Perhaps the most apparent is to increase precision, which is a function of manufacturing and assembly tolerances, gear tooth surface finish, and the center distance of the tooth mesh. Sound can be suffering from gear and housing components as well as lubricants. In general, expect to spend more for quieter, smoother gears.
Don’t make the error of over-specifying the motor. Remember, the insight pinion on the planetary must be able deal with the motor’s low backlash gearbox result torque. What’s more, if you’re utilizing a multi-stage gearhead, the result stage should be strong enough to soak up the developed torque. Obviously, using a more powerful motor than necessary will require a bigger and more costly gearhead.
Consider current 
limiting to safely impose limits on gearbox size. With servomotors, output torque is definitely a linear function of current. Therefore besides protecting the gearbox, current limiting also defends the motor and drive by clipping peak torque, which can be from 2.5 to 3.5 times continuous torque.
In each planetary stage, five gears are concurrently in mesh. Although you can’t really totally eliminate noise from such an assembly, there are many ways to reduce it.
As an ancillary benefit, the geometry of planetaries matches the form of electric motors. Hence the gearhead can be close in diameter to the servomotor, with the result shaft in-line.
Highly rigid (servo grade) gearheads are usually more costly than lighter duty types. However, for rapid acceleration and deceleration, a servo-grade gearhead may be the only wise choice. In such applications, the gearhead may be seen as a mechanical spring. The torsional deflection caused by the spring action adds to backlash, compounding the effects of free shaft movement.
Servo-grade gearheads incorporate a number of construction features to reduce torsional stress and deflection. Among the more common are large diameter result shafts and beefed up support for satellite-gear shafts. Stiff or “rigid” gearheads have a tendency to be the most costly of planetaries.
The type of bearings supporting the output shaft depends upon the strain. High radial or axial loads generally necessitate rolling element bearings. Small planetaries can often get by with low-cost sleeve bearings or other economical types with relatively low axial and radial load capacity. For larger and servo-grade gearheads, durable output shaft bearings are often required.
Like most gears, planetaries make noise. And the faster they run, the louder they obtain.
Low-backlash planetary gears are also obtainable in lower ratios. While some types of gears are usually limited to about 50:1 and up, planetary gearheads expand from 3:1 (solitary stage) to 175:1 or more, depending on the amount of stages.