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 and also lubricants. In general, be prepared to pay more for quieter, smoother gears.
Don’t make the error of over-specifying the engine. Remember, the input pinion on the planetary should be able handle the motor’s output torque. What’s more, if you’re using a multi-stage gearhead, the low backlash planetary gearbox 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 safeguarding the gearbox, current limiting also protects the electric motor and drive by clipping peak torque, which may be anywhere 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 remove noise from this assembly, there are several methods to reduce it.
As an ancillary benefit, the geometry of planetaries matches the shape of electric motors. Thus the gearhead can 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 fast 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 resulting from the spring action adds to backlash, compounding the consequences 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 costliest of planetaries.
The kind of bearings supporting the output shaft depends upon the load. High radial or axial loads usually necessitate rolling component bearings. Small planetaries can often manage with low-cost sleeve bearings or various other economical types with fairly low axial and radial load capability. For larger and servo-grade gearheads, heavy duty result shaft bearings are often required.
Like the majority of gears, planetaries make sound. And the quicker they operate, the louder they get.
Low-backlash planetary gears are also available in lower ratios. While some types of gears are usually limited to about 50:1 and up, planetary gearheads extend from 3:1 (single stage) to 175:1 or even more, depending on the amount of stages.
