Worm gears are used when large gear planetary gearbox reductions are needed. It is common for worm gears to possess reductions of 20:1, and even up to 300:1 or greater.
Many worm gears have an interesting property that no additional gear established 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 attempts to spin it, the friction between the equipment and the worm retains the worm in place.
This feature is useful for machines such as for example conveyor systems, in which the locking feature 
can act as a brake for the conveyor when the motor isn’t turning. An added very interesting utilization of worm gears is usually in the Torsen differential, which is used on some high-performance cars and trucks.