The measurement of lubricant–film thickness using ultrasound

Abstract

Ultrasound is re®ected from a liquid layer between two solid bodies. This re®ection depends on the ultrasonic frequency, the acoustic properties of the liquid and solid, and the layer thickness. If the wavelength is much greater than the liquid-layer thickness, then the response is governed by the sti¬ness of the layer. If the wavelength and layer thickness are similar, then the interaction of ultrasound with the layer is controlled by its resonant behaviour. This sti¬ness governed response and resonant response can be used to determine the thickness of the liquid layer, if the other parameters are known.In this paper, ultrasound has been developed as a method to determine the thickness of lubricating lms in bearing systems. An ultrasonic transducer is positioned on the outside of a bearing shell such that the wave is focused on the lubricant-lm layer. The transducer is used to both emit and receive wide-band ultrasonic pulses. For a particular lubricant lm, the re®ected pulse is processed to give a re®ection-coe¯cient spectrum. The lubricant-lm thickness is then obtained from either the layer sti¬ness or the resonant frequency.The method has been validated using ®uid wedges at ambient pressure between ®at and curved surfaces. Experiments on the elastohydrodynamic lm formed between a sliding ball and a ®at surface were performed. Film-thickness values in the range 50{ 500 nm were recorded, which agreed well with theoretical lm-formation predictions. Similar measurements have been made on the oil lm between the balls and outer raceway of a deep-groove ball bearing.

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