>> THE TURNING OF THE SCREW - How a costly bearing
failure got the simple spanner treatment.
Two
turns of a bolt was all that was eventually necessary to solve a
costly fan bearing problem at a major UK tobacco factory.
Condition
Monitoring experts Monition, have shown that small mechanical problems,
can be both expensive and difficult to detect, after a fan bearing
which repeatedly failed and defied analysis, but was found by Monition
to be a simple case of balancing the anti-vibration mounts which
hold the fan bearing casing.
Monition’s
Senior Consulting Engineer, Bob Patterson explained that it wasn’t
quite as simple as it sounds however. “It wasn’t an
obvious cause of failure, and because the main symptom was excessive
heat build-up the natural assumption is that the cause is an internal
mechanical or bearing problem”.
The
faulty fan, part of a suite of three identical units, is used in
the drying process at Imperial Tobacco’s Nottingham plant.
The other two fan bearings were operating normally and recording
an operating temperature of around 35‘C. The faulty unit would
reach 102‘C and would regularly fail – costing in the
region of £500 for new bearings plus installation and productivity
losses.
Vibration
readings for all three bearings revealed similar vibration amplitudes
and frequencies and therefore suggested that it was unlikely to
be a poor fitting or lubrication fault. However, to try to manage
the problem and help to extend the bearing life, improved, pressure
fed, high temperature lubricant was installed. Investigations by
bearing and fan engineers had also been undertaken, still no solution
could be found.
Then
Monition’s engineers carried out a phase analysis on the fan
units to map out patterns of energy. Results compared to normal
fan conditions showed high level frequency being created directly
under the bearing frame, which was the cause of the excessive heat
build up and its transfer to the bearing.
The
cause was then easy to identify – the casing was slightly
twisted and the anti-vibration mounts were in effect unbalanced,
causing the very high vibration and heat build-up.
Nothing
more technical than two 19mm spanners and an adjustment of the bolts
by backing the two centre mounts off by two turns was required.
Eventually the operating temperature of the unit dropped to 28‘C
and the problem has now gone away.
Colin
Little, Imperial Tobacco’s Primary Manager told us that finding
the root cause of the failure has removed an expensive problem.
“It goes to show that diagnosis may be complicated but the
remedy is often simple”.
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