Abstract: A valve operator assembly is provided for a valve. The assembly includes a housing adapted to be mounted on a bonnet of the valve, an input member rotatably mounted with respect to the housing and an inverted roller screw mechanism and comprising a screw adapted to be connected to a valve stem of the valve, a nut surrounding and coaxial with the screw, the nut being connected to the input member, and a plurality of rollers radially disposed between the screw and the nut.

Abstract: A screw mechanism comprising a screw provided with an outer thread, a nut surrounding and coaxial with the screw, the nut being provided with an inner thread, and a plurality of rolling elements radially disposed between the screw and the nut and engaged in both of the outer and inner threads. A contact diameter Dcontact between the rolling elements and the nut is defined by: D contact ? L ? × tan ? ( ? ) wherein L corresponds to the lead of the screw mechanism and ? corresponds to a determined non-back-driving factor which is chosen from 0°<??1°. The contact diameter is provided in order to prevent back-driving of the mechanism.