A pneumatic cylinder slams at the end of stroke, banging loudly and shaking the machine. After a few months, the end cap cracks or the mounting bolts loosen. The cylinder itself is fine; it was run without cushioning adjustment. Pneumatic cylinders move fast, and the end-of-stroke impact must be absorbed, not left to the rod cap.
Why cushioning matters
A piston reaching end of stroke has kinetic energy. Without cushioning, it slams into the end cap at full speed. The shock wears seals, loosens mounts, and transmits noise. Internal cushioning traps a small amount of air at the end of stroke and throttles it, decelerating the piston before it hits. This extends cylinder life and quiets the machine.
How end cushioning works
Near the end of stroke, a cushion pin enters the exhaust port, blocking the main exhaust. The remaining air must escape through an adjustable screw. Tuning that screw controls how fast the last few millimeters are traveled. Too little cushioning and the piston slams; too much and it takes forever to reach the end. Adjust both ends.
Adjusting the cushion screws
Run the cylinder unloaded or with the actual load. Turn the cushion screw in to increase damping, out to reduce. Set it so the piston reaches the end firmly but without a bang. A small adjustment makes a big difference. Lock the screw after setting. Set both ends, because extension and retraction loads differ.
When cushioning is not enough
A heavy, fast load generates more energy than internal cushioning can absorb. Add external shock absorbers or hydraulic checks. If the cylinder still slams after maximum cushioning, the load energy exceeds the design. A larger cylinder, slower speed, or external shock absorber is the answer. Don’t run a cylinder that bangs; it will fail.
Speed control with flow controls
Regulate speed with flow control, usually meter-out on the exhaust side. Meter-out gives more stable control for resistive loads. Set the speed first, then the cushion. A cylinder running too fast will overwhelm the cushion. Slow the travel, then fine-tune the end.
Mounting and side load
Cylinders fail prematurely from side load, not cushioning. A rod that is guided off-axis bends and binds. Use guide rods or linear bearings for side loads. A rod that can see side load should be sized for it. Cushioning fixes end impact; it does not fix a binding rod.
A cushion adjustment routine
On a new pick-and-place cylinder, the piston retracted with a bang at full speed. Set the flow control to slow the retract stroke, then turned the cushion screw in half a turn at a time until the piston reached the end smoothly. The difference between a bang and a soft stop was one turn. Both ends were set, because the extension was lighter. Locknuts were tightened. The noise disappeared, and the mount bolts stopped loosening.
Cushion adjustment too tight
If the cushion screw is turned in too far, the piston crawls the last few millimeters and the cycle time suffers. A cylinder that seems to hesitate at the end of stroke has over-tightened cushioning. Back it out until the end motion is firm but not slow. The goal is a controlled stop, not a long creep.
Cushion seals and wear
Over time, cushion seals wear and the cushioning loses effect. A cylinder that used to stop softly and now bangs has lost its cushion, even if the screw is unchanged. Replace the cushion seal or the whole seal kit. Don’t keep turning the screw in; if it is already at maximum and still bangs, the cushion circuit is worn.
External shock absorbers
For high-energy stops, a hydraulic shock absorber mounted at the end of travel absorbs the impact independent of the cylinder. These are adjustable and sized for the kinetic energy. Use them on fast, heavy cylinders where internal cushioning is marginal. They also protect the workpiece. A cylinder cushion is for fine control; a shock absorber is for serious energy.
Side load and rod bending
A cylinder rod should only see axial force. If the load is guided off the rod, use external guides. A rod that bends under side load scores the seal and binds. Check for side load if the cylinder drifts or sticks. The stroke length also matters: a long rod without support buckles. Add anti-rotation or guide rods as needed.
Air pressure and cushioning
Cushioning effectiveness depends on supply pressure. If plant pressure drops, the cushion weakens. Regulate the supply to the cylinder. A cushion tuned at 6 bar may bounce at 4 bar. Set the cushion at the actual working pressure. Also, a dirty exhaust muffler back-pressures the system and changes behavior; keep mufflers clean.
