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