Gear Oil Viscosity Selection by Pitch Line Speed and Temperature

This answer is for enclosed spur and helical industrial gearboxes on viscosity index 90 mineral gear oil with no grade from the maker. Worm and hypoid gears need their own charts. ANSI/AGMA 9005-F16 does not address grease-lubricated enclosed gearboxes and is not intended to replace AGMA application standards, including those for industrial enclosed drives and wind turbines (AGMA, 2026).

The gear oil viscosity selection table by speed and temperature

The gear oil viscosity grade for an industrial gearbox rises as pitch line velocity falls and as sump temperature rises. The table below decides the ISO viscosity grade (ISO VG) for viscosity index 90 mineral gear oil by speed band and bulk oil temperature. Rows marked F16 come from the current ANSI/AGMA 9005-F16 tables; rows marked E02 come from the 2002 edition, which F16 replaced, as reproduced by STLE. Read the band for the slowest mesh, then the row for the sump temperature measured under normal load.

The key row is the third: at 1.0 to 2.5 m/s and a 50 °C sump, ANSI/AGMA 9005-F16 Table B.1 gives ISO VG 320 for VI 90 mineral oil (Errichello, 2018).

How do you calculate pitch line velocity for a gear set?

Pitch line velocity for a gear set is pi times the pitch diameter times the shaft speed, converted to meters per second: PLV = π × d × n / 60, with d in meters and n in rpm. STLE writes it as PLV = π × D × N and notes that rpm must be converted to seconds to give meters per second (Johnson, 2008). Gear speed is based on pitch line velocity, not the simple shaft speed, because pitch line velocity indirectly characterizes the contact time between the gear teeth (Johnson, 2008).

Worked example (illustrative values): a helical output stage has a pinion pitch diameter of 150 mm turning at 200 rpm. PLV = 3.1416 × 0.150 m × 200 rpm / 60 = 1.57 m/s, which falls in the 1.0 to 2.5 m/s band. With a measured sump temperature of 50 °C, the table gives ISO VG 320 for VI 90 mineral oil.

When a gearbox has several gear sets, STLE advises considering the lowest speed mesh and, if there is any difference, the most critical reduction set (Johnson, 2008).

How do you use the gear oil viscosity table?

You use the gear oil viscosity table only after checking the gearbox nameplate and manual, because ANSI/AGMA 9005-F16 states it is "not intended to supplant any specific recommendations of gear manufacturers" (AGMA, 2026). If no grade is given:

1. Calculate the pitch line velocity of the lowest speed mesh.

2. Measure the operating temperature, which Errichello defines as the bulk oil temperature of the oil supplied to the gear teeth (Errichello, 2018).

3. Read the viscosity index from the oil's product data sheet, where STLE notes it is typically available (Johnson, 2008).

4. Read the ISO VG from the matching row. Between two rows, taking the hotter row is a conservative choice (our reading, not a table rule).

5. Check the pour point. Errichello recommends a pour point at least 5 °C below the minimum expected ambient temperature at start-up (Errichello, 2018).

6. Check that the chosen grade still meets the minimum viscosity of the shaft bearings, a concern STLE says grows as gear sump temperatures approach 100 °C (Johnson, 2008).

Which ISO VG grade fits a slow, heavily loaded gearbox?

A slow, heavily loaded industrial gearbox needs the grade for the lowest pitch line velocity band, 1.0 to 2.5 m/s, at its measured sump temperature: ISO VG 320 at 50 °C, VG 460 at 55 to 60 °C and VG 680 at 65 °C for VI 90 mineral oil (Errichello, 2018). Low pitch line velocity goes with high loads and low speeds, and STLE notes that lower PLV values tend to require higher operating viscosities and surface-active antiwear and extreme pressure additives (Johnson, 2008).

Below 1.0 m/s, ANSI/AGMA 9005-F16 has no table row. In a 2018 case with a slowest gear at 0.71 m/s, Errichello used the 1.0 to 2.5 m/s values, the lowest velocity the standard gives, and judged that a 65 °C operating temperature requires ISO VG 680 (Errichello, 2018).

