Anti-back drive device

An anti-back drive device comprising a rotary drive member coupled to a rotary driven member to enable the rotary drive member to drive the rotary driven member, the device further comprising a ring which surrounds the rotary driven member, the rotary driven member being provided with at least two cam recesses on its periphery containing respective lock members between respective cam surfaces of the cam recesses and the said ring, each recess being sufficiently deep towards one of its ends to accommodate its associated lock member but not at locations towards its other end where the recess is shallower relative to the ring, whereby drive of the driven member by the drive member will be permitted by each lock member in a given sense of rotation of the driven member relative to the ring but each lock member will inhibit back drive rotation of the driven member in the opposite sense of rotation relative to the ring, wherein the said at least two cam recesses extend from their deeper to their shallower locations in opposite directions around the driven member, wherein the coupling between the drive member and the driven member is a free-play coupling whereby a reversal in the sense of rotation of drive results in the disengagement of the coupling between those two members, a predetermined relative rotation between those two members in which they are disengaged from one another, and then re-engagement between them, and wherein respective protuberances extend from the drive member into the said at least two cam recesses to retain one of the lock members associated with one of those recesses at the deeper location thereof whilst permitting movement of the other lock member towards the shallower location of its associated recess, and, upon reversal of the sense of rotation of the driven member by the drive member, to retain the said other of the said lock members at the deeper location of its associated recess whilst permitting movement of the said one of the lock members towards the shallower location of its associated recess, so that the device is a bi-directional anti-back drive device.