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Railroad steam locomotive history

Railroad steam locomotive historyEvery locomotive consists of a framework or cassis supported by springs on weels. The framework carries in its turn a boiler, and an engine with two, three, or four horizontal or nearly horizontal cylinders, two being the usual number. The framing may be regarded as the link between all the various parts of the whole locomotive. There are two types of framing, namely, the plalte frame and the bar frame. The latter was very little used in the U.K.; the former very little used in the United States. In certain cases it is not easy to say to which type the framing belongs; but these are very exceptional.

The plate frame is a rectangular steel structure, composed mainly of two plates extending from the leading to the trailing end of the engine. Their depth and thickness vary in different designs; but it may be taken generally that the plates are 1 inch to 1.25 inch thick, and 18 inches to 2 feet deep. They are secured to each other by cross plates and angle steels. These main frames are usually supplemented by secondary frame plates much lighter and narrower, on top of which rests a flat steel plate, known as the "running board," along which the driver can walk, and so oil and inspect his engine while it is running. Little or nothing of the main frame can he seen in many engines, because it is concealed by the wheels, splashers, running board, etc.

It was of the utmost importance to the good and safe running of the engine that the framework always remained quite rigid; that the angles between the longitudinal and the cross plates should be true right angles; and that, in a word, no twisting in any plane could take place. If the track were a dead level there would be no risk of twisting; but it is not level, and one corner of the engine may be raised by a wheel on a ridge, while another is lowered because the nearest wheel is in a hollow. Changes in the amount and direction of the stress occur every moment.

The stresses are far too complicated to permit of mathematical treatment. The designer never attempts to calculate their amounts. He adapts the proportions, and method of riveting or bolting, which have been found by experience to be the best. Any considerable change in design involves something of an experiment. Risks are got over, however, by the simple expedient of making things very strong.

Frames may be either "inside" or "outside." In the first case the journals of the axles are inside the wheels. In the latter case they are outside the wheels. Tile distance between the bosses or hubs of the wheels cannot for a line of 4 feet 8.5 inches gauge be more than 4 feet 5.25 inches, and with inside cranks this reduces the length of the bearing or journal within narrow limits: If the journals are placed outside, then the bearing can, of course, be made as long within reasonable limits as may be desired; the load per square inch is reduced, and a substantial advantage gained. But the cross breaking stress on the crank axle is augmented; and besides, with coupled engines, cranks fitted on the ends of the axles become necessary, and the design of the engine ceases to be compact. With inside frames no crank arms are used, the pins being secured in radial prolongations of the wheel bosses.

So long as engines remained small, and particularly with single engines, either the outside bearing or a combination of the outside and inside bearings remained in favour. The combination was in a way a compromise. Two short journals were used, one inside, the other outside the wheel, which was then so far supported that even if the axle broke anywhere but in a journal the wheel could still carry its load, and the engine would not be derailed. The advent of the big coupled engine, however, gave the coucp de grace to outside bearings, and they are very seldom seen now except on old locomotives. But from the first there was trouble. The crank axles of those days were not very trustworthy forgings, and as far back as 1838 we find Robert Stehhenson putting in no fewer than four inside frames, which were thus described by Mr. W.N. Marshall many years ago.

This description and the illustrations, Fig. 1, were worth producing, because the inside frame to sustain the crank shaft against the thrust and hull of the connecting-rod is still used. The axle box also shows the system of wedges for tightening the bearings on the shaft, and also in the horn-plates. All driving axle boxes are fitted with wedges to take up wear between the axle boxes and the faces of the horn-plates, but only a single wedge is used, as the small longitudinal displacement cannot affect the running of the engine.