229. The ordinary locomotive consists of a pair of direct-acting horizontal of nearly horizontal engines, fixed in a rigid frame under the front end of a boiler of the type described in § 133, and coupled to the same shaft by cranks at right angles, each with a single slide-valve worked by a link-motion, or by a form of radial gear. The engine is non-condensing, except in special cases, and the exhaust steam, delivered at the base of the funnel through a blast-pipe, serves to produce a draught of air through the furnace. In some instances a portion of the exhaust steam, amounting to about one-fifth of the whole, is diverted to heat the feed-water. In tank engines the feed-water is carried in tanks on the engine itself; in other engines it is carried behind in a tender.
230. On the shaft are a pair of driving-wheels, whose frictional adhesion to the rails furnishes the necessary tractive force. In some engines a single pair of driving-wheels are used; in many more a greater fractive force is secured by having two equal driving-wheels on each side, connected by a coupling-rod between pins on the outside of the wheels. In goods engines a still greater proportion of the whole weight is utilized to give tractive force by coupling three and even four wheels on each side. These arrangements are distinguished by the terms "four-coupled," and "eight-coupled" applied to the engines. In inside-cylinder engines the cylinders are placed side by side within the frame of the engine, and their connecting-rods work on cranks in the driving shaft. In outside-cylinder engines the cylinders are apart far enough to lie outside the frame of the engine and to work on crank-pins on the outsides of the driving wheels. These dispenses with the cranked axle, which is the weakest part of a locomotive engine. Owing to the frequent alternation of strain to which it is subject, a locomotive crank axle is peculiarly liable to rupture, and has to be removed after a certain amount of use.
In some locomotives the leading wheels are coupled to driving wheels behind them, but it is now generally preferred to have under the front of the engine two or four smaller wheels which do not form part of the driving system. These are carried in a bogie, that is, a small truck upon which the front end of the boiler rests by a swivel-pin or plate which allows the bogie to turn, so as to adapt itself to curves in the line, and thus obviate the grinding of types and danger of derailment which would be caused by using a long rigid wheel-base. The bogie appears to have been of English origin;2 it was brought into general use in America, and is now common in English as well as in American practice. Instead of a four-wheeled bogie, single pair of leading wheels are also used, carried by a Bissel pony truck, which has a swing-bolster pivoted by a radius bar about a point some distance behind the axis of the wheels. This has the advantage of combining lateral with radial movement of the wheels, both being required if the wheel base is to be properly accommodated to the curve. Another method of getting lateral and radial freedom is the plan used by Mr. Webb of carrying the leading axle in a box curved to the arc of a circle, and free to slide laterally for a short distance, under the control of springs, in curved guides.3
231. In inside-cylinder engines the slide-valves are frequently placed back to back in a single valve-chest between the cylinders. The width of the engine within the frame leaves little room for them there, and they are reduced to the flattest possible form, in some cases with split ports, half above and half below a partition in a central horizontal plane. In some of Mr. Stroudleys engines the valves are below the cylinders, with faces sloping down towards the front, while the cylinders themselves slope slightly up. In many engines the valves work on horizontal planes above the cylinders; this position is specially suitable when Joys or some other form of radial gear is used instead of the link-motion. Radial valve-gears have the advantage, which is of considerable moment in slide-cylinder engines, that the part of the shafts length which would otherwise be needed for eccentrics is available to increase the width of main bearings and crank-pins, and to strengthen the crank-cheeks. Walshaerts gear is very extensively used on Continental locomotives, and Joys has now been applied to a large number of British engines.
