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 can be suffering from gear and housing components as well as lubricants. In general, expect to spend more for quieter, smoother gears.
Don’t make the error of over-specifying the electric motor. Remember, the input pinion on the planetary must be able handle the motor’s result torque. What’s more, if you’re using a multi-stage gearhead, the result stage must be strong enough to absorb the developed torque. Certainly, using a more powerful motor than required will require a bigger 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. So besides safeguarding the gearbox, current limiting also protects the motor and drive by clipping peak torque, which may be 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 eliminate noise from this 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 could be close in diameter to the servomotor, with the output shaft in-line.
Highly rigid (servo grade) gearheads are usually more expensive 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 spring. The torsional deflection resulting from the spring action increases backlash, compounding the effects of free shaft motion.
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 have a tendency to be the costliest of planetaries.
The kind of bearings supporting the output shaft depends on the strain. High radial or axial loads usually necessitate rolling component bearings. Small planetaries can often get by with low-price 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 usually required.
Like most gears, planetaries make sound. And the faster they operate, the louder they get.
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