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Viscosity
Pressure Coefficient Table of Contents Physical & Chemical Properties Viscosity increases with pressure because the molecules are squeezed together forcing greater interaction. In an EHL contact where the pressure can be 2.1 GPa (300,000 psi) the viscosity is so high that the oil is considered a plastic-like solid. Viscosity at high pressures is measured by flow through pressurized capillary tubes, or a ball falling down a pressurized tube. The higher the temperature the lower the viscosity increase due to pressure. Viscosity pressure coefficient is the slope of lines on graphs of the log of viscosity vs pressure. The unit for pressure viscosity coefficient is the reciprocal of pressure. The SI units are 1/Pa or m2 N-1. Reference 7 gives the pressure viscosity coefficients of several mineral oil showing a variation from 1.6 to 2.68 X 10-8 Pa-1. The coefficient increases with viscosity, and can vary by a factor of 3. Pressure viscosity coefficient can also be measured from oil film thickness and other parameters from a transparent disk-on-ball apparatus. Pressure viscosity coefficient is used in the calculation of oil film thickness in tribological contacts. For example, in EHL contacts, oil film thickness is directly proportional to the 0.74 power of the pressure viscosity coefficient. For more information call 1-888-HERGUTH (437-4884) |
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