Eccentric, in machinery, a contrivance for converting the rotating motion of a shaft into an oscillatory straight-line motion. It is in very general use for working the steam-valves of engines. It consists of a circular disc fitted on to the rotating shaft in such a way that it is perpendicular to the axis of the shaft without this axis passing through the centre of the disc. Thus its motion is ex-centric, the centre of the disc describing a small circle around the axis of the shaft.
Round the edge of the disc is fitted a metal ring or strap, to which is rigidly fixed the eccentric-rod.
The farther end of this rod is constrained to move in a straight line, and therefore has a backward and forward motion in that line imparted to it.
The eccentric may, therefore, be regarded as a special kind of crank; the disc corresponds to the crank-pin, though considerably enlarged; the eccentric-rod corresponds to the connecting-rod, the strap to the sleeve that fits on the ordinary crank-pin.