What viscosity is and how it is measured
Viscosity is the resistance of a fluid to flow — in practical terms, it is the 'consistency' of the oil. It is measured in two ways: dynamic viscosity (cP or mPa·s) and kinematic viscosity (cSt or mm²/s). For industrial lubricants, kinematic viscosity measured at 40°C is used.
Kinematic viscosity is obtained by dividing dynamic viscosity by the fluid density. Laboratories measure it by flowing the oil through a calibrated capillary tube and measuring the time — the longer the time, the higher the viscosity.
What the ISO VG number means
ISO VG (Viscosity Grade) is the classification system of ISO standard 3448. The number is the nominal kinematic viscosity of the oil measured at 40°C, in cSt. ISO VG 46 has a viscosity of 46 cSt at 40°C, with a tolerance of ±10% (between 41.4 and 50.6 cSt).
- ISO VG 2 → 2 cSt at 40°C (very fluid, for precision instrumentation)
- ISO VG 10 → 10 cSt (high-speed spindle oils)
- ISO VG 32 → 32 cSt (light hydraulic, screw compressors)
- ISO VG 46 → 46 cSt (standard hydraulic, world's best seller)
- ISO VG 68 → 68 cSt (light gears, vane compressors)
- ISO VG 100 → 100 cSt (medium gears, low-speed reducers)
- ISO VG 220 → 220 cSt (industrial reducers, bevel gears)
- ISO VG 680 → 680 cSt (worm gears, very slow open gears)
Why choosing the wrong grade damages the equipment
Viscosity too low: the lubricating film is insufficient to separate metal surfaces. Result: adhesive wear, elevated temperature at contact points, reduced equipment service life.
Viscosity too high: the oil generates resistance to flow, viscous friction power losses, higher reservoir temperature, and possible pump cavitation due to suction difficulties.
The correct viscosity is that which, at the actual operating temperature of the oil, maintains the hydrodynamic film at the contact point of the most demanding component in the system.
How viscosity changes with temperature
All oils become less viscous when heated and more viscous when cooled. The magnitude of this change is expressed with the Viscosity Index (VI): the higher the VI, the lower the viscosity variation with temperature.
A conventional mineral oil has a VI of 90-100. A hydrocracking mineral oil reaches 110-130. A PAO synthetic can exceed 150. In applications with wide thermal cycles (outdoor machinery, for example), the VI is as important as the ISO VG grade.
The correct ISO VG grade is selected so that the viscosity in operation (not at the 40°C reference, but at the actual system temperature) is within the optimal range of the most sensitive component. If you don't know that range, the equipment manufacturer specifies it in the maintenance manual — and if it is not there, it is a technical enquiry worth making before the next oil change.
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