The Rotary Actuator That Slammed at the Ends: Pneumatic Cushioning, Not the Actuator
A rotary pneumatic actuator (180° rotation) slamming at both ends. It made a loud bang every cycle. The customer thought the actuator was broken. It was fine. It had no end-position cushioning. The rotation was at full speed until the internal stop. This is about pneumatic rotary actuator cushioning, not about the actuator being defective.
Why rotary actuators slam
A basic rotary actuator (vane type) rotates at full speed until the vane hits the end stop. The kinetic energy is absorbed by the stop. At high pressure (6 bar), the rotation speed is high. The impact is loud and damaging. The stop wears out. The vane cracks over time.
The cushioning options
| Type | How it works | Speed control |
|---|---|---|
| No cushioning (basic) | Full speed to end stop | None — slams |
| Adjustable pneumatic cushion | Air trapped at the end, bled through a needle valve | Good (adjustable) |
| External shock absorber | A hydraulic shock absorber at the end position | Very good (high energy) |
| Internal rubber bumpers | Elastomer pads at the stops | Moderate (low speed only) |
| Speed controller (flow control) | Restricts air flow, slows rotation overall | Good (but slows the whole move) |
What we did
1. Installed adjustable cushioning. The actuator had adjustable end-position cushioning (a needle valve at each end). We adjusted the valves to slow the last 10° of rotation. The actuator approaches the end at 20% speed, then gently seats. The slam disappeared.
2. Added flow controls. We added flow control valves (one-way restrictors) on the air exhaust. The rotation speed is controlled (not full speed). The actuator rotates at a set speed (we set it to 90°/s instead of 180°/s). The kinetic energy at the end is lower. The cushioning has less to absorb.
3. Considered a shock absorber. For high-energy applications (heavy load, fast rotation), an external hydraulic shock absorber is better than pneumatic cushioning. We added one at the end stop. It absorbs the impact regardless of the load. The actuator now reaches the end softly, every time.
The adjustment
Adjust the cushioning needle valve: turn it in (clockwise) to slow the approach, out to speed it up. The right setting is: the actuator reaches the end with a soft “thud,” not a “bang.” If it’s too slow, the cycle time suffers. If it’s too fast, it slams. Set it so the last 10° takes 0.2 seconds.
The rotary actuator rule: A slamming rotary actuator needs end-position cushioning. Adjust the needle valves. Add flow controls to slow the rotation. Use a hydraulic shock absorber for high-energy loads. The actuator that slammed wasn’t broken — it was running at full speed into a hard stop.
