What to Check First When a Gearbox Leaks Oil

Do not reach for a new oil seal. Four steps, in this order: decide whether it is a real leak at all — condensation, assembly residue and oil from nearby equipment are routinely read as a gearbox leak; trace upward from the wet spot to the source, because oil runs downhill and the lowest wet spot is usually the path, not the origin; check oil level and breather next, since an overfilled housing or a blocked, wrongly oriented breather pushes oil out past an intact seal; only then suspect the seal itself.

In the catalogue's troubleshooting section, "vent the reducer" and "correct the oil quantity" are not general advice. They are the first-line action assigned to particular leak positions. Do those two before you open anything up. The order is the whole argument: skip a step and you end up replacing a part that was never at fault.

First, the precondition: stopped, depressurised, cool

No oil plug, sight glass or breather may be opened until the machine is stopped, depressurised and cooled.

A gearbox that has been running holds hot oil under pressure. A gearbox that has just stopped still has hot oil in it and a housing that has not equalised. Loosen a drain plug, a sight glass or a breather at either moment and hot oil can be ejected. That is a personal-injury risk, not an inconvenience.

Every "check the breather", "read the oil level" and "correct the oil quantity" instruction below therefore starts from a stopped, depressurised, cool machine. Loosening a drain plug is never the way to release pressure. On site the move that hurts people is the one behind the words *"I'll just crack it open and take a look."* It takes a second, it looks harmless, and it is the one that sends someone to hospital.

Step 1 — Decide whether it is leaking at all

Before any teardown is considered, four kinds of false leak have to be excluded.

The test is mechanical rather than clever: wipe the area clean, note the position and the time, run the machine for a while, then come back and look. No recurrence means no leak, and the investigation ends there. Recurrence means the trail is real, and you go to step 2.

Skipping this step is expensive in a quiet way. A unit can be opened for a leak that was condensation on a cold morning, and a cover that was never at fault is resealed by hand instead of by the procedure that belonged to it.

Step 2 — Trace upward from the wet spot

Oil runs downhill. A wet spot at the lowest point of a housing is therefore usually the route the oil took, not the point it escaped from. The source sits above it, and sometimes above the gearbox altogether.

Start at the top of the housing and follow the trail upward until it stops. The boundary where the surface goes from wet to dry is the starting point, and that is the position the rest of this article is about. The order that finds it fastest:

1. Breather — the highest point, and the one most easily overlooked.
2. Reducer cover and the screws on the outer covers.
3. Input and output shaft seals.
4. Housing split line and end-cover joint faces.
5. Sight glass, drain plug, filler plug.
6. Driven shaft or coupling side — an oil source outside the gearbox.

What makes the method work is that it converts "what is the cause" into "where is the position". A position can be observed; a cause cannot. Position first, action second. Nothing in this step calls for a guess about which cause is likeliest, and no such guess is offered here.

One habit defeats the method: cleaning the whole housing and then watching the lowest point. The trail has been erased along with the evidence. Wipe a strip, not the machine — or wipe everything, but only after the starting point has been recorded.

Step 3 — Oil level and breather, before the seal

These two come before the seal for one reason: both can produce a leak while the seal is perfectly intact. A seal replaced without them being checked is a part fitted against a cause that has not moved.

A blocked or wrongly oriented breather. Running raises the oil temperature, the oil expands, and the housing pressure rises with it. The breather exists to hold the housing open to atmosphere and equalise that pressure. Block it, or fit it in an orientation where oil submerges it, and the housing pressurises; the pressure escapes at the weakest seal and takes oil with it. Fitting a new seal at that point changes nothing, because the new seal is pushed open in exactly the same way, for exactly the same reason. It belongs here, ahead of the seal, because no seal material can outrun a housing that is holding pressure.

Oil level too high. Fill above the design level and oil works its way out past the output shaft seal or alongside the vent valve. The reflex on seeing seepage is to top up "for safety", which raises the level and increases the leak. Topping up a leaking unit without finding the position is the one action guaranteed to make the next reading worse.

One boundary has to be stated here. Different mounting orientations use different oil-level references, and that derivation belongs to the mounting-position article, not to this one. The job at this point is narrower than it sounds: read the level against the reference for the orientation marked on this unit, and not against habit, nor against whatever the last machine on the line needed.

Step 4 — Only now, the seal itself

By the time the seal is a suspect, three things have been done: the leak has been confirmed as real, the trail has been traced to its starting point, and the breather and the oil level have been checked and found sound.

That sequence is what makes the seal a reasonable suspect at all. A seal is the last component in the housing's pressure path, so it is the last thing that should be accused. Suspect it when the starting point is on a shaft seal itself, or when venting and level correction have been done and the same position still weeps. The table below says what belongs to each position.

The six leak positions, and what actually fixes each

Organised by leak position, from the catalogue's troubleshooting section on oil leakage. The catalogue groups by where the oil appears; it does not rank those positions by frequency, and neither does this table.

The value of this table is concentrated in the last column. Three of these six positions are fixed first by venting or by correcting the oil quantity — not by replacing a seal. Vent the housing and set the level, and some leaks close without the unit being opened at all.

Read the middle column again as well. Even where the position is a shaft seal, the cause the catalogue lists is pressure or oil quantity, not the seal material. That is the practical form of the opening rule: the seal is the messenger, and the message is in the housing.

Foaming from frequent cold starts

Repeated cold starting churns the oil and mixes air into it, and the oil foams. Foam occupies more volume than the same mass of liquid oil, so the level reads high and oil escapes alongside the vent valve. A machine that starts and stops often can therefore leak from the same place with its seal untouched, and the leak will return after every seal replacement until the starting pattern or the level is dealt with.

