Abstract: A constant speed actuator significantly characterized by being able to produce a displacement at constant speed, even though the loading on a hydraulically damped piston is substantially varied. The actuator consists of a pneumatic driving pressure which acts against a volume of hydraulic fluid, through a interface with a free piston. The hydraulic fluid, thus acted upon by the pneumatic pressure, flows to the opposite side of the hydraulic load piston through a hydraulic fluid by-pass at a constant volumetric rate regardless of the loading on the hydraulic piston. The constant volume by-pass flow is automatically controlled by employing a constant volume valve in the by-pass. Consequently, a consistent, predictable speed of actuation is achieved over a wide range of hydraulic piston loadings and pneumatic supply pressure.