Steam Locomotive Construction and Maintenance/Chapter XI

by E. L. Ahrons

CHAPTER XI

LOCOMOTIVE MAINTENANCE AND REPAIRS

The locomotive, when in traffic, is under the charge of one of the running sheds or districts, into which the railway is divided for locomotive running purposes. The usual practice is for ordinary running defects and minor repairs to be attended to in the small workshops attached to the sheds. The facilities provided for this purpose depend upon the size of the running shed, and in some important sheds many “heavy repairs” are now executed. Generally speaking, running shed repairs include re-turning the tyres, refitting axleboxes and bearings, repairs to the motion and brake gear, and the fitting of new springs. The springs and other parts are, of course, supplied from headquarters. Minor boiler repairs such as the replacement of stays are also done in the running sheds, but when the boiler requires heavy repairs the engine is sent away to the principal works.

Records are kept of the mileage and of every repair done to the engine, however small. Further, periodical examinations are made of most of the important parts of the engine after it has run a definite number of miles, or for a certain length of time. Thus the firebox would be examined about once a month, and the tyres and axles of express engines after running about 4,500 miles, but bogies, springs and slide valves would be allowed to run 20,000 miles between each periodical examination. Before the engine is sent into the principal shops for heavy repairs, a special report is sent there some weeks in advance, giving full particulars of the condition of the principal portions, so that the new parts are ready and no unnecessary delay occurs after the engine has been sent in.

An engine may be in service from one to two years between its visits to the principal shops. The time depends upon the class of engine, the service upon which it has been employed, the mileage run, and the general condition in which it is.

Boiler. The boiler requires more supervision than any other part of the locomotive, and is the most expensive in the matter of upkeep. Sir John Aspinall, an eminent locomotive engineer, once made a statement^[1] which will be endorsed by every locomotive man that “there was only one thing which caused trouble, namely, the boiler. The engine part was quite satisfactory, and never gave any trouble, but the boiler was an everlasting trouble.”

To make a thorough examination of the boiler, the tubes are withdrawn, and the incrustation due to the water is removed from the plates by means of suitable chipping tools, in the use of which great care must be taken not to “nick” or damage the plates, or subsequent fractures are liable to result.

The steel boiler and firebox casing plates are liable to corrosion and “pitting,” cracks, and “grooving.” The corrosion takes the form of a uniform wasting of the plates, whereby they become thinner, and eventually unable to withstand the pressure required. This form of corrosion is usually found on a longitudinal belt from 1 ft. to 2 ft. wide near the water level. Pitting is the more usual form of corrosion, in which the plate is honeycombed with small cavities either isolated, or running into one another to form depressions of considerable size. This defect is probably caused by combined chemical and galvanic action due to dissolved acids in the water. The scale deposited by the feed water, if thin, helps to protect the plates, but the alternate expansion and contraction of the plates, as they are heated and cooled, helps to detach pieces of the scale, leaving the plates exposed to the action of any acid in the water. It may here be remarked that most boiler and firebox troubles are caused by the alternate expansion and contraction of the different parts, and that its wear and tear are due to causes quite different from those which operate in the cylinders and motion. “Grooving,” usually found on the smokebox tubeplate and at the foundation ring which unites the copper firebox to the firebox shell, is also due to the same cause. As the tubes expand they tend to push out the centre of the tubeplate, whilst it is rigidly held at the edges, especially where it is jointed to the flange of the cylinders. The constant bending and unbending causes a slight crack in the plate which ultimately develops

Fig. 46.—Oval Tube Holes (A) and Cracked Tube Plate (B).

into a groove, the latter being enlarged by the action of acid in the water.

A badly grooved or wasted plate is renewed, but if the defects are not so serious, the plate is repaired by riveting a patch over the defective portion. Rivet heads are liable to corrosion, and defective rivets are knocked out and replaced.

Firebox. The firebox is particularly subject to defects. The copper plates, especially at the firehole or at the flanges where the plates are united, are wasted away by the action of the flame. Cracks and fractures as at B, Fig. 46, are frequently found across the tubeplate between the holes in which the tubes are secured; such cracks also appear between the stay holes in all the plates. As these cracks extend rapidly great care is taken in looking for them. As the tubeplate expands in a vertical direction when the boiler warms up, the tube holes gradually assume an oval shape and leakage then occurs round the tubes. Overheating of the plates, due to occasional shortness of water, causes burning of the plates. Expansion and contraction cause frequent breakages of the stays which unite the roof and sides to the firebox casing, and bulging-in of the plates may then result. The presence of defective side stays is detected by tapping the heads with a light hammer, when an experienced man can tell them by the sound. Stay heads are generally burnt away by the flame.

