Gearbox oil viscosity calculator (ISO VG)
Recommended ISO VG for enclosed gear drives. Method 1 looks up the ideal ISO VG from lubrication type, reduction type, power and output speed (optional temperature adjustment); Method 2 uses the AGMA 9005-F16 Annex B tables (pitch-line velocity, bulk oil operating temperature and oil viscosity index).
Ideal ISO VG from the type of lubrication, type of reduction, transmitted power and output speed.
Fill in the tile above to get a recommended ISO VG, then rank real industrial gear oils that match it at your operating temperature.
Compares the power you transmit with the drive's application-adjusted thermal rating, PTA = B1 × B2 × B3 × B4 × B5 × PT (Falk A-Plus catalog 131-110). PT is the basic thermal rating printed in your drive maker's catalog for the size, ratio and cooling option.
How this calculator works
The calculator offers two methods.
Method 1, By Power / Speed / Reduction (the default), looks up an ideal ISO VG in a rule-of-thumb table. The lookup uses the type of lubrication (splash or pump), the type of reduction (single, ratio below 10:1, or multiple, above 10:1), the transmitted power band in HP and the output speed band in RPM. Some light grades come with an oil type, such as a hydraulic (AW) or R&O oil. An optional temperature rule can step the grade along the ISO VG ladder. Without an oil cooler, it adds 2 grades above 122 °F ambient and 1 grade above 95 °F, and subtracts 1 grade below 50 °F and 2 grades below 10 °F. A sump temperature above 150 °F raises the step to at least +1, and above 185 °F to at least +2. With a cooler, only the sump temperature is used. This table is not an AGMA method.
Method 2, By Pitch Line Velocity / Temp, reads ANSI/AGMA 9005-F16 Annex B, Tables B.1–B.4, for spur, helical and bevel gears. There is one table for each oil viscosity index: VI 90 mineral, VI 120 mineral/PAO, VI 160 PAO and VI 240 PAG. Rows are bulk oil temperature in 5 °C steps from 10 to 100 °C. Columns are pitch-line velocity from 1.0 to 30 m/s. A temperature between rows reads the next warmer row, and a speed between columns reads the slower column. Both choices give the heavier, conservative grade, and the neighbouring cells are listed. Pitch-line velocity is π × pitch diameter (m) × RPM ÷ 60, in m/s. The calculator warns about grades below ISO VG 68 or above 680, blank table cells, speeds below 1 or above 30 m/s, and temperatures outside 10–100 °C.
The Lubricant Recommendation tile below the result ranks industrial gear oils that match the target grade at your operating temperature.
Inputs explained
- Method 1: Type of lubrication (splash or pump), type of reduction, power (HP band) and output speed (RPM band). The optional ambient temperature, sump temperature and oil cooler fields only add the temperature step.
- Oil type / viscosity index (Method 2): selects Table B.1 (VI 90), B.2 (VI 120), B.3 (VI 160) or B.4 (VI 240).
- Bulk oil operating temperature (Method 2): the stabilized sump temperature, in °C or °F.
- Pitch line velocity (Method 2): enter it in m/s, or enter the gear pitch diameter (mm or inches) and the RPM of the slower gear. For a multi-stage gearbox, use the lowest-speed mesh.
Worked example
Method 2: a helical gearbox whose slower gear has a 400 mm pitch diameter and turns at 250 RPM. Mineral oil (VI 90), bulk oil at 65 °C.
| Pitch-line velocity | π × 0.4000 m × 250 RPM ÷ 60 = 5.24 m/s → 5 m/s column |
|---|---|
| Table | B.1 (VI 90, mineral), 65 °C row |
| Neighbouring cell | 10 m/s column: VG 220 |
Calculator output: Recommended ISO VG 320.
Method 1: splash lubrication, single reduction, 10–20 HP, output speed 0–150 RPM. Calculator output: Ideal viscosity ISO VG 460. Adding a 100 °F ambient with no cooler gives “Ambient >95°F (35°C): +1 grade” and “Temperature-adjusted: ISO VG 680”.
Frequently asked questions
How do I select gear oil viscosity for an enclosed gearbox?
Use the gearbox maker's recommendation first. Without one, this calculator offers two methods. ANSI/AGMA 9005-F16 Annex B gives an ISO VG from the pitch-line velocity of the slowest gear mesh, the bulk oil operating temperature and the oil's viscosity index (mineral VI 90 up to PAG VI 240). A rule-of-thumb lookup gives one from the lubrication type, reduction type, power and output speed.
How do I calculate pitch-line velocity?
Pitch-line velocity in m/s = π × pitch diameter in metres × RPM ÷ 60, using the pitch diameter and speed of the slower gear. For a multi-stage gearbox use the lowest-speed mesh. Enter the pitch diameter (mm or inches) and gear RPM, and the calculator works it out.
What if I don't know the gearbox oil temperature?
The AGMA method needs the stabilized bulk (sump) oil temperature. As the note under the temperature field says, AGMA's tables assume roughly 45 °C above ambient when it isn't known. The Thermal Check tile also gives a rule-of-thumb sump estimate, not from the catalog: it assumes the sump reaches 200 °F (93 °C) when the transmitted power equals the drive's application-adjusted thermal rating PTA, and that the rise above ambient scales with P/PTA. For a better result, measure the sump temperature once the gearbox is at steady state.
Why does the result pick the heavier grade between table values?
AGMA tabulates rows in 5 °C steps and a fixed set of speed columns, with no interpolation rule. The calculator reads the next warmer row and the slower speed column, which both give the heavier, conservative grade, and it lists the neighbouring cells.