Rotary shaft seal troubleshooting: why an oil seal leaks

The lip is hard, glazed, or cracked around its circumference

What you see: The lip has lost its flexibility. It may be shiny where it ran on the shaft, and cracks may run around the lip rather than across it.

Likely causes, most common first

1. The lip ran without an oil film. The lip needs a thin layer of oil between it and the shaft. Run dry, it rubs until it overheats, and the heat leaves it hard and cracked.
2. The lip ran hotter than the oil did. The temperature at the sealing edge is the temperature of the fluid nearest the seal plus the heat friction generates under the lip. Measuring the sump tells you only half of it.
3. A dust lip was added to a duty that was already marginal. A dust lip adds friction heat, so the sealing edge runs hotter than on a seal without one. A second lip buys dirt exclusion and costs heat.
4. Shaft speed raised the under-lip temperature. Speed raises the under-lip temperature as well as the sump temperature, and over time the heat hardens the lip until it can no longer keep contact all the way round the shaft.
5. The compound was run near its ceiling for most of its life. Where the running time at the maximum normal temperature is more than about a third of the total, one maker derates the allowable temperature by twenty degrees.
6. A nitrile lip was run in a synthetic oil. In polyalkylene glycols and polyalphaolefins the ceiling for nitrile drops well below its figure in mineral oil. For FKM the maker will not state a number at all and asks for a trial.

Fix: Get the lip temperature, not the sump temperature. If the duty is genuinely hot, move up the material ladder rather than accepting shorter life, and check whether the dust lip is earning its heat.

Lip materials by family: Nitrile (NBR), HNBR, Polyacrylate (ACM), FKM (fluorocarbon), Silicone, PTFE.

Sources: Parker® Rotary Seal Design Guide, Catalog EPS 5350/USA, read 2026-09-17; NOK® Oil Seals catalogue 014E, read 2026-09-16; Freudenberg Technical Manual for Simmerring® seals and rotary seals, read 2026-09-16.

The lip is swollen, soft, or has lost its shape

What you see: The seal may be hard to remove, the lip looks bulked out, and the material feels spongy rather than resilient.

Likely causes, most common first

1. The compound is not compatible with the fluid. Swelling is chemical attack, not heat. A standard nitrile lip holds up poorly against steam or hot water, brake fluid, ketones and ozone; silicone should not see steam, aliphatic or aromatic hydrocarbons, halogenated hydrocarbons, phosphate esters or polar solvents; EPDM must never see petroleum oil.
2. The fluid is right but the temperature is not. A compound is not compatible with a fluid in the abstract. Makers state a continuous temperature per medium, and the figure for a hypoid gear oil is lower than for an engine oil in the same compound.
3. An additive package rather than the base oil. Extreme-pressure lubricants are their own duty, and are the reason one family of lip compounds exists.

Fix: Match the compound to the actual fluid and its temperature, not to the fluid family. Send the fluid name and the running temperature with the dimensions.

Sources: Parker Rotary Seal Design Guide, Catalog EPS 5350/USA, read 2026-09-17; Freudenberg Technical Manual for Simmerring seals and rotary seals, read 2026-09-16.

A groove is worn into the shaft where the lip ran

What you see: A visible track in the shaft under the lip. A new seal fitted in the same place will leak down the groove.

Likely causes, most common first

1. The shaft is too soft. Bronze, aluminum and plastic shafts wear heavily under a lip. The preferred counterface is a carbon steel at the hardness the makers give, and abrasive service asks for more.
2. Abrasive contamination reached the lip. Where abrasives are present, or the lip compound itself carries abrasive additives, or a high-pressure design is used, the makers raise the shaft hardness requirement.
3. The shaft surface was too rough. Roughness above the makers' band breaks through the lubricant film, which leaks and wears the lip at the same time.
4. Carbonized oil built up at the lip. A lubricant that is not compatible with the under-lip temperature leaves abrasive carbonized particles at the sealing edge, which then act like grit.

Fix: A grooved shaft is not a seal problem any more. Either repair the surface, or fit a seal designed to run on a new track, or a wear sleeve that gives the lip a fresh counterface.

The shaft counterface the makers ask for: plunge ground to 10 to 17 µin Ra (0.25 to 0.43 µm Ra), at least Rockwell C30 (Rockwell C45 where there are abrasives, an abrasive lip compound or a high-pressure design), and no machining lead. The finish band is where the ranges Parker and Timken® publish overlap.

A grooved shaft can take a shaft repair sleeve pressed over the old track, which gives the lip a new counterface. Send the part number or drawing and we quote it.

