A gear is a wheel with teeth on the outside rim. Gears are usually used in groups of two or more. The teeth of one rotating gear will be used to push the teeth of other gears, causing them to rotate too. When the teeth on two gears are placed like that, the gears are said to be meshed.

Gears can be used to transfer force, increase or reduce speed, and change the direction of rotary motion. The gear you turn (called the driver) causes the other gear (called the follower) to turn.
92_gears___ animation___ final_by_

This video introduces gears.

Click on the player to watch.

Facts about gears...

Two gears which are meshed together turn in opposite directions. files/animsp/ 00005000/5267/ three_gears_ turning_A_md_wm.gif

An idler gear in the middle makes neighboring gears turn in the same direction. The idler gear is blue in this animation. wikipedia/ commons/d/ d1/Gears_ animation_A-B.gif

When two gears of different size are meshed, the smaller one turns faster and the larger one turns slower. If A is the driver, it will make B turn faster (gearing down). If B is the driver, it will make A turn faster (gearing up). fileuploads/ 9192/ 919209236b00 3417a172380f9b 47a410da00dac.gif

A crown gear can change the direction of rotary motion through a 90 degree angle.

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