Fatigue failure of the bolts connecting a Francis turbine with the shaft

This paper describes the failure analysis of bolts used to connect a turbine to the shaft of a hydroelectric power generator. Three of ten bolts were found broken during a disassembly process to replace the turbine. The methodology included visual inspection of the thread to detect eventual stress concentrations. Using loads measured on the turbine shaft for different power levels and the actual power generation history for a period of one year, a stress history on the bolts was obtained. With this stress history, damage was calculated for three preload values. The results showed some scratches on the root of the thread produced during the machining process and by subsequent corrosion during operation, which likely significantly affected the fatigue life. A preload that was smaller than the recommended one was found in the maintenance report, which put the bolts at risk of loosening. A fatigue life of only 16.4 years was estimated if loosening occurs on the bolts. Better preload control during the assembly process and a change in the bolt material to stainless steel are the primary recommendations from this analysis.