According to this invention, backlash is eliminated through the use of a split-worm that is preloaded by having a first part of the worm fixed in a stationary position on an input shaft and the second part of the split-worm mounted for limited axial movement along the input shaft relative to the stationary part of the split-worm. Both parts of the split-worm rotate with the input shaft, unlike the prior art. The ball gear uses recirculating balls instead of gear teeth to transmit the tangential load from one gear member to the other. The balls roll through ground grooves in the worm halves and thus drive the ball gear. The balls recirculate through an internal passage extending through the two parts of the split-worm. The ends of the internal passage communicate with the grooves on the exterior of the split-worm.