link-motion

from The Century Dictionary.

• noun A system of pieces pivoted together, and turning about pivots attached to a fixed base, all the rotations being in the same plane or parallel planes, so that all the points describe definite curves; a complete linkwork.
• noun Specifically In steam-engines, a system of gearing for controlling the valves for the purpose of starting or reversing the engine, and for controlling the cut-off. See valve-gear.
• noun another set for backward motion;
• noun two eccentric-rods or -blades from these two eccentrics to the top and bottom respectively of
• noun the link or slot-link; and
• noun the sliding-block in the slot of this link which carries the pin to which the valve-stem
• noun is fitted, either directly or through a rocker-shaft and arms. When the link is lowered, the forward eccentric drives ; when the link is raised, the backward eccentric drives . In intermediate positions the block is actuated partly by both and , and, as these are in opposite phase or nearly so, the motion of becomes less, and the port is opened during a smaller portion of the piston-stroke, and an earlier cut-off results. Stephenson's is differentiated from other link-motions by having the link raised and lowered, and with it the eccentric-blades or -rods, to vary cut-off and reverse; hence, the slot-link has a curvature formed by an arc struck with the center of the engine-shaft as a center. The swing of the eccentric-straps and -blades around the eccentrics in the cutting-off and reversing adjustments of the link makes the lead of the valve increase, as the cut-off in forward gear is made earlier.