Metallurgical failure analysis was requested on a mobile crane that had failed, separating the crane boom from the mobile carrier (motorized vehicle that transports the crane). An overall view of the crane accident scene is shown in Photograph 1 below.
Photograph 1 Crane failure accident scene.
Visual examination of the turntable bearing (which is called a slewing bearing) race revealed that the ball bearing loading port was aligned at the 5 o’clock position. The slewing bearing allows the crane to rotate a full 360 degrees. The bearing manufacturers instructions required that the ball bearing loading port, through which bearing balls are inserted during original assembly of the turntable (slewing) bearing, be oriented at the 3 o’clock or 9 o’clock position. This variance of the loading port orientation caused excessive stresses to be transmitted to the slewing bearing during crane usage. The excessive stress was in the area of the ball bearing loading port. An overall view of the failed slewing bearing, looking from the 12 o’clock position (behind operator cab) toward the 6 o’clock position (toward rear of carrier unit) is shown in Photograph 2 below.
Photograph 2 Failed slewing bearing.
Microscopic examination of the turntable (slewing) bearing fracture revealed several important features. The fracture surface between the bearing attachment bolt hole and the ball bearing loading port exhibited several beach marks. Beach marks are created by a pause or abrupt change in the stress being experienced by the component during cracking prior to complete separation of the component. Photograph 3 below shows the fracture surface with red arrows denoting various individual beach marks.
Photograph 3 Beach marks, denoted with read arrows, on slewing bearing fracture.
It is quite apparent that the turntable bearing fracture at this location exhibits a “thumbnail within a thumbnail”, indicating the fracture was caused by at least two distinct and separate overstress events. The inner thumbnail exhibits a discolored surface (possibly corrosion and/or lubricant staining) indicating that the inner thumbnail was formed at a much earlier time than the outer thumbnail. An imbedded particle also appears to be located at the origin at the inner thumbnail. Photograph 4 shows the “thumbnail within a thumbnail” and an arrow denotes the discolored area.
Photograph 4 Thumbnail within a thumbnail on slewing bearing fracture surface.
Scanning electron microscopy (SEM) revealed that crack arrest marks were in areas surrounded by dimple rupture. These crack arrest areas were found to have cleavage fracture on both sides of the arrest lines, indicating fracture occurred as a series of propagation events.
A metallographic examination was conducted around the ball bearing loading hole. An overall view of the metallographic mount is shown in Photograph 5. The metallographic mount shows two surface cracks (denoted with yellow arrows) and a subsurface crack associated with subsurface flaws (denoted with white arrows).
Photograph 5 Metallographic section from subject failed
slewing bearing. Yellow arrows denote two surface cracks.
White arrows denote two sub surface flaws.
Based on the results of the metallurgical failure analysis, it was opined that the crane accident resulted from failure of the subject turntable bearing. The turntable bearing failure was the result of improper placement of the ball bearing loading port, incorporated metallurgical flaws in the forging of the slewing ring, and long term improper and inadequate maintenance of the slewing bearing. The operator errored in reading the load chart which was also determined to be contributors to the subject crane accident.