A Study on Early Stages of White Etching Crack Formation under Full Lubrication Conditions

Walter Holweger,Viktorija Rumpf,Alexander Schwedt,Ling Wang,Joachim Mayer,Christof Bohnert,Jürgen Wranik,Joshua Spille

doi:10.3390/lubricants10020024

Walter Holweger, Viktorija Rumpf + Show 6 more

Open Access

https://doi.org/10.3390/lubricants10020024

Copy DOI

Abstract

The appearance of White Etching Cracks (WEC), not covered by the ISO 281 modified failure rate calculation, leads to difficulties in predicting bearing reliability. This uncertainty in bearing applications leads to a worldwide activity in order to understand and prevent this situation since the WEC failure mode deviates from the traditional Rolling Contact Fatigue (RCF) mode. Plenty of factors have been found to influence this phenomenon over the years, however the precise initiation of the WEC is still under debate. In order to understand the initiation and analyze the temporal evolution, interrupted tests on the same material were performed under conditions that were known to lead to WEC formation and RCF. To avoid the added complexity of boundary lubrication, a Deep Groove Ball Bearing (DGBB) test rig under full lubrication (Elastohydrodynamic Lubrication, EHL) was chosen. Within a standard operating mode, named Mode 1 (RCF), the bearings are solely subjected to a radial load. By suspending the tests at different time steps, a continuous progress of changes in the subsurface material structure seen as equiaxed grains with low dislocation densities, identified as ferrite, is observed. The bearings did not fail up to load cycles of 109. In contrast, a Mode 2 Electrical Charged Contact Fatigue (ECCF) test provoked the early formation of cracks and crack networks, first without WEA, then later with WEA. It became obvious when comparing Mode 1 (RCF) with Mode 2 (ECCF) that Mode 2 (ECCF) achieves far fewer load cycles until failure occurs.

Highlights

Bearings are key elements in any drive train in industrial and automotive applications.Their basic rating life is defined by standards such as ISO 281 or ISO/TS 16281
In the course of the experiment these structures grow (Figure 4, 200 h,300 h, 670 h) and the elongated grains get accompanied by lenticular carbides with a ladder-like substructure (best visible in the image of the sample tested for 300 h (Figure 4, 300 h))
This study shows that the fatigue behavior of the bearings from Mode 1 (RCF) tests differ from those apparent in the conditions leading to Mode 2 (ECCF)

Summary

Introduction

Bearings are key elements in any drive train in industrial and automotive applications. Their basic rating life is defined by standards such as ISO 281 or ISO/TS 16281. It became evident within the past that the appearance of failures such as White Etching Cracks (WEC) do not correspond to the ISO 281, making calculations of cumulated failure probabilities for bearings under specific operating conditions impossible. WEC features were first reported in 1966 [1] where cracks with structural changes adjacent to them were observed after the sectioning of a bearing. WEC appearing as subsurface crack networks have been reported in bearings within most industrial and automotive applications [2,3,4,5,6,7]. Due to its unpredictable nature, WEC has been suggested to be one of the most critical causes leading to large downtimes of wind turbines, resulting in repair costs of >250,000 € per failure [8]

Methods

Results

Discussion

Conclusion