Spur gears are wheels with teeth that mesh together. Spur gears are used to change the speed and force of a rotating axle.


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How much the speed and force change depends on the gear ratio. The gear ratio is the ratio of the number of teeth on the pair of gears that are meshed. The first gear in the pair is on the input axle. For example, this could be the gear on the axle of the motor. The second gear in the pair is on the output axle. This could be the axle of the wheel. The gear ratio is the ratio number of teeth of the gear on the output axle to the number of teeth of the gear on the input axle. For example, this picture shows an 8 tooth spur gear meshed with a 40 tooth spur gear. If the 8 tooth gear is on the input axle, and the 40 tooth gear is on the output axle, then the gear ratio for this gear pair is 40 to 8. This can be simplified to 5 to 1.

What this means is it takes 5 revolutions of the input gear to make 1 revolution of the output gear. This results in a slowdown of the output gear by a factor of 5. It also increases the force of the output gear by a factor of 5. If the input and output axles are reversed, then the gear ratio would be 1 to 5. That means the output axle would rotate 5 times faster than the input axle, but have 5 times as less force.

Spur gears change the direction of rotation. If the input axle rotates clockwise, then the output axle would rotate counter clockwise.

See also: Bevel gear, Worm gear, Slip clutch, Belts and pulleys, Compound Gear Ratio, Gears and Speed, Gears and Strength