Fatigue life of rotor shafts in theory, practice and in a full scale testing environment

On one hand rotor shafts of wind turbines seem to be larger and heavier than they need to be, due to a lack of empirical data from full scale fatigue testing and a compensation through safety factors. On the other hand wear mechanisms at the contact region between the rotor shaft and the shrink fitted inner ring of the main bearing can be observed. These wear mechanisms are also known from other industries. Shear stresses in the contact region that result from the rotating bending moment at the rotor shaft eventually overcome the frictional connection and lead to a relative motion between rotor shaft and inner ring. These relative motions can cause surface deterioration and thus a degradation of the fatigue strength of the component. The components life span, estimated by common life time assessment methods, will decrease unnoticed. To investigate both the fatigue behavior of the whole component and wear mechanisms a full scale fatigue test has to be carried out. This paper will deal with the common assessment of the fatigue life of rotor shafts as well as the different wear mechanisms and it will show how those and other boundary conditions influence the test strategy for a full scale fatigue test of the rotor shaft