Ultrasonic Measurement of Rolling Bearing Lubrication Using Piezoelectric Thin Films

This paper describes the measurement of lubricant-film thickness in a rolling element bearing using a piezoelectric thin film transducer to excite and receive ultrasonic signals. High frequency (200 MHz) ultrasound is generated using a piezoelectric aluminum nitride film deposited in the form of a very thin layer onto the outer bearing raceway. This creates a transducer and electrode combination of total thickness of less than $10μm$⁠. In this way the bearing is instrumented with minimal disruption to the housing geometry and the oil-film can be measured noninvasively. The high frequency transducer generates a fine columnar beam of ultrasound that has dimensions less than the typical lubricated contact ellipse. The reflection coefficient from the lubricant-layer is then measured from within the lubricated contact and the oil-film thickness extracted via a quasistatic spring model. The results are described on a deep groove 6016 ball bearing supporting an 80 mm shaft under normal operating conditions. Good agreement is shown over a range of loads and speeds with lubricant-film thickness extracted from elastohydrodynamic lubrication theory.