Optical Elastohydrodynamic Fluid Testing

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Abstract
A very-wide-ranging series of lubricants have been tested by optical elastohydrodynamics. The pressure viscosity coefficients so found agree well with those determined in conventional high-pressure viscometers. Deviations from Newtonian behavior show up clearly, and some oils containing viscosity index improvers have, at these high shear rates, an effective viscosity only a few percent higher than their base oils. The perfluorinated alkyd polymers all have the same pressure viscosity coefficient irrespective of their base viscosity. Oils with surfactants do not show any modified surface viscosities; the conflict of this result with other measurements is discussed; and a possible resolution of the difficulty suggested. The aging of oils can be detected from the test results. The change from boundary to full fluid lubrication can be studied by this method. Its use as a secondary viscometer, using an accurate equation derived experimentally, is discussed.