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A cylinder starts weeping oil. A week later the rod is wet, the floor is slick, and the machine is down. The seal takes the blame.
Most of the time, it doesn't deserve it. In the field, a failed seal is usually the last link in a chain: dirty fluid, a scored rod, too much heat, a rushed rebuild. The seal was the messenger.
Replace the seal without finding the cause and the new one fails the same way — often faster, because the next failure has less material to eat through. Here are the twelve causes repair shops and maintenance teams run into most often: what is actually happening, how to recognize it on the failed part, and what to fix so it doesn't come back.
As one experienced hand put it when asked how to diagnose a hydraulic problem: "Don't make assumptions without concrete evidence to support it." The failed seal is your evidence. Start there.
Before ordering anything, put the failed seal on the bench and look at it. The damage usually announces the cause.
|
What you see on the failed seal |
What it usually means |
What to check first |
|
Circumferential scratches on the rod, lip worn flat |
Contamination — particles grinding between seal and metal |
Filtration, wipers, breathers, fluid condition |
|
Hard, cracked, glassy surface |
Heat, or long-term aging |
Real operating temperature vs material rating |
|
Fresh cuts, nicks, spiral tear |
Installation damage |
Sleeves, lubrication, fitting method |
|
Ragged, "chewed" edge |
Extrusion from pressure or too much clearance |
System pressure, extrusion gap, backup support |
|
Swollen, shrunken or discolored |
Fluid and material don't match |
Fluid type vs seal material |
|
Weeps at low pressure only |
Compression set — the seal stopped springing back |
Material selection, groove depth, seal age |
|
Looks perfect, still leaks after a rebuild |
Internal bypass, or the seal went in the wrong way |
Piston seal orientation, seal seat dimensions |
|
Fine black sludge, polished seal face |
Dry running, poor lubricity, or the wrong seat surface finish |
Lubrication, surface finish, running speed |
What's happening: dirt, metal particles, water and air in the oil act like grinding paste between the seal lip and the metal it runs against. Ask anyone who rebuilds cylinders what kills seals first and the answer is always the same — contamination.
How to spot it: the lip is worn flat or has fine circumferential scratches, and the rod is scored in the same pattern. Milky or cloudy oil points to water; a pitted surface points to air bubbles collapsing under pressure.
What to do: filter and sample the oil, replace wipers before they harden, cap open breathers, and don't assume new oil is clean oil. A seal replaced into dirty fluid is a seal you will replace again.
→ The running surface matters too: wear rings take the side load so the seal doesn't have to.
What's happening: heat hardens the seal material until it stops sealing and starts cracking. It comes from hot fluid, high ambient temperature, or friction from fast stroking. Cold does the opposite: the material turns brittle and can crack on the first cold start.
How to spot it: a hard, glossy, cracked seal; elastomer that looks baked or browned; leaks that only appear once the machine has been running hot for a while.
What to do: match the material to the temperature you actually run at, not the number on a spec sheet. If the system itself is overheating, a new seal buys you a few weeks — the cooling problem is the real job.
→ Where the temperature window is wide: PTFE seals hold up where rubber grades give up.
What's happening: the oil can slowly eat the seal. Swelling, shrinking, hardening and color change are all signs the fluid and the material were never a real match — and the damage takes months to show up, which is why it gets missed.
How to spot it: a swollen or shrunken seal, a color that no longer matches a new one, or a seal that went soft and gummy. The classic case is a seal that worked fine until the fluid or the additive package changed.
What to do: confirm the fluid first, then the material. Compatibility is a two-way match — temperature rating alone won't save a seal sitting in the wrong medium.
→ What are PTFE seals? covers the chemical-resistance side in more detail.
What's happening: when system pressure goes past what the seal and the groove were designed for, the seal is squeezed into the gap and deformed. Pressure spikes from shock loads and sudden valve movements do this without ever showing on a gauge you happen to be watching.
How to spot it: a ragged or "chewed" edge, a seal with a nibbled lip, or a seal that has taken on the shape of the gap it was pushed into.
What to do: check real peak pressure — not the relief setting — and check the extrusion gap. Where pressure genuinely runs high, the answer is a harder material plus proper backup support, not a thicker seal.
What's happening: the gap between the piston and the bore, or the rod and the gland, is the seal's weakest point. As the gap opens up — through wear, a worn bore, or side load — the seal extrudes into it a little more on every stroke.
How to spot it: extrusion damage on the low-pressure side of the seal, a bore with visible wear steps, or a history of repeated failures on the same unit.
What to do: measure the gap rather than eyeball it, and take the side load off the seal. That is what wear rings are for: they carry the load and keep the metal off the metal.
What's happening: a damaged groove, a burred port, a scored rod or a seal seat machined to the wrong dimension will destroy a good seal in a short time. This is a hardware fault, not a seal fault.
How to spot it: a new seal fails as fast as the old one; the seal is damaged in the same place every time; the rod shows a scratch that lines up with the leak.
