It comprises a carriage (4) displaceable forwards and backwards by means of a spindle (3) actuated by a motor (2), the carriage (4) containing clutch pins (12) so that in the forward displacement the engagement takes place with a clutch disc (22) integral with which is an eccentric, whose rotation will cause the actuation of the lock in which the unit is fitted. In its displacements the carriage (4) is guided in rods (8) provided in the corresponding cover (5) of a rotor (1) in which the motor (2) and carriage (4) assembly is housed. The clutch pins (12) are connected to pushing springs (13) for pushing them towards the emergency position and therefore that of engagement when those pins (12) are facing the holes (23) of the disc (22).