The right way to keep away from over- or under-sizing a servo gearbox

Sponsored by GAM.

When sizing a servo gearbox, it’s tempting to deal with a single quantity. In lots of instances, engineers make their choice based mostly totally on torque, velocity, or inertia matching. However contemplating just one specification as an alternative of how a number of parameters work together can result in quite a lot of points—similar to a gearbox and servo motor which might be both outsized or undersized for the appliance.

Oversizing means paying for extra gearbox than the appliance wants, whereas undersizing may end up in downtime and sudden prices when the gearbox fails. Even with out outright failure, an undersized gearbox is prone to overheat or trigger poor machine efficiency.

“Our joke within the office is that the gearbox acts as the fuse in the system,” says Matt Ruggles, senior design engineer at GAM, a U.S.-based producer of servo gear reducers and different movement management elements. “If something goes wrong, the gearbox is the thing that breaks.”

A number of specs come into play when deciding on a servo gearbox. Torque demand is without doubt one of the first components to think about, encompassing nominal torque, acceleration torque, and emergency torque.

Nominal torque refers back to the steady output torque a gearbox can transmit throughout regular operation with out overheating or extreme put on. In cyclical functions, RMS output torque accounts for variations within the obligation cycle. Acceleration torque displays the brief bursts of torque required to hurry up or decelerate the load. Emergency torque defines the utmost degree the gearbox can stand up to briefly throughout irregular occasions similar to emergency stops or crashes, and is often restricted to a set variety of occasions.

Radial and axial load capability should even be evaluated. These rankings are based mostly on the capability of the gearbox bearings and point out how a lot load the output shaft can carry.

After which there’s inertia, which describes how resistant a load is to modifications in movement. Gearboxes are usually used to assist match the inertia of the servo motor to that of the pushed load.

The setting the place the system operates can introduce further necessities, similar to temperature limits, IP rankings, or corrosion resistance. In food-processing environments, for instance, gearboxes might have to make use of food-safe supplies.

Stainless-steel inline planetary gearbox for washdown or clear environments. (Picture: GAM.)

In response to Ruggles, gearbox choice ought to begin by figuring out what the appliance is definitely demanding from the system; that is likely to be radial load capability, torque transmission, or one other constraint. The remaining specs can then be addressed in sequence.

“I haven’t seen a gearbox that can do everything, as much as I would like there to be,” says Ruggles. “There’s always going to be compromises, and it’s just a question of what is really the thing you were trying to achieve at the end of the day.”

Focusing too closely on one specification can create unintended penalties elsewhere within the system. Ruggles factors to bearing capability for example. If radial load is the one issue thought-about, the result’s usually a gearbox that’s too giant, with an undersized motor compared. When that imbalance happens, the motor can wrestle to carry out and find yourself stalling.

“You have this tiny little motor trying to turn this big gearbox,” says Ruggles. “It’s the little engine that couldn’t. Something that we can do at GAM is if you have a very high radial load, we can provide additional support at the output without having to enlarge the main size of the gearbox. We can custom tailor it to match the application instead of just going into the catalog and saying, ‘Here’s the biggest gearbox we have—good luck.’”

One other state of affairs can come up when functions depend on extraordinarily excessive gearbox ratios to realize nice positioning or inertia matching. “With motors, they’re never perfectly smooth in their torque,” says Ruggles. “There’s some torque ripple. Those small ripples can result in large torque variations at the output. If you’re not careful, it can be very easy to over-torque the gearbox or overpower what’s happening at the output.”

Excessive ratios can even make it tougher to know what is occurring inside the system throughout tuning or troubleshooting, since variations occurring on the load could also be dampened earlier than they attain the motor.

“Having a good, broad understanding of what’s happening in the application can impact other aspects of the gearbox that you weren’t anticipating,” provides Ruggles. “Sometimes it’s the environment, sometimes it’s an unexpected shock load, sometimes it’s as simple as the person assembling the machine having trouble reaching a bolt.”

A custom-made inline gearbox (proper) has a shorter size in comparison with the usual (left). This small change helped simplify a machine design, for a big financial savings to the builder. (Picture: GAM.)

In follow, the appliance doesn’t all the time behave the best way the unique calculations counsel. Methods could also be sized based mostly on the anticipated torque required to maneuver a load, just for the precise load to be bigger or have a unique inertia than anticipated. These variations can tip the steadiness of what the motor and gearbox are able to dealing with, particularly in designs that function near their limits.

“Application sizing can be really tricky,” says Ruggles. “Being proud and thinking you’ve got it all sometimes can bring down a whole project. We’ve got a lot of experience at GAM, and we’re happy to help people with their application sizing.”

To study extra, go to GAM.

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