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.

[21] The term _blast-orifice_ is also often used to designate these parts of locomotives.

[22] Colburn’s Locomotive Engineering.

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.

[23] In speaking of “American” locomotives, we mean locomotives like that shown in Plate I, with four driving-wheels and a four-wheeled truck, and shall so use the term hereafter.

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.

[24] The width is dependent upon the distance between the rails, or _gauge_ of the road, as it is called. The above size is for a 4 feet 8¹⁄₂ in. gauge.

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.

[Illustration: _Fig. 41._

Scale ¹⁄₄ in. = 1 foot.]

[Illustration: _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.

[Illustration: 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_.

[25] This method of staying crown-sheets has been extensively used on the Baltimore & Ohio and Reading railroads, and is now very generally used in Europe.

[26] Colburn’s Locomotive Engineering.

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.

[Illustration: Fig. 44. Scale ¹⁄₄.]

[Illustration: Fig. 45. Scale ¹⁄₄.]

QUESTION 97. _How are the tubes fastened and made water-tight in the tube-sheets?_

[Illustration: Fig. 46. Scale ¹⁄₄.]

[Illustration: 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.

[Illustration: Fig. 48. Scale ¹⁄₄.]

[Illustration: 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.

[Illustration: Fig. 50.]

[Illustration: 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.

[Illustration: 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.

[27] A force exerted to pull any material apart is called a _tensile strain_, and if exerted to compress it is called a _compressive strain_.

[28] It should be explained that in the process of manufacturing iron by rolling, the iron is stretched out into fibres in the direction in which it passes between the rolls.

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.

[Illustration: _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.

[29] In the following calculations all the dimensions have for convenience been reduced to decimals.

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.

[Illustration: _Fig. 54._]

[Illustration: _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.

[Illustration: _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.

[30] Boiler plates should always be so arranged that the greatest strain will come on them in the direction of their greatest strength, which is parallel with the fibres of the metal.

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:

Area of ¹¹⁄₁₆ rivet = .3712 × 4 × 50,000 = 74,240.

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:

Plates through rivet holes (tearing) = 71,200 lbs. Rivets (shearing) = 72,240 lbs. Plates in front of rivets (crushing) = 92,812 lbs. Solid plate (tearing) = 140,625 lbs.

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.

[Illustration: _Fig. 57._]

[Illustration: _Fig. 58._]

[Illustration: _Fig. 59._]

[Illustration: _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:

Plates through rivet holes (tearing) 118,125 lbs. Rivets (shearing) 120,260 lbs. Plates in front of rivets (crushing) 118,125 lbs. Solid plates (tearing) 196,875 lbs.

[31] It has been necessary to take for an illustration, plates of a different width from the preceding example, in order to get an even number of spaces between the rivets in each case.

[Illustration: _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.

[Illustration: _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.

[Illustration: _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:

Plates through rivet-holes (tearing) 110,742 lbs. Rivets (shearing) 110,425 lbs. Plates in front of rivets (crushing) 126,562 lbs. Solid plates (tearing) 158,205 lbs.

[32] That and even greater distances between the edges of the holes are now used successfully with ³⁄₈-inch plates.

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:

Plates through rivet-holes (tearing) 147,656 lbs. Rivets (shearing) 150,325 lbs. Plates in front of rivets (crushing) 147,656 lbs. Solid plates (tearing) 205,078 lbs.

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.

[Illustration: _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:

Plates through outside rows of rivet-holes (tearing) 91,880 lbs. Rivets in one outside and middle row (shearing) 111,360 lbs. Plates in front of rivets (crushing) 139,218 lbs. Solid plates (tearing) 140,625 lbs.

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:

Percentage of strength compared with solid plate. Single-riveted seam, punched holes ¹¹⁄₁₆ rivets 50.6 Single-riveted seam, drilled holes ⁷⁄₈ rivets 60. Double-riveted seam, drilled zigzag holes ³⁄₄ rivets 70. Double-riveted seam, drilled zigzag holes ⁷⁄₈ rivets 72. Single riveted seam, punched holes with covering plate 65.3

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.

[Illustration: _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.