The Effect of Shear Rate and Shear Stress on Viscosity Determinations at Low Temperatures for Engine Oils

The viscosity of engine lubricants was measured at multiple shear rates by a unique Scanning Brookfield apparatus. These oils included both Pumpability Reference Oils (PRO) and a selection of today's commercial multigrade oils. It was found that the viscosity of “flow-limited” oils remained constant when the shear rate decreased. Oils with “air-binding” properties exhibited an increase in viscosity with decreasing shear rate. The magnitude of a change in “slope of the temperature/viscosity profile was found to suggest the degree of air-binding character of an oil.

The flow characteristics of PRO 1, 3, 9, 11, and 16 were measured at several shear stresses and temperatures in the Mini-Rotary Viscometer. The Scanning Brookfield technique and the Mini-Rotary Viscometer were found to yield similar results when the shear stress of the Mini-Rotary Viscometer was reduced from 525 to 35 Pascals.