What Viscosity Measures
The kinematic viscosity ν describes the flow resistance of an oil under its own weight—unit mm²/s (cSt), measured at 40 °C. The dynamic viscosity η is the absolute flow resistance and is calculated as η = ν · ρ (mm²/s · g/cm³ = mPa·s). In mechanical engineering, the kinematic viscosity is the commonly used reference value.
ISO-VG classes according to ISO 3448 (DIN ISO 3448)
ISO 3448 defines 18 standard viscosity grades (VG 2 through VG 1500) in geometric increments with a factor of ≈ 1.5 The VG value is the nominal viscosity at 40 °C, with a tolerance of ±10% for each class. ISO VG 220 (198–242 mm²/s) is the standard class for industrial gearboxes; lighter classes (VG 32–68) are suitable for high speeds, while heavier classes (VG 320–680) are suitable for low-speed, high-load applications such as worm gearboxes.
Viscosity Index and Gear Type
The viscosity index (VI) according to DIN ISO 2909 describes how well an oil maintains its viscosity under temperature changes. Mineral oils typically have a VI of 90–105, while synthetic PAO or ester oils have a VI of 130–160, thereby enabling a wider operating temperature range and longer change intervals. For rolling bearings, the following speed rule of thumb also applies: High speed (dn > 300,000) → ISO VG 10–32; Medium speed (dn 100,000–300,000) → ISO VG 32–68; Low speed (dn < 100,000) → ISO VG 100–220.
Mnemonic
Worked example: Viscosity at operating temperature
A gear oil is specified as ISO VG 220—this applies at 40 °C. During operation, the gearbox heats up to about 100 °C.
According to the learning unit, the viscosity of a mineral oil decreases by a factor of about 10 as the temperature rises from 40 °C to 100 °C—VG 220 effectively becomes a viscosity in the range of VG 22. This is precisely why the printed VG class is a reference value at 40 °C and does not indicate the lubricating film thickness at operating temperature—the actual operating temperature must always be taken into account when making a selection.
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