F. LOCOMOTIVE POWER
Locomotive Power. Types of Engine.
Locomotives may broadly be reduced to two classes, according to the situation of the working cylinders. In the first class these are within the framing, under the boiler, with the main driving axle cranked at two points to receive the power from the two cylinders; in the second class they are outside the framing, and connected, not to the axle, which is straight, but to crank-pins fixed between the spokes of the wheels, in connexion with the nave. From these distinguishing features the two types of engines are known respectively as "inside cylinder locomotives" and "outside cylinder locomotives." In the latter the general contour of the cylinder is usually visible at the fore-end of the machine. The tenders have six or four wheels, according to the taste of the designer, and they are supplied with powerful brakes, worked by screws, with blocks of wood placed against each wheel. A water-tank forms the upper part of the tender, namely, the two sides and the back, usually in the form of a horse-shoe, holding from 1000 to 3000 gallons; and in the hollow of the shoe the fuel is deposited, of which a full charge may weigh form 30 cwt. to 3 1/2 tons. The engine and the tender are sustained on springs placed over the axle-bearings. Again, there is the general classification of locomotives into passenger engines and goods and mineral engines. As the power of the engine is brought into action through the grip of the diving wheels upon the rails, it is necessary, for the exertion of maximum power in goods engines, to make two or more pairs of the wheels of one size, and transmit the driving force from the central pair of wheels to the front and back pairs by means of coupling-rods attached to crank-pins at the naves of the wheels. Such engines are called "six-coupled," and for them the most convenient combination is with inside cylinders. When the cylinders are outside it is usual to couple only the hind pair of wheels to the driving wheels, making a "four-coupled" engine, the leading or front wheels being of smaller diameter than the driving-wheels, and so leaving room for the convenient placement of the cylinders. The six-coupled engine can take the heaviest train on a good straight railway,that is, one free for the most part from curves; but four-coupled engines work more economically on lines with frequent curves, and may be made so as to take, in average practice, as great a load as six-coupled engines. Passenger locomotives have usually been constructed with a single pair of driving-wheels, for free running at high speeds; but as traffic became heavier four-coupled-wheel passenger engines came into vogue; and express trains are now for the most part worked with four-coupled engines. In recent years the forepart of the engines has in many cases been placed on a four-wheeled truck connected by a central bolt or pivot to the frame of the engine, so that the fore-wheels can swing to the curves of the line. On the Metropolitan, Metropolitan District, and North London Railways entirely, and on many large railway systems partially, where sharp curves are frequent, bogie-engines are employed, and with great advantage in facilitating traction. Another device for the same purpose is the use of radial axles,that is, axles either at the forepart or the back of the engine, which by their axle-boxes slide laterally between circularly formed guides on entering and on leaving curved parts of the way, and so maintain a radial position at right angles to the line of rails.
American practice, many years since, arrived at two leading types of locomotives for passenger and for goods traffic. The passenger locomotive has eight wheels, of which four in front are framed in a bogie, and the four wheels behind are coupled drivers. This is the type to which English practice has been approximating. The tender is carried on eight wheels, disposed under two trucks or bogies, fore and aft. Goods locomotives are made with eight wheels and with ten wheels, of which, in each case, the leading pair of wheels are connected with a swing bolster and radius bar, to conform laterally and radially to curves.
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