Power Transmission / Gear Reducers

Gearbox Still Leaking After an Output Shaft Seal Replacement?

Doc: gearbox-still-leaks-after-seal-replacement
Section: power-transmission/gear-reducers
Revised: 2026-08-17

A wet seal is not proof that the seal caused the leak. A standard radial lip seal is a low-pressure part, and it will lose to housing pressure, to an oil level that stands above it, or to the same worn shaft track that killed the first one. The second seal fails the same week when the crew replaces the part nearest the oil instead of the condition that pushed oil past it.

Key Specifications

Specification	Value	Source
Dry when cold, weeps after warm-up or at speed	Suspect housing pressure, not the lip. General-purpose radial shaft seals are rated at max 0.05 MPa (7 psi) operating pressure and up to 14 m/s (2 755 ft/min) surface speed. Prove the breather fitting and the drilled housing passage separately.	SKF, Maximizing performance — Radial shaft seals HMS5 and HMSA10: https://cdn.skfmediahub.skf.com/api/public/0901d196802a343c/pdf_preview_medium/0901d196802a343c_pdf_preview_medium.pdf
Wet before the shaft turns, oil stands at the seal	Check the fill level against the actual mounting position. Reducers ship with an increased oil level that must be adjusted to the required mounting position before startup, so a unit installed in a different orientation can run with the output seal submerged.	SEW-EURODRIVE, Gear Units and Gearmotors manual — mounting positions and oil fill: https://automatedpt.com/SEW-Eurodrive-Gear-Units-Gearmotors-Manual.pdf
New lip runs on a groove or a hand-polished shaft	Seal counterface spec is a plunge-ground, non-oriented finish of Ra 0.2–0.5 µm (8–20 µin) per ISO 6194-1 recommendation, or Ra 0.2–0.8 µm (8–32 µin) per DIN 3760, with h11 tolerance and minimum 45 HRC. Emery-cloth polishing leaves a spiral lead that pumps oil past the lip.	SKF, Maximizing performance — Radial shaft seals HMS5 and HMSA10, shaft recommendations: https://cdn.skfmediahub.skf.com/api/public/0901d196802a343c/pdf_preview_medium/0901d196802a343c_pdf_preview_medium.pdf
Seal turns in the bore or weeps at its outside diameter	Check the housing bore, not the shaft. Recommended bore is H8 with Ra 1.6–3.2 µm (63–126 µin). A scored or oversize bore leaks around the case, which reads as a lip leak from the outside.	SKF, Maximizing performance — Radial shaft seals HMS5 and HMSA10, housing bore recommendations: https://cdn.skfmediahub.skf.com/api/public/0901d196802a343c/pdf_preview_medium/0901d196802a343c_pdf_preview_medium.pdf
Lip is cut, spring displaced, or wear is heavy on one sector	Installation or runout, not a defective seal. Protect the keyway with a sleeve, press square on the supported case, and measure runout at the seal track. Careful installation to the seal installation standard is a prerequisite for the seal to work at all.	SKF, Industrial shaft seals — installation and troubleshooting: https://www.skf.com/skf/campaign/sealsno/media/10919_Industrial_shaft_seals_EN.pdf

Types & Variations

Standard radial lip seal

A spring-loaded elastomer lip runs directly on the shaft counterface; single-lip and auxiliary-lip versions cover most gearbox output shafts.

Use: Use where the shaft track, lubricant, temperature, speed, and housing pressure stay inside the seal's rating — roughly 7 psi (0.05 MPa) and 14 m/s (2 755 ft/min) for general-purpose types.

Radial lip seal with a shaft repair sleeve

A thin drawn sleeve presses over the worn track and gives the lip a new counterface without machining or replacing the shaft.

Use: Use when the groove is measurable but the shaft is otherwise sound, and the sleeve is approved for the shaft diameter, the final lip position, and the interference fit.

Cassette or labyrinth-style upgrade

A multi-element cartridge adds contamination exclusion and moves the dynamic contact off the original shaft surface.

Use: Consider it only with the reducer maker's approval, where washdown or abrasive dust keeps destroying lips faster than a standard seal can be justified.

