When Planetary Gears meet Servomotors

Feb. 22, 2001
Integrated planetary-geared servomotors are a compact choice for high-performance applications.

Ben Hoffmann
Designer, PSA Series Actuators
Exlar Corp.
Chanhassen, Minn.

Edited by Victoria Reitz

Exlar Corp.'s PSA Series planetary-geared servomotors are for high-torque, high-power applications.

Designers generally turn to gear reducers in servopositioning applications for two reasons. First, many applications require less speed and more torque than economically available from a servo-motor alone. A gear reducer trades top-end speed, which may not be required, for higher output torque. Servomotors typically run at 3,000 to 5,000 rpm although for compact, high-power applications, some motors are wound to operate at higher speeds.

Second, a servocontrolled system has optimal performance when load inertia and motor inertia are similar. Settling-time delays often result if load inertia is high compared with motor-armature inertia. Gear reducers solve this common problem, reducing reflected inertia (the load inertia seen by the control system) by the square of the gear reduction ratio. For example, a 5:1 reduction ratio provides a 25:1 reduction in the reflected load-inertia ratio, ensuring stable system operation and optimal machine performance.

When high performance is required, one good option is integral planetary gears, where the gear set is part of the actuator unit. In other systems gears bolt on to the motor and are not always planetary. An integrated servoactuator gives one of the highest power densities available in an electric-actuator system and is useful in applications that require high torque and/or power while minimizing weight and space. Typical uses include robotics, material handling, packaging machinery, and process control.

Planetary gearheads have high torque ratings because several gears uniformly share the load. The mixing motion of the planet gears with the outer ring gear keeps all teeth well lubricated. A single drop of oil on one tooth evenly redistributes over the entire gear set, which is not the case for nonplanetary gear reducers. Planetary reducers also offer low backlash, which can be important for positioning accuracy and servosystem stability.

An integrated planetary-geared servoactuator offers all the benefits of typical servomotors and bolt-on planetary gear-heads with few of the drawbacks. Add-on servogearheads use a collar clamp to attach the pinion gear and motor shaft, which causes problems for some users. The pinion gear may mount too far in or out and decrease the mesh with other gears. It can be misaligned, which reduces gear-head life. And incorrect fastener torque can lead to failure under load. Geared servomotors without clamps eliminate these problems and increase the system stiffness. Integrated planetary-geared actuators can also reduce inertia by eliminating unneeded components. This can improve performance through lower RMS current and quicker setting times.

Fewer components also offer compact packaging compared to bolt-on reducers. Bolt-on gear-heads frequently use right-angle gears to reduce the length a reducer extends beyond a machine. Right-angle gears, however, require an additional set of bevel gears to make the turn. These gears increase the solution's size, cost, complexity, and total backlash since another mechanism is placed between the load and the motor. Use of beveled gears defeats the advantage of a planetary gearhead altogether.

Some planetary-geared servo-motors offer additional options. For instance, oil-flow lubrication and cooling lets units operate continuously at up to three times the nominal torque and power rating. Oil-cooling also keeps the actuator cool in high-temperature environments. Other options include explosionproof designs, alternative materials and coatings, custom shafts, nonstandard motor voltages such as 24 or 48 Vdc, special feedback devices, and special electrical connectors to operate with most amplifiers.

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