Endurance test strategies for pitch bearings of wind turbines

Abstract

Modern wind turbines turn their blades along their primary axis to control the conversion of aerodynamic power. The pitch bearings connect the rotor hub of the turbine with the blades and allow this rotation. They are large slewing bearings with diameters in the range of 2 - 7 m. In comparison to bearings in other applications, pitch bearings suffer from three particularities: Oscillating movements instead of full rotations, dynamic and stochastic loads in five degrees of freedom instead of controlled loads, and interface parts with low and complex stiffness along the circumference instead of almost stiff connections. The capital costs of the bearings and even more the costs connected to failures make it paramount to mitigate operational risks of new designs. As there are no established calculation methods for the most dominant damage modes of pitch bearings, bearing tests are of critical value to this risk mitigation. This work covers the design of endurance test runs for pitch bearings. A true endurance test run of pitch bearings should result in a profound evaluation of the risk of failure during the lifetime of the wind turbine. This goal makes the design of an endurance run a substantial task. It gets even more challenging when such a test run needs to fit into the overall turbine design process, both in matters of timing and finance. This work results in a test run profile that covers wear and fatigue damage modes of the bearing rollers and raceways, among others. It also covers the development and commissioning of the test environment for different bearing sizes. The IWT7.5-164 reference turbine serves as example for the design of the test environment and the test run.