Study of load distribution and ellipse truncation for angular contact pitch bearings-blade assembly subjected to external loads

Abstract

Pitch bearings are components used in variable-pitch rotors to connect the hub and the blade, while allowing the blade to rotate around its long axis. Most of the studies concerning pitch bearings focus on four-point contact bearings that are widely used in large size wind turbines. However, very few studies consider duplex pairs of angular contact ball bearings that can be found in variable-pitch propellers or small-sized wind turbines. An efficient five degrees of freedom calculation method is proposed to study the load distribution in such bearings while considering the specific design of the blade-hub assembly. The external loads due to aerodynamic and centrifugal effects are prescribed on the blade, and then distributed between the bearings supporting the blade. The governing equation of the blade bending is considered, to further solve the initially statically indeterminate problem. The performance of three bearing arrangements (back-to-back, face-to-face and tandem) is compared in terms of load distribution, with the tandem arrangement being very specific to variable-pitch rotor application, where two angular bearings mounted in the same direction are loaded by the centrifugal force. Additionally, a novel method is proposed to predict the risk of ellipse truncation for each arrangement.