Abstract
The EHD friction properties of a wide range of base fluids have been measured and compared in mixed sliding–rolling conditions at three temperatures and two pressures. The use of tungsten carbide ball and disc specimens enabled high mean contact pressures of 1.5 and 2.0 GPa to be obtained, comparable to those present in many rolling bearings. The measurements confirm the importance of molecular structure of the base fluid in determining EHD friction. Liquids having linear-shaped molecules with flexible bonds give considerably lower friction than liquids based on molecules with bulky side groups or rings. EHD friction also increases with viscosity for liquids having similar molecular structures. Using pure ester fluids, it is shown that quite small differences in molecular structure can have considerable effects on EHD friction. The importance of temperature rise in reducing EHD friction at slide–roll ratios above about 5% has been shown. By measuring EHD friction at several temperatures and pressures as well as EHD film thickness, approximate corrections to measured EHD friction data have been made to obtain isothermal shear stress and thus EHD friction curves. These show that under the conditions tested most low molecular weight base fluids do not reach a limiting friction coefficient and thus shear stress. However, two high traction base fluids appear to reach limiting values, while three linear polymeric base fluids may also do so. Constants of best fit to a linear/logarithmic isothermal shear stress/strain rate relationship have been provided to enable reconstruction of isothermal EHD friction behaviour for most of the fluids tested.
Highlights
Elastohydrodynamic (EHD) contacts are present in many lubricated machine components including rolling bearings, gears, cam/follower systems, and CV joints
EHD friction increases with viscosity for liquids having similar molecular structures
It is shown that quite small differences in molecular structure can have considerable effects on EHD friction
Summary
Elastohydrodynamic (EHD) contacts are present in many lubricated machine components including rolling bearings, gears, cam/follower systems, and CV joints. In most of these systems it is important to achieve as low friction as possible in order to maximise machine efficiency. The friction of EHD contacts is determined by the high non-Newtonian rheological response of the thin lubricant film to the combination of high strain rate and very high pressure present within such contacts. It is recognised that EHD friction is determined primarily by the structure and flexibility of the individual molecules of the base fluid since these control how molecular layers move past one another during shear at high pressure
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