endless screw, screw fixed so that it cannot move longitudinally as it rotates. Thus, instead of the usual action in which a screw advances itself through a medium as it turns, the screw remains stationary and forces the medium or a mechanical part to advance. When a screw of this type is arranged to drive a cogwheel whose teeth mesh with its thread, the screw is called a worm gear. See Archimedes' screw.