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 easily turn the gear, however the gear cannot turn the worm. This is because the angle on the worm is indeed shallow that when the gear tries to spin it, the friction between your planetary gearbox equipment and the worm retains the worm set up.
This feature pays to 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. One other very interesting utilization of worm gears is usually in the Torsen differential, which is utilized on some high-performance vehicles.