Sources: Parker Rotary Seal Design Guide, Catalog EPS 5350/USA, read 2026-09-17; Timken, Recommendations for Bearing Seals, The Timken Company, read 2026-09-17.

A new seal leaks on a shaft that looks perfect

What you see: No damage, no groove, no obvious fault. The seal leaks anyway, sometimes from new.

Likely causes, most common first

1. The shaft is too smooth. A finish finer than the makers' band cannot hold the small pockets of oil the lip needs. The lip sticks and slips instead of riding a film, which lets fluid out and heats the lip.
2. The shaft is too rough. Peaks tall enough to break through the lubricant film cause leakage and accelerate lip wear at the same time.
3. The grinding left a lead. A machining lead acts like a screw thread under the lip and pumps fluid along the shaft, whichever way the shaft turns. One maker asks for a lead angle below a twentieth of a degree, the other for none at all.
4. The finish was specified by Ra alone. Two shafts with the same Ra can behave differently, which is why a peak-to-valley requirement is specified alongside it.

Fix: Plunge grind to the band, and check for lead. The string test will not prove lead is absent but it reliably finds a significant one.

The shaft counterface the makers ask for: plunge ground to 10 to 17 µin Ra (0.25 to 0.43 µm Ra), at least Rockwell C30 (Rockwell C45 where there are abrasives, an abrasive lip compound or a high-pressure design), and no machining lead. The finish band is where the ranges Parker and Timken publish overlap.

Sources: Parker Rotary Seal Design Guide, Catalog EPS 5350/USA, read 2026-09-17; Timken, Recommendations for Bearing Seals, The Timken Company, read 2026-09-17.

The seal leaks from new, and the garter spring is missing or out of its groove

What you see: The spring is lying in the housing, hooked on the lip, or gone altogether. The lip has no radial load.

Likely causes, most common first

1. The spring was already out before fitting. The makers put verifying the spring second in their installation sequence, right after checking the seal is the right one and undamaged.
2. The seal was pushed on over the shaft without protecting the lip. Driving a seal over a keyway, a splined end or a sharp shoulder can lift the spring out of its groove and roll the lip at the same time.

Fix: Check the spring is seated before the seal goes anywhere near the shaft, and use a fitting sleeve over any step, keyway or spline.

Sources: Parker Rotary Seal Design Guide, Catalog EPS 5350/USA, read 2026-09-17.

Oil appears around the outside of the seal rather than at the shaft

What you see: The shaft and lip look fine. The oil track starts at the housing bore, not at the lip.

Likely causes, most common first

1. The bore finish is too coarse. A coarse bore can be a leak path on its own, with nothing else wrong.
2. Burrs or sharp edges scored the seal outside diameter on assembly. A burr in the bore scars the metal outside diameter as the seal goes in, and the scar is the leak path.
3. A metal outside diameter in a duty that generates pressure. One maker is explicit that a metal outer periphery cannot be used where pressure is generated, because it bleeds and leaks there.
4. A metal outside diameter in a housing that moves with heat. A metal case is only of limited use in a split housing or one with high thermal expansion, and it can score a light alloy bore going in. A rubber-covered outside diameter grips the bore roughness instead.
5. A rubber outside diameter in a housing that deforms with heat. The opposite failure. A rubber outer periphery must not go into a housing prone to severe heat deformation, such as aluminum, and under pressure it needs a retaining structure so it cannot push out.

Fix: Decide the outside diameter type from the housing and the duty rather than from what was in there before. The two types fail in opposite conditions.

The outside diameter type and the lip form are part of the designation. See the cross-reference before matching a replacement on dimensions alone. Oil seal cross-reference.

Sources: Parker Rotary Seal Design Guide, Catalog EPS 5350/USA, read 2026-09-17; NOK Oil Seals catalogue 014E, read 2026-09-16; Freudenberg Technical Manual for Simmerring seals and rotary seals, read 2026-09-16.

The seal leaks as soon as it is fitted, and the lip is turned back

What you see: The lip is inverted or folded, sometimes only over part of the circumference. It will never recover.

Likely causes, most common first

1. The lip was not lubricated before fitting. The makers prelubricate the lip with the system lubricant before the seal goes on.
2. The seal was driven in square onto an unprotected shaft end. Anything the lip has to climb over, a step, a keyway, a spline or a sharp chamfer, can turn it back.
3. The wrong lubricant went on a composite outside diameter. A rubber outside diameter is lightly lubricated. One composite outside-diameter design must NOT be lubricated at all.

