Perhaps the most apparent is to increase precision, which is a function of manufacturing and assembly tolerances, gear tooth surface finish, and the guts distance of the tooth mesh. Sound is also affected by gear and housing materials along with lubricants. In general, be prepared 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 handle the motor’s output torque. What’s more, if you’re using a multi-stage gearhead, the output stage must be strong enough to soak up the developed torque. Certainly, using a more powerful motor than necessary will require a larger and more expensive gearhead.
Consider current limiting to safely impose limits on gearbox size. With servomotors, result torque is definitely a linear function of current. Therefore besides protecting the gearbox, current limiting also protects the engine 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 simultaneously in mesh. Although it’s impossible to totally get rid of noise from such an assembly, there are several methods to reduce it.
As an ancillary benefit, the geometry of planetaries matches the shape of electric motors. Hence the gearhead could be close in diameter to the servomotor, with the result shaft in-line.
Highly rigid (servo grade) gearheads are generally more costly than lighter duty types. However, for fast acceleration and deceleration, a servo-grade gearhead could be the only sensible choice. In this kind of applications, the gearhead could be seen as a mechanical spring. The torsional deflection caused by the spring action adds to backlash, compounding the effects of free shaft motion.
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 costliest of planetaries.
The kind of bearings supporting the output shaft depends on the strain. High radial or axial loads usually necessitate rolling component bearings. Small planetaries can often get by with low-cost sleeve bearings or other economical types with fairly low axial and radial load capacity. For larger and servo-grade gearheads, heavy duty output shaft bearings are often required.
Like the majority of gears, planetaries make noise. And the faster they operate, the louder they get.
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