Ultrasonic Measurement of Lubricant Film Thickness

Prof. Rob Dwyer-Joyce
Prof Bruce Drinkwater, University of Bristol

Engineering machinery relies on adequate lubrication to function correctly. Under ideal conditions a lubricant film forms between sliding or rolling surfaces. Components like journal bearings, rolling bearings, and gears would not function without a separating lubricant film. Engines, transmissions, process plant, and production machinery all rely on lubricated contacts to function; and assessment of these contacts is an essential part of machine condition monitoring.

This new method (invented and patented by the Sheffield Tribology group in collaboration with the Bristol NDT group) involves bouncing a sound wave off the oil film. A transducer is mounted on the bearing shell and emits an ultrasonic pulse directly onto the lubricated interface. The reflected signal is then recorded and digitally processed to obtain the lubricant film thickness. The digital signal processing method is tailored to the lubrication regime under measurement.

Depending on the thickness of the lubricant film, different signal processing techniques are required (and different frequency transducers employed). The method works well with hydrodynamic (journal bearing type) films of 10 microns and upwards. By focusing the pulse it is also possible to measure elastohydrodynamic films 50 nanometers and upwards. This has been used to measure the oil film in an operating ball bearing. Research is continuing to expand the operating limits of the method and extend the range of industrial applications.

Ultrasound can also be propagated through most bearing shell materials (although polymeric materials tend to attenuate and require special transducer technology). So measurement can be carried out on real bearing components. This approach has been used on a wide range of machine elements including; face seals, ball bearings, thrust pads, journal bearings, prosthetic hip joints and piston rings.

Want to know more? See Tribosonics Ltd; the spin-out company set up to market this technology. (click logo to open Tribosonics home page in a new tab)