boiler with the steam, because water will be discharged so slowly from the cylinders that there is not time for it to escape before the piston must complete its stroke, so that the cylinder-heads will be “knocked out,” or the cylinder itself or the piston will be broken. The reason for drawing or “taking” steam from a point considerably above the water is because there is less spray there than there is near the surface, and the hottest steam, which is also the dryest, ascends to the highest part of the steam space.

Question 116. Where is the dome usually placed?

Answer. In this country it is usually placed over the fire-box, but in Europe it is placed further forward, either about the centre of the boiler or near the front end of the tubes. Sometimes two domes are used on engines, in this country, one over the fire-box and another near the front end.

Question 117. How is the steam conducted from the dome to the cylinders?

Answer. By a pipe I m m, fig. 41, called the dry-pipe, which extends from the top of the dome to the front tube-plate. On the front side of the tube-plate and inside the smoke-box two curved pipes, O, fig. 41, (shown also in fig. 40,) called steam-pipes, are attached to the dry-pipe at one end, and to the cylinders at the other. The vertical portion of the dry-pipe in the dome, sometimes called the throttle-pipe, is usually made of cast iron, the horizontal part of wrought iron, and the steam pipes of cast iron.

Question 118. How is the loss of heat from locomotive boilers by radiation and convection prevented?

Answer. By covering the boiler and dome with wood, called lagging, about ⁷⁄₈ inch thick, which is a poor conductor of heat, and then covering the outside of the wood with Russia iron, the smooth, polished surface of which is a poor radiator of heat.

Question 119. What is the smoke-box for?

Answer. The smoke-box Q is simply a convenient receptacle for the smoke before it escapes into the chimney or smoke-stack, which is attached to the top of the smoke-box. It also affords a convenient place for the steam and exhaust pipes, where they are surrounded with hot air and smoke, and not exposed to loss of heat by radiation. The front end of the smoke-box is usually made of cast iron, with a large door in the centre which affords access to the inside.

Question 120. How are the chimneys or smoke-stacks of locomotives constructed?

Answer. The forms of smoke-stacks which have been used are almost numberless. For burning bituminous coal and wood they are generally made with a central pipe, R, fig. 41, and a conical-shaped cast-iron plate, S, called the cone or spark deflector, which, as the latter name implies, is intended to deflect the motion of the sparks and cinders which are carried up with the ascending current of smoke and air in the pipe R, so as to prevent them from escaping into the open air while they are incandescent, or “alive.” A wire netting, t t, is also provided, which is intended as a sort of sieve to enclose the sparks and cinders, and at the same time allow the smoke to escape. The receptacle h h is intended as a chamber in which the burning cinders will be extinguished before they escape. For burning anthracite coal, a simple straight pipe, without a deflector or wire netting, is ordinarily used.

Question 121. What are the proportions and materials usually employed in the construction of smoke-stacks?

Answer. The inside pipe R, fig. 41, is usually made of the same diameter as the cylinders, or an inch or two smaller. For the other dimensions there are no established rules, excepting for the height of the top of the chimney above the rail, which is usually from 14 to 15 feet. The outsides of smoke-stacks are made of sheet iron, but the upper part is now sometimes made of cast iron, so as to withstand the abrasion of the sparks and cinders longer than sheet iron will. For very warm and damp climates, the outsides of smoke-stacks are sometimes made of copper to resist corrosion, which is very destructive to all iron structures in those countries. The wire netting is made of iron or steel wire from ¹⁄₁₀ to ¹⁄₃₀ of an inch in diameter, and with from 3 to 4 meshes to the inch.

Question 122. What is the pipe N, fig. 41, intended for?

Answer. It is intended to conduct the exhaust steam and a portion of the smoke from the bottom of the smoke-box, where the steam escapes, to the base of the smoke-stack. In nearly all European engines, the exhaust steam escapes at the top of the smoke-box just below the aperture of the smoke-stack. There is, however, often difficulty in equalizing the draft in the tubes, that is, to get an equal amount of smoke to pass through all of them. By the use of the pipe P, called the inside pipe or petticoat pipe, it is thought that the draft in the tubes can be equalized much better, as a part of the smoke is drawn out of the smoke-box at the top and part at the bottom of the smoke-box. The pipe is usually made in two parts, which slide into each other like a telescope. The distance of the upper end from the top of the smoke-box and that of the lower end from the bottom can thus be increased or diminished, and if the draft is greater through the upper tubes than through the lower ones, or vice versa, it can be regulated or equalized by simply raising or lowering the top or bottom of the petticoat pipe. Sometimes this pipe is made with openings and a kind of deflectors over them between the two ends. It is then called a flounced petticoat pipe, for obvious reasons.

No exact theory can be stated regarding the proportions of these pipes, or the results effected by them, which can be determined only by practical experience. Some more accurate knowledge concerning them is, however, much needed.

PART VIII. THE BOILER ATTACHMENTS.

Question 123. How is water supplied to the boiler to replace that which is converted into steam?

Answer. It is usually forced into the boiler against the steam pressure by force pumps, but another instrument called an injector is now much used.

Question 124. What is the form of construction and principle of the operation of such force pumps?

Answer. The ordinary single-acting force pump, fig. 66, used on locomotive and other steam engines consists of a pump-barrel, A A, which is a cast-iron or brass cylinder in which a tight-fitting piston, B B, called the pump-plunger, works. This piston or plunger is simply a round rod which works air-tight through what is called a stuffing box, C, whose construction will be fully explained hereafter. The plunger receives a reciprocating motion, usually from the piston-rod of the engine, but is sometimes worked by a small crank attached to one of the crank-pins, or by an eccentric on one of the axles. The pump-barrel is connected with the water-tank of the tender by the suction-pipe, D, and with the water-space of the boiler by the feed-pipe, E E. Over the suction-pipe D is a valve, F, called the suction-valve, which opens upward, and below the feed-pipe, E, is another valve, G, called the pressure-valve. These valves are cylindrical and made of brass, and rest on brass seats, g, g, to which they are fitted so as to be water-tight. They work in guides, k, k, called cages, the form of which is more clearly shown in the section, fig. 67, and the plan, fig. 68. When the plunger is drawn out of the pump-cylinder it creates a vacuum behind it, and the pressure above the valve G closes it, while the atmospheric pressure on the water in the tank forces it into the suction-pipe, opens the valve F, and fills the pump-cylinder. When the plunger is forced back again the force with which it presses against the water in the pump-barrel, A, closes the valve F, and opens the pressure-valve G, and the water is then forced through the feed-pipe into the boiler. In order to be certain that the water in the boiler will not flow back into the pump, and also to prevent all the water and steam in the boiler from escaping in case of accident to either the feed-pipe or pump, another valve, H, fig. 66, called a check-valve, is placed between the feed-pipe and the boiler. The construction of this valve is similar to that of the pressure and suction valves. It is inclosed in a cast-iron or brass case, I I. All of these valves have cages in which they work and which also act as stops, which prevent them from rising from their seats further than a certain distance. This distance is called their lift, and the successful working of the pumps depends very much on the amount of lift which the valves have. This is usually from ³⁄₁₆ to ¹⁄₂ inch.

Fig. 66.

Fig. 67.

Fig. 68.

 

Over the pressure-valve G is a chamber, J, called an air chamber. When water is forced into this chamber, it is obvious that as soon as it rises above the mouth of the pipe E, the air above the surface, c d, of the water will be confined in this chamber. This confined air, being elastic, will be compressed and expanded by the pressure of the water, so that it forms a sort of cushion, which relieves the pump and the pipes from the sudden shocks to which they are subject, owing to the rapid motion of the pump-plunger.

Another air-chamber, K K, is sometimes placed below the suction-valve F. The object of this is to supply a cushion to relieve the suction-pipe from the shock which is caused by the sudden arrest of the motion of the water in the pipe when the valve F is closed. When the pump-plunger is drawn out, the water flows through the valve F to fill the vacuum in the pump-barrel, A A, and consequently all the water in the suction-pipe is put in motion. As soon as the plunger returns, the valve F is closed and the motion of the water is suddenly arrested, thus producing more or less of a shock in the pipe D. When the water in the air chamber K K rises above the line a b, it is evident that the air above that line will be confined in the space surrounding the pipe L. This air then forms a cushion in the same way as that in the upper air chamber J does, which has already been explained. The advantages of the lower air chamber are, it is thought, more imaginary than real.

Question 125. How can the pump be taken apart and the valves examined?

Answer. By removing the bolts e, e, the upper air chamber can be taken off, and by taking out the bolts f, f, the lower one can be taken down, and the valves and cages removed. The check-valve H can be taken out by removing the bolts l, l, which hold up the valve-seat h and the valve and cage.

Question 126. How can it be known whether the pump is forcing water into the boiler?

Answer. To show this a cock, called a pet-cock, is attached to the upper air chamber in the position shown by the dotted circle m.[33] By opening this cock, if the pump is working, a strong jet of water will be discharged from it during the backward stroke of the pump-plunger. If the pump is not forcing water into the boiler, or is working imperfectly, the stream discharged from the pet-cock will be weak, and the backward and forward strokes of the plunger will thus not be very definitely indicated by the discharge from the pet-cock.

Another small cock is often attached to the lower air chamber, or to the feed-pipe, to allow the water to escape from the pump in cold weather, when the engine is not working, so as to prevent it from freezing.

Question 127. Why is it necessary to be able to regulate the quantity of water which is forced into the boiler by the pumps?

Answer. Because when the engine is working hard, that is, pulling a heavy load up a grade, more steam and consequently more water are consumed than when it is not working so hard, and therefore more water must be forced in to supply the place of that which is used in the form of steam. If more water is forced in than is consumed, the water will rise and fill the steam-space and a part of it will then be carried into the cylinders without being evaporated. If too little water is forced into the boiler, the heating surface will not be covered, and there will consequently be danger that those portions which are exposed to the fire will be overheated and injured.

Question 128. How is the supply of water which is fed into the boiler by the pump regulated?

Answer. By a cock in the suction-pipe called a feed-cock, which can be regulated by the locomotive runner, so that more or less water is supplied to the pump. There is also a valve in the water tank by which the supply of water can be regulated.

Question 129. On what part of the locomotive are the pumps usually placed?

Answer. They are usually attached to the frames behind the cylinders, and are worked by the piston-rod, as will be more fully explained hereafter; but they are sometimes placed inside of the frames, that is, between the wheels, and worked from an eccentric on one of the axles, and sometimes they are placed outside of the wheels near the back part of the locomotive, and worked from short cranks attached to the crank-pins.

Question 130. What provision is made for preventing the water in the pumps from freezing in cold weather?

Answer. Pipes which communicate with the steam-space of the boiler are attached to each of the suction-pipes, so that, by opening valves in the former, steam is admitted into the suction-pipes to heat the water in them. By admitting this hot water into the pump, it is kept warm, and the water is thus prevented from freezing.

Fig. 69.

Question 131. What is an “injector”?

Answer. It is an instrument in which a jet of steam from the boiler mingles with and forces a continuous jet of water into the same boiler against its own pressure.

Question 132. What is the action of the injector and what are the names of its essential parts?

Answer. All injectors have certain parts in common. These may be shown in the simplest form of instrument, as in the fixed-nozzle injector, a section of which (omitting all detail of construction) is shown in fig. 69.

The steam from the boiler, passing through the pipe A, enters the receiving-tube C. Here it is joined by the water which enters the pipe B. The water condenses the steam in the combining-tube D, and a water jet is formed which is driven across the overflow space F F, and enters the delivery-tube H, thence past the check-valve I into the boiler. During the passage of the water from D to H, as it passes across the overflow space F, if too much water has been supplied to the steam, some will escape at this point and flow out through the overflow nozzle G, while if too little water has been supplied, air will be drawn in at G, and carried into the boiler with the water. The names of the essential parts seem very applicable when we notice that steam is received from the boiler at C, combines with the water at D, and both are delivered to boiler through H.

Question 133. How is the operation of the injector explained?

Answer. Steam escaping from under pressure has a much higher velocity than water would have under the same pressure and condition. The escaping steam from the receiving-tube unites with the feed-water in the combining-tube, and gives to this water a velocity greater than it would have if escaping directly from the water-space in the boiler. The power of this water to enter the boiler comes from its weight moving at the velocity acquired from the steam, and it is thus enabled to overcome the boiler pressure.

This can be illustrated with a wooden croquet ball, which will float on the surface of water and will require considerable force to make it sink. If, however, it is thrown violently into the water, it will sink to a considerable depth before its buoyancy will overcome its momentum, or actual energy. If, however, we were to take a very light, hollow wooden or india-rubber ball, no matter how violently we throw it into the water, it will not sink, because the total actual energy of any body IS PROPORTIONAL TO ITS WEIGHT MULTIPLIED BY THE SQUARE OF ITS VELOCITY, and therefore if we throw the hollow ball at the same velocity as the solid one, the former will still have much less energy than the latter. Now, as already stated, steam under a given pressure escapes from an orifice with a very much greater velocity than water. But steam being very light, if its weight is multiplied by its velocity its total energy will be comparatively small. Now in the injector, a portion of the high velocity of steam is imparted to the heavy water, because this water is presented to the action of the steam, not in a mass, as in the boiler, but in small quantity and in such a position that it can easily escape, so that it gradually acquires as high a velocity as the escaping steam can impart, and at the same time the steam is condensed, and therefore there is a heavy substance with a high velocity, whose actual energy is sufficient to overcome the pressure in the boiler. If the steam were not condensed we would have a comparatively light substance moving at a high velocity, which, as has already been explained, would have little actual energy, and would therefore not overcome the boiler pressure.

Question 134. Does this involve any principle like a perpetual motion, or of work done without consumption of power?

Answer. No, the steam escapes as steam, and is returned to the boiler as water with its bulk reduced, say 1,000 times, and if it carries with it twenty times its weight of fresh feed-water, there would still be a loss of pressure or effective force in the boiler sufficient to do the work required in introducing the water.

Question 135. Will the injector feed hot water?

Answer. The instrument will not work when the feed-water is too hot to condense the steam, for the reasons given above, and the amount of water thrown is always the greatest when the feed-water is the coldest. Steam at a low pressure can be condensed more readily than steam of higher pressure, because it contains less heat. The feed-water may be used hotter to condense low steam than to condense high steam. In using the injector, the lower the boiler pressure the hotter may be the water within certain limits, the limit being the possible condensation of the steam.

Question 136. Will a “fixed-nozzle” injector, such as has been described, answer as a boiler feeder on locomotives?

Answer. It will answer at some one pressure of steam, to which pressure it may have been adapted in making the instrument, and at that pressure it will work admirably; but it will not work satisfactorily at any other pressure, either higher or lower, and has not much range in quantity of water delivered.

Question 137. What is required to make an injector work at different pressures?

Answer. The instrument must be so made that the water passage between the receiving tube and the combining tube can be varied in size. This is usually done by making the combining and receiving tubes conical and moving the former to or from the latter, thus contracting or enlarging the water space. Such adjustment must be made at each change of steam pressure in the boiler. If this adjustment is made by hand, as in some kinds of injectors, it requires constant attention, if the steam pressure varies frequently.

Question 138. How has this regulation been accomplished without such attention?

Answer. In the SELF-REGULATING INJECTOR, fig. 70, by using the escape water at overflow to push the combining tube towards the receiving tube and the indraught of water at the same place to pull the combining tube away from the receiving tube. This can be explained as follows:

The case G of the instrument has two inlets, one for steam, the other for water, the two being separated by the plate F F. Steam passes into the receiving tube A, and its escape is regulated by a taper-plug in the end of the rod B, moved by the handle H. At the upper end of the combining tube C, where it swells out into a bell mouth, is a piston N N, sliding in the case. The lower end at C is guided by the upper end of the delivery tube D. The delivery tube D is stationary. The overflow opening is at O. The action of this instrument may be thus described: Steam entering the receiving tube A escapes through its lower end when the plug B has been drawn back. It unites with the water surrounding it in the space N N, is condensed and passes with the feed-water into the delivery tube D, and thence into the boiler. If too much water enters the combining tube, some will escape at the overflow O, and filling the space below the piston N N, will force the combining tube up toward the receiving tube, and, thus contracting the space between them, will diminish the water supply; while, if it gets too little water in this space, it will take some in at the overflow O, and thus draw down the piston N N, and enlarge the space, giving more water to the instrument. This self-regulating principle enables the instrument to continue working efficiently, no matter how much the steam pressure in the boiler varies.

Fig. 70.

Question 139. How do you start this kind of injector?

Answer. The instrument just described is the latest form of self-regulating injector, manufactured by Messrs. Wm. Sellers & Co., and is called by its makers the Injector of 1876. It is started and stopped by a simple movement of the lever H. This lever H moves a cross-head I, on the guide-rod J; it also, by means of stops T and Q, on rod L, opens and closes the starting-valve K. In fig. 70 the instrument is shown shut off, and with the starting-valve K open. On the rod B are two valves, a small one W and a larger one X. A stop or collar is shown a short distance beyond the large valve X. If the lever H be drawn back until this collar comes in contact with the valve K, it will have raised the valve W from its seat, and steam will escape through a small passage in the centre of the conical regulating plug. The steam so admitted is sufficient to lift the water, which will then be driven through D, and past the valve K, and escape at P. Drawing back the lever H to the end of its stroke, after the water has been lifted, the large valve X is raised, and the lug on the lever H, coming in contact with the stop T, on the rod L, the valve K is closed. The jet is fully established and the water is driven into the boiler. At the entire end of stroke of the lever H, the latch V falls into notches on the rod J, when, as the lever is moved forward toward the position shown in fig. 70, this latch will click over the notches and hold the lever in any desired position between the maximum and minimum delivery.

Question 140. What attachments are needed besides the instrument to render it effective?

Answer. A globe-valve should be placed in the steam pipe leading to the injector, to be closed only when there is occasion to remove the injector when steam is up, and in cold weather, to prevent the condensation of steam in the pipes at the end of its trips. During all the working time of the injector, this valve should be wide open.

Question 141. In what position, and in what location on the engine should the injector be placed?

Answer. On the right hand side, high enough up to have an air chamber below the injector, above the top of the water in the tank when the latter is full.

Question 142. What is required to keep the instrument in working order?

Answer. Constant use is better than occasional use. Having two injectors on the same engine, one on each side, the one on the runner’s side will be used while running. The one on the other side should be used when standing. All pipe connections must be tight, so as to prevent the leaking of air. The pipe carrying steam to the instrument should be from such part of the boiler as will insure the use of dry steam, and the waste pipe must not be contracted. The instrument represented in the engraving is the injector of 1876, manufactured by Messrs. William Sellers & Co. of Philadelphia. Beside this, Mack’s and several other kinds of injectors are now used.

Fig. 71.
Scale, ³⁄₈ in. = 1 foot.

LIST OF PARTS DESIGNATED BY LETTERS OF REFERENCE IN FIG. 71.

Question 143. How can the height of the water in the boiler be known?

Answer. Two appliances are used by which the height of the water in the boiler can be observed. These are: 1. Gauge or try cocks. 2. The glass water gauge.

Fig. 72. Scale ¹⁄₄.

Fig. 73.
Scale, 3 in. = 1 foot.

Every locomotive is provided with four or more gauge-cocks, which are usually placed at the back end of the boiler, where they can easily be seen and reached. These cocks, s, s, s, s, are shown in fig. 71, which represents the back end of a locomotive, and to which frequent reference will be made. They are also shown on a larger scale in fig. 72, which represents the end plate of the boiler in section. They communicate with the inside of the boiler and are so placed that one is three or four inches above the other. The two upper cocks are placed above the point where the surface of the water should be when the engine is working, and the two lower ones below it, so that the upper ones communicate with the steam space and the lower ones with the water. When these cocks are opened, if the water is at its proper height, steam is discharged from the two upper ones, and water from the two lower ones.

When a gauge-cock which communicates with the steam space is first opened, it is usually filled with condensed water, so that it should usually be kept open for a little while until this water is discharged. If the upper cocks are opened and continue to discharge water, they indicate that there is too much water in the boiler; on the other hand, if steam is discharged when the lower cocks are opened, then there is too little water in the boiler, and the heating surface is in danger of being exposed to the fire without being covered with water, and consequently overheated, or as it is called “burned,” and so injured as to become too weak to bear the strain to which it is subjected by the pressure of the steam. There is then great danger that the crown sheet may be crushed down by the pressure of the steam above it, or that the boiler may be exploded. Even if no accident occur, the boiler is in great danger of permanent injury from overheating when the water is allowed to get too low.

Below the gauge-cocks s, s, s, s, fig. 71, an inclined cylinder, R, called a drip-pipe, is placed with openings to receive the water and steam which are discharged from the cocks. This water is conducted away by the pipe p.

The water-gauge, P, fig. 71, which is shown in section in fig. 73, consists of an upright[34] glass tube, a a, which is from one-half to three-quarters of an inch in diameter, and from 12 to 15 inches long. The glass is about one-eighth of an inch thick. At its ends it communicates with the steam and water of the boiler through brass elbows, b, c. The openings in these elbows, which communicate with the boiler, are closed by the valves or plugs, d, e, which are worked by screws and handles, f, g. The glass tube, when it is attached to the elbows, is made steam-tight by rubber rings, which are pressed tight around the tube by packing-nuts, h, i. The elbows are provided with the valves, d, e, so that in case the glass tube breaks the steam and water can be shut off, so as not to escape through the elbows. The lower elbow is provided with a blow-off cock, k, through which any sediment or dirt which collects in the glass tube or elbows can be blown out. When the valves in the upper and lower elbows are opened the steam flows into the glass tube through the upper one, and water through the lower one, and the water assumes a position in the glass tube on a level with the surface of that in the inside of the boiler; that is, the position of the water in the boiler becomes visible in the glass tube. On account of the constant variations of the water in the boiler, the column of water in the glass never remains stationary, but plays up and down as long as the boiler is working. But if the communication between the glass tube and the boiler is closed, then the water in the tube becomes stationary and the water gauge is useless. In order that there may be no obstruction of the glass tube by mud or dirt from the water, it must be blown out often. To do this the lower valve, e, is closed, and the blow-off cock, k, and the steam valve, d, are opened. The steam pressure in the tube on top of the column of water will force it out of the blow-off cock, and the mud and dirt will be carried with it.

If from any cause the glass tube is broken, first of all the water-valve e should be closed and then the steam-valve d, so as to prevent the hot water and steam which will escape from the broken glass from scalding those who are working the engine. By unscrewing the nuts h and i the old glass can easily be removed and a new one substituted in its place.[35] Care should be taken in putting in new glasses not to screw the packing nuts down any more than just sufficiently to make the rubber rings steam-tight around the glass tubes. If they are screwed too tight they are apt to produce a strain on the tube, so that the slightest expansion by heat or contraction from cold will break it.

Fig. 74.
Scale 1¹⁄₂ in. = 1 foot.

Fig. 75.
Scale 1¹⁄₂ in. = 1 foot.

Question 144. How is the steam pressure in boilers prevented from exceeding a certain limit?

Answer. By what are called safety valves. These consist of circular openings, a, fig. 74, about three inches in diameter placed usually on the top of the dome,[36] and covered by a valve, b, which is pressed down either by a lever, c c′, and spring, d, as shown in fig. 74, or by a spring alone, as in fig. 76. Two of these valves are usually placed on the top of the dome, so that if one gets out of order the other one will allow the steam to escape as soon as its pressure exceeds that which, it has been decided, the boiler can safely bear. This pressure, in locomotive boilers, is usually from 100 to 130 pounds per square inch.

Question 145. How is the amount of pressure which must bear on top of a safety-valve determined?

Answer. This pressure is determined BY MULTIPLYING THE AREA OF THE OPENING FOR THE VALVE IN SQUARE INCHES BY THE GREATEST STEAM PRESSURE, IN POUNDS PER SQUARE INCH, WHICH THE BOILER IS INTENDED TO BEAR. Thus, if the opening for a safety-valve is three inches in diameter, its area will be seven square inches, and, therefore, if the greatest steam pressure which it is intended that the boiler shall bear is 100 lbs. per square inch, the valve must be pressed down with a pressure equivalent to 7 × 100 = 700 pounds. If the pressure on the valve is produced by a lever, as in fig. 74,[37] then the total weight of the safety-valve must be MULTIPLIED BY THE SHORT ARM OF THE LEVER, (or the distance A between the centre of the fulcrum e and that of the load f,) AND DIVIDED BY B, THE TOTAL LENGTH OF THE LEVER. In fig. 74 the short arm of the lever is 3¹⁄₂ inches, and the whole length 35 inches; therefore if the valve is to be pressed down with a pressure of 700 pounds, the pressure on the end of the lever would be calculated as follows:

The spring d must therefore pull down on the end of the lever with a tension equal to 70 pounds. When the pressure of the spring bears directly on the valve, as shown in fig. 76, then the tension of the spring must be just equal to the pressure on the valve. This tension is produced by screwing down the nuts, c, c. The spring d, which produces the requisite pressure on the end of the safety-valve lever, fig. 74, is arranged inside of two cylinders, g and h, which slide over or into each other like the sections of a telescope. This arrangement is called a spring-balance. The spring, d, is attached to the covered ends and draws them towards each other. The upper cylinder g is connected by a rod, i, to the flattened end of the lever c′, which has a hole drilled through it to receive the rod. The other end of the rod is screwed into the upper cylinder g. This rod is sometimes arranged so that it can be either lengthened or shortened by the nut j. By lengthening or shortening the distance, the tension of the spring is either diminished or increased. The lower cylinder of the spring-balance, represented in fig. 74, is attached to a lever, m, which is fastened to the back of the steam-gauge k. This is shown more clearly by fig. 75, which represents the back of the gauge, and also the lever, l m, whose fulcrum is at m. The spring-balance is attached to the lever at k. By drawing down the lever the tension of the spring is increased, and by raising it up it is diminished. The lever is held in any desired position by the latch, n, and the ratchet r r. By this contrivance, which is employed on the engines built at the Grant and also at the Baldwin Locomotive Works, the pressure on the valve can at once be either increased or diminished, which it is often desirable to do, especially when an engine is not at work. The spring-balance is shown in fig. 71, and is indicated by the letter M and the lever by N. Unless the pressure of the steam exceeds that on top of the valve, it will of course not be opened. As there is always danger that a safety-valve or some of its attachments may become corroded or otherwise disordered, so that it will not act promptly or with certainty, it is desirable to open it frequently, so as to be sure that it is in good working order. To do this the pressure on the valve must be reduced below that in the boiler, which can very conveniently be done with the spring-balance lever which has been described.

The lower cylinder of the spring-balance sometimes carries an index or pointer, t, fig. 74, which protrudes through a slot in the cylinder g, and indicates the amount of pressure of the spring on a scale marked along the slot on the outside of the cylinder. If it is desired that the safety-valve should open when the steam pressure reaches 100 or any other number of pounds per square inch, the spring-balance is subjected to a tension which will bring an amount of pressure on the top of the safety-valve equal in pounds per square inch of its surface to that of the steam pressure desired.[38]

There should always be some provision made which will render it impossible to increase the steam pressure beyond that which it has been determined that the boiler will safely bear. This is usually done by arranging one of the safety-valves with a lever, as shown in fig. 74, and the other without, like that in fig. 76. The latter is often covered and sealed or locked up, so as to be beyond the control of the locomotive runner.

The safety-valves are usually fitted into conical seats, S S, figs. 74 and 76, so as to be perfectly steam-tight, and are made with wings or guides, t, t, the form of which is shown in the sectional plan A, figs. 74 and 76, under the valve. These guides are intended to keep the valves in the proper positions in relation to their seats.

Fig. 76. Scale ¹⁄₈.

As soon as the steam pressure under the valves becomes greater than the pressure of the springs on top of them, the valves will be lifted up and the steam will escape until the pressure in the boiler is relieved. It will be seen, however, that although the surface of the valve which is exposed to the pressure of the steam is equal to the area of the opening for the valve, after it is lifted from its seat and the steam escapes all around the edge, a larger surface will be exposed to the pressure of the escaping steam. For this reason, it will be found that after a valve is opened steam will escape, or “blow off,” as it is termed, until the pressure is several pounds lower than it was when the valve first opened. Advantage has been taken of this fact in the valve shown in fig. 76, which is called the “Richardson” valve, which is now much used. The top of this valve is made larger in diameter, so as to expose more area to the escaping steam. Grooves are also made around the edge of the valve and the seat. These, it is claimed, produce some sort of reflex action of the steam, which keeps the valve open longer than it otherwise would be.

Fig. 78.

Fig. 77.

 

Question 146. How is the steam pressure in the boiler indicated?

