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Developing a Motor Bearing Regreasing Strategy
Wayne Ferguson, Reliability Engineer, Eli Lilly
and Company
At my plant site this was a real problem. Most bearings that
were being greased were based on guess work. Many were not greased
at all. I decided to form a team consisting of our lubricator,
his supervisor, a vibration technician and myself.
We started first with our most critical machines. We then collected
the bearing sizes and used the formula G=[(0.114)X(D)X(B)] to
determine the relube amount needed in ounces of grease. We then
determined the speed of the bearing and the bearing type (ball,
roller, etc) and used an SKF bearing chart to determine the interval
for the relube.
We now were able to determine how much grease we needed and how
often. We set up PMs with the proper frequency and listed the
amount to add for each machine (In many of the bearings we were
lubricating, we found that we had been adding 4 to 10 times too
much grease). In order for this to work the lubricator needed
to know how much grease his grease gun was putting out. We measured
the amount in two ways,first, we determined the weight per stroke
(His grease gun puts out .062 oz per stroke).
Secondly, we made up a grease block to determine how many strokes
per cubic inch. Many times the motor manufacturer will give you
a spec such as, add 2 cubic inches of grease every 6 months. Now
we know what that is in strokes of our lubricators grease gun
(11 pumps per cubic inch).
We noted in the PM that only the lubricators grease gun may be
used (All of our grease guns must be checked in this way). The
grease gun is marked with this information. After setting up the
PMs our vibration analyst closely monitored the bearings for problems.
If he noticed that a bearing was consistently needing grease we
updated the PM amount.
The formula is very accurate,we only had to update 3 PMs and
then only by 1 extra pump. We did this in 1997 on more than a
one hundred pieces of equipment, and as far as we have been able
to determine we have not had another lubricant related failure.
We also noted that this is for normal operations. Vertically
mounted motors or motors subjected to dirty environments or other
adverse conditions would have to be determined separately. Note:
We understand that the proper way to grease a bearing would be
to shutdown the machine, remove the bearing, clean and repack.
However, this is not possible in our business, so we must lube
with the unit running.
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