Improved infrared temperature mapping of elastohydrodynamic contacts

Abstract

An effective means of studying lubricant rheology within elastohydrodynamic contacts is by detailed mapping of the temperature of the fluid and the bounding surfaces within the lubricated contact area. In the current work, the experimental approach initially developed by Sanborn and Winer and then by Spikes et al., has been advanced to include a high specification infrared (IR) camera and microscope. Besides the instantaneous capture of full field measurements, this has the advantage of increased sensitivity and higher spatial resolution than previous systems used. The increased sensitivity enables a much larger range of testable operating conditions: namely lower loads, speeds, and reduced sliding. In addition, the range of test lubricants can be extended beyond high shearing traction fluids. These new possibilities have been used to investigate and compare the rheological properties of a range of lubricants: namely a group I and group II mineral oil, a polyalphaolephin (group IV), the traction fluid Santotrac 50, and 5P4E, a five-ring polyphenyl-ether. As expected, contact temperatures increased with lubricant refinement, for the mineral base oils tested. Using moving heat source theory, the measured temperature distributions were converted into maps showing rate of heat input into each surface, from which shear stresses were calculated. The technique could therefore be validated by integrating these shear stress maps, and comparing them with traction values obtained by direct measurement. Generally there was good agreement between the two approaches, with the only significant differences occurring for 5P4E, where the traction that was deduced from the temperature over-predicted the traction by roughly 15 per cent. Of the lubricants tested, Santotrac 50 showed the highest average traction over the contact; however, 5P4E showed the highest maximum traction. This observation is only possible using the IR mapping technique, and is obscured when measuring the traction directly. Both techniques showed the effect of shear heating causing a reduction in traction.