Eccentric 1 rotates about fixed axis A and has wedge-shaped grooves a containing rollers 3. Eccentric 1 is encircled by collar b of link 2 which is oscillated by a drive (not shown). When link 2 turns counterclockwise, rollers 3 are jammed between eccentric 1 and collar b so that eccentric 1 is turned clockwise through a certain angle. As link 2 swings back, rollers 3 are released and eccentric 1 and its shaft remain stationary. Since, as eccentric 1 is turned about axis A, distance A͞B is gradually varied from minimum to maximum and back again, the angle of rotation of the eccentric and its shaft varies likewise.