The primary task of a gear is to mesh teeth with another toothed part to transmit torque or to translate rotation. Thus, geared devices can change the speed, torque, and direction of a power source. When two gears of unequal number of teeth combine, it results in a mechanical advantage, with both the rotational speeds and the torques of the two gears differing in a simple relationship.

A number of devices use gears in some mechanical arrangement and these devices go by a number of names, depending on the branding of the manufacturer. Here is a look at some of the more common terms.

Gear Reducers

Gear reducers, also known as speed reducers, are a component of many mechanical, electrical, and hydraulic motors. Essentially it is a gear or series of gears combined in such a manner to alter the torque of a motor. Typically, the torque increases in direct proportion to the reduction of rotations per unit of time.

Speed reducers come in two varieties: base mounted and shaft mounted. The shaft-mounted type, in turn, has two variations. One is truly shaft mounted in that it is supported entirely by the input shaft of the drive machine, with torque reaction absorbed by a special link. The other is mounted to the driven-machine housing so the input shaft does not absorb reducer weight or torque reaction. By AGMA definition, the term “speed reducer” is applied to units operating at pinion speeds below 3,600 rpm or pitch-line velocities below

5,000 fpm. Reducers operating at speeds higher than these are called “high-speed units.” Catalog ratings and engineering specifications for speed reducers are generally based on AGMA standards.

There are as many types of speed reducers as there are types of gears. Worm gear reducers are used in low to moderate horsepower applications. They offer low initial cost, high ratios, and high output torque in a small package, along with a higher tolerance for shock loading then helical gear reducers. The ratio of a worm gear set is the ratio of the number of teeth in the gear to the number of threads (starts or leads) on the worm.

Helical reducers are used in higher horsepower applications where long-term operational efficiency is more important than initial cost. The ratio of a helical or bevel gear set is simply the number of teeth in the larger gear divided by the number of teeth in the smaller gear.

Spur gear reducers have a gear wheel with radial teeth parallel to the axle. In bevel and spiral bevel gears, the gear wheel meshes with another at an angle between 90o and 180o.


A gearhead is often another term for gear reducer; however, it does not limit the unit’s function to speed reduction. Gearheads are mainly used wherever an application calls for high torque at low speed. It reduces a load’s reflected mass inertia, which makes accelerating heavy loads easier, enabling a design to use a smaller motor.

Gearheads come in a variety of styles from basic spur gearheads to more complex planetary gearheads and harmonic-type gearheads, each with their own characteristics and suitable applications.

In some applications, gearhead backlash may become an issue. In this case, consider using a gearhead with low backlash or zero backlash.


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