Abstract: A rotor rotating device connected to an oil supply system for turning the rotor of a rotary machine, which rotor is fitted with a gear wheel, the device including: a hydraulic motor; a hub mounted to be rotated by the motor; a pinion mounted on the hub and dimensioned to mesh with the gear wheel; a screw shift drive operatively associated with the pinion for permitting movement of the pinion in the direction of the axis of rotation of the hub between a first position in which the pinion is disengaged from the gear wheel and a second position in which the pinion meshes with the gear wheel; a compression spring disposed to urge the pinion toward the first position; and a piston connected to the pinion and arranged to be actuated by oil under pressure from the oil supply system for moving the pinion into the second position.