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

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

Question 247. How may the wheels be classified?

Answer. As driving and carrying or truck wheels.

Question 248. What service must the driving wheels perform?

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

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

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

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

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

Question 251. What service does the truck perform?

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

Question 252. How does it perform the latter service?

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

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.

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.

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.

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.

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.

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]

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.

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.

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.

Question 273. How are the truck wheels made?

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

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

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

Question 275. How are the frames for locomotives constructed?

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

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

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.