Choose industrial gearbox oil in this order: the ISO VG grade from the speed and load of the gear stage, then the base oil from the gear type and the temperature the sump actually reaches, then the additive specification that goes on the purchase order — and, before any change from what is in the unit now, a compatibility check. The grade is the decision that matters; the other three are conditions on it.
If the rating plate or the manual names an oil group, base oil and viscosity class, that is the answer, and deviating from it is what voids the warranty. The steps below are for the unit whose plate is blank, whose manual is gone, or whose duty has changed since it was commissioned.
1. Choose the Correct Viscosity First
Pick the ISO VG grade before anything else, from the pitch-line speed of the loaded gear stage: viscosity decides whether an oil film separates the teeth, and base oil and additives only work inside that film. The table gives the starting bands. In a multi-stage unit the slow, high-torque output stage sets the grade; the input stage runs without trouble on the heavier oil.
| Operating condition | Typical starting range |
|---|---|
| High-speed, lightly loaded gears | ISO VG 68–150 |
| Medium-speed industrial gearbox | ISO VG 150–220 |
| Moderate/low speed, heavier load | ISO VG 220–320 |
| Slow-speed, heavily loaded gears | ISO VG 320–460 |
| Very slow, high-load equipment | ISO VG 460–680+ |
The rows are keyed on speed. Load is the reason to sit at the top of a band rather than the bottom — a thicker film raises the scuffing load capacity — but it is not a second axis. When the gear data is available, pitch-line speed places the stage in a row:
v = π · d1 · n1 / 60000
Here d1 is the reference diameter of the gear in mm, n1 its speed in min⁻¹, and v comes out in m/s. The band boundaries then fall at roughly 10, 5, 2.5 and 1 m/s from the top row down.
In doubt, take the next grade up. Too thick costs churning heat; too thin brings mixed friction and wear on the teeth, and wear is the more expensive of the two. The one exception is the high-speed end, where an over-heavy oil overheats the unit.
2. Consider Gearbox Type
Gear type decides whether a standard EP industrial gear oil is enough — it is for spur, helical, bevel and planetary units — and flags the two cases, worm and hypoid, where base oil and additive chemistry need a separate decision.
| Gear type | Typical lubricant consideration |
|---|---|
| Spur gears | EP industrial gear oil |
| Helical gears | EP industrial gear oil |
| Bevel gears | EP industrial gear oil |
| Planetary gears | EP oil with strong load/micropitting protection |
| Worm gears | Often require special consideration |
| Hypoid gears | Follow OEM specification carefully |
Worm gears are the special case because the contact is almost pure sliding, steel worm on bronze wheel. That calls for a low-friction oil with an EP package that does not attack bronze (the copper-strip line in step 4).
Lubricant makers prefer polyglycol for a worm gearbox; a worm unit usually leaves the factory with a mineral fill in the 460–680 band, and PG is an option the OEM wants to be consulted on. Follow the OEM, and if you do go to PG, read step 6 first.
Planetary stages run high loads at low pitch-line speed, the combination that produces micropitting, so the oil for a planetary gearbox needs a proven micropitting rating (the FVA 54 line in step 4), not just a scuffing rating. Hypoid gears combine worm-like sliding with helical-like loads; their oil is set by the unit’s maker and there is no general rule.
3. Select Mineral or Synthetic Oil
Mineral oil is the right choice for a gearbox that runs a sump under 80 °C in a normal ambient. A synthetic is bought for one of three reasons: a wider temperature window, a longer drain interval, or lower friction in a worm or hypoid stage.
| Base oil | Best suited for | Main advantages |
|---|---|---|
| Mineral | Normal industrial conditions | Lower cost, widely available |
| PAO synthetic | Wide temperatures, long service intervals | Good oxidation resistance and low-temperature performance |
| PAG synthetic | Many worm gears and high-temperature applications | Low friction and potentially higher efficiency |
| Synthetic ester | Specialized/high-temperature applications | High thermal performance; some products offer biodegradability |
Synthetics have a flatter viscosity–temperature curve and age more slowly: at an 80 °C sump a PAO or PG fill is rated for about twice the service life of a mineral one. When temperature or drain interval is the reason, PAO is the safe synthetic — it tolerates residual mineral oil and suits the same seals and paints, so the changeover in step 6 is straightforward.
PG is the low-friction option. In a worm or hypoid stage a change from mineral to synthetic can raise efficiency by more than 20 %, and PG gives the largest gain of the three; the running temperature drops with it. PG also brings the compatibility problems of step 6.
4. Check the Additive Requirements
For any standard helical, bevel or planetary unit, write “CLP gear oil to DIN 51517-3” on the order — the same oil is labelled ISO-L-CKC or an AGMA EP grade under the other designation systems — and the additive question is settled.
