Step-by-step mechanism with locking device

A two way stepping ratchet drive, e.g. for seat adjustment in a vehicle, has an operating lever (22) pivot mounted (38) on a support frame (20) and with two gear segments (32) pivot mounted on the lever to drive a ratchet wheel (30) in either direction. The gear segments are spring loaded (44) to rest against a stop (42) and have pegs (46) which interact with two sprung levers which hold the gear segments in the mid position. Pressing the operating lever in either direction moves the ratchet wheel by a set amount, which is much greater than is obtained by single tooth ratchet drives.