The lubricant performance capability for aero propulsion drive systems is derived from the physical properties of the oil and performance attributes associated with the chemical properties of the oil. Physical properties, such as viscosity, pressure-viscosity coefficient and full-film traction coefficient are inherent properties of the lubricating fluid. Chemical attributes are critical for the formation of protective boundary lubricating films on the surfaces to prevent wear and scuffing. These attributes are also associated with surface initiated fatigue (micropitting). To assure performance and to provide required information for engineering design, methodology for at least five oil properties are being studied: (1) pressure-viscosity coefficient, (2) full-film traction coefficient, (3) scuffing resistance, (4) wear resistance, and (5) micropitting propensity. The pressure-viscosity coefficient can be measured either directly by assessing viscosity as a function of pressure using high-pressure apparatus, or indirectly by measuring film thickness in an optical interferometer. This document (ARP6157) describes the following two alternative test instruments for calculating the pressure viscosity coefficient by measuring the fluid film thickness:
PCS EHD2 test instrument
WAM test machine
Both methods have been shown to give similar results, therefore either method can be used to calculate the pressure viscosity coefficient of a sample lubricant. For details of how to set up and run either method, see Section 7 (PCS-EHD) or Section 8 (WAM).
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.