Perhaps the most apparent is to increase precision, which 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 along with lubricants. In general, expect to spend more for quieter, smoother gears.
Don’t make the mistake of over-specifying the motor. Remember, the input pinion on the planetary should be able handle the motor’s output torque. Also, if you’re utilizing a multi-stage gearhead, the result stage must be strong enough to absorb the developed torque. Obviously, using a more powerful motor than required will require a larger and more expensive gearhead.
Consider current limiting to safely impose limitations on gearbox size. With servomotors, output torque is usually a linear function of current. Therefore besides protecting the gearbox, current limiting also protects the engine and drive by clipping peak torque, which can be anywhere from 2.5 to 3.5 times continuous torque.
In each planetary stage, five gears are at the same time in mesh. Although it’s impossible to totally remove noise from such an assembly, there are many ways to reduce it.
As an ancillary benefit, the geometry of planetaries matches the shape of electric motors. Hence the gearhead can be close in diameter to the servomotor, with the result shaft in-line.
Highly rigid (servo grade) gearheads are generally more costly than lighter duty types. However, for rapid acceleration and deceleration, a servo-grade gearhead could be the only wise choice. In this kind of applications, the gearhead could be seen as a mechanical spring. The torsional deflection caused by the spring action adds to backlash, compounding the effects of free shaft low backlash planetary gearbox movement.
Servo-grade gearheads incorporate a number of construction features to reduce torsional stress and deflection. Among the more prevalent are large diameter output shafts and beefed up support for satellite-gear shafts. Stiff or “rigid” gearheads tend to be the costliest 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 get by with low-cost sleeve bearings or various other economical types with relatively 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 noise. And the quicker they operate, the louder they obtain.
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 expand from 3:1 (one stage) to 175:1 or even more, depending on the number of stages.