Answer. By an instrument called a steam-gauge. There are a great variety of such instruments made, but they may all be divided into two classes, and they all operate upon one of two principles. In the one class the pressure of the steam acts upon a diaphragm or plate of some kind, as shown in fig. 77, which represents a section of a gauge of this kind; a b is a metal plate made with circular corrugations, as shown in section and also by the shading in fig. 78, which represents a front view of the gauge with a part of the dial-plate removed. The steam enters by the pipe c and the small opening d, and fills the chamber e behind the metal plate or diaphragm. The corrugations of the latter give it sufficient elasticity, so that when the pressure is exerted behind it, it will be pressed outward by the steam. If it were flat, it is plain that it would not yield, or only to a very slight degree, to the pressure of the steam. In the centre of the diaphragm on the outside is a pin or stud, f, which bears against the plate. This stud is attached to a bent lever or “bell-crank,”[39] g h k, whose fulcrum is at h. To the outer end, k, a rod, l, figs. 77 and 78, is attached, the lower end of which is connected to the short arm m of a toothed segment, n, whose fulcrum is at o. This segment gears into a small pinion, p, which is attached to a spindle or shaft, which carries a pointer, fig. 78. It is obvious now that if the diaphragm, a b, is pressed outward, it will move the bent lever k h g, the motion of which will be communicated by the rod l to the toothed segment n, which will in turn revolve the pinion p, and thus move the hand or index. We have selected for this illustration one of many forms of this kind of gauge. The mechanical appliances for communicating the motion of the diaphragm to the index or pointer are different in the gauges made by different manufacturers. The form of the diaphragm also differs. In some cases it is made of a metal plate; in others a spiral spring is used, covered with india-rubber to make it steam-tight. The steam-gauge represented by figs. 77 and 78 is the form manufactured by M. B. Edson, of New York.

Fig. 79.

Fig. 80.

In the other class of gauges, shown in fig. 79, the steam acts upon a bent metal tube, a b c, usually of a flattened or elliptical section. It may not be known to all readers that if a tube bent, say in the form of the letter U or C, is subjected to the pressure of a liquid or gas on the inside, the force exerted by the pressure has a tendency to straighten out the tube. This is due to the tendency which a tube of an elliptical or flat section has to change the shape of the latter and approximate to a circular form when the inside is subjected to a pressure. Thus let A B, fig. 80, represent a cross section, and a b d c, a longitudinal section of a part of such a tube contained between two radii, O a and O b, drawn from the centre O of the curve in which the tube is bent. If now we subject the inside of A B to a pressure it will have a tendency to assume the form of the circle C D, and would then be represented in the longitudinal section by the dotted lines a′ b′ d′ c′. If now we draw radial lines through a′ c′ and b′ d′, it will be found that they intersect at O′, instead of O, which was the original centre of the curve of the tube. It will be seen that as the section of the tube approximates to the form of a circle, the portion a b which is outside the curve will be moved farther from the centre, while the other side, c d, is moved nearer to it. As indicated by the radial lines, when this occurs either the outside must be lengthened and the inside shortened, to conform to the radial lines a O and b O, or else the tube will be straightened so that the radial lines will assume the position a′ O′ and b′ O′.

The phenomenon of the straightening of the bent tubes of steam gauges is frequently attributed to the difference between the area of the inside and outside of the curve. This error was shared by the writer, until the fallacy of the reasoning which supported it was pointed out to him.

In the gauge represented in fig. 79, (in which the dial-plate is removed), one end, c, of the tube is attached at d to a lever which has a toothed segment, e, at the other end. The end a of the tube is connected with the lever at f. The connection at d, therefore, forms the fulcrum of the lever. It is obvious that as the two ends of the bent tube are forced apart by the steam pressure, the lever and the segment have motion imparted to them. The latter gears into a pinion on the spindle of the index or pointer, r r, which thus indicates on the dial the degree of pressure in the tube. The latter is connected with the boiler by a tube attached at g. Various forms of this kind of steam-gauge are also made, but all act on essentially the same principle.

The position of the steam-gauge on the engine is shown at M, in fig. 71.

Question 147. Why is the pipe which connects the steam-gauge with the boiler bent as shown in fig. 71?

Answer. To prevent the hot steam from coming in contact with the metal plate or tube, as it is found that the heat of the steam affects their elasticity. When a bent tube is used, the steam from the boiler is condensed and fills the bent portion so that when the steam pressure comes on the surface of the water it forces it up the other leg of the tube into the gauge. A cock is attached to this pipe so that the steam can be shut off in case the gauge should get out of order or require to be removed while there is steam in the boiler.

Question 148. How can the accuracy of a steam-gauge be tested?

Answer. When the gauge is in good working order, the index or pointer moves easily with every change of pressure in the boiler, and if the steam is shut off from the gauge, the index should always go back to 0. In order to determine the accuracy of its indications, however, they should be tested with a column of mercury. This consists of a long, vertical tube, terminating at its base in a closed vessel filled with mercury. The gauge is then attached to the top of this vessel and water or oil is forced into the vessel on top of the mercury and into the gauge. A pressure of one pound per square inch will force up the column of the mercury 2.04 inches, so that by graduating the tube into spaces that distance apart, the divisions will indicate the pressure in pounds per square inch. Thus, a pressure of 50 pounds would force up the column of mercury 102 inches, and with 100 pounds pressure the column would rise 204 inches, and therefore, when the mercury reaches these or any other points, the steam-gauge, if it is accurate, should indicate equivalent pressures.

The ordinary steam-gauges are very liable to get out of order, and therefore they should be frequently tested to ascertain whether their indications are correct.

Question 149. What is the steam whistle, and for what purpose is it used?

Answer. The steam-whistle, W, fig. 71, and shown in section on a larger scale in fig. 81, consists of an inverted metal cup or bell, A, made usually of brass. The lower edge of this cup is placed immediately over an annular opening, a a, from which the steam escapes and strikes the edge of the cup or bell, which produces a deep or shrill sound, according to the size or proportions of the whistle. The annular opening a a is formed by the plate or cover, a a, which nearly fills the mouth of the cup B, which is attached to the stem c. The latter is screwed into the top, D, of the dome. Communication with the steam-space of the boiler is either opened or closed by a valve, b, which is attached to a sort of spindle, d, which extends upward inside of the stem c. This spindle does not entirely fill the opening in the stem c, so that the steam which enters when the valve b is opened rises and escapes through the holes e, e, e, into the cup B and out through the annular opening a a. The valve is opened by the lever E, whose fulcrum is at f. The end g of this lever is connected by a rod, h, figs. 81 and 71, with the cab, and by a suitable handle or lever, h′ h′, fig. 71, it can be opened and the whistle be blown at any time by the locomotive runner or fireman to give signals to the trainmen or of the approach of a train to a station, or to warn persons to get off of the track.

Fig. 81. Scale 1¹⁄₂ in. = 1 foot.

Question 150. How is a locomotive boiler emptied and cleaned?

Answer. One or two large cocks, called blow-off cocks, X, fig. 71, are placed near the bottom of the fire-box, either in front or behind, and sometimes on the side. By opening either of these the water in the boiler is blown out, and much of the loose mud and dirt is carried out with the water. The cock X, fig. 71, is opened by a handle, w, which is connected with the cock by a rod.

In order to clean out the mud and scale which are not entirely loose, what are called mud-holes or hand-holes are placed in the corners of the fire-box near the bottom. These are oval-shaped holes, about 4¹⁄₂ inches long and 2¹⁄₂ inches wide, and covered with two metal plates, one of which is put inside the boiler and the other outside, and fastened with a bolt through both. Another hand-hole is sometimes placed in the bottom of the front tube-sheet. When the boiler is emptied of water these hand-holes are uncovered, and as much dirt is removed as can be scraped out of these holes. A hose pipe is then inserted and a strong stream of water is forced in, which washes out nearly all the loose dirt, so as to leave the boiler comparatively clean.

When the water is very impure, what is called a mud-drum, M, fig. 41, is used. Much of the mud and dirt is deposited in this receptacle, from which it can easily be removed by taking off the cast-iron cover on the bottom of the drum. The cover is also provided with a blow-off cock, which is shown in the figure referred to.

Question 151. What other attachments are there to the boiler of a locomotive?

Answer. There are two cocks, a, a, fig. 71, called heater-cocks, which are connected with pipes to the feed-pipes D D, to admit steam to the latter to prevent the water in them from freezing. There is also another cock, b, called a blower-cock, which is connected to the smoke-stack by a pipe b, b. Steam is conducted through this pipe and escapes up the chimney in a jet, thus producing a draft when the engine is not working. This arrangement is called a blower and is used to blow the fire when the engine is standing still. The action of the jet is similar to that of the exhaust steam which escapes up the chimney, excepting that the steam from the jet escapes in a continuous stream instead of distinct “puffs,” as it does when it is liberated alternately from one end of the cylinders and then from the other.

T′ is a handle which is connected by a rod, T′ T, with the feed-cock (not shown in the engraving) in the pipe D. This cock can be opened or closed by the handle, and the supply of water fed into the boiler by the pump can thus be regulated. J is a handle on the other side of the engine, for regulating the working of the pump on that side.

e, e are handles, also connected by rods with the pet-cocks on the pumps. These cocks can thus be opened or closed, and it can then be known whether the pumps are working.

A is the furnace door, which is fastened by a latch. The latter has a chain, Q, attached to it by which it can be conveniently opened or closed. The door also has a circular register with six holes to admit air into the furnace. These holes can be opened or closed by the revolving circular disc shown in the engraving.

Fig. 82. Scale ³⁄₄ in. = 1 foot.

Fig. 83. Scale ³⁄₄ in. = 1 foot.

Fig. 84. Scale ³⁄₄ in. = 1 foot.

Question 152. How are the grates constructed?

Answer. As has already been explained, they are made usually of cast-iron bars,[40] A, A, A, figs. 82 and 83, called grate-bars. Fig. 82 is a plan, and fig. 83 a horizontal section of one form of grate. The bars in this kind of grate are usually cast in pairs, or some times three or more are cast together. They are made wider on the top than on the bottom edges, as shown in the section, fig. 83, so that cinders and ashes will fall through easily, and also to give free access to the air from below. They are usually from ³⁄₄ to 1¹⁄₂ inches wide on the top, and about ³⁄₄ inch on the lower edges. The spaces between the bars are made from ¹⁄₂ to 1¹⁄₄ inches wide. For burning wood the bars are placed comparatively close together and are stationary, but for burning bituminous coal they are usually made so that they can be moved, in order to shake or stir up the fire, just as is necessary in an ordinary stove or grate fire. In the grate we have illustrated the bars, A, A, are cast in pairs, and run crosswise of the fire-box. The ends are made with a sort of journals, b, b, which rest on two supports, B, B, called bearing bars, which have suitable indentations to receive the ends of the grate-bars. The latter have arms, C, C, fig. 83, cast on the under side, to which a bar, D D, is attached. By moving this bar back and forth, the grate-bars have a rocking motion imparted to them, as shown in fig. 84. It is evident that in this way the fire over the whole surface of the grates will be disturbed or shaken. The bar, D D, is moved by a lever, m m, shown in fig. 71. An extension piece, not shown in fig. 71, is used with the lever m m, so as to increase its length; but it is removed after it has been used, so as not to be in the way of the fireman. Grates which have movable bars are called shaking or rocking grates. A great variety of such grates are made and in use, to describe which would require more room than is available here.

For burning anthracite coal what are called water-grates are used. These consist of wrought-iron tubes, 2 inches in diameter outside, which are fastened in the front and back plates of the fire-box and are inclined upward from the front end, so that there will be a continued circulation of water through them to keep them cool and thus prevent them from being burned out by the intense heat of the fire.

Question 153. How is the fire removed from the fire-box when it is necessary to do so?

Answer. In bituminous coal burning engines, what is called a drop-door, E E, figs. 82, 83 and 84, is provided for that purpose. This door is supported partly on journals, d, d, similar to those in the grate-bars, on which it can turn, and is held up or prevented from dropping by arms, e, e, attached to a shaft, F F. This shaft is operated by a lever, f f, fig. 82, outside the fire-box.

When the arms are in the position represented in fig. 83, the drop-door is held up in the place in which it is shown; but when they are turned as in fig. 84, the door falls down so that the burning coal can be taken out of the opening at G, and, by raising up the ash-pan damper, H, fig. 84, can be raked out on the track or into suitable pits usually provided for this purpose. The drop-doors are sometimes perforated so as to admit air to the fuel on top of them.

The grates for burning anthracite coal usually have about four solid wrought-iron bars between that number of tubes. These bars can be withdrawn, and the fire then falls into the ash-pan through the opening left by the withdrawal of the tubes.

Question 154. How are the dampers of the ash-pan operated?

Answer. They are connected by suitable rods and levers with two handles, l, l, fig. 71, which are raised or lowered, thus opening or closing the dampers.

PART IX. THE THROTTLE-VALVE AND STEAM PIPES.

Question 155. How is the steam admitted to and the supply regulated or shut off from the cylinders?

Answer. By a valve, h, fig. 41, called a throttle-valve, which is usually placed at the end of the pipe I, near the top of the dome. Throttle-valves are sometimes placed in the smoke-box at the front end, n, fig. 41, of the dry-pipe. Until within a few years they consisted of plain slide-valves which covered openings similar in form to the steam-ports, but smaller in size. The pressure on such valves is of course greatest when there is no steam underneath, which is the case when the valves are closed. It is then very difficult to open them, and as it is important that the supply of steam admitted to the cylinders when the locomotive is started should be easily regulated, such valves are objectionable, and therefore the form has been introduced which is illustrated in fig. 41, and also on a larger scale in fig. 85, which represents a longitudinal section of the throttle-pipe and valve. This is what is called a double-poppet valve, and consists of two circular discs, a and b, which cover two corresponding openings in the end of the pipe I. When these discs are raised up, as shown in fig. 85, steam flows in around their edges, as represented by the darts. It will be observed that the steam pressure in the boiler comes on top of the disc a and against the under side of b. The pressure on the one thus neutralizes or balances that on the other. If the two discs were of the same size, the pressure of the one would be exactly the same as on the other; but as they are joined together and are made to fit steam-tight on their seats by what are called countersunk joints, their diameters must be somewhat larger than the openings they cover. The only practicable way, therefore, by which the lower disc b can be introduced into the end h of the pipe I so as to cover the lower opening is through the upper opening a. For this reason the lower disc must be made smaller than the upper one, and therefore the pressure on the upper one, being in proportion to its size, has a constant tendency to close the valve. As it is of the greatest importance that a throttle-valve should remain closed after steam is shut off, and never be opened at any time accidentally, the arrangement described accomplishes just what is needed—that is, makes the valve work comparatively easily, and at the same time keeps it closed after the steam has been shut off.

Fig. 85. Scale ³⁄₄ in. = 1 foot.

Fig. 86. Scale ³⁄₄ in. = 1 foot.

Question 156. How is the valve opened and closed?

Answer. By a lever, O′ O, called a throttle-lever, figs. 85, 86[41] and 71. This lever is connected by a rod, d, called the throttle-stem, with a bell crank, i, the other arm of which works the rod c, to which the throttle-valve is attached. The rod d works through a steam-tight stuffing-box, f, in the back end of the boiler. The end of the throttle-lever is attached to two links, g, fig. 86, which are fastened by a pin to the stud h. These links have a slight vibratory motion, which enables the pin k, by which the lever O′ O, is fastened to the rod d, to move in a straight line, which is necessary in order that the rod d may work steam-tight in the stuffing-box, f. The throttle-lever has a latch, l, which gears into a curved rack, n, so as to hold the lever and valve in any required position. This latch is operated by a trigger, m. Various other devices are used to fasten the throttle-lever and thus hold it in any position required.

Question 157. How are the steam pipes constructed?

Answer. The steam, after it is admitted by the throttle-valve, as was explained in answer to Question 155, passes into the throttle-pipe I and the dry-pipe m m, fig. 41. At the front end of the dry-pipe a pipe, n, figs. 40 and 41, which divides into two branches like the top of the letter T and is therefore called a T-pipe, is attached. The steam-pipes o, o, fig. 40, are connected to each of the two branches of the T-pipe at one end and to the cylinders at the other.[42]

Fig. 87. Scale 1¹⁄₂ in. = 1 foot.

Fig. 88. Scale 1¹⁄₂ in. = 1 foot.

These pipes, being in the smoke-box, are exposed to great changes of temperature, and are therefore subjected to expansion by heat and contraction by cold. The joints are therefore constantly subject to disturbance by the contraction and expansion of the pipes and so are difficult to keep tight. It is also practically impossible to construct the boiler, the cylinders and the pipes with perfect accuracy, and therefore a small amount of adjustability and flexibility is necessary in the joints of the pipes. If, for example, the opening x in the cylinder, fig. 40, were either too near or too far from the cylinder of the engine, it would be necessary to move the end of the pipe o either to the right or to the left in order to connect it with x. If the joint of the upper end of the steam-pipe were attached to the T-pipe with a flat joint like that shown at a b, fig. 87, it would be impossible to move the lower end of the steam-pipe either to the right or to the left without disturbing the joint and causing it to leak. For this reason these pipes are connected with what are called ball joints, fig. 88, that is, the end a b of one of the pipes is turned into the form of a part of a sphere,[43] and the other end into a corresponding concave form. It is known that a sphere will fit into a corresponding socket in any position; for example, an acorn in its cup or the bones at the hip or shoulder joints. If, therefore, the pipes are joined with such spherical or ball-joints as they are called, the lower end can be moved sideways several inches either way, and the joint will still be steam-tight if it is then firmly bolted together. Even after it is bolted together it will have so much flexibility that the expansion and contraction of the pipes will not cause it to leak.

There is, however, still another difficulty. Although the lower end of the pipe, o o, fig. 40, can, with a ball-joint above, be moved in any direction horizontally, yet if the pipe is too long or too short it is obvious such a joint will not permit it to be moved up or down. A joint with a flat surface, like that shown in fig. 87, would, however, permit such motion in the pipe without leaking. If, for example, the steam-pipe were ¹⁄₈ of an inch too short, it might be drawn down that distance, and if the upper joint were then screwed up it would still be steam-tight. In order, then, to get both vertical and lateral flexibility in the joints of the steam-pipes, a ring, a b, fig. 89, is interposed between the pipes. One side of this ring is spherical and the other flat, so that the pipes can move either around the spherical part or slip up or down or sideways on the flat surface of the ring. In this way the pipes are flexible and adjustable in every direction, and for all kinds of motion caused by expansion, or which may be needed when the parts are put together. Sometimes the joints at one end only of the steam-pipes are made in this way, and the other is connected with a simple ball-joint.

Fig. 89. Scale 1¹⁄₂ in. = 1 foot.

In designing these joints their form should be drawn with a radius, c d, fig. 88, from one centre, c, so that the surface of the joint will form a part of a sphere. If they are drawn from two centres, as is sometimes done, it is obvious that the surface of the joint will not be a part of a sphere, and therefore will not have the requisite flexibility. The surfaces of the joints are carefully turned to the proper form, and then made steam-tight by scraping or grinding them with emery and oil, and the pipes are then fastened together with bolts, g, g, fig. 89, and flanges, f, f, cast on the pipes.

Question 158. How are the exhaust pipes constructed?

Fig. 90. Scale 1¹⁄₂ in. = 1 foot.

Answer. They are made of cast iron. When two nozzles are used they are generally cast together, as shown in fig. 90. When only one is used, the form of the pipes resembles somewhat that of an inverted letter ⅄, as shown in fig. 91, so as to cover the two openings which connect with the cylinders. The tops of these pipes have rings or bushings, a a, fitted into them, which are held by set screws, b, so that they can easily be removed and others with larger or smaller openings be substituted. If the openings in the exhaust-nozzles are small, the steam must be discharged at a higher rate of speed, in order to exhaust all that is in the cylinders, than if the blast orifices are larger. Therefore, if the latter are reduced in size, the draft becomes more violent, but at the same time the back-pressure in the cylinder (which will be explained hereafter) is increased. It therefore becomes necessary to adjust the size of the blast orifices with the greatest care, so as to have them just small enough to produce the required draft and yet leave them as large as possible, so as to reduce the back-pressure. For these reasons what are called variable exhausts are sometimes used. In these the blast orifice can be increased or diminished at pleasure, and thus regulated to suit the conditions under which an engine is working. A great variety of such devices has been used, but now nearly all have been abandoned for the simpler arrangement described, which is not variable when the engine is working.

Fig. 91. Scale 1¹⁄₂ in. = 1 foot.

PART X. THE CYLINDERS, PISTONS, GUIDE-RODS AND CONNECTING-RODS.

Question 159. How are the steam cylinders constructed?

Answer. They are made of hard cast iron, and have the steam and exhaust ports and valve-seats cast with them. The harder the iron the better will the cylinders withstand the wear of the pistons and valves, but they must at the same time be made soft enough, so that after they are cast the inside can be bored out perfectly cylindrical, the ends turned off, the bolt-holes drilled, and the valve-seats planed smooth.

Fig. 92 represents a longitudinal section through the centre of the cylinder and steam-chest. Fig. 94 is a plan of the same parts with the cover of the steam-chest and the valve removed. The left-hand side of fig. 95 shows a transverse section through the centre c d, fig. 92, of the cylinders, and the right side is a section through the steam-pipe G h, fig. 94. The same letters indicate like parts in the three different views.

The cylinders of locomotives in this country are now universally placed on the outside of the wheels, as has already been described. In order to fasten them securely together and to the boiler, they are attached to what is called a bed-plate or bed-casting, D D, figs. 94 and 95, which is placed between them. Some builders make this bed-casting in a separate piece, and the cylinders are then bolted to it on the outside, about at the dotted lines, l, m, fig. 95. Others cast one-half of it with each cylinder, as shown in our engravings, and then bolt them together at the line i, j, which is the centre of the engine. The bed-casting is also bolted to the smoke-box by the flanges E, E. The cylinders are bolted to the frame F with bolts, m and k, fig. 95.

After the cylinders are bored out, and the ends turned off, heads, A and B, figs. 92 and 94, are fitted with steam-tight joints to each end. These heads are fastened with bolts and nuts, a, a, a, to flanges, C, C.

Question 160. How is the steam conducted to and from the cylinders?

Answer. Two pipes or passages are cast in each cylinder, the one, G G′, fig. 95, for admitting steam into the steam-chest, and the other, H H′, for exhausting it from the cylinders. The one G G′ is called the steam-passage, and the other, H H′, the exhaust-passage. The steam-passage terminates at one end with a round opening, G, figs. 94 and 95, to which the steam-pipe o, figs. 40 and 95, is attached inside of the smoke-box. At the other end it divides into two branches, G′, G′, fig. 94, each of which terminates in an opening, g, g′, inside of the steam-chest. The steam is thus delivered at both ends of the chest, and can pass freely into each of the steam-ports. By making the cylinders in this way, they are exactly alike for each side of the engine, or, to use a shop phrase, there are “no rights and lefts,” so that a cylinder casting can be used for either side of the engine. This method of making cylinders has been adopted by a number of the principal builders in this country, but is by no means universal.

Fig. 92. Scale ³⁄₄ in. = 1 foot.

Fig. 93.

Fig. 94. Scale ³⁄₄ in. = 1 foot.

Fig. 95. Scale ³⁄₄ in. = 1 foot.

Question 161. How is the steam-chest constructed?

Answer. It usually consists of two castings, one of which, J, figs. 92 and 94, is a square cast-iron box made open at the top and bottom. This rests on the top of the cylinder casting and is joined to the latter with a steam-tight joint. On top of it is a cast-iron cover, K K. The steam-chest and cover are held down by bolts, p, p, which are screwed into the cylinder casting and have nuts on top.

Question 162. How are the slide-valves made to work steam-tight on the valve-seats?

Answer. They are first planed off smooth, and then filed and scraped until the two touch each other over the whole of their surfaces in contact. The valve-stem v, fig. 92, works steam-tight through a stuffing-box on the steam-chest.

Question 163. How are the valves and pistons oiled?

Answer. The oil is usually introduced into the steam-chest through a cock, c, fig. 92, called an oil-cock. From this cock it flows down upon the valve and is conducted by suitable holes and channels to the valve-face and from there through the steam-ports to the cylinder and piston. Sometimes, for greater convenience, oil-cocks c, c, fig. 71, are placed inside of the cab and communicate by pipes with the steam-chests.

The valves are oiled by pouring oil or melted tallow into the oil-cocks when the steam is shut off from the steam-chests and cylinders. When the pistons are working in the cylinders without steam, they create a partial vacuum, so that if oil is then poured into the oil-cocks it will be sucked into the steam-chests, or, in other words, it will be forced in by the pressure of the air above it. Q, fig. 71, is a shelf attached to the boiler to receive an oil-can filled with oil or tallow, which is thus melted or kept in a fluid condition by the heat of the boiler.

Question 164. How are the cylinders and steam-chests protected so as to prevent, as far as possible, the heat in the steam from being lost?

Answer. The sides of the cylinders are covered with wood, w, w, w, fig. 95, called the cylinder lagging, and the wood is covered outside with Russia iron or brass, which is called the cylinder-casing. The ends of the cylinders have light metal covers, called cylinder-head covers, made of cast iron, brass or sheet metal. The steam-chest is covered in a similar way so as to be surrounded either with a covering of wood or of confined air. Sometimes coarse felt is used for the purpose. The covering, excepting the cylinder lagging, is not shown in the engravings.

Question 165. For what purpose are the cocks C, C, figs. 92 and 95, at each end of the cylinder, used?

Answer. They are used to exhaust the water which collects in the cylinders. When the engine is not working the cylinders and steam-pipes are all cooled off, so that when steam is first introduced into them a great deal of it is condensed until they become warmed. Water is also frequently carried over from the boiler with the steam. When this occurs the boiler is said to prime, or to “work water.” This water and that produced by the condensation of steam collect in the bottom of the cylinder and will not escape through the exhaust-pipes until the piston moves up so near to the end of the cylinder that the water will fill the whole space between it and the cylinder-head. As has already been stated, it will then escape so slowly that the momentum of the piston and other machinery is liable to “knock out” the cylinder-heads or even break the cylinder itself. The cocks C, C, called cylinder-cocks, are therefore placed in the under side of the cylinder, so that when they are open if there is any water in the cylinder it will escape through the cocks. They are therefore always opened when the engine is starting, or at any other time when there is any indication that there is water in the cylinders.

Question 166. How are these cocks opened and closed?

Answer. A shaft, R R, figs. 92 and 94, which extends across the frames, has an arm, R S, fig. 92, at each end. These arms are connected by rods, S T, with the handles of the cylinder-cocks. The shaft also has a vertical arm, R U, the upper end of which is connected by a rod with the cab. At the end of the rod is a suitable handle, f, fig. 71, by which the cocks can be either opened or closed at pleasure by the locomotive runner.

Question 167. How is the piston-rod fastened to the piston?

Answer. It fits into a straight or tapered hole in the piston-head, in which it is fastened either with a key, k k, as shown in figs. 96 and 97, [44] or by a nut on the front side of the piston.

 

Fig. 96.

Fig. 97.

 

Scale 1¹⁄₂ in. = 1 foot.

Question 168. How is the piston constructed?

Answer. It is made of two cast-iron pieces, B and C, fig. 97, the one, B, called the piston-head or spider, to which the piston-rod D is attached. The other part, C, called the follower-plate, is bolted to the piston-head by the bolts c, c, called follower-bolts. The piston-head has lugs or projections, d, d, d, fig. 96, cast on the inside, to which the follower plate is bolted. Hollow spaces are thus left between these lugs.

Question 169. How is the piston made to work steam-tight in the cylinder?

Answer. By means of two rings, A, A, figs. 96 and 97, called packing-rings. These rings are turned of the same size or a little larger in diameter than the cylinder. They are then cut open at one point in their circumference so that they can be pressed apart or expanded by the springs a, a, called packing springs, on the inside of the rings. These springs are pressed out by the nuts and bolts b, b, called packing-bolts and packing-nuts, so that when the rings wear they can be expanded so as to fill the cylinder completely. The place where the one ring is cut is placed opposite that of the opening in the other ring, or they are made to break joints, as it is called. This is done to prevent the steam which leaks through the opening where the one ring is cut from passing through to the other side of the piston. These rings are usually made of brass and have grooves, c, c, fig. 97, turned in them, which are filled with what is called Babbitt’s metal. This metal is used because it is less liable to scratch the cylinders than brass alone. Another ring, l l, made of cast iron and as wide as the two brass rings, is placed inside of the latter and is intended to furnish a bearing for the springs, and thus distribute the pressure of the springs equally on the packing rings. This iron ring is also cut open at one point.

Question 170. How is the piston-rod made to work steam-tight through the cylinder-head?

Answer. By what is called a stuffing-box. This consists of a cylindrical chamber, r r, fig. 92, which is made about 1¹⁄₂ inches larger in diameter than the piston-rod. This leaves a space ³⁄₄ of an inch wide all around the rod. This space is filled with hemp or some other fibrous material, called packing, saturated with oil or melted tallow. This packing is compressed by a hollow cylinder, s s, called a gland, the inside of which fits the piston-rod and the outside the stuffing-box. This gland is forced into the stuffing-box by nuts, t, t, which are screwed down on a flange, u, attached to the gland. The packing is thus compressed in the stuffing-box and forced against the piston-rod, which is made smooth and perfectly round and straight, and against the side of the stuffing-box, so that no steam can escape around the piston-rod. A brass ring or “bushing” is often put into the cylinder-head and in the gland where it touches the piston-rod,[45] because brass will bear the friction of the rod better than cast iron, and when it is worn out it can be removed and a new one substituted in its place.

Question 171. Why is the end of the piston-rod made to work in guides?

