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What are the common types of gears?

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Spur gears are the most common type of gears. They have straight teeth and are mounted on parallel shafts. Sometimes, many spur gears are used at once to create very large gear reductions.

Spur gears are used in many devices that you can see all over HowStuffWorks, like the electric screwdriver, dancing monster, oscillating sprinkler, windup alarm clock, washing machine and clothes dryer. But you won’t find many in your car.

This is because the spur gear can be really loud. Each time a gear tooth engages a tooth on the other gear, the teeth collide, and this impact makes a noise. It also increases the stress on the gear teeth.

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To reduce the noise and stress in the gears, most of the gears in your car are helical, which we’ll explain next.

The teeth on helical gears are cut at an angle to the face of the gear. When two teeth on a helical gear system engage, the contact starts at one end of the tooth and gradually increases as the gears rotate, until the two teeth are in full engagement.


The gradual engagement of helical gears makes them operate much more smoothly and quietly than spur gears. For this reason, helical gears are used in almost all car transmissions.

Because of the angle of the teeth on helical gears, they create a thrust load on the gear when they mesh. Devices that use helical gears have bearings that can support this thrust load.

The bent teeth of helical gears means they have to be staggered, with the teeth of the next gear going in the opposite direction so the teeth can mesh. Each gear is called “right-handed” or “left-handed” when it meshes with another gear on parallel shafts. If the angles of the gear teeth are correct, helical gears can also be mounted on perpendicular shafts, adjusting the rotation angle by 90 degre.

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Worm gears are cylinders with a spiral thread wrapped around the outside that meshes with another gear to turn it. They’re used when large gear reductions are needed. It is common for worm gears to have reductions of 20:1, and even up to 300:1 or greater.

Many worm gears have an interesting property that no other gearset 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 tries to spin it, the friction between the gear and the worm holds the worm in place.

This feature is useful for machines such as conveyor systems, in which the locking feature can act as a brake for the conveyor when the motor is not turning. Worm gears are also used in the Torsen differential, which increases torque for some high-performance cars and trucks.

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