Viscosity and viscosity index

Viscosity expresses the inner friction and the fluidity of the lubricant. It describes the hydrodynamic lubricity.

• Usually given as kinematic viscosity η at +40 °C
• Unit mm²/s (old: cSt)
• Accord. ASTM D7042, ASTM D445, DIN 51562, DIN 51659
• Conversion by the density ρ into the dynamic viscosity η possible v = η / ρ
• Depending on the temperature (see VT diagram) and pressure (only relevant above 500 bar)

Simplified selection criteria for the viscosity for an application:

• At high pressure → chose a higher viscosity
• At high rotational speed → chose a low viscosity

Impacts of a wrongly chosen viscosity:

To low

• Lubricating film is too thin
• Enhanced wear and reduced lifetime

To high

• Lubricant is unsuited for the conveying system and does not reach the lubrication points
• Deficient lubrication and wear
• Enhanced inner friction with increased energy consumption

The viscosity index is given by the temperature-viscosity relation (VT behaviour)

• No unit
• Determination acc. to DIN ISO 2909
• Allows calculation of kinematic viscosity at various temperatures
• Higher VI values are of advantage since temperature changes do not result in large viscosity changes i.e. less viscosity change at a varying temperature → Oil covers larger temperature range.

The density is the ration from mass to volume

ρ = m/V

• Unit g/cm³
• Crucial for the conversion of dynamic and kinematic viscosity
• Allows conversion of mass m to volume V and vice versa