How to avoid over- or under-sizing a servo gearbox

Sponsored by GAM.

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

Oversizing means paying for extra gearbox than the applying wants, whereas undersizing can lead to downtime and surprising prices when the gearbox fails. Even with out outright failure, an undersized gearbox is prone to overheat or trigger poor machine efficiency.

“Our joke inside the workplace is that the gearbox acts because the fuse within 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 one thing goes unsuitable, the gearbox is the factor that breaks.”

A number of specs come into play when deciding on a servo gearbox. Torque demand is without doubt one of the first elements to contemplate, 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 quick bursts of torque required to hurry up or decelerate the load. Emergency torque defines the utmost stage the gearbox can face up to briefly throughout irregular occasions akin to emergency stops or crashes, and is often restricted to a hard and fast variety of occasions.

Radial and axial load capability should even be evaluated. These scores 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 atmosphere the place the system operates can introduce further necessities, akin to temperature limits, IP scores, or corrosion resistance. In food-processing environments, for instance, gearboxes may have to make use of food-safe supplies.

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

In line with Ruggles, gearbox choice ought to begin by figuring out what the applying 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 may do every part, as a lot as I would love there to be,” says Ruggles. “There’s at all times going to be compromises, and it’s only a query of what’s actually the factor you had been attempting to realize on the finish of the day.”

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

“You might have this tiny little motor attempting to show this huge gearbox,” says Ruggles. “It’s the little engine that couldn’t. One thing that we are able to do at GAM is when you’ve got a really excessive radial load, we are able to present further help on the output with out having to enlarge the principle dimension of the gearbox. We will customized tailor it to match the applying as a substitute of simply going into the catalog and saying, ‘Right here’s the largest gearbox we now have—good luck.’”

One other scenario can come up when functions depend on extraordinarily excessive gearbox ratios to realize high-quality positioning or inertia matching. “With motors, they’re by no means completely easy of their torque,” says Ruggles. “There’s some torque ripple. These small ripples can lead to massive torque variations on the output. If you happen to’re not cautious, it may be very straightforward to over-torque the gearbox or overpower what’s occurring on the output.”

Excessive ratios may make it tougher to grasp what is going on 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 great, broad understanding of what’s occurring within the utility can impression different elements of the gearbox that you just weren’t anticipating,” provides Ruggles. “Typically it’s the atmosphere, generally it’s an surprising shock load, generally it’s so simple as the particular person assembling the machine having hassle 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 applying doesn’t at all times 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 distinct 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.

“Utility sizing may be actually difficult,” says Ruggles. “Being proud and considering you’ve acquired all of it generally can convey down a complete undertaking. We’ve acquired a variety of expertise at GAM, and we’re comfortable to assist folks with their utility sizing.”

To be taught extra, visit GAM.

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