a hydraulic push drive for pusher centrifuges , having a rotating body with a cylindrical bore . the rotating body is provided with a piston rod and piston body therein . a rotary control valve is disposed in the rotating body . the rotary control valve is driven by a hydraulic motor and controls the application of pressure . a valve is slideably mounted in a coaxial bore in the piston rod in order to control the stroke center of the piston by pulse duration modulation of a controlled leakage flow .