The on-line measurement of lubricant film thickness for condition monitoring

The ultrasonic reflectivity of a lubricant layer between two solid bodies depends on the ultrasonic frequency, the acoustic properties of the liquid and solid, and the layer thickness. In this paper, ultrasonic reflectivity measurements are used as a method for determining the thickness of lubricating films in bearing systems. An ultrasonic transducer is positioned on the outside of a bearing shell such that the wave is focused on the lubricant film layer. The transducer is used to both emit and receive wide-band ultrasonic pulses. For a particular lubricant film the reflected pulse is processed to give a reflection coefficient spectrum. The lubricant film thickness is then obtained from either the layer stiffness or the resonant frequency. The method has been validated using fluid wedges at ambient pressure between flat and curved surfaces. Experiments on the elastohydrodynamic film formed between a ball sliding on a flat were performed. Film thickness values in the range 50-500 nm were recorded which agreed well with theoretical film formation predictions. Similar measurements have been made on the oil film between the balls and outer raceway of a deep groove ball bearing.