Effect of particle contamination in lubricants on electrically induced failures in cylindrical roller bearings

M.Sc. Jörn Christian Harling

Summary

Premature, electrically induced bearing failures in wind turbines (WT) significantly reduce their reliability, increase maintenance costs and therefore the levelized cost of electricity. While filtration systems typically exclude particles smaller than 6 µm due to presumed low mechanical risk, such fine particles can alter lubricant electrical properties and thus affect bearing stress from current passage. However, their direct impact on electrically induced premature damages – such as grey frosting, electro erosion, white etching cracks – remains unclear.

This study aims to clarify how particle contamination in lubricants influences electrically induced premature bearing damages. Experiments are conducted on a radial test rig with NU 206 bearings under constant current loading, while allowing the voltage to adjust freely. The lubricant is deliberately contaminated with iron particles sized 4-6 µm at concentration levels typical for WT applications. To assess the influence of different electrical loading conditions, selected tests are conducted under alternating current as well. The surfaces of the raceway and rolling elements are evaluated using microscopy and tactile roughness measurements after a defined test duration; ultrasonic testing will further investigate subsurface damage such as white etching cracks.

The results demonstrate that even fine iron particles can significantly influence the surface condition and damage patterns of bearings under electrical loading. Despite their impact, such particles are currently not addressed in existing design standards, which typically only consider larger particles due to their mechanical effects on bearing damage. This work highlights the importance of considering electrically induced damage mechanisms when defining filtration requirements. Further research is needed to transfer these findings to full-scale WT gearbox bearings under realistic electrical loading conditions and to adapt filtration guidelines accordingly.