Figs. 124, 125, 126. Valve Regulation and Feed Rolls
[Fig. 127]. Link-motion valve gear of a locomotive; two eccentrics are used for one valve, one for the forward and the other for the backward movement of the engine. The extremities of the eccentric rods are jointed to a curved slotted bar, or, as it is termed, a link, which can be raised or lowered by an arrangement of levers terminating in a handle, as shown. In the slot of the link is a slide and pin connected with an arrangement of levers terminating in the valve stem. The link, in moving with the action of the eccentrics, carries with it the slide, and thence motion is communicated to the valve. Suppose the link raised so that the slide is in the middle, then the link will oscillate on the pin of the slide, and consequently the valve will be at rest. If the link is moved so that the slide is at one of the extremities, the whole throw of the eccentric connected with that extremity will be given to it, the valve and steam ports will be opened to the full, and it will only be toward the end of the stroke that they will be totally shut; consequently the steam will have been admitted to the cylinder during almost the entire length of each stroke. But if the slide is between the middle and the extremity of the slot, as shown in the figure, it receives only a part of the throw of the eccentric and the steam ports will only be partially opened, and quickly closed again, so that the admission of steam ceases some time before the termination of the stroke, and the steam is worked expansively. The nearer the slide is to the middle of the slot the greater will be the expansion, and vice versa.
Figs. 127, 128. Link and other motions
[Fig. 128] represents a mode of obtaining motion from rolling contact. The teeth are for making the motion continuous, or it would cease at the point of contact shown in the figure. The fork catch is to guide the teeth into proper contact.
[Fig. 129]. What is called the Geneva-stop, used in Swiss watches to limit the number of revolutions in winding-up; the convex curved part of the wheel serving as the stop.
[Fig. 130]. A continuous rotary motion of the large wheel gives an intermittent rotary motion to the pinion-shaft. The part of the pinion shown next the wheel is cut of the same curve as the plain portion of the circumference of the wheel, and therefore serves as a lock while the wheel makes a part of a revolution, and until the pin upon the wheel strikes the guide-piece upon the pinion, when the pinion-shaft commences another revolution.