Fix: Prelubricate the lip, protect it over every step, and press on the seal case rather than on the lip. Check the lip has not turned back before the shaft goes in.

Sources: Parker Rotary Seal Design Guide, Catalog EPS 5350/USA, read 2026-09-17.

The seal leaks at speed but holds when stopped, or wears unevenly around the lip

What you see: Wear concentrated on one side of the lip, or a lip that looks evenly worn but leaks only when running.

Likely causes, most common first

1. Dynamic shaft runout beyond what the lip can follow. When the shaft does not rotate about its own center, the lip has to move in and out to follow it. Past a point it cannot keep contact, and it leaks.
2. Shaft-to-bore eccentricity. When the center of shaft rotation is not the center of the bore, the shaft presses harder on one side of the lip than the other. That side wears quickly while the opposite side loses contact.
3. Both at once. Cyclical deflection from runout is superimposed on the static deflection from eccentricity, which is why the two are assessed together.

Fix: Measure runout and shaft-to-bore alignment before fitting another seal. Designs exist for misaligned duty; a standard seal is not one of them.

Sources: Parker Rotary Seal Design Guide, Catalog EPS 5350/USA, read 2026-09-17.

The lip is extruded, badly worn, or the seal has been pushed out of the bore

What you see: The lip is deformed against the shaft, worn far more than its hours suggest, or the whole seal has moved in the housing.

Likely causes, most common first

1. A standard seal was used where there is pressure. A standard rotary lip seal is a lubricant retainer, not a pressure seal, and the makers put its limit in fractions of a bar. It is the single most common wrong assumption about these seals.
2. Pressure forced the lip onto the shaft. Excess pressure pushes the lip harder against the shaft, which raises friction, heat and wear together.
3. Trapped pressure from heating, with nowhere to vent. Where the unit warms enough for the enclosed air to become pressurized, one maker advises a vent rather than a stronger seal.
4. A grease-duty type used where there is any pressure at all. The grease and dust types are stated by their maker as unusable for pressure applications, full stop.

Fix: If there is pressure, say so when you ask for the seal. Pressure-rated rotary designs exist across a wide range, and a supported lip or a back-up ring is the other route.

NOK rates its standard oil seal types to 0.3 bar (4.4 psi), and Freudenberg's manual gives 0.2 to 0.5 bar (2.9 to 7.3 psi) for its standard lips. Where the duty has pressure, the pressure-rated types are the TCV stiffened lip and the TCN and TCZ caged lips. TCV, TCN and TCZ, Pressure ratings by design.

Sources: NOK Oil Seals catalogue 014E, read 2026-09-16; Freudenberg Technical Manual for Simmerring seals and rotary seals, read 2026-09-16; Parker Rotary Seal Design Guide, Catalog EPS 5350/USA, read 2026-09-17.

A replacement seal leaks although the original did not, on the same shaft

What you see: Nothing obviously wrong. The seal is the right size and the shaft is good.

Likely causes, most common first

1. A directional seal was fitted to a shaft that turns the other way. Some lips carry hydrodynamic features that pump fluid back under rotation. They work in one direction of rotation, and a plain lip does not have this behavior at all.
2. The replacement was matched on dimensions alone. A designation match is not a fit, form and function match, and lip type is one of the things a dimensional cross-reference does not carry.
3. A machining lead on the shaft is pumping the wrong way. A lead on the shaft acts like a pump thread under the lip regardless of which seal is fitted.

Fix: Check the direction of rotation and whether the original lip was directional. Some designs are explicitly independent of the direction of rotation; if yours must be, say so.

The outside diameter type and the lip form are part of the designation. See the cross-reference before matching a replacement on dimensions alone. Oil seal cross-reference.

Sources: SKF® Industrial Seals, Product Overview, read 2026-09-16; NOK Oil Seals catalogue 014E, read 2026-09-16; Parker Rotary Seal Design Guide, Catalog EPS 5350/USA, read 2026-09-17; Timken, Recommendations for Bearing Seals, The Timken Company, read 2026-09-17.

Conquest Seal supplies radial shaft seals, also sold as rotary shaft seals, rotary lip seals and oil seals, in inch and metric sizes. Send the part number or drawing and we quote it.

NOK is a trademark of NOK Corporation; SKF is a trademark of Aktiebolaget SKF; Simmerring is a trademark of Carl Freudenberg KG; Parker is a trademark of Parker Intangibles LLC; Timken is a trademark of The Timken Company. The maker designations on this page are the makers' own. No affiliation or endorsement is implied.