Abstract: An oscillating fluid-driven actuator for a shaft, such as a ship rudder shaft, a butterfly valve spindle or the like. The actuator comprises (i) a stationary actuator housing defining an annular chamber coaxial with the shaft and having a suitable cross section, and (ii) an annular piston arranged coaxially with the shaft in the chamber and connected fixedly with the shaft by means of an arm mounted intermediate the ends of the annular piston by means of a central piston hub. The piston hub is made with a coaxial bearing surface or surfaces of convex spherical shape and the actuator housing has a central lower hollow hub defining an inner concave spherical bearing surface to support the piston hub by engagement with the spherical surface thereof. A corresponding concave spherical bearing surface is arranged in the upper portion of the housing to cooperate with the corresponding spherical surface of the upper portion of the hub.