232. In a powerful locomotive of the ordinary type the cylinders are 17 to 19 inches in diameter, with a stroke of about 26 inches. The steam pressure is 130 to 175 _. The horse-power ranges up to about 700. A passenger engine for express service has driving-wheels form 7 to 8 feet in diameter, and weighs, without tender, about 40 tons. Of this nearly 15 tons is borne by each driving axle.4
Fig. 138 shows a half section through the smoke-box and one cylinder of an inside-cylinder engine (of the Midland Railway), and illustrates how in an engine of this type the cylinders are situated with regard to the frame, which consists of a single pair of steel plates, extending from end to end and united by other transverse plates, one of which, called the motion-plate, gives support to the guide-bars, and another holds the draw-bar. Another form of frame is built up to two longitudinal plates on each side. In the engine illustrated the valves are above the cylinders, and are worked by Joys gear. A bogie truck appears in section below the engine. S is the steam-pipe, and B the blast-pipe, which is tapered in the fore-and-aft plane.
233. The outside-cylinder type is adopted by several British makers; in America it is universal. There the cylinders are in castings which are bolted together to form a saddle on which the bottom of the smoke-box sits. The slide-valves are on the tops of the cylinders, and are worked through rocking levers from an ordinary link-motion. Other features by which American practice is distinguished are the use of bars instead of plates for the frames, of cast-iron wheels with chilled rims instead of wrought-iron wheels with steel rims shrunk or forced on, and steel fire-boxes and wrought-iron tubes instead of copper fire-boxes and brass tubes. Fig. 139, which is a half section through one cylinder of an American locomotive, by the Baldwin Company of Philadelphia, shows the position of the cylinders and valves.
234. Locomotive engines have been compounded in several ways. In 1876 M.A. Mallet1 introduced, on the Bayonne and Biarritz Railway, a type of compound locomotive which one small high-pressure cylinder and one large low-pressure cylinder were used in place of the two equal cylinders of a common locomotive. Outside cylinders were used in the first instances, but Mallets system is also applied to inside-cylinder engines. The pipe from the high to the low-pressure cylinder takes a winding course through the smoke-box; this gives a sufficient volume of intermediate receiver, and also dries the steam before it enters the large cylinder. A reducing valve is provided through which steam of a pressure lower than that of the boiler can be admitted direct to the low-pressure cylinder to facilitate starting. The reserving gear is arranged to act on both cylinders by one movement, and also to permit a separate adjustment of the cut-off in each. Engines on Mallets system have been successfully used on other Continental railways and in India, in some instances by conversion from the non-compound form.2 His plan has the advantage of permitting this (in certain cases), and of requiring scarcely any more working parts than are needed in a common locomotive; but it gives an unsymmetrical engine. He has also proposed an engine with four cylinders,one high-pressure cylinder tandem with one low-pressure cylinder on each side. Another symmetrical form has been used, in which a pair of outside high-pressure cylinders are compounded with a pair of inside low-pressure cylinders.
235. The most important experiment yet made in the compounding of locomotives is that which Mr. F. W. Webb, of the London and North-Western Railway, has been conducting on a large scale since 1881.3 In Mr. Webbs system three cylinders are used. Two equal high-pressure cylinders are fixed outside the frames, and drive the rear driving axle by crank-pins at right angles to one another. A single low-pressure cylinder of very large size is set beneath the smoke-box, and drives a crank in the middle of the forward driving axle. The driving axles are not coupled, and the phase-relation of the low-pressure to the high-pressure stroke is liable to alter through unequal slip on the part of the wheels. This, however, is of no material consequence, on account of the large size of the intermediate receiver and the uniformity with which the two high-pressure cylinders delivers steam to it. The receiver is formed, as in M. Mallets arrangement, by leading long connecting pipes through the smoke box. All three slide-valves are worked by Joys gear. Those of the low pressure cylinders are placed below the cylinders (an arrangement which has the advantage of letting the valve fall away from the port-face when the engine is running down hill with the steam-valve closed); the valve of the large cylinders is above it. The arrangement is completely symmetrical; it has the important mechanical advantage of dispensing with coupling rods, while retaining the greater tractive power of four drivers; only one axle is cranked, and that with a single crank in the centre, which leaves ample room for long bearings. A plan of Mr. Webbs engine, half in section, is given in fig. 140. The results of Mr. Webbs experiments have been, in his judgment, so satisfactory that for express passenger service he is now building engines only of the compound type. In some recent examples the small cylinders are 14 inches, and the large cylinder 30 inches in diameter, with a stroke of 24 inches, and the boiler pressure is 175 _. Engines of the same type are also being introduced in India, South America, and the continent of Europe.
