Not long ago, you'd be hard pressed to find even one servomotor on a manufacturing line. Today, thanks to technology, they're everywhere.

As you might expect, advances in electronics helped open the door, but just as pivotal was the development of a new class of gearing known as lowbacklash planetary (LBP) gearheads. Today, up to 50% of the servomotors in industry rely on LBP gearheads, and as prices come down, the numbers are expected to go up. If you're not familiar with this new technology, here's your chance to learn.

What's a gearhead?

Gearboxes are used with motors to multiply torque and reduce speed. In recent years, the term "gearhead" has emerged describing low-backlash planetary gearboxes specifically designed to be integrated with servomotors. For the sake of discussion, a "servomotor" application is understood to be one where a load is repetitively accelerated and decelerated under the control of a microprocessor- based electronic driver.

The term gearhead helps differentiate low-backlash planetaries from separable gearboxes used in combination with conventional "prime mover" types of electric motors. An example would be an ac motor driving a conveyor at constant speed, whose only control is an on-off switch. Gear types typically used in such applications include right-angle worm gears and parallel-shaft spur/helicals, foot-mounted or mated with NEMA C-face, 48-frame (6-in. diameter) or larger motors.

The amount of backlash in a standard gearbox is determined by the manufacturer and represents the usual trade-off between cost and function. Although in their catalogs, many manufacturers don't even mention backlash, you can expect anywhere from 90 to 180 arc-min (1.5 to 38) of play. By comparison, backlash in the average LBP gearhead is less than 30 arc-min.

In positioning applications, minimizing backlash is usually a good idea. The less backlash, the less error. For that reason, many designers try to avoid mechanical gearing altogether. But some motion control tasks, such as accelerating and decelerating large inertial loads, absolutely require gears. Fortunately, you can minimize the free rotational movement that causes these errors by specifying low-backlash components.

Low backlash also pays off in dynamic reversing applications, mainly by minimizing shock loads. With less shock load to contend with, a smaller LBP gearhead can often replace a larger conventional gearbox in many reversing applications.

Why planetary

Although it's possible to achieve low backlash in many types of gears, there are several advantages to taking the planetary route.

Planetary gears are more compact and less expensive than other types. And for a given output torque, their power transmission capability per unit volume is unmatched. Efficiencies of 95% per stage or better (at rated load) are typical.

Low-backlash planetary gears are also available in lower ratios. While some types of gears are generally limited to about 50:1 and up, planetary gearheads extend from 3:1 (single stage) to 175:1 or more, depending on the number of stages.

As an ancillary benefit, the geometry of planetaries matches the shape of electric motors. Thus the gearhead can be close in diameter to the servomotor, with the output shaft in-line.

Selection factors

Backlash: For a given size gearhead, the lower the backlash, the more you'll pay. So, don't specify 5 arc-min when 20 or 25 arc-min will do. On the other hand, don't expect to save money above 30 arc-min, the nominal limit for planetary gears. You can specify 45 or 60 arc-min if you wish, but you'll pay the same as if you specified 30 arc-min. A cost savings isn't realized until about 90 arcmin, which is the lower end of parallel- shaft gears.

Stages: Ratios of 3:1 to 10:1 are generally available in a single stage gear. For higher ratios, planetaries must be ganged, with the first stage feeding the second and so on. Ratios of up to 100:1 are achievable in two stages, but three stages makes for a stronger gearhead. The more stages, of course, the higher the cost.

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