What is "link motion"

Link motion \Link" mo"tion\ (l[i^][ng]k" m[=o]"sh[u^]n) n. (Steam Engine) A valve gear, consisting of two eccentrics with their rods, giving motion to a slide valve by an adjustable connecting bar, called the link, in such a way that the motion of the engine can be reversed, or the cut-off varied, at will; -- used very generally in locomotives and marine engines.

Note: The illustration shows a link motion for a vertical engine, c representing the shaft carrying two eccentrics, a and b, for making the engine run forward and backward, respectively, their rods e and d being jointed to opposite ends of the slotted link f, in the opening of which is a pin g which is attached to the valve rod h. The valve will receive the motion of the forward eccentric when the link is in the position shown, and the motion of the backward eccentric when the link is shifted so far to the right as to bring e in line with h, or a compound motion derived from both eccentrics when the link is shifted to intermediate positions, the compound motion causing the valve to cut off the steam at a point determined by the position to which the link may have been shifted.

n. (context engineering dated English) A valve gear consisting of two eccentrics with their rods, giving motion to a slide valve by an adjustable connecting bar (the link) in such a way that the motion of the engine can be reversed, or the cut-off varied.