Ultrasonic oil-film thickness measurement: An angular spectrum approach to assess performance limits

Abstract

The performance of ultrasonic oil-film thickness measurement is explored. A ball bearing (type 6016, shaft diameter 80 mm, ball diameter 12.7 mm) is used with a 50 MHz focused ultrasonic transducer mounted on the static shell of the bearing and focused on the oil film. In order to explore the lowest reflection coefficient and hence the thinnest oil-film thickness that the system can measure, three kinds of lubricant oils (Shell T68, VG15 and VG5) with different viscosities were tested. The results show a minimum reflection coefficient of 0.07 for both oil VG15 and VG5 and 0.09 for oil T68. This corresponds to an oil-film thickness of 0.4 µm for T68 oil. An angular spectrum approach is used to analyse the performance of this configuration.The effect of the key measurement parameters (transducer aperture, focal length and centre frequency) is quantified. The simulation shows that for a focused transducer the reflection coefficient tends to a finite value at small oil-film thickness. For the transducer used in this paper the limiting reflection coefficient is shown to be 0.05 and that the oil-film measurement errors increase as the reflection coefficient approaches this value. The implications for improved measurement systems are then discussed.Ultrasonic oil-film thickness measurement 3

Review of ultrasonic-based technology for oil film thickness measurement in lubrication

“…With a high-frequency focusing sensor, Zhang et al has used the angular spectrum method to study the interaction between the sound field and the thin oil film based on the spring model [65].…”