A valve operator assembly having a tubular housing extending along a central axis, an input member rotatably mounted with the housing, a transmission mechanism arranged in an inner bore of the housing and extending along the central axis, the transmission mechanism providing a rotating element connected to the input member and a translating element, the transmission mechanism adapted to convert applied rotation of the input member to the rotating element into axial translation of the translating element. At least one rolling bearing is radially arranged between an inner bore and rotating element to support in rotation and guide the rotating element within the housing. The valve operator assembly includes a clutch mechanism to permit transmission of driving torque from the input member to the rotating element in both directions around the central axis, and prevent any back-driving torque from the rotating element to the input member in either direction.