Additive type also matters. For steel-on-steel gears, STLE says an extreme pressure additive system would be appropriate, with exceptions for very high pitch line velocities and steel-on-bronze worm gears (Johnson, 2008).

Does a hot sump need a higher gear oil viscosity grade?

Yes, a hotter sump needs a higher gear oil viscosity grade, because oil thins as it heats. At 1.0 to 2.5 m/s, ANSI/AGMA 9005-F16 Table B.1 moves VI 90 mineral oil from ISO VG 320 at 50 °C to VG 460 at 55 to 60 °C and VG 680 at 65 °C (Errichello, 2018). Each step is one ISO grade, and one grade is roughly 50 % more viscous than the one below (Anton Paar, accessed 2026).

Errichello notes that VG 320 is adequate only if the operating temperature is at or below 50 °C, that such a low temperature will likely require a cooler, and that a more typical operating temperature of 65 °C requires ISO VG 680 (Errichello, 2018). A gearbox delivered with VG 320 and running at 65 °C is a question for the maker.

Before raising the grade, find out why the sump is hot and whether a fitted cooler still works. The heavier grade must still have a pour point at least 5 °C below the minimum start-up ambient temperature (Errichello, 2018).

Why do AGMA and ASM viscosity methods give different grades?

AGMA and ASM viscosity methods give different gear oil grades because they use different inputs: the AGMA tables combine pitch line velocity, sump temperature and viscosity index, while the ASM formula derives a viscosity at 40 °C from speed and pitch diameter (Johnson, 2008). STLE gives the ASM formula as V40 = 7000 / V^0.5, where V = 0.262 × n × d is the pitch line velocity in ft/min, n the driver speed in rpm and d the pitch diameter in inches (Johnson, 2008). Where the operating temperature is appreciably above or below 40 °C, STLE says the engineer must account for the difference (Johnson, 2008).

For the illustrative pinion above, the ASM formula gives about 398 mm2/s at 40 °C before any temperature adjustment, while the AGMA table gives VG 320 at a 50 °C sump. In STLE's wire-drawing example, the ASM target for the mean speed was somewhat less than the ISO 320 grade from the AGMA method, while the minimum speed gave about 514 mm2/s (Johnson, 2008).

STLE concludes that AGMA methods "provide stronger advice where multiple types of lubricant options exist", while ASM methods "provide more exacting advice for specific operating states" but give little help on base stocks (Johnson, 2008). The AGMA tables are also under review: in 2023, answering a film thickness challenge to the synthetic grades, the lubrication committee said it plans to review the tables, and update them as needed, during the next revision of AGMA 9005 (AGMA, 2023).

Where the gear oil viscosity table does not apply

The gear oil viscosity table does not apply when the gearbox maker gives a grade, when the gear type is not spur or helical, or when the oil is not VI 90 mineral oil. Grease-lubricated enclosed gearboxes and drives with their own AGMA application standard fall outside ANSI/AGMA 9005-F16 (AGMA, 2026), and AGMA 955-A22 does not cover hypoid gears (AGMA, 2022). AGMA 955-A22 (2022), created as the first step of splitting lubrication guidance out of ANSI/AGMA 9005-F16, gives AGMA's general lubrication guidance and should be read alongside F16 (AGMA, 2022).

Synthetic gear oils use separate tables: in the 2002 edition, four Annex B tables covered VI 90 and VI 120 mineral oil, VI 160 PAO and VI 240 PAG (Johnson, 2008). A 2023 request for interpretation to AGMA reads F16 Annex B as giving, at 5 m/s and 50 °C, VG 150 for VI 90 mineral oil and VG 100 for PAO and PAG (AGMA, 2023), so the mineral column cannot be applied to a synthetic.

Call the gearbox maker or a lubrication engineer when the output stage runs below 1.0 m/s or the sump above 65 °C, outside the F16 Table B.1 excerpt (Errichello, 2018), when the nameplate and table disagree, or when the gearbox has a history of pitting or scuffing. Lubinpla can work that question from your own data sheets and site history and return one cited document for your review.