A rotary drive member drives a driven member. A ring surrounding the driven member has two cam recesses containing lock members between cam surfaces of cam recesses and the ring. Each recess accommodates an associated lock member at different locations where the recess is shallower. Driving the driven member is permitted in a given rotation relative to the ring, but each lock member inhibits rotation of the driven member in the opposite sense. The recesses extend in opposite directions. Coupling between the driven members is free-play whereby reversal in the rotation disengages members and reengages. Protuberances extending into cam recesses retain a lock member associated with one recess at the deeper location permitting movement of the other lock member towards the shallower location. Upon reversal of the rotation, the protuberances retain another lock member at the deeper location permitting movement of one lock member.