Why do pump bearings
What do we mean by good bearing life? Most of us
change the bearings every time we disassemble the equipment to
replace the mechanical seal or the packing sleeve. Is this really a
sensible thing to do? If you think about it for a minute there is
nothing in a bearing to wear out; there are no sacrificial
Bearing life is determined by the number of hours
it will take for the metal to "fatigue" and that is a function of the
load on the bearing, the number of rotations, and the amount of
lubrication that the bearing receives. Pump companies predict bearing
life measured in years. As an example, the Duriron pump company
anticipates a three hundred year life for the radial bearing on their
3 x 2 x 10 pump ( 75 mm. x 50 mm. x 250 mm.) when pumping a liquid
with a specific gravity of "one" (fresh water).
the term "fatigue" we'll conduct an experiment:
- Straighten out a standard paper
- Flex it a little and then let it go. You'll
notice that it returns to the straightened position. You could
repeat this cycle many times (many years actually) without
breaking (fatiguing) the metal because you are cycling the metal
in its "elastic range" ( it has a memory similar to piece of
- Now we will bend (stress) the paper clip a lot
further and you will note that it did not return to the
straightened position. This time you stressed the metal in its'
"plastic range" where it did not have a memory.
- If you bend the metal back and forth in this
plastic range it will crack and break in less than twenty cycles.
The metal fatigued more quickly because it "work hardened" and
became brittle. The more you stress the metal by flexing it the
quicker it will work harden and break.
- You have just demonstrated that fatigue is a
function of stress and cycles.
- When the bearing is pressed on a rotating
shaft the load passes from the inner race( inside ring) through
the balls to the bearing outer race (the outside
- Each ball carries a portion of the stress as
the balls roll under the load. It's this stress that will
eventually fatigue the metal parts.
When a pump is operating at its best efficiency
point (bep) the only loads the bearing have to carry are:
- The weight of the rotating
- The stress caused by the interference fit on
- Any bearing preload specified by the
fact is that most bearings become overloaded because of:
- The wrong interference fit between the bearing
and the shaft ( the shaft was out of tolerance).
- Misalignment between the pump and its'
- Bent shafts.
- An unbalanced rotating element.
- Pushing the bearing too far up a tapered
- Operating the pump off of its best efficiency
- Shaft radial thermal expansion.
- A futile attempt to cool the bearings by
cooling the bearing housing with a water hose or some other
similar system. Cooling the outside diameter of a bearing causes
it to shrink, increasing the interference and causing additional
- Water hammer.
- Axial thrust.
- The bearing housing is sometimes out of
- Pulley driven designs.
- Vibration of almost any form.
- The impeller is located too far away from the
bearing. This is a common problem in many mixer/ agitator
- A bad bearing was supplied. This is becoming
more of a problem with the increase in counterfeit parts we're
finding in industry.
This overloading will cause heat to be generated,
and heat is another common cause of premature bearing
will cause the lubricant to:
- Decrease in viscosity, causing more heat as it
loses its ability to support the load.
- Form a "varnish" residue and then "coke" at
the elevated temperature. This "coking" will destroy the ability
of the grease or oil to lubricate the bearing. It will also
introduce solid particles into the lubricant.
In addition to the heat generated by overloading
we get additional heat from:
- The oil level is too high or too low. Too
often pumps are aligned but not leveled.
- The bearing was over greased.
- The shaft material is conducting heat from the
pumpage back to the bearing housing. This is a common problem in
heat transfer oil pumps, or any time a metal bellows seal is used
in an application and the stuffing box cooling jacket is shut off
- A loss of barrier fluid between double seals
causing a temperature rise that conducts heat back to the
- A failed cooling jacket in the bearing housing
around the stuffing box or built into the seal gland.
- Grease or lip seal contact on the shaft, right
next to the bearings.
