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.

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

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