Failure Investigation of Spline-Shaft of an Under Slung Crane

Investigation of the causes of premature failure of a spline-shaft used in the hoist gear box assembly of an under slung crane has been presented. The investigation consisted of visual inspection, chemical analysis, characterization of microstructures by optical and scanning electron microscopes, energy dispersive spectroscopy (EDS), and hardness measurement. Visual observation of the fracture surface of the shaft revealed multiple cracks all of which initiated at the sharp corners of the spline. Fractography of the shaft showed striations confirming fatigue. The chemical composition of the shaft was found to be close to EN-24 grade of hardened and tempered steel which is specified as per IS 5517. Microstructural examination showed branched-out cracks and few of them were associated with oxide layer or scale which was confirmed by EDS analysis. Since the shaft was not exposed to high temperature in service, formation of scale along the crack suggested that it was pre-existing in the component. Base matrix of the shaft revealed bainitic microstructure. Hardness values were found to be lower than that obtained for EN-24 grade in hardened and tempered condition. Analyses of the results suggest that the component suffered from fatigue under reversed torsional loading, which initiated at pre-existing cracks in the component.