April - Library

1
What is Predictive Maintenance
Vol 20 No 2
FIGURE 3 Motor Limestone Ball Mill FIGURE 4 Inner Race Damage
Oil analysis identified a sleeve bearing problem on one of our forced draft fan motors. These are 3000 HP, oiled,
sleeve bearing, 6.9kv motors. Oil samples are taken quarterly based on previous track record for this
equipment. We had a spike to 35ppm tin in the sample of January 27, 2005 (Figure 5). A unit shut
down would be required to remove this equipment from service, so a bleed and feed was performed
until the oil tested clean (patch test). We increased sampling and patch tested for presence of tin. A
5 micron patch didn’t reveal much, but tin was very evident with a 1.2 micron patch. We performed a
couple more bleed and feeds before a scheduled shut down on February 19, 2005. Inspection of the
removed bearing showed one area in the leading edge of the lower half with loose and missing
babbitt (Figure 6). A repaired bearing was installed and the motor returned to normal service.
A patch test was run after
of couple weeks run time
with no tin seen. Quarterly
sampling was resumed with
tin slightly trended up to
9ppm by the September 05
sample, by January 03, 2006
the tin had jumped to 67ppm
(Figure 5). These results
came in a couple days before
an un-scheduled shut down
for a tube leak repair, so the
bearing was replaced during
that time.
FIGURE 5 Tin Levels From Oil Sample
This motor was removed for a 5 year rebuild in March 2006. During the inspection process the recently
replaced inboard bearing was found to already have similar damage to it after less than three months run
time. No external causes such as, water for hydrogen embrittlement, improper clearances, damaged shaft
journals, or any other problems, could be found during the motor rebuild to have caused these bearing failures.
FIGURE 6 Loose and Missing Babbitt
During the run in of the rebuilt motor we experienced
another bearing failure, this time on the outboard end. All
4 of the failed bearings were almost identical in nature and
all had been repaired at the same facility, this prompted
some in depth conversations with the repair facility and
their quality control measures.
On the subject of sleeve bearing oiled motors, we have
almost all of our oiled, 6.9kv motors equipped with sight
glass oilers which are a bulls eye sight glass with a 4 oz
make up oiler. This not only adds to the comfort zone over
the bulls eye sight glass, it maintains the correct oil level
as long as there is oil in the reservoir and reduces the