The Indexing Table That Overshot Every Cycle: Acceleration Tuning, Not the Table
A pneumatic indexing table rotating 90° every cycle. It overshot by 5 degrees every time. The customer thought the table was worn. The table was fine. The pneumatic motor was accelerating too fast. It didn’t decelerate at the end of travel. This is about pneumatic rotary actuator tuning, not about the table being defective.
Why indexing tables overshoot
A pneumatic rotary actuator (vane or gear type) rotates at full speed until it hits the end stop. If the rotation is fast and the load is heavy (the table with parts), the momentum carries past the stop. The table overshoots. The stop (a mechanical pin) engages, but the load has already moved past the correct position.
The tuning options
| Method | How it works |
|---|---|
| Flow control valve | Restricts air, slows the rotation overall |
| Adjustable cushioning | Slows the last 10° of rotation |
| External shock absorber | Hydraulic shock absorbs the momentum |
| Pneumatic brake | Applies braking air before the stop |
| Servo indexer (electric) | Closed-loop, programmable acceleration/deceleration |
What we did
1. Slowed the rotation. We added flow control valves on the exhaust. The rotation speed dropped from 300°/s to 100°/s. The momentum at the end was 1/3 of the original. The table reached the stop without overshooting.
2. Added cushioning. We adjusted the end cushioning needle valve. The last 10° of rotation slows to 20°/s. The table approaches the stop softly. It seats at exactly 90°.
3. Added a hydraulic shock absorber. For the heavy-load case (20 kg of parts on the table), we added a shock absorber at the 90° position. It absorbs the remaining momentum. The table doesn’t bounce back. It stays at 90°.
The repeatability
After tuning, the table indexed to 90° ±0.1°. That’s repeatable enough for the station (which needs ±0.5°). The customer needed 0.5° accuracy. We met it. The table wasn’t worn — it was running too fast. Slowing it down fixed the overshoot.
The indexing table rule: If the table overshoots, slow the rotation. Add cushioning for the last 10°. Use a shock absorber for heavy loads. The table isn’t worn — it’s accelerating too fast. Pneumatic actuators don’t have built-in deceleration. You provide it with flow controls and cushioning.
