Gear oil selection: viscosity, EP additives and when synthetic pays

An enclosed industrial gearbox is one of the few pieces of plant where the lubricant is genuinely part of the design. The oil film separating two gear teeth is a few micrometres thick and is regenerated thousands of times a minute. Selecting it on the basis of what the last supplier delivered is how gearboxes acquire a reputation for failing early.

Viscosity comes from speed, load and temperature

The film between two gear teeth is formed by the oil being dragged into a converging contact. How thick that film gets depends on how fast the surfaces move, how hard they are pressed together, and how viscous the oil is at the temperature it is at when it gets there.

That gives the two rules that govern the choice. Higher pitch line velocity generates film more easily, so faster gearing generally takes a lower viscosity. Higher load and lower speed make the film harder to form and easier to squeeze out, so heavily loaded slow gearing takes a higher viscosity. Bulk oil temperature moves the whole picture, which is why a gearbox in a foundry and the same gearbox outdoors are not the same application.

The gearbox manufacturer publishes the selection, usually as a table against speed and ambient temperature, and standards such as AGMA and DIN 51509 set out the method. Use that. A general rule of thumb produces a grade that is right often enough to be dangerous.

What EP additives do, and where they do harm

Extreme pressure additives are typically sulphur-phosphorus compounds that are chemically inert at normal temperature. At the high local temperature and pressure of a tooth contact under heavy or shock load, they react with the steel surface to form a soft sacrificial film. That film is sheared away instead of the metal underneath.

It is a genuinely useful mechanism, and heavily loaded spur, helical and bevel gearing needs it. It also has a well-known limitation: the same chemistry that reacts with steel can attack copper alloys. Where a gearbox contains a bronze worm wheel, an active-sulphur EP oil is exactly the wrong choice, and worm drives generally take a different lubricant entirely.

Check what is inside the box before specifying EP. A bronze component and an aggressive EP oil is a slow failure that looks like nothing at all until the wheel is worn.

Micropitting is a separate failure

Classic pitting is a fatigue failure that produces visible craters. Micropitting is different: a grey, matt patch on the tooth flank made up of very fine surface cracks, and it is driven by an oil film that is too thin relative to the surface roughness rather than by overload.

It matters because the response is not the same. Micropitting is addressed by raising the specific film thickness — higher viscosity at operating temperature, smoother surfaces, lower bulk temperature — and by additive systems formulated for it. Adding more EP to a micropitting problem does not help and can make it worse.

When a synthetic actually pays

Synthetic gear oils cost several times what a mineral EP oil costs, and in a mild indoor application on a short drain interval they will never return it. The cases where they do are specific, and they are visible in the data. Two grades from our range, both ISO VG 220:

The 100 °C figure is the interesting one. At operating temperature the synthetic is close to fifty percent thicker than the mineral oil of the same nominal grade, which is film thickness where it is actually needed. The pour point is twenty-four degrees lower, which is what a gearbox starting outdoors in winter is asking for. And the thermal stability of a PAO base is what makes an extended drain interval a defensible decision rather than a hopeful one.

Four conditions, any one of which can justify the change:

• Wide ambient range or outdoor cold starts — the pour point and the flat viscosity curve do the work.
• Sustained high operating temperature, where a mineral oil's oxidation life becomes the limiting factor.
• Drain intervals where the downtime to change the oil costs more than the oil.
• A micropitting history, where the higher film thickness at temperature is a direct intervention.

Before you change anything

• Take the manufacturer's grade recommendation for the actual speed, load and ambient range.
• Confirm whether there is bronze in the box before specifying an active EP oil.
• If wear is the reason for the review, identify the failure mode — micropitting, pitting, scuffing and abrasive wear point in different directions.
• For a synthetic conversion, get seal and paint compatibility in writing, and agree the flushing procedure.

Full published data for the mineral and synthetic gear ranges, including the figures above, is on the technical data page.

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