Fig. 13.
The relative positions of link and eccentric centers of an engine, when the crank is on the forward center, are shown in [Fig. 13]; the link being represented with the block in the center, which represents mid gear. It will be observed that the centers of the eccentrics f and b, from which the rods receive direct influence, are both some distance ahead of the center of the axle, the one above, the other below. The eccentric-straps to which the rods are connected sweep round the eccentric circles, and are controlled thereby. When the link is moved up or down, each eccentric-rod pin, where it attaches to the link, describes the arc of a circle with a radius drawn from its own eccentric. If both rods were worked with a radius from the axle-center, the link could be raised and lowered when the engine stands on the dead center, without moving the rocker-pin at all; but, under the existing arrangement, the link is influenced directly by one or other of the eccentrics, whatever position in the link the block may stand. When the engine is standing on the forward center, with the link in mid gear, as shown in [Fig. 13], it will be readily perceived that the block stands at its farthest point away from the axle; for the rods are so placed to reach their greatest horizontal distance ahead, and consequently in this position the lead opening is greatest. If the link be now lowered, the backing eccentric-rod will immediately begin to pull the link back: and, as the pin of the forward eccentric-rod approaches the central line of motion, it will also keep drawing the link back; so that, by the time the link is in full gear, the lead opening will be considerably reduced.
Fig. 14.
When the engine stands on the back dead center, as shown in [Fig. 14], the eccentric centers will be on the other side of the axle, and the eccentric-rods will be crossed. While in mid gear, the link-block is drawn closer to the axle than it would be in any other position of the link; and consequently the lead opening is greatest. If the link be now lowered, the forward eccentric-rod will approach its horizontal position, and consequently reaches farther on the central line of motion, so it will push the link-block away from the axle, thereby decreasing the lead. Pulling the link into back gear has a similar effect.
The tendency of a link-motion to increase the lead towards the center is made greater by shortening the eccentric-rods. Increasing the throw of eccentric inclines to accelerate the lead towards the center, since it throws the eccentric centers farther apart. For slow running, hard-pulling locomotives, where increase of lead is a disadvantage, the tendency to increase the lead is sometimes restrained in forward gear by reducing the angular advance of the backing eccentric. This expedient is, however, not necessary where proper care and intelligence have been bestowed in the original design of the motion.
In studying this part of the valve-motion, a young machinist or engineer will obtain valuable assistance by cutting a link template out of a piece of pasteboard, and using strips of wood as eccentric-rods. With these he can test on a drawing-board or table the various positions of the link, and note, in a way that is easily understood, the effect of changing the link into different positions.