A model for the reflection of shear ultrasonic waves at a thin liquid film and its application to viscometry in a journal bearing

Abstract

The apparent viscosity of oils in the thin layers that exist in machine elements such as gears and bearings is very different to that in the bulk. In addition, oils in lubricating layers are characterized by non-Newtonian behaviour due to the severe thermodynamic conditions that arise. It is this viscosity that determines the film thickness in lubricated mechanical components. This paper describes a novel methodology based on an ultrasonic approach to determine viscosity in situ in a lubricated contact. The methodology considers the lubricant at the solid boundary as a Maxwell viscoelastic fluid and determines its response to an ultrasonic wave. This approach is then compared with existing methodologies in both a static contact and in a rotating journal bearing. The obtained results have shown that the algorithm proposed in this study is most suitable to study lubricants in the range of 0.3–3 Pas and the measurement error has been found to be less than 10%. This viscosity range is common in components such as cam-follower, CVT transmissions and highly loaded journal bearings. At lower viscosities, the measurement method suffers from excessive error caused by the acoustic mismatch between the bearing component and the oil film and the resulting difficulty in obtaining a high enough signal-to-noise ratio.