Modelling of roller indent marks on wind turbine bearings

Wind turbine bearings may experience conditions where they are subjected to short-term high-load events. These events could lead to the rollers of the bearing exerting significant loads on the raceway. If these forces exceed a critical threshold, the resulting high-stress state at the roller-raceway contact interface can induce plastic deformation [1], potentially leading to the formation of indentation marks on the raceway surface. Deviations in stress states from the normal stress states in an optimally functioning bearing due to certain smaller indentations have been shown to cause a reduction in the bearing life [2], which may also be the case for the bigger roller indentations being studied here. In this paper, an aspect of this indentation phenomenon of the bearing is studied by attempting to ascertain the stress states of the indented area to understand further its impact on the fatigue life of the bearing. The process involves comparing the finite element method (FEM) model to the actual indented geometry of the bearing by iteratively adjusting the material properties to replicate similar indentation shapes and depths. The Hertzian stress equation [3] can normally be used for contacting curved surfaces to obtain stress states and deflections along the contact region. However, these equations are derived on the assumption that the material's yield point is not reached for any of the surfaces and, therefore, that plasticity is not induced. Since the formation of indentation is to be studied here, the plasticity must be considered. It, therefore, becomes necessary to calibrate the material properties using Finite Element Modelling (FEM) to obtain an accurate representation of deformation behaviour and the stress states.