When calculating film thickness and friction in elastohydrodynamically lubricated contacts, assuming a non-Newtonian fluid, the lubricant limiting shear stress is an essential parameter. It influences minimum film thickness and determines traction in the contact. The limiting shear stress is pressure dependent according to the Johnson and Tevaarwerk equation:
$τL=τ0+γp$
The limiting shear stress-pressure coefficient γ has in a previous screening investigation been shown to depend on several parameters: oil type, oil viscosity at + 40°C, maximum contact pressure and temperature. In the present investigation, the preliminary data is used together with response surface methodology. With these results in mind, further experiments are made and an empirical model is built. This paper presents a new model for γ which is valid for two types of oil (a polyalphaolefine with diester and a naphthenic oil) with different viscosities at +40°C. The model incorporates the influence of maximum contact pressure and oil temperature on γ. The measurements on which the model is based were carried out at temperatures ranging from −20 to + 110°C. The pressure range was 5.8–7 GPa and the shear rate was about 106 s−1.
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