A gear speed reducer is a representative exemplory case of quickness changers, and presently used units can be categorized by the kind of gears, shaft positions and set up of gears into (1) gear reducer with parallel axes, (2) equipment reducer with orthogonal axes, (3) gear reducer with planetary gearbox perpendicular nonintersecting axes, and (4) gear reducer with coaxial axes.

Types and mechanisms of equipment reducers with parallel axes
The gear reducers with parallel axes use spur gears, helical gears, or herringbone gears. Their input and output shafts are parallel. As for decrease ratios, 1/1 – 1/7 for one-stage shafts, 1/10 – 1/30 for two-stage shafts, and 1/5 – 1/200 for more than three-stage shafts are commercially available. The overall characteristics of gear reducers with parallel axes are as follows :

For high precision gears, the transmission efficiency is very high. (98 to 95% for one-stage equipment reducer)
When correctly lubricated, it can be used for a long time.
Could be produced relatively cheaply as standardized gears are used.
Gear reducers with spur gears are used for increasing acceleration.
The sizes of gear reducers with spur gears are is usually large. In comparison to worm gear reducers with the same velocity ratio, their outer shapes are huge, and the amount of parts increases leading to constructional disadvantages. Therefore, it is utilized for machines with high rotation on the strain side, or which require higher output rotation compared to the primary movers (for increasing speed). The gear types are demonstrated in Table 2.1.

The apparatus reducers with parallel axes usually use helical gears. They are found in steel facilities, ships, cranes, elevators, and conveyors. For automation machines, these gear reducers are also known for geared motors which are equipment reducers with directly connected motors.