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Developing a Motor Bearing Regreasing Strategy
Fred Spaid, Sr. Product Service Engineer, Emerson
Motor Company
Regreasing shielded and sealed bearings is a waste of time and
grease. You should never regrease a sealed bearing and when trying
to regrease a shielded bearing, you could do much more harm than
good. You could very easily blow the shield into the bearing from
the psi generated by the grease gun. The amount of oil that may
migrate from the outside of the bearing, past the shield and eventually
work its way to the rolling elements is pretty small.
You cannot treat motors as a collective group. You need to treat
each motor as an individual. No two motors operate exactly the
same. If you treat them all the same, you will generally do one
of two things, over grease them or under grease them.
You need to take at least these seven, and possibly more depending
on the situation, concerns into account when you set up a motor
relubricating plan. I am assuming compatible grease is being used.
1. Operating temperature. How hot are the bearings running? What
is the ambient temperature within three to five feet of the motor?
2. Motor speed. Is this a two pole motor or a ten pole motor?
Bearings operating at faster speeds tend to work harder than those
working at slower speeds.
3. Bearing size and type. Again, is it a smaller bearing operating
at high speed or a large bearing operating at slow speed? Is it
a ball bearing or a roller bearing? Roller bearings require more
frequent lubrication than ball bearings.
4. Duty cycle. Is this motor operating 24/7 or once a week?
5. Vibration levels. Hopefully, there has been a base line vibration
analysis taken on each motor. Is there a history of the motors
vibration available to the maintenance people? Is it trended?
Is there a high or low ambient vibration at the motor's location?
6. Environmental conditions / contaminants. Where is the motor
operating? Is it in a clean room, coalmine, cement plant, boiler
room, irrigation ditch, etc.?
7. Critical nature of the application. If this motor fails, will
it bring the plant down or will it just be a minor inconvenience?
All of the above need to be considered when setting up a relubricating
plan. It is not easy finding the correct time to regrease a motor,
but it can be done. By greasing the motor properly, when it needs
it, it will last a very long time. If you regrease it when you
want to, you many end up shorting its life.
Now that you have determined when the motor needs to be relubricated,
giving it the correct amount of grease it just as important. Motor
bearings require different amounts of grease based on their size.
This may range from tenths of an ounce to four or five ounces.
The worst thing you can do is to over grease the bearing. Excess
grease generally ends up in the motor winding. Then you have two
problems.
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