Bucket chain excavator: Failure analysis and redesign of the counterweight boom supporting truss columns

The supporting truss of the counterweight boom is a vital part of the bucket chain excavator's (BCE) superstructure. The occurrence and propagation of cracks in the supporting truss columns' heads may lead to BCE collapse. The goals of the study presented in this paper were to: (a) diagnose the cause of cracks occurrence; (b) define the reconstruction design of the supporting truss columns and (c) validate the reconstructed structure by numerical-experimental analysis. In order to clarify the causes of cracks occurrence, experimental investigations were performed with the purpose of defining the chemical composition, tensile properties, impact toughness and macrohardness of the columns' material. Metallographic examinations were also conducted. Based on the results of finite elements analyses (FEA) and experimental analyses it can be concluded that cracks are caused by the 'design-in' defects. The redesign solution enabled repair and reconstruction in field conditions, without previously dismantling any substructure of the BCE superstructure. Thereby the time required for performing reconstruction is shortened and indirect costs due to the BCE downtime are considerably diminished. Both the experimental analysis of the stress state of the reconstructed columns in regular working conditions and the failure-free exploitation have confirmed the validity of the reconstruction design, while the BCE excavated approximately 8.5 x 10(6) t of coal and 1.8 x 10(6) m(3) of overburden after the reconstruction