Answer. Because it must move in a straight line if it and the piston work steam-tight in the cylinder. By referring back to fig. 2, it is obvious that if a pressure be exerted against the piston B and communicated to the crank-pin N by the connecting-rod E, the latter, excepting at the dead-points, will exert a pressure either upward or downward, according to the direction the piston is moving. This pressure would bend the piston-rod if no provision were made to prevent it. For this reason, therefore, the end of the piston-rod is attached to what is called a cross-head, L, figs. 92 and 94, which works in guides, M, M. The cross-head is made of cast iron and has slides, N, N, figs. 93 and 94, one on each side, each of which works between a pair of guide-bars or rods, M, M, shown in section in fig. 93.[46] These guide-rods, or guides as they are called, are planed and finished with great accuracy so as to be straight and smooth, and are attached to the cylinder-head at one end, and to a support, O, called the guide-yoke, which is fastened to the frame at F, fig. 94, and also usually attached to the boiler. The guides are set with great care, so as to be exactly parallel with the axis or centre line of the cylinder, so that the cross-head will slide in exactly the same path that the piston-rod will if it moves in a straight line. If then the piston-rod and the connecting-rod are attached to the cross-head, all the strain produced by the obliquity of the connecting-rod will be borne by the guides, thus relieving the piston-rod, and making it certain that it will move in a straight line.

Question 172. How are the piston and connecting-rods attached to the cross-head?

Answer. The end of the piston-rod fits into a tapered hole in the cross-head and is held by a key, w, figs. 92 and 94. The connecting-rod is attached to a pin, Q, called a wrist-pin, which is cast with the cross-head.

Question 173. How is the wear of the slides lessened and compensated?

Answer. Sometimes they are made with brass wearing pieces called gibs, shown at N, N, fig. 93, which are placed between the slides and the guides. These gibs can either be removed and new ones substituted when they become very much worn, or by inserting thin pieces of metal, called liners, between them and the cross-head, they will be spread apart so as to fill the space between the slides. The slides are now, however, oftener made without gibs, and have recesses either cast or drilled in them, which are filled with either Babbitt’s metal or glass bearings, which latter are said to wear very well. The guides are bolted at each end to blocks, x, x, called guide-blocks, which can be planed off so as to bring the guides nearer together when they and the slides are worn. Sometimes liners are placed between the blocks and the guides, which can be removed when it is necessary to bring the guides nearer together.

Question 174. Are the top and bottom guides worn alike?

Answer. No: the top guide in ordinary engines is worn the most, because the pressure of the slides is always on the top guide when the engine is running forward, and on the bottom guide when it is running backward. This will be understood by referring back to the series of figures from 11 to 24. It will be noticed that in the backward stroke of the piston, represented by figs. 11 to 17, the strain on the connecting-rod tends to push the cross-head upward, and in the forward part of the stroke, figs. 18 to 24, the connecting rod pulls the cross-head in the same direction. If the crank turned the opposite way, this action would be reversed and the cross-head would then be alternately pushed and pulled downward, and the bottom guides would then be worn the most. As nearly all locomotives run forward more than backward, the top guides are usually worn the most.

Question 175. How are the slides oiled?

Answer. Oil cups, n, o, figs. 92, 93 and 94, are placed about the middle of the top guide. These cups usually have a reservoir to hold a supply of oil, and are so constructed that it will be gradually fed on the slides, which are thus constantly and regularly lubricated.

Question 176. How are the pumps worked from the piston-rods?

Answer. The pump-plunger is attached to a projection, W, figs. 92 and 93, called the pump-lug, cast on one of the slides of the cross-head. The plunger thus receives a reciprocating motion from the piston.

Question 177. How are the connecting-rods made?

Answer. They are made of flat bars of wrought iron. The rods which connect the cross-heads with the driving-wheels are called main connecting-rods, and the rods which connect or couple the driving-wheels together are called coupling-rods.[47] Fig. 98 represents a side view and a plan of a main connecting-rod. In the side view the end B, and in the plan both ends of the rod are shown in section. It is attached to the wrist-pin at A and to the crank-pin at B. Fig. 99 represents similar views of a coupling-rod. To save room in the engraving each of these rods is represented with a part of the middle broken away. The main rods are usually made wider at G, next the crank-pin, than at the other end, as it has been found that they are most liable to break at that end. The coupling-rods are now made either straight or somewhat wider in the centre.

Fig. 98.

Fig. 99.
Scale ³⁄₄ in. = 1 foot.

Question 178. How are these rods prevented from getting loose on the pins from the wear of the latter in the inside of the holes of the rods?

Answer. The ends of the rods are provided with what are called brass-bearings,[48] or “brasses,” c, d, and e, f. These brasses are made in pairs, so as to embrace the pins, from each side. They are held by ⊃-shaped clamps, s s s, called straps, which are bolted to the rods. When the brass-bearings become worn, they are taken out of the straps, and a portion of their surfaces of contact with each other is filed away, thus allowing them to come nearer together, and thereby reducing the size of the hole which receives the pin or journal. In order to prevent their being loose in the straps, tapered or wedge-shaped keys, k, k, which bear against the brasses, are fitted in the straps and rods. By driving down these keys the brass bearings are forced together, thus reducing the size of the hole for the journal, and making the rods fit tightly on the pins. A hard steel plate, shown by dark shading in the engraving, is sometimes interposed between the keys and the brasses to prevent the key from indenting the surface of the soft brass. As the keys are very liable to get loose and fall out, they are held either by screws and nuts, x, x, as shown in the engraving, or by set-screws on the side of the rods. The whole arrangement of straps, keys and brasses is called a stub-end.

Question 179. How are the journals of the crank-pins oiled?

Answer. By oil-cups attached to the straps, above the journals, similar to the cups used on the guide-rods, but which are not shown in the engravings of the connecting-rods. Sometimes oil-cellars, as they are called, are attached to the under side of the straps. These are metal boxes, which are filled with oil, which is agitated violently by the rapid motion of the rods, and is thus applied to the journals through holes drilled in the straps. In order to confine the oil and prevent its leaking out around the journals of the coupling-rods, the brasses at n, n, fig. 99, are usually made so as to enclose the outside end of the crank-pin, which thus not only keeps the oil in, but excludes the dust. The brasses are usually lined with Babbitt’s or some other kind of soft metal, which is thought to be less liable to heat from the friction of the journals.

Question 180. Are the coupling-rods always made with stub-ends?

Answer. No; their ends are sometimes made in one piece—that is, without straps or keys. The holes which receive the crank-pins then have brass rings or bushings, as they are called, which fit tightly and are driven into the holes, and form the bearings on the pins. When these rings become worn they are driven out and new ones put in.

Question 181. What is meant by the term lost motion?

Answer. It is used to designate the wear of machinery, which causes a loss of motion in some of the parts. Thus if the bearings of the main connecting-rods are worn, the piston must move a distance equal to the wear at each end of the stroke before it moves the crank-pin. Lost motion might therefore be called the looseness of the parts. When we speak of taking up the lost motion, we mean making parts which were loose fit tightly.

PART XI. THE VALVE-GEAR.

Question 182. What is meant by the valve-gear of a locomotive?

Answer. By the valve-gear is meant the arrangement of eccentrics, rods, links, rockers, etc., by which the valves are moved and their motion regulated.

Question 183. What is required of the valve-gear in working a locomotive?

Answer. It must be so arranged that the locomotive can be run either backward or forward, and so that the motion of the wheels can be reversed quickly and with certainty. It should enable the runner to employ the greatest power of the engine by admitting steam into the cylinders during the whole or nearly the whole of the stroke of the pistons, or when less power is required, to use the steam more economically by working it expansively, which latter is accomplished with the present appliances by changing the travel of the valve.

Question 184. How is the valve-gear constructed so as to run the engine either backward or forward?

Answer. As already explained, in answer to question 76, two eccentrics are provided for each cylinder. These are set so that one of each pair will run the locomotive in one direction, and the other two the reverse way.

Question 185. How must the eccentrics for each cylinder be set in order that the one may run the engine forward and the other backward?

Answer. This can be best explained by reference to fig. 100, in which the piston, P, is represented at the beginning of the backward stroke, and the valve V has the requisite lead and is just about to open the front steam-port. It is obvious that, in order to complete the backward stroke of the piston, the front port must be opened to admit steam into the front end of the cylinder, and therefore the valve must be moved in the direction indicated by the dart a. To do this, the upper arm of the rocker r must move in the same direction, and the lower arm must be moved the reverse way, as indicated by the dart e. If the crank is intended to move in the direction indicated by the dart N, then the centre of the eccentric must be above the centre of the shaft or axle, in order to move the rocker in the direction indicated by the dart e. Supposing, however, it was intended to move the crank the reverse direction, as shown by the dart N in fig. 101; it is evident in that case that the valve must be moved in the same direction as before, in order to open the front steam-port and thus admit steam to force the piston back. But if the crank turns in the direction shown by the dart N, fig. 101, then the centre of the eccentric must be placed below the centre of the axle in order to move the lower rocker arm in the direction of the dart e and the valve in that indicated by a. It will thus be seen that the centres of the eccentric for running forward and that of the one for running backward must be placed, the one above and the other below the centre of the axle at the beginning of the stroke of the piston, as shown in figure 101.

Fig. 100.

Fig. 101.

Fig. 102.
Scale ³⁄₈ in. = 1 foot.

Question 186. Why is it that the centres of the eccentrics are not placed opposite to each other on the axle?

Answer. Because before the beginning of the stroke of the piston it is necessary to move the valve from its middle position a distance equal to the lap before the steam-port begins to open. If we have a valve like that shown in fig. 10—that is, without any lap—the centres of the eccentrics could be placed at right angles, or, as mechanics say, “square” with the crank, as was shown in fig. 11, and exactly opposite to each other, because such a valve begins to take steam as soon as it moves from the middle of the valve-face. If, however, we have a valve like that shown in fig. 27, it is plain that before it will admit or take steam, as it is called, in either of the steam-ports, it must be moved from the centre of the valve-face, or its middle position, a distance equal to the lap, L. For this reason, therefore, the eccentric, instead of being placed at half-throw,[49] as it is called, must be so far ahead of the middle position as to have moved the valve a distance equal to the lap, and if any lead is given to the valve, equal to the lap and lead together. In figs. 100 and 101, f g is a vertical line at right angles to the crank at the beginning of the stroke. It will be seen that the centre of each of the eccentrics is set far enough ahead of this line to give the valve the required lead. When the piston reaches the back end of the cylinder, the two eccentrics will occupy the position shown in fig. 102, in which position the lower one would move the valve so as to turn the crank in the direction of the dart N, and the upper one in the reverse direction. It will be seen that in this position both of the eccentrics are again ahead of half-throw, when the piston is at that end of its stroke.

Fig. 103.

Scale ³⁄₄ in. = 1 foot.

Question 187. How is the motion of either eccentric communicated to the valve?

Answer. The ends of each pair of eccentric-rods are connected together by a link, a b, fig. 103. This link has a curved groove or slot, a b, in it, in which a block, B, fits accurately, so that it can slide freely from one end to the other. This block is attached to the lower rocker-arm by a pin, c, which works freely in the block. The two eccentric-rods C and D are attached to the ends of the link at e and f by pins and knuckle-joints. It is apparent that if the link is down, or in the position shown in fig. 103 and also on a smaller scale in fig. 104, the motion of the upper eccentric-rod, which is usually used for the forward motion, will be imparted to the rocker, and thus to the valve, and when the link is in the position shown in fig. 105, that the valve will be moved by the lower or backward eccentric-rod B. In order to reverse the engine, it is then only necessary to provide the means of raising and lowering the links. This is done by a shaft, A, fig. 103, called a lifting-shaft, which has two horizontal arms, E,[50] one for each link, and a vertical arm, F. The links are suspended from the ends of the horizontal arms by rods or bars, g h, called link-hangers, which are connected to the links and to the arms above by pins, which enable the hangers to vibrate freely. The lower pin is attached to a plate, L d, called a link-saddle, which is bolted to the link. The vertical arm of the lifting-shaft is connected by a rod, G G, called the reverse-rod, to a lever O, O, Plate II. in the cab called a reverse-lever, the construction of which will be explained hereafter. This lever is worked by the locomotive runner, and by moving the upper end of it forward, the link will be lowered, and the rocker and valve will be moved by the forward eccentric; and if the reverse-lever is moved back, the link will be raised, and the backward eccentric will move the valve. When this is done, the valve-gear is said to be thrown into the forward or backward motion, or forward or back gear.

Fig. 104.

Fig. 105.

Scale, ³⁄₈ inch = 1 foot.

Fig. 106.

Fig. 107.

Scale, ³⁄₈ inch = 1 foot.

Question 188. How is the travel of the valve changed by the motion of the link?

Answer. By either raising or lowering the link, so that the link-block and rocker-pin will be some distance above or below the eccentric-rods. Thus in fig. 104, the motion of the upper eccentric-rod, and in fig. 105 that of the lower or back eccentric-rod is communicated to the rocker-pin and the valve. If, however, the link should be raised so that the link-block and rocker-pin are somewhat below the upper or forward eccentric-rod, as shown in fig. 106, then the motion imparted to the rocker and valve will partake somewhat of that of the upper and also of the lower eccentric-rod. So long as the rocker-pin is above the centre of the link, the motion of the valve will partake most of that of the upper or forward rod, and the engine will then run forward, but when the rocker-pin is below the centre of the link, its motion will be influenced more by the back eccentric-rod, and the engine will then run backward.

The motion of the link, which is somewhat complex and difficult to understand clearly, will perhaps be understood better if we represent it in a number of successive positions of the whole stroke of the piston, as was done to show the motion of the eccentric in figs. 11 to 24. We will therefore suppose that the link is in what is called full gear forward, as shown in figs. 103 and 104. In fig. 108 the link is in the position it would occupy at the beginning of the stroke of the piston; in fig. 109 it is in that which it will be in when the piston has moved four inches; in fig. 110, when it has moved eight inches; in fig. 111, twelve; and in figs. 112, 113 and 114, sixteen, twenty and twenty-four inches. Figs. 114 to 119 represent the successive positions of the link during the return stroke. In order to show the different positions of the link we have represented on a larger scale, in fig. 120, the successive positions of the centre line of the link, which will indicate the motion imparted by it to the rocker. In order to designate each of these positions, the centre lines in fig. 120 are numbered + and -0, 4, 8, etc., etc., to correspond with similar numbers in figs. 108 to 119.

Fig. 108.

Fig. 109.

Fig. 110.

Fig. 111.

Fig. 112.

Fig. 113.

Fig. 119.

Fig. 118.

Fig. 117.

Fig. 116.

Fig. 115.

Fig. 114.

Scale ³⁄₈ in. = 1 foot.

Fig. 120.

Scale ³⁄₁₆ in. = 1 inch.

Fig. 121.

Scale ³⁄₁₆ in. = 1 inch.

Fig. 122.

Scale ³⁄₁₆ in. = 1 inch.

Thus the line -0 -0 represents the position of the centre of the link which it occupies at the beginning of the stroke as shown in fig. 108. The line -4 -4, that represented by fig. 109, when the piston has moved 4 in. The lines -8 -8, -12 -12, -16 -16, -20 -20, 24 24, +4 +4, etc., the successive positions of the centre of the link represented in figs. 108 to 119. The dotted lines h a and h b represent the two extreme positions into which the rocker-arm would be moved by the action of the link. It will be seen that when the link is in the position shown, it imparts the full stroke of the eccentrics to the rocker-pin and consequently to the valve. We will now suppose that the link is raised up as shown in fig. 106, so that the position of the rocker-pin is just half-way between the end of the eccentric-rod and the centre of the link. This position is called half-gear. In fig. 121 the different positions of the centre line of the link and of the rocker have been laid out for half-gear in the same way as was done for full-gear before. From this it will be seen that the travel, a b, imparted to the rocker-pin and valve by the link when it is in the position shown, instead of being 5 in. is only 3¹⁄₂ in. In fig. 107 the link is raised up, so that the rocker-pin is in the centre of it or midway between the eccentrics. This position is called mid-gear. The successive positions of the centre line of the link in this position have been laid down in fig. 122 in the same way as was done for full and half-gear. The movement of the rocker, it will be seen, is, for mid-gear, only 2¹⁄₂ in. These diagrams show that when the rocker-pin is opposite the eccentric-rod, the valve receives the full throw of the eccentric, and that the motion imparted by the eccentric diminishes as the rocker-pin approaches the centre of the link, so that, with eccentrics having 5 in. throw and a valve with ⁷⁄₈ lap and ¹⁄₈ in. lead, we can increase or diminish the travel of the valve from 2¹⁄₂ to 5 in. by simply raising or lowering the link, which is done by the reverse-lever.

Question 189. What is the effect of this variation of travel on the working of the valve and the admission and release of steam to and from the cylinder?

Answer. It is almost precisely the same as that which is effected by increasing or diminishing the throw of the eccentric, which was explained in the answer to Question 52. In order to show this effect more clearly, we have represented by motion-curves,[51] fig. 123, the movement imparted to the valve by the link when it is in full, half and mid-gear, as illustrated in the preceding figures. The curve for full-gear is engraved in full heavy lines; that for half-gear in lighter lines, and for mid-gear in dotted lines. From these curves it will be seen that when the valve is worked in full-gear the steam-port is opened wide at 2 in. of the stroke and steam cut off at 21 inches. When the valve is worked in half-gear the port is not at any time opened wide and steam is then cut off at 17¹⁄₂ in. of the stroke, and when worked in mid-gear the greatest opening of the steam-port is no greater than the lead and the cut-off occurs at 4 inches of the stroke.

It is of course possible to work the link in any intermediate position between those which we have represented. Usually the reverse-lever is arranged so that the steam will be cut off at 6, 8, 10, 12, 15, 18, and 20 inches of the stroke.

Question 190. What is the greatest and the least admission of steam possible with the ordinary link motion?

Answer. With 24 in. stroke of piston and 5 in. travel and ⁷⁄₈ in. lap, steam can be admitted as shown by the motion-curves during 21 in. or 87¹⁄₂ per cent. of the stroke, and can be cut off at about 4 in. or 16²⁄₃ per cent. It will be seen, however, that in mid-gear the motion-curve becomes a straight line, and that the pre-admission of steam, that is the admission of steam before the piston reaches the end of the stroke, is equal to that admitted after, so that it is impossible to work the locomotive with the link in that position. Practically it is found that no useful work can be done with a link if the steam is cut off at less than six inches, or one-fourth of the stroke. Even then the opening of the steam-ports is so small that the steam which enters the cylinders is very much wire-drawn.

Question 191. How are the curves drawn which represent the motion of the valve?

Answer. These motion-curves as produced by the link-motion are very difficult to draw, as the motion of the link is extremely complicated. It is doubtful, therefore, whether those who have no knowledge of mechanical drawing will be able to understand the following description of the method of doing it, which we will try to make as clear as possible.

Fig. 123.

Scale ³⁄₁₆ in. = 1 inch.

Fig. 124.

Scale ³⁄₄ in. = 1 foot.

In the first place, the centre S, fig. 124, of the axle, A, of the rocker, and B of the lifting-shaft, must be laid down in their proper positions. If, now, the valve has ⁷⁄₈ in. lap and ¹⁄₈ lead, the lower rocker-pin must be one inch ahead of its middle position when the piston is at the front end of the cylinder, and at the beginning of the backward stroke. We will, therefore, mark the centre, a, of the rocker-pin in this position. If from the centre of the axle a circle, c d e, be drawn whose diameter is equal to the throw of the eccentrics, this circle will represent the path in which the centres of the eccentrics will revolve. If, now, the distance from the centre of the axle to the centre of the lower rocker-pin, a, when the latter is in its middle position, be taken for a radius,[52] and from the position of the rocker-pin at the beginning of the stroke as a centre, the circle representing the path of the eccentrics be intersected at two points, c and d, the points of intersection will represent the positions of the centres of the forward and backward eccentrics. Having determined these positions, draw arcs of circles, f and g, from these centres with a radius equal to the distance from the centres of the eccentrics to the centres of the pins which connect the rods to the link. It is evident that at the beginning of the stroke the centres of the pins in the link must each be in one of these arcs. But the link is suspended by the hanger, i h, which oscillates from the end, i, of the lifting-arm, which for any one point of cut-off is stationary; and therefore the point of suspension of the link must always be in the arc, j k, described from the centre of the pin, i, in the lifting-arm, with a radius equal to the length of the hanger. There are, therefore, three points in the link, each of which must be in one of the arcs which have been drawn, and which will determine the position of the link. This can be done easiest by drawing the link, L, fig. 125, on a stiff piece of paper, m n, and cutting off the back, p q, of it through the centres of the pins, s and t, and also cutting out a triangular piece, u, the apex of which will correspond with the centre of the point of suspension, o. By placing this piece of paper on the drawing it can be moved, so that the three centre points, s, t and o, will respectively conform with the arcs, f g and j k, fig. 124. In this position the piece of paper will then be in the position of the link for the point of the stroke represented. By marking the centres of the link-pins on the arcs f and g, and from them as centres, with the length of the rods used to draw the arcs, two other arcs, v, w, be drawn intersecting each other, the point d, where they intersect will be the centre from which the centre line of the link can be drawn with a radius, l d, equal to the distance from the centre of the eccentrics to the centre of the link. This will give the first position -o -o, of the centre line of the link. As the rocker-pin must always be in the centre of the link, it is obvious that the point at which the centre-line of the link intersects the arc in which the rocker-pin oscillates must be the position of the centre of the rocker-pin. With this determined the position of the valve can easily be located.

Fig. 125.

Scale, ³⁄₄ in. = 1 foot.

In order to represent the link at any other point of the stroke, say after the piston has moved four inches, the position of the crank must first be laid down. To do this, allowance must be made for the irregularity due to the angularity of the connecting-rod, which was explained in answer to Question 54. From the centre of the axle, a circle, C D, whose diameter is equal to the stroke of the piston, is first drawn, which will represent the path of the crank-pin. A horizontal centre line, E F, should also be drawn through the centre of the axle and the centre of the cylinder. The intersection o of this line with the path of the crank-pin will be the position of the latter at the beginning of the stroke. If from this point a distance, o o, be laid off on the centre line equal to the length of the connecting-rod,[53] it will give the position of the wrist-pin at the beginning of the stroke, so that from this its successive positions for each inch of the stroke can be laid off. From its position after the piston has made say four inches of the stroke as a centre, and the length of the connecting-rod as a radius, if the path of the crank-pin be intersected at -4, the point of intersection will represent the position of the crank-pin at four inches of the stroke. The distance from o to -4 is equal to 44 degrees of the whole circle. The eccentrics, being attached to the axle, of course move the same number of degrees that the crank does, and therefore, in order to determine their position when the crank has moved any distance, it is only necessary to move them as many degrees as the crank has. This can be done very easily by extending the radii of the eccentrics, when they are in the first position, until they intersect the path of the crank-pin at c′ and d′. By stepping off from the latter points of intersection a distance c′ c‴ and d′ d‴, equal to o -4, which the crank has moved, and then drawing other radii from the two points c‴, d‴, their intersection, c″ d″, with the path of the eccentrics will represent the position of the centres of the eccentrics when the crank is at -4. Having determined the position of the eccentrics, the link can be laid down as before, that is, from c″ and d″ as centres and with the length of the eccentric-rods as a radius arcs, f″ and g″, are drawn. Then with the paper template the positions of the centres of the link-pins in these arcs are determined and marked, and from them with the length of the eccentric rods as a radius, two intersecting arcs, v″, w″, are drawn, whose intersection gives the centre of the link from which its centre line, -4 -4, is drawn. This will give the position of the rocker-pin for another point of the stroke. In a similar manner its position can be determined for any number of points of the stroke, from which the position of the valve can easily be determined and laid down on the diagram for the motion-curve as was described in the answer to Question 44. Of course the valve will be moved from its middle position the same distance that the rocker-pin is,[54] only in an opposite direction. In order to lay down the position of the valve on the diagram for motion-curves, it is, therefore, only necessary to draw it in the same relative position as that of the rocker-pin which is given by the point of intersection of the center line of the link with the path in which the rocker-pin oscillates. To construct the motion-curves it is necessary to determine the positions of the valve for different points of the stroke and mark them on the horizontal lines which represent the respective positions of the piston. Curves are then drawn through these points, either by hand or by constructing templates. The more points there are determined, the more accurate will be the curves. It is, therefore, best to lay down the position of the valve for each inch of the stroke of the piston. They should also be drawn full size, which of course was impossible for the illustrations which are given herewith.

Question 192. Is there any other method of drawing these motion curves?

Answer. Yes: models which show the working of the valve-gear have been constructed with a pencil, to which the reciprocating motion of the valve is imparted, and which traces a curve on a surface having the same motion as the piston. This method has been employed by the writer in an instrument which he has applied to the locomotive itself. The principle upon which it works will be understood by supposing that the steam and exhaust-ports as represented in the diagram for motion-curves, fig. 123, be drawn on a board, A B C D, fig. 126, but instead of standing vertical, as in fig. 123, they are represented in a horizontal position, and the board on which they are drawn is fastened to the cross-head, L, so that the former will move backward and forward simultaneously with the latter and the piston. A small shaft, F, is attached to suitable supports, j, which are fastened to the guides. This shaft has two arms, G and E, one vertical and the other horizontal and of the same length. The upper end of the vertical one, G, is then attached to the valve-stem or rocker-arm by a short connecting-rod, H, or other suitable means, so that the movement of the valve-stem will be imparted to the arm and shaft. Of course the end of the horizontal shaft then has exactly the same motion vertically that the valve-stem and valve have horizontally, with the very trifling inaccuracy due to the fact that the movement of the one is in a straight line, whereas the other is in the arc of a circle.

Now if a pencil, P, is attached to the end of the horizontal arm, E, and is set so that its point indicates the exact position of the steam edge, h, of the valve, as shown in fig. 123, it is obvious that when the piston and board have moved four inches, the pencil will have moved downward and have drawn the portion of the motion-curve from h to i; and when the piston has moved eight inches the curve will be drawn to j, and at 12, 16, 20 and 24 inches of the stroke the curve will be drawn to k, l, m and n. During the return stroke a corresponding curve, n o h, will, of course, be drawn. With such an instrument curves can be drawn for any position of the link, and they will show the exact movement of the valve during the whole stroke, and will indicate all the defects resulting from bad proportions or construction, lost motion in the parts, or other causes of error or irregularity.

Fig. 126.—Scale ³⁄₄ in. = 1 foot.

In using this instrument, however, it is impracticable to attach a board to the inside of the cross-head, and it must therefore be fastened to the outside. The horizontal arm E should be made of thin steel, so as to form a spring. The end has a small boss[55] with a hole in it ³⁄₁₆ of an inch in diameter. This hole has a screw thread cut in it, into which an ordinary hard drawing pencil is screwed. The spring is so arranged that the pencil will not be in contact with the board unless it be pressed against it. The locomotive is then placed on a smooth piece of track with steam on and run very slowly, so that a person walking alongside can press the pencil against the surface of the board, which should be covered with drawing paper. By watching the cross-head when it reaches the end of the stroke, the pencil can then be pressed against the paper and kept in contact through the whole stroke and instantly released when the motion-curve is completed. The link can then be placed in another position, and thus any number of curves can be drawn, which will furnish the most accurate means of analyzing the motion of the valve.

In practice it is best not to draw the lines which represent the edges of the ports, until after the curves are drawn and the paper removed from the board. A centre line must, however, be drawn on the engine from which to lay off the ports. This can be done by placing the valve in its middle position, and then fastening the shaft F in that position with a nut which should be provided for that purpose on the end of the shaft. After it is fastened in this position, detach the connecting-rod H, and with one stroke of the piston a centre line can be drawn with the pencil P. From this centre line the edges of the ports can easily be laid off and drawn on the paper after it is taken off the engine.

Question 193. Can the position of each edge of the valve, with any given amount of travel, be shown in its relation to the ports by one motion-curve, or is it necessary to draw such curves for each edge of the valve, as shown in fig. 28?

Answer. One motion-curve is sufficient to represent the position of any part of the valve during the entire stroke. This will be apparent if it is remembered that each motion-curve is exactly like the others, as shown in fig. 28, the only difference being that the ports occupy different positions in relation to the curves. It is, therefore, only necessary to draw lines to represent the relative positions of the ports to the other curves to show the entire motion of the valve by one curve. To illustrate this it will be assumed that a motion-curve, h i j k l m n o p, and a centre line, a b, fig. 127, have been drawn with the instrument described in the answer to the previous question. The centre line a b, which will be equal in length to the stroke of the piston, should then be divided into inches, and lines ff, 23 1, 22 2, etc., should be drawn through the points of division and at right angles to a b. If, now, we want to show the movement of the front steam edge of the valve in relation to the corresponding steam port, a line, t, should be drawn perpendicular to f f, to represent that edge of the valve at the beginning of the stroke. As it is impossible to determine accurately the position of this steam edge at the beginning of the stroke from the motion-curve, which is then tangent[56] to the line f f, we must lay it off from the centre line, a b. This can readily be done if we remember that if a valve has ⁷⁄₈ in. lap when it is in the middle position, as shown in fig. 27, and ¹⁄₁₆ in. lead at the beginning of the stroke, it must have moved ¹⁵⁄₁₆ in. from the middle position at the beginning of the stroke as shown in fig. 28. The line t must therefore be drawn ¹⁵⁄₁₆ in. from a b to represent its proper position in relation to the motion-curve, and as it has ¹⁄₁₆ in. lead, the steam edge, h h′, of the steam-port must be drawn at that distance from t. Another line, m m′, can then be drawn to represent the width of the front steam-port, c c′. From these lines the movement of the valve in relation to the front port, c c′, and the admission of steam are shown as clearly as in fig. 28.

