Reliability Engineering Snapshot TM

Illustrated Case Studies in the Maintenance Reliability Engineering World of Failure Analysis, Predictive Maintenance, and Non Destructive Evaluation



Lubrication - Case Study No. 2: Corrosive Oil

Close Up of Pitted Ball BearingOil doesn't have to look bad to actually be bad. Oil should be routinely monitored, or at the very least, replaced on a schedule. The picture on the left shows a ball bearing which was taken out of a process pump assembly.

This oil had actually turned corrosive as indicated by the numerous pits. The pits on the top are new. As time continued, these pits acted like miniature hydraulic pumps and the oil film was compressed into the pits at a much higher pressure; this increase in hydraulic pressure caused an implosion which subsequently caused the surrounding metal around the initial pit to break away. Some of these pits are clearly visible directly underneath the top cluster of pits.

Close Up of Galling on Ball BearingThis bearing was caught long before it self destructed. Routine vibration diagnostics had indicated that the bearings were starting to deteriorate long before the pump assembly made noise or even heated up. As the oil deteriorated it lost its ability to develop an adequate oil film and support a load. The picture on the right shows what happened as the oil allowed this ball bearing to come into contact with the bearing raceway. There is metal to metal contact and the ball metal smeared in the direction of rotation (center - left to right). This type of phenomenon is known as galling, and can happen when another condition known as "boundary lubrication" occurs. What happens during boundary lubrication is that for one of many possible reasons, the balance between oil viscosity and the bearing loading has either changed or been disrupted, and the oil can no longer support the load.

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