Worm gears are used when huge gear reductions are needed. It is common for 
worm gears to possess reductions of 20:1, and also up to 300:1 or greater.
Many worm gears have a fascinating property that no other gear arranged has: the worm can certainly turn the gear, but the gear cannot turn the worm. That is because the position on the worm is so shallow that when the apparatus tries to spin it, the friction between the gear and the worm holds the worm set up.
This feature pays to for machines such as for example China Pulley conveyor systems, in which the locking feature can become a brake for the conveyor when the motor isn’t turning. One other very interesting usage of worm gears is in the Torsen differential, which is used on some high-performance cars and trucks.