Numerical Analysis of the Wind Turbine Pitch Bearing Raceway Tribo-contact due to Cyclic Loading Under Constant Pitch Angle

Pitch control movements in wind turbines are commonly short and slow. Moreover, some operating conditions may result in no pitch movement for large time periods. Variable loads coming from wind and the own weight of the blades will turn into micro-oscillations at ball to raceway contact as consequence of ring and contact deformations. Control oscillating movements have been extensively studied in literature. However, no research has been found relating motionless operating point with wear and false brinelling damage. In the present work, a comprehensive analysis at ball-raceway contact taking a 4-point contact ball bearing of 5 MW NREL offshore reference wind turbine as case of study. A simplified finite element model allows to simulate different bearing parameters and loading conditions in order to determinate their effects on the ball and raceway contact behavior. The numerical results highlight the limitations of classic analytical contact mechanics equations which do not predict any tangential stress for non-conformal spherical bodies of same material under normal loading. Furthermore, results reveal the occurrence of micro-slip once a pressure threshold is reached and the effects of some bearing parameters such as raceway conformity or contact friction coefficient. As consequence of this analysis, it is numerically demonstrated that a cyclic normal load without any rolling can lead to local frictional wear, false brinelling or fretting corrosion damage at the bearing raceway.