All these defects, of which some are sure to be found, would become dangerous if left, and for this reason the firebox is thoroughly examined at frequent regular periods. The high pressures used in modern locomotives, from 170 to 225  lbs. per sq. in., increase the tendency to firebox troubles, and some engineers take advantage of the benefit due to super-heating by reducing the pressure to 160 lbs. per sq. in.

To repair cracked plates, patches of various sizes and shapes are used. It is essential in patching to cut away the defective portion of the plate first. This, leaves an oval or rectangular hole, and a piece of new copper plate is marked off and cut out slightly larger than the hole, so that it forms a lap all round. The patch is then carefully bedded down and the stay holes and rivet holes marked off in the new piece after

Fig. 47—Patch on Firebox Plate.

which it is riveted or fastened by stud bolts according to the position which it occupies, for in some parts of the firebox riveting cannot be done. It should be noted that such patches are never put over the cracks in the old plate, but the latter is always cut out, for the reason that were the old plate left there would, after the patch has been put on, be a double thickness of plate, which would reduce the conductivity, so that overheating and burning would result. If a firebox plate be in bad condition, the lower half may be cut away, and a new half-side, or half tube plate may be riveted to the old upper half. When tube holes are oval as at A, Fig. 46, and the plate between the holes is cracked as at B, the holes are enlarged with a “rose-bit” tool,

Fig. 18.—Bushed Tube Holes in Tube Plate

and specially turned and screwed plugs are inserted tightly into the holes. These plugs are then riveted over on each side of the tube plate to cover the cracks in the plate as far as possible. The plugs, if not left solid, are then drilled to receive new tubes, which are somewhat smaller than the tubes which were previously in these holes. Fig. 48 shows two of these bushed holes in section.

Every five or six years an entirely new firebox is required. This involves stripping the whole of the interior of the boiler, cutting through and knocking out all the stays, and removing the foundation ring. The boiler, when undergoing this repair, is turned upside down and the new firebox put in as in a new boiler (see Chapter II). The whole boiler will require renewal after 12 to 16 years; this is a comparatively simple matter, the old boiler being lifted away from the frames, and the new one placed on them as described in Chapter VIII.

Firebox Stays. Leaky and broken firebox stays are a constant source of trouble. Leaky copper stays, if not in too bad a condition, may be attended to in the running shed, the heads being lightly riveted over and “caulked” with a light tool which has a circular head. The principal trouble, however, is with broken stays. This

Fig. 49.—Cracked and Broken Stays, with Heads Wasted Away Inside Firebox

defect is shown in Fig. 49. The copper or inside firebox expands in a vertical direction as the steam is being raised, and rises so that the stay is bent or inclined as shown at A, since the outer steel firebox shell to which the stays connect it does not rise so rapidly or to the same extent. The constant bending and re-straightening of the stays causes them to crack as at B and finally fracture as at C. When putting in new stays the holes are slightly enlarged, tapped for new screw threads, and fitted with correspondingly larger stays.

Tubes. The chief defects of steel tubes are (1) pitting of the outsides due to the same cause as pitting of the boiler plates, and more particularly (2) leakage at the firebox tube-plate. In former years leakages were stopped temporarily by driving a conical tool, known as a “drift,” into the open firebox-end of the tube. This forced the metal of the tube against the walls of the tube-holes, but was very injurious to the tubeplate, and its use is now forbidden. Instead, a special “tube-expander” is used. This consists of a casing which will enter the tube, and in it are slots through which a number of small rollers protrude slightly, the axes of the rollers being nearly parallel to the axis of the tube. An internal conical mandril can be pushed in by the operator so that the cone gradually forces the rollers outwards against the inside wall of the tube, and as the tool revolves the rollers roll the metal of the tube against that of the tube-plate, thus making a tight joint. In some cases a thin soft copper ring or ferrule is placed between the outside of the tube and the hole in the tube-plate, the ferrule being practically squeezed between the tube and the plate. The outer ends of the tubes project into the firebox about ⅜ in. and are afterwards “beaded” or worked over against the face of the tube-plate.

