Hydraulic device for producing a to-and-fro movement

A hydraulic device for producing a to-and-fro movement of a first member, the device comprising a double-acting piston-cylinder unit to which the first member is connected, a pressure fluid distributor including a control member for alternately feeding pressure fluid to one side or to the other side of the piston in the piston/cylinder unit to move the first member alternately in opposite directions, and a hydraulically actuable control device for controlling the distributor, the control device including a cylinder, a reciprocable piston member movable in the cylinder and a motion transmitting member, the piston member having a central large diameter section between two reduced diameter sections, and the motion transmitting member being in contact with the surface of the piston member so as to move transversely of the cylinder first in one direction and then in the opposite direction when the piston member moves relative to the motion transmitting member between two end positions, the motion transmitting member also being in contact with the control member of the distributor to transmit motion thereto, the control device also including an independently actuable single-acting piston movable into a position where it holds the piston member with its large diameter section registering with the motion transmitting member.