the drive mechanism for a paper tape punch and reader assembly utilizes tape drive assemblies which move the paper tape in response to the application of continuous periodic motion and control signals . the periodic motion serves to oscillate a rocker plate between first and second limits and the control signls serve to selectively engage and disengage a detent which effects coupling between the drive shaft and the source of periodic motion . the periodic motion is imparted to the drive assembly by a connecting link which in turn is driven by an adjustable eccentric . the eccentric comprises a pulley plate which is fixed to a shaft . a circular body is adjustably movable with respect to the pulley plate and is arranged to engage the race of a ball bearing assembly which further comprises balls and a retaining ring . the connecting link has a circular opening which provides a snug fit to the exterior of the retaining ring . the pulley plate and the body are held in a fixed physical relation by two locking screws and the relative position of the two is established by an eccentric which is mounted on one of the locking screws .