A Root Cause Failure Analysis Team was
formed to determine the root cause(s) of the pinion shaft failures.
Several problems were identified that could be attributed to shorter
than design pinion life. Some of these were:
- Too high of a rod charge level.
- Improper pressure switches
that allowed the mill to be started with inadequate oil pressure.
- An inadequate startup procedure. The
original procedure called for the mill to come to a complete stop
before restarting it. The revised procedure did not include this
statement. Operators had been trained using the revised procedure.
- The inert gas pressure in the clutch
was too high.
- Improper clutch engagement time. The
clutch manufacturer said the clutch should smoke 5-7 seconds to
fully engage. Faster times will overload the pinion shaft and
motor. The clutch is designed to be the wear part of the system,
not the pinion shaft.
During the root cause investigation,
one of the verification steps was to determine the clutch engagement
time. The Motor Analysis Team (MAT) proposed the idea of monitoring
the current on the motor to attempt to determine the clutch engagement
time. The MAT used the current analysis in-rush test function on the
PdMA analyzer to capture the motor current data. From this data, we
were able to calculate the engagement time for the clutch. The initial
test indicated a very rapid clutch engagement time, approximately 2.25
seconds, similar to an across the line start for the motor and clutch
(Figure 3).
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