Measuring lubricant viscosity at a surface and in a bearing film using shear-horizontal surface acoustic waves

The lubricating effectiveness of an oil film in a journal bearing depends on the dynamic viscosity of the oil. The viscosity in turn depends on the local operating temperature, pressure, and shear rate. Reproducing these conditions in a laboratory viscometer to investigate the lubricant behaviour is a challenging task. As a result, methods that allow oil viscosity measurement in-situ in a film, would be preferred.

Ultrasound technology utilising shear bulk acoustic waves (BAW) has been used to measure liquid viscosity in the bulk, as well as in-situ in a film; the reflection of a shear BAW from a solid-liquid interface depends on the liquid viscosity. Surface acoustic waves (SAW) have been also used for measuring bulk liquid viscosity. In this paper, shear-horizontal surface acoustic waves (SH-SAWs) were explored for measuring oil film viscosity, as they present good coupling with liquids and sensitivity to surface changes. The main objectives of this work were to generate SH-SAWs on metallic media, investigate the wave response at the metal-oil interface, relate the wave response to viscosity with the aim to apply this knowledge to a journal bearing application for measuring viscosity in-situ the lubricant film.

Initially, the SH-SAW response was investigated at a solid-liquid interface. SH-SAWs attenuate at the solid-liquid interface, due to the liquid viscosity. This was modelled as a function of the liquid properties, material and geometry of the medium, and wave frequency. The SH-SAW attenuation-viscosity model was used to calculate the viscosity (in the range of ∼3 to 4600 cP) of different oils at a free surface, which agreed with the viscosity values from datasheets and bench-top viscosity measurements.

This approach was then implemented in-situ in a journal bearing application. A bearing sleeve was instrumented with a pair of SH-SAW transducers and a shear BAW transducer installed inside the rotating journal. These two approaches were used to measure the film viscosity of 4 lubricants blended with different additives in two ways; the former by the leakage of the surface wave, and the latter by the reflection of the bulk wave. Both approaches were found to be in good agreement. They successfully distinguished the chemistry of the oil test samples according to their viscosities under various loading conditions and constant speed, and were able to monitor changes in the oil film viscosity in the loaded region.

The SH-SAW sensors used were low cost and small sized and so can be fitted relatively conveniently into a bearing sleeve, requiring nothing but a function generator and digitiser to operate. This approach could then be used to evaluate lubricant formulations for their performance actually inside a bearing, rather than through the extrapolation of data from a conventional bench top viscometer.