Perhaps the most apparent is to increase precision, which really 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 as well as lubricants. In general, expect to pay out more for quieter, smoother gears.
Don’t make the mistake of over-specifying the engine. Remember, the input pinion on the planetary should be able manage the motor’s output 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 better motor than required will require a bigger and more expensive gearhead.
Consider current limiting to safely impose limits on gearbox size. With servomotors, output torque is certainly a linear function of current. Therefore besides protecting the gearbox, current limiting also defends the electric 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 remove noise from this assembly, there are many ways to reduce it.
As an ancillary benefit, the geometry of planetaries fits the form 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 generally more expensive than lighter duty types. However, for speedy acceleration and deceleration, a servo-grade gearhead may be the only sensible choice. In such applications, the gearhead may be seen as a mechanical springtime. The torsional deflection resulting from the spring action low backlash gearbox increases backlash, compounding the consequences of free shaft movement.
Servo-grade gearheads incorporate many 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 tend to be the most costly of planetaries.
The kind of bearings supporting the output shaft depends on the load. High radial or axial loads generally necessitate 
rolling element bearings. Small planetaries could manage with low-price sleeve bearings or various other economical types with fairly low axial and radial load capability. For bigger and servo-grade gearheads, durable result shaft bearings are usually required.
Like the majority of gears, planetaries make noise. And the quicker they run, the louder they obtain.
Low-backlash planetary gears are also obtainable in lower ratios. While some types of gears are usually limited by about 50:1 and up, planetary gearheads prolong from 3:1 (solitary stage) to 175:1 or even more, depending on the amount of stages.