The Gearbox That Reversed Into Position: Backlash and Positioning Accuracy
A rotary actuator with a gearbox. It oscillates (rotates back and forth). The position accuracy was off by 2 degrees. The customer thought the motor was losing steps. The motor was fine. The gearbox backlash was 1.5 degrees. The oscillation (reversing direction) amplified the backlash. This is about backlash in oscillating loads, not about the motor being defective.
Why backlash matters for oscillation
A gearbox has backlash (play between the gears). When the shaft reverses direction, there’s a dead zone where the motor turns but the load doesn’t move (the gear teeth re-engage). For a 1.5-degree backlash, the motor has to turn 1.5 degrees before the load moves. In one direction, the load is at position X. In the other direction, it’s at position X minus 1.5 degrees. The oscillating load is always off by the backlash amount.
The backlash values
| Gearbox type | Backlash | Use for |
|---|---|---|
| Worm gear | 1-3 degrees | Non-critical, low speed |
| Planetary (standard) | 0.5-1 degree | General motion |
| Planetary (precision) | 0.1-0.3 degree | Reversing, positioning |
| Harmonic drive | < 0.01 degree | High precision, robotics |
| Direct drive (no gearbox) | 0 | Ultra high precision |
What we did
1. Switched to a precision gearbox. We replaced the standard planetary (1 degree backlash) with a precision planetary (0.1 degree backlash). The dead zone dropped from 1 degree to 0.1 degree. The position accuracy improved from 2 degrees to 0.3 degrees. The customer needed 0.5 degrees. We met it.
2. Added a preload. For the oscillating load, we added a slight spring preload (a torsion spring) on the output shaft. The preload keeps the gear teeth engaged in one direction. When the motor reverses, the preload takes up the backlash immediately. No dead zone. The load moves when the motor moves.
3. Used a closed-loop positioner. We added a position sensor (encoder on the output shaft, not the motor). The PLC measures the actual output position and corrects. The backlash is compensated in software — the motor overshoots slightly in the reversing direction to account for the dead zone.
The backlash rule: An oscillating load with a gearbox will have backlash error. Use a precision gearbox (0.1 degree or less). Add a preload to take up the play. Use an output encoder for closed-loop correction. The gearbox that reversed into position wasn’t failing — it had standard backlash in a reversing application. That’s the wrong gearbox for the job.
