Fluorescence imaging of lubricants in microtextured bearings

Abstract

The work described here concerns the possibility that textured surfaces may reduce coefficients of friction and increase load support within hydrodynamic bearings. This topic is of particular interest in the context of the potential of texturing to reduce friction in internal combustion engines using techniques adapted to suit high volume production, such as Laser Honing. Despite the existence of a number of theoretical explanations (which include shear rate reduction, inlet suction, effect of the presence of cavitation and the mitigation of starvation) and numerous experimental reports as to the effect of texture on friction, there has been no simultaneous study of friction, cavitation and oil film thickness in textured hydrodynamic bearings. This is because such studies are difficult. One must address certain barriers which include the large range of oil film thicknesses encountered and the need for accurately known feature dimensions on the surfaces in question. A new test rig has been designed and constructed that enables the simultaneous measurement of friction, load and oil film thickness. Novel features include a rotating glass disc on a silicon pad, a high-sensitivity fluorescence imaging microscope system to determine the oil film thickness and a non-contact displacement sensor for friction measurement. The process of photolithography was used to accurately produce predetermined textures on silicon surfaces. Tests were conducted on convergent plain and textured silicon pads whilst concurrently monitoring friction, load, cavitation (if present) and oil film thicknesses. The results were compared to theoretical predictions based upon the solution of Reynolds equation, with cavitation, in two dimensions. Results show broad agreement with the theoretical predictions and suggest that textured surfaces may be either beneficial or detrimental according to both their geometry and the operating conditions.