A servo gearbox must be sized from the complete motion profile, not selected from motor power or nominal output torque alone. By the end, you will have a practical checklist for comparing ratios, torque ratings, inertia, precision, loads, construction, mounting, and operating conditions before requesting a quotation.
Key takeaways
- Calculate the ratio from required output speed, not motor speed alone.
- Size continuous, peak, emergency, and thermal torque separately.
- Check reflected inertia, backlash, torsional stiffness, and reversal duty.
- Confirm output loads, mounting dimensions, environment, and the final datasheet.
Start With the Motion Profile and the Required Gear Ratio
A gearbox ratio is only suitable if it fits the machine’s complete motion profile. Record the required output speed and torque, stroke or travel, acceleration and deceleration times, reversal frequency, load inertia, friction or process torque, axis orientation, duty cycle, available space, and mounting constraints.
From the motor datasheet, capture motor rated torque, peak torque, rated speed and maximum speed, rotor inertia, encoder limits, and continuous and intermittent operating limits.
1. Calculate the target ratio from speed: N = motor speed ÷ required output speed. Choose the motor operating speed needed for the cycle, then check that the resulting ratio keeps the motor below maximum speed while using rated speed efficiently.
2. Reflect the load to the motor shaft: Jreflected = Jload ÷ N². A higher ratio reduces reflected inertia, but it also lowers output speed and makes encoder resolution, backlash, and torsional compliance more significant at the load.
3. Calculate acceleration torque from motor-side inertia, reflected load inertia, acceleration, load torque, and gearbox losses. Do not compare models using output torque alone; efficiency varies with ratio, speed, load, lubrication, and temperature, so assuming 100% efficiency can understate motor torque and regenerative braking demand.
4. Check the resulting motor torque against motor rated torque and peak torque across the cycle. For cyclic motion, calculate RMS torque for thermal sizing rather than using arithmetic average torque, which can hide overheating during rapid, repeated moves.
For a vertical axis, also record holding torque, brake behaviour, back-driving risk, and loss-of-power requirements before treating any ratio as a safety solution.
Separate Continuous, Peak, Emergency, and Thermal Torque
Do not size the gearbox from rated load torque alone: calculate acceleration, reversal, braking, and emergency-stop cases from the complete motion profile.
- Continuous torque is the torque the gearbox and servo must sustain throughout the running portion of the cycle. Check it against the gearbox continuous rating and the motor’s continuous torque.
- Peak torque covers acceleration, deceleration, reversing, and repeated short events. Confirm the permitted duration and repetition rate; a peak rating is not a continuous operating point.
- Emergency-stop torque is the worst transient produced by the commanded stop, brake, or jam. Check the gearbox’s allowable emergency value and the motor’s peak limit, including the stopping time and any load torque.
- RMS torque represents the heating effect of the entire duty cycle. Compare it with the gearbox thermal rating and the motor’s continuous capability, because a cycle can pass every instantaneous torque check and still overheat.
Calculate required motor torque from motor-side inertia, reflected load inertia, acceleration, load torque, and gearbox losses. Include efficiency at the selected ratio, speed, load, and temperature; assuming 100% efficiency understates acceleration torque and regenerative braking energy.
Then test the actual cycle: acceleration, constant speed, reversal, deceleration, dwell, and emergency stop. Record torque duration and repetition, and verify the servo remains within continuous and peak speed limits. Recheck the calculation after installation: shaft misalignment, coupling forces, housing reaction torque, flange error, and loose fasteners can add loads absent from the ideal model.
Match Inertia, Backlash, Stiffness, and Gearbox Architecture
A 10:1 gearbox makes reflected load inertia approximately J_load/N², so a 100 kg·cm² load appears as 1 kg·cm² at the motor. That improves acceleration matching, but it does not erase compliance: a high ratio also increases output sensitivity to motor encoder resolution and gearbox torsional compliance.
Compare the motor rotor inertia with the reflected load before tuning. The gearbox’s torsional stiffness and the combined motor/load inertia establish the mechanical resonance; a rigid, low-backlash gearbox can still have a low-frequency torsional mode when output inertia is large. A large mismatch narrows usable servo bandwidth, causing oscillation or slow settling.
Treat these as separate checks:
- Backlash is clearance during reversal. It creates a position jump that damages indexing and bidirectional positioning.
- Lost motion is the measured output movement that fails to follow a reversal command. Measure the complete drivetrain; catalog backlash alone does not predict it.
