Catechism of the Locomotive | Project Gutenberg

[72] The above answer and much of the material referring to springs has been translated from “Die Schule des Locomotivführers,” by Messrs. J. Brosius and R. Koch.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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?

[73] The following rules for calculating the proportion and strength of steel springs are from Clark’s Railway Machinery.

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.

[74] The span is the distance between the centres of the spring-hangers when the spring is loaded.

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.

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.

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?

[75] This lever is called an equalizing lever or beam, or, more briefly, an equalizer.

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.

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.

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.

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.

[76] In some rare cases three pairs of wheels are employed for locomotive trucks. Six-wheeled trucks are very commonly used under passenger cars.

[77] The right half is a section through the centre of the axle, or of G g, of fig. 186, and the left half a section through the centre of the truck, or on g g, of fig. 186.

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.”

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.