eccentric, in mechanics, device for changing rotary to back-and-forth motion. A disk is mounted off center on a shaft. One flat, open, circular end of a rod fits around the edge of the disk; the other end is usually attached to a block that slides in a slot. As the shaft rotates the block slides back and forth, carrying along whatever is attached to it, e.g., a valve. The distance between the center of the shaft and the center of the disk is the eccentricity. The so-called throw may mean either the eccentricity or the distance the block moves, which is twice the eccentricity. Cams and cranks perform the same function as the eccentric, which designers often prefer to the crank for short motions.