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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 is also suffering from gear and housing components and also lubricants. In general, expect to pay 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 output torque. What’s more, if you’re utilizing a multi-stage gearhead, the result stage must be strong enough to soak up the developed torque. Obviously, using a better motor than necessary 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. So besides protecting the gearbox, current limiting also low backlash planetary gearbox defends the motor 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 get rid of noise from this assembly, there are several ways to reduce it.

As an ancillary benefit, the geometry of planetaries matches the shape of electric motors. Therefore 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 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 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 type of bearings supporting the output shaft depends on the load. High radial or axial loads generally necessitate rolling element bearings. Small planetaries can often get by with low-cost sleeve bearings or various other economical types with fairly low axial and radial load capacity. For bigger and servo-grade gearheads, heavy duty output 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 obtainable in lower ratios. While some types of gears are usually limited by about 50:1 and up, planetary gearheads lengthen from 3:1 (one stage) to 175:1 or even more, depending on the number of stages.