the radius of curvature of each slot region is centered on the axis of rotation 24 of the valve stem in the example actuator shown in fig2 guide slot 40 is formed of two slot regions region 42 having a radius of curvature 44 and region 46 having a radius of curvature 48 greater than radius 44 it is also noted that the arc length of slot region 42 is smaller than the arc length of slot region 46 for reasons described in detail below the slot regions 42 and 46 are connected to one another by a radially extending slot region 50 guide slot 40 may also have an enlarged slot region 52 positioned at one end the enlarged slot region is sized to permit the retaining head 38 to pass through the plate 14 allowing assembly and disassembly of the actuator 12 it is also feasible to make the retaining head 38 removable from the cam 28 by attaching it to the cam by screw threads for example if the retaining head is removable then there is no need for the enlarged slot region 52 in another embodiment shown in fig2 a guide slot 40 is formed of three slot regions 43 45 and 47 the three slot regions are connected to one another by two radially extending slot regions 49 and 51 slot regions 43 45 and 47 subtend an angle of about 180 degrees and such configurations are used with valves having a longer throw such as diverted.