Abstract: A fluid pressure-operated device has a rotor mounted inside a main housing, radial slots circumferentially spaced in the rotor, radially sliding vanes in the slots, springs urging the vanes radially inwardly to engage at their inner ends with a cam at the center of the rotor, a control arm pivotally mounted on the housing for changing the position of the cam to control radial displacement of the vanes and opening and closing of a valve having inlet/outlet ports connected to an annular chamber in which the vanes rotate so that fluid pressure from the valve will drive the vanes and thus the rotor in either direction of rotation, depending upon the position of the control arm and cam.