Ultrasonic Phase and Amplitude and the Measurement of Oil Film Thickness

Abstract

The reflection of ultrasound at an oil film can be used to determine the film thickness. A thin oil film reflects less ultrasound than a thick film. When the film is thin there is a simple relationship between oil film thickness and the proportion of the wave amplitude reflected. The reflection coefficient is in fact a complex quantity with both magnitude and phase. A model for how both the phase and amplitude vary with oil film thickness (and the properties of the bearing materials) has been developed. It has been shown that both can be used to determine film thickness. Tests have been performed to determine the oil film thickness and explore the relationship between reflection amplitude and phase. Experiments are performed both on a static oil film between flat plates, and on an operating journal bearing. Both methods provide a simple accurate method for the measurement of oil film thickness.