Steam Locomotive Construction and Maintenance/Chapter VI

From Wikisource
Jump to: navigation, search
Steam Locomotive Construction and Maintenance by E. L. Ahrons
Chapter VI. Machine Shops — Axles and Wheels

CHAPTER VI

MACHINE SHOPS — AXLES AND WHEELS

Axles. Straight axles are turned to gauges in special axle lathes. Formerly these lathes were of the ordinary form with a headstock at the left-hand end and a poppet head or tailstock at the right. The most modern form of lathe is of the centre-driven type, in which the axle after having been turned in the middle is gripped
Ahrons (1921) Steam Locomotive Construction and Maintenance Fig23.png

Fig. 23. — Straight Axle.

and driven there, supporting tail-stocks being used at each end. This method gives a more rigid support for the axle, and enables cutting tools to be used at both ends. Fig. 24 shows a lathe of this type, which admits axles 9 ft. long. A straight axle is illustrated in Fig. 23, in which A A are the “wheel seats,” i.e., the portions on to which the wheel centres are pressed, and B B are the “journals” which revolve in the bearings in the axle-boxes. The collars C C keep the journals from moving laterally. The wheel seats and journals are turned smooth and truly circular and the journals are finished with
Ahrons (1921) Steam Locomotive Construction and Maintenance Fig24.jpg

Fig. 24. — Centre Driven Axle Lathe.
By Messrs. Hulse & Co., Ltd. (Manchester).

burnishing rollers. These are revolving steel rollers secured in the tool posts, or carried by attachments at the back of the lathe (as in Fig. 24); they are pressed against the revolving journals, and impart to the latter a perfectly even and polished surface. Grinding machinery is now frequently used for axles, more especially for finishing the wheel seats.

Crank axles are turned in heavy lathes of the ordinary pattern, as the position of the crank webs does not allow of centre driving. The inside faces of the jaws of the crank webs are slotted out, or more usually milled out by a large revolving disc, into the periphery of which a number of cutters are inserted. A machine of this type is shown in Fig. 25. This has two centring headstocks at the back (one of which is hidden from view), in which the crank axle is held in special chucks, and there is also the revolving disc provided with cutters. The latter is slowly fed into the crank between the crank arms, and cuts these out to the dimensions required. This part of the work completed, the headstocks are set in motion to revolve the crank axle, which is set so that the crank pin is rounded by a cutting tool to a circular form. The axle is then turned round end to end in the centring headstocks and the other crank arms are milled out and the pin turned.

Many crank axles of large modern engines, instead of being forged in one piece, are now built up of nine pieces, two ends and one middle
Ahrons (1921) Steam Locomotive Construction and Maintenance Fig25.jpg

Fig. 25. — Crank Sweep Milling Machine.
By Messrs. Craven Bros. (Manchester) Ltd.

piece, two pieces for the crank pins, all the above being of circular form, and four flat slabs for the webs. The slabs are milled round in a vertical milling machine, the holes for the pins and parts of the axle bored. They are then heated and shrunk on to the circular parts and crank pins of the axle, and finally secured by round plugs screwed into the joints in a direction parallel to the length of the axle.

Wheel Centres. The term “wheel centre” refers to that portion of the wheel which includes the central boss, and the spokes and rim, but does not include the axle or tyre. Wheel centres are now generally made of cast steel. They are turned outside, and also bored for the axles either in a wheel lathe, or — more conveniently — on the revolving table of a horizontal boring mill of the type shown in Fig. 26.

The wheel centres are forced on to the axles in a hydraulic press, the pressure required being from 60 to 120 tons, according to the size of wheel and axle. Small bogie and tender wheels are pressed on by 60 tons or so, but large driving wheels require from 90 to 120 tons. To obtain the necessary grip, the part of the axle or “wheel seat,” on which the wheel fits, is turned slightly larger than the bore of the wheel centre. The amount of this difference or “allowance” for an axle 8¾ in. diameter is about ²¹⁄₁₀₀₀ in., the hydraulic pressure then required being 10 to 12 tons per inch diameter of the axle. If the pressure
Ahrons (1921) Steam Locomotive Construction and Maintenance Fig26.jpg

Fig. 26. — Horizontal Turning and Boring Mill.
By Messrs. Craven Bros. (Manchester) Ltd.

registered on the gauge or recorder attached to the press is too small the wheel is taken off again, and another substituted. In the case of driving and coupled wheels, additional security is provided by driving keys into keyways cut into both wheel and axle, but in the case of other wheels no keys are used.
Ahrons (1921) Steam Locomotive Construction and Maintenance Fig27.png

Fig. 27. — Boring Tyres on Horizontal Boring Mill.

Tyres. The tyres, which come from the steel manufacturers, are rolled without weld. They are bored inside to an internal diameter slightly less than the outside diameter of the wheel centre, on to which they have to be shrunk, the allowance being about ¹⁄₁₀₀₀ of the diameter of the wheel centre. Formerly tyres were bored in a wheel lathe, but it is found much more convenient and expeditious to bore them on a boring and turning mill of the type shown in Fig. 26. The tyres are placed horizontally on a revolving table as shown in Fig. 27. The tool holders shown are special ones which allow several tools to be used at once.

