Measurement of the rheology of lubricant films within elastohydrodynamic contacts H. A. Spikes*, V. Anghel, and R. Glovnea Tribology Section, Department of Mechanical Engineering, Imperial College, London SW7 2BX, UK Received 23 November 2003; Accepted 7 March 2004 There is growing need for a reliable model of the rheological response of lubricants in elastohydrodynamic (EHD) contacts, not only to predict behaviour in full-film EHD conditions, but also for use in modelling mixed-film lubrication. One barrier to developing such a model is that measurements of friction actually represent averaged values over the whole, lubricated contact under study. However the fluid film conditions of temperature, pressure and strain rate generally vary over such contacts, which makes it difficult to determine constitutive shear-stress equations from friction measurements. This paper examines the various different techniques used to study the origins of EHD friction and the underlying film rheology. It then describes and applies a technique for obtaining the temperature rise maps of both solid surfaces in a rolling-sliding EHD contacts and thus shear-stress and friction maps. The work shows that the shear stress of the traction fluid studied increases approximately linearly with pres- sure and decreases approximately linearly with temperature in the high-pressure central region of EHD contacts. KEY WORDS: EHD, rheology, infrared emission, shear stress, traction, traction fluid, lubricant 1. Introduction Because of growing concern to increase the energy- efficiency of machine components it is of considerable practical performance to be able to predict the friction (often called ‘‘traction’’) of elastohydrodynamic (EHD) lubricant films. In many cases the aim is to reduce this friction to a minimum by appropriate combination of lubricant and operating conditions. In some applica- tions however, such as traction drives, high EHD fric- tion over a wide range of conditions is sought. The friction in a full-film contact is determined by the integral over the contact area of the shear stresses generated in the lubricant film and is thus dependent on the rheological response of the lubricant film to the conditions within the contact. In rolling/sliding EHD contacts these conditions are extraordinarily severe, involving the short-duration application to the lubri- cant film of both extremely high pressures, of the order of 1–3 GPa, and very rapid strain rates, typically 10 6 to 10 8 s )1 . The high pressure produces a very large increase in the viscosity of the lubricant within the film and the combination of this with the high, applied strain rate, results in the oil experiencing a very high shear stress. At such high shear stresses the lubricant behaves in a highly non-Newtonian fashion. To under- stand and thus be able to predict friction in EHD con- tacts we need to have an accurate rheological model to describe this non-Newtonian behaviour. The require- ment for an accurate model has recently become even more acute with the interest of many researchers in numerically simulating mixed-film lubrication, where the surface roughness is of the same order as the EHD film thickness. In such conditions it is essential to have a reliable model for the dependence of shear stress on temperature, pressure, strain rate and time of strain, in order to accurately predict film behaviour in asperity conjunction regions. A number of constitutive equations have been pro- posed to describe the rheology of liquids in EHD con- ditions [1], but there is still considerable dispute about their validity. High-pressure viscometry studies on non-polymeric liquids have tended to focus on Newto- nian behaviour, curtailed, at high shear stress by a lim- iting shear-stress response, although shear thinning has also been observed [2]. EHD studies have generally placed more emphasis on shear thinning above a cer- tain shear stress and on viscoelastic behaviour [3,4]. The main problem in developing and validating equations of rheology in EHD conditions is that it is very difficult to measure the shear-stress properties of lubricants at conditions that are both representative of those found in EHD contacts and are accurately defined. The current paper first examines the main methods used to study lubricant rheology in EHD and thus the origins of EHD friction. It then develops a technique for mapping shear stress in contacts, based on measurement of surface temperature rise. *To whom correspondence should be addressed. E-mail: h.spikes@imperial.ac.uk 1023-8883/04/1000–0593/0 Ó 2004 Springer Science+Business Media, Inc. Tribology Letters, Vol. 17, No. 3, October 2004 (Ó 2004) 593