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]

[67] A switch is a movable pair of rails, by which a locomotive is enabled to run from one track to another.

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.

[Illustration: _Fig. 149._

_Fig. 150._

Scale, ³⁄₈ inch = 1 foot.]

[Illustration:

_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.

[Illustration:

_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.

[68] The periphery is the outside surface on which the wheel rolls. This part of a wheel is usually called the “_tread_.”

[69] That is, that their centre lines incline towards each other, and if extended far enough would meet at the centre of the curve.

[Illustration:

_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.

[Illustration: _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.

[Illustration:

_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_.

[Illustration: 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]

[70] It is exclusively used on the Baltimore and Ohio Railroad.

[Illustration: 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.

[Illustration: _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.

[Illustration: 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.

[Illustration: 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]

[71] It should be mentioned here that it is only certain kinds of cast iron which will be hardened in this way, or will “_chill_,” as it is called. The cause to which this chilling property is due is not known.

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.

[Illustration: _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.

[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.

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.

[Illustration: _Fig. 170._]

[Illustration: _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.

[Illustration: _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.

[Illustration: _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.

[Illustration: _Fig. 179._

Scale 6 in. = 1 foot.]

[Illustration: _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.

[Illustration: _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.

[Illustration: _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?_

[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.

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?_

[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.

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.

[Illustration: _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.

[Illustration: _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.

[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_.”

[Illustration: _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.