Rheology Extracted from Traction Curves-Is There Enough Information?

Most published high-pressure rheological properties of EHL lubricants came about from the Roelands/Eyring framework applied to measured traction. Sometimes Roelands is replaced with a similar function which also lacks the previtreous response. Using the real thermophysical properties of a well-characterized mineral oil, it is demonstrated that the Roelands/Eyring framework can reproduce a traction curve which was generated from the experimentally validated Hybrid/Carreau framework. Two different sets of rheological models with very different piezoviscous and shear-thinning responses can generate the same traction curve within experimental error leading to the conclusion that there is insufficient information in a traction curve to extract the rheological properties of the oil. Even worse, the fluid parameters extracted from traction curves are system dependent. It is shown here that the properties are different for line and circular contacts.