Worms and worm gears are a basic gear type. The worm resembles a screw. The direction of rotation of the worm gear (also called worm wheel) depends on the direction of rotation of the worm and upon whether the worm teeth are cut right-hand or left-hand.

Worm gearsets are also made so that the teeth of one or both wrap partly around the other. Such sets are called single-enveloping and double-enveloping worm gearsets. Worm gearsets are mostly used when the speed ratios of the two shafts are quite high (3 or more).

Key points:

  • Non-intersecting orthogonal axes
  • Can design them to prevent “backdriving”
  • Poor efficiency (40 - 85%)
  • High ratios obtainable in single stage
  • High torque capacity

Nomenclature and Handedness

  • Worm is not really a gear (more like a screw), so its pitch diameter is not related to gear ratio (). Instead, we have , where is the number of worm teeth (or start number).
  • The axial pitch is the same as the circular pitch of the gear, such that
  • One of the important parameters for Worm Gear Load Analysis is the lead angle , or its tangent value given as .

  • A RH worm is mated with a RH worm wheel (in contrast to helical gears)

Example