Ultrasonic Measurement of Bearing Film Thickness

Dedi Gasni
M. Kamel Wan Ibrahim
Prof. Rob Dywer-Joyce

The thickness of oil films in ball bearings is a key parameter for ensureing prolonged bearing life. If the film is too thin, then surface contact can result in high friction and wear. The film is usually so thin that it is small compared to elastic distortions of the bearing elements. The ball bearing relies on lubricated contacts to function; and assessment of these contacts is an essential part of machine condition monitoring before failure occurs.

Ultrasound has been developed as a method to determine the thickness of lubricating films in bearing systems. An ultrasonic transducer is positioned on the outside of a bearing shell such that the wave is focused on the lubricant film layer. The transducer is used to both emit and receive wide-band ultrasonic pulses. For a particular lubricant film, the reflected pulse is processed to give a reflection coefficient spectrum. The lubricant film thickness is then obtained from either the layer stiffness or the resonant frequency. The thickness of the lubricant film determines both the signal processing techniques required and the frequency of transducer employed. The method can measure film-thickness values as low as 50 nm.

The aim of this research is to observe film thickness behavior during ball bearing running, especially under non-steady conditions.


  1. Drinkwater, B.W., Zhang, K, Kirk, K.J., Elgoyhen, J, Dwyer-Joyce, R.S., (2008), "Measurement of rolling bearing lubrication using piezoelectric thin films", ASME Journal of Tribology, in press.
  2. Zhang, J, Drinkwater, B.W., and Dwyer-Joyce, R.S., (2006), "Monitoring of Lubricant Film Failure in a Ball Bearing Using Ultrasound", ASME Journal of Tribology, Vol. 128, pp. 612-618.
  3. Zhang, J, Drinkwater, B.W., and Dwyer-Joyce, R.S., (2005), "Acoustic Measurement of Lubricant Film Thickness Distribution in Ball Bearings", The Journal of the Acoustical Society of America, Vol. 119, issue 2, pp. 863-871.
  4. Dwyer-Joyce, R.S., Reddyhoff, T., and Drinkwater, B., (2004), "Operating Limits for Acoustic Measurement of Rolling Bearing Oil Film Thickness", STLE Tribology Transactions, pp. 366-375, Vol. 47, No. 3.