False Brinelling and Fretting Wear in Wind Turbine Pitch Bearings

Pitch bearings connect the blades to the rotor hub of a wind turbine and allow the blades to rotate and optimise their position under different wind speeds. The rotation of the blades is necessary to change the angle of attack to control power production and loads acting on wind turbine components. Accurate positioning of pitch bearings is essential to maximise the productivity of wind turbines and ensure their operational safety. Double-rowed eight-point contact ball bearing is the most widespread bearing solution used in pitch systems. Wear in wind turbine pitch bearings is a tribological issue that has attracted more research interest in recent years, motivated by the challenge of improving the reliability of components and systems of modern large-scale wind turbines, which are rapidly increasing their size with demanding lightweight design requirements. False Brinelling and fretting corrosion are two wear modes affecting pitch bearings produced by oscillatory movements of small-amplitude. The characteristics and differences of False Brinelling and fretting corrosion in pitch bearings are not clearly defined and variations exist among different research publications. Moreover, the effect of operating conditions on these wear modes is not fully understood. This research project aims to characterise the damage modes in wind turbine pitch bearings produced by small-amplitude oscillations and investigate the effect of the most relevant variables involved in these failure modes. To achieve the research aims, a series of simulations and experimental studies were conducted. Firstly, a finite element analysis of the load distribution in a pitch bearing assembly and the damage investigation of a failed pitch bearing section were conducted. The finite element simulation and damage observation results provided information about the damage characteristics of the bearing under actual operating conditions and allowed the estimation of the conditions that produced the damage found on the raceway surface. Secondly, two experimental studies were carried out to reproduce fretting wear under simplified pitch bearing conditions and analyse the effect of different testing parameters, such as the oscillation amplitude, contact force, oscillation frequency, presence of lubricant, contact geometry and number of cycles. The first experimental study consisted of a small-scale configuration using a standard tribometer consisting in ball-in-flat rolling/sliding contact with a 9~mm diameter ball. The results showed limitations of the experimental setup due to the scale and the kinematics of the ball. These limitations were overcome by designing a bespoke medium-scale fretting test rig where the balls are in contact with two grooved samples, allowing a more realistic movement of the balls. Finally, a finite element analysis of the ball contact considering the geometry of both experimental configurations was developed implementing two different approaches to predict the occurrence of wear and to gain a better understanding of contact variables to complement the experimental results. Based on the main results and conclusions obtained in this research project, it was possible to define the main characteristics of fretting wear produced under pitch bearing testing conditions. These characteristics correspond to adhesive and abrasive wear combined with oxidation, whose shape and profile depend on the operating conditions and the sliding regime. However, it was not possible to provide a better differentiation between false Brinelling and fretting corrosion. The experimental studies and finite element analyses allowed different effects of the key testing parameters to be understood which made it possible to outline a proposal to mitigate fretting wear under pitch bearing conditions. The main recommendations consisted of decreasing the oscillation amplitude when actively pitching, reducing the contact force by improving the load distribution, and optimising the contact geometry. Lubrication was not the main focus of this PhD work, therefore, conducting an in-depth analysis of greases and their properties was suggested as possible future work. It is also recommended to conduct an investigation of the effect of variable oscillation amplitude and force, because in this research, all the tests were performed considering these parameters constant. The novelty of the work presented in this thesis lies in the experimental methods where the testing conditions were defined to be representative of wind turbine pitch bearing conditions and the test rigs were designed to account for the complex kinematics taking place in these kinds of bearings, which are not the same normally assumed in standard fretting tests.