Development of Ultrasonic Techniques for Rolling Element Bearing Monitoring

Ultrasound techniques cover almost the whole spectrum of performance criteria for rolling element bearings including lubrication and load measurements. Many areas that are not covered are also current areas of research. It is clearly attractive to be able to use the same data acquisition hardware and processing techniques to acquire multiple indicators of bearing performance. Of course, the technology has its advantages and disadvantages. Film thickness measurements are not as accurate as optical techniques, measurements of contact load are made indirectly through detection of raceway deflection and converting to load through knowledge of contact mechanics, and some minor modifications are often required to bearings in order for measurements to be made. There are also issues with distortion of signals with bonded sensors called fringe effects. However,
instrumentation can be much less invasive than other measurement methods and it is arguable that film thickness measurement is most practical with ultrasound, in part due to the ease of calibration through naturally occurring reference signals. Additional insights into lubricant behaviour between roller passes can also be obtained from ultrasonic techniques.

The broad aim of this work was to develop ultrasonic rolling bearing techniques. There were several areas of development in this work. These were error in measurements, measurement of bearing load distribution, the use of shear sensors, investigation into distortion of bonded sensor signals (fringe effects) and development of focussing techniques using bonded sensors to eliminate fringe effects.

Quantification of error in ultrasonic measurements was used to identify possible transient events in data from an operational wind turbine, most likely caused by wind gusts. Movement of the loaded zone in this wind turbine was also observed and linked to these transient events. A method for detection and severity monitoring of rolling bearing misalignment was developed using ultrasonic techniques in a lab setting using a full scale wind turbine bearing test rig. Shear sensors were used to observe areas of a contact that were fully separated and areas that were in mixed lubrication. They were also used to observe possible lubricant solidification in the bearing test rig. k-space modelling of the interaction of an ultrasonic wave with a contact determined that fringe effects were most likely a result of both reflections from either side of the contact interfering with each other when the beam width was larger than the contact, and scattering of the beam at the edges of the contact. Modelling also suggested that fringe effects could be removed from signals through focussing of the ultrasonic beam. Finally, the total focussing method was applied to bearing sized contacts, both through k-space modelling and experimentally, in order to remove fringe effects from signals. Convergence methods were also proposed to ensure sufficient focussing.