Development of a novel bearing concept for improved wind turbine gearbox reliability

The wind industry has experienced rapid growth in recent years as a result of ever increasing concerns over fossil fuel power generation and the associated impact this has upon climate change. Wind turbine technology has however displayed relatively poor reliability from the outset, largely due to the highly variable nature of the wind, which subsequently places system components under extremely harsh loading conditions. Such reliability issues are becoming increasingly problematic for the wind turbine operator, as wind farms – and indeed the turbines themselves – continue to be up-scaled thus increasing the complexity and cost of maintenance, particularly in off-shore environments. Gearbox failures have been found to be responsible for a large proportion of wind turbine downtime, and this is very often linked to the failure of bearings at certain locations within the gearbox. One such critical location is the epicyclic stage, where planetary support bearings are often found to exhibit damage to a localised portion of their inner raceways, corresponding to the location of the applied load. A concept has been proposed to extend the life of such bearings by periodically rotating the normally static inner raceway so as to avoid the build of damage to one localised region. The concept has been termed the MultiLife(TM) mechanism and a key aim of this thesis was to establish proof-of-concept for this system. Initial analytical work was performed, through which it was identified that a five-fold enhancement to bearing life would be theoretically achievable. The concept was subsequently validated experimentally through testing on a bespoke test platform. In addition to this, the ultrasound technique has been explored as a means to provide early warning of bearing failure and thus support the operation of the MultiLife system. This technique has previously been proven as a method to measure the thickness of the oil films that form within bearing contacts. It was identified for this study that oil films would be too small to measure due to the low viscosity lubricants and high bearing loads utilised to accelerate bearing failures. Nonetheless, a general reduction in the amount of ultrasonic energy reflected from the rolling contact was observed during bearing failure, which was linked to a breakdown of the lubricating oil layer due to degradation of the rolling surface. In this case the ultrasound technique did not provide any advanced warning of bearing failure over more classical condition monitoring techniques; however, it was identified that the use of higher resolution ultrasound sensor arrays would enhance the capabilities considerably. Currently, very few monitoring techniques are applied to wind turbine gearbox bearings, and as a further part of this work a condition monitoring system has been installed within an operational 600kW wind turbine. A high speed shaft bearing has been instrumented with a variety of sensors, including ultrasound instrumentation, to assess the potential of such a system in providing early warning of impending gearbox failures.