Detection of lubrication state in a field operational wind turbine gearbox bearing using ultrasonic reflectometry

Fully flooded lubrication is the ideal state for a rolling bearing; this is especially true in the aggressive environment of a wind turbine transmission where bearings are subject to intermittent operation and highly variable loading. In this paper, a novel ultrasonic reflection method is used to detect the presence of oil between rollers in the bearing. Ultrasonic sensors were instrumented on the static inner (lab) and outer (field) bearing raceways and reflections were captured as the rollers travelled past the sensor. The proportion of the sound wave reflected (known as the reflection coefficient, R) is dependent on the acoustic mismatch of the materials either side of the interface. Changes in R indicate either a steel–air or steel–oil interface as R values transitioned from 1 to 0.95, respectively, and even lower for a steel–roller interface. Consequently, it was possible to detect the presence of lubricant on the raceway between roller passes. From the laboratory measurements, the recurring reflection coefficient patterns between roller passes were used to identify the lubrication condition of the raceway. An absence of these patterns between roller passes indicated the absence of lubricant on the bearing surface. For the field measurements, three bearing lubrication conditions (partial, insufficient, and fully lubricated) were observed. Partially and insufficiently lubricated datasets were found to occur mostly during transient operation. As transient operation is often accompanied by overloading and torque reversals, coupled with the lubrication issues, these all act to increase the risk of premature bearing failure.