What causes white etching cracks to develop?
In looking at bearing failure analysis, the root cause of white etching cracks (WEC) in bearings can be traced to bearing rolling contact fatigue as well as accelerated rolling contact fatigue.
In prematurely spalled bearings, two physical parameters will accelerate rolling contact fatigue:
Stresses higher than anticipated
Strength lower than anticipated
The material strength of a bearing can be negatively influenced by environmental factors that are suspected to generate hydrogen. These could include water contamination, corrosion, electrical stray currents and more. In these cases, moderate loading conditions may lead to a premature failure (fig.1).
While still being discussed in the material science community, the findings of bearing failure analysis detailed below strongly support that WEC occur at the end of the failure chain, and are a natural consequence of crack networks in failed bearings.
Testing provides insights on premature bearing failure and WEC
Research on premature bearing failure and WECs has been conducted with several external partners, including SKF University Technology Centers. Investigations were conducted related to field bearings, endurance testing and WEC testing. Although not fully clarified and part of ongoing research, it was shown that WEC can be reproduced, found and associated with the following test conditions:
- in bearing endurance tests of medium-to-larger bearings (rolling contact fatigue)
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in bearing tests with rings being exposed to higher-than-normal structural tensile stresses
- in bearing tests being exposed to brief, heavy loads
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in bearing tests being exposed to contamination with water (fig.6)
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in bearing tests running under mixed friction and high kinematic slip conditions using specific lubricants (fig.7)
- in bearing tests with hydrogen charged components
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in bearings that suffered from electrical current damage (electrical erosion)
Findings
- Each premature bearing failure is unique: The "one & only root cause" does not exist and each failure case needs to be reviewed in the light of the corresponding operating conditions.
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Counter measures can be identified to significantly increase bearing performance.