Maintenance Steps

1. Isolate the reducer and preserve the leak evidence
Photograph the wet path and note the operating state when it appears. Shut down, apply lockout/tagout (LOTO), block rotation and any suspended or gravity load, and let hot oil cool. Relieve stored pressure and verify a zero-energy state before removing a guard, plug, breather, coupling, or seal.
A stopped motor is not isolation. A back-driven conveyor, a raised load, hot oil, or a pressurized housing can still move or discharge.
2. Clean and prove the leak origin
Degrease and dry the housing, cover joint, plugs, breather, seal outside diameter, shaft exit, and keyway. Apply leak-detection powder, restore guards, and run only the minimum authorized test needed to see the first wet point. Where powder cannot resolve the path, an approved fluorescent dye and UV inspection works, provided the dye is compatible with the lubricant.
Check the SDS for solvent and cleaner, ventilate, remove ignition sources, and wear eye and skin protection. Never reach past a guard or wipe a rotating shaft.
3. Check breathing, mounting position, and oil condition
Under LOTO, remove the breather and prove that both the fitting and the drilled housing passage are open. Confirm the installed orientation against the nameplate mounting position, then check the settled oil level and lubricant grade against the manual. Sample oil that is foamy, milky, contaminated, or rising.
Crack plugs slowly from beside the port, never in front of it, after the housing has cooled and pressure has been relieved. Hot oil can be ejected by trapped pressure.
4. Measure the shaft system before choosing another seal
Pull the coupling with the specified puller or hydraulic method. Inspect the seal track, bore, shoulder, keyway, and the removed lip. Measure the groove depth, runout at the seal track, and output bearing play, then compare each with the reducer and seal documentation.
Rig and support hub weight on rated lifting points, keep hands clear of puller jaws and the reaction path, and remove the dial indicator and magnetic base before anything can be energized. Do not heat a hub before checking lubricant, seal material, and fire load.
5. Repair the cause and install the seal square
Correct the vent, oil level, lubricant, bearing, runout, bore, or shaft defect you found. Protect keyways and threads with an installation sleeve, orient the lip toward the retained oil unless the drawing says otherwise, lubricate as directed, and drive the seal to its specified depth with a square tool bearing on the supported case.
Do not push a lip over an exposed keyway, spline, thread, or burr. A seal that was cocked, cut, dropped, or lost its garter spring is scrap, not a candidate for another try.
6. Refill and verify under controlled conditions
Refill with the approved oil to the level for the actual mounting position. Restore guards, clear tools and people, release LOTO through the site procedure, and run a short unloaded check where practical. Recheck the seal track, breather, temperature, noise, and settled level, then inspect again after the first normal duty cycle.
Stop and re-isolate on a growing wet ring, abnormal temperature, noise, or vibration. Never tighten, wipe, or adjust anything while the shaft is turning.

Safety Warnings

Before removing guards, plugs, the breather, coupling, or seal, apply lockout/tagout (LOTO) to every energy source, block rotation and gravity movement, relieve residual pressure, let hot oil cool, and verify a zero-energy state.
Never pressurize an assembled gearbox with shop air to find a leak. Line pressure is far above what the housing seals are built for and can eject oil, plugs, seals, or a cover.
If a temporary pressure indicator is fitted at the breather port, let the housing cool first, use a gauge and hose rated for the oil and temperature, point the connection away from personnel, and relieve pressure before removing it.
A guarded diagnostic run is a separate operating state. Follow the site's controlled-testing procedure, keep everyone out of the line of fire, then de-energize and reapply LOTO before touching the reducer.
Oil on floors and guards is a slip and fire hazard. Contain it immediately and dispose of oil, solvent, wipes, and the old seal under site and environmental procedures.

Prove the oil is leaving at the lip before you pull anything

Degrease the end cover, shaft, breather, sight glass, fill and drain plugs, and the housing joint above the shaft. Dry everything and dust the suspected paths with leak-detection powder. Follow the highest clean-to-wet boundary, not the lowest drop: oil from a breather, cover joint, or loose plug runs down the housing and arrives at the shaft looking exactly like a seal leak. Oil can also travel out along a keyway and surface behind the coupling hub.

Then time it. A gearbox that is wet before it turns points to a cut or reversed lip, wrong seal size, damaged bore, deep shaft groove, or an oil level standing above the sealing point. A gearbox that stays dry cold and starts weeping after twenty minutes of running is telling you about pressure, foaming, and oil throw instead. That single observation splits the whole job in two, and it costs nothing.

Check the two things that need no teardown

The air space inside the housing heats up and has to breathe somewhere. Standard radial lip seals for general gearbox duty are rated for roughly 7 psi (0.05 MPa) of operating pressure, which a painted-over or sludge-packed breather clears easily. Test the fitting and the drilled housing passage as two separate items. Crews replace the breather, miss the paint in the boss below it, and hand the new seal the same job the old one lost.

