S136 Operational Residual Stress Formation in Vibration-Loaded Rolling Contact
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In several applications like paper making machines, fans or wind turbines, rolling bearings are operated under external mechanical vibrations that cause near-surface fatigue. This hitherto unnoticed loading is illustrated by case examples. Smoothing of the machining structure reveals mixed friction. Material aging is shifted towards the surface despite almost indentation-free raceways. Two distinct types of residual stress depth profiles occur. The established micro friction model partitions the contact area into sections of different friction coefficients. Additional tangential load then shifts the equivalent stress either closer to or directly on the surface. Orange skin formation on the raceway by plastic deformation and spot-like dark etching regions in the outermost edge zone confirm nonuniform material loading in the contact area. Both types of residual stress profiles are reproduced on a self-developed vibration test rig for grease or oil lubricated rolling bearings. Cracks on the raceway surface are initiated by tribochemical dissolution of MnS inclusion lines. Acidification of the lubricant due to aging reactions is proven. The novel rig also generates gray staining. The vibration-induced temperature rise of the test bearing depends on the type of lubricant and increases linearly with the contact area-related frictional power loss. PRACTICE-DRIVEN FAILURE RESEARCH In the past years, several X-ray diffraction (XRD) material response analyses of failed or rigtested rolling bearings have revealed the formation of compressive residual stresses near the surface despite an geometrically undisturbed raceway, i.e. without densely distributed indentations (statistical waviness) that could explain this finding by Hertzian micro contacts. In such cases, four of which are discussed below, the running conditions regularly suggest causative additional loading by external mechanical vibrations stemming, e.g., from adjacent machines or engines. The typical example of a XRD depth profile measurement after a shaking rig test of 1200 h is presented in Fig. 1a. In the initial state, only the values of the residual stresses σres (tangential component determined) and the full width at half maximum, FWHM, of the (211) ferrite diffraction reflection on the raceway surface, which are respectively built up to about −550 MPa and reduced by around 0.2° by the finishing process of grinding and honing, differ from the core. The taper roller bearing (TRB) is used in an automotive engine fan unit. Compressive residual stresses are formed up to a depth z of 40 μm below the raceway of the inner ring (IR). Figure 1b shows a scanning electron microscope (SEM) image taken in the secondary electron (SE) mode. The surface reveals a partly smoothed honing structure but virtually no indentations. Mixed friction running conditions caused by the vibrations are reflected in polishing wear on the raceway. Compressive residual stresses at the surface are built up in operation to more than 700 MPa. The line 722 Copyright ©JCPDS-International Centre for Diffraction Data 2009 ISSN 1097-0002