Figs. 20 (above) and 21.—Sections of a
Locomotive Cylinder.
From what has been said it is obvious, too, that every time the piston moves from one end of the cylinder to the other the valve must also be moved back and forth in the steam-chest. This is done by what is called an eccentric.
An "eccentric" is a disk or wheel (Fig. 22) with a hole, S, the size of the axle of the locomotive to which it is attached. The centre n of the outside periphery of the eccentric is some distance from S, the centre of the shaft. A metal ring, K K (Fig. 23), made in two halves, embraces the eccentric, and the latter revolves inside of this ring. A rod, L, is attached to the strap, and is connected with the valve so that the motion of the eccentric is communicated to it. It is obvious that if the eccentric revolves it will impart a reciprocating motion to the rod L, which is communicated to the valve.
Fig. 22.—Eccentric.
Fig. 23.—Eccentric and Strap.
If properly adjusted on the axle the eccentric will run the engine in one direction. To run the opposite way another eccentric must be provided. Therefore locomotives always have two eccentrics for each cylinder. These, J and K, are shown in Figure 24, which represents the "valve-gear" of a locomotive. S is a section of the main driving-axle, to which the eccentrics are attached by keys or screws. C is the eccentric rod of the forward-motion eccentric and D that of the one for running backward. As a locomotive must be run either backward or forward, and, as the one eccentric moves the valve to run forward and the other to run backward, we must be able to connect or disconnect the rods to and from the valve at will. The eccentric rods of the early locomotives had hooks on the ends by which they were attached to or detached from suitable pins connected with the valves. But these hooks were very uncertain in their action and therefore were abandoned, and now what is known as the "link-motion" is almost universally used for the valve-gear of locomotives. It consists of a "link" (a b, Fig. 24) which has a curved opening or slot, k, in it in which a block, B, fits accurately, so that it can slide from end to end of the link. This block has a hole bored in the middle which receives a pin, c, which is attached to the end of the arm N of the "rocker" M O N. The rocker has a shaft, O, which can turn in a suitable bearing, and two arms, M and N; the latter, as explained, is connected to the link by the pin c and block B. The upper arm M has another pin, V, on its end, which is connected by a rod, v V, to the main slide-valve V. The rocker-arms, as will be seen, can vibrate about the shaft O.
Fig. 24.—Valve Gear.
The link is hung by a pendulous bar, g h, to the end g of the arm E, attached to the shaft A. This shaft has another upright arm, F, which is connected by a rod or bar, G G′, to a lever, H I, called a reverse lever, whose fulcrum is at I. To save room, in the engraving this lever and the cylinder G are drawn nearer to the main axle S than they would be on an engine. The lever is located inside the cab of the locomotive, and is indicated by the numbers 17 17′ in Figure 36 on [p. 133], which is a view looking from the tender at the back end of a locomotive. The lever has a trigger (t, Fig. 24) which is connected by a rod, r, to a latch, l, which engages in the notches of the sector S S′. This latch holds the lever in any desired position and can be disengaged from the notches by grasping the upper end of the lever and the trigger.
Fig. 24.—Valve Gear.