Abstract: A valve actuator has at least one piston in a cylindrical chamber that has a segmental skirt provided with transverse teeth on its chordal face meshing with lengthwise teeth on the hub of a rotor extending diametrically through the chamber with one rotor end exposed and provided with a portion for mating engagement with a valve stem portion when the valve is clamped to the actuator. Air is delivered into the chamber on opposite sides of the piston and the skirt is provided with anti-friction elements. Single and dual piston embodiments are disclosed as well as the use of springs by which the valve is operated in a wanted manner in the event of loss of piston operating pressures.