- A failed cooling "quench" in an A.P.I. type
A leading bearing manufacturer states that the
life of bearing oil is directly related to heat. Non contaminated oil
cannot wear out and has a useful life of about thirty years at thirty
degrees centigrade (86 F.). They further state that the life of the
bearing oil is cut in half for each ten degree centigrade rise (18
F.) in temperature of the oil.
This means that oil temperature regulation is
critical in any attempt to increase the useful life of anti friction
major cause of premature bearing failure is the contamination of the
bearing lubrication by moisture and solids. As little as 0.002% water
in the lubricant can reduce bearing life by 48%. Six percent water
can reduce bearing life by 83% percent.
There are several methods used by pump companies
to keep this water and moisture out of the bearing
- A flinger ring to deflect packing or seal
leakage away from the bearings. A silly arrangement at
- Keeping the bearing oil hot to prevent the
forming of condensation inside the bearing case. A ridiculous
system when you consider that bearing life is directly related to
- The use of "so called" sealed bearings. You
can call them any thing you want, but the seals will not seal
anything, especially moisture or water.
- Grease or lip seals that have a useful life of
about two thousand hours (84 days at 24 hours per day) and will
cut the expensive shaft directly under the seal lip. Double lip
seals will cut the shaft in two places.
- Labyrinth seals that are superior to lip seals
but not totally effective because you are still trying to seal
with non contacting surfaces that are useless
moisture comes from multiple sources:
- Packing leakage flows back to the bearing
- Because of packing leakage a water hose is
used to wash down the area. This washing splashes on to the pump
bearing case also.
- Aspiration, moist air enters through the lip
or labyrinth seals when the bearing case cools down.
- A seal quench gland that often has steam,
condensate or cooling water leaking out and directed at the radial
The moisture causes several problems:
- Pitting and corrosion of the bearing races and
rolling elements that will increase the fatigue of the metal
- Free atomic hydrogen, in the water, appears to
cause hydrogen embrittlement of the bearing metal accelerating the
- A water and oil emulsion does not provide a
good lubricating film.
We get solids
into the lubricant from several sources:
- Metal seal cage wear. This is the part the
separates the balls that are held between the bearing races. It is
often manufactured from brass or a non metallic
- Abrasive particles leach out of the bearing
- Often solid particles were already
contaminating the grease or oil we are using for the
- Solids were introduced into the system during
the assembly process because of a lack of cleanliness.
- Airborne particles penetrate the bearing
- Particles worn off of the grease or lip seals
penetrate into the bearings.
to keep solids and moisture out of the bearing housing.
- Seal the inside of the bearing housing with
epoxy or some other suitable material to stop rusting and to
prevent solids from leaching out of the metal case. If you do this
be careful about using some of the new high detergent lubricants.
They might be powerful enough to remove this protective
- Replace the grease or labyrinth seals with
positive face seals. In the future, you are going to need these
seals to prevent hydrocarbon fugitive emissions.
- Install an expansion chamber outside of the
bearing casing to accept the air (approximately 16 oz. or 475 ml.
in a typical process pump) that expands as the bearing casing
increases in temperature. Without this expansion chamber
approximately one atmosphere of pressure will build up in the
bearing housing. This is not a problem for a mechanical seal, but
during long periods of shut down the pressure could be
- Clean the oil in the bearing casing by
installing a simple oil circulating and filtering system, or
change the oil frequently.
do you go from anti-friction ball and roller bearings to hydrodynamic
(sleeve) bearings in a centrifugal pump?
- Any time the DN number exceeds 300,000
(Bearing bore (mm.) times rpm)
- If the standard bearings fail to meet an
L10 life of 25.000 hours in continuous operation or
16,000 hours at maximum axial and radial load and rated
- If the product of the pump horsepower and
speed in rpm is 2.7 million or greater.
The past several years have seen a decrease in the
quality of the bearings available for rotating equipment. We find
pre-packed bearing being shipped with too much, or no grease at all.
Stabilization temperatures have changed and overall quality has
diminished. If you adopt the above suggestions you should not have to
be changing your bearings as frequently as you do now.
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