The worm gear is a special type of gear that looks like a screw.

Worm gears have some unique properties.

1. Worm gears change the direction of rotation. The output axle is perpendicular to the input axle. This is similar to bevel gears.

2. Worm gears provide a very large gear ratio. This means that worm gears can produce a very large increase in force, but at the cost of a slowdown in speed. Every time the worm gear makes one revolution, it advances just 1 tooth on the spur gear that is meshed with it. That means if a 24 tooth spur gear is meshed with a worm gear, the gear ratio is 24 to 1!

3. Worm gears can only be driven in one direction. A worm gear can turn the spur gear, but the spur gear cannot turn the worm gear. The worm gear can only be on the input axle. This makes the worm gear act like a ratchet. This is useful if you need something like a robot arm to stay in one place, even though gravity might be trying to pull it down.

See also: Gears, Spur gear, Bevel gear, Ratchet, Compound Gear Ratio