Page 180 - Know and Understand Centrifugal Pumps

Bearings

The foam and froth in the bearing oil, increases the volume and
artificially raises the oil level, which leaks through the seals. When
enough has leaked to stop foaming, the air bubbles leave the oil
resulting in inadequate oil levels. Too much friction heat and failure is
the result.

Bearing maintenance

Cleaning bearings and relubrication
A lubricant, either oil or grease, should always be present in the
bearings in small quantities. If not, the life of the bearing will be
compromised by damage to the bearing surfaces. This damage can be
avoided with proper cleaning and relubrication. The intervals for
cleaning and re-lubing the bearings are generally long periods.
It’s easy to see when a bearing needs oil. Check the oil site level
indicator. It’s different with grease. It’s impossible to determine when a
bearing needs more grease. This is because the grease in the bearing
does not suddenly lose its lubricating properties. These properties are
lost gradually over time. Previous operating experience (history) is a
good guide to determine when to add more grease. The intervals
depend on the grease properties, the size and design of the bearing, the
operating speed, the temperature, and humidity.
In important process pumps, the grease in a bearing should be changed
every 12 to 18 months. This will assure a reliable pump operation and
service because time alone causes certain deterioration in the
lubricating ability of grease.

The intervals for cleaning and re-lubricating bearings should be more
frequent if water or moisture is able to enter into the bearing chamber.
Bearings can become contaminated from rain, hose-downs, pumps
located under dripping equipment, dew, fog and condensation.
Entrance points could be through inadequate, worn or failed shaft seals,
the breather cap, and the lube oil fill port. Be sure the new grease or oil
is not contaminated.
Grease is normally injected through a port called a zerk (or zirk) fitting,
or by removing the bearing end cover or housing cap. When injecting
grease mechanically or hydraulically, remember to open the drain or
expel port. The new grease will expel the old grease under pressure.
Also remember to close the drain port afterward. The amount of grease
to be added is a function of the housing size and design and the size of
the bearing. The grease should completely impregnate the bearing and
fill the housing about 25% full. Too much grease leads to overheating.

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