Fatigue initiated overload failure of a 4130 alloy steel pump shaft

Abstract

This paper describes the failure investigation of a vertically mounted tank cleaning pump system from an off-shore oil platform. The incident involved the catastrophic failure of an intermediate shaft, connecting pump shaft and surrounding components. Corrosion to the shaft ball bearings on the pump shaft generated excessive vibrations which transferred via a universal joint to the connecting shaft which resulted in a secondary couple. The secondary couple caused rotating unidirectional bending in the intermediate shaft at a constrained point; the located bearing. A machined fillet adjacent to the located bearing created a stress concentration point where fatigue cracks initiated due to the loading. The fatigue propagated to only a relatively shallow depth around the outer surface of the shaft through cyclic vibrations and bending moment. Propagation of the crack effectively created a large notch within the fillet allowing sudden overload failure of the intermediate shaft through the remaining section, most likely at start-up. The material properties of both shafts were shown to be within specification and material defects were found. It was concluded that root cause of failure was due to bearing damage imparting vibration and bending moments on the intermediate shaft. These elements initiated and propagated the fracture and ultimate failure leading to the secondary failures of the pump shaft and bearing houses.