236. Experiments on the saving of fuel by compounding locomotives point to an economy of from 10 to 20 per cent. It may be expected, for reasons which have been discussed above, that a compound engine, even when working at the high speed of a locomotive, will have a somewhat higher efficiency than a non-compound engine. But, apart from this, an important merit of the compound system is that, while it absolutely prevents the grade of expansion from being reduced below a certain minimum, depending on the ratio of cylinder volumes, it also permits a comparatively high degree of expansion, which in an ordinary locomotive would involve the use of specially large cylinders and a separate cut-off valve. Experiments on the steam-jacketing of locomotive cylinder have not hitherto been attended by success.
237. Tramway Locomotives for the most part resemble railway locomotives in the general features of their design. The boiler is of the usual locomotive type. A pair of cylinders in front, either inside or outside the frames, are connected directly to the hindmost of two coupled driving axles. Owing to the smallness of the driving-wheels, the axle is lie near the road, and the cylinders are set sloping at a considerable angle upwards to keep them clear of dirt. To prevent the discharge of steam into the atmosphere, the exhaust steam is often led into an atmospheric condenser, consisting of a large number of pipes set on the air. In some instances the common locomotive type is widely departed from: a mixed vertical and horizontal boiler is used, and the engine is connected to the driving axle by worm-wheel or other gear, or by a rocking lever between the connecting-rod and the crank.1
238. In the "fireless" tramway locomotive of M. Leon Francq, a reservoir which takes the place of an ordinary boiler is charged at the beginning of the journey with water heated under pressure by injecting steam from stationary boilers at a pressure of 15 atmospheres. The thermal capacity of the water is sufficientwithout further addition of heatto supply steam to the engine during the journey, at a pressure which gradually falls off.2 The system has not come into general use.
239. Several forms of tramway engine have been devised in which the motive power is supplied by compressed ari.3 In the Mekarski system the compressed air, on its way from the reservoir to the cylinders, passes through a vessel containing hot water and steam under pressure (charged, as in Francqs system, by injecting steam at a station). In this way the air is heated, and may then expand in the cylinder without having its temperature lowered to an objectionable degree.
240. Steam road-locomotives or traction-engines have usually a boiler of the locomotive type, with a cylinder or compound pair of cylinders, generally on the top, driving a shaft from which motion is taken by a gearing chain or spur-wheels to a single driving axle at the fire-box end. The engine is steered by means of a leading axle, whose direction is controlled by a hand-wheel and chain-gear. To facilitate rapid turning the driving-wheels are connected to their axle by a differential or compensating gear which allows them to revolve at different speeds. This is a set of four bevel-wheels like Whites dynamometer coupling: the outside bevel-wheels are attached to the driving-wheels; the intermediate ones, which gear with these, turn in bearings in a revolving wheel driven by the engine. So long as both driving-wheels are equally resisted both are driven at the same speed but if one is retarded (as the inner wheel is in turning a curve) it acts to some extent as a fulcrum to the bevel gear, and the outer wheel takes a greater share of the motion. An important feature in traction engines is the elasticity of the driving-wheels. Many devices have been employed, partly to give the wheels an extended tread, or arc of contact with the ground, an partly to avoid shocks in passing over rough ground. Both objects are accomplished by Mr. R.W. Thomsons plan of surrounding each wheel with a thicker tyre of India-rubber, protected on the outside by armour of small plates. In most modern traction-engines the rim is itself rigid, but is connected to the nave through a system of springs which allow it to take up an eccentric position, and the tyres have skew bars on the surface to increase their adhesion to the road.
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