A rust-and-oxidation oil without EP (ISO CKB) belongs only in lightly loaded gears, not in a reducer that sees shock load.
Three test lines are worth adding, because “EP” on a label says nothing about how much protection there is:
- Scuffing: FZG test A/8.3/90 to ISO 14635-1, fail stage at least 12 — the minimum for a CLP oil.
- Micropitting: FVA 54, GF fail stage 10 or higher — for planetary stages and any slow, heavily loaded stage.
- Bronze compatibility: copper strip to ISO 2160, rating 1a or 1b — for any unit with a bronze worm wheel, because active-sulphur EP chemistry corrodes copper alloys.
For a unit whose sump runs hot, specify the CKD category: the same package with reinforced oxidation stability.
5. Account for Operating Temperature
Choose grade and base oil for the temperature the oil reaches in the sump, not the ambient in the plant. Without a measured sump temperature, correct the grade from step 1 by ambient:
- No oil cooler: go up one ISO VG grade when ambient exceeds 35 °C, two grades above 50 °C.
- With an oil cooler: go up one grade when the oil exceeds 65 °C, two grades above 85 °C.
Above 90 °C in the sump the answer is no longer a heavier oil: the unit is past its thermal rating and needs a cooler, or a much shorter drain interval.
The base oil is chosen for the same temperature. Every 10 °C of sump temperature roughly halves oil life, because the oxidation rate doubles; that is why a hot unit goes synthetic even when a heavier mineral grade would still give a thick enough film.
| Base oil | Oil temperature window (approx.) |
|---|---|
| Mineral | −10 °C to +90 °C (short term +100 °C) |
| PAO or PG synthetic | −20 °C to +100 °C (short term +110 °C) |
| Synthetic ester | −20 °C to +90 °C |
At the cold end the constraint reverses: the oil has to flow at start-up, so the lowest temperature the unit sees must stay several degrees above the pour point. A heavy mineral grade in an outdoor worm unit that starts below freezing is the typical case; there the synthetic’s low pour point, not its high-temperature rating, is the reason to buy it.
6. Check Compatibility Before Changing Oils
Refill with the same make and type of oil already in the unit; change base oil or additive technology only for a reason you can name, and then drain, flush, and check seals, paint and aluminium parts before the new oil goes in.
PAO tolerates the mineral residue a normal drain leaves, as long as it stays under 5 % of the fill. PG mixes with neither mineral oil nor the other synthetics, so a change to PG needs a complete drain and a flush with the new oil — a lower grade of the same PG type flushes better, and the flushing charge is never left in as the fill. Esters mix with mineral and PAO, only partly with PG.
Two mineral CLP oils from different makers can also carry different additive chemistries, which is why the first sentence says same make, not just same grade. Only the oil maker can judge whether two packages are compatible.
Then check what the oil touches. NBR shaft seals are fine with PAO and PG below an 80 °C sump; above that the unit needs FKM or PTFE seals before a synthetic goes in. Ordinary interior primers resist mineral oil but not always PG; two-pack epoxy resists all of them. PG under high sliding load can wear aluminium — bearing cages, or a worm wheel with aluminium in the alloy.
A Practical Example
A helical-bevel gearbox drives a belt conveyor without an oil cooler in a plant where summer ambient passes 40 °C, and it is currently filled with a mineral CLP VG 220. The six steps run as follows.
- Viscosity: Its stages sit in the medium-speed row, so the table starts at VG 150–220. That is the baseline before temperature is corrected.
- Gear type: Helical-bevel — an EP industrial gear oil, nothing special.
- Base oil: Summer ambient past 40 °C leaves two options. Step up to mineral VG 320 — the no-cooler rule adds one grade above 35 °C ambient — or keep VG 220 and change to PAO. Mineral VG 320 costs less; PAO VG 220 gives about twice the drain interval and starts more easily in winter.
- Additives: The order line reads “CLP gear oil to DIN 51517-3, ISO VG 320” — or “PAO-based CLP, ISO VG 220” — with FZG A/8.3/90 fail stage 12 minimum and, because a conveyor’s output stage is slow and heavily loaded, FVA 54 GF stage 10 or higher. No bronze in the unit, so no copper-strip line.
- Temperature: Measure the sump at full load on a hot afternoon. Under 80 °C either choice holds. Between 80 and 90 °C the mineral fill is at its limit and PAO is the safer buy. Over 90 °C, buy a cooler, not a different oil.
- Compatibility: The mineral VG 320 is the same base oil; from the same maker and CLP line it is a hot drain and refill. The PAO needs the drain to leave under 5 % residual mineral oil — a flush with the new PAO removes the doubt — and NBR seals are fine with it as long as the sump stays under 80 °C. Either way, confirm the change with the gearbox maker before the order goes out.
