Fixing an industrial gearbox oil leak means finding the exact point where the oil gets out — a shaft seal, the housing joint or a cover, a plug or fitting, or the breather — and then correcting the condition that pushed it out, not only the part it came through. A new seal fitted to a unit with a blocked breather, a grooved shaft or a loose bearing leaks again within weeks, and that is where most repeat leaks come from.
The seven steps below run in the order that finds the cause with the least disassembly: lock out, locate, oil level, breather, and only then the seal, joint or plug.
1. Shut Down and Lock Out the Machine
Stop the drive, isolate and lock the disconnect, release any stored energy, and prove the unit cannot start before you loosen a plug, a cover or a seal housing.
Two forms of stored energy belong to gear drives in particular. A hoist, a screw jack or a loaded inclined conveyor back-drives the output shaft the moment the brake is released, so the load has to be blocked or lowered, not only the motor isolated. And a unit with a blocked breather holds pressure while the oil is hot: let it cool and crack the fill plug first, before any drain plug or seal cover comes off.
2. Locate the Exact Leak Point
Clean the housing completely, run the unit to operating temperature under load, and trace the wet track back to the highest point it starts from — oil runs down and along the housing, so the place it drips from is rarely the place it leaves. A dusting of talc on the clean housing, or UV dye in the oil, shows the first wet point clearly.
Rule out the leaks that only look like a seal leak. Oil around the output shaft may have run down from a top cover or the breather; oil at the input side may be motor-bearing grease thrown off the coupling rather than gear oil at all; and a thin film over the whole unit is usually mist from a breather passing oil, not a seal.
Record the pattern as well as the place: one seal or several, only when hot or all the time, a drip or a mist. Several seals at once is a pressure problem, not a seal problem.
3. Check the Oil Level
Check the level against the mark for the unit’s actual mounting position, stopped and with the oil settled, before you touch any seal: an overfilled gearbox leaks through a clear breather and past good seals, and an underfilled one tells you how much oil the leak has already taken. The usual story behind a unit that started leaking after a service is that someone topped it up to the top of the sight glass.
Too much oil raises churning, and churning raises temperature, foam and internal pressure; the oil then leaves through the breather and under the seal lips together. Drain to the correct level and run the unit again before you change anything else.
The level mark and the fill quantity are set for the mounting position on the nameplate. A unit that was remounted — foot to flange, horizontal to vertical — needs the quantity and level plug of the new position, and a sight glass or dipstick left over from the original position reads wrong.
4. Inspect the Breather
Take the breather out and confirm air passes through it freely; if it is blocked, painted over or has been swapped for a solid plug, the housing pressurises as the oil warms and expands, and that alone pushes oil past every seal on the unit. A standard radial lip seal is rated for 0.03 to 0.05 MPa of internal pressure — 0.3 to 0.5 bar — and a plugged vent on a warm gearbox reaches that without difficulty.
Pressure does more than push oil under the lip; it presses the lip onto the shaft. At 0.2 MPa the lip contact force is about four times its unpressurised value, so a unit that ran for months with a blocked breather has worn seals as well as leaking ones. Clear the breather first, then plan on replacing the seals that ran under that load rather than waiting to see whether they dry up.
Two breather faults are not blockages. A breather screwed into the wrong port for the mounting position sits below the oil level and weeps continuously; it belongs at the highest point of the housing in the position the unit actually runs in. And a breather that blows oil rather than air, with the level correct, means the oil is foaming — water or the wrong oil in the sump — which is an oil problem to fix before any seal.
Where washdown, dust or fibre blocks the breather repeatedly, fit one designed for that environment instead of cleaning it again.
5. If a Shaft Seal Is Leaking, Inspect More Than the Seal
Before you fit a new seal, measure the shaft under the lip, check the bearing behind it for play, and confirm the seal material suits the oil temperature.
The Shaft Under the Lip
Run a fingernail across the shaft where the old lip ran. A groove you can feel is a wear track, and the new lip must not run in it: fit a thin repair sleeve over the track, re-machine or replace the shaft, put a spacer ring behind the seal, or press the seal to a different depth in the bore so the lip lands on fresh surface. The sleeve is the usual choice on site because the shaft stays in the unit.
A repair sleeve is stainless steel 0.28 mm thick, made for shafts from 12 to 200 mm, so the same seal size goes back on with no machining. It fails only when fitted wrong: position it to cover the wear track rather than flush with the shaft end, fill a deep groove with metal-filled epoxy before sliding it on, and treat it as single-use — a sleeve that has been pulled off is scrap.
If the shaft is re-machined, the surface the lip runs on needs Ra 0.2 to 0.8 µm to DIN 3760, plunge-ground, at 45 HRC minimum. The lower roughness figure is a minimum, not a target.
Never dress the shaft with emery cloth, by hand or in the lathe. It leaves a fine spiral pattern — a lead — that works as a screw thread under the lip and pumps oil out in one direction of rotation, which is why plunge grinding, with the wheel run until it stops sparking, is the specified finish.
