Free contact angles in pitch bearings and their impact on contact and stress conditions

The pitch bearing, which connects the hub and the blades, allows the required oscillating movements of the blade, to change the angle of attack thus reducing the lift and drag coefficients there by controlling power and loads of the wind turbine. During the service life of the turbine of approximately 20 years the pitch bearing is loaded dynamically. In a double row four-point contact ball bearing, which is often used for this application, the described loads lead to radial and axial displacements between the inner and outer ring and deformations. These displacements and deformations lead to contact angles which vary from the mounted contact angle which is given by the supplier of the bearing. These divergent angles are called free contact angles. For a four-point contact ball bearing the mounted contact angle in the presented example is 45°. Deviations from the mounted contact angle can lead to reduced fatigue life of the bearing and increased wear, arising from the modified contact behavior. Under axial loads and bending moments the contact area shifts perpendicular to the rolling direction. In the worst case the contact area is truncated, which leads to concentrated and comparatively high stresses. This paper shows how the free contact angles of a double row four-point contact ball bearing of a modern wind turbine with 7.5 MW behaves during the service life and the effects of contact angle variations to the stress distribution. For the analysis a FE-Model of the bearing which includes the stiffness of the bearing and adjacent components is used.