Cracks on bearing rings VICopyright: © IWM
Figure 1: (left) Various heat-treated (martensitic and bainitic, respectively) rolling bearings that have failed due to axial cracking or chipping. The macroscopic damage (right) is in each case due to WEA/WEC below the raceway surface.
In rolling bearing applications, failures currently occur well before the calculated fatigue life.Early bearing damage often takes the form of axial cracking or pockmarking on the raceway surface of the bearing rings. Below the raceway surface, bearing failures are characterized by areas with a white etching appearance and are therefore referred to as white etching areas, or WEA. Cracks that lead to bearing damage run along these WEA and are consequently called "White Etching Cracks", or WEC for short. Typical WEC damage patterns of large-diameter bearings from wind turbine gearboxes with extensive networks of "white etching areas/cracks" are shown in Figure 2. Although multiple explanations for this phenomenon are given and controversially discussed in the literature, the reasons for the occurrence of WT/WEC are still unclear. In this respect, the rolling bearing damage caused by WTW/WEC cannot be predicted with the aid of standard bearing life calculation methods (see DIN ISO 281) or taken into account in the design of rolling bearings.
Effective and sustainable measures to prevent bearing premature failures require an in-depth understanding of the chemical-physical mechanisms that lead to damage. To date, however, there is no generally accepted damage hypothesis that explains the formation of WEA/WEC. The main causes for the formation of WEA/WEC are controversially discussed in the materials engineering community, Thereby, the mechanism in the formation of WEA/WEC is currently described by two basic hypotheses: Either crack initiation precedes the formation of structurally altered regions, or crack initiation occurs after the formation of the white etchable regions. All the hypotheses researched have in common that contact parameters, reinforcing factors as well as material and lubricant properties influence the formation of the microstructural changes or crack initiation. These include slip, pressure, temperature, hydrogen input, electric current and lubricant composition.
Due to the variety of explanations as well as the reinforcing factors leading to the above rolling bearing early failures, the key to preventing the damage is not only to investigate the damage development mechanisms but also to determine the critical parameters of the reinforcing factors leading to accelerated failure.
- To understand and be able to evaluate the favoring of WEA/WEC occurrence by different lubricant formulations.
Other sub-objectives of the project are:
- Derive a screening method to evaluate the favoring of WEA/WEC emergence of a lubricant formulation.
- Identify WEA/WEC-critical lubricant formulations and differentiate the extent to which they favor or influence WEA/WEC emergence.
- Identify and understand relationships between lubricant formulation, triboreaction and -mutation layer formation, hydrogen input and WEA/WEC emergence.
- Identify alternative additives of the same additive group to modify a lubricant formulation toward no/less favorable to WEA/WEC formation
- Demonstrate transferability of findings between different test beds for the same conditions
Research and project partners
- Institut für Maschinenelemente und Systementwicklung, RWTH Aachen University, short iMSE
- Gemeinschaftslabor für Elektronmikroskopie, RWTH Aachen University, short GFE