After having been bored the tyres are placed on the floor inside a shallow pit, which has a circular gas ring round it, by which they are heated so that they expand. The wheel centre on the axle is then taken up by a crane and lowered into the tyre, which as it cools, shrinks and grips the wheel.

Ahrons (1921) Steam Locomotive Construction and Maintenance Fig28+29.png

Figs. 28 and 29. — Tyre Fastenings.

Figs. 28 and 29 show the most usual methods of securing the tyres to the wheels. T is a section through the tyre, and W one through the rim of the wheel. In both examples, there is a lip A which prevents the tyre from sliding laterally over the wheel rim towards the inside of the rail. The flange F, which runs against the rail, prevents the tyre from sliding towards the outside. In Fig. 28 the tyre is secured by set screws S which pass through the rim of the wheel. Some engineers object to set screws, as the tyres tend to break across the holes. In the alternative construction, Fig. 29, the tyre is held by a retaining ring R, which is heated and driven into the recessed groove shown, and the lip B of the tyre is then hammered down all round the wheel on to the ring.

The outside crank pins, upon which work the coupling rods, and also the connecting rods of outside cylinder engines, are turned and then ground on a grinding machine, and pressed into the holes bored into the wheel boss to receive them. As the pin on one side of the engine has to be fixed at exactly 90° with the pin on the opposite side, the boring of the holes in the wheels is done on a special “quartering machine,” in which both holes are bored simultaneously in the opposite wheels. The arrangement of the machine is such that the 90° angle is automatically maintained.

Finally the complete set of wheels and axle are lifted into a heavy wheel lathe, of the type shown in Fig. 30, to have the tyres turned on the outside. These lathes are very rigid and powerful machines, since the extremely hard steel of the tyres places a heavy stress on both the cutting tools and the machine. Figs. 31 to 34 show a set of sheet iron gauges used for turning the tyres. Figs. 31 to 33 are used to see that the cone of the head of the tyre (usually 1 in 20) and the height of the flange are correct. Fig. 34 tests the proper distance apart or the “gauge” of the two tyres on a pair of wheels.
Ahrons (1921) Steam Locomotive Construction and Maintenance Fig30.jpg

Fig. 30. — Locomotive Driving Wheel Lathe.
By Messrs. Craven Bros. (Manchester) Ltd.

Wheel Balancing. In some shops the wheels when finished are balanced by revolving them at high speed in a special machine, and adding small weights until they revolve with perfect steadiness. A machine for this purpose designed by Mr. G. J. Churchward, chief mechanical engineer of the Great Western Railway, and used at Swindon Works,
framless

Fig. 31–34. — Tyre Gauges.

is illustrated in Fig. 35. The wheels and axle rest in bearings supported by springs, and are driven at a speed corresponding to 60 m.p.h. by means of a 55 h.p. electric motor. The speed is registered by the counter on the wall at the back of the machine. The hand wheel actuates the brake blocks by means of a screw, so that the wheels can be stopped quickly when required after the current has been cut off. The excess or deficiency of balance, as found by the small weights attached during the test to secure steady running, is afterwards rectified in the balance weights. Axleboxes. The account of the work done in the wheel shop may conveniently include the axleboxes. These are most frequently machined in the general machine shop, but as they have to be fitted to the journals they are, in some modern works, machined in the wheel shop, where they are close to the wheels, and do not then require to be taken from one shop to another. An alternative method, used in other works, is to fit and bed the axleboxes on to a short “dummy” axle journal, kept in the machine shop for this purpose, the axleboxes then being taken straight to the erecting shop.

Axleboxes are either of gun-metal or of cast steel fitted with gun-metal bearings, the latter form being now more usual. The “brasses” which bear on the journals are provided with recesses, into which is run “white” or antifriction metal, consisting of about 80 per cent. of tin, 10 per cent. of copper, and 10 per cent. of antimony.[1] This prevents the journal of the axle from being injured in case the bearing runs hot.

The boxes are arranged on a planing machine, in two rows of about ten boxes in each row, and planed between the jaws to receive the brasses and axlebox keeps, which are machined in turn and fitted into the boxes. The brasses must be fitted very carefully, and they must be well bedded down to avoid any rocking action of the
Ahrons (1921) Steam Locomotive Construction and Maintenance Fig35.jpg

Fig. 35. — Wheel Balancing Machine.
G.W.R. Swindon Works.

brass in the body of the box. The holes are then broached out for the pin, which secures the “keep” in the box. Following this the boxes are fixed to a long planed casting or angle plate on the bed of a planing machine, about twelve boxes, six on each side, and are planed on one face, and along the recess at the side which fits on to the hornblocks. The boxes are then reversed and the other face and recess planed. They are now bored four or six together on a special boring machine. In some works the boring and the facing are done together on a double boring mill, two boxes being machined at once. Oil grooves, which were formerly chipped out with a chisel, are now frequently grooved out on a small milling machine. Finally the fitters take the boxes and bed the brasses down on to the journals or dummies, by using red “marking” and filing down the high spots.
  1. The composition of “white metals” varies considerably. The above figures give one mixture used for locomotive bearings. Many white metals contain from 40 to 70 per cent. of lead, which replaces the tin.