If now we want to represent the motion and relative position of the back steam edge of the valve in relation to its port, it is only necessary to assume that the line t represents that edge, and that the curve h i j k l m n o h represents its motion, and to draw the back steam-port in its proper relation to it. When the valve is in its middle position, as shown in fig. 27, the outside edge of the port h is ⁷⁄₈ in., or a distance equal to the lap, from the steam edge q of the valve. As the center line a b, fig. 127, represents the middle position of the edge of the valve, it is only necessary to draw a line, n n′, ⁷⁄₈ in., or the same distance from the centre line a b that the outer edge of the port d is from q in fig. 28, to represent this edge of the port in fig. 127, and another b b′, at a distance from the former equal to the width of the port, to represent its inner edge. A line, q, below the line 0 24, will represent the edge of the valve at the beginning of the forward stroke. The curves in relation to the port d will then show the motion of the valve in relation to this port, in the same way that the dotted curve d f does in fig. 28.

Fig. 127.

Scale ³⁄₁₆ inch = 1 inch.

If it is desired to represent the motion of the exhaust edge h′, fig. 28, of the valve, it is only necessary to imagine that the line t, fig. 127, represents that edge, and then draw in the port d in the same relation to it that it bears to the edge h′, in fig. 28. This has been done in dotted lines, c c′ and e e′, in fig. 127.

If the reader will cut a paper section of a valve like that shown in fig. 27 and place the different edges, h, i, h′ and q, so that they will successively correspond with the line t in fig. 127, the diagram will perhaps be more clear. If, for example, the paper section be placed to the right of the line t, so that the edge h will correspond with t, then it will be seen that the port c occupies the same relation to it that it does in fig. 28. If the valve be placed to the left, so that the edge q corresponds with t, then the port d will be in the same relation to it that it has in fig. 28. If the edges i and h′ be made to correspond with t, then the ports drawn in dotted lines in fig. 127 will represent the ports c and d in fig. 28.

The position of the ports in relation to the centre line of the motion-curve can be determined, if it is kept in mind that the centre line a b, fig. 127, represents the position of the different edges of the valve when the latter is in the middle of the valve-face as shown in fig. 27, and that the ports must be on the same side, and the same distance from the centre line that they are from the edge of the valve whose motion is represented. Thus if the movement of the steam edge h in relation to its port, c, was represented, the edge of the latter must be drawn on the motion diagram the same distance from the centre line that it is from h when the valve is in its middle position as shown in fig. 27. This distance is of course just equal to the lap of the valve. If the motion of the exhaust edge h′ was represented in relation to the steam port d, then the inside edge of the latter would be drawn the same distance from the centre line a b in the diagram that the inner edge of the port is from the edge h′ of the valve, which is equal to the inside lap. The exhaust port could also be drawn in the same way, but it would be liable to confuse a diagram made to so small a scale as that which has been employed for the accompanying illustrations, and it has therefore been omitted. Diagrams of this kind which are made full size will, of course, show the movement of the valve more distinctly than is possible in the space occupied by the illustrations herewith. When they are made of full size, the lines indicating the ports should be drawn of different colors, so as to distinguish them from each other easily. Such diagrams will show the position of the valve in relation to the ports, and indicate the distribution of the steam during the whole stroke. It is only necessary to refer the curve to the proper line to determine the position of the valve in relation to either of the ports for either the admission or release of the steam. If, for example, we want to observe how the admission of steam is governed by the valve, by referring to fig. 127 we see that at the beginning of the backward stroke the valve has ¹⁄₁₆ inch lead; that at 1³⁄₄ inches of the stroke the port c is wide open, as shown by the intersection of the motion-curve with the line m m′; that the valve has received its maximum backward travel at 9 inches of the stroke, and begins to close the port at 15¹⁄₂ inches, and completely closes it at 21 inches of the stroke. By referring the motion-curve to the lines n n′ and b b′, we see that the valve as shown by the line q at n′ again has ¹⁄₁₆ inch lead at the beginning of the forward stroke; that the steam port is wide open at 1³⁄₈ inches of the stroke; begins to close at 16¹⁄₄ inches, and is completely closed at 21 inches. By referring the curve to the lines e e′ and c c′ we see that the front port begins to open to the exhaust before the piston has completed its forward stroke and when it has nearly an inch to move, that it is wide open almost immediately after the piston begins its stroke, does not begin to close until the piston has moved 19¹⁄₂ inches of its stroke, and is completely closed at 23 inches of the stroke. By referring the curve to the lines d d′ and g g′, almost the same phenomena will be observed for the forward stroke. In fact from such a diagram the whole motion of the valve can be studied and analyzed with the greatest accuracy; and, as has already been shown, the motion imparted to a slide valve by a link is of so complicated a nature that it is almost or quite impossible to observe its exact nature without such diagrams.

Question 194. Can a motion diagram be constructed to represent the motion of the valve with different amounts of travel?

Answer. Yes; it is only necessary to construct motion-curves for the same diagram for each distance traveled, and they will show the movement of the valve for the given amount of travel represented by the curves. This has been done in fig. 128, which is a reduced copy of a series of motion-curves taken from a locomotive. From this diagram the movement of a slide-valve worked by the link-motion can be seen from the highest to the lowest practicable point of cut-off. For convenience of reference the curves have been numbered.

Fig. 128.

Scale ³⁄₁₆ inch = 1 inch.

The smallest travel of the valve represented by curve No. 1 is a little less than 2¹⁄₂ in., and the ports are then opened only about ⁵⁄₁₆ in., and the steam is cut off at 8 in. on the backward and 6³⁄₄ in. on the forward stroke. The exhaust is opened or the steam is released during the backward stroke at 17 in., and during the forward stroke at 16⁵⁄₈. When the valve works with its greatest travel, as represented by curve 8, it travels 5 in., and opens the steam port wide at 3 in. of the backward stroke and 2¹⁄₄ in. of the forward stroke. The steam is cut off at 20³⁄₄ and 20¹⁄₂ in., and its release takes place at 23¹⁄₈ in. of each stroke. The following table gives the greatest width of opening, the point of cut-off, the point of release, and the lead for each motion-curve on the diagram. This table has been made up from the motion-curves drawn with the instrument described in answer to Question 191, on a locomotive which had been running about eighteen months and whose valve-gear consequently was considerably worn, as s indicated by the flatness of the motion-curves on each side at the point when the motion of the valve was reversed. This flatness was caused by the lost motion in the valve-gear, the pencil remaining for a time stationary when the motion was reversed and while the parts were moving from their bearings on one side to those on the other. The curves and the table therefore show the operation not of a theoretically perfect valve-gear, but are examples of actual practice, with such imperfections as are incidental to ordinary locomotives. It will be seen that the instrument shows not only what the valve-gear should, but what it actually does do, and delineates all its imperfections.

Question 195. What are the chief dimensions of the valve-gear represented in fig. 128?

Answer. The throw of eccentrics was 5 in., the steam-ports were 1¹⁄₄ in., and the exhaust-port 2³⁄₄ in. wide, the valve had ⁷⁄₈ in. outside and ¹⁄₁₆ inside lap and ¹⁄₁₆ in. lead at full stroke.

Question 196. What relation is there between the distance which the ports are opened by the valve, and its travel when worked by a link?

Answer. As explained in the answer to Question 52, the width which the steam-ports are opened by the valve for the admission of steam diminishes with the travel of the valve. This is shown very clearly by the motion-curves, and also in the above table, from both of which it will be seen that when the valve travels only 2¹⁄₂ in. the steam-ports are opened only ¹¹⁄₃₂ in. for the back stroke and ⁵⁄₁₆ for the front. With 2⁵⁄₈ travel the opening is ⁷⁄₁₆ and ¹³⁄₃₂ in. With 4 in. travel the port is opened 1¹⁄₈ and 1³⁄₃₂ in. and with 4¹⁄₄ in. travel they would be opened wide. With 4¹⁄₂ and 5 in. travel, as will be seen from the motion diagram, the ports are not only opened wide, but the valve throws “over” them, or travels beyond their inner edges.

Question 197. How is the point of cut-off affected by the link?

Answer. Changing the travel of a valve with a link has a very similar effect to that produced by eccentrics of different throw—that is, the period of admission is increased with the throw of the eccentric and that for expansion lessened. This is shown clearly in both the motion diagram and the table. With the first curve and a travel of 2¹⁄₂ in. the steam is cut off at 8 in. for the backward stroke and 6³⁄₄ in. for the front, and with 5 in. travel steam is admitted during 20⁵⁄₈ in. of the backward and 20¹⁄₂ in. of the forward stroke.

Question 198. How is the point of release or exhaust of the steam affected by the link?

Answer. As the travel increases, it is delayed until later in the stroke. Thus, with 2¹⁄₂ in. travel the steam is exhausted or released from the cylinder during the backward stroke when the piston has moved 17 in., and on the return stroke at 16⁵⁄₈ in., whereas, with 5 in. travel of the valve, the release is delayed until 23¹⁄₈ in. of the stroke. An examination of the diagram and table will show very clearly the relation of the point of release to the travel.

Question 199. How is the lead affected by the ordinary link motion?

Answer. It is increased as the travel is diminished, as is shown in the table, and also by the inclination of the curves at the top and bottom of the diagram.

Fig. 129.

Fig. 130.

Fig. 131.
Scale ³⁄₄ in. = 1 foot.

Question 200. What is the cause of this change of the amount of lead?

Answer. This can be best explained by reference to fig. 129, which represents a link with very short eccentric rods. If now the centre from which the link was drawn was in the centre of the axle S, and the eccentric straps embraced the axle instead of the eccentrics, their ends c and d would each describe the same arc, a b, parallel with the centre line, x y, of the link, and the latter could then obviously be raised and lowered without moving the rocker-pin at all. But the eccentric straps being attached to the eccentrics, as shown by the dotted lines, when the rods are raised or lowered they describe arcs, c e and g h, from the centres s and t of the eccentrics, and not from the centre of the axle. When the link is raised then, the end of the upper rod obviously moves in the arc c e, and the top of the link is moved from the axle, as shown in fig. 130, a distance equal to the interval between the arc, a b, drawn from the centre of the axle, and e f, which the rod describes from the centre of its eccentric. When the link is lowered from back to mid-gear, a similar action takes place, as the end, d, fig. 130, of the lower rod describes an arc, f g, so that the whole link is thrown from the axle a distance equal to the space between the arcs described from the centre of the axle and the centres of the eccentrics. When the position of the eccentrics is reversed, as shown in fig. 131, the link is moved towards the axle, thus causing an increase of lead on the opposite side of the valve. We have employed for our illustrations very short eccentric rods, in order to make this action apparent by exaggerating it. It is obvious from the engravings that the difference in the lead is increased as the eccentric rods are shortened, and also as the distance between the points of connection of the rods with the link is increased. It will also be plain that increasing the throw of the eccentrics, that is, increasing the distance of the centres s, s, of the eccentrics from the centre S of the axle will also increase the variation in the lead in full and mid-gear.

Question 201. What is meant by the distribution of steam in the cylinder?

Answer. It means the admission and exhaust of steam to and from the cylinder in relation to the stroke of the piston or the revolution of the crank.

Question 202. What are the principal periods or elements of the distribution of steam by the slide-valve and link motion?

Answer. They are:

1. The pre-admission or lead, that is, the admission of steam into the cylinders in front of the piston before it has completed its stroke.

2. The admission of steam after the piston has commenced its stroke.

3. The expansion of steam in the cylinder.

4. The pre-release, or exhaust of steam before the piston has completed its stroke.

5. The release, or exhaust during the return stroke of the piston.

6. The compression of steam, or closing the exhaust before the piston has completed its return stroke.

Question 203. What is meant by the clearance of the piston?

Answer. It is the space between the piston and the cylinder-head when the former is at the end of the stroke. If the piston touched the cylinder-head at the end of each stroke, it would cause a concussion or “thump” which would injure these parts. Owing to the impossibility of constructing machinery with absolute accuracy, it is therefore necessary to leave a space, usually from ¹⁄₂ to ¹⁄₄ in. wide, between the piston and the cylinder-heads, so as to be certain that they will not strike each other should there be any slight inaccuracies in the length of the piston-rods, connecting-rods, frames or other parts.

Question 204. Why is it desirable to open the steam-port and admit steam at the end of the cylinder towards which the piston is moving BEFORE the latter has completed its stroke?

Answer. Because it is essential, in order to insure a good action of the steam, that the maximum cylinder pressure should be attained at the very commencement of the stroke. If the steam-port was not opened until after the piston had commenced its stroke, some appreciable time would be consumed in filling the clearance space and the steam-way with steam.[57] It is also found, especially if an engine is working at a high speed, that a slide-valve worked by the ordinary link-motion will not open the steam-port rapidly enough to enable steam of the maximum boiler pressure to fill the space after the receding piston, unless the valve begins to open the port before the piston reaches the end of its stroke.

Another advantage resulting from the pre-admission of steam consists in the smooth working of the engine at high speeds, a circumstance which reduces greatly the wear and tear of the working gear. As the piston approaches the end of its stroke, the pre-admitted steam forms a kind of elastic cushion, which is well calculated to absorb the momentum of the reciprocating parts at that instant. The pressure due to the momentum of these parts will, of course, depend upon their weight and the speed of working, increasing directly as the square of the speed. It follows from this that the lead should increase with the speed, and that it should be greatest at high speeds. As has been shown before, this condition is fully accomplished by the ordinary shifting-link motion.

Question 205. Upon what does the admission of steam into the cylinder depend?

Answer. It depends in the first place upon the opening of the throttle-valve, and the size of the pipes and passages through which it is conveyed from the boiler to the cylinder. In the second place, it depends upon the time and amount of opening of the steam-port by the valve.

Question 206. What should be the pressure of the steam in the cylinder during admission?

Answer. In order that the steam may be used to most advantage, it should be admitted and maintained in the cylinder at full boiler pressure during the whole period of admission. If the opening of either the throttle-valve or the steam-ports is not sufficient to allow the steam to flow into the cylinder at full boiler pressure, the steam is said to be wire-drawn, and much of the advantage of using it expansively as has already been explained in answer to Question 59, is then lost.

Question 207. Why is it difficult to admit and maintain steam at the full boiler pressure in the cylinder during admission?

Answer. Because it is necessary to reduce the travel of the slide-valve in order to cut off the steam “short,” or soon after the beginning of the stroke of the piston. When the travel is reduced, the valve opens the port only a small distance, so that the area of the opening is not then sufficient to allow the steam to flow into the cylinder with sufficient rapidity to fill it at full boiler pressure, especially if the engine is working at a high speed. Thus, by referring to the table given on page 216 and to the motion curves in fig. 128, it will be seen that when the steam is cut off at from ¹⁄₄ to ¹⁄₂ stroke, the port is opened for the admission of steam only from ¹⁄₄ to ¹⁄₂ inch wide. From the curves it will also be seen that the valve then acquires its maximum travel and the steam-port its greatest width of opening very soon after the piston begins its stroke; after which the port is gradually closed, so that before the steam is entirely cut off the opening is so much reduced in area that the steam cannot flow through it rapidly enough to maintain the steam at full boiler pressure in the cylinder when the engine is working at high speeds.

Question 208. What means are used to overcome this difficulty and thus admit steam at full boiler pressure when the valve is cutting off short?

Answer. In the first place, the steam-ports are made from ten to twelve times as long as they are wide, so that a narrow opening will have a comparatively large area. In the second place, by giving the valve lead, not only are the clearance space and the steam-way filled with steam when the piston begins its stroke, but the port is then open a distance equal to the lead. With the ordinary link motion, as has already been shown, this lead increases as the travel and period of admission diminish, so that the smaller the total distance that the port is opened, the greater is its opening at the beginning of the stroke. As the steam is usually cut off short when locomotives run at high speeds, it will be seen that the increased lead which is imparted to the valve by the shifting link is an advantage rather than a disadvantage. But while it is often possible in this way to secure a pressure of steam in the cylinder at the beginning of the stroke equal or nearly so to that in the boiler, yet it is almost impossible to maintain this pressure during the whole period of admission, when the steam is cut off short and the engine working at a high speed. To obviate this evil what is called the Allen valve was designed, which is represented in fig. 132. This valve has a channel or supplementary port, a a, which passes over the exhaust cavity, and has two openings, b, b′, in the valve-face. When the valve begins to admit or “take” steam at c, as shown in fig. 133, it will be seen that it also uncovers the opening b′ at e and admits steam at b′, which passes through the channel b′ a a b and enters the steam-port c at b, and in this way there is a double opening for the admission of steam. The opening b of the supplementary port is closed as the valve advances, but when this takes place the steam-port is uncovered far enough to admit all the steam that is required. This form of valve is very efficient when the travel and point of cut-off are very short. It then gives just twice as much opening as the ordinary valve for the admission of steam. This improved valve has been much used in Europe; but, although it is an American invention, has not received the attention in this country which its merit deserves.

Fig. 132.

Fig. 133.
Scale ³⁄₁₆ in. = 1 inch.

Question 209. What is meant by the pre-release of steam?

Answer. It is the release of the steam before the piston has completed its stroke. If it is confined until the piston has reached the end of the cylinder, there will not be time nor will it be possible, with a slide valve and link-motion, to secure a sufficiently large opening of the port to permit the steam to escape from the cylinder before the piston begins its return stroke. If there were no pre-release, there would therefore be more or less back pressure on the piston.

Question 210. Upon what does the amount of pre-release depend?

Answer. First, as has already been explained in answer to Question 51, on the amount of inside lap; and second, on the outside lap of the valve and lead of the eccentrics; and third, on the travel of the valve. The less the inside lap, the greater the outside lap and consequent lead of the eccentrics; and the shorter the travel of the valve, the earlier will be the release. The proper amount of this pre-release depends upon the velocity of the piston and the quantity of steam to be discharged or the degree of expansion. From the motion-curves in fig. 128 it will be seen that it is a marked feature of the shifting-link motion that the pre-release occurs earlier in the stroke as the link approaches mid-gear, or as the travel of the valve diminishes. As the link is usually worked near that position when the engine is run at a high speed, it will be seen that in this respect again the link-motion is well adapted for working the slide-valves of locomotives.

Question 211. What governs the period of release?

Answer. The release like pre-release is dependent upon the amount of inside lap, the outside lap and consequent lead of the eccentrics, and the travel of the valve.

The addition of inside lap has the effect of closing the port earlier than it would be closed without, and thus shortening the period of release and also of reducing the area of the opening of the port. This will be apparent by referring to fig. 128, in which the valve had ¹⁄₁₆ in. lead. The dotted lines which represent the edges of the ports in relation to the exhaust edges of the valve are therefore drawn ¹⁄₁₆ in. from the centre line a b. If, however, there had been no inside lap, then the edges of the ports would have conformed to the line a b. It will be observed that the first curve crosses the dotted line g g′ at 15¹⁄₂ in. of the forward stroke, which is the point at which the port is closed to the exhaust, or where the period of release ends and compression begins. If there had been no lap and the line g g′ had therefore occupied the same position as a b, then the motion-curve would not have crossed it until the piston had reached 16 in. of its stroke, thus showing that the period of release had been lengthened and compression delayed. As the width of the opening of the port is represented by the distance of the motion-curve from the right hand side of the line g g′, which represents the edge of the port, it is obvious that if there had been no lap, so that the position of the line representing the edge of the port had occupied the position of a b, then the space between it and the motion-curve would have been greater, thus showing that the port would have been opened wider if there had been no inside lap. The width of the opening of the port to the exhaust is in fact always diminished by an amount equal to the inside lap.

With the same travel, increase of outside lap and lead shortens the period of release, but has no effect on the width of the opening of the port to the exhaust.

Increase of travel, with the same outside lap, lengthens the period of release and also increases the width of the opening of the port to the exhaust.

Question 212. What governs the period of compression?

Answer. As compression begins when release ends, or when the port is closed to the exhaust, it is controlled by exactly the same causes, and as the two events occur simultaneously, of course whatever shortens the period of release lengthens that of compression.

Question 213. What effect do the clearance spaces and steam-ways have upon the compression of the confined steam?

Answer. By referring to the motion-curves in fig. 128, it will be seen that the steam-port is closed by the exhaust edge of the valve, or compression begins some time before the piston reaches the end of the stroke. The result is that the remaining portion of the cylinder, through which the piston must move after the port is closed to the exhaust, is filled with steam of atmospheric pressure, or possibly a little above that pressure. As this is confined in the cylinder, it is compressed by the advance of the piston. If there was no room between it and the cylinder at the end of the stroke, then either the cylinder would be burst or the valve would lift so as to allow the compressed steam to flow back into the steam-chest. The clearance and the steam-passages, however, afford considerable room, into which the confined steam can be compressed without danger of bursting the cylinder, or of raising the slide-valve when there is steam in the steam-chest. As the clearance spaces and steam-ways must be filled with high-pressure steam at the beginning of each stroke, it must be obtained either by taking a supply of “live”[58] steam from the boiler, or by compressing into the clearance spaces the low-pressure steam that still remained in the cylinder when the port was closed to the exhaust. By the latter process, a certain quantity of steam is saved at the expense of increased back pressure. It should be borne in mind also that the total heat of the compressed steam increases with its pressure, and as this latter approaches that in the boiler, the temperature of the former must have been raised from that due to about atmospheric pressure to nearer the temperature of that in the boiler. These changes of temperature which the steam undergoes will affect the surface of the metal with which the steam is in contact during the period of compression; it follows from this, that the ends of the cylinder principally comprising the clearance spaces must acquire a higher temperature than those parts where expansion only takes place. This is an important consideration, since the fresh steam from the boiler comes first in contact with these spaces, and by touching surfaces which have thus previously been heated, as it were, by the high temperature of the compressed steam, less heat will be abstracted from the fresh steam, and therefore a less amount of water will be deposited in the cylinder.[59]

It will thus be seen that the effect of compression is to fill the clearance spaces and steam-ways with compressed steam before pre-admission begins. As already stated, this is done at the expense of back pressure in the cylinder. It must be remembered that all the energy, excepting that part which is wasted by loss of heat, friction, etc., which is consumed in compressing the confined steam, is again given out to the piston by expansion. The confined steam also acts as an elastic cushion to receive the piston, just as the steam which is admitted before the end of the stroke would if there were no compression. Compression, therefore, has the effect of saving the quantity of live steam which it would otherwise be necessary to admit before the end of the stroke to fill the clearance spaces and steam-ways and also to “cushion” the piston. As already stated, the momentum of the piston and other parts depends upon their weight and the speed at which they are working, increasing directly as the square of the speed, from which it follows that the compression should increase rapidly with the speed, and should be the greatest at high speeds. As the ports are prematurely closed to the exhaust with the shifting-link motion, and as the lead increases rapidly as the link approaches mid-gear, and the amount of compression is at the same time correspondingly augmented, it will be seen that the shifting-link motion fulfills these conditions very perfectly.

The pressure to which the confined steam will rise depends of course upon the amount of the period of compression, and also on the size of the clearance spaces. As it is possible to have such an amount of compression that it will exceed the boiler pressure, and thus raise the valve from its seat and be forced back into the steam-chest, some care must be exercised to proportion the one to the other, so that the degree of the confined steam may not be excessive.

Question 214. How can the effect of the distribution of the steam upon its action in the cylinder be determined by experiment?

Answer. As already explained in answer to Question 55, this can be done by an instrument called a steam indicator.

Fig. 134.

Fig. 135.

 

Scale 3 in. = 1 foot.

Question 215. What is the construction of this instrument?

Answer. The indicator now ordinarily used is the Richards indicator, the outside of which is represented in fig. 134 and a section in fig. 135. It consists of a cylinder, B, into which a piston, C, is accurately fitted, but so that it will move freely in the cylinder. The piston rod is surrounded with a spiral spring, D, the lower end of which is attached to the top of the piston, and the upper end to the cylinder cover. When steam is introduced below the piston it pushes it up in the cylinder and the spring is compressed. If there should be a vacuum below the piston, the air above it will press the piston downward and extend the spring. This latter occurs only when the indicator is used on condensing engines. Of course the distance which the piston is forced up by the steam pressure below it depends upon the amount of pressure and also on the tension of the spring; and therefore by attaching a pencil to the piston-rod so that it can mark on a moving card in front of it, a diagram will be drawn which would indicate the steam pressure, as was explained in answer to Question 55. But there are some practical difficulties in the way of doing this. It is found that if the pencil is attached directly to the piston-rod of the indicator, the distance through which they must move, in order to make the scale of the diagram sufficiently large to be clear, is so great that the momentum of the parts carries them further than the pressure of the steam alone would move them. The distance through which the pistons or instruments move, moreover, makes it impossible that the changes of pressure should be indicated simultaneously with the position of the piston; the latter must travel while the action is taking place, and thus the diagram shows changes of pressure later or more gradually than they occur.[60] To overcome these and other difficulties, the piston-rod of the indicator which we have illustrated is attached at h to the short arm of a lever, F G, and to the end of the long arm a piece, F I, is attached, which carries a pencil, J. By this means the piston has only one-fourth of the motion that it imparts to the pencil, so that the momentum of the moving parts is comparatively slight. If the pencil was attached directly to the end of the lever, it is obvious that it would move in the arc of a circle, and that this would be a source of error in the diagram. To avoid this the pencil is attached to what is called a “parallel motion.” This consists of a coupling-rod, F I, which connects the ends of two levers, F G and I H. The centre of the rod F I, to which the pencil is attached, will with this arrangement move in a straight line. The levers and all the parts are of course all made as light as possible, so that their weight will have little effect on the motion of the indicator piston.

The paper or card on which the diagram is drawn is wrapped around a brass cylinder, A A. This cylinder is made to revolve part of the way around by a strong twine, a b, which is wrapped around a pulley, b, at the bottom of the cylinder. The twine is attached to a lever, similar to that shown in fig. 30, which receives a reciprocating motion from the piston of the engine. The twine can of course move the cylinder only in one direction, and therefore a coiled spring similar to a clock spring is placed inside of the cylinder to draw it back when the twine is relaxed. In this way the paper cylinder or drum receives a part of a revolution at each stroke of the piston, and moves simultaneously with it. This drum is used instead of a flat card, on account of the practical difficulties of employing the latter. The motion of the paper on this drum will, however, be exactly the same in relation to the pencil as the motion of a flat card would be.

Fig. 136a.

The method of attaching an indicator to a locomotive is represented in fig. 136a. It will be seen from this that it is placed over the center of the steam chest and connected to each end of the cylinder with ³⁄₄-inch pipes. A globe valve was in the case represented placed on each side of the indicator, so that it could be put into communication with either end of the cylinder, or could be completely shut off from both. A better plan, however, is to have a three-way cock at the point where the horizontal pipe connects with the vertical one leading to the indicator, as the passages in a three-way cock are more direct than those in globe valves. The arrangement of the levers for giving motion to the indicator drum, and of the seat, which is very requisite for the experimenter, will be readily understood from the engraving without further explanation. It is thought by some engineers that the indicator should be applied as near to each end of the cylinder as possible. It is believed, however, that if the pipes, cocks, and their connections are made large enough so as not to impede the motion of the steam, no appreciable error will arise from the method illustrated in fig. 136a.

Question 216. What should be the form of an indicator diagram, if the steam is distributed by a link motion so as to produce the best practicable action in the cylinders?

Answer. It should approximate to that shown in fig. 136b. In this diagram the vertical lines represent inches of the stroke, and the scale on the left the steam pressure in pounds per square inch. The atmospheric and vacuum lines are also indicated, as already explained in answer to Question 55. The points at which the different periods of the distribution begin are indicated by the letters a, b, c, d, e and f. These are in the order in which they occur: a, pre-admission; b, admission; c, expansion; d, pre-release; e, release; and f, compression. The lines forming the outline of the diagram will be designated for convenience of description as follows:

The line from a to b, the admission line.

The line from b to c, the steam line.

The line from c to d, the line or curve of expansion.

The line from d to e, the exhaust line.

The line from e to f, the line of back pressure.

The line from f to a, the line or curve of compression.

Fig. 136b.

The diagram represents a distribution of steam produced by a valve having ⁷⁄₈ in. outside and ¹⁄₁₆ inside lap, and operated by the link motion represented in fig. 103. The eccentrics have 5 in. throw, and the steam-ports are 1¹⁄₄ and the exhaust 2³⁄₄ in. wide. The valve as shown by the diagram is cutting off at 8 in., or one-third of the stroke. Pre-admission begins when the piston still has ¹⁄₂ in. to move before reaching the end of its stroke. Admission of course begins with the stroke, expansion at 8 in., pre-release at 18¹⁄₂ in., release at the end of the stroke, and compression at 17¹⁄₂ in. of the return stroke. The valve is supposed to be set without any lead, or “line and line,”[61] as it is called at full stroke. When the steam is cut off at 8 in. of the stroke, the valve has 2⁵⁄₈ in. travel and ³⁄₁₆ in. lead. The steam pressure in the boiler is supposed to be 100 pounds above the atmosphere. Of course, when the valve cuts off at different points of the stroke, the periods of distribution will be somewhat changed; but from the above diagram the principal features of a good distribution can be explained.

