Papers by Author: Wolfgang Nierlich

Abstract: Mixed friction acting in a rolling contact increases the v. Mises equivalent stress and shifts the maximum towards the surface. Tangential stresses are superimposed to the stress distribution. The resulting position of the maximum v. Mises stress depends on the magnitude of the friction coefficient and is located directly on the surface from values of about 0.25 upwards. The impact of three-dimensional machine vibrations on rolling bearings in operation can cause severe mixed friction running conditions. Residual stress distributions measured on indentation-free raceways indicate high friction coefficients of up to greater than 0.25. The surfaces reveal smoothing of the finishing structure but no adhesive wear. The simulation of the vibrationally loaded rolling-sliding contact is based on the tribological model of localized friction coefficient. This approach avoids seizing by allowing for increased friction only in intermittently changing subareas of the contact at low sliding speed. The macroscopic friction coefficient, meeting a mixing rule, does not exceed 0.1. The finite element method (FEM) is used for the stress analysis. In the first step, a simplified FEM model involves a circumferentially oriented band of high friction coefficient from 0.2 to 0.5 within a cylindrical roller contact. The resulting depth distributions of the v. Mises equivalent stress during overrolling and the corresponding residual stresses are evaluated below the inner ring raceway of the bearing. The features of the FEM model are discussed in detail. The increased sliding friction in the band shifts the maximum of the v. Mises equivalent stress to the surface. Compressive residual stresses are induced in the edge zone. Depending on the applied Hertzian pressure, an additional subsurface peak occurs. First results of the finite element analysis are presented.

Abstract: Rolling bearings in wind turbine gearboxes occasionally fail prematurely by so-called white etching cracks. The appearance of the damage indicates brittle spontaneous tensile stress induced surface cracking followed by corrosion fatigue driven crack growth. An X-ray diffraction based residual stress analysis reveals vibrations in service as the root cause. The occurrence of high local friction coefficients in the rolling contact is described by a tribological model. Depth profiles of the equivalent shear and normal stresses are compared with residual stress patterns and a relevant fracture strength, respectively. White etching crack failures are reproduced on a rolling contact fatigue test rig under increased mixed friction. Causative vibration loading is evident from residual stress measurements. Cold working compressive residual stresses are an effective countermeasure.

Abstract: The accelerating effect of absorbed hydrogen on subsurface rolling contact fatigue is studied by bearing rig testing under HF electric current passage. The additional chemical loading enhances dislocation mobility and microplasticity. Structural changes occur in a premature stage, as shown by comparative XRD residual stress analysis. Dark etching regions appear to be a precursor for phase transformation in regular white etching bands. Fatigue crack initiation is demonstrated.

Abstract: By X-ray diffraction (XRD) measurements, material conditions of rolling bearings are red that point to a variety of load possibilities especially at raceway surfaces. Due to unambiguously distinguishable damage symptoms, according to H. Muro, it is differentiated between the surface and sub-surface failure mode in the literature. Surface distress of different intensity can be generated by particle-contaminated lubricants that result in raceway indentations. These micro-Hertzian contacts may lead to changes in residual stress and line broadening and in the microstructure. Another cause of surface distress is boundary lubrication. Relevant position and nature of the failure mechanisms are characterized. In case of initial material stabilization, the time alterations of the XRD parameters correlate with the statistical parameter of the 10 % bearing life. Contrary to the L10 value for sub-surface fatigue, which leads to spalling only a long time after incipient material softening, in the surface damage mode the L10 life roughly coincides with the beginning of the instability phase. Surface pitting or gray staining turns up frequently with low XRD indication of material aging. Here, scanning electron microscopy and electron microprobe analysis point to corrosion fatigue. The interaction between material and lubricant opens research in the field of tribology.