Abstract: An actuator for driving a rotatable component includes a first member comprising a screw thread and rotatable about an axis (X), and a second member comprising a screw thread configured to cooperate with the screw thread on the first, rotating member. The second member is restrained against rotational movement about the axis (X) such that rotation of said first member causes movement of said second member along the axis (X), and the second member comprises one or more helical grooves. The actuator also includes a third member rotatable about the axis (X) and restrained against axial movement, wherein the third member comprises one or more helical rails, each configured to ride within a respective one of the helical grooves such that movement of the second member along the axis (X) causes rotational movement of the third member about the axis (X).