These are: First, that the steam pressure should rise rapidly during the period of pre-admission, so that there will be full boiler pressure in the cylinder at the beginning of the stroke. When this occurs, the pre-admission line will rise from a to b, to such a point at b which will indicate full boiler pressure in the cylinder. The same pressure should then be maintained in the cylinder during the whole period of admission, and the admission line from b to c should therefore be a straight horizontal line, as shown in fig. 136b. When expansion begins, the pressure will fall, as was explained in answer to Question 55. The expansion line should approximate a hyperbolic curve, but if there is much loss of heat by radiation or other causes, the diagram will fall considerably below the theoretical curve. With cylinders well protected and with dry steam the expansion line will fall slightly below a hyperbolic curve at the beginning of the period of expansion, and rise above it during the latter part of the same period. The reason of this is that the cylinder is heated by the admission of live steam of comparatively high pressure and temperature, so that, when the pressure becomes reduced by expansion, a part of the water which is condensed in the cylinder will be re-evaporated by the heat in the latter. From the point of the pre-release, d, to the end of the stroke, e, the exhaust line should fall rapidly, so that there will be no pressure behind the piston during its return stroke. To explain the theoretical form of the exhaust line would lead us into a very abstruse discussion, which would be out of place here. It will be sufficient for our purpose to call attention to the fact that the pre-release should allow all the steam in the cylinder to escape before the piston reaches the end of the stroke, so that the back pressure during the return stroke may be as low as possible. It is, however, only at comparatively slow speeds that the steam in locomotive cylinders escapes during the period of pre-release, so that the back pressure is reduced to that of the atmosphere. It is necessary in locomotives, as has already been explained, to contract the area of the blast orifices or exhaust nozzles, in order to stimulate the draft through the fire, so that the steam cannot escape with sufficient rapidity to reduce the back pressure to that of the atmosphere if the engine is running fast. Of course every pound of back pressure on the piston is so much loss of energy, and a reduction of the amount of work done by the engine; but it is a sacrifice which must be made in order to be able to generate the requisite quantity of steam. In studying the distribution of steam, however, every effort should be made to reduce the back pressure as much as is practicable, and yet maintain a sufficient supply of steam, and therefore the line of back pressure should conform as closely as possible to the atmospheric line. The compression line should be a hyperbolic curve, beginning with the period of compression. In calculating both the compression and expansion, allowance must be made for the clearance space and steam-way. In a cylinder like that illustrated in fig. 92, their contents would be about equal to that of two inches of the cylinder. Therefore, when steam is cut off at 8 in. of the stroke, instead of having a quantity of steam which will fill a cylinder 16 in. diameter and 8 in. long, we have as much as would fill a cylinder of that diameter and 10 in. long. The same thing is true of the compression. This must occur in the above example when the piston has 6¹⁄₂ in. more to move before completing its stroke. There is therefore a quantity of steam in front of it sufficient to fill a cylinder 8¹⁄₂ in. in diameter. This steam is of course compressed by the advance of the piston, and if its pressure when compression begins is the same as that of the atmosphere, then it will be 0.9 lbs. above it when the piston has only 6 in. to move and 3.2, 6.2, 10.5, 16.9, and 27.5 lbs. effective pressure when the piston has 5, 4, 3, 2 and 1 inches to move respectively, and when pre-admission begins, the pressure will have risen to 48.7 lbs. If the back pressure is above that of the atmosphere, of course the compression will be correspondingly increased. It will also be seen that, without any or with very little clearance space, the compression would at the end of each stroke rise above the boiler pressure. It being a peculiarity of the ordinary shifting-link motion that as the period of admission is reduced that of compression is lengthened, the latter becomes very excessive when the steam is cut off at less than one-third or one-fourth of the stroke.

Question 217. In what respect would a diagram made by an indicator differ from the theoretical form represented in fig. 136b?

Answer. It would be drawn with less exactness; that is, the corners instead of being sharply defined, as in fig. 136b, would be more or less rounded, as in fig. 137, and the curves and straight lines would vary somewhat from the exact mathematical form indicated in fig. 136b. The higher the speed at which the engine is working when the diagrams are taken, the greater will be the variation from the theoretical form.

Fig. 137.

Question 218. If the amount of pre-admission is insufficient, how will it be shown in the indicator diagram?

Answer. The effect of too little pre-admission is to lower the pressure of the steam at the beginning of the stroke, and at high speeds there will not be time enough nor sufficient opening of the steam-port to supply the deficiency after the stroke has commenced. The corner of the diagram at b will then be very much rounded, as shown in fig. 138. This is apt to be the case when steam is admitted during a considerable part of the stroke, as a shifting-link motion then gives less lead than when it is worked nearer mid-gear. If the steam is cut off short, then the pressure in the cylinder during admission is very much below boiler pressure, and is apt to fall rapidly after the commencement of the stroke, as shown in fig. 138.

Fig. 138.

Question 219. If the opening of the steam-ports during admission is too small, what will be the form of the diagram?

Answer. The effect will be very much the same as that produced by too little pre-admission or lead; that is, the pressure in the cylinder will be much lower than in the boiler and will fall rapidly during the periods of admission, as shown in fig. 138.

Question 220. What defects will be indicated by the expansion curve of indicator diagrams?

Answer. If the cylinders are not well protected, and there is much loss of heat from radiation, there will be a rapid fall of pressure during the period of expansion, which will be shown by the expansion curve falling below the theoretical curve shown in fig. 136b. If, on the contrary, the indicator curve is much above the theoretical curve, it may be caused by a leak in the valve. As steam is quite as likely to leak from the steam-port into the exhaust as from the steam-chest into the steam-port, a valve which is not tight may produce just the contrary effect upon the indicator diagram. As it is usually quite easy to detect a leak in the valve by other means, the use of the indicator for this purpose is unnecessary. Attention is called to it, however, to show the impossibility of getting results of any value with the indicator if the valves are not steam-tight.

Question 221. What should be observed regarding the exhaust line of the indicator diagram?

Answer. The most important point to be observed is, whether the pressure at the end of the stroke is reduced as low as possible, as at high speeds it is usually much more difficult to exhaust the steam from than to admit it into the cylinder. As already stated, the blast in the chimney makes it almost impossible to exhaust the steam to atmospheric pressure when the locomotive is running fast. If the steam is released too late in the stroke, as already explained, there will not be time enough nor sufficient opening of the port to allow the confined steam to escape from the cylinder before the end of the stroke, and this will be indicated on the diagram by the space between the line of back pressure and the atmospheric line during the commencement of the return stroke, as shown in fig. 138.

Question 222. What should be observed regarding the line of back pressure?

Answer. The most important point is, that it should approximate as closely as possible to the atmospheric line, as all the back pressure not only diminishes the efficiency of the engine, but is a total loss of energy. Too much inside lap will increase the amount of back pressure, but generally it is more influenced by the area of the blast orifices than by any other cause. Every effort should be made, therefore, to have them as large as possible and yet have the boiler make as much steam as is needed.

When only one blast orifice is used for both cylinders, it often happens that when the steam is exhausted from the one cylinder it “blows” over into the other, and thus produces an additional amount of back pressure. This is shown by a rise or “hump” in the line of back pressure, as indicated in fig. 138.

Question 223. Can the amount of compression which is needed be determined by calculation?

Answer. Yes; but it involves more abstruse principles of mathematics than it is thought best to introduce here. Some of the reasons can, however, be given, which will make the subject clearer, and enable the reader, if he has sufficient knowledge of mathematics, to investigate the subject still further.

Fig. 140.

In the first place it is a well-known fact that the motion of a piston in the cylinder of a steam engine is not a uniform one, but increases in speed from the beginning of the stroke to the middle, and diminishes in speed from the middle to the opposite end. The cause of this is that the crank revolves at a uniform speed during the entire revolution, but the piston moves much less at the beginning of the stroke, with a given amount of revolution of the crank, than it does at the middle. This is shown in fig. 140, in which A is a cylinder and B the piston and a b c d the path of the crank. Now while the crank moves from a to 1, or ¹⁄₁₂ of a revolution, the piston has moved 1³⁄₈ in., or a distance equal to that from a to 1′ or to the base of a perpendicular drawn from 1 to the centre line a c. While the crank moves from 1 to 2, or through the second twelfth of a revolution, the piston has moved from 1′ to 2′, or 4³⁄₈ in., or 2³⁄₄ in. further than during the first twelfth of the crank’s revolution. During the third twelfth of the revolution the piston moves from 2′ to 3′, or 6 in., thus showing that it continues to increase in the distance moved during each period of the revolution of the crank until the latter has made a quarter revolution. The speed of the piston then begins to diminish until it reaches the end of the stroke. It is slightly affected by the angularity of the connecting-rod, as already explained, but for the present this is disregarded. It is obvious now that if the momentum, or actual energy stored up in the piston and other reciprocating parts after they have passed the middle of the stroke, added to the pressure behind the piston, is greater than the resistance offered by the crank, the motion of the latter will then be accelerated and thus conveyed to the moving engine and train. If, however, there is any momentum in the piston when it reaches the end of the stroke, evidently it can exert no power to cause the crank to revolve, but must be expended by producing a pressure on the crank-pin and thus on the axle-boxes. Not only will such a pressure not cause the crank to revolve, but it will be more difficult to turn the crank with such a pressure against it than it would be without. The momentum of the piston and other reciprocating parts at the dead point therefore creates a resistance to the movement of the crank instead of helping to turn it. It will also be observed that after the crank has moved slightly from the dead point, any pressure on the piston will exert very little force which will tend to turn the crank. In fact the nearer the piston is to the end of the stroke the greater is the proportion which the friction of the crank-pin and axle bears to the useful effect of the strain in causing the crank to turn. Calculation shows that for about three degrees on either side of the dead points the effect of pressure on the crank-pin is actually to retard the engine. If now the piston reaches the end of the stroke with a certain amount of unexpended momentum stored up in it, if this energy is expended by producing pressure on the crank, then it will not only be a waste of energy but a double waste by retarding the motion of the crank. If, however, this energy can be absorbed by compressing steam which will fill the clearance spaces, it will not only prevent the retarding effect referred to, but the energy in the piston and other parts will be converted into steam pressure, which will be given out in useful work during the next stroke. It would, of course, be impossible to arrest the motion of the piston instantly, and therefore its momentum is gradually absorbed from the time compression begins until it reaches the end of the stroke. As the energy of a moving body is equal to its weight multiplied by the square of its speed, it is obvious that to overcome this a different amount of compression would be required for each speed, and also that it must be adjusted to the weight of the moving parts. Such exact adaptation is not practicable on locomotives, nor does the link motion enable us to alter the amount of compression with so much exactness: but the explanation shows the value of increasing the amount of compression with the speed, which fortunately the peculiarities of the shifting-link motion enable us to do without difficulty.

Fig. 139.

Question 224. What cause produces the form of diagram represented by Fig. 139?

Answer. It is produced by excessive compression, which causes the pressure in the cylinder to rise above boiler pressure before pre-admission begins. As soon as the port is opened, part of the steam in the cylinder flows back into the steam-chest, and thus the pressure is reduced, as shown by the diagram.

Question 225. How can we determine whether the steam is distributed in the cylinders to the best advantage, and how can we discover the fault, if there is one, in the link motion?

Answer. The indicator will show the action of the steam in the cylinder, and motion-curves drawn with the instrument described in answer to Question 192 will show the exact movement of the valve. By comparing the indicator diagram with the motion-curves, the one will show the defects in the other.[62]

Question 226. To what extent can the movement of the valve be modified by alterations in the proportions of the link motion?

Answer. The motion of the valve is susceptible of an almost infinite number of changes, by different variations and combinations of proportions of the working parts of the link motion. These changes are, however, limited by the general laws which govern the motion of eccentrics, and therefore cannot influence the motion of the valve beyond certain limits. Hardly any variation can be made either in the proportions or arrangement of the working parts which will not have some influence upon the movement of the valve. Aside from the proportions of the valve itself, which have already been discussed, the throw of the eccentrics, the length of the rods and of the link, the point of connection of the rods with the link, the point of suspension, the position of the lifting shaft, the length of the arms, the length and position of the rocker arms will each of them effect the distribution of steam. The number of combinations of all these different proportions is of course almost infinite, and therefore any full discussion of them will be impossible here.

Question 227. What are the most important points which require attention in designing a link motion?

Answer. It should be proportioned so that—

First, the lead and the period of admission should be the same for each end of the cylinder, for each point of cut-off, and, if possible, in back as well as forward gear.

Second, the width of opening for both admission and exhaust should be as large as possible when steam is cut off short.

Third, the exhaust or pre-release should occur early enough and be maintained long enough to reduce back-pressure as low as possible.

Question 228. How can the lead and period of admission be equalized?

Answer. It is impossible to make the periods of admission absolutely alike for every point of cut-off in both fore and back gear. It is therefore customary to disregard the back gear, as engines are worked but little with the link in that position. Even for forward gear the periods of admission cannot be made exactly alike for each end of the cylinder and for each point of cut-off, and therefore it is usual to make the periods of admission alike for half-gear forward, in which position the link is worked most.

The periods of admission for the front and back ends of the cylinder can be changed most in relation to each other by altering the position of the point of suspension on the link. This can be done either by moving this point up or down, or horizontally. Usually links are suspended from a point halfway between the points of connection of the eccentric-rods and from ¹⁄₄ to ³⁄₄ in. back of the centre line of the slot in the link. A somewhat better distribution can be secured by suspending it about 3 in. above the centre, but the suspending link must then be made so short that it is subjected to very great strains by the motion of the link, and this evil is usually considered much greater than the advantage which is gained thereby in the more equal distribution. The point at which the upper end of the suspension link is hung also influences the relative amount of admission front and back. This point, of course, varies as the end of the lifting arm is raised or lowered. In designing valve gear it is usually tested by a full-sized model, which will show the exact motion of all the parts. The best position for the lifting shaft and the length of its arm can be determined perhaps most satisfactorily by placing the link in full gear forward, then moving the point of suspension of the upper end of the link-hanger horizontally so that the front and back admission will be alike, and then marking this position. The same process should then be repeated for half gear and for the shortest point of cut-off. If the position of the lifting shaft and the length of its arm are then so arranged that the end of the latter will move through the three points which have been thus determined, the admission will be very nearly equal for each end of the cylinder. Usually, however, it is impossible to arrange the shaft and arm so that they will conform exactly to these conditions, and therefore an approximation is made which will come as near as possible to what is required. It may be stated, however, that the lifting shaft should be kept as low as possible, so as not to interfere with the eccentric-rods. In some cases the shaft has been suspended from the boiler, so that the outside eccentric-rod would work past or over the end of the lifting shaft, thus allowing the latter to be located lower than would otherwise be possible.

Question 229. Which parts of the link-motion have the greatest influence on the distribution of steam?

Answer. The lap of the valve and the throw of the eccentrics. The effect of any change of these upon the distribution is very similar to that produced if a single eccentric is used, which was explained in the answers to Questions 49, 50 and 52.

Fig. 141.

Question 230. What is the effect upon the admission of increasing the throw of the eccentrics with the same lap?

Answer. As already explained, the effect is to increase the period of admission, or in other words to cut off later in the stroke, and also to increase the width of the opening of the steam-port or the distance which the valve throws over the port. This has an important influence upon the admission, when the link-motion is used.

Question 231. What is meant by the angular advance of the eccentrics?

Answer. It is the angle which a line, e f, fig. 141, drawn through the centre of the axle and the centre of the eccentric makes with a vertical line a b, when the crank is on one of the dead-points or centres. Thus in fig. 141 the crank A is represented on the front centre. In order to give the valve the necessary lead the eccentric must be moved ahead of the vertical line a b. The angle c which the line e f (drawn through the centre of g of the eccentric and f of the axle) makes with the vertical line is called the angular advance.

Question 232. What is meant by linear advance?

Answer. By linear advance is meant the distance which the valve has moved from its middle position at the beginning of the stroke of the piston. This, when the two rocker arms are the same length, is the same as the distance of the centre of the eccentric g from the vertical line a b, fig. 141.

Question 233. Why does the cut-off occur earlier with an eccentric having a short throw than with one which gives more travel to the valve?

Answer. Because it is necessary to give the eccentric with the short throw more angular advance in order to give the valve the required lead. This is illustrated in fig. 142, in which a section of a valve, V, and ports c, g, and d, are represented. In order to simplify the diagram as much as possible the rocker is left out and the valve is supposed to be moved by the rod R directly from the centre a of the eccentric.[63] The effect of the angularity of the connecting rod and eccentric rod is also neglected. The circle a b e f represents the path of the centre of an eccentric having 5 in. throw, and h i j the path of one having 3¹⁄₂ in. throw. In order to give the valve the required lead, which is supposed to be just line-and-line at the beginning of the stroke, the linear advance of the valve must be equal to the lap, or ⁷⁄₈ in. If therefore we draw a line, p a, parallel to the vertical centre line, e k, and ⁷⁄₈ in. from it, the intersection of p a at a and h with the paths of the eccentric will be the centres of the eccentrics. If through these centres and the centre of the circle, lines, o a and o p, be drawn, the angles which they make with the vertical e k will be the angular advance. It will be seen from these lines that in order to give the valve the required lead it is necessary to give the eccentric with the small travel more angular advance than is necessary for the one with the larger throw. It is obvious, too, that when the centre of the larger eccentric has reached the point b the valve will have received its greatest travel, and that when it reaches p the steam-port c will again be closed or the steam cut off. If the small eccentric is employed, the valve will then have its maximum travel when the centre h reaches s, and the port will be closed when it reaches i. By drawing lines, o p and o n, through i and p, it will be seen that from the beginning of the stroke until the steam is cut off, if the large eccentric is employed, it, and consequently the shaft and crank, must move over an angle measured by the arc q t p. If the small eccentric is used, it and the crank must move through an angle measured by the arc u t n. In other words, the crank must turn a considerably greater distance before steam is cut off with an eccentric having a large than with one having a small throw.

Fig. 142.

Fig. 143.
Scale ³⁄₁₆ in. = 1 inch.

It is also quite obvious from fig. 142 why the port is opened a shorter distance with a small than with a large eccentric. The distances o s and o b are equal to half the throws of the eccentrics, or 1³⁄₄ and 2¹⁄₂ in. The linear advance o r is in both cases ⁷⁄₈ in., and therefore after the port begins to open the valve will be moved by the small eccentric a distance which is equal to 1³⁄₄ - ⁷⁄₈ = ⁷⁄₈ in., and by the large one 2¹⁄₂ - ⁷⁄₈ = 1⁵⁄₈ in.

Fig. 144.

Scale ³⁄₁₆ in. = 1 inch.

Question 234. What is the effect on the admission of giving an eccentric with a small throw the same angular advance as one with a large throw, and then reducing the lap of the valve so that the lead will be the same in both cases?

Answer. The admission and the cut-off will then occur at the same points of the stroke, but the ports will not be opened so wide. This is illustrated in fig. 143, in which the paths of two eccentrics having the same throw as those in fig. 142 are represented. The centre, a, of the larger eccentric is represented in the same position in fig. 143 as in fig. 142. If a line is drawn from the centre of the larger eccentric to that of the axle, and if the centre, h, of the smaller eccentric is located on the intersection of this line with the circle representing its path, then the smaller eccentric will have the same angular advance, but the linear advance measured by the distance o t will be only ⁵⁄₈ in. If the valve have the same lap as in fig. 142, its steam edges at the beginning of the stroke, if the small eccentric is employed, will occupy the position represented by the dotted lines A and B. If these edges are cut off, as shown by the full lines and shading, then the valve will have the same lead as in fig. 142. It is obvious, too, that if the smaller eccentric has the same angular advance it will reach the point v, at which, with the reduced lap, the steam will be cut off, at the same time that the centre, a, of the large eccentric will reach p, at which point it cuts off the steam with the valve having the large lap. There is, however, this difference in the distribution, that in the one case the valve opens the port a distance equal to t s, and in the other a distance equal to r b. As o t is equal to the linear advance of the small eccentric, or ⁵⁄₈ in., and o s to half the throw of the eccentric, or 1³⁄₄, t s is equal to 1³⁄₄ - ⁵⁄₈ = 1¹⁄₈ in. The distance r b, as shown above, is equal to 2¹⁄₂ - ⁷⁄₈ = 1⁵⁄₈ in., so that the effect produced upon the admission of using an eccentric with a small throw and corresponding amount of lap is, that the ports are not opened so wide as with an eccentric having a larger throw.

Fig. 145.

Scale ³⁄₁₆ in. = 1 foot.

Question 235. How do eccentrics with a short throw, and valves with a corresponding amount of lap, affect the admission with a link motion as compared with eccentrics having a larger amount of throw and greater lap of valve?

Answer. The chief difference is that the ports are not opened so wide for the same period of admission. Thus in fig. 144 is a series of motion-curves drawn with a model of a link motion like that illustrated in fig. 103. The eccentrics had 5 in. throw, and the valve ⁷⁄₈ in. lap outside and ¹⁄₁₆ in. inside. Fig. 145 represents a series of curves, drawn with the same arrangement of valve-gear, excepting that the eccentrics had 3¹⁄₂ in. throw and the valve ¹⁄₂ in. lap. In both cases the curves represent the motion of the valve when cutting off at the same point of the stroke. The following table will show the relative amount of opening of the port.

It will be seen from this that the eccentric with 5 in. throw gives a greater width of opening for every point of cut-off than the one with 3¹⁄₂ in. throw. For the higher admissions this is not important, but when steam is cut off short it will be observed that the width of the opening is very small. At high speeds the small opening is a great disadvantage.

Question 236. Has it been determined what amount of opening is required for given speeds of the piston?

Answer. Not with any degree of accuracy. It is customary to make the area of the ports about one-tenth that of the piston. It is certain, however, that with steam-ports of this proportion an opening considerably less than their whole area is sufficient to maintain steam at boiler pressure in the cylinders. One of the defects of the link motion is that the opening of the port is very small when the steam is cut off short. It is best, therefore, to secure the largest practicable opening of the ports for the lower points of cut-off.

Question 237. What are the proportions of the valves and eccentrics used in the ordinary practice in this country?

Answer. The following report made by a committee of the Master Mechanics’ Association will show the proportions used on thirty-five different railroads, and is a fair indication of the common practice.

Question 238. What should be the width of the bridge between the steam and exhaust ports?

Answer. It is usually made about the same thickness as the sides of the cylinder, in order to secure a good casting; but sometimes it is necessary to make it wider, in order to prevent steam from escaping from the steam-chest into the exhaust, which is apt to be the case if a valve has little lap and a long travel.

Question 239. What determines the width of the exhaust-port?

Answer. The throw of the valve. This will be clear if we refer to fig. 146, which represents a valve with a travel of 5¹⁄₂ in. It will be seen that when it is in the extreme position in which it is shown the width A of the opening of the exhaust-port is very small. If this opening is contracted too much it will of course interfere with the free escape of the exhaust steam. It is therefore best to make the exhaust-port so wide that with the greatest travel of the valve the width of its opening will be either quite or very nearly equal to the width of the steam-port.

Fig. 146.

Scale ³⁄₁₆ in. = 1 foot.

Fig. 148.

Fig. 147.

 

Scale ³⁄₄ in. = 1 foot.

Question 240. Where is the reverse lever located and how is it constructed?

Answer. It is located on the foot-board[65] K′ K′, as shown in plate II. It consists of a lever O, O, with the fulcrum at the lower end. The reverse-rod e k, which connects the lever with the vertical arm k of the lifting-shaft, is attached above the fulcrum of the reverse lever. Figs. 147 and 148 represent side and end views of the lever on an enlarged scale and with some of the details attached which are omitted on plate II. C, C are two curved bars, which in this country are usually called quadrants, but in England are called (and more properly) sectors. These are placed on each side of the reverse-lever and are fastened to some portion of the engine. They have notches, n, n, n, cut in them to receive the latch L, which slides in a clamp, H, and holds the reverse-lever in the notches in which it is placed. This latch is operated by a trigger, D, which is grasped by the locomotive runner when he takes hold of the handle A of the reverse-lever. The trigger works on a pin, E, as a fulcrum and is attached to the latch by a rod, r r. When the trigger is pressed up against the handle, the latch is raised out of the notches by the rod r r, and is pressed into them again by the spring s when the trigger is released. F is a set-screw which presses against a gib, G, and is intended to keep the latch tight and prevent the reverse-lever from shaking.

Question 241. How are the notches in the sector arranged?

Answer. They are usually arranged so that the steam will be cut off at some full number of inches of the stroke when the reverse-lever is in each one of the notches. They are therefore located so that the steam will be cut off at 6, 9, 12, 15, 18 and 21 inches, or at 6, 8, 10, 12, 15, 18 and 21 inches of the stroke. A notch is also placed so as to hold the link in mid-gear. In some cases as many notches as there is room for are put into the sectors. The latter seems to be much the best plan, as it gives more gradations in which the valve-gear can be worked, and it is a matter of no consequence whatever in the working of an engine whether the steam is cut off at some full or some fractional number of inches of the stroke. By referring to fig. 144 it will be seen how very great the difference of the distribution of steam must be, as indicated by the 5th, 6th and 7th motion-curves.

Question 242. How long should the reverse-lever be?

Answer. The lever should be sufficiently long so that in throwing the link from full gear forward to full gear backward the handle A will move not less than four times the distance that the link is moved. It is much better to give the end of the handle A five or even six times the motion of the link, as there will then be a much easier action in reversing the engine. This will also make it possible to use longer sectors, and give room for more notches.

Question 243. What provision is made in the reversing gear for overcoming or neutralizing the weight of the link and other parts of the valve-gear?

Answer. Their weight is counterbalanced by the pressure of a spring of some kind. In fig. 103 the two volute springs enclosed in a case, H, are used for this purpose. These are compressed by the rod m, which is attached to a short arm l, on the reverse shaft A, when the link is lowered, and consequently the tension of the spring resists the weight of the link when the latter is down or in forward gear. Different kinds of springs are used for this purpose and sometimes are attached to the reverse-lever instead of to the lifting-shaft.

Question 244. What is meant by “setting” a slide-valve?

Answer. It is to fasten the eccentrics in the right position on the axle and to adjust the length of the eccentric-rods and valve-stem so that the valves will give the required distribution of steam.

Question 245. How are the valves of a locomotive set?

Answer. After the wheels, axles, main connecting-rods and valve-gear are connected together, put the rocker-arm in its middle position, and lengthen or shorten the valve-stem so that the valve will be in the centre of the valve-face. Then place the crank on the forward centre and the full part of the forward motion eccentric above and that of the backward motion eccentric below the axle, and fasten them to the axle temporarily by tightening up the set-screws. Then throw the link down until the block comes opposite to the end of the eccentric-rod, and turn the wheels,[66] and at the same time, observe whether the travel of the valve is equal to the throw of the eccentric and also whether it travels equally on each side of the centre of the valve-face. If its travel is greater than the throw of the eccentric, raise the link up; if less, lower it down until the two are just equal, and then mark the position for the notches on the sections or quadrants to receive the latch of the reverse-lever. If the valve does not travel equally on each side of the centre of the valve-face, either lengthen or shorten the eccentric-rod, as may be necessary. Repeat this operation for the backward motion, by raising the link up until the block is opposite the end of the lower eccentric-rod. After having done this, go over the whole process again to see whether it is all correct. Now with the crank on the forward centre, and the link in full gear forward, loosen the set-screws in the forward eccentric, and move it around the axle so that the valve will have the required lead and then fasten it again. Now raise the link up into full back gear, and set the backward eccentric in the same way. Then turn the wheels so as to bring the crank on the back centre, and observe whether the lead is correct for the back end of the cylinder. If it is not, lengthen or shorten the valve-stem or eccentric-rod so as to make the lead alike at both ends, and if it is then too much or too little, it can be increased or diminished by moving the eccentrics on the axle.

Great care must be taken in setting valves to be sure that the cranks are exactly on the centres or dead points, and it is impossible to set them in that position with sufficient accuracy from the motion of the piston or cross-head, and therefore the centres of the crank-pins should always be set so as to conform to a line drawn through the centre of the cylinder and the axle. When the cylinders are horizontal, it is of course only necessary to place the cranks on a horizontal line drawn through the centre of the axle.

When the valves are set it should also be noticed whether the axle-boxes (whose construction will be explained hereafter) are in the centre of the jaws, and if not they should be moved to the centre by driving wooden wedges between them and the frames, either above or below, as may be required. The position of the boxes has a very material influence on the valve-gear.

If it is intended to lay off the notches on the sectors so as to cut off steam at certain definite points of the stroke, these points should be laid off in the guides from the motion of the cross-head. The latter being placed in any of the required positions at which steam is to be cut off, the reverse-lever should then be moved so that the link will just close the admission port. The lever can then be clamped to the sectors, and the wheels turned so as to show whether its position is correct for each end of the stroke. As before stated it is impossible to get the ordinary link-motion to cut off at exactly the same points at both ends of the cylinder, but a very close approximation can be made by proportioning the different parts properly. As has already been stated, it is believed to be a much better plan to put as many notches in the sectors as possible, than to locate them for certain definite points of the stroke.

In setting the valves of locomotives, care must be taken to turn the wheels forward for the forward motion and backward for the backward motion.

After the valves are set the position of the eccentrics on the shaft should be marked, so that in case they become loose on the road they can easily be set again. It is usual, too, to mark the position of the valves with centre-punch marks on the valve-stem and on the stuffing-box of the steam-chest, so that with a gauge made for the purpose the position of the valve can be determined without taking off the steam-chest cover.

In some cases the eccentrics are keyed on, which is done after their position is determined by setting the valves. The ends of the set-screws which are used to fasten the eccentrics should be cup-shaped and case-hardened, so as to hold as securely as possible to the axle when they are screwed down.

