SAE Standards Works
E-34 Propulsion Lubricants Committee
ARP6157 - Pressure-Viscosity Coefficient Measurement
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Lavern D. Wedeven
Pressure-Viscosity Coefficient Measurement
The rate at which viscosity increases with pressure is an inherent and fundamental property of lubricating oils. Viscosity increase with pressure contributes substantially to the generation of elastohydrodynamic (EHD) pressure that separate surfaces with an EHD film operating under high stress and relative motion. Viscosity rises exponentially with pressure and the rate of viscosity increase is generally given as an exponent, alpha, assigned to a pressure, p. The measurement of pressure-viscosity coefficient is accomplished by accurately measuring the thickness of a dynamic EHD film between lubricated surfaces under controlled speed, temperature and pressure conditions. The test method uses a WAM test machine available at bearing companies, acedemic institutions, small business and in some government laboratories. Film thickness is measured with optical interferometry using an optical system (AChILES) that provides high resolution 3D mapping of the oil film. Similar test machines with optical interference capability for measurement of dynamic EHD films can also be used. The test method identifies the recommended operating conditions for pressure-viscosity coefficient measurements.
The pressure-viscosity coefficient of lubricating oils is an inherent property for generating EHD films. It is used in rolling element bearing and gear design calculations for predicting EHD film thickness and for estimating wear life, scuffing resistance and contact fatigue life. Pressure-viscosity is a report item in AS5780 and the measurement method is currently described in an appendix of AS5780. Improvements in the measurement technique and the identification of the operating speeds to avoid thermal effects have provided recommended test conditions for measurement. The SAE E-34 committee has made recommendation for the test method to be documented as recommended practice.