Experimental Research On The Rheological Model Of Lubricating Oil For Vehicles Over A Wide Shear Range

As Vehicle transmission is evolving towards higher speed, there is a growing need for lubricant characteristics under high pressure and shear. Gears, worm gears and worms, bearings in vehicle transmission systems usually work in high pressure、high temperature and high speed. Lubrication film thickness in these higher pairs is very thin and pressure is on the order of 109 Pa. What’s more, the shear strain rate is up to 106 s-1. So lubricants show the characteristics of non-Newtonian fluid and the oil film is easy to crack especially in the case of high loads. Unfortunately, the classical fluid rheological model is able to describe the relation between shear stress and shear strain rate under low shearing conditions accurately, but low accuracy under extremely atrocious conditions like high pressure and high shear.This article focuses on UB-3 traction oil、15W/40 oil、150N base oil,which are wildly used in the auto industry. UMT double disc friction machine was improved by high-precision eddy current displacement sensors to measure oil film thickness between double discs under high pressure. The film thickness is only about 0.1 micrometer to 1 micrometer. So, rheological experimental curves of these three lubricating oil for vehicles were drew. The curves show that change rules of rheology conform with classical Bair-Winer、Circular、Evans-Johnson rheological model under low shear. Shear stress increases quickly with shear strain rate linearly at first, then nonlinear slowly rises to the maximum value and keep constant. However, the curves don’t accord with classical rheological model anymore, but present a different change trend in case of high shear. Shear stress rises to the maximum value and then drops to a certain value on account of heating effect. Eventually it also remains constant.Viscosity-temperature characteristics of lubricating oils were discussed as well with MCR101 rotational rheometer. The result indicates that at certain experimental conditions, the viscosities of lubricating oils all decrease with the increase of temperature according to exponential laws. Furthermore, the rheological models of lubricating oils for vehicles over a wide shear range under different temperature were established. Rheological experimental curves of UB-3 traction oil have only minor differences under high temperature relative to 15W/40 oil and 150 N base oil. So UB-3 traction oil shows high stability at high temperature. At last, the effect of temperature and pressure to ultimate shear stress and slide shear strain rate is also discussed. Slide shear strain rate declines with the increase of pressure, but increases along with the temperature rise. Ultimate shear stress rises vary approximately linearly with pressure, but decreases with temperature slightly.