PART XII. THE RUNNING GEAR.

Question 246. What is meant by the running gear of a locomotive?

Answer. It means those parts, such as the wheels, axles and frames, which carry the other parts of the engine. As the Germans express it, it is the “wagon” of the locomotive.

Question 247. How may the wheels be classified?

Answer. As driving and carrying or truck wheels.

Question 248. What service must the driving wheels perform?

Answer. The driving wheels, as indicated by their name, “drive” or move the locomotive on the track, as was explained in answer to Questions 64, 65 and 66. As their adhesion depends upon the pressure with which they bear upon the rails, they must carry either a part or the whole of the weight of the engine.

Question 249. What proportion of the weight of ordinary locomotives is usually carried on the driving wheels?

Answer. Eight-wheeled “American” locomotives, which are most commonly used in this country, have about two-thirds of their weight on the driving wheels.

Question 250. What is meant by the “truck” of a locomotive?

Answer. It means one or more pairs of wheels which are attached to a separate frame and to the locomotive by a flexible connection, so that the axles are not held rigidly at right angles to the main frame, but can assume positions which approximate to that of radii of the curves of the track. In plates I, II and III, E E are the truck wheels, b′ b′ the truck frame, and y, plate II, and fig. 40, the centre-pin, around which the truck frame turns.

Question 251. What service does the truck perform?

Answer. It carries the weight of the front end of the locomotive, and also guides it into and around curves and switches.[67]

Question 252. How does it perform the latter service?

Answer. It does it very much in the same way as the front wheels of an ordinary wagon enable it to turn around corners; that is, the truck wheels being attached to a separate frame, which is connected to the locomotive by a centre-pin, just as the front axle of an ordinary wagon is connected by the king-bolt, can turn.

Fig. 149.

Fig. 150.

Scale, ³⁄₈ inch = 1 foot.

Fig. 151.

Question 253. Why are two pairs of wheels used on a locomotive instead of one, as on an ordinary wagon?

Answer. Because it is necessary to have one pair of wheels guide the other. In an ordinary wagon the front axle is guided by the pole or shafts. Nearly every one knows the difficulty of moving a wagon when the pole or shafts are removed, especially if it be pushed from behind. The movement of the front axle is then uncontrolled, and it is impossible to direct the motion of the vehicle. The same thing would occur with a locomotive if a single pair of wheels were used, and attached in the same way as the front axle of a wagon. Thus if a single pair of wheels were connected to a locomotive by a centre-pin, a, fig. 149, so that the axle would be free to move around this pin, then if one of the wheels should strike an obstruction, say a stone, s, fig. 150, there would be nothing to prevent the axle from being thrown into the position shown in fig. 150, and the wheels would be quite sure to leave the track. When two pairs of wheels are used and both axles attached to the same frame, which is connected to the engine by a centre-pin, s, fig. 151, between the two axles, then the wheels in moving round the centre-pin must move around the centre s in arcs of circles, m n, m n, described from the centre s. These arcs, it will be observed, cross the rails. Now if the wheels should move in that direction, the flange of one of them would come in contact with the rail and prevent it from moving any farther. It is therefore evident that wheels arranged in that way can only move about the centre-pin as far as the curvature of the track will permit. Trucks are sometimes used with only one pair of wheels, but the centre-pin is then placed some distance behind the centre of the axle, or in the same relation to it that the centre s is to the axle a a′ in fig. 151. It is evident that if the frame for such a truck turns around the centre-pin, the wheels must move across the track in the same way as represented by the arcs m n, in fig. 151. The construction and operation of trucks with a single pair of wheels will be more fully explained hereafter.

Fig. 152.

Fig. 153.
Scale
³⁄₁₆ Inch = 1 Foot.

Fig. 154.

Fig. 155.

 

Fig. 156.

Question 254. Why will a locomotive run around curves easier if the front axles are attached to a truck frame which is connected to the locomotive by a flexible connection?

Answer. Because the truck axles can then assume positions which conform very nearly to the radii of the curves of the track, and it is well known that if two or more axles, each with a pair of wheels on it, are attached to a frame with their centre lines parallel with each other, as shown in fig. 152, they will roll in a straight line, but if the centre lines of the axles are inclined to each other, as shown in fig. 153, the tendency will be to roll in a curve, the radius of which will depend upon the degree of inclination of the axles to each other. In order to make the wheels in fig. 152 roll on the curves a b and c d, it will be necessary to slide them laterally a distance equal to that between the curves and the straight lines m o and p r, and as the length of the outside curve is greater than the inside one, if the wheels are fastened to the axles so they cannot turn on them and roll on the curves, either the wheels on the inside or those on the outside must slip a distance equal to the difference in the length of the two curves. Considerable force will therefore be required to overcome the resistance due to the combined lateral and circumferential sliding of the wheels, so that more power will be needed to make them roll in a curve than is necessary to make them roll in a straight line. If, however, the axles are inclined to each other, then the wheels will naturally roll on a curved path, and it will not be necessary to slide them sideways to make them conform to such a path. But if the wheels are all attached to the axles so that those on the same axle cannot turn independently of each other and are all of the same diameter, then either the inside or the outside ones must slip, because the path in which the outside ones roll is longer than the inside curve, so that even if the axles are inclined to each other more power will be needed to roll the truck in a curved path than to roll the wheels shown in fig. 152 in a straight line. It is, however, a fact that a cone or a portion of a cone like that shown in fig. 154 will of itself roll on a curve. It will do the same thing if the middle is cut away, as indicated by the dotted lines in fig. 154 and as shown in fig. 155. If now the wheels are made so that their peripheries[68] form portions of a cone and the axles are inclined to each other as shown in fig. 156, then there will be no slipping on the track, because the outside wheel, being larger in diameter than the inside one, advances further in one revolution than the latter does, and thus rolls on the longest path in the same time that the inside or smaller wheel does on the shorter one. When this is the case, such wheels will roll in a curve as easily as those in fig. 152 will in a straight line. The degree of inclination of the axles and of the sides of the cone must, however, vary with the radius of the curve. But if the axles are parallel to each other, and the wheels conical, as represented in fig. 157, they will not roll either in a straight line or in a curve without great difficulty, because if they roll in a straight line, the wheels on one side being larger in diameter than those on the other, either the larger or the smaller ones must slip on the path in which they roll. If they roll on a curve, then each pair of wheels has a tendency to roll in a curve independent of the other, and therefore the wheels must slip laterally if both pairs roll on the same track. Thus, suppose two pairs of wheels, a, a′, and b, b′, fig. 157, to be made conical and attached to a frame so that their axles are parallel to each other. Now each pair of such wheels will have a tendency to roll in circular paths, a′ i, a h, and b′ k, b j, the centres of which are at m and n, or at the apices of the cones of which their peripheries form a part. If they are made to roll in circular paths, c d, e f, described from a point g, then each pair of wheels must slip laterally over the space between the paths a′ i, a h, in which they would naturally roll and that in which they are made to roll. Thus the wheel a would slide laterally the distance between the curve a h and a f, and a′ that between a′ i and c d; b would slide from b j to b f and b′ from b′ k to b′ d. It will thus be seen that in order that two pairs of wheels may roll with equal ease in a straight line and in curves, the wheels in the one case must be of equal diameters and the axles parallel, and in the other case the wheels must be of unequal diameters and their axles be radial[69] to the curve. This is equally true of any number of pairs of wheels. If we have three, four, or any number of axles, with wheels all attached to the same frame, if their axles are parallel, and the wheels of the same diameter, they will roll in a straight line; but if their wheels are conical and their axles radial, they will roll in a curve.

Fig. 157.
Scale
³⁄₁₆ In. = 1 Foot.

For the preceding reasons it is therefore sufficiently obvious that if a locomotive is to run on both straight and curved tracks, on the former the wheels should be of the same diameter and the axles parallel, and on the latter the wheels should be conical and the axles radial.

Fig. 158.

Fig. 159.
Scale ³⁄₈ in. = 1 foot.

Question 255. How are the wheels made so that in curves they will act as though they were of the conical form described and on a straight track all be of the same diameters?

Answer. The periphery or tread of each wheel is made conical, but of the same size as the other, and with the small diameter of the cone outside, as shown in fig. 158. On a straight track if the position of the wheels on the rails is such that their two flanges are equally distant from the rails, as shown, then obviously at the points of contact with the rails the wheels are of the same diameter. That is, a is equal to b. But in running on a curved track, if the wheels are of the same diameter, as has been shown, they will roll in a straight line and consequently towards the outside of the curve. The flange c—supposing it to be at the outside of the curve—will therefore roll towards the rail, and consequently the outside wheel will rest on the rail at a point nearer the flange, as shown in fig. 159, where the diameter a is larger, and the inside one further from the flange where the diameter b is smaller than at a and b in fig. 158; and consequently the action of the wheels is the same as though their peripheries were made of the form shown in fig. 157.

Question 256. How are the axles of locomotives made to assume a position radial to the curves in the track?

Answer. This is only done approximately, as the mechanical difficulties in the way of doing it perfectly are so great as to render it impracticable. By attaching the truck to the locomotive by a flexible connection or centre-pin, s, as shown in fig. 160 (which represents a plan of the wheels of an ordinary locomotive), it is plain that the truck axles e f and g h, instead of remaining parallel to the driving-axles a b and c d, will, by turning around the centre-pin, s, adjust themselves to the curve so as to approximate as closely to radii as is possible for two axles which are held parallel to each other. Of course the further apart they are the greater will be their divergence from the position of radii, and whether the tread of the wheels be cylindrical or conical the further apart their axles are the greater will be the divergence of the paths in which they would naturally roll from that of any curve on which they must roll. Thus, if the axles were twice as far apart as they are represented in fig. 157, and in the position shown in the dotted lines l l′ and o o′, the wheels, if they are conical, would then naturally roll in curves drawn from the centres p and q. If the wheels are cylindrical, they would roll in straight lines. In either case the divergence of their paths l s and l r from the curve of the track is greater than a h and a′ i, the paths in which they would roll if their axles were nearer together. This divergence increases with the distance between the axles, and therefore the lateral slip of the wheels must be in the same proportion.

Fig. 160.
Scale ³⁄₁₆ Inch — 1 Foot.

Question 257. Is the resistance to rolling diminished by placing the truck axles nearer together?

Answer. It is, within certain limits. The nearer each other they are placed, the closer will the centre-pin of the truck be to the centre of the axles. The closer it is to the centre of the axle, the greater is the tendency of the wheels to become “slewed,” or to assume a diagonal position to the rails as represented in fig. 150, and thus increase the resistance and also the danger of running off the track. The increase of resistance from this cause, after the axles reach a certain distance from each other, is greater than the decrease from a closer approximation to the position of radii. In ordinary locomotives it is necessary to place the truck wheels from 5 ft. 6 in. to 6 ft. apart, in order to get the cylinders between them in a horizontal position. This distance apart works very well in ordinary practice.

Question 258. What is meant by flange friction?

Answer. It is the friction of the flanges of the wheels against the head of the rails. Thus if two pairs of wheels, a a′, b b′, fig. 151, be placed on a curve and rolled in the direction of the dart, the wheel a will roll towards the outside of the curve until the flange comes in contact with the rail. As already explained, if two axles are parallel to each other, no matter whether the wheels are conical or cylindrical, they must slip laterally in order to roll in a curved path. As the flange must follow the curve of the rail, it forces the wheel laterally and thus compels it to roll in the curved path into which the rail is bent. As the wheel offers considerable resistance to sliding there is a corresponding pressure of the flange against the rail, and consequently the revolutions of the wheel produce an abrasive action between the two. This action is obviously increased with the distance between the axles, because, as has been shown, the lateral slip of the wheels is then greater than when they are nearer together. It is also obvious that if the wheels are parallel with the rails there will be no abrasive action of the flanges, but that the greater the angle at which the wheels stand to the rails the harder will the flanges rub against the rails, and the greater will be the flange friction. With the aid of geometry it can very easily be proved that the farther apart two parallel axles are, the greater will be the angle of the wheels to the rails on a curved track, and, therefore, the greater will be their flange friction. It must, however, be remembered that if the wheels are so close together that they are liable to become “slewed,” or assume a diagonal position across the rails, as shown in fig. 150, the angle at which the wheels would stand to the rails would thus be very much increased. It has therefore come to be a very generally recognized rule that the centres of axles should never be placed nearer together than the distance between the rails.

Question 259. Is the flange friction of all the wheels of a truck the same on any given curve?

Answer. No; of the front wheels obviously only the flange of the one on the outside of the curve comes in contact with the rail. As the centrifugal force of the engine presses the back pair of wheels towards the outside of the curve, the flange of the outside wheel alone comes in contact with the rail. But as this wheel is constantly rolling away from the rail, as shown by the dotted lines h g, fig. 151, obviously the friction of its flange is less than that of the front outside wheel, which always rolls towards the rail. The flange of the back inside wheel is carried outwards by the centrifugal force and also by the tendency of the wheels to roll on their largest diameters on a curve, so that its flange will not touch the rail.

Question 260. Can the axles of driving wheels assume positions radial to the track?

Answer. In ordinary engines they cannot. Various plans have been devised for the purpose of enabling them to do so, but it is only recently that they have met with any success. Some of these plans will be described hereafter. It is, however, of less importance that the driving axles, when they are behind the centre of the locomotive, should assume positions radial to a curved track than that the front wheels should. This is illustrated by a common road wagon, as all know the ease with which such a vehicle can turn a corner if we run it with the front axle ahead, and the difficulty of doing so when the back axle is in front. In the case of a locomotive the reason for it is very much the same as that which makes the flange friction of the back wheels of a truck less than that of the front ones. From fig. 160 it will be seen that the outside driving-wheels, when the engine is running with the truck in front, are rolling from the rail and not against it. As stated before, the centrifugal force of the engine when in motion has a tendency to throw the wheels towards the outside of the curve. It will also be noticed that the front driving axle is near the centre of that portion of the curve which lies between the centre s of the truck and the centre k of the back axle. If it were in the middle between them, it would be exactly radial to the curve; being near the middle, it approximates closely to that position, and therefore the flange friction of its wheels is very slight. It will be noticed that if the flange of the back or trailing-wheel on the inside of the curve were not kept away from the rail it would roll toward and impinge against that rail. But it will be noticed that the flange of the front driving-wheel will come in contact with the inside rail before that on the back wheel can touch it. For this reason, and also on account of the effect of the centrifugal force exerted on the engine and the tendency of the wheels to roll on their largest diameters, the flange of the inside back wheel is kept out of contact with the rail, and as the back wheel on the outside of the curve rolls away from the rail there is very little friction of the flanges of the back driving-wheels.

It will also be noticed from fig. 160, that if the radius of the curve is very short, the bend of the rails between the back pair of driving-wheels and the centre of the truck is so great that the inside rail will press hard against the flange of the front or main driving-wheel next that rail. This of course produces a great deal of friction, and if the curve is excessively short the flange will mount on top of the rail and the tread of the opposite wheel will fall off from its rail. For this reason the centre-pin of the truck is sometimes arranged so that it can move laterally, that is cross-wise of the track. In fig. 160 the centre-pin is represented as having moved some distance from the actual centre of the truck, which is represented with dotted lines. The front wheels of locomotives are also sometimes made with wide “flat” tires, that is, tires without flanges, so that there will be no friction against the one rail and no danger of falling off the other.

Another action also takes place which facilitates the motion of the driving-wheels of ordinary engines around curves. Every one knows how easy the direction in which the front wheels of a common wagon can be controlled by taking hold of the end of the tongue or pole. With the leverage which it gives the wheels and axle can easily be directed wherever it is desired. A similar action takes place in an ordinary locomotive. The front driving-axles are guided by the truck, which is attached to the frame ten or twelve feet in front of the driving-axle, and thus the track exerts a leverage to guide the movement of the driving-axles, just as a common wagon can be guided by the pole.

If the locomotive is run backward, then none of these advantages exist, and the flange friction of the back driving-wheels is excessive. Engines such as construction locomotives, which run backward as much as forward, wear out the flanges of the back wheels very rapidly on crooked roads.

Question 261. What is meant by the “spread” of the wheels or axles?

Answer. It is the distance between the centres of two axles.

Question 262. What is the “wheel-base” of a locomotive?

Answer. It is the distance between the centres of the front and back or trailing-wheels. On ordinary engines, such as that illustrated in plate I, it is the distance from the centre of the front truck to the centre of the back driving-wheels.

Question 263. Is the “coning” of the tread of the wheels of much practical importance?

Answer. There is great difference of opinion regarding it, but even if its action is very beneficial, the advantage is very soon lost, owing to the wear of the wheels. It is, therefore, believed that the advantage is more apparent in theory than in practice.

Question 264. How are the driving-wheels of locomotives constructed?

Answer. They are made of cast iron with wrought-iron or steel tires around the outside. Fig. 161 represents a perspective view of a pair of locomotive wheels and axle. The central portion of the wheel, that is the hub, spokes and rim, are cast in one piece. Usually the hub and the rim, and sometimes the spokes, are cast hollow. The central portion of the wheel, that is the part which is made of cast iron, is called the wheel-centre.

Fig. 161.

Question 265. How are the tires fastened on the wheel-centres?

Answer. The insides of the tires are usually turned out somewhat smaller than the outside of the wheel-centre. The tire is then heated so that it will expand enough to go on the centre. It is then cooled off, and the contraction of the metal binds it firmly around the cast iron part of the wheel. As an additional security bolts or set-screws, a, a, fig. 161, are screwed through the rim and into the tire to prevent it from slipping off in case it becomes loose. In some cases the wheel-centre and the inside of the tire are turned conical, and the tires are then put on cold and held on with hook-headed bolts, C, as shown in fig. 162, which is a section of the tire and the rim of the wheel. The wheel-centre is made largest on the inside. As the strain against the flange of the tire is inward, the cone of the wheel-centre resists this strain. If it was curved or tapered the reverse way, the strain would come on the bolts, and it would also be impossible to remove the tires without first taking the wheels off the axles. This method of putting on tires has the advantage that they can be removed quickly and without heating the tires.[70]

Fig. 162. Scale 3 in. = 1 foot.

Question 266. Are there any standard sizes for the inside diameters of tires?

Answer. Yes. To avoid the great inconvenience arising from the diversity in the inside diameters of tires, the American Railway Master Mechanics’ Association has recommended that the inside diameter of tires should be made 36, 40, 44, 50, 56 and 62 inches. The thickness for the first three sizes to be 3 in. and the last three 2¹⁄₂ in.

Question 267. How are the driving-wheels fastened on the axles?

Answer. The hubs are accurately bored out to receive the axles, and the latter are turned off so as to fit the hole bored in the wheel. The axles are then forced into the wheel by a powerful pressure produced either with a hydraulic or screw press, made for the purpose. In order to prevent the strain upon the crank-pins from turning the wheels upon the axle, they are keyed fast with square keys driven into grooves cut in the axle and in the wheel to receive them. The ends of these keys are shown at b, fig. 161.

Question 268. How are the crank-pins made?

Answer. They are made of wrought iron or steel and accurately turned to the size required for the journals for the connecting-rods. Fig. 164 represents one of the main crank-pins, and fig. 163 a back pin for an American engine. The main pin has two journals, one, B, to which the main connecting-rod is attached, and the other, A, receiving the coupling-rod. The back pin has only one journal, A, for the coupling-rod.

Question 269. How are the crank-pins fastened to the wheels?

Answer. They are turned so as to fit accurately holes which are bored in the wheels, and are usually “straight” or cylindrical. The pins are then either driven in with blows from a heavy weight swung from the end of a rope, or else pressed in with a screw or hydraulic press. Sometimes the holes are bored tapered or conical and the pins turned to the same form. They are then ground in with emery and oil, so as to fit perfectly, and are secured by a large nut and key on the inside of the wheel.

Fig. 163.

Fig. 164.

Scale 1¹⁄₂ in. = 1 foot.

Question 270. What are the pieces A, A, fig. 161, between the spokes of the wheel for?

Answer. They are called counterbalance weights, and are put in the wheels to balance the weight of the crank-pins, connecting-rods and pistons. The principle of their action will be explained hereafter.

Question 271. On what part of the axle does the weight of the engine rest?

Answer. It rests on the driving-axle boxes L, fig. 161, which are placed just inside and close to the wheel.

Fig. 165.

Question 272. What are the driving-axle boxes for and how are they made?

Answer. They are cast iron blocks, L, fig. 161, which embrace and rest on the axle. The part of the axle on which the box bears is called the journal. Each box has a brass bearing, c, fig. 165, which bears on top of the journal and which is consequently exposed to the friction and wear. Fig. 165 is a perspective view of a driving-box, which shows what is called the oil-cellar, d. This is a receptacle underneath the axle which is filled with wool or cotton waste and saturated with oil for the purpose of lubricating the journal. The oil-cellar is held in its position by two bolts, f, f, which pass through it and the driving-box casting. By removing the bolts the oil-cellar can easily be removed and the box can then be taken off the axle.

Fig. 166.

Fig. 167.

 

Scale ¹⁄₂ in. = 1 foot.

Question 273. How are the truck wheels made?

Answer. They are made of cast iron, usually in one piece. Figs. 166 and 167 represent sections of two forms of wheels used for cars. Those used for locomotive trucks are similar to these, excepting that they are usually a little smaller in diameter. They are made with a disc or plate which unites the tire to the hub, and in some cases they have ribs cast in the inside, as shown in the two figures. Some are made with single and others with double plates, as shown in the engravings, and still others with spokes similar to the driving-wheels. The tread of the wheel is hardened by a process called chilling. This is done by pouring the melted cast iron into a mould of the form of the tread of the wheel. The mould is also made of cast iron, but being cold cools the melted iron very suddenly, and thus hardens it somewhat as steel is hardened when it is heated and plunged into cold water.[71]

Question 274. How are the boxes, journals and journal-bearings of the truck-wheels made?

Answer. They are very similar to those for the driving-wheels, their chief difference being that those for the truck-wheels are smaller than those for the driving-wheels.

Question 275. How are the frames for locomotives constructed?

Answer. The frames, H H H, plates I, II and III, are made of bars of wrought iron from 3 to 4 inches thick and about the same in width. They are usually made in two parts, the one at the back part of the engine, to which the driving-boxes and axles are attached, and the other at the front end, to which the cylinders are bolted. The back part, or main frame, as it is called, is represented in figs. 168 and 169, and consists of a top bar, H H, to which pieces, a, a′, b, b′, called frame-legs, are welded. Two of these form what is called a jaw, which receives the axle-box, as shown in fig. 169. To the bottom of each jaw a clamp, c, is bolted to hold the two legs together. The two legs, a and b′, are united by a brace, d d, welded to the bottom of the legs. A brace, m, unites the back end of the frame with the leg b, and is welded to each.

The front part of each frame consists of a single bar, e, which is bolted to the back end, as represented in figs. 168 and 169, which show the construction clearer than any description would. These front bars extend forward to the front end of the engine, and a heavy timber, called a bumper-timber, E′ E′, plates I, II and III, extends across from one to the other and is bolted to each of them. This timber is intended to receive the shock or blow when the locomotive runs against any object, such as a car. The cow-catcher or pilot, S, is fastened to this timber.

Fig. 168.

Fig. 169.

Scale ³⁄₈ in. = 1 foot.

The front bar of the frames also has usually two lugs or projections forged on it, between which the cylinders are attached. The latter are securely held in their position by wedges, which are driven in between the lugs and the cylinder castings.

The frames, as already stated, are made of wrought iron and are accurately planed off over their whole surface.

Question 276. How are the frames fastened to the boiler?

Answer. As already stated, they are fastened to the cylinders with wedges and bolts, and as the cylinders are bolted to the smoke-box the frames are thus rigidly attached to the front end of the boiler. In order to strengthen those portions of the frames which extend beyond the front of the smoke-box and to which the bumper-timber is attached, diagonal braces, r′ r′, plates I, II and III are bolted both to the timber and to each of the frames at their lower ends. The upper ends are bolted to the smoke-box. Other braces, d′, plate II, are also fastened to the frames and to the barrel of the boiler. The frames are fastened to the fire-box by clamps, I, I, plate I, called expansion clamps. These clamps embrace the frames so that the latter can slide through the former longitudinally. There are also usually two diagonal braces not shown in plate II, the upper ends of which are fastened to the back end of the shell of the fire-box at about the level of the crown-sheet, and the lower ends to the back ends of the frames. There are also usually transverse braces attached to the lower part of the frames, thus uniting the two together. The guide-yokes, j j, plate I, are also usually bolted to the frames and to the boiler. In many cases one only is used, which extends across from one frame to the other and is fastened to the boiler.

Question 277. Why are the frames attached to the shell of the fire-box so as to slide longitudinally through the fastenings?

Answer. Because when the boiler becomes heated it expands, and if it could not move independent of the frames its expansion would create a great strain on both itself and the frames. The fastenings to the fire-box are therefore made so that the frames can move freely through them lengthwise, but in no other direction.

Question 278. How much more will the boiler expand than the frames in getting up steam?

Answer. From ¹⁄₄ to ⁵⁄₁₆ of an inch.

Question 279. Why is it necessary to support the engine on springs?

Answer. [72]Because, however well a road may be kept up, there will always be shocks in running over it; these occur at the rail joints and especially when the ballasting of the ties is not quite perfect. These shocks affect the wheels first, and by them are transferred through the axle-boxes to the frame, the engine and the boiler. The faster the locomotive runs, the more powerful do they become, and therefore the more destructive to the engine and road, and consequently the faster a locomotive has to run the more perfect should be the arrangement of the springs.

If we strike repeatedly with a hammer on a rail, the latter is soon destroyed, while it can bear without damage a much greater weight than the hammer lying quietly on it. The axles, axle-boxes and wheels strike like a hammer on the rails at each shock, while the shock of the rest of the parts of the engine first reaches and bends the springs, but on the rails has only the effect of a load greater than usual resting on them. Another comparison will make still plainer the lessening by the springs of the injurious effect which the weight of the boiler, etc., exercises on the rails.

A light blow with a hammer on a pane of glass is sufficient to shatter it. If, however, on the pane of glass is laid some elastic substance, such as india-rubber, and we strike on that, the force of the blow or the weight of the hammer must be considerably increased before producing the above-named effect. If the locomotive boiler is put in place of the hammer, the springs in place of the india-rubber, and the rails in place of the glass, the comparison will agree with the case above. From this consideration it will be seen how important it is to make the weights of the axles, axle-boxes and wheels as light as possible.

Question 280. How are the driving-axle boxes arranged so that the weight of the engine will rest on springs?

Answer. They are arranged so as to slide up and down in the jaws. Springs, B, B′, fig. 169, are then placed over the axle-boxes and above the frames. These springs rest on ∩-shaped saddles, G′, G′, which bear on the top of the axle-boxes. The frames are suspended to the ends of the springs by rods or bars, g, g′, g, g′, called spring-hangers. As the boiler and most of the other parts of the engine are fastened to the frames, their weight is suspended on the ends of the springs, which, being flexible, yield to the weight which they bear.

Question 281. How are the frames protected from the wear of the axle-boxes which results from their sliding up and down in the jaws?

Answer. The insides of the legs, a, a′, b, b′, are protected with shoes or wedges, h, h, which are held stationary, and the box slides against the faces of the shoes, thus wearing the shoe or wedge but not the frame.

Question 282. Why are the shoes usually made wedge-shaped?

Answer. They are made in that way so that when they become worn, by moving one or both of them up in the jaws, the space between them is narrowed and the lost motion is taken up. They are moved by the screws i, i. If the boxes should become loose from wear, it would cause the engine to thump at each revolution of the wheels or stroke of the piston.

Question 283. How are the springs for the driving-wheels made?

Answer. They are made of steel plates which are placed one on top of the other. These plates are of different lengths, as shown at B, B′, in fig. 169, and are from 3 to 4 in. wide and ⁵⁄₁₆ to ⁷⁄₁₆ thick. The length of the springs measured from the centre of one hanger to the centre of the other is usually about three feet.

Fig. 170.

Fig. 171.

Question 284. What determines the amount which a spring will bend under a given load?

Answer. The number of plates, their thickness, length and breadth, and of course the material of which they are made. This can be explained if we suppose we have a spring plate of a uniform thickness, h, and a triangular form, of which fig. 170 is a side view and 171 a plan, and that it is clamped fast at its base, b. It is a well known mechanical law that any material of this form and under these conditions will have a uniform strength through its whole length to support any load, P, suspended at its end, and also that it will bend or deflect in the form of an arc of a circle.

Fig. 172.

Fig. 173.

Fig. 174.

Fig. 175.
Scale ³⁄₄ in. = 1 foot.

Question 285. How are locomotive springs usually made?

Answer. In locomotives the arrangement of springs is always such that they are either supported in the middle and moved at the two ends, or such that they are supported at the two ends and loaded in the middle; for our consideration it is indifferent which of the two kinds of springs is taken for the present illustration. That shown in plan and elevation in figs. 172 and 173, which is formed of a wide plate placed diagonally, and which in reality consists of two such triangular pieces as were represented in fig. 171 united at their bases m m, and loaded at two opposite corners, e and f, would answer the requirements mentioned if the great breadth, m m, were not an obstacle. This breadth is obviated by cutting the spring into several strips, a a, b b, c c, d d, ... i, fig. 173, of equal width, and placing these not side by side, but one over the other, as shown in figs. 174 and 175.

