Choose calculator option
Select one option first. The active calculator is highlighted and updates the result panel.
In general, lubricants used in industry are subject to high pressures, up to several GPa. At high pressures, lubricant viscosity increases significantly, and this increase should be taken into account when calculating central film thickness or friction in the contact. This calculator estimates viscosity for a given pressure using two common equations: Barus and Roelands. More background is available in the TriboNet article on lubricant viscosity.
Barus equation
This equation is very simple and therefore widely used. It links the viscosity of the oil to the pressure as follows:
where μ0 is the viscosity at ambient pressure and α is the pressure-viscosity coefficient. For lubricating oils, α is typically about 0.1 × 10-8 Pa-1 to 4 × 10-8 Pa-1.
The Barus equation is known to become inaccurate at high pressure, above about 1 GPa, where it can predict a larger viscosity increase rate than is observed in practice.
Roelands equation
A more accurate relation, which can also include temperature dependence, is given by Roelands:
Here:
- pr = 1.962 × 108 Pa
- T0 is the temperature at which μ0 was measured, in kelvins
- S0 is the viscosity-temperature index, commonly 1.0 to 1.5
- z is the pressure-viscosity index, commonly 0 to 0.8
- μ and μ0 are given in Pa·s in this equation
The pressure-viscosity coefficient α used in the Barus equation is related to the pressure-viscosity index z in the Roelands equation as follows:
More tribology calculators
Continue with another engineering tool or open the full searchable directory.
Contact Mechanics
Lubrication and Viscosity
Wear and Bearing Screening
Hardness and Material Properties
Indentation and Rough Surfaces
- Adhesion Calculator for Rough Surfaces – Bearing Area Model
- Cone with a Parabolic Cap
- Indentation by a Flat Elliptical Punch
- Indentation of Elastic Body by Sinusoidal Surface
- Indentation of Elastic Plane by Truncated Cone
- Rigid Cone on Elastic Flat
- Rigid Flat Punch on Flat
- Surface Roughness Unit / Parameter Helper
- Truncated Paraboloid on Elastic Flat
Test Data and Thermal Screening
