FESI Seminar Day at ESR Technology (September 2014)
The second FESI seminar on rolling bearing fatigue was held in September 2014 following on from the first one in November 2011. Presentations were made to a capacity audience that consisted of delegates from academia, wind turbine, aerospace and bearing manufacturing industries (amongst others). The focus of this year’s seminar was to debate the relevance of current methods to quantify the cleanliness of bearing steels towards describing the fatigue performance of rolling bearings.
An alternative risk-based approach using the Generalised Pareto Distribution was described by Leicester University based upon work performed in the early 2000s. Essentially, it may not be the largest inclusion present in the steel that initiates bearing failure, but the more numerous inclusions whose size is smaller and who are more likely to be present in the stressed zone of the Hertzian contact.
The inadequacy of current methods such as ASTM E45 and ISO 2283 was highlighted by SKF who prefer to use push-pull fatigue testing to search out the largest damaging inclusions in a bearing steel batch.
Due to the increasing interest in the fatigue performance of induction and case hardened steels, ESR Technology made a presentation on their metallurgy and the nature of problems and faults that arise in these surface hardened steels. A presentation was made by Leicester University on a potential method to assess the fatigue behaviour of a large ball bearing using GPD and a fracture mechanics limit stress equation developed by ESR Technology. The method allows the effects of different steel cleanliness and maximum inclusion sizes to be assessed, as well as the likely improvement by introducing residual compressive stress by means of surface hardening.
The method to develop the limit stress equation using a fracture mechanics approach was described by ESR Technology, based upon Professor Murakami’s work. Newcastle University described the large bearing test rig that has been constructed to evaluate the fatigue performance of bearings used in wind turbine gearboxes. They have a planned programme of work for the rig over the next few years.
To close the seminar and initiate discussion, Vestas presented an end user’s perspective on the issues arising in large rolling element bearings used in wind turbine applications. A lively discussion ensued that produced a list of topics for consideration at the next seminar. In the meantime, some of the topics may be covered in a seminar entitled “Fatigue Crack Initiation in Rolling Bearing Steels” planned for Spring 2015.