Failure of threaded joints: Results from a survey on italian companies

Threaded joints play an important role in the definition of the reliability and safety performance of any industrial product: from earthmoving machinery to orthopaedic prostheses, from aircraft to machine tools. This joining methodology is, nonetheless, highly sensitive to several design, manufacturing and control parameters: if only one link of this chain fails, the entire system may come to unexpected failure, sometimes with severe consequences. On the one hand, the literature encompasses several review studies that analyse the main root causes of failure: These studies are necessarily focused on failures documented in the scientific literature. On the other hand, it is rather difficult to find adequately organized field-data that can help the scientific community to orient the research towards the most impelling industrial needs. Moreover, the few data available is almost entirely gathered from the automotive and aerospace sectors and dating back to the nineties of the last century and the early two-Thousands. This research is based on an anonymous survey that has been submitted to Italian industries or to Italian facilities of multinational corporations, operating in several fields (automotive, earthmoving, machine tools, oil & gas and many others). The survey aimed at assessing the most frequent root causes of failure, whether the failure of the fastening system involved subsequent failure or not, whether the failures were mainly recorded upon assembly or quality control or during service life of the goods, the materials of the fasteners and/or components more prone to failure. The results have been analysed by means of statistical methods to clarify the effectiveness of each of the analysed variables. Among the most prominent findings: (i) the relative abundance of vibrational induced failures (loosening); (ii) the high frequency of assembly-related issues, mostly due to the lack of knowledge of the actual friction coefficients upon assembly; (iii) the non-negligible amount of failures related to an inexact knowledge of the actual operational loads.