Effect of Base Oil Structure on Elastohydrodynamic Friction

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.

Advances in nonequilibrium molecular dynamics simulations of lubricants and additives

“…For example, when the applied conditions yield shear localisation, the friction coefficient of L−J fluids can decrease with increasing pressure and shear rate. Such behaviour is not commonly observed experimentally for lubricants, but is more similar to how traction fluids behave [204] above their critical shear stress [185]. The nonequilibrium phase diagrams and friction maps for L−J fluids were also found to be sensitive to the degree of the wall roughness on the atomic scale [188].…”

The Influence of Base Oil Properties on the Friction Behaviour of Lithium Greases in Rolling/Sliding Concentrated Contacts

“…Low shear strength, and thus low friction, is favoured by oils containing linear, aligned and flexible molecules. These are all common features of the alkyl chains that compose the PAO oils, which consequently are expected to produce lower friction than mineral oils [37]. However, the reasons for the higher friction observed with the mineral oil-based greases within the low-speed, thickener-dominated region are somewhat less obvious, given that all greases were formulated with the same lithium thickener.…”

“…Such behaviour is expected at high speeds, where grease friction is determined by its base oil properties as discussed above. Indeed, EHL friction with oil is known to be strongly dependant on the molecular structure of the oil, including molecular shape and flexibility, since these characteristics determine the 'fluid' friction arising from shearing the film molecular layers at high pressure [35][36][37]. Low shear strength, and thus low friction, is favoured by oils containing linear, aligned and flexible molecules.…”

Stress-augmented thermal activation: Tribology feels the force

“…Thus while many processes exhibit the relationship stress  log(rate), others do not. For example in EHD at high strain rates most simple liquids show friction that increases with log(strain rate) in isothermal conditions, but traction fluids reach a constant value [146]. In boundary friction Briscoe and Evans found that fatty acid films showed friction proportional to log(sliding speed) but this was not the case for fatty acid soaps [84].…”