General Arrangement.—Since the time of Robert Stephenson and his co-workers, the structure of the European locomotive has essentially become like that shown in longitudinal section in Figure r, and in perspective, though on a reduced scale, in Figure 2 (Pk 98). The car riage consists of two plate-iron frames, parallel with the track, joined by the buffer-beam fi and stiffened in several places by cross and diagonal struts. They enclose and are supported by the axle-boxes (t/. 97, fig-. 2; pl. 98, fig. 7), in which rest the journals of the driving- and running wheels /. These wheels differ from ordinary wagon-wheels in being fixedly connected to their axles, so that the axles rotate with them. Further, their peripheries are flanged on the inside (fi/. 97, figs. 2, 3), and of late years have been slightly tapered conically toward the outside, although this practice is going out of use.
The frame does not rest directly upon the axle-boxes, but is supported on the ends of springs suspended from the boxes by means of a rod placed in their centre. The axle-boxes are movable vertically in the jaws hold ing them, but not horizontally; hence, with the locomotive upon a hori zontal track, they shift in accurately vertical planes, so that, to allow for shocks caused by the unevenness of the track, the frame can move up and down, but its horizontal shifting upon the axles, crosswise of the track, is impossible. The motor, which is secured to the frame, is generally a twin steam-engine—that is, consists of two reciprocating engines of equal dimen sions which act upon the driving-axle through their connecting-rods. Some recent locomotives have compound engines, the high-pressure cylinder upon one side and the low-pressure cylinder upon the other. It is neces sary to have at least two engines, since the locomotive must be put in motion at every position of the piston and crank, including, of course, the two dead centres or stroke-ends of each engine. The two cylinders of this twin engine are secured on some engines outside and on others inside the frame, the outside connection being the exclusive American usage. Their pistons act, by means of piston-rods and connecting-rods, upon the cranks K (bl. ji g 1), which, in the case of outside-cylinder engines, are placed outside on the driving-wheels, and with inside-cylinder engines are placed inside on the driving-axles, which are bent or otherwise " cranked " for this purpose. The cylinders are horizontal, or nearly so, to prevent undue compression and expansion of the bearing-spring in.
The driving-axle generally carries for each of the two engines two eccen tric sheaves (fig. 4), of which it is usually said that one is for the forward and one for the backward motion of the locomotive, although really both are always rotating and acting. By means of eccentric-straps and eccen tric-rods they act much like a crank mechanism upon the link-motion, which, according as it is raised or lowered by means of a bell-crank and lever (n ; r), operates a rocker-arm fastened to the valve-stem, and thus regulates, through the distributing slide-valve, the admission, the degree of expansion, the exhaust, and the degree of compression. The
present tendency appears to be in the direction of substituting for the eccentrics and their sheaves, as a means of working the valve-rods, what are known as "radial gears," which are driven from the cross-head, con necting-rod, or other reciprocating part, and not from any rotary piece. Of these the Joy gear appears to be the most promising and successful in England, while that of Brown of Winterthur, in many modifications, has been tried with satisfaction on the Continent. The movement has not gained much headway in America, although radial valve-gears are giving good satisfaction in marine work. Figure 5 shows, in section, the plan of two inside-connected cylinders (F), with piston (j), cross-head, steam chest (C;), steam-passages, three-port slide-valves, valve-rods (a), and guide bars (1). (Sec under head of locomotive slide-valves, p. 202); also under general subject of slide-valves, p. 254)•) Upon the carriage and between the sides of the frame the steam boiler (fig. 1) is so secured as to allow it to expand lengthwise by heat without springing the frame. It has an inside fire-box (a) and flues (b), frequently over two hundred, placed between the fire-box a and the smoke-box c. There is also an outside fire-box (q), enclosing the fire-box proper, a cylindrical shell (f) surrounding the flues, a dome (e), and a smokestack (a'). 'flue water is contained in the large cylindrical shell and in the space or "leg" between the inside and the outside fire-boxes, which arc fastened together by a number of stay-bolts. The steam-pipe f•, whose outlet can be opened and closed from the cal) by a throttle-valve and rod (f), conducts the steam into the steam-chests, while the pipe b allows the exhaust steam to pass from the cylinders and through the comparatively small exhaust nozzles into the stack. The effect of this exhaust is that the outer atmo spheric pressure forces air into the ash-pan and through the grate re, driv ing the gases of combustion through the flues b. There is a safety-valve weighted with a spiral spring (1), and a feed-pipe (w) which conducts the feed-water from the tender through a flexible coupling (at w) between the locomotive and tender. This tender (pi. 101, fig. 5) is attached to the rear of the locomotive, and has a tank for water and bunkers for fuel. For throwing off obstructions there is further provided a pilot or catcher (0), which frequently carries vertical wire brooms to sweep the top surface of the rail-heads. The engineer's post on the footboard in all American engines is enclosed by a cab, and in most others is at least protected by a roof.