INTRODUCTION : Industrial scale drivetrain bearings, particularly those used in wind turbines, often exhibit premature spalling or macro-pitting well before reaching the rolling contact fatigue (RCF) design life of the bearing (1). In many of these cases, the premature failures are caused by broad branching crack networks surrounded by local regions of nano-grained, microstructurally altered steel (2). When bearings containing these cracks are sectioned and etched with Nital (nitric acid and ethanol), the nano-grained regions resist the etchant, and appear white in contrast with the surrounding steel. Because of this, these failures are often referred to as “White etching cracks” (WECs).
The cause of WEC networks in field bearings is unknown, because of this, multiple avenues have been taken with regards to recreating these crack networks in accelerated benchtop tests, these include but are not limited to: excessive slip, the application of electrical load, and impact loading. However, in a majority of these replication tests, a specific oil is used which is thought to embrittle the test samples and aid in the formation of WECs. Additionally, no researcher has ever been able to show the formation of WECs within a wind turbine gearbox lubricant. (3)
In recent work (4), it was identified that WECs within wind turbine bearings preferentially initiate subsurface at dual-phase inclusions which contain both a manganese sulfide component, and an aluminum oxide component. These types of inclusions are far more prevalent in larger bearings, and therefor almost never exist in small benchtop test specimens. The presented work will investigate the hypothesis that, if the steel from a wind turbine gearbox bearing is tested at a benchtop scale, WECs may form within commercially available wind turbine lubricants.
Methods: All tests within this study were performed on the PCS Instruments micro-pitting rig (MPR). The MPR utilizes a three ring on roller splash lubricated contact and can be operated at various conditions ranging from pure rolling to pure sliding This test rig and lubricant have been used in multiple studies examining the formation of WECs, most notably in (5,6).
The presented tests utilize two different steel samples, the first being the PCS supplied AISI52100 through hardened benchtop test specimens, these samples have been the standard used for WEC testing over the past 4 years. The second set of samples was fabricated from a wind turbine gearbox bearing using electrical discharge machining. These samples were also made of AISI 52100 steel, with an identical surface hardness to that of the PCS supplied samples. These two sets of samples were under sets of identical conditions within a commercially available wind turbine bearing gearbox oil.