Journal bearing oil film measurement using ultrasonic reflection

A wave of ultrasound will reflect from a lubricant film between two bearing components. The response of the film to a sound wave depends on the acoustic and geometrical properties of the oil layer. For thin lubricant films the layer behaves as a spring and the proportion of the wave reflected (or the 'reflection coefficient') is related to the stiffness of the oil layer. This provides a method for determining the film thickness. An ultrasonic transducer has been mounted on the outside of a hydrodynamic journal bearing bush. Pulses are emitted through the bearing bush and reflected back from the oil film. The reflected pulses are picked up by the same transducer and processed to determine the proportion of the wave amplitude reflected. The film thickness has then been calculated from the reflection spectra. The film thickness has been determined for a range of steady state bearing operating conditions and the results compared, qualitatively, with published numerical solutions. The method has been shown to be robust and flexible and demonstrates the potential of an ultrasonic based lubricant film monitoring device.