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)