In order that the separate strips and layers of the spring so made, figs. 174 and 175, may not slip out of place, the strips a a, b b, etc., are made in one piece, and all the plates are inclosed with a strap, F, figs. 176 and 177. The plates, instead of being cut from a piece like that represented in fig. 173, are, however, made out of steel of the proper width, and the ends, instead of being cut off pointed as represented, are sometimes drawn out thinner on the ends, like the point of a chisel, or oftener still cut off straight, as shown in fig. 177.

Fig. 176.

Fig. 177.

Fig. 178.

Scale ³⁄₄ in. = 1 foot.

In order to hold the plates together a band, F, is put around the middle. This is put on hot, and becomes tight by contracting as it cools. The centre of the spring has a hole drilled through it with a pin, s, fig. 178 (which shows a cross section of a spring), to prevent the plates from sliding endwise. The plates at each end usually have a depression, a, fig. 179 (which is a cross section of a plate on a larger scale than the preceding figure), made in them on one side, and a corresponding elevation, b, on the other. The elevation on one plate fits into the depression on the other, and thus prevents the plates from slipping sideways.

Fig. 179.

Scale 6 in. = 1 foot.

Fig. 180.

Scale ³⁄₄ in. = 1 foot.

Question 286. How should springs be curved?

Answer. Springs should be curved so that when they bear the greatest load which they must carry they will be straight. If they are curved too much they are subjected not only to a strain which bends the plates, but to one which has a tendency to compress them endwise. Thus if a spring like that represented in fig. 180 is bent into a half-circle, it is obvious that the strain at the ends has no tendency at all to bend the plates, but only to compress them endwise. Near the middle the strain will of course bend the spring. In the one direction the spring is flexible and elastic, and in the other it is not; and as the strain of compression depends on the amount of curvature, the greater the latter is, the less flexibility and elasticity the spring will have.

Springs are often given a double curve, as shown in fig. 181. This is not to be recommended, because when a spring bends the plates must slide on each other. If they have but a single curve, they will do so and remain in contact through their whole length, but if they have two curves they will separate and therefore “gape,” as it is called.

Fig. 181.

Scale ³⁄₄ in. = 1 foot.

Question 287. What is meant by the elasticity of a spring?

Answer. It is the amount which a spring will deflect or bend under a given load without having its form permanently changed. If the bending is so great that the spring does not recover its original form when the load is removed, then the strain to which it is subjected is said to exceed the limits of elasticity, and if repeated often it will ultimately break the spring.

Question 288. What is meant by the elastic strength and the ultimate strength of a spring?

Answer. The elastic strength is the strain it will bear without being strained beyond the limits of elasticity, and the ultimate strength is the strain which will break it.

Question 289. What determines the strength of a spring?

Answer. It depends of course (1) upon the material of which the spring is made; (2) its strength increases in proportion to the number of plates, and (3) to their width, and (4) in proportion to the square of their thickness, and (5) as the length diminishes.

Thus, if we wanted to double the strength of a spring like that shown in figs. 170 and 171, it could be done in either of the following ways: (1) by making it of material twice as strong; (2) by putting another plate just like it on top; (3) by doubling the width of the base b, which would make the strength of the whole plate twice what it was before; (4) by making the whole plate about four-tenths thicker, which would increase its strength, as already stated, in proportion to the square of the thickness as 1.4 × 1.4 = 2 nearly; (5) by reducing the length to one-half what it is in fig. 170.

Question 290. What determines the elasticity of a spring?

Answer. (1) The material of which it is made; with the same material the elasticity increases (2) as the number, and (3) as the width of the plates diminishes, and (4) with the cube of the length, and (5) decreases with the cube of the thickness of plate.

Thus, supposing the plate in figs. 170 and 171 to be ³⁄₈ in. thick and the deflection d 2¹⁄₂ in.; the latter would be only half as much or 1¹⁄₄ in. (1) if it were made of material twice as stiff, or (2) with two such plates, or (3) with one twice as wide at the base. If (4) the length were doubled, the deflection would be equal to 2 × 2 × 2 = 8 times what it was before, or in proportion to the cube of the length. If (5) the thickness were doubled the deflection would be reduced in the same proportion, and would be only one-eighth of 2¹⁄₂ in., or ⁵⁄₁₆ in.

Fig. 182.

Fig. 183.

Scale ³⁄₄ in. = 1 foot.

Question 291. What should be the proportion of the plates of a spring in relation to each other?

Answer. The lower plates should diminish regularly in their lengths. The reason for this will be apparent from the fact which has already been stated, that if a triangular plate of uniform thickness is clamped fast at its base, it will, if loaded at the end, be of uniform strength throughout its whole length. It is immaterial what the length of the base of such a triangle is, if the two sides are of equal length and the thickness of the plate is uniform, not only its strength but the amount of deflection or bending from any load will be equal all through its length. If, therefore, we make a spring by cutting a plate formed of two such triangular pieces united at their bases into strips, as has already been explained, evidently the spring made of them will have a uniform strength throughout its whole length. As the strips thus made diminish in length regularly, it is evident that if the spring plates are made of steel rolled of the requisite width, their length should be the same as that of those cut from the plate referred to above. When this is the case the lower outline, a b b a, fig. 183, of the spring will, when the spring is not bent, be straight lines. Sometimes the lower outline of springs is made curved, as shown in fig. 182. This gives too much strength near the strap F, and too little near the ends. In drawing springs, therefore, it is best to lay them out with the plates straight, as shown in figs. 182 and 183, and after determining the thickness, drawing a straight line from the strap to the end of the longest plate will give the best form of the spring and the length of each of the plates. It is necessary, however, to put a sufficient number of long plates in each spring to give it the required strength next to the attachment of the hanger. Sometimes one or more of these long plates are made heavier than the rest. The evil of this method of construction will be apparent if it is remembered that the greatest permissible deflection up to the breaking of the spring decreases with the cube of the thickness of the plate and its strength increases with the square of the thickness. Now if we have a spring with say ten plates ³⁄₈ in. thick and one on top ³⁄₄ in. thick, the thick plate will have a strength four times that of the thin plates, but its elasticity will be only one-eighth that of the thin plates, and therefore it will require eight times as much load to bend it any given distance as is needed to bend the thinner plates the same distance. But its strength is only four times that of the thin plates, so that for any given amount of elasticity the thick plate must bear twice as much load as it has strength to carry. This shows what a great mistake is committed if some of the plates are made thicker than others, a conclusion which is supported by practical experience, as it is found that if the top plates are made thicker than others, the thick ones break most frequently, which is the necessary result of the supposed strengthening by increasing the thickness of the top plates.

Question 292. [73]How can we find by calculation the elasticity or deflection of a given steel spring?

Answer. By multiplying the breadth of the plates in inches by the cube of the thickness in sixteenths, and by the number of plates: divide the cube of the span[74] in inches by the product so found, and multiply by 1.66. The result is the elasticity in sixteenths of an inch per ton of load.

Question 293. How can we find the span due to a given elasticity and number and size of plates?

Answer. By multiplying the elasticity in sixteenths per ton by the breadth of plate in inches, and by the cube of the thickness in sixteenths, and by the number of plates: divide by 1.66, and find the cube root of the quotient. The result is the span in inches.

Question 294. How can we find the number of plates due to a given elasticity, span, and size of plate?

Answer. By multiplying the cube of the span in inches by 1.66; then multiplying the elasticity in sixteenths by the breadth of plate in inches, and by the cube of the thickness in sixteenths: divide the former product by the latter. The quotient is the number of plates.

Question 295. How can we find the working strength, that is the greatest weight it should bear in practice, of a given steel-plate spring?

Answer. By multiplying the breadth of plates in inches by the square of the thickness in sixteenths, and by the number of plates; multiply, also, the working span in inches by 11.3: divide the former product by the latter. The result is the working strength in tons (of 2,240 pounds) burden.

Question 296. How can we find the span due to a given strength, and number and size of plate?

Answer. By multiplying the breadth of plate in inches by the square of the thickness in sixteenths, and by the number of plates; multiply, also, the strength in tons by 11.3: divide the former product by the latter. The result is the working span in inches.

Question 297. How can we find the number of plates due to a given strength, span and size of plates?

Answer. By multiplying the strength in tons by the span in inches, and by 11.3; multiply also, the breadth of plate in inches by the square of the thickness in sixteenths: divide the former product by the latter. The result is the number of plates.

Question 298. How can we find the required amount of curvature or set of the spring before it is loaded?

Answer. By multiplying the elasticity, per ton, in inches, by the working strength in tons; add the product to the desired working compass. The sum is the whole original set, to which an allowance of ¹⁄₈ to ³⁄₈ in. should be added to the permanent setting of the spring.

Question 299. How are the spring-hangers attached to the ends of the springs?

Answer. A great variety of methods have been used. The most common ones are those shown in fig. 169. There the hanger embraces the spring at the ends, g, g, (shown on an enlarged scale at a, in figs. 176 and 177.) The end of the spring has two projections forged on its end to receive the upper end of the hanger, which is made to fit the groove thus formed between the two projections. The other end, b, of the spring, figs. 176 and 177, has an eye cut in it which receives the hanger b. The latter is made of a single bar, and also has an eye, c, to receive a key which sustains the weight suspended on the hanger b. The back end of the front springs and the front end of the back springs are made in this way because they come on the side of the fire-box, and if their width was increased by the thickness of the hanger, as shown at a in fig. 177, it would rub against and wear the outer shell of the fire-box.

Question 300. How are the lower ends of the hangers attached?

Answer. The front hanger, g, fig. 169, of the front spring, and the back hanger, g, of the back spring have eyes and pins in their lower ends, k, as shown in the engraving. The pins are supported by rubber springs, l, l, which are held between two concave castings, n, k, one of each of which rests against the frames. The object of the rubber springs is to relieve the spring-hangers from sudden shocks and strains. The benefit derived from their use is believed to be purely imaginary, as the spring itself, if sufficiently elastic, should absorb the sudden shocks which the wheels and axles will convey to the hangers.

Question 301. Why are the ends g′, g′ of the springs attached to the lever[75] A A?

Answer. Because if there is a spring for every axle and the hangers are fastened to the frame, then evidently the locomotive has as many points of support as it has axle-boxes. Every shock from the rails is transferred through the wheel and the axle to the nearest axle-box and the spring belonging to it, and the latter must be made strong enough to receive and dispose of the whole of it. If the adjacent hangers, g′, g′, fig. 169, of the adjoining springs, B and B′, are connected by an equalizing lever, A A, which turns on the fixed point C, then the shock which affects one wheel will be transferred first to the corresponding spring. From this spring a part of the shock will be transferred to the frame by the hanger g, and a part by the hanger g′ to the equalizer, which will transfer the pressure to the adjoining spring B′. If by some unevenness of the road or a powerful oscillation of the locomotive, a spring is momentarily burdened, the equalizer thus causes the next wheel to receive part of this load.

The advantages of this arrangement are evident: since the springs have to receive only a part of the shocks, they can be made less strong and therefore more flexible. The danger of running off the track and that of breaking axles, springs and hangers, is therefore reduced by the use of equalizing levers.

Question 302. How are the equalizing levers constructed?

Answer. They are made of wrought iron and are supported in the centre by a fulcrum, C, which is fastened to the frame or boiler or both. The spring-hangers g′, g′ are usually attached to the lever by eyes and keys. Sometimes eyes are made in the lever, as shown in fig. 169, and the hanger is inserted into the eye and held either with a key or else with projections which are forged on the hanger below the lever. In other cases the hangers are made with an eye which embraces the end of the lever.

Question 303. How is the distribution of weight of the engine affected by the equalizing levers?

Answer. The weight is equally distributed on all the driving-wheels. This is apparent if it is observed that the weight suspended from each of the spring-hangers of each spring in fig. 169 must be the same; for if the weights in the two hangers, g′ and g′, were unequal, then the end of the spring which supports the heaviest weight would be drawn down until the pressure was equalized. If the weights suspended from the two hangers, g′ and g′, attached to the equalizing lever were unequal, then the one supporting the greatest load would draw up its end of the equalizer until the weights were again in equilibrium.

Fig. 184.

Fig. 185.

Scale ³⁄₈ in. = 1 foot.

Another effect of the equalizing levers is that each side of the locomotive is supported in such a way that the action is the same as it would be if it was supported on one point. If, for example, we have a heavy beam, say a piece of timber like that shown by A B, fig. 184, suspended at one point, C, in its centre, to the middle, a, of a long spring, D E, the ends of which rest on two supports, F and G, it is evident that if the point of suspension is at the middle, C, of the timber and a of the spring, the weight of the timber will rest equally on the two supports, F and G, and that the ends of the timber can move up or down or vibrate about the point of suspension, C, without affecting the distribution of weight on the supports, F and G. If, now, the timber is suspended from three points, A, C and B, fig. 185, that is, its middle and two ends, as shown in fig. 185, the ends, A and B, being attached to the ends of the springs b c and d e, the latter resting on the supports F and G, and connected at their opposite ends to an equalizer, f g, whose fulcrum is at a, it is evident that each of the end hangers must support one-half of that part of the weight of the timber between it and the middle, and that the centre hanger must support one-half the weight between the middle and the two ends. Thus the hanger A b must support one-half the weight of the timber between A and C, and B e must support one-half of that between B and C; in other words, the end hangers would each sustain one-fourth of the weight of the timber and the middle one-half of its weight. If the weight of the timber is 1,000 pounds, the end hangers would each sustain 250 and the middle one 500 pounds. The weight of the middle of the timber is hung on the equalizer, and one-half, or 250 pounds of it is thus transferred to each of its ends f and g, and thence to the hangers f c and g d, and thus to the springs, so that the ends, c and d, of the springs, sustain a weight of 250 pounds, therefore, as the opposite ends also sustain the same weight, it is evident that each of the springs bears a total load of 500 pounds, or one-half of the weight of the timber, which is the same load they sustained in fig. 164. If the ends of a timber supported as shown in fig. 185 are moved up or down about the centre point of suspension, it is evident that the distribution of weight would not be affected any more than it was in fig. 164 by a similar movement, because if the ends of the timber move as shown by the dotted lines around the centre point of suspension C, the end A will ascend as much as B descends. The same thing is true of the ends b and e of the springs and of their opposite ends c and d, and also of the ends of the equalizer, so that when the timber, springs and equalizer are in the position shown by the dotted lines, it is in equilibrium, just as it was when the timber was horizontal; and therefore the weight on the supports is the same in both cases, thus showing that the load A B can move about the centre of suspension when supported as shown in fig. 185 as freely as it can if arranged as shown in fig. 184. It therefore follows that in the distribution of the weight of each side of the locomotive on the wheels and on the track, it may be regarded the same as though it was supported at one point, which is the fulcrum of the equalizing-lever.

Question 304. What advantage results from supporting the weight of the back part of the locomotive on two points?

Answer. If the back part of the locomotive rests on only two points and the front end on the centre of the truck, then the whole weight of the engine will be sustained on three points. Now it is a well known fact that any tripod, like that on which an engineer’s level is mounted, or a three-legged stool, will adjust itself to any surface, however uneven, and stand firmly in any position; whereas if there are more than three points of support, if they are all of the same length the surface on which they rest must be a plane, otherwise some of them will not touch. All railroad tracks have inequalities of surface, and therefore it is of the utmost importance that a locomotive should be able to adjust itself on its points of support to any unevenness of the track on which it must run. This is possible only when the weight rests on three points of support.

Fig. 186.

Fig. 187.

Fig. 188.
Scale, ³⁄₈ inch = 1 foot.

Question 305. How is the truck constructed?

Answer. It consists, as has already been stated, of two pairs of wheels.[76] These are attached to a frame, b′ b′, plates I, II and III. The axles have boxes called truck-boxes, and brass bearings similar to those used on the driving-axles. These boxes work in jaws, also similar to those on the main engine frame, excepting that they have no attachment to prevent them from being worn by the motion of the boxes up and down in the jaws. Fig. 186 is a horizontal section, fig. 187 a plan, and fig. 188 a transverse section[77] of a truck. The frame C D E F, fig. 187, shown also at h′ h′, figs. 186 and 188, is of rectangular form and is forged in one piece. The legs f f which form the jaws for the boxes, are bolted to the frame as shown in fig. 186. To the lower end of these legs a brace, g g g, is bolted, which thus unites them together. On each side one spring, S F S, is placed under the frame and in the reverse or inverted position to that of the driving-springs. A pair of equalizing levers, G G G, is placed on each side of the truck, one of them on the inside of the frame and the other on the outside, as shown in the plan. The ends of these equalizers rest on the top of the truck-boxes, and the springs are attached to the levers at i i by the hangers, j j. The truck-frame rests on the top of the spring-strap, F, which is made of the form of an arc of a circle, or “rounded,” as it is termed by workmen, so that it can move freely about the point of support. It is evident that this arrangement of spring and equalizer operates in the same way as that employed for the driving-wheels in distributing the weight on each of the wheels, and that the truck-frame is supported on two points, k, k, figs. 186 and 187. The weight of the front end of the engine rests on a cast iron centre-plate, H H. This centre-plate rests on four bars, l l, l l, and m m, m m, two of which are bolted to the frame transversely and the other two longitudinally, as shown in the plan. These bars are elevated in the centre as shown in figs. 186 and 188. The transverse bars are trussed with two corresponding bars, n n, fig. 188, below. These truss-bars as they are called, are bolted to the upper bars with bolts, o, o, but are separated from the top-bars by distance pieces, P, P, figs. 186 and 188. The centre-plate H H, called the lower centre-plate, has an annular groove in it, which receives a corresponding projection on the casting K K, called the upper centre-plate, which is bolted to the bed-plate of the cylinders, as shown in plate II. The upper centre-plate has a pin, y, called a centre-pin, fig. 186 and plate II, attached to it, which passes through the lower centre-plate, and has a key underneath the latter plate. This key is intended to prevent the engine from “jumping” off of the truck on a rough track or in case of accident. The annular groove and the projection which fits into it are intended to receive the strain which otherwise would bear against the centre-pin and would be liable to break or bend it.

From this description it will be seen that while the truck-frame rests on two points, k and k, the weight of the engine is supported by the centre-plate of the truck. As the back part substantially rests on the centres of the two equalizers, it will be seen that this distribution of the weight fulfills the conditions of the tripod, or as it has been called, the “three-legged principle.”

Fig. 189.

Fig. 190.

 

 

Fig. 191.

 

Scale ³⁄₈ in. = 1 foot.

Question 306. How are trucks arranged so as to give them lateral motion?

Answer. When this is done, the lower centre-plate is usually suspended in some way from the truck-frame on links or hangers, so that it can swing laterally. One method of doing this is shown in figs. 189, 190 and 191. Fig. 190 is a front view, fig. 191 a plan, and fig. 189 a transverse section of such an arrangement. The centre-plate H H has cast with it an extension, B B, the ends of which are suspended on links, L, L, called suspension-links, the upper ends of which are attached to bars, m, m, which are set edgeways and extend across the truck-frames. It is evident that with this arrangement the lower centre casting can swing crosswise of the track on the links L, L, and that the front end of the engine will thus have a lateral motion independent of the truck.

PART XIII. ADHESION AND TRACTION.

Question 307. What is meant by the “adhesion” of a locomotive?

Answer. It is the resistance which prevents or opposes the slipping of the driving-wheels on the rails, and is due to the friction of the former on the latter.

Question 308. On what does the amount of this friction depend?

Answer. Like all friction it depends upon the weight or pressure of the surfaces in contact, and consequently upon the load which rests on each wheel. It also depends upon the condition of the rails, and probably to some extent upon the material of which they and the tires on the wheels are made.

Fig. 192.

Scale ¹⁄₄ in. = 1 foot.

Question 309. How much force is required to make the driving-wheels of a locomotive slip on an ordinary railroad track?

Answer. The force required to make them slip will, as already stated, vary very much with the condition of the rails. If they are quite dry and clean it will require a force equal to about one-fourth the weight on the wheels. That is, supposing we have a wheel, A B, fig. 192, attached to a frame which is fastened so that it cannot move, and that the wheel rests on a rail and is loaded with say 10,000 pounds, if now a rope or chain could be attached at a point, B, exactly at the tread of the wheel, and carried over a pulley, C, then it would require a weight, D, of about 2,500 pounds attached to the end of the rope to make the wheel slip. If the rails were sanded, the adhesion would be somewhat greater, and if they were wet or muddy or greasy, considerably less. For ordinary circumstances the adhesion may safely be assumed to be one-fifth of the weight of the driving-wheels. Of course the total weight on all the driving-wheels must be taken in calculating the adhesion. Thus, if a locomotive has four driving-wheels and each one of them bears a load of 10,000 pounds, then the total weight on the driving-wheels, or adhesive weight, as it is called, will be 10,000 × 4 = 40,000 pounds, and the adhesion will be

Question 310. What is meant by the tractive power of a locomotive?

Answer. It is the force with which the locomotive is urged in a horizontal direction by the pressure of the steam in the cylinders, and which therefore tends to move the locomotive and draw the load attached to it.

The tractive power is of course due to the pressure of steam on the piston, and therefore its amount is dependent upon the average steam pressure in the cylinder, on the area of the piston, and also on the distance through which the pressure is exerted, or, in other words, on the stroke of the piston. Thus if we have a cylinder 16 inches in diameter and two feet stroke and an average steam pressure of 50 pounds per square inch, then as the area of such a piston would be 201 square inches, the average pressure on it would be 201 × 50 = 10,050 pounds, and as each piston moves through four feet during one revolution of the wheels, the number of foot-pounds of energy exerted by it would be 10,050 × 4 = 40,200, and for the two cylinders of a locomotive double that amount, or 80,400 foot-pounds. Now if the driving-wheels are five feet in diameter, their circumference will be 15.7 feet, and therefore the locomotive will move that distance on the rails during one revolution, if the wheels do not slip. The 80,400 foot-pounds of energy is therefore exerted through a distance of 15.7 feet, and therefore

which is the force exerted through each foot that the circumference of the wheel revolves and the locomotive moves. If the wheels were only half the diameter, or 2¹⁄₂ feet, then their circumference would be 7.85 feet and the tractive power would be

or double what it was before. It will be seen, then, that the tractive force of a locomotive is dependent upon (1) the average steam pressure in the cylinders, (2) the area, (3) the stroke of the pistons, and (4) the diameter of the driving-wheels.

Question 311. How is the tractive power of a locomotive calculated?

Answer. By multiplying together the area of the piston in square inches, the average steam pressure in pounds per square inch on the piston during the whole stroke, and four times the length of the stroke of the piston,[78] and dividing the product by the circumference of the wheel. The result will be the tractive power exerted in pounds. The adhesion must of course always exceed the tractive force, otherwise the wheels will slip.

Question 312. How is the locomotive made to advance by causing the wheels to revolve?

Answer. The pressure of steam in the cylinders is exerted in one direction against the piston, and in the opposite direction against the cylinder head, as shown in fig. 192, in which the steam is represented by the dotted shading in the back end of the cylinder, and the direction of the pressure by the darts s, s. The pressure against the piston is communicated by the connecting-rod to the crank-pin E, and that on the cylinder-head is carried to the axle by the frame F F′, and the direction of the two forces is indicated by the two darts, a and b. We may now regard the spokes of the wheels as acting as levers, and assume that the fulcrum is either at the centre G of the axle, or at B, the point of contact of the wheel with the rail.[79] We will assume that it is at the centre G of the axle and for the sake of even figures that the wheel is six feet in diameter and cylinders have two feet stroke. We will also suppose that the engine is supported so that the wheels do not touch the rails, and that a chain or rope passing over a pulley C is attached to the wheels at B and with a weight at D. We now have a force, a, of 10,000 pounds exerted on the crank-pin, or at the end of the short arm E G of the lever E G B. As E G is one foot and G B three feet long, 10,000 would be balanced by

at B. In other words, it would require 3,333 pounds suspended from the chain at D to resist the strain at E. But when this is the case, the pressure of the axle at the fulcrum, in the direction of the dart c, is equal to the pressure against the crank-pin E added to that exerted by the weight D at B, or 10,000 + 3,333 = 13,333 pounds.

As the pressure against the axle in the opposite direction, b, is only 10,000 pounds, there will be an unbalanced force of 3,333 pounds acting in the direction of the dart c, and tending to move it that way. As the axle is attached to the locomotive frame, this force will of course have a tendency to move the whole machine, and is really the tractive force of the engine.

If, on the other hand, we regard the point of contact B of the wheel with the rail as a fulcrum, we have a force of 10,000 pounds acting against a lever, E G B, four feet long. There would therefore be a force, c, of

exerted at G, and as the pressure in the direction of the dart b is only 10,000, there would be an unbalanced strain of 13,333 - 10,000 = 3,333 pounds acting against the axle in the direction of the dart c, or, in other words, there is 3,333 pounds more of force pulling the axle forward than there is pushing it backward.

When the crank-pin is below the axle, in the position shown in fig. 193, then if the centre of the axle is regarded as the fulcrum we have a pressure of 10,000 pounds pushing against the front cylinder-head, which is transferred to the axle by the frames, and acts in the direction of the dart c, and we also have a pressure against the crank-pin E in the direction of the dart a. Now 10,000 pounds at E would exert a force of

at B and

at G in the direction of b. But the pressure on the cylinder-head pulls against the axle G in the direction c with a force of 10,000, so that the excess of strain in the direction b will be equal to 10,000 - 6,666 = 3,333 pounds. If we regard B as the fulcrum, the calculation is exactly the same, as 10,000 pounds at E exerts a force at G equal to

and the difference between it and the pressure exerted by the strain against the front cylinder-head is the force which urges the axle and with it the locomotive forward.

It will be seen, then, that it is immaterial which point is regarded as the fulcrum, as the result of the calculations is exactly the same.

Fig. 193.

Scale ¹⁄₄ in. = 1 foot.

It must not, however, be hastily supposed from what has been said that the total pressure against the axle can be greater than its resistance to the pressure. As soon as the one exceeds the other it will move. But supposing that it requires a force equal to 3,333 pounds to draw a train coupled to the engine, as soon as the difference between the force exerted against the axle by the piston to move it forward and that which presses it back exceeds 3,333 pounds, the locomotive will move the train. If it continues to exceed it the speed of the train will be accelerated and thus the resistance which holds the engine back and that which pushes it forward will always be equal.

Question 313. Does the fact that the piston is working from the end of a longer lever E G B, fig. 192, when the crank-pin is above the axle, enable the locomotive to start a heavier train than when the crank-pin is below the axle and the piston is working against a shorter lever, G E B, fig. 193?

Answer. No; because, as has already been shown, the pressure against the axle is the same in both cases. It is in fact only during the forward stroke that the pressure on the crank-pin moves the engine forward. The forward pressure which is exerted by the crank-pin at the axle is then greater than that exerted against the latter in the opposite direction by the cylinder-head and frames. It is this excess of crank pressure which moves the engine and which is the tractive force during the forward stroke. During the backward stroke the piston is pushing the axle backward, and the pressure against the front cylinder-head is pulling it forward. The latter then exceeds the former, and the difference between the two is the force which moves the engine forward. As has been shown, this difference is the same in both positions of the crank, and therefore the locomotive can not from this cause pull more when the crank is above the axle than when it is below.

PART XIV. INTERNAL DISTURBING FORCES IN THE LOCOMOTIVE.

Question 314. What are the internal disturbing forces in a locomotive?

Answer. They are: 1, the momentum of the parts which have a reciprocating motion; 2, those due to the varying pressure of the steam in the cylinder-heads; 3, those caused by the thrust of the connecting-rods against the guide-bars; and 4, those produced by unbalanced revolving parts.

Question 315. How can the effects of these disturbing forces be neutralized?

Answer. By putting counterweights, A, A, fig. 161, in the driving-wheels opposite the crank-pins. The motion of these will then be in the reverse direction to that of the weight of the parts attached to the crank-pins, and the motion of the one will thus, to some extent, at least, neutralize the disturbing influence of the other.

Question 316. Can the weight of these counterweights be calculated for any locomotive?

Answer. It can probably be calculated, but it is an exceedingly complicated problem, and one about which there is much difference of opinion. The difficulty is also increased by the fact that while counterweights may be heavy enough for one speed, they may be too heavy or too light for a slower or faster speed, and quite disproportioned when the engine is not working steam. The following rules are given in “Clark’s Railway Machinery,” and are perhaps sufficiently close to find a first approximation to the requisite position and weight of the counterweights; but the final adjustment should be made by trial. This can be done by suspending the locomotive by chains attached to the four corners of its frame, and setting the machinery in motion at the speed it is intended to run. By attaching a pencil to one or to each of the four corners of the frame, and arranging it so that it will mark on a horizontal fixed card, a diagram will be drawn, being usually an oval, which will show the amount and form of the oscillations. The counterweights can then be adjusted so that the diagram drawn by the pencil is reduced to the least possible size. When the adjustment is successful, the diameter of the diagram is reduced to about ¹⁄₁₆ of an inch.[80] Another and simpler, but less accurate, way is to place a pail or other vessel filled with water on the front of the engine and run the locomotive on a smooth track at a high speed, and adjust the counterweights so that the least amount of water will be spilled.

Question 317. How can the centre of gravity of a counterweight in one segment be found?

Answer. By cutting a wooden templet of uniform thickness to the form of the surface, and freely suspending it by one of the corners, a, as in fig. 194; a plummet-line, P a, dropped from the same point of suspension in front of the templet will intersect the centre line b c at the centre of gravity C.

