Hydraulic actuating drive

A hydraulic actuating drive is described which has a cylinder and a piston axially displaceable in the cylinder under the action of a pressure medium, the actuating forces being passed to the outside via a piston rod, provided with a tooth system, and a pinion. A tooth segment displaceable in the opposite direction to the piston rod meshes with the pinion. The zero position and the end position are defined and can be varied by means of adjusting screws. A centre position of the piston is reached when the initiated inflow of pressure medium is interrupted by the valve ball resting on the valve-ball seat. The centre position thus reached can be changed by axial adjustment of the valve chamber. Reversal of the solenoid valve permits a further inflow of pressure medium via the second inflow line, as a result of which the piston passes from the centre position into the end position.