This paper describes the failure investigation of a shaft that is part of a hammer drill assembly. The fracture surface was completely damaged during and subsequent to the failure process thus obliterating fractographic features. However, careful consideration of the application and operation of the assembly, distribution and nature of stresses on the shaft, design aspects, manufacturing process characteristics, metallography, visual observations and SEM fractography revealed many clues that pointed to the root cause of failure of the shaft. The basic failure mode was identified as a combination of rotating bending and torsional fatigue failure originating along the inside diameter of the shaft. The recommendations to prevent such failures in future were manyfold including improving surface finish on the bore diameter, reducing dry sliding friction, decreasing the overall level of dynamic loads by appropriate design changes and adding a surface strengthening heat treatment.