A gear quickness reducer is a representative exemplory case of speed 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) gear reducer with orthogonal axes, (3) gear reducer with perpendicular nonintersecting axes, and (4) gear reducer with coaxial axes.
Types and mechanisms of gear reducers with parallel axes
The apparatus 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 obtainable. The overall characteristics of equipment reducers with parallel axes are the following :
For high precision gears, the tranny efficiency is very high. (98 to 95% for one-stage gear reducer)
When correctly lubricated, it can be used for a long time.
Could be produced relatively cheaply because standardized gears are used.
Gear reducers with spur gears are used for increasing acceleration.
The sizes of gear reducers with spur gears are is generally large. In comparison to worm gear reducers with the same speed ratio, their outer forms are large, and the number of parts increases resulting in constructional disadvantages. Consequently, it is utilized for devices with high rotation on the strain side, or which need higher output rotation compared to the prime movers (for increasing speed). The apparatus types are shown in Table 2.1.
The gear 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 planetary gearbox motors which are gear reducers with directly linked motors.