raised the height of the cam ramp surface 72 and pushes against the surface 57 on the pinion 52 through a washer 58 thereby raising the pinion by an identical distance to disengage the jaw clutch teeth 54 and 55 the spring 51 is compressed slightly to allow such movement the appropriate force to rotate the lower collar 71 is applied to the lugs 74a and 74b by the shifter fork 96 the shifter fork 96 is operated by the eccentric 95 and is located between the rack 92 and the main drive gear 82 see fig3 and 4 a recess 100 is provided at one end of the shifter fork 96 so that the shifter fork 96 may partially surround the upper collar 63 a pair of tynes 101a and 101b extend downwardly from the shifter 96 and are arranged to abut lugs 74a and 74b respectively operation of the shifter fork is demonstrated in fig5 and 8 the position of the tynes 101a and 101b with respect to the lower collar 71 is determined by the direction of rotation of the eccentric 95 the shifter fork 96 is acted upon by the frictional drag forces created between the shifter fork 96 the rack 92 the upper surface of the drive gear 83 and the outer peripheral surface.