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 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 engine. Remember, the input pinion on the planetary must be able manage the motor’s result 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 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. So besides protecting the gearbox, current limiting also protects 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 simultaneously in mesh. Although you can’t really totally remove noise from such an assembly, there are several methods to reduce it.
As an ancillary benefit, the geometry of planetaries fits 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 usually more expensive than lighter duty types. However, for rapid acceleration and deceleration, a servo-grade gearhead could be the only sensible choice. In such applications, the gearhead could be viewed as a mechanical springtime. The torsional deflection resulting from the spring action adds to backlash, compounding the consequences of free shaft motion.
Servo-grade gearheads incorporate a number of construction features to minimize torsional stress and deflection. Among the more prevalent 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 on the load. High radial or axial loads usually necessitate rolling component bearings. Small planetaries can often get by with low-cost sleeve bearings or additional 
economical types with relatively low axial and radial load capacity. For larger and servo-grade gearheads, durable output shaft bearings are usually required.
Like most gears, planetaries make noise. And the quicker they operate, the louder they obtain.
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