What to do: inspect the hardware before fitting anything — rod condition, bore, groove depth and width, surface finish, and chamfers. Polish or replace what is damaged. A rebuilt cylinder that goes back together with a scored rod will be back on your bench.
What's happening: a sharp edge, a dry seal, or a seal rolled on with a screwdriver will cut or twist the lip. The seal can survive the rebuild and fail in service weeks later.
How to spot it: fresh cuts, nicks, a spiral tear, or a lip that has been rolled over. There is no wear pattern, because nothing has worn yet.
What to do: cover threads and grooves with a sleeve or tape, lubricate the seal with the system fluid, use the right fitting tool, and look at the seal after it's seated — you should not be able to see a single mark on it.
What's happening: reusing a seal during a rebuild is the classic short-term saving that costs a second teardown. The second mistake is orientation: a rod seal and a piston seal are not interchangeable, and a seal fitted backwards will leak from day one.
How to spot it: a used seal that looks "still fine"; a leak that starts immediately after a rebuild; a seal that was clearly working in the wrong direction.
What to do: new seals every rebuild, no exceptions. Check the lip symmetry and direction against the seal you removed, and check the seal seat dimensions while the cylinder is open.
→ Related: What are wear rings?
What's happening: when two seals face the same direction, fluid can be trapped between them with nowhere to go. Pressure builds in that closed space and pushes the seals outward — sometimes in a circuit that also has a load-holding valve, which is how seals fail on machines that look perfectly normal.
How to spot it: seals deformed in the middle of a pair, damage on the side facing the trapped volume, or a cylinder that fails repeatedly in the same position.
What to do: confirm the seal arrangement, check whether the space between seals can vent, and review the circuit rather than the seal. If you can't see the circuit, don't guess — ask.
What's happening: over months of service, an elastomer seal loses its memory and stops springing back. It still holds under pressure, but at low pressure or at rest there isn't enough force to keep the lip against the metal — so it weeps.
How to spot it: the machine is dry under load and leaves a puddle overnight; a seal that looks fine but has taken a set in the shape of the groove.
What to do: choose a material with better compression-set resistance for the temperature you run, and check the groove — an over-deep groove reduces the squeeze the seal was designed to have.
What's happening: a cylinder that never moves can fail faster than one that runs. The rod stays extended and corrodes, the fluid picks up moisture, and the seal dries out against the metal.
How to spot it: rust or pitting on the exposed rod, a dry-looking seal, and leaks that appear after a machine has been parked for months.
What to do: keep rods retracted or protected during storage, exercise idle equipment occasionally, and expect to service seals on machines that sit far more than they run.
What's happening: seals need a film of fluid. When lubrication is lost — a starved pump, a dry start, poor lubricity in the fluid, or a component specified for oil and run on something thinner — friction rises and the seal face polishes, hardens and burns.
How to spot it: a polished, glazed or slightly burnt face, black sludge, and a component that runs hotter than its neighbour.
What to do: check lubrication and startup conditions first, then the surface finish of the mating part. If the application genuinely runs dry, that changes the material and the seal profile — it is a different specification, not a better version of the same seal.
Sometimes the seal is fine and the leak is somewhere else. A piston seal that bypasses internally will not leave a drop on the floor — the machine just loses force or drifts. A pump or motor shaft seal can fail because of pressure in a cavity that shouldn't be pressurised, or because the shaft is running faster than the seal was built for. A worn bore, a bent rod or side load will kill every seal you fit, one after another.
The pattern to watch for is the same failure, in the same place, more than once. That is not a seal problem. That is a system problem, and no seal on the shelf will fix it.
If you have cleaned the fluid, checked the hardware and confirmed the material, and the seal still fails, the answer is probably not a tougher version of the same seal. It may be a different profile, a different material, or a component specified for the pressure, temperature, speed and duty cycle you actually run.
Why does my hydraulic cylinder keep leaking after a rebuild? The most common reasons are a seal fitted in the wrong direction or in the wrong position, a damaged rod or groove that damages the new seal, and reused seals. If the cylinder is dry at full pressure and wet at rest, look at compression set instead.
How can I tell whether the seal failed or the rod is damaged? Look at both. A scored rod leaves matching circumferential marks on the seal lip. A seal that failed on its own usually shows heat, hardening or extrusion damage without matching marks on the metal.
Why do seals wear out so fast in one machine and last for years in another? Usually contamination, heat or misalignment — not the seal. Compare the fluid condition, the real operating temperature and the bore condition of the two machines before comparing the seals.
Can I replace a PTFE seal with a rubber one to save cost? Only if the operating window allows it. PTFE is normally chosen for high temperature, aggressive media or low-friction running where rubber grades degrade — swapping it for rubber in the same application usually shortens life rather than saving money.
Send us the failed seal, your drawing and your operating conditions — pressure, temperature, fluid, speed and duty cycle. Our engineers will tell you what failed and what will last in that application, whether or not it is a standard part.
