A Worm gear is an arrangement in which a worm (a gear in the form of a screw) engages with a worm wheel usually used for large gear reductions.

An interesting property of worm gears is that the worm can turn the wheel, but the wheel cannot turn the worm.

This is because there is such a large friction between the gears holding the worm in place due to the angle between the two segments being so small – giving it a self-locking system. Also worm gears are used in car differentials.