Valve operator assembly comprising: - a tubular housing extending along a central axis, - an input member rotatably mounted with respect to said housing, - a transmission mechanism (3; 46) arranged in an inner bore of said housing and extending along said central axis, said transmission mechanism (3; 46) comprising a rotating element (6; 48, 52) connected to the input member and a translating element (5; 47), said transmission mechanism (3, 46) being adapted to convert applied rotation of the input member to said rotating element (6; 48, 52) into axial translation of said translating element (5; 47), - at least one rolling bearing (18, 19) radially arranged between an inner bore of said housing and rotating element of the transmission mechanism (3; 46) to support in rotation and guide said rotating element (6; 48, 52) within said housing, wherein the valve operator assembly further comprises a clutch mechanism to permit transmission of driving torque from the input member to the rotating element (6; 48, 52) in both directions around the central axis, but prevent any back-driving torque from said rotating element (6; 48, 52) to the input member in either direction, said clutch mechanism extending along the central axis, arranged between the housing and said rotating element (6; 48, 52), and provided with: - a casing mounted on the housing by fixing means (21, 22), - a first axial sleeve mounted on the rotating element (6; 48) and a second axial sleeve mounted with the input member, said axial sleeves (23, 24) being axially adjacent one to the other, - locking members arranged between said first sleeve and second sleeve, each of the locking members being in contact with the first sleeve and with the second sleeve, in such a way that each of the locking members is in a locked position to lock the second sleeve with the first sleeve when a driving torque in either direction around the central axis is applied to the second sleeve so as to transmit said braking torque from said second sleeve to the first sleeve, said locking members being in an unlocked position when said first sleeve is in back-driving torque in either direction around the central axis, and - a first rolling bearing being radially arranged between the first sleeve and an inner bore of the casing, and a second rolling bearing is radially arranged between the second sleeve and the inner bore of the casing. - a tubular housing extending along a central axis, - an input member rotatably mounted with respect to said housing, - a transmission mechanism (3; 46) arranged in an inner bore of said housing and extending along said central axis, said transmission mechanism (3; 46) comprising a rotating element (6; 48, 52) connected to the input member and a translating element (5; 47), said transmission mechanism (3, 46) being adapted to convert applied rotation of the input member to said rotating element (6; 48, 52) into axial translation of said translating element (5; 47), - at least one rolling bearing (18, 19) radially arranged between an inner bore of said housing and rotating element of the transmission mechanism (3; 46) to support in rotation and guide said rotating element (6; 48, 52) within said housing, - a casing mounted on the housing by fixing means (21, 22), - a first axial sleeve mounted on the rotating element (6; 48) and a second axial sleeve mounted with the input member, said axial sleeves (23, 24) being axially adjacent one to the other, - locking members arranged between said first sleeve and second sleeve, each of the locking members being in contact with the first sleeve and with the second sleeve, in such a way that each of the locking members is in a locked position to lock the second sleeve with the first sleeve when a driving torque in either direction around the central axis is applied to the second sleeve so as to transmit said braking torque from said second sleeve to the first sleeve, said locking members being in an unlocked position when said first sleeve is in back-driving torque in either direction around the central axis, and - a first rolling bearing being radially arranged between the first sleeve and an inner bore of the casing, and a second rolling bearing is radially arranged between the second sleeve and the inner bore of the casing.