Perhaps the most apparent is to improve precision, which really is a function of manufacturing and assembly tolerances, gear tooth surface finish, and the center distance of the tooth mesh. Sound can be affected by gear and housing components as well as lubricants. In general, expect to pay more for quieter, smoother gears.
Don’t make the mistake of over-specifying the motor. low backlash gearbox Remember, the input pinion on the planetary must be able deal with the 
motor’s result torque. Also, if you’re utilizing a multi-stage gearhead, the result stage should be strong enough to absorb the developed torque. Obviously, using a better motor than required will require a bigger and more costly gearhead.
Consider current limiting to safely impose limits on gearbox size. With servomotors, result torque is certainly a linear function of current. Therefore besides safeguarding the gearbox, current limiting also protects the motor and drive by clipping peak torque, which may be from 2.5 to 3.5 times continuous torque.
In each planetary stage, five gears are at the same time in mesh. Although you can’t really totally remove noise from this assembly, there are several ways to reduce it.
As an ancillary benefit, the geometry of planetaries fits the shape of electric motors. Hence the gearhead can be close in diameter to the servomotor, with the output shaft in-line.
Highly rigid (servo grade) gearheads are usually more expensive than lighter duty types. However, for quick acceleration and deceleration, a servo-grade gearhead may be the only wise choice. In such applications, the gearhead could be seen as a mechanical spring. The torsional deflection resulting from the spring action increases backlash, compounding the effects of free shaft motion.
Servo-grade gearheads incorporate several construction features to reduce torsional stress and deflection. Among the more prevalent are large diameter output shafts and beefed up support for satellite-equipment shafts. Stiff or “rigid” gearheads tend to be the most costly of planetaries.
The type of bearings supporting the output shaft depends on the load. High radial or axial loads usually necessitate rolling element bearings. Small planetaries could get by with low-price sleeve bearings or various other economical types with relatively low axial and radial load ability. For larger and servo-grade gearheads, durable output shaft bearings are often required.
Like the majority of gears, planetaries make sound. And the faster they run, the louder they obtain.
Low-backlash planetary gears are also obtainable in lower ratios. Although some types of gears are generally limited to about 50:1 or more, planetary gearheads expand from 3:1 (solitary stage) to 175:1 or more, depending on the number of stages.