Why the Cylinder Seized After 30 Days: What Happens Inside
A Ø32 mm pneumatic cylinder on a clamping station. It cycled every 6 seconds. After about a month, it stopped moving. The piston was stuck in the barrel. We pulled it apart. The seals were fine. The barrel was scratched. The air supply had water in it. This is what a seized cylinder looks like from the inside out.
Day 1 to Day 10: lubrication loss
The cylinder came with a grease-packed seal. At installation, we connected the air supply without a lubricator. That’s fine for a modern cylinder — the seals are pre-lubricated for life. But the air was dry. The FRL filter removed moisture, but the compressor was producing wet air. Water condensed in the air line and reached the cylinder. The water washed the grease off the seal and the barrel wall.
Without lubrication, the seal rides dry on the barrel. The PTFE-filled seal material can handle dry running for a while, but the barrel wall (anodized aluminum) and the seal start wearing against each other. Fine aluminum particles from the barrel mix with the remaining grease and water — turning it into an abrasive paste.
Day 10 to Day 25: scoring starts
The abrasive paste scratches the barrel wall. The scratches run along the stroke direction. The seal catches on the scratches. Friction rises. The cylinder starts moving slower, then stuttering. The customer notices the clamping force is weaker — the cylinder can’t reach end-of-stroke.
At this stage, the fix is easy: clean the cylinder, replace the seal, add a lubricator. But the machine is running and the customer doesn’t want to stop it. They turn up the air pressure to push harder. Higher pressure forces the piston past the scratch. It works for a week.
Day 25 to Day 30: the seizure
One morning, the cylinder doesn’t move. The piston is stuck at mid-stroke. The barrel has a groove worn into it, about 0.05 mm deep, at the mid-stroke position where the cylinder spends most of its time. The seal has ridden over that groove so many times that it’s torn. The air bypasses the piston. No pressure, no motion.
What we changed
Drain the air supply. We added a water trap (auto-drain filter) before the FRL. The compressor’s condensate was going straight into the line. The auto-drain removes it automatically. The customer’s compressor room was 5°C below ambient — cold air condenses water. A dryer (refrigerated) would be the full fix, but the auto-drain stopped the worst of it.
Don’t turn up the pressure. When a cylinder gets slow, the answer isn’t more air pressure. It’s finding out why it’s slow. We now have a rule: if a cylinder’s cycle time increases by 20%, stop and inspect. Don’t compensate with pressure.
Spec the right cylinder for wet air. If the air supply can’t be dried, use a stainless-bore cylinder (not anodized aluminum). Stainless doesn’t corrode, and the barrel surface is harder. It costs 20% more. It lasts 10× longer in wet-air conditions.
Wear patterns to look for
| What you see | What it means | Fix |
|---|---|---|
| Scratches along stroke | Dry-running seal, lubrication lost | Lubricator, replace seal |
| Pitting on barrel | Corrosion from wet air | Dryer, stainless bore |
| Uneven wear (mid-stroke groove) | Cylinder spends most time at one position | Redesign stroke, or use a guided cylinder |
| Seal extruded past ring | Pressure spikes (water hammer) | Flow control, check for back-pressure |
| Rod pitting (visible rust) | External corrosion, washdown or wet environment | Stainless rod, wipers |
The cycle that should have caught it
A seized cylinder takes weeks to develop. The friction rise is gradual. If the machine monitored the cylinder’s extend time (via a reed switch at each end), it would see the slowdown before the seizure. A 20% increase in extend time over a week is the warning. We added that monitor — a timer between the retract and extend reed switches — and the customer gets an alarm before the cylinder sticks.
The maintenance rule: Wet air kills cylinders. Keep the air dry (dryer + auto-drain). Watch cycle time as the wear indicator. Don’t compensate slow cylinders with more pressure. A seized cylinder at day 30 was actually telling you something at day 15.