The clue is in the sight glass: foam, rather than a continuous oil surface. A level read on foam is not a level. Topping up against such a reading raises the real level further, and the seepage gets worse rather than better.

Seal seepage during the run-in period is normal

The catalogue's troubleshooting section says it plainly: during the run-in period, shaft seals may show short-term oil or grease seepage.

That sentence changes what to do on site. On a new unit, or on one that has just had a shaft seal replaced, this kind of seepage during run-in does not establish that the seal has failed, and it does not justify a teardown. That conclusion has a clear edge: light seepage inside the run-in period is normal, but once the run-in period is over and the unit is still seeping, it has to be dealt with as a fault, not as a run-in effect. The sequence is:

1. Wipe the area clean and record the position, the date and the quantity.
2. Watch whether the seepage falls away as running hours accumulate.
3. If it does not fall away, or it increases, work through the position table above.

The reverse handling is the one to avoid. Tearing the unit down during run-in discards a seal that was behaving as the catalogue documents, and the teardown is not a neutral act: a cover opened and reclosed introduces new risk at every joint and every fastener it touches, and the leak that follows may well be one the machine did not have before.

When the machine has to be stopped

Not every seepage calls for a stop. Stop immediately when:

1. the oil level has dropped below the lowest permitted mark;
2. the leak runs as a stream rather than a seep or a drip;
3. the leak comes with abnormal temperature rise;
4. the leak comes with abnormal noise, vibration or output behaviour;
5. the leak sits close to a heat source or to electrical components, where fire or short-circuit risk exists.

Outside those conditions, light seepage can be recorded and dealt with at a planned stop. It should not simply be filed away, though, because "it still runs" is not the same as "it can be left". A continuing seep lowers the level by degrees and surfaces eventually as under-lubricated bearings, and by then the damage has already happened — quietly, in a part nobody was watching.

Put a number on the rate rather than an adjective on the report:

Q = V / t

V is the oil added between two top-ups (L), t is the interval between them (h), and Q is the leakage rate (L/h). Recording Q is far more useful than recording "a bit of seepage" — it shows whether the leak is accelerating or steady, which is the difference between an entry in a logbook and a decision about the machine.

Conditions on site and what they mean

On-site checklist

Observable while running — dismantle nothing

1. Record the sight glass reading, and note whether it is a continuous oil surface or foam.
2. Record the leak form: seep, drip or stream.
3. Record the top-up quantity and the interval, then work out Q = V / t.
4. Watch for temperature rise, abnormal noise and vibration alongside the leak.

After stopping, depressurising and cooling

1. Wipe the trail clean, mark the position, run the machine and look again to see whether it recurs.
2. Follow the trail from high to low and find the starting point.
3. Check whether the breather is clear and whether its orientation is the one marked for the unit.
4. Check the oil level against the reference for the mounted orientation.
5. Check the outer cover screws, the split line, the sight glass, the drain plug and the filler plug for looseness.

Qualified personnel only

1. Deciding whether a shaft seal or any other sealing element actually has to be replaced.
2. Any work touching the housing split line, the bearings or a damaged shaft journal.

Common mistakes and the limits of this article

Common mistakes

1. Replacing the seal because the lowest point is wet, with the source sitting above it the whole time.
2. Not checking a blocked breather, and replacing seals one after another — each new one is pushed open by the same pressure.
3. Seeing seepage and topping up "for safety", raising the level until it leaks more than it did.
4. Ignoring what the mounting orientation does to the oil-level reference, and filling by habit.
5. Tearing a new unit down during run-in, and discarding a seal that was behaving as documented.
6. Cracking a plug, sight glass or breather open on a hot or running machine "just to look" — hot oil ejection and personal injury.
7. Recording "a bit of seepage" instead of a rate, leaving no way to tell acceleration from steady state.

Where this stops

• The check order, the position table and the stop criteria here apply to oil-leak troubleshooting on general-purpose gear reducers.
• This article gives no oil seal part number, no seal specification, no tightening torque, no oil-level mark value and no breather model. All of those have to be confirmed against the specific unit and its technical documentation.
• This article does not judge which cause is most common. The distribution differs between machines, installations and duties, and an attribution without a statistical source misdirects the search rather than shortening it.
• Nothing here is advice to dismantle a pressurised machine. Every action that opens a plug, a sight glass or a breather is preceded by stopping, depressurising and cooling.
• This article does not replace the installation and maintenance manual supplied with the unit.

Data to send with a service request

Items 9 and 10 are the two that get skipped most often, and they are the two that decide whether this is a level problem, a run-in effect or a genuine seal failure. Leave them out and the other eight can be reported for nothing.

About this article and the publisher

This article was written by the technical team of Chengfa Reducer (Changzhou) Co., Ltd. (CHINFAR) and released after technical review. CHINFAR gear reducers are built and delivered through the joint manufacturing system shared with HUIGUANG.

Three related subjects are held by separate articles and are not repeated here:

• Gearbox mounting positions M1–M6: which parameters change with orientation
• Gearbox lubrication guide: oil type, fill quantity and change interval
• Industrial gearbox installation and first-run checklist
• Talk to the CHINFAR technical team

Next step

Send the ten items above to sales@chinfar.com. If all you need is a first split of the problem, send three: leak position, leak form, and the quantity of the most recent fill. Those three separate "oil level or venting" from "the seal itself" without anyone opening the machine.

One last time: stop the machine, depressurise it and let it cool before any inspection. This article does not replace the manual supplied with the unit, and it does not replace a safety review of the specific equipment.