Analysis of the cause of the girth gear tooth fracture occurrence at the bucket wheel excavator

Abstract

Premature damages and fractures of components and structures of bucket-wheel excavators at open pit mine often occur during exploitation, which is caused by inadequate designing, insufficient knowledge that refers to the properties of materials and welded joints and flaws in technology of component production. Apart from direct damage caused by damages and fractures, disorder in the production of electricity could cause significant indirect damage. Bucket-wheel excavator SRs 2000x32/5.0, manufactured by company TAKRAF from Germany, was employed on the excavation of barren soil for 5.000 h (a few weeks more than a year after assembly) when the fracture of the tooth of the girth gear which enables circular motion of the upper structure of the bucket-wheel excavator occurred, and during that period it excavated 11.000.000 t of barren soil at the open pit mine ’Kostolac’, Serbia. The gear was, according to the documentation of the manufacturer, made of cast steel GS 40 MnCrSi3 V. This paper contains the calculation of the number of cycles of stress variations for one tooth, as well as parameters of stress mechanics – critical stress intensity factor and critical crack length. It was established that the fracture of the tooth occurred due to an initial crack existing in its base, which originated during gear manufacturing, i.e. during so-called "manufacturing in defect".