Several successful gears have recently been brought out, in which the valve is driven from some recipro cating part of the engine.
One of the best known of these is the Joy valve-gear, which has been largely used both for locomotive and marine engines. Figs.
7t.; and 79 illustrate a sim ple form of this g,ear ap plied to a horizontal stationary engine. A vibrating rod or link B is attached at one end to a point A, near the middle of the connecting-rod ; while the lower end is joined to the radius rod C, which compels B to more in a vertical plane. To a point D in the link B is jointed the end of the long arm of a lever E F, of which the end of the small arm works the valve rod G, and the fulcrum F is attached to a block which slides in the curved slot J. This slot is formed in a disk, the center of which is the position of the fulcrum when the piston is at either end of its stroke. The radius of the slot is equal to the length of the valve-rod G.
The disk can be made to rotate through an arc by means of the worm and wheel shown. Thus the slot can be inclined to either side of the vertical. The slot allows the fulcrum of the lever to move up and down with the motion of the point A of the con necting-rod. The forward or backward motion of the engine and the into of ex pansion are controlled by inclining the slot to one or other side of the vertical, the central position con sponding with mid - gear. If the end 1) of the lever were attached direct to the connecting-rod, the motion of the fulcrum F about the center of the slot would not be symmetrical, and the result would be that the cut-off would be une qual in the two strokes. This error is corrected by attaching the end of the lever to the point
U of the vibrating link: for, while the point A on the connecting-rod describes a nearly true ellipse, as shown in Fig. 81, the point 1) describes a bulged figure, and the amount of the bulge is so regulated as to correct the unequal motion of the fulcrum above and below its central position. It is obvious that by shifting the point D the amount of the bulge may be altered, and thus the error may be corrected too little or too much, and by taking advantage of this circumstance a later cut-off may be given to either end of the cyl inder, if found desirable.
which has recently been fitted to a large num ber of marine engines, is shown in Fig. 80. In this system only one ec centric is used, the end of the eccen tric rod being attached to a rod hung from a pin on the reversing-shaft lever R, by which it is constrained to move in an arc of a circle inclined to the center line. To an intermediate point P in the eccentric-rod a con necting link is attached, which com municates the necessa-ry motion to the slide-valve rod, By adjusting the position of the reverse lever R any desired degree of expansion can be obtained, or the engineS reversed, as required. There are few working parts, and distribution of steam both for full power and for expansive working is satisfactory.