It means a pair of worm which consists of one or several spiral helix and worm wheel the center of whose face width is concave, and tooth space of worm wheel is engraved at a rakish angle like helical gear, so it is driven by a screw thread. Worm gear can gain large reduction gear ratio of around 1/300 and cannot be driven by worm wheel.

It is gear in which pinion in the form of cone with screws emerged and gear with large spiral angle value are driven in a pair. With it, you can obtain high strength, high rate and high efficiency, and it is used in the place where the adjustment of backlash is important.