The planetary way

Sept. 1, 2000
Low-backlash planetary gearheads are installed on nearly half the servomotors in industry. Find out if they make sense for you, and how to choose the best one for your application.

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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Rigidity: Highly rigid (servo grade) gearheads are generally more expensive than lighter duty types. However, for rapid acceleration and deceleration, a servo-grade gearhead may be the only sensible choice. In such applications, the gearhead may be viewed as a mechanical spring. The torsional deflection resulting from the spring action adds to backlash, compounding the effects of free shaft movement.

Construction: Servo-grade gearheads incorporate several construction features to minimize torsional stress and deflection. Among the more common are large diameter output shafts and beefed up support for satellite-gear shafts. Stiff or "rigid" gearheads tend to be the most costly of planetaries.

Bearings: The type of bearings supporting the output shaft depends on the load. High radial or axial loads usually necessitate rolling element bearings. Small planetaries can often get by with low-cost sleeve bearings or other economical types with relatively low axial and radial load capability. For larger and servo-grade gearheads, heavy duty output shaft bearings are usually required.

Acoustic noise: Like most gears, planetaries make noise. And the faster they run, the louder they get.

In each planetary stage, five gears are simultaneously in mesh. Although it's impossible to totally eliminate noise from such an assembly, there are several ways to reduce it.

Perhaps the most obvious 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 affected by gear and housing materials as well as lubricants. In general, expect to pay more for quieter, smoother gears.

Motor: Don't make the mistake of over-specifying the motor. Remember, the input pinion on the planetary must be able handle the motor's output torque. What's more, if you're using a multi-stage gearhead, the output stage must be strong enough to absorb the developed torque. Obviously, using a more powerful motor than necessary will require a larger and more costly gearhead.

Consider current limiting to safely impose limits on gearbox size. With servomotors, output torque is a linear function of current. So besides protecting the gearbox, current limiting also protects the motor and drive by clipping peak torque, which can be anywhere from 2.5 to 3.5 times continuous torque.

Ratios: Choose "strong" ratios whenever possible. Typical ratios for single-stage planetaries are 3:1, 4:1, 5:1, 7:1, and 10:1. For a given gearhead diameter, the 3:1, 4:1, and 5:1 "baseline" ratios are relatively strong. By contrast, the 7:1 ratio achieves approximately 80% of the baseline torque rating, while the rating for 10:1 is only about 65%. These relationships are geometry dependent and inherent in all planetaries.

Another way to understand it is to realize that for a given gearhead diameter, the 10:1 ratio has the smallest input pinion. The strength of the input pinion obviously imposes a limit on torque. It follows then that a 10 x 10 (100:1) gearhead is not as strong as a 5 x 5 x 4 (100:1). Likewise, a twostage 3 x 3 (9:1) or 4 x 3 (12:1) gearhead is stronger than a 10:1 single-stage.

Temperature: Planetary gearheads tend to run hot. Since operating temperatures are a function of duty cycle, cyclic applications with little time to rest generate the most heat.

Sevomotors also get warm when they run, and some of the heat flows into the gearhead. Gears also generate heat due to inefficiencies. Friction from ball bearings, tightly preloaded for rigidity, is yet another heat source.

Cooling: Because of their compact design, planetary gearheads have relatively little surface area to dissipate heat. Thus, lubrication can be very important. In general, oil transmits heat through the housing more evenly and efficiently than grease.

There's a catch though. Heat can shorten the life of lubricants and seals, so you can't rely entirely on oil or grease. To safely limit temperature, you might consider using a larger or more efficient gearhead, or an external cooling fan or heat sink.

Tooth profile: Unless you absolutely need exotic gearing, your best bet is to stick with standard spur gears with straight teeth.

Exotic gears — sometimes found in right-angle gearheads — are generally used to achieve quietness or a slightly more compact design for a given output torque. They derive their benefits from higher tooth contact ratios as compared to spur gears. The tradeoffs are that exotic gears are more expensive to make and they introduce bi-directional internal axial thrust forces, which often necessitate more expensive bearings.

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Precisely manufactured spur gearing, especially with hardened and computer controlled ground tooth shapes, usually offer just as much quietness and life as exotic gears.

Couplings: Input couplings are supplied by the gearhead manufacturer. Along with output couplings, they are as important as any other consideration.

Couplings should be selected based on the magnitude and duty cycle of the required torque. In small gearheads that don't have to deliver a lot of torque, input pinions typically attach to the motor shaft with a permanent adhesive. For moderatetorque applications, consider using a pinion that clamps onto the motor shaft.

For high-torque and servo applications, most gearheads have their input pinion integrated into the gearhead. This method — usually based on two clamping screws — locates the pinion quickly and easily on the motor shaft. If the motor has a keyway, it's not a bad idea to fill it with a half-key to prevent the input pinion from loosening due to the sides of the keyway collapsing.

Output shafts on servo gearheads are usually not keyed. Keys are considered unreliable in high-torque applications involving rapid reversal or frequent starting and stopping. Any degree of rotational looseness that may develop enables the key to pound loose and eventually fail. That's why many coupling manufacturers have developed zero-backlash couplings that allow for some misalignment.

Motor interface: Except for gearheads standardized for NEMA dimensioned motors, the gearhead manufacturer should be given the dimensions of the motor interface. Some manufacturers will machine adapter flanges and provide pinions to fit the motor shaft.

Mounting: Grease lubricated gearheads are usually safe in any orientation if the gears don't get hot. Even if they do, horizontal mounting may be okay because planetary gearing circulates lubricant well. However, in a hot-running vertical mount, the oil in the grease could drain out over time. Here, an oil lubricated gearhead with seals may be better.

Measuring backlash

Backlash specifications make sense only in the context of the way in which they are measured. One way to measure backlash is to attach an encoder on the back of a motor and observe the free play in the system. But this method is valid only for single-stage planetary gearheads.

Another method, one used by many gearhead manufacturers, is to lock the input pinion using some sort of fixture. Next, you would clamp a balanced (symmetrical) lever arm to the gearhead output shaft. By applying weights alternately on both sides of the arm — such that the resulting torque is about 3 to 5% of the rated torque — you can create a slight rotation S as measured with a dial indicator at a distance R.

Backlash is computed as:

θ = S/R (rad)

Heavier torque loads (> 5%) would begin to measure torsional deflection. Most of the torsional deflection is in the output shaft. In servo-grade gearheads, relatively large-diameter output shafts and other features minimize this potential error source.

Backlash and size

Contrary to what you might think, backlash is toughest to control in small gearheads. Consider the relationship between backlash and gear dimensions:

θ = S/R

where u = backlash or arc of travel (rad), S = arc distance (in.), and R = radius of measurement (in.).

Gearhead makers indirectly control S, the distance between gear teeth, with their machinery. It follows then that if a gearhead manufacturing process has a finite limit on the smallness of S, backlash is a function of R. Hence, it is more difficult and costly to achieve low backlash in small gearheads.

Tom Provencher is Director of Marketing and Sales for Mijno Precision Gearing, Park Ridge, Ill.

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