- Torsional stiffness is resistance to elastic twist under torque. Low stiffness causes contour error and ringing even when backlash is small.
Match gearbox architecture to the motion:
- A precision planetary gearbox suits high dynamic response when its specified lost motion and stiffness meet the contouring requirement.
- A strain-wave gearbox can suit compact, low-backlash axes, but verify stiffness and settling under the actual output inertia.
- A cycloidal gearbox suits applications where shock loading matters; check its servo accuracy and reversal behaviour rather than assuming architecture guarantees precision.
For pick-and-place, prioritise settling time after each move. For contouring, verify following error during acceleration and reversal, not only static accuracy.
Check Output Loads and Mechanical Compatibility
An output shaft can meet the gearbox torque rating and still fail under belt tension, chain pull, or an eccentric tool. Confirm the complete load case at the actual output speed, direction, duty cycle, and distance from the nearest bearing.
- Calculate the radial load from pulleys, sprockets, pinions, belts, and chains, then add the axial load from thrust forces, helical gears, and the machine’s axis orientation.
- Calculate the overturning moment from each offset load. Compare it with the manufacturer’s allowable moment and overhung load at your exact bearing distance; a catalog value at a reference location does not apply unchanged to a longer shaft extension.
- Check the output shaft diameter, key or clamp connection, bearing ratings, and housing strength against the combined radial load, axial load, and moment. Gear-tooth capacity does not prove bearing life or shaft strength.
- Match the motor flange and pilot diameter to the gearbox input interface. Verify bolt-circle dimensions, register engagement, shaft diameter, keyway, and motor weight; a loose pilot or incorrect flange fit can create coupling forces.
- Confirm mounting face flatness, locating features, fastener stiffness, and housing reaction torque. Control shaft alignment and coupling misalignment instead of using the gearbox bearings to absorb installation error.
Request a load diagram or calculation from the supplier using your force directions, bearing spacing, speed, and duty cycle. Also check whether the machine structure, coupling, and mounting plate deflect enough to add load during acceleration or reversal.
Validate Construction, Operating Conditions, and the Final Datasheet
Confirm the gearbox architecture before comparing part numbers. A planetary gearbox suits high torque density and low backlash; a helical gearbox suits efficient inline transmission; a bevel gearbox suits right-angle layouts; a cycloidal gearbox suits high shock loads and frequent reversals. Compare these choices against output speed, stiffness, noise, efficiency, and available space.
| Architecture | Strength to verify | Main check |
|---|---|---|
| Planetary | Torque density and precision | Peak torque and torsional stiffness |
| Helical | Efficient inline drive | Ratio, heat, and mounting space |
| Bevel | Right-angle transmission | Efficiency, backlash, and shaft direction |
| Cycloidal gearbox | Shock-load tolerance | Speed, ripple, and servo tuning |
The datasheet must state continuous torque, peak torque with duration and repetition, service factor, rated and maximum speed, thermal limits, efficiency, backlash, torsional stiffness, bearing loads, and permissible shaft or coupling misalignment. Select the service factor from starts per hour, operating hours, shock, load variation, reversing frequency, and the driven machine—not motor power alone.
Check lubricant type, fill level, ambient temperature, speed range, lubrication and installation orientation. Vertical axes require static holding torque, brake behaviour, back-driving, and loss-of-power analysis; ratio or nominal efficiency is not a safety brake. Control flange flatness, shaft alignment, housing reaction torque, and fastener stiffness.
Before ordering, request the dimensional drawing, motor-adapter drawing, rating curves, thermal data, installation manual, lubrication schedule, and test or inspection records. If OLE Automation Inc coordinates the motion package, provide those documents alongside the rail, coupling, and motor-interface dimensions.
Frequently asked questions
How do you determine the required gearbox ratio for a servo motor?
Use the required output speed and the servo motor’s operating speed, then confirm that the ratio supports acceleration, deceleration, travel, reversals, and the machine’s torque demand.
What torque ratings must you compare when selecting a servo gearbox?
Compare continuous torque, peak torque during acceleration or process loads, emergency or stopping torque, and thermal torque capacity for the actual duty cycle.
Why do inertia, backlash, and torsional stiffness matter in servo gearbox selection?
They affect tuning, settling time, positioning accuracy, reversal response, and the servo’s ability to control the reflected load inertia.
What mechanical checks are required before installing a servo gearbox?
Verify output radial and axial loads, shaft and flange dimensions, mounting orientation, coupling fit, lubrication, sealing, operating temperature, and available space.
