Gear units are used to increase or decrease the speed of rotating machines. These gears have to be designed to withstand the torque produced by their own weight and can also accommodate the load applied to the rotating device. Gear units can also be used to transfer energy. There are two types of gear units: spiral bevel gears and helical gears.

Gear units come in various sizes. The external dimensions of a gear unit are typically 160x90x200mm. These units are usually enclosed in an aluminium housing. A lubricant is usually pumped into the housing to avoid splashing and causing damage to the components. Bearings in a gear unit should not be submerged in the sump. The oil projections from the rotating elements help to lubricate the bearings.

The input and output shafts are connected to the gear unit through shafts. These shafts are locked on by couplings or keys. Bearings are used to support the shafts and reduce friction. The housing of a gear unit encloses the gear drive assembly and contains the shafts. The housing may also contain flanges for mounting.