Ball Bearing Lubrication in Centrifugal Pumps
Ball Bearing Lubrication in Centrifugal Pumps
Ball Bearing Lubrication in Centrifugal Pumps
The manufacturer using the bearing in his equipment, not the ball bearing manufacturer,
determines the anticipated life of a ball bearing. This life, once determined, is called the
L10 life of the bearing and it is based on the premise that 90% of the bearings will last a
certain amount of revolutions before they experience metal fatigue.
Fatigue is a weakening and eventual breaking of metals due to a prolonged strain. Since
the manufacturer of the equipment that uses the bearing is the only one that know the
operating conditions, he sets the L10 life. It is usually measured in years. This fatigue or
L10 life is determined from:
Duriron pump company literature states that the radial bearing in their 2 x 3 x 10 Mark
two, group two pump has a L10 life of 300 years. In other words 90% of those bearings
would be expected to run 300 years before they would experience a fatigue failure.
Since the pump end user is not experiencing anything like that type of life, what is
causing the premature failure? Is it a manufacturing, installation, maintenance or
operation problem?
As little as 0.002% water in the bearing oil will reduce bearing life 48%. The water enters
from packing leakage, wash down hoses and aspiration caused by the temperature
cooling down in the bearing casing after shutdown, and moisture laden air entering the
bearing case. A 6% water content in the oil will reduce bearing life by as much as 83%.
The water or moisture contamination comes from three main sources:
Packing leakage.
Water hoses used to wash down the base plate area because of packing leakage.
Aspiration or moisture in the air entering the bearing case especially when the
pump is stopped.
In paper 13-9 we talked about the seals you can use to keep this moisture out of your
bearing case. In this paper we will investigate the second reason bearings fail.
Excessive heat!
A couple of paragraphs above I said that over lubrication would cause high heat. What is
the problem with over lubrication? If a little lubrication were good, wouldn't a lot be
better? Not really! Think about it this way. Picture yourself on a hot day walking along
the beach. You go into the water up to your ankles, and as you walk along rapidly you
feel cool and refreshed. Now walk rapidly in water up to your waist and you see the
problem