Safety Valves. These and other steam valves wear slightly on the valves and seats, so that they leak and have to be “ground in.” A little fine emery and oil is sprinkled on the valve and its seat, and the valve is rotated backwards and forwards until both are ground together to a perfect seating, which can be seen from the dark colour all round their bearing faces.

Frames. Frames occasionally give way by breaking from the inside corners of the recesses into which the driving hornblocks fit, as at A in Fig. 17. The alternate thrust and pull due to the steam action is felt most severely at these corners. To repair them the frames may be either patched or welded. A patch is a piece of plate about ⅞ in. thick cut out to embrace the upper portion of the hornblock and cover the fracture. Welding is now becoming more usual either by the oxy-acetylene or the electric arc process. In both processes, the crack is chipped out, and a V groove formed, into which molten metal is deposited by the flame or arc to produce a solid “weld.” Great care has to be taken, especially with the oxy-acetylene process, that the contraction of the frame when cooling after the operation does not cause a new breakage.

Cylinders. The defects which arise in cylinders are principally oval wear of the bore, wear of the port faces, over which the valves work, and cracked or broken cylinders. In the case of cylinders with circular ports for piston valves, such as those shown in Fig. 22, these ports are provided with internal circular liners, through which the necessary port holes are cut. As these wear through the action of the rings of the piston valves, they can be replaced by new liners, and the cylinder casting itself remains uninjured and intact.

For refacing the port faces of slide valve engines, and for reboring the cylinders themselves, the latter are not taken out of the engine frames. To do this would be too expensive an operation. Portable facing and boring machines are used which are fixed in front of the cylinders, the buffer beam having been taken down. These machines were formerly worked by hand, but now an electric motor actuates them through suitable gearing. The bores are carefully callipered, and just sufficient metal is removed to make them truly circular. New piston heads of greater diameter are required when the cylinders have been re-bored.

For cracks and breakages the cylinders must be removed from the frames, and if the cracks are serious the cylinders are replaced by new ones. Some cracks on the exterior are patched with gun metal patches cast from a wooden pattern specially made to fit the contour of the defective part. If the tops of the cylinders are corroded by the smokebox ashes, which always contain sulphur, a patch of copper plate is made to fit and bolted on. If the port faces are badly worn they are faced-up and plates or false faces are screwed on, which are machined to the original dimensions. Cracked “bridges” between the ports may be repaired by cutting away the metal on each side of the crack and dovetailing a piece into the space formed.

Acetylene welding has been used, chiefly in America, for repairing cylinders, especially in cases where flanges are broken.

Wheels and Axles. With the exception of the boiler these require more attention than any other part of the engine. The wheel “centres” themselves very rarely crack now that they are of cast steel. The former wrought iron driving wheels used sometimes to crack between the spokes at the boss, but these are not made now, though a considerable number are still in service. Broken or flawed tyres are replaced immediately, no attempt being made to repair them by welding. The chief defect in tyres is the ordinary wear in service, through which the “tread” on the

Fig. 50.—Tread of Tyre Showing Wear and Section After Re-Turning.

rail decreases in diameter. The flange therefore becomes deeper relatively to the tread as shown by the dotted line B, Fig. 50, and when it becomes too deep it is liable to catch in crossings on the line and so become a source of danger. The tyres are not removed from the wheels, but the whole set is put into a wheel lathe and re-turned. The re-turning has also to be done when flat places occur on the tyres. Very powerful wheel lathes are required, since a number of exceedingly hard spots are always found on the treads of tyres that have been some time in service. An unavoidable but considerable waste of metal occurs when tyres are re-turned. In the first place much metal has to be turned off to produce the new proper contour C of the tread and shape of flange, and secondly, in the case of engines with four or six wheels coupled, the wheels wear down to unequal diameters, so that when the smallest wheel of the set of say six wheels has been turned down, all the other five wheels must be turned to exactly the same diameter, and more metal has to be removed than would be necessary if each wheel were independent. Coupled wheels of even slightly unequal diameters cause a great strain on the coupling rods, and also produce a “nosing” or side to side action of the engine when running. In Fig. 50 A is the contour of the tread of the tyre when new. This wears down to line B, and the tyre is then re-turned to the new contour C.

