Lubricant base oils exhibit a reversible viscosity increase with the application of high pressure, due to molecular mobility restrictions imposed by the forces being exerted. This fact is especially important because in most mechanical applications films of fluid are compressed between sliding or rolling surfaces under very high loads. Both, the film thickness determined from a values and a/p, the ratio of the viscosity-pressure derivative (partial derivη/partial deriv p)_T and viscosity-temperature derivative (partial derivr η/partial deriv T)p, are useful tools to determine the most appropriate oil for the different lubricated applications. The a coefficient could be measured either directly by assessing viscosity as a function of pressure using high-pressure apparatus, or indirectly by measuring film thickness in an optical interferometer. Coefficient values obtained using the direct and indirect approaches can show large discrepancies. The American Gear Manufacturers Association proposes to use a values derived from optical interferometry (AGMA 925-A03), due to direct measurements of a are not ready available for many lubricants. In this work, a revision of the different definitions of the viscosity-pressure coefficient is presented by using experimental viscosity data previously measured in our laboratory for squalane and four pure pentaerythritol ester base lubricants.
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