The Cylinder That Slammed Home on Air-Up: End-of-Stroke Cushioning
A Ø80 mm cylinder, 500 mm stroke, pushing a heavy slide. When it reached end-of-stroke, the slide slammed hard. The machine rattled. After two weeks, the cylinder’s end cap bolts loosened. The customer thought the air pressure was too high. We checked the cushioning. It was closed.
What cushioning does
Modern pneumatic cylinders have adjustable cushioning at both ends. Near the end of the stroke, the piston covers the main exhaust port. The remaining air must escape through a small needle valve (the cushion adjustment). This trapped air creates a back-pressure that decelerates the piston before it hits the end cap.
If the cushion needle is fully open, there’s no back-pressure. The piston arrives at full speed and slams. If the needle is fully closed, the piston decelerates too early and takes too long to reach end-of-stroke. The right setting is between: the piston lands softly but arrives on time.
How to adjust it
Run the cylinder at operating pressure, no load. Adjust the cushion needle at the end where the slamming occurs. Turn it in (clockwise) 1/4 turn at a time. Run the cycle. Listen and feel. The piston should arrive at end-of-stroke with a soft “thud,” not a “bang.” If it takes more than 1 second longer to reach the end, the cushion is too tight — back it off.
For our Ø80 cylinder, we adjusted the cap-end cushion to 1.5 turns open from fully closed. The piston decelerated in the last 20 mm of stroke. It landed softly. The end-cap bolts stopped loosening.
The physics of the slam
The kinetic energy at end-of-stroke must be absorbed by the cushion. For a 50 kg slide moving at 0.5 m/s:
E = ½ · m · v² = ½ × 50 × 0.5² = 6.25 J
The cushion needle must absorb this 6.25 J over the last 20 mm of stroke. The back-pressure force is F = E / d = 6.25 / 0.02 = 312 N. That’s the force the cushion air exerts to stop the slide. If the cushion is open, there’s no back-pressure, and the 6.25 J goes into the end cap. The bolts see it. The frame sees it.
When cushioning isn’t enough
If the load is heavy or the speed is high, the cylinder’s internal cushioning may not be sufficient. Add external shock absorbers (oil-filled) at the end of stroke. These are mounted on the machine frame, not the cylinder. They absorb the energy hydraulically. For a 50 kg slide at 0.5 m/s, an industrial shock absorber (MC30) handles 10 J per cycle — enough.
| Energy at end-of-stroke | Solution |
|---|---|
| < 1 J | Cylinder cushioning only |
| 1–10 J | Cylinder cushioning + adjust carefully |
| 10–50 J | External shock absorber |
| > 50 J | Hydraulic damper + slow the speed |
The other end
We adjusted the cap end (extend). The rod end (retract) also has cushioning. The retract was lighter (the slide pulled back by its own weight) and didn’t slam. But we adjusted it anyway — 1 turn open. The cylinder was symmetric. Both ends landed softly.
The end-cap bolts were torqued to 12 Nm (not just finger-tight). The jam nuts were locked. The slamming was the root cause of the loosening; the cushion adjustment fixed the root cause.
The cushion rule: If a cylinder slams at end-of-stroke, adjust the cushion needle first. It’s a 2-minute job. If the load is heavy (10+ J), add an external shock absorber. Don’t blame air pressure — the cushion is what stops the piston, not the regulator.
