CHAPTER XXII.
THE JOY VALVE-GEAR.
DESCRIPTION OF MOTION.
In this form of valve-gear, eccentrics and their equivalents are entirely dispensed with. The motion for the valve is taken direct from the connecting rod; and by utilizing independently the backward and forward action of the rod, due to the reciprocation of the piston, and combining this with the vibrating action of the rod up and down, a movement results which is employed to actuate the valves of engines using any combination of lap and lead desired, and giving an almost mathematically correct cut-off for both sides of the piston for forward and backward motion, and for all points of expansion intermediately. The general outline of the motion can be understood by an examination of [Fig. 28], which gives three views of the gear. The sub-figures 1, 2, and 3 show the motion in elevation, plan, and transverse section respectively.
Fig. 28.
HOW TO APPLY THIS GEAR TO AMERICAN LOCOMOTIVES.
To apply this gear, the valve and steam-chest are placed, as is usually the case in American locomotives, over the cylinders; the valve-stem center being, however, in the same vertical plane as the cylinder center line, immediately over it. All the valve-motion is thus arranged on this plane,—perfectly central,—and there are no crooked or side strains. To a point A on diagram, about two-fifths along the connecting rod from the piston end, a small hook-link is pinned. From the lower end of this at D is jointed a lever E, which, of course, moves with the connecting rod both backward and forward, and up and down; the point D, in fact, moving in a very irregular ellipse, hereafter explained. The fulcrum F of the lever E is carried in a block which slides in a slot-link JK, and from the upper end of this lever E the valve-stem rod G is carried to the valve. This slot-link JK is centered so as to be able to be inclined from the vertical either way. When standing in the vertical position, the engine is in mid-gear, and the valve will only be opened for lead; but this will be exact for each end, and exact also on either side of the center line of the connecting rod. When the slot-link is inclined forward to the front of the engine, the engine is in full gear forward; and, when inclined backwards, it is in full gear for going back. Placed in any intermediate position, the cut-off is regulated for any required degree of expansion; the front and back of the piston receiving equal cut-off, or equal amounts of steam.
CONSTRUCTION DIRECTIONS.
In laying out this valve-motion, it is necessary to make the center or fulcrum F of the lever E coincide, when the crank is at either extreme stroke, with the center on which the slot-link JK vibrates: then these two centers coinciding when the engine stands at lead (that is, at either end of the stroke), the slot-link JK, which is, in fact, the reversing agent, may be put forward or backward through all degrees, without moving the valve, which thus has constant lead at all grades of expansion.
If, however, it is desired to give increasing or decreasing lead in the grades of expansion, this may be given by varying the coincidence of these two centers accordingly, or by under-correcting or over-correcting the action of the lever E, by varying the position of D on the link B.