A gearbox oil change interval is set by temperature and contamination first, and by the calendar only as a rough fallback. For an industrial gear unit running continuously on a correct mineral oil, the defensible baseline is about 8,000 running hours - one year of three-shift operation - while a fully synthetic gear oil in the same duty is usually extended to 20,000 hours or more. Neither number survives a hot sump, water ingress or a breather that drinks dust, and that is exactly why the interval deserves an argument rather than a sticker.

This guide covers the decisions behind the interval: which viscosity the case actually needs, when a synthetic gear oil pays for itself, how temperature rewrites the schedule, and what the oil you drain is trying to tell you about the gears.

What the Oil Actually Does in a Gear Case

The oil in a gear unit has four jobs, and they are not equally obvious. It separates the tooth surfaces with a film that carries the contact load, it removes heat from the meshes to the case wall, it carries wear debris and contamination to the drain or the magnetic plug, and it protects the internals from corrosion between runs.

Only the first job happens at the contact. The other three happen in the sump, which is why the sump - its volume, its position, its temperature - matters as much as the oil grade printed on the datasheet. A case with a generous oil volume can absorb a hot week without the oil degrading; a case filled to the minimum line has no such reserve.

It is worth remembering what the oil does not do: it does not repair a failed seal, and it does not forgive a case that stands two degrees out of level with the upper bearings out of the bath. Lubrication problems are frequently mounting problems wearing a lubrication disguise.

Viscosity: The First Decision and the One That Matters Most

Gear oil viscosity is chosen so that the film at the tooth contact stays thick enough at the sump's normal operating temperature. Industrial gear units overwhelmingly take ISO VG 220 or ISO VG 320 mineral or synthetic oils; smaller high-speed units sometimes drop to VG 150, and heavily loaded low-speed stages go to VG 460.

Two rules do most of the work:

• Hotter sump, higher grade. If the oil film is too thin at working temperature, the contact runs in boundary lubrication and the wear rate multiplies. Moving one viscosity grade up is the standard correction.
• Colder start, lower grade or synthetic. An oil that is correct at forty degrees can be nearly solid at minus fifteen, and the first start after a cold weekend drags the train through it. Synthetic base oils hold viscosity flatter across the temperature range, which is why they dominate in outdoor and unheated installations.

The mistake to avoid is doubling up: a plant that upgraded to a heavier oil to quieten a noisy stage and then also added a cooler has, in effect, over-thickened the oil for everything except the hottest hour of the day. Viscosity is a compromise, and one correction at a time is the discipline.

Mineral, Semi-Synthetic or Fully Synthetic

The base oil decision is a cost-per-hour decision, and it is usually made wrongly because it is made on price per litre.

Mineral gear oils meet the needs of the majority of indoor, correctly sized industrial gearboxes. They are the reference against which the standard gearbox oil change interval was written, and they remain the sensible choice where the sump runs cool and the breather stays clean.

Fully synthetic gear oils earn their premium in four situations: sump temperatures consistently above about 85 °C, ambient ranges wider than roughly forty degrees between cold start and full load, extended intervals that make the oil change itself expensive in downtime, and heavily loaded worm stages where the efficiency gain of a synthetic compound is measurable in motor kilowatts. In those duties the synthetic's oxidation resistance roughly triples the practical drain interval, which usually pays the premium back before the first year ends.

Semi-synthetic blends sit in the middle and are honest products, but they rarely win an argument: they cost most of what a full synthetic costs and extend the interval by a fraction of what a full synthetic does.

What Determines the Change Interval

Beyond base oil, four variables set the real interval, and the first of them dwarfs the rest.

Temperature. Oxidation roughly doubles for every ten degrees of sustained sump temperature rise. An oil specified for 8,000 hours at 70 °C is a 4,000-hour oil at 80 °C, and a 2,000-hour oil at 90 °C. This is the whole reason a case bolted beside a furnace or over a hot product needs its interval argued from the local temperature, not the catalogue one - the same reasoning that drives the oil decision on a unit hung above heated product, such as a vertical agitator gearbox over a bitumen tank.

Contamination. Water from a condensing breather, dust from an ambient breather, or wear metal from a stage in trouble all shorten the interval. A case that lives in a wash-down bay is a different lubrication problem from the same case in a dry hall.

Duty. Intermittent running extends the calendar but not the hours. A drive that runs 2,000 hours a year can reasonably go two to three years between changes on the hours count, provided the standing periods are not condensing water into the case.

Oil volume and age history. A small sump concentrates whatever arrives in the air. A unit whose oil has been topped up repeatedly with different grades carries an unknown blend, and blends have no datasheet.

The practical method is to start from the table above, then confirm with oil analysis after the first interval. The analysis, not the calendar, becomes the schedule.

