Jörg Loos

WEC Formation in Rolling Bearings under Mixed Friction: Influences and “Friction Energy Accumulation” as Indicator

ABSTRACT White etching cracks (WEC) can lead to premature rolling contact fatigue. Possible drivers of WEC can be mixed friction, sliding between rolling elements and raceways, electrical current, critical additives, and water-contaminated lubricant. With respect to WEC failures induced by sliding between rolling elements and raceways under mixed friction, an approach is presented that can explain the experimentally observed failure characteristics of cylindrical roller thrust bearings. Variants of the bearing were tested using a WEC-critical lubricant. The tests showed that not only the contact pressure and sliding between rolling elements and raceways but also the lubrication conditions (specific film thickness) and the frequency of the contact load cycles have an influence on WEC life. These influences are reflected best by a newly introduced characteristic parameter termed friction energy accumulation. As far as WEC failures induced by sliding under mixed friction are concerned the friction energy accumulation could be used for a comparative assessment of the WEC risk of arbitrary rolling bearing applications. A link between the friction energy accumulation and the absorption of hydrogen is discussed and can provide further explanations for the susceptibility of bearing components to WEC formation.

Non‐destructive subsurface damage monitoring in bearings failure mode using fractal dimension analysis

Purpose – Bearings in field applications with high dynamic loading, e.g. wind energy plants, suffer from sudden failure initiated by subsurface material transformation, known as white etching cracks in a typical scale of μm, preferably around the maximum Hertzian stress zone. Despite many investigations in this field no precise knowledge about the root cause of those failures is available, due to the fact that failure under real service conditions of wind energy plants differs from what is known from test rig results in terms of contact loading, lubrication or dynamics. The purpose of this paper is to apply Barkhausen noise measurement to a full bearing test ring running under conditions of elastohydrodynamic lubrication (EHL) with high radial preload.Design/methodology/approach – Full bearing tests are carried out by use of DGBB (Deep Grove Ball Bearings) with 6206 specification, material set constant as 100Cr6, martensitic hardening, 10‐12 percent maximum retained austenite and radial preload of 3500 MP...

White Etching Cracking—Simulation in Bearing Rig and Bench Tests

ABSTRACT White etching cracking (WEC) is a contact fatigue bearing failure commonly observed in wind turbine applications. It can lead to fatigue lifetimes more than an order of magnitude shorter than expected lifetimes. Though various mechanical and chemical factors have shown direct or indirect impacts of on WEC failure, the correlations between these factors are yet to be fully understood. The critical intersection among various lubricant- and non-lubricant-related parameters and their influence on hydrogen diffusion and WEC formation are discussed in this article. Experimental results are shown under diverse operating conditions and contact configurations using three test rigs. This study confirms that the mechanical properties of a rolling contact and lubrication parameters alone cannot predict WEC failure. The formation of a tribofilm and accumulation of atomic hydrogen below the contact surface can be essential to explain WEC events. Higher hydrogen concentration in the WEC zone depends on contact area size, the presence of metal-containing additives in lubricants, and higher frictional energy dissipation. Finally, a mechanism of WEC failure has been proposed that intersects the overlap of hydrogen and subsurface shear stress.