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Rolling Contact Fatigue of Ball Bearings

Background: A fire occurred in a pump unit at a lubrication factory. The pump failed due to increased vibration that was traced back to the bearings. The client wanted to determine the cause of the increased vibration within the bearings.

Figure 1: Optical Micrograph (Left) and SEM Micrograph (Right) of Signs of RCF on the Bearing Balls

Summary: The failure of the ball bearings was due to the onset of rolling contact fatigue within the bearings. Figure 1 shows an optical micrograph of a white etching region (WEL) on the left and an SEM micrograph of spalling (the removal of material) on the bearing surface on the right. The WEL is one of the first and most characteristic signs of the onset of RCF. Though it is not fully understood what microstructural changes cause the layer to have the distinct light etching appearance seen in the figure, it is recognized as being a telling sign of RCF. The spalling seen in the SEM micrograph is the likely cause of the increased vibration in the bearings. Further analysis revealed that the bearing material had significant inclusions which likely served as stress risers and increased the rate at which RCF progressed in the bearings.
Spalling in RCF is caused by the initiation of fatigue cracks due to the RCF that can either be initiated at the surface or have sub-surface initiation. If the cracks are initiated at the surface, then pits begin to form in the material, and as these pits grow they become spalled regions. If the cracks initiate below the surface, then they travel through the material until they reach the surface and then larger amounts of material are removed, resulting in spalling.

Take Away: Increased vibration in bearings can lead to catastrophic failure of the equipment. A metallurgical examination of the bearings can reveal the damage mechanisms leading to the failure, and provide insight into how to successfully correct the problem.

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