Temperature Is the Whole Story

Because the oxidation rule is so dominant, it is worth being specific about where the number comes from. The sump temperature that matters is the steady value an hour into full load on a hot afternoon, measured at the drain or with a probe through the fill plug - not the temperature of the case surface, which runs cooler than the oil a few millimetres inside.

Three cases deserve attention. A case that runs at 65 °C in winter and 88 °C in summer needs a summer interval, not an average one. A case whose temperature has drifted upward over two years is telling the plant about fouling on the case wall, a dragging seal or a bearing beginning to fail - the oil is the messenger. And a case that runs hot only on start-up, then settles, is probably over-filled or has the wrong grade for the cold condition.

If the steady sump temperature cannot be brought under about 85 °C by grade and volume alone, the fixes in order of cost are: a synthetic gear oil, a larger sump, a finned case, and a forced-oil or fan-cooled arrangement. The fourth is rarer than it should be and cheaper than the gear set it protects.

Reading the Oil You Drain

The moment of draining is the cheapest diagnostic opportunity the unit will offer all year, and it is usually wasted on a bucket.

What to look at, in the order it tells you things:

• Colour and clarity. Milky oil means water, from condensation, a wash-down hose or a failed seal. Grey-black oil with a burnt smell means oxidation has gone further than the interval assumed.
• Magnetic drain plug. A fine grey paste on the magnet is normal running wear. Visible steel particles or flakes are not; they mean a surface is failing now.
• Sediment in the sump. Sludge at the bottom means the case never runs long enough or hot enough to keep contamination suspended, or that the last drain was not complete.
• The filter or mesh if one exists. Bronze swarf in a filter upstream of nothing is a worm wheel telling you its remaining life.

A sample taken mid-drain into a clean bottle, sent for basic analysis, costs little and converts the next change interval from a guess into a measurement. Plants that do this twice a year stop being surprised by gearboxes.

Water, Dust and the Breather

Two of the three enemies of gear oil enter through the same hole: the breather. Condensation cycles draw humid air in during the night and push it out as the case warms; dust follows the same path in dry climates; and a wash-down hose aimed anywhere near a case top will find the breather first.

The cheap defences, all order-stage decisions: a breather piped to a clean dry point away from sprays, a desiccant breather where the case stands idle between runs, and a case arrangement whose covers and shaft seals do not offer the hose a direct route inside. Where water keeps appearing despite all three, the leak is usually a shaft seal running on a worn sleeve, and no oil change will fix it.

Change intervals shortened by contamination are shortened for a reason: water in the oil at a few tenths of a per cent already attacks the film, and at half a per cent the oil is failing. If analysis shows water repeatedly, fix the entry path before extending or shortening any schedule.

A Change Procedure That Actually Helps

The change itself is where intervals are quietly ruined. Oil drained hot carries most of the debris and sludge out; oil drained cold leaves it behind. The case should be drained warm, flushed if the sump shows sediment, and refilled to the level mark with the same grade the datasheet names - not the nearest grade the store had.

Three habits make the difference between a routine and a repair:

1. Record the grade, the brand and the hours at every change, in the same log as the temperature readings. The next engineer inherits data instead of folklore.
2. Clean the fill cap and the area around it. A funnel balanced on a dusty cover undoes the whole point of the exercise.
3. Check the breather at every change. It is the one component whose failure guarantees the next interval will be short.

FAQ

How often should gearbox oil be changed?

On a correct mineral oil in normal indoor duty, about every 8,000 running hours - roughly annually for a continuously running unit. Fully synthetic gear oil in the same duty is commonly extended to 20,000 hours. High sump temperature, water or dust shorten both numbers, and oil analysis is the honest way to set the final figure.

Can I extend the interval by switching to synthetic gear oil?

Usually yes - synthetics resist oxidation far better, which is exactly why they are specified for hot sumps and wide ambient ranges. The extension should still be confirmed with one oil analysis at the planned interval before the schedule is made official.

What happens if the oil grade is wrong?

Too thin and the tooth contact runs metal-close at working temperature, multiplying wear; too thick and the first cold start drags the train through near-solid oil while churning losses run the sump hotter in normal operation. One grade in either direction is the maximum correction, and it should follow a temperature measurement, not a noise complaint.

Is an oil change needed after a gearbox has been standing idle?

Not on hours, but often on condition. A case that stood through a humid season may have condensed water into the sump, and the oil should be checked for milkiness before the unit returns to load. Standing time is also the right moment to check the breather and the seal lips, which age whether the shaft turns or not.

A gearbox oil change interval is a temperature-and-contamination decision that the calendar only approximates. Set the baseline from the duty, measure the sump, sample the oil, and let the analysis - not the sticker - schedule the next drain. When in doubt about what grade and volume a new unit should carry, the online selection system returns the frame and its datasheet together, and the datasheet is where the argument should start.