Measurement of EHD Friction at Very High Contact Pressures
EHD friction curves have been measured up to very high pressure (pmean = 5 GPa, pmax = 7.5 GPa) using a newly developed, rolling-sliding, ball on disc machine, the ETM. Six base fluids have been studied, spanning the API base oil categories Group I to Group V. At high pressures, thermal effects become substantial even at quite modest slide-roll ratios, and these must be considered when analysing friction measurements in terms of the underlying rheological properties of the oils. By comparing measurements from steel/steel and WC/WC ball and disc combinations with very different thermal conductivities, the use of thermal correction to derive isothermal friction curves has been validated. At relatively low pressures (mean pressure = 1 GPa), there are substantial differences between the EHD friction properties of the various API Group base oils, but as pressure is raised these diminish and the EHD friction coefficients of all the Groups approach a similar maximum value at a given temperature. EHD friction continues to be quite strongly temperature dependent even at very high pressure. As pressure is increased, EHD friction curves become progressively steeper, so that friction coefficients at very low slide-roll ratios (1 to 2% SRR) become several times greater at high than at low pressure. This has important practical implications for the efficiency of rolling element bearings at high pressures since these components normally operate in this SRR range. There is no evidence of any of the base oils reaching a limiting shear stress over the whole pressure and temperature range studied. Instead, shear stress continues to increase with log(strain rate) in accord with the Eyring-activated flow model up to very high pressures.