An eccentric is a contrivance for taking an alternating rectilinear motion from a revolving shaft. It consists of a disk or circular frame, fixed on a revolving shaft or axis which doesn't pass through the center of the disk. The disk is surrounded by a hoop of metal, and to this hoop a rod is attached. When the shaft revolves, carrying the eccentric with it, the rod attached to the encircling hoop receives a reciprocating motion, just as it would do if attached to a crank in the shaft.