efficiency increases

Conventional spiral bevel gearing, meshing in the position shown at the bottom of the drawing, has a purely rolling meshing action that is mechanically very efficient.  Its drawback is that it offers the smallest total tooth contact area, so its torque throughput capacity is lower.  Single-stage spiral bevel gearing is limited to about a 6:1 reduction ratio.   It’s easy to get higher ratios with multiple-stage configurations, but the additional gear stage lowers mechanical efficiency, increases backlash, consumes space and weight and reduces reliability.

Conventional worm gearing, meshing in the position shown at the top of the drawing, has a very high total tooth contact area.  While it offers high torque throughput and high ratio reduction, worm gearing has the lowest mechanical efficiency, due to the friction generated by its high component of sliding action. ...