Screw actuator

A screw actuator comprising a rotatable input member and a non-rotatable primary output member, wherein the input member is provided with a screw thread formation within which a plurality of spherical members are received to provide a ball screw coupling between the input member and the primary output member, a secondary output arrangement including first and second screw threaded parts in screw threaded engagement with the input member to provide a screw thread coupling between the input member and the secondary output arrangement, said first and second screw threaded parts being capable of relative angular and axial movement, and said secondary output arrangement incorporating an axial drive mechanism for generating relative axial movement of said first and second screw threaded parts in response to relative angular movement of said first and second screw threaded parts, whereby, in the event that the ball screw coupling fails, relative angular movement of the first and second screw threaded parts occurs resulting in relative axial movement of said first and second screw threaded parts serving to lock the first and second screw threaded parts onto the input member, thereby to stall the actuator.