Fatigue Cracking of a 3-MW Electric Motor Shaft

After approximately fifteen months of service, cracking was detected in the journal section of a 3-MW electric motor shaft used to drive a reciprocating gas compressor. A detailed failure analysis revealed sulfide inclusion and quench crack-initiated fatigue cracks. Two crack orientations were observed, axial and angular. The axial cracks are believed to be quench cracks which initiated along sulfide stringers; the angular cracks were found to be high-cycle fatigue cracks emanating from the axial cracks. Recommendations included the use of a reduced sulfur content shaft steel (0.010 wt.% max) and a pre-service non-destructive inspection to detect the presence of quench cracks or other surface imperfections.