Fig. 194.

Question 318. How can the centre of gravity of a counterweight in three segments be found?

Answer. Find the centre of gravity C, fig. 195, of one of the counterweights, as above; through C strike an arc from the centre, a, of the wheel, crossing the centre lines of the other segments at their centres, C′ C″; draw C′ C″ meeting A B at D, and set off D E, one-third of the interval D C. Then E is the common centre of gravity of the three segments.

Fig. 195.

Question 319. How can the centre of gravity of a counterweight in two segments be found?

Answer. This is required when the crank is opposite to a spoke, as in fig. 196. Find the centre of gravity, C, of one segment as before, and by an arc find the other centre C′; draw C C′, cutting A B at D, which is the common centre of gravity.

Fig. 196.

Question 320. How can the centre of gravity of a counterweight in four segments be found?

Answer. Find, as before, the centres C, C′, C″, C‴, fig. 197, of the segments; draw C″ C′ and C‴ C, cutting the line A B; bisect the interval so inclosed at E for the common centre of gravity.

Fig. 197.

Question 321. How is the counterweight for outside-cylinder engines with a single pair of driving-wheels calculated?

Answer. Find the total weight, in pounds, of the revolving and reciprocating masses for one side, namely, the piston and appendages, connecting-rod, crank-pin and crank-pin boss; multiply by the length of crank in inches, and divide by the distance in inches of the centre of gravity of the space to be occupied by the counterweight. The result is the counterweight in pounds, to be placed exactly opposite to the crank.

Question 322. How is the counterweight for outside-cylinder engines with coupled driving-wheels calculated?

Answer. Find the separate revolving weights, in pounds, of crank-pin, crank-pin boss, coupling-rods and connecting-rod, for each wheel, also the reciprocating weight of the piston and appendages, and half the connecting-rod; divide the reciprocating weight equally between the coupled wheels, and add the part, so allotted, to the revolving weight on each wheel; the sums so obtained are the weights to be balanced at the several wheels, for which the necessary counterweight may be found by the preceding rule.

Question 323. How are the counterweights constructed?

Answer. They are usually made of cast iron, so as to fill or partly fill the spaces between the spokes of the wheel, as shown at A A in fig. 161. Each of the segments consists of two pieces, one of which is put in from the outside of the wheel and the other from the inside. The two are then bolted together with bolts, which are shown in the above figure. In some cases the counterweights are cast solid with the wheels, and in others, cavities are cast in the wheels which are filled with lead, which is poured in when melted. Some locomotive builders make the rims and spokes of the wheels hollow and fill them with lead opposite to the crank-pins. It is difficult, however, to get the required weight in such wheels unless they are of large diameter.

Question 324. What effect has the strain on the draw-bar, when the engine is pulling a load, upon the distribution of the weight on the wheels?

Answer. It has the effect of pulling the back end of the engine down, and, as it is balanced on the fulcrums of the equalizing levers, it at the same time lifts up the front end. In this way the harder a locomotive is pulling the greater will be the weight which is thrown on the driving-wheels, and that on the truck will be correspondingly diminished. The higher the draw-bar is above the rails the greater will be the tendency to pull the engine down behind and up in front.

PART XV. MISCELLANEOUS.

Question 325. What is the sand-box, V, plates I and II for, and how is it constructed?

Answer. It is intended to carry a supply of dry sand, which is scattered on the rails in front of the driving-wheels when the latter are liable to slip. This is done by two pipes, e′, e′, plate I, one on each side of the engine. They lead from the sand-box to within a few inches of the rail. At the upper end and inside the sand-box they each have a valve which is operated by a lever which is connected to the cab by a rod which enables the locomotive runner to open or close the valve at pleasure.

The sand-box is usually made of sheet iron with a cast iron base, and a top of a more or less ornamental design. It also has an opening on top through which the sand is supplied to the box. This opening has a loose cover to exclude rain and dirt from the sand. The sand-box is usually located on top of the boiler in front of the front driving-wheels.

Question 326. What is the bell U for?

Answer. It is used for giving signals of the starting or approach of the engine. It also is located on top of the boiler and is usually hung on a cast iron frame and rung with a rope connecting it with the cab.

Question 327. What is the weight of an ordinary locomotive bell?

Answer. From 50 to 100 pounds.

Question 328. What is a locomotive head-light?

Answer. It is a large lamp, T, plates I and II, placed in front of the locomotive to signal its approach at night and also to illuminate the track for the locomotive runner.

Fig. 198.

Scale 1¹⁄₂ in. = 1 foot.

Question 329. How is a head-light constructed?

Answer. The lamp has what is called an Argand burner; that is, a burner with a hollow cylindrical wick, through the centre of which a current of air circulates, which thus supplies the flame with a larger quantity of air than is possible if the latter can come in contact with the former only from the outside. The result is that the combustion is much more brilliant than with ordinary burners. In order to throw all the light on the track the burner is placed inside of a concave reflector, a b c, fig. 198, which is of a parabolic form. One of the peculiarities of this form of reflector is that if a light is placed in its focus f the rays will be reflected from its surface in parallel lines. Thus, let a b c, fig. 198, represent a section of such a reflector. Now, if a light be placed in the focus f the rays will strike against the reflector, be thrown in the direction of the dotted lines f 1 x, f 2 x ... f 9 x, etc., and thus be thrown directly in front of the engine. The reflectors are usually made of copper and plated with silver.

The lamps and reflectors for head-lights are inclosed in a rectangular case which is supported on two brackets bolted to the front of the smoke-box. On these brackets a wooden shelf is fastened on which the head-light rests.

Question 330. What are the running-boards and hand-rails?

Answer. The running-boards are planks, i i, plate I, placed on each side of the boiler to enable the locomotive runner or fireman to go from the cab to the front end of the engine when it is running. The hand-rails, m m′ m′, are brass or iron pipes attached to the top of the boiler and extending from the cab to the smoke-box, and are placed there, as their name indicates, to support or for a hand-hold for persons on the running-board.

Question 331. What provision is made for removing from the track such obstacles as cattle or fallen rocks, which may be in front of locomotives?

Answer. What is called a cow-catcher or pilot, S, plates I and II, is attached to the front of the locomotive. This is usually made of wood, and consists of a triangular frame at the bottom which is supported about four inches above the tops of the rails. Straight pieces of wood of about 2¹⁄₂ × 4 inches section are fastened to this frame and also to a horizontal piece which is bolted to the bumper-timber E′. These pieces arranged in this way and only a few inches apart give to the cow-catcher a peculiar curved form somewhat resembling that of the mould-board of a plow, which is very well adapted for throwing any obstacles from the track. Sometimes these pieces are placed horizontally instead of being inclined up and down. Cow-catchers are also in some cases made of round iron bars or angle iron. They are always bolted securely to the bumper-timber and strengthened by strong iron braces attached to the bottom frame at the front and back. These braces are usually fastened at the other end to the bumper-timber, but are sometimes attached to the bed-plates of the cylinder.

There is also usually a strong pushing-bar attached with a bolt and hinged joint to the bumper-timber. This is shown in plates II and III in the position it occupies when not in use. It is used in pushing cars, as very often there is not room for the pilot under the end of the car. In using it is raised up, and the front end is then coupled to the draw-head of the car.

Fig. 199.

Question 332. What is the foot-board or foot-plate of a locomotive?

Answer. It is a wrought or cast iron plate which extends across and rests upon the two frames at the back part of the locomotive and behind the boiler, and on which the locomotive runner and fireman stand. It also unites the two frames very securely, and furnishes an attachment for the draw-bar. The foot-board is often made much heavier than is necessary for strength, in order to increase the weight, and thus the adhesion, on the driving-wheels. It is a fact often not suspected that any weight placed on the back end of an ordinary locomotive will increase the load on the driving-wheels by an amount considerably greater than that of the weight itself. The reason of this is that the locomotive rests on the centre of the truck and the centres of the equalizers, and therefore the weight, if applied to the back end of the engine, gains considerable leverage. This will be clear if we take a beam, A B, and rest it on two supports, m and n, fig. 199. If now we put a weight W on the end, overhanging the point of support, the weight which will rest on n will be equal to that of W multiplied by its distance C from m and divided by the distance D between m and n. Thus if a foot-board weighs 1,000 pounds, and its centre of gravity is 5¹⁄₂ feet behind the centre of the equalizer, and the latter 14 feet from the centre of the truck, then the weight thrown on the driving-wheels will be equal to

The same thing is of course true of any other weight placed on the back end of the engine.

Question 333. What are the “wheel-guards” of a locomotive?

Answer. They are sheet iron covers over the upper half of the periphery or tread of the wheels, and are placed there to protect the engine from the dirt and mud which adhere to the wheels and are then thrown off on the machinery by the centrifugal force.

Question 334. What are “check” or “safety chains?”

Answer. There are two kinds of such chains, the one attached to the trucks and frames of the locomotive and the tender. The object of these chains is to prevent the trucks from turning around and getting crosswise of the track if the trucks should leave the rails. The other kind of safety chains connects the engine to the tender, so that, in case the draw-bar or coupling-pins should break, the two will not separate. Great care should be exercised to attach the truck chains so as to be strong enough to resist the strains to which they will be subjected in case the trucks run off the track. The grossest carelessness and ignorance are often shown in the construction of these parts.

PART XVI. SCREW-THREADS, BOLTS AND NUTS.

Question 335. How must the screws of bolts and nuts be made, in order to fit each other?

Answer. Each size of screw must be made of exactly the same diameter, and their threads of the same form and proportions and pitch.

Question 336. What is meant by the “pitch” of a thread?

Answer. It is the distance the thread progresses lengthwise of the screw in one revolution. Thus if a single-threaded screw has one-eighth of an inch pitch, it means that the threads are ¹⁄₈ of an inch apart measured from the centre of one thread to the centre of that next to it, and therefore there are eight threads to each inch in length of the screw.

Question 337. What is meant by a “single-threaded” screw?

Answer. It means a screw with but one thread instead of two or more. Thus if we take a string and wind it around a pencil, it will represent a single-threaded screw, and if we take two or three strings and wind them parallel to each other, they will represent a double or treble-threaded screw. The latter kinds are seldom or never used on locomotives, so that in the following discussion only single-threaded screws will be referred to.

Fig. 200.

Full Size.

Question 338. What is the usual form of the threads of screws?

Answer. The most common is what is called the V-thread, represented in fig. 200, which is made sharp at both the top and bottom. If such a thread for one screw is made very pointed and that for another is blunt, it is plain that the nut for the one will not fit the other accurately, and also that if a nut has eight threads to the inch, it will not fit on a bolt with nine. Owing to the fact that, until within a few years, no common standard has been agreed upon for the form, proportions or pitch of screws there has been very great diversity in these respects in the screws which have been used in the construction of locomotives and other machinery. In 1864 the inconvenience and confusion from this cause became so great that it attracted the attention of the Franklin Institute of Philadelphia, and a committee was appointed by that association to investigate and report on the subject. That committee recommended the adoption of the Sellers system of screw-threads and bolts, which was devised by Mr. William Sellers, of Philadelphia. This same system was subsequently adopted as the standard by both the Army and Navy Departments of the United States, and then by the Master Mechanics’ and Master Car Builders’ associations, so that it may now be regarded, and in fact is called, the United States standard, but the design is due to Mr. Sellers.

Question 339. In establishing a standard system of screws and threads, what is the first thing which must be determined?

Answer. The number of threads to the inch, or the pitch of the threads for screws of different diameters.

Fig. 201. Full size.

Question 340. What is the standard for the number of threads to the inch for the different sized screws of the Sellers system?

Answer. The number of threads with their other proportion is given in the table at the end of this chapter on page 348.

Question 341. What is the form of the thread of this standard?

Answer. The form is shown in fig. 201, and is similar to the V-thread, excepting that it is flattened at the top and bottom.

Question 342. What are the reasons for the adoption of this form of thread?

Answer. It has already been pointed out that if a screw is made with a “blunt” thread it will not fit a nut with very acute or “sharp” threads; or, if the thread of the bolt was acute and that in the nut obtuse, they would fit imperfectly. It is therefore necessary in a standard system to fix upon the angle which the sides of the thread shall bear to each other. This in the United States standard system was determined by Mr. Sellers at 60 degrees, because that angle is easily laid off without special instruments[81] and is perhaps as good as or better than any other form for the threads.

It is obvious that if a tool is ground with its sides at an angle of 60 degrees to each other, if the point is made sharp after a very little use it will be worn more or less so that the bottom of the thread will not be cut perfectly sharp, and therefore it will be difficult to make bolts and nuts with sharp threads fit each other accurately. It is also plain that the sharp edge of a thread gives very little strength to the screw, and yet diminishes that of the bolt very materially. It will also be impossible to measure the diameter of the screw at the bottom of the thread if it is made sharp, as its depth will vary as the point of the tool wears, and it is almost impossible to measure the diameter of such a screw accurately with ordinary calipers. A sharp-edged thread in a bolt is also very liable to be injured and bruised by coming in contact with other objects. To obviate these evils the standard threads are therefore made flat on the top, and it is evident that a similar shape at the bottom will give increased strength to the bolt as well as conform to and fit the thread in the nut. To give this form requires only that the point of the cutting tool shall be taken off, and then it is evident this form of thread can be cut in a lathe with the same tool and in the same manner as the sharp thread. The width of the flat top and bottom should of course bear a definite proportion to the size or pitch of the thread.

Fig. 202.

Question 343. What are the proportions of the standard threads?

Answer. The rule given by Mr. Sellers for proportioning the thread is as follows: “Divide the pitch, or, what is the same thing, the side of the thread, into eight equal parts; take off one part from the top and fill in one part in the bottom of the thread: then the flat top and bottom will equal one-eighth of the pitch, the wearing surface will be three-quarters of the pitch and the diameter of screw at bottom of the thread will be expressed by the formula:

In order to make the form and proportions of this standard thread as plain as possible, we have had an enlarged diagram, fig. 202, engraved, so as to represent the different parts clearly. x represents the pitch, d the diameter of the screw in inches, v the number of threads to the inch, n the diameter at bottom of thread, m the width of back part of thread at the top and bottom, and s the length of the side of thread.

From Mr. Sellers’ rule the following formulæ can be deduced:

From this rule any thread can be constructed, it being only necessary to know the pitch or number of threads to the inch. This, with all the dimensions of the standard threads for bolts from ¹⁄₄ to 2 inch diameter, is given in the table on page 348.

For practical use in the shop a gauge like that shown in fig. 203 will be found most convenient for grinding the tools to the proper form for making the standard screws. With this gauge the screw-cutting tool can first be ground to the proper angle by fitting it to the deepest notch, and the requisite quantity should then be taken off the point by fitting it to the notch representing the form of thread for the sized bolt or number of threads to the inch which it is intended to cut.

Wherever this standard for threads is used, if any pretense at all is made to accuracy of workmanship, careful attention should be given to the form and proportion of the threads as well as to the number to the inch. In buying taps and dies the purchaser should see that they conform in every respect to the standard, and in making specifications for new work similar care should be exercised to secure the true standard, form and proportion of screws. In many shops the workmen who have the care of those tools are entirely ignorant of the peculiarities of the Sellers system, and have only the vague idea that so long as they get the proper number of threads to the inch they are doing all that is necessary to secure uniformity. Unless, therefore, some care is exercised to insure accuracy of workmanship in this department, the adoption of a “standard” for screws will not insure the advantages which would result from uniformity of screws and threads.

Fig. 203. Full size.

Question 344. How thick must a nut be, measuring lengthwise of the bolt, so that the thread in the nut will be of equal strength to that of the bolt?

Answer. Its thickness must be equal to the outside diameter of the screw.

TABLE GIVING PROPORTIONS OF THE UNITED STATES OR SELLERS STANDARD SYSTEM OF SCREW-THREADS.

PART XVII. TENDERS.

Question 345. What are locomotive tenders for?

Answer. They carry a supply of fuel and water for locomotives while they are running.

Question 346. How are they usually constructed?

Answer. Their construction is represented in figs. 204, 205 and 206. Fig. 204 is a side view, fig. 205 a longitudinal section, and fig. 206 a plan of an ordinary tender, which consists of a frame, A A A A, made of wood or iron,[82] mounted on a pair of trucks, T, T. The top of the frame is covered with planks, B, B, which form the floor of the tender. On top of this floor a sheet-iron tank, C C C, is placed, which carries the supply of water. This tank is made somewhat in the form of a letter ⊃, as shown in the plan. It is made in this way so that the space between the two branches, C, C, or “legs,” as they are called, will give room for fuel. Around the upper edge of the tank a sheet-iron rim, D D, is riveted, so as to prevent the fuel from falling off when it is filled up above the top of the tank.

Question 347. How is the tank filled with water?

Answer. There is a round opening, E, called a man-hole, on top. Into this the end of a leather or canvas hose is introduced, which is attached to a stationary tank, at a water station, and a stream of water is then allowed to flow through the hose into the tank of the tender.

Question 348. How is the water conducted from the tender to the engine?

Answer. To each side of the front end of the tank, one end of a piece of rubber hose, F F, is attached, which is connected at the other end to the pipe on the engine which supplies the pump with water. The opening inside the tank through which the water flows to the hose is covered with a valve, which is not shown in the engraving, but which is operated by a lever or handle, G. The valve is covered with a hood or strainer, perforated with small holes, which is intended to prevent dirt from entering the hose and thus getting into and obstructing the pump. The hose is connected to the supply pipe by a screw-coupling similar to that used with ordinary fire-engine hose.

Question 349. How are the flat sides of the tank strengthened so as to resist the pressure and weight of the water?

Answer. They are sometimes braced or stayed with rods or bars, a, a, and h, h, fig. 205, extending from one side to the other and from the top to the bottom, and angle or ⊤ iron is also riveted to the sides to stiffen them.

Question 350. How is the tender connected to the engine?

Answer. By the draw-bar, H, and coupling-pin, o, fig. 205, and also by the safety chains, d.

Fig. 204. Scale ¹⁄₄ inch = 1 foot.

Fig. 205. Scale ¹⁄₄ inch = 1 foot.

Fig. 206. Scale ¹⁄₄ inch = 1 foot.

Question 351. In what respect do the tender trucks differ from the engine truck?

Answer. Chiefly in having the journal-bearings and frames outside of the wheels.

Fig. 207. Scale ³⁄₁₆ in. = 1 inch.

Fig. 208. Scale ³⁄₁₆ in. = 1 inch.

Question 352. Why are the bearings placed outside instead of inside?

Answer. Because they are then more accessible than if they are inside, and the oil-boxes on the axles can be entirely closed over the ends of the axles, so that no oil can leak out, whereas if the boxes are inside, they must be left open at both ends. When the boxes are on the outside, they can be oiled, or a journal-bearing can be removed and a new one put in its place, with much less difficulty than if the boxes were on the inside of the wheels. The only reason why the bearings of engine truck-axles are placed inside the wheels is because they would be in the way of the cylinders if they were outside.

Question 353. How are the axle-boxes for the tender axle constructed?

Fig. 209. Scale ³⁄₁₆ in. = 1 inch.

Answer. Their construction is similar to that of a car axle-box, the standard form of which is represented in figs. 207, 208 and 209. Fig. 207 is a section lengthwise of the axle, fig. 208 a section crosswise of the axle, and fig. 209 a sectional plan. A is the journal of the axle, which is inclosed by a cast iron box, K K, which is open in front and at the back. The front has a cover, H, which is either fastened by a spring, as shown in the illustrations, or is bolted to the box. The axle enters the box from the back, I, and has either a wood or leather packing, J J, called a dust-guard, to keep the dust from getting in and the oil from leaking out of the box. D is a brass journal-bearing, which rests against a cast iron bearing piece or key, E, which is put in so that by removing it through the opening F F, the brass bearing can be raised up high enough to clear the collar, G, on the end of the axle and thus be removed in the same way. The lower portion, L, of the box under the axle is usually filled with cotton or woolen waste saturated with oil. This constantly presses against the axle, and thus keeps it oiled.

Question 354. How are the tender trucks constructed?

Answer. They are made of various patterns, some of which have wooden frames and in other cases the frames are made of iron. The truck illustrated in our engraving, figs. 204 and 205, is made of iron and is very similar to the engine truck, excepting, as already stated, the frames are outside instead of inside of the wheels.

Question 355. How is the tender supported on the trucks?

Answer. It rests on the centre of the front truck and on a bearing, n, fig. 204, on the frames on each side of the back truck. This arrangement gives three bearing points, the advantages of which have already been explained. A truck which supports the load which it carries in the centre is said to be centre-bearing, and if the load is carried on each side, side-bearing.

Question 356. How are the brakes attached to the tender?

Answer. They are attached usually to the back truck alone, but in some cases to both trucks, and are operated by a wheel, N, fig. 204, and a shaft which extends below the tender frame and on which a chain attached to the brakes is wound.

Question 357. How much water and coal does an ordinary tender carry?

Answer. A tender for a thirty-ton engine carries from 1,500 to 2,000 gallons of water and three to four tons of coal, and weighs from 40,000 to 45,000 pounds loaded. An engine of this kind and its tender therefore weigh about 100,000 pounds, which, being even figures, can easily be remembered.

PART XVIII. FRICTION AND LUBRICATION.

Question 358. What is meant by friction?

Answer. Friction is the resistance between two bodies in contact which opposes the sliding of the one on the other. Thus if a brick is placed on a board with a slight inclination, it will not slide because the friction between them, or the resistance opposed to motion, is greater than the force exerted by the weight of the brick to move it downward. If, however, the inclination of the board is increased sufficiently so that a larger proportion of the weight of the brick urges it downward, then the friction will be overcome, and it will slide. When the brake-blocks of a car are pressed against the wheels, they produce friction, which resists the revolving motion of the wheels and will ultimately stop the car; and when the weight of an engine is supported on the driving-wheels and they rest on the rails, the friction between them, as has already been pointed out, resists their slipping on each other, and thus enables a locomotive to exert tractive force. Friction also resists the turning of an axle on its journal and therefore makes the tractive force of the locomotive necessary to move a train of cars.

Question 359. On what does the amount of friction depend?

Answer. The amount of friction of two bodies in contact depends (1) UPON THE PRESSURE OF THE ONE ON THE OTHER, AND IS INDEPENDENT OF THE AREA OF THE SURFACES IN CONTACT; (2) ON THE NATURE OF THE MATERIALS IN CONTACT; (3) ON THE NATURE OF THE SUBSTANCE, SUCH AS OIL OR OTHER LUBRICANT, WHICH IS INTERPOSED BETWEEN THEM. Thus, a brick will slide down an inclined board as easily if it is laid on its broadest side as it will if placed edgewise; and if a cast iron plate, say 10 inches square, is planed and scraped, so as to be as nearly a perfect plane surface as it is possible to make it, it will, if loaded with say a hundred pounds weight, slide on a similar true surface as easily as another plate with half as much area and loaded with the same weight. A shaft resting against a long bearing will require no more power to turn it than would be needed if the bearing was short.[83]

Question 360. What is meant by the “co-efficient of friction?”

Answer. It is the proportion which the resistance to sliding motion bears to the force pressing the surfaces together. Thus a smooth, clean and dry cast iron plate loaded with 100 pounds will require a force of about 15 pounds, or fifteen one-hundredths of the weight or pressure of the plates, to slide them on each other. The co-efficient of friction is therefore said to be 0.15, and with any other weight or pressure on the plates we could determine the force required to slide them on each other by multiplying the pressure by the co-efficient of friction. Thus, if the plates were loaded with 250 pounds, the force required to slide the one on the other would be equal to 250 × .15 = 37.5 pounds. The co-efficient of friction, however, varies for different materials. Thus, while the co-efficient of friction between two pieces of smooth, clean and dry cast iron is 0.15, that of a piece of brass on cast iron, under similar conditions, is 0.22, and of two pieces of wood about 0.4.

Question 361. What is the effect of introducing some unguent or lubricating material, such as oil, between the surfaces in contact?

Answer. The co-efficient of friction is very much reduced thereby. Thus the co-efficient of friction of the cast iron plates, if their surfaces are greased with tallow, is 0.1; if lubricated with lard 0.07, with olive oil 0.064, and with lard and plumbago 0.055, thus showing that the amount of the friction depends very much upon the nature of the lubricant which is used, as well as on that of the materials in contact.

Question 362. What effect on the amount of friction has the manner of applying the lubricating material to the surfaces in contact?

Answer. The more perfect the lubrication the less will be the co-efficient of friction. It has, for example, been found by experiments made with cast iron shafts turning on bearings of the same material that when the lubricating material was applied so that the surfaces were only “unctuous,” that is slightly greasy, the co-efficient of friction was very little less than when they were dry, that is when there was no lubricating substance between them, and that when they were greased “from time to time” the co-efficient was reduced to 0.07 and 0.08; but when they were continually oiled it averaged 0.05, and sometimes fell as low as 0.025, showing that with the best lubrication the friction was only one-sixth what it was when the surfaces were only “unctuous.” Between these two limits there is every degree of frictional resistance, according to the condition of lubrication. This shows how important it is that the oiling fixtures should be kept in the most perfect condition and the utmost care be exercised in keeping every part of a locomotive thoroughly lubricated.

Question 363. What effect does the pressure per square inch of the surfaces in contact have upon the lubrication?

Answer. The tendency is, when this pressure becomes excessive, to press out the lubricant which is between the two surfaces, and ordinary experience proves that the greater the weight or the force per square inch with which two bodies are pressed together, the greater is the difficulty of keeping them perfectly lubricated.

Thus it is easier to keep the journals of a car well lubricated when it is empty than when it is heavily loaded, and the guide-bars of a locomotive are more liable to be cut when the engine is pulling a heavy load than with a light one.

Question 364. What effect has the velocity of the surfaces in contact on the friction and lubrication?

Answer. With the surfaces in the same condition, the friction is independent of the velocity of motion of the surfaces against each other, but perfect lubrication becomes more difficult as the velocity increases, so that an increase of velocity will often increase indirectly the amount of friction. Thus, taking our previous illustrations, it is more difficult to keep the journals of a car or engine well lubricated when running fast than when running slow, and the same thing is true of the guide-bars.

Question 365. What considerations should govern the proportions of frictional bearings for locomotives and other machines?

Answer. The dimensions to be given them should not be determined from a consideration solely of their resistance to rupture,[84] but they should be made so large that the pressure they must bear will be distributed over so much surface that the proportion borne by each square inch will be comparatively small, thus making good lubrication much less difficult, and consequently reducing the co-efficient of friction.

Question 366. Is not the amount of energy required to overcome the friction on a journal of large diameter greater than would be required if the journal was smaller?

Answer. If the co-efficient of friction in the two cases is the same, undoubtedly the large journal will require the greatest expenditure of energy to turn it, because its periphery moves further than that of the small one; but the advantage attributed to large journals is that they can be lubricated more perfectly, because their surfaces being larger the pressure is not so great per square inch, and thus the gain from the reduction of the co-efficient of friction is greater than the loss attributable to the increase of the diameter of the journal. Thus if a car journal is 3¹⁄₄ inches in diameter × 5¹⁄₂ inches long, the available surface exposed to friction is equal to that of a longitudinal section of the journal, or 3¹⁄₄ × 5¹⁄₂ = 17.875 square inches.[85] Supposing now that the journal is loaded with 5,000 pounds, and the average co-efficient of friction is 0.085. In one revolution of the wheel the journal will move 0.85 of a foot, and therefore 5,000 × .085 = 361¹⁄₄ foot-pounds of work. If now the journal is made, as has been proposed, 3³⁄₄ × 7 inches, then its effective surface will be equal to 26¹⁄₄ square inches, but the journal will move 0.98 of a foot in one revolution. If, however, the lubrication is improved by the increased area of the journal so that the co-efficient of friction is reduced from 0.085 to 0.07, then the energy consumed in one revolution will be equal to 5,000 × 0.7 × .98 = 343 foot-pounds, or less than was consumed with the small journals. The co-efficient of friction is assumed, and could only be determined by experiment, but the assumption shows how the resistance of the large journals may be less than that of the small ones. Of course it would be better to give the increased bearing surface by adding to the length of the journal, but nearly all locomotives and car journals must be increased in diameter as well as in length when they are enlarged, in order to have the requisite strength to carry the loads they must bear.

Question 367. Is the law that FRICTION IS IN PROPORTION TO THE PRESSURE ON EACH OTHER BY THE SURFACES OF CONTACT true under all circumstances?

Answer. No; there is a limit to the exactness of the above law, when the pressure becomes so intense as to crush or grind the parts of the bodies at and near their surfaces of contact. At and beyond that limit the friction increases more rapidly than the pressure;[86] and the friction then becomes very irregular.

Question 368. In what cases is the limit referred to probably reached?

Answer. Probably in some locomotives the pressure of the driving-wheels on the rails is sufficient to partly crush the latter.

Question 369. What effect has the nature of the materials in contact on the friction?

Answer. The amount of friction and also the lubrication is very much influenced by the nature of the bearing surface and also by the material used as a lubricant. Some metals, such as brass and other alloys, are much less liable to abrasion and seem to retain lubricants on their surfaces better than other metals, and are therefore much used for journal and other bearings. Some substances, especially oils, are good lubricants, while other materials of apparently similar nature are not. The reason why these materials possess these properties while others are without them is not known, and the value of any material as a lubricant, or the degree to which another will resist friction without abrasion, can only be tested by experiment.

PART XIX. COMBUSTION.