A lead screw (5), a lead screw nut and a drive member are aligned with an axis (4). A coupling between the lead screw and the lead screw nut allows a helical movement of the lead screw. The drive member is rotationally locked with the lead screw nut. The lead screw is coupled with the drive member to generate a helical movement of the lead screw when the drive member is moved in one axial direction. The coupling is overridden when the drive member is moved in the opposite axial direction. Spline features (25) are arranged on the lead screw in a row (20) parallel to the axis with alternatingly small and large gaps between succeeding spline features. A stop feature of the drive member has a dimension in the direction of the axis which is larger than the small gaps and at most as large as the large gaps.