Tyres are usually about 3 ins. thick when new but, owing to wear and re-turning, they eventually attain a thickness which will not allow them to be safely re-turned. Wheels up to about 5 ft. 6 ins. may be re-turned so long as the tyres are in sound condition and provided that the tyres will not be less than 1½ in. thick after re-turning but, for wheels larger than 5 ft. 6 ins., the limit of thickness after re-turning is about 1¾ ins.

Axles, especially driving crank axles, are subject to very heavy bending and twisting forces, and are watched very carefully for flaws and cracks. If a minute crack begins to show itself the axle may be allowed to run until the crack develops, when the axle is immediately condemned, the wheels forced off by hydraulic pressure, and a new axle substituted. Other than flaws, the chief running defect is oval wear of the journals which work inside the axleboxes, and a similar oval wear of the crank pins of driving axles. To restore the journals to a true circular form the wheels are put into a wheel lathe, and a very light cut is taken off the axle journals. For truing up crank pins of crank axles, a portable machine is clamped to the crank webs. The axle remains stationary, and the cutting tool revolves round the crank pin.

Axleboxes. These wear on the flanged sides which are constantly moving up and down in the hornblock guides under the action of the springs. The thrust and pull of the rods on driving and coupled axles accentuate this wear on the axleboxes of such axles. As the hornblock guides also wear, the latter are faced up first. This may be done in place on the engine either by filing them up, using a surface plate to detect the high places, or by means of a portable facing machine, or the horn blocks may be removed and re-ground. The axleboxes are then “lined up,” i.e. the deficient width across the faces of the hornblocks is made up by riveting a brass liner on one side or by white metalling this side to the proper thickness. The side is then planed and the box fitted into the hornblocks. The crown of the brass which bears on the journals will be worn down and the box will generally require a new brass; if not too badly worn, the old brass may be re-metalled with white metal. The centres are marked off when the boxes are in place on the engine, and the boxes are then sent to the machine for re-boring. The centres must be properly located so that the distance apart of the different axles is exactly correct.

The Motion, etc. Connecting and coupling rods are examined for flaws, and if any are found in the bodies the rods are at once condemned. Usually flaws in connecting rods occur in the “big-end” straps at the corners and bolt holes. These straps being smaller independent pieces are not so expensive to renew. The rods are also tested for straightness with a straight-edge. The straps will require filing up and perhaps closing a little at the jaws, and the brasses, frequently new, must be a perfect fit in the straps, and be made perfectly square with the rods. The brasses having been fitted and bolted up the ends are placed on the rods and the length between centres of the big and small ends on each rod is trammelled to the dimensions shown on the drawings. The centres are marked and the ends sent to the machine to be bored out to the size of the crank pin journal, to which they are then fitted. Coupling rods are more easily repaired by having new white metal circular bushes pressed into the solid ends of the rods. This work is done in a small hydraulic press.

Slide bars are re-ground on a grinding machine, and set up on the engine in a similar manner to that for a new engine in the erecting shop. Slide valves are re-faced, or if they are worn too thin or are otherwise defective they are renewed. Piston rods and valve spindles require re-turning and re-grinding, as they wear unevenly in the glands, and are often found to be badly scored and grooved. Screw threads and cotter holes in them are examined for flaws, which if found, cause the rods to be condemned.

The various pins in the valve gear wear slack in their holes, and the holes in the links through which they pass are found to be enlarged. The metal being case-hardened these holes cannot be re-bored, but they may be made true by lapping them out with a lead mandrel using emery and oil. An alternative and more rapid method now generally employed is to use a special grinding machine with a very small wheel. If the enlarged holes be used as they are, new pins are required, but in many cases the holes are bushed with hardened steel bushes. The wearing surfaces of the quadrant links are re-ground or lapped out. Great care should be taken that all oil holes are in their proper positions and thoroughly cleaned out.

The brake-gear is overhauled and adjusted and new cast iron brake blocks are provided. There are various other details too numerous for description in this primer, and it remains only to be mentioned that the putting together again, of the engine is very similar to the erection of a new engine as described in Chapter VIII.

1. Proceedings Institution of Mechanical Engineers, July, 1909.