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 is also suffering from gear and housing components in addition to lubricants. In general, expect to pay more for quieter, smoother gears.
Don’t make the error of over-specifying the electric motor. Remember, the input pinion on the planetary should be able deal with the motor’s output torque. Also, if you’re using a multi-stage gearhead, the output stage must be strong enough to absorb the developed torque. Obviously, using a more powerful motor than necessary will require a larger and more expensive gearhead.
Consider current limiting to safely impose limits on gearbox size. With servomotors, output torque is definitely a linear function of current. So besides protecting the gearbox, current limiting also protects 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 at the same time in mesh. Although you can’t really totally remove noise from such an assembly, there are many methods 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 low backlash planetary gearbox servomotor, with the result shaft in-line.
Highly rigid (servo grade) gearheads are generally more costly than lighter duty types. However, for fast 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 caused by the spring action increases backlash, compounding the effects of free shaft motion.
Servo-grade gearheads incorporate several 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 costliest of planetaries.
The type of bearings supporting the output shaft depends on the strain. High radial or axial loads generally necessitate rolling element bearings. Small planetaries could get by with low-cost sleeve bearings or various other economical types with fairly low axial and radial load ability. For larger and servo-grade gearheads, heavy duty output shaft bearings are often required.
Like the majority of gears, planetaries make sound. And the faster they run, 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 and up, planetary gearheads prolong from 3:1 (solitary stage) to 175:1 or more, depending on the number of stages.