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 can be suffering from gear and housing materials along with lubricants. In general, expect to pay out more for quieter, smoother gears.
Don’t make the mistake of over-specifying the engine. Remember, the insight pinion on the planetary must be able handle the motor’s output torque. Also, if you’re utilizing a multi-stage gearhead, the result stage should be strong enough to soak up 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 definitely a linear function of current. Therefore besides protecting the gearbox, current limiting also defends the engine and drive by clipping peak torque, which can be anywhere 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 get rid of noise from this assembly, there are many methods to reduce it.
As an ancillary benefit, the geometry of planetaries matches the form of electric motors. Thus the gearhead could be close in diameter to the servomotor, with the output shaft in-line.
Highly rigid (servo grade) gearheads are generally 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 springtime. 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 output shafts 
and beefed up support for satellite-equipment shafts. Stiff or “rigid” gearheads have a tendency to be the most costly of planetaries.
The kind of bearings supporting the output shaft depends upon the strain. High radial or axial loads generally necessitate rolling element bearings. Small planetaries could manage with low-price sleeve bearings or additional economical types with relatively low axial and radial load capability. For larger and servo-grade gearheads, durable output shaft bearings are usually required.
Like most gears, planetaries make sound. And the quicker they run, the louder they obtain.
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