> How Gears Work
  › Introduction
  › Basics
  › Helical Gears
  › Bevel Gears
  › Worm Gears

> What is a Gear Ratio?
> How to select a Gearbox?
> Thermal Limits of Gearboxes
> Synthetic Gear Oil Selection
> Controlling Gearbox Contamination
> Avoiding Gear Maintanance Errors
> Failure Analysis for Gearing

Worm Gears
Worm gears are used when large gear reductions are needed. It is common for worm gears to have reductions of 20:1, and even up to 300:1 or greater.

Photo courtesy Emerson Power Transmission Corp. Figure 8. Worm gear

Many worm gears have an interesting property that no other gear set has: the worm can easily turn the gear, but the gear cannot turn the worm. This is because the angle on the worm is so shallow that when the gear tries to spin it, the friction between the gear and the worm holds the worm in place.

This feature is useful for machines such as conveyor systems, in which the locking feature can act as a brake for the conveyor when the motor is not turning. One other very interesting usage of worm gears is in the Torsen differential, which is used on some high-performance cars and trucks.