Oil level is the other free check, and it catches people because reducers ship deliberately overfilled. SEW-EURODRIVE, for one, states that its gear units are supplied with an increased oil level and that the level must be adjusted to the required mounting position before startup. Mount a unit foot-down that was filled for a shaft-up arrangement and the output seal sits underneath a standing column of oil. Check the level with the unit stopped, settled, and in the orientation it actually runs in, and compare it against the mounting position on the nameplate. Milky, foamy, or rising oil is its own answer: water, mixed lubricant, or overfill.

Measure the shaft before you buy another seal

Mark the old sealing line before you clean the shaft, then look across it under angled light and drag a fingernail over it. Any catch is a groove, and that groove hands the new lip the same escape path. The counterface also has a specification, not just an appearance. Seal makers ask for a plunge-ground, non-oriented finish inside a narrow roughness band and a hard shaft; a shaft that was polished with emery cloth wrapped around it now has a spiral lead that pumps oil out from under the lip no matter how good the seal is.

Put a dial indicator on the seal track itself and turn the shaft by hand under lockout to read runout, then check radial play at the output bearing. Remove the indicator and its magnetic base before anything is re-energized. Shafts that look perfect can still run eccentric enough to make the lip chase them.

Read the removed seal last, because it records what happened. Heavier wear over one sector of the circumference means runout or a cocked installation. A displaced or missing garter spring means the lip was dragged over something. Cuts that line up with the keyway, a thread, or a burr say the lip crossed an unprotected edge. Hardening, glazing, or swelling points at temperature or lubricant compatibility instead.

Fix the cause, then install one seal

Restore the vent path and set the oil level before any seal work, and correct bearing looseness, runout, bore damage, and contaminated lubricant while the unit is open. Shaft repair sleeves are the normal answer to a worn track where the supplier approves one for the shaft size and duty. After fitting, confirm the final outside diameter, the lip position on the sleeve, the surface direction, and that the sleeve flange clears the housing shoulder. Locating the new lip on an unworn track slightly inboard or outboard is the other option, but only where the seal remains fully supported. Deep wear or a cracked shaft is a shaft job.

Match the replacement on dimensions, lip direction, case style, elastomer, temperature, speed, and pressure duty. Cover the keyway with an installation sleeve, deburr, lubricate the lip, keep the spring seated, and drive the seal square on the supported part of its case. One hammered edge scraps it. Refill with the approved lubricant, rotate by hand, restore guards, and make the first powered run short and watched from outside the hazard zone.

Then close the loop so it does not come back. Put the breather on the same inspection route as the oil level instead of treating it as a lifetime fitting, and protect it from paint, dust, and washdown. Record the mounting position, lubricant, settled level, seal identification, lip direction, and the runout you measured on the work order. Trend stain width, oil level, foam, and output shaft movement; all four move before the next lip goes.

FAQ

How do I tell whether the leak is gearbox pressure or too much oil?

Use temperature. Pressure leaks are dry on a cold start and appear once the housing warms and the air space expands, usually growing with speed and load. An overfill or wrong mounting position leaves oil standing at the seal, so the unit is often wet before it turns and stays wet at idle. Check the breather passage and the settled level against the nameplate mounting position before removing anything.

Can a clogged breather really push oil past a good seal?

Yes, and it does not take much pressure. General-purpose radial shaft seals are rated around 7 psi (0.05 MPa), so a vent packed with paint, dust, or sludge makes the lip the cheapest relief path in the housing. Prove the fitting and the drilled passage in the housing separately — replacing only the breather is the common miss.

Can I install a new oil seal over a worn shaft groove?

No. A new lip running in the old groove has the same escape path the last one had. Measure the groove and pick a repair that suits it: a correctly fitted shaft repair sleeve, a small axial offset onto an unworn track where the seal is still fully supported, or shaft repair. Check runout and bearing play before deciding.

Why did the seal fail again after I polished the shaft with emery cloth?

Because hand polishing with the cloth wrapped around a turning shaft leaves a directional, spiral finish. That lead acts like a screw thread and pumps oil out from under the lip. Seal counterfaces are supposed to be plunge ground so the finish has no direction; if the track is already damaged, a repair sleeve gives you the correct surface instead.

References

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