Failure of an Induction Hardened Shaft due to Rotating Bending Fatigue

We investigated the failure of an induction hardened shaft that transmits torque from a drive motor to a pulley to raise and lower a platform lift in a distribution warehouse. After securing a load, the lift failed and fell several stories to the ground. The drive assembly comprises a frictionally engaged clamp securing a belt-driving pulley to a drive shaft that is directly coupled to a motor on one end and supported by self-aligning pillow block bearings on either side of the pulley. Fractography indicated the drive shaft failed by fatigue due to reversed rotating-bending loads during normal operation of the lift. The shaft fracture initiated at the edge of contact between the pulley clamping mechanism and the shaft; inspection of the initiation area revealed damage consistent with fretting fatigue and galling due to micro-movements of the clamp on the shaft. Contributing factors of the failure were loosening of the pulley clamp fasteners that led to fretting fatigue at the pulley clamp edge of contact and improper assembly of one of the bearings that led to unanticipated bending stress in the shaft during operation.