Worms and Worm Gears

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 $d_W$ is not related to gear ratio ($\frac{N_G}{N_W}\ne \frac{d_G}{d_W}$). Instead, we have $L=N_W\cdot p_x$, where $N_W$ is the number of worm teeth (or start number).
• The axial pitch $p_x$ is the same as the circular pitch of the gear, such that $p_x=p_G$
• One of the important parameters for Worm Gear Load Analysis is the lead angle $\lambda$, or its tangent value given as $\tan \lambda =\frac{L}{\pi d_W}$.

Example