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Viscosity
Shear Rates Table of Contents Physical & Chemical Properties Mineral oil viscosity does not change much with shear rate, that is, they are Newtonian fluids. However, the viscosity of multi-grade, non-Newtonian oils usually decrease with shear rate because of the temporary alignment or breaking down of long chain hydrocarbon molecules to form shorter molecules. Shear rate is speed divided by oil film thickness: Shear rate =ms-1/m = s-1, or reciprocal seconds. For example, with a speed of 1 ms-1 and an oil film 1 micrometer thick, the shear rate is 106 s-1. Shear stability is defined as the ability of a lubricant to withstand shearing without breaking of the long chain hydrocarbon molecules. In lubrication, the viscosity of an oil at high shear rates is important to understanding performance in high speed, thin oil film equipment. An example is a large tilting pad thrust bearing in an hydroelectric generator. Viscosity, as a function of shear rate, is measured by various rotating instruments. The instruments measure the force resisting the flow of oil films of known thickness and speeds. ASTM method D 4683-90 prescribes a tapered roller rotating in a matched tapered stator with a known oil film thickness between them. Results are reported as: viscosity in cP (at 150 degree C and a shear rate of 106 s-1). Another rotating apparatus is the Couette Rheometer, where a precision cylinder rotates at high speed in a larger cylinder with an oil film of known thickness between them. Viscosity at high shear rates is also measured with an ultrasonic shear tester, and a high shear rate capillary at specified frequency, temperature and time. Many original equipment manufacturers(OEM) now require a minimum shear stability. Some original equipment manufacturers (OEM) now require a viscosity of 2.9 cP (at 150 degree C and 106 s-1). For more information call 1-888-HERGUTH (437-4884) |
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