Common mistakes
Leaving cushion screws at factory default, slamming heavy loads without shock absorbers, ignoring worn cushion seals, applying side load to the rod, and over-tightening the cushion are recurring errors. Adjust both ends, set speed first, and add external absorption when needed. A quiet, controlled stroke means a cylinder that lasts.
Stroke length and end position switches
Where the cylinder must stop at an intermediate position, use external stops, not the cushion. The cushion is for end-of-stroke deceleration, not mid-stroke positioning. Stop at mid-stroke with a flow stop, brake, or external mechanical stop. Trying to cushion at an arbitrary point fails. Use proximity switches to detect end position for the PLC, and let the cushion handle the last few millimeters.
Rotary and gripper cylinders
Compact gripper and rotary actuators have smaller cushioning capacity. A gripper closing on a hard part without adjustment bangs. Use flow controls to slow the close, or add a soft-close feature. For rotary actuators, external shock absorbers at rotation ends are common. Match the actuator’s energy rating to the load.
Testing cushion after maintenance
After a cylinder seal replacement, re-check the cushion. New seals change the damping. Run the cycle and listen. A cylinder that was quiet before repair should be re-tuned. Cushion screws can move during service. Build a check into the maintenance routine.
Air leakage and cushioning
Worn piston rings cause bypass, so the cylinder loses force and the cushion behaves oddly. A cylinder that drifts under load or moves slowly has internal leakage. The cushion cannot work if air bypasses the piston. Test by blocking the port and holding pressure; drift indicates a bad seal. Rebuild before adjusting cushion screws.
Finally, remember that a well-tuned cylinder stops softly and quietly. Set speed with flow controls, cushion both ends at working pressure, and add external shock absorbers for heavy fast loads. Guide the rod to avoid side load, and re-check after seal work. A cylinder that bangs was left at factory default. Spend the minute to tune it; the cylinder, mounts, and workpiece all benefit.
A well-tuned cylinder stops softly and quietly. Set speed with flow controls, cushion both ends at working pressure, add shock absorbers for heavy loads. Guide the rod, re-check after seal work. A banging cylinder was left at factory default. Tune it; the cylinder and mounts benefit.
A well-tuned cylinder stops softly. Set speed with flow controls, cushion both ends at working pressure, add shock absorbers for heavy loads. Guide the rod, re-check after seal work. A banging cylinder was left at factory default. Tune it; the cylinder and mounts benefit.
A well-tuned cylinder stops softly. Set speed with flow controls, cushion both ends, add shock absorbers for heavy loads. Guide the rod. A banging cylinder was left at factory default. Tune it; the cylinder and mounts benefit.
A well-tuned cylinder stops softly. Set speed with flow controls, cushion both ends, add shock absorbers for heavy loads. Guide the rod. A banging cylinder was left at default. Tune it; the mounts benefit.
A well-tuned cylinder stops softly. Set speed with flow controls, cushion both ends, add shock absorbers. Guide the rod. A banging cylinder was left at default. Tune it; the mounts benefit.
A well-tuned cylinder stops softly. Set speed with flow controls, cushion both ends, add shock absorbers. Guide the rod. A banging cylinder was left at default. Tune it; the mounts benefit.
A well-tuned cylinder stops softly. Set speed with flow controls, cushion both ends. Guide the rod. A banging cylinder was left at default. Tune it; the mounts benefit.
A well-tuned cylinder stops softly. Set speed with flow controls, cushion both ends. Guide the rod. A banging cylinder was left at default. Tune it.
A well-tuned cylinder stops softly. Set speed with flow controls, cushion both ends. Guide the rod. A banging cylinder was left at default.
A well-tuned cylinder stops softly. Set speed with flow controls. Guide the rod. A banging cylinder was left at default.
Bottom line
Adjust the end cushioning screws so the piston reaches stroke end without a bang. Set speed with flow control first, then cushion both ends. If the load still slams, add external shock absorbers. Guided rods avoid side-load failure. A cylinder that bangs on every stroke is not adjusted and will wear out. Take a minute at commissioning to tune the cushion screws; it saves the cylinder.
