How Gears Work

Gears are used in many mechanical devices to provide speed reduction in motorized equipment. A small, rapidly-turning motor can provide enough power for some devices, but not enough torque.


Gears are used in many mechanical devices to provide speed reduction in motorized equipment. A small, rapidly-turning motor can provide enough power for some devices, but not enough torque. An electric screwdriver has a large gear reduction because it needs torque to turn screws, but the motor produces a small amount of torque at high speed. With gear reduction, the output speed is reduced while the torque is increased.

Gears can also change rotational direction. Automobile differentials use bevel gears to turn the power from the drive shaft 90 deg and send it to the wheels.

The distance from the center of the gear to the point of contact determines gear ratio. If one gear is twice the diameter of the other, the ratio is 2:1.

Spur gears

Spur gears are the most common type. They have straight teeth and are mounted on parallel shafts. A sequence of spur gears can be used to create very large gear reductions.

Spur gears can be noisy. Each time a tooth engages a tooth on the other gear, they collide, making noise and increasing the stress on the teeth.

Helical gears

Helical gear teeth are cut at an angle to the face of the gear. More than one tooth on a helical gear system is always engaged. Contact starts at one end of a tooth and gradually slides across the face as the gears rotate.

This gradual engagement and multiple tooth contact make helical gears operate smoother and quieter than spur gears. Helical gears are usually mounted on parallel shafts, but if the gear tooth angles are correct, they can be mounted on perpendicular shafts, changing the rotation angle by 90 deg.

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

Bevel gears

Bevel gears are useful when the direction of the rotation axis must be changed. They are typically mounted on shafts 90-deg apart, but can be designed to work at other angles.

Teeth on bevel gears can be straight or spiral. Straight bevel gear teeth have the same problem as straight spur gear teeth-as each tooth engages, it contacts the corresponding tooth all at once.

The solution to this problem is to curve the gear teeth. These spiral teeth engage just like helical gear teeth. Contact starts at one end of the gear and progressively slides across the whole tooth.

On straight and spiral bevel gears, the shafts must be mounted in the same plane so that the gear centers are aligned.

Worm gears

Worm gears are used when large gear reductions are necessary. Worm gears can have reductions of 20:1; some exceed 300:1.

Many worm gears have a unique property-the worm can easily turn the gear, but the gear cannot turn the worm. This difference 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 property is useful for conveyer systems because the locking feature can act as a brake for the conveyer when the motor is not turning.

Involute gear profile

Today, virtually all gears use a tooth profile called an involute. This specially curved profile maintains a constant ratio of rotational speed between the two gears. As the gears spin, the point of contact moves, but the tooth profile continually compensates for the movement.

Marshall Brain, founder and CEO of HowStuff, will be at the Association for Facilities Engineering (AFE) booth Monday and Tuesday (March 5-6) of National Manufacturing Week.

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