Proceedings of WTC2005 World Tribology Congress III 12-16, Sept 2005, Washington, D.C., USA WTC2005-63789 ULTRASONIC PHASE AND AMPLITUDE AND THE MEASUREMENT OF OIL FILM THICKNESS R S Dwyer-Joyce Department of Mechanical Engineering, University of Sheffield, Sheffield, S1 3JD T Reddyhoff Department of Mechanical Engineering, University of Sheffield, Sheffield, S1 3JD B Drinkwater Department of Mechanical Engineering, University of Bristol, Bristol, BS8 1TR 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. INTRODUCTION The thickness of the oil film in tribological components is a key parameter. If the film is too thin then surface contact can occur resulting in high friction and wear. Measurement of these oil films is difficult because the film is so thin. Usually this requires the electrical isolation of the bearing components or the use of some form of surface mounted contact. One method that shows potential for non-invasive oil film measurement, is the use of ultrasonic reflection. When an ultrasonic wave is incident on an oil film it is partially reflected and partially transmitted. The proportion of the wave reflected, known as the reflection coefficient, depends on, amongst other parameters, the thickness of the oil film. This approach has been validated for application to oil films between moving metal surfaces [1]. This has subsequently been used to measure the oil film in rolling element bearings [2], hydrodynamic journal bearings [3], and piston ring liner contacts [4]. In previous work, only the variation of reflection coefficient amplitude was used to determine the oil film thickness. In this work the use of the phase component of a reflected ultrasonic wave in investigated. ULTRASONIC REFLECTION AT AN INTERFACE The proportion of a wave amplitude that is reflected is known as the reflection coefficient. This is a complex number and has both magnitude and phase. The analysis in this work shows that the film thickness can be related to the magnitude, R and phase, Φ R by the expressions: ( ) ( ) 2 2 2 1 2 2 1 2 2 1 2 1 R z z z z R z z c h − − − + = ω ρ ( ) ( ) ( ) ( ) ( ) ( ) 2 2 1 2 2 2 1 2 2 2 2 1 2 2 1 2 2 2 1 2 tan tan z z z z z z z z z z c h R R ω ω ω ρ − Φ − ± − Φ = where ω is the angular frequency of the ultrasonic wave and z is the acoustic impedance (the product of the wave speed, c and the density ρ) and the subscripts 1 and 2 refer to the materials either side of the layer. 1 Copyright © 2005 by ASME