Perhaps the most apparent is to improve 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 components as well as lubricants. In general, expect to pay out more for quieter, smoother gears.
Don’t make the mistake of over-specifying the electric motor. Remember, the input pinion on the planetary should be able deal with the motor’s result torque. Also, if you’re using a multi-stage gearhead, the result stage should be strong enough to absorb the developed torque. Certainly, using a more powerful motor than necessary will require a larger and more costly gearhead.
Consider current limiting to safely impose limitations on gearbox size. With servomotors, result torque is a linear function of current. So besides protecting the gearbox, current limiting also defends the engine 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 concurrently in mesh. Although you can’t really totally eliminate noise from such an assembly, there are several ways to reduce it.
As an ancillary benefit, the geometry of planetaries fits the shape of electric motors. Therefore the gearhead can be close in diameter to the servomotor, with the output shaft in-line.
Highly rigid (servo grade) gearheads are usually more costly than lighter duty types. However, for rapid acceleration and deceleration, a servo-grade gearhead may be the only wise choice. In such applications, the gearhead may be viewed as a mechanical spring. The torsional low backlash planetary gearbox deflection caused by the spring action adds to backlash, compounding the consequences of free shaft movement.
Servo-grade gearheads incorporate many 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 tend to be the most costly of planetaries.
The kind of bearings supporting the output shaft depends upon the load. High radial or axial loads generally necessitate rolling element bearings. Small planetaries can often manage with low-price sleeve bearings or various other economical types with relatively low axial and radial load capacity. For larger and servo-grade gearheads, durable output shaft bearings are often required.
Like most gears, planetaries make noise. And the faster they operate, the louder they get.
Low-backlash planetary gears are also obtainable in lower ratios. While some types of gears are usually limited by about 50:1 or more, planetary gearheads expand from 3:1 (solitary stage) to 175:1 or even more, depending on the amount of stages.