The Bearing Behind the Seal
Lift the shaft with a bar and feel for play, then turn it and watch the seal area for eccentric running. Radial play or a misaligned shaft loads one side of the lip and unloads the other, and at speed the lip cannot follow a shaft that is running out; oil leaves through the gap on the unloaded side. Such a seal wears on one side and leaks whatever is done to it until the bearing is replaced.
A worn output bearing usually has a cause outside the gearbox: belt tension or coupling misalignment applying more overhung load than the shaft is rated for. Correct that when the bearing is changed, or the new bearing wears out the same way.
The Seal Material and the Oil Temperature
A standard nitrile (NBR) seal is rated for 80 °C continuous in gear oil, not the 100 °C printed as the lip’s maximum, and with PAG or PAO synthetic oils the seal makers hold it to 80 °C as well. A unit that leaks only when hot, or whose old lip comes out hardened and cracked rather than worn, has been running over that limit.
Replace the NBR seal with a fluoro rubber (FKM) seal of the same design, rated 150 °C continuous in gear oil, and find out why the unit runs hot. The FKM seal survives that heat; it does not remove it.
Fitting the New Seal
Note which way the old seal faces before levering it out, and never refit it. Choose the replacement to the original design and material unless the temperature check above says otherwise, wipe the shaft and the new lip with the gearbox’s own oil, and protect the lip with a sleeve or tape wherever it must pass a keyway, spline or thread.
Press the seal in square with a tool that bears on its outer diameter, never a hammer on the seal face, and keep the lip in sight as it goes on so you can see the garter spring stay in its groove and the lip not fold back. A lip turned under leaks immediately.
6. For Housing or Cover Leaks
A leak at the split line or a cover is lost bolt clamp, or a gasket or sealant that has failed. Retorque the bolts to the unit’s specification in a cross pattern, clean the joint and run the unit; if it still weeps, strip the joint, clean both faces to bare metal and reseal.
Reseal with the type the unit was built with: a cut gasket where there was a gasket, an oil-resistant anaerobic sealant where the faces were metal-to-metal. On a bearing cover the gasket thickness is part of the shim stack that sets the bearing clearance, so swapping one type for the other changes the clearance.
Before resealing, establish why the joint opened. Bolts that were tight over a joint that still leaked point to a warped cover or a nicked face; a joint that started leaking after a shock load may be a crack at a bolt boss; a joint weeping at several points at once is usually the breather again. A cracked housing is not a sealing job but a housing replacement.
7. For Leaking Plugs or Fittings
A leaking drain, fill or level plug is almost always a missing, flattened or hardened sealing washer or O-ring, or a tapered plug refitted dry. Renew the sealing element rather than tightening harder — over-torquing a plug in a cast housing cracks the boss, and that is a far bigger repair than the leak was.
Parallel-thread plugs seal under the head on the washer or O-ring, so the thread itself never seals; tapered plugs seal on the thread and need sealant or PTFE tape on every refit. Sight glasses leak at their O-ring or gasket, or crack from over-tightening or a knock, and are replaced complete. Cooler lines and pipe fittings that leak after vibration are remade with a new ferrule or hose end, not retightened.
A Useful Diagnostic Guide
Read the table from the pattern you recorded in step 2.
| Leak symptom | Likely cause | Typical fix |
|---|---|---|
| Oil around output shaft | Worn seal lip; shaft grooved under the lip; bearing play letting the shaft run out | Replace the seal; sleeve or re-machine the shaft so the new lip runs on fresh surface; replace the bearing if there is play |
| Several seals leaking | Blocked breather pressurising the housing (a standard lip seal holds only 0.03–0.05 MPa), or overfill | Correct the oil level; clean or replace the breather; then replace the seals that ran under pressure |
| Oil from housing joint | Bolts have lost clamp; gasket or sealant failed; cover warped or face nicked | Retorque in a cross pattern; if it still weeps, strip, clean to bare metal and reseal with the original type |
| Oil leaking only when hot | Pressure from a blocked breather; level too high once the oil expands; NBR seal running above its 80 °C limit in gear oil | Check breather and level; if the lip is hardened, fit an FKM seal and find why the unit runs hot |
| Oil leaking from sight glass | O-ring or gasket hardened; glass cracked by over-tightening or a knock | Replace the O-ring or the complete sight glass; do not over-tighten |
| New seal quickly leaks again | New lip running in the old wear track; lead on the shaft from emery dressing; bearing play or misalignment; lip damaged or folded on installation | Correct the mechanical cause first — sleeve or re-machine the shaft, replace the bearing, align the drive — then fit the seal square with the lip protected |
| Oil comes out of breather | Overfill; foaming from water or the wrong oil; breather at the wrong port for the mounting position; contamination | Drain to the correct level; drain and refill if the oil is foaming or wet; move the breather to the highest port |
