Thin oil-film thickness distribution measurement using ultrasonic arrays

This paper describes an approach for calculating the oil-film thickness distribution in machine elements using an ultrasonic array, based on the measurement of normal and oblique incidence reflection coefficients. An experimental system is described in which a high-precision digital piezoelectric translator (DPT) is used to controllably displace the surfaces in a steel–oil–steel system and hence alter the oil-film thickness by a known amount. This three-layer system was chosen to be representative of a typical lubricated contact found in various machine elements such as bearings, gears and seals. In such lubricated systems the oil-film thickness typically ranges from 0.1 to 100 μm and this paper explores the range 2–9 μm experimentally. In the measurements described, reflection coefficients were obtained from transmitter–receiver pairs of an ultrasonic array. In this way, each reflection coefficient measurement corresponds to a point on the oil-film and is related to a specific incidence angle. The oil-film thickness distributions were then extracted from these reflection coefficients via a multi-layer model. The measured oil-film thicknesses are shown to be in good quantitative agreement with the known displacements. This demonstrates the potential of this approach for the measurement of oil-film thickness distribution in lubricated contacts.