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Impact of imperfections on bearing performance

Impact of imperfections on bearing performance

Phil Burge of SKF discusses research into the measurement of microstructure and pre-existing imperfections of bearing materials and how these measurements are aiding the development of predictive modelling tools to study the effects of metallurgical imperfections on the long term performance of bearings.

Modern machinery is expected to work at or near its design limits, which places a considerable burden of responsibility on manufacturers of critical components such as bearings, to ensure that their materials of construction are resilient against such phenomena as rolling contact fatigue and white edge cracking, and are thus able to perform reliably in the long term under highly stressed conditions. Research on these fatigue mechanisms is thus fundamental to advancing bearing technology for challenging applications, and to better understand the effects of microstructure and pre-existing metallurgical imperfections on bearing performance.

Premature bearing failure is rare these days – some estimates put this at around 0.5% of all bearings in service – but modern bearings do sometimes fail prematurely due to rolling contact fatigue (RCF) and white etching cracking. Gaining an understanding of the failure mechanisms involved is fundamental to the continuous improvement in bearing metallurgy and, ultimately, the long term performance of bearings in high power density applications.

RCF is rare; indeed, the final achieved life of a rolling bearing is usually in excess of its calculated rating life. There are instances, however, where in specific applications a bearing may fail prematurely, usually as the result of failure of the weakest link in its construction. Apart from the effects of poor lubrication of the bearing's rolling surfaces (the culprit for nearly a third of all bearing failures in service), premature failure due to RCF is likely to be the result of imperfections in the subsurface material, non-metallic inclusions, for example, which are an inevitable consequence of steel production.

The stress concentration near an inclusion can cause local plasticity of the steel, inducing a localised tensile residual stress, which, together with other components of the local stress near the inclusion, may cause crack initiation and eventually crack propagation.

SKF has developed a modelling technique using the finite element method (FEM) to study the behaviour of subsurface inclusions under surface loading. The method considers different types of inclusion and their bonding to the surrounding steel crystalline structure. This FE simulation also takes account of the elastic-plastic behaviour of bearing steels to account for the localised plasticity in the vicinity of the inclusion.

Modelling shows that under a specified contact pressure, initiation of a crack from an inclusion depends primarily on the local stress near the inclusion. It is governed by the type rather than the size of the inclusion, the stress raising factor being independent of the inclusion size. Moreover, a crack initiated from an inclusion doesn't always go on to develop a spall and in some cases the crack may be permanently arrested.

A bearing subject to RCF often reveals signs of an extensive subsurface crack network within its steel microstructure known as white etching cracks (WECs). Affected areas consist of ultra-fine, nano-crystalline, carbide-free ferrite, or ferrite with a very fine distribution of carbide particles that appear white when viewed under a microscope. Over time, WECs will propagate towards the bearing surfaces, and may typically result in raceway spalling.

Considered a symptom of fatigue failure rather than the root cause, WECs occur due to a number of factors, principal among which is RCF. Another cause of WECs is accelerated fatigue (premature spalling) resulting from higher stress and lower material strength. Bearings are subject to higher stresses as a result of transient high loadings or temperature effects. Examples include structural stress in the bulk material of the bearing caused by factors such as misalignment, and increased stress on the raceways due to tribological factors, including reduced lubricant film thicknesses.

The material strength of a bearing may also be negatively influenced by environmental factors that are suspected to generate hydrogen, including water contamination, corrosion and electrical stray currents. In these cases, even moderate loading conditions can lead to premature bearing failure.

White etching is a universal phenomenon, found in all types of industry, all types of bearing and all types of heat treatment. WECs occur at the end of the failure chain and are a natural consequence of crack networks in prematurely failed bearings. The key to identifying the root causes for bearing premature failure is not only to study WECs, but rather to discover the relevant weakening effects (the higher stresses or lower material strength mentioned above) that lead to accelerated fatigue.

Each premature bearing failure is unique and the reasons for premature spalling can be very different. A single root cause does not exist, and each failure case needs to be reviewed in the light of the corresponding operating conditions. Higher stress or lower material strength have been identified as principal weakening drivers and within these broad categories, general recommendations can help to identify a likely cause of premature failure.

However, for a truly accurate diagnosis, this general advice should be underpinned either with a deep knowledge of the mechanisms of bearing failure, or by seeking help from an acknowledged expert with the tools and research background to support their diagnostic capabilities.

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