BEARING MAINTENANCE
Proper bearing lubrication is a critical part of getting the designed life out of pump and motor
bearings. As strange as it may sound, more bearings have failed from over-lubrication than from
lack of lubrication. In fact, some bearings never require lubrication and may fail if they are greased.
Shielded and sealed bearings come factory-lubricated and have sufficient lubricant to last the life of
the bearing. Shielded bearings have a metal skirt that is attached to the outer race. It covers the
rollers but doesn't touch the inner race. Sealed bearings have a rubber skirt that does touch the inner
race. Bearings that do require periodic grease lubrication use a surprisingly small amount of grease
when compared to the bearing housing size. A properly greased bearing will have a bearing housing
that is never more than 25-30% full. The grease is responsible for lubricating and cooling the
bearing.
Grease that is inside the bearing will get hot as the bearing heats up. When the grease gets hot it
becomes more fluid and is thrown out of the bearing and onto the wall of the bearing housing,
where it cools. Grease that is outside the bearing is drawn into the race, where it again heats up and
is thrown out. This process keeps the bearing lubricated and removes heat from the bearing. If the
bearing housing is full of grease there is no way for the hot grease to get out of the bearing. The
lubricant inside the bearing overheats and breaks down. Bearings overheat and fail when this
happens.
As bearings heat up and cool down, the races and rollers expand and contract. Bearings are
temperature stabilized to about 2500
F. This means they will assume their original dimensions as
long as the temperature does not rise above 2500
F. This is the reason small electric motors should
not operate above 220o
F or 1050
C. Lubrication schedules for low-speed (under 2500 rpm) antifriction bearing applications are based on the operating temperature of the bearing. Always refer to
the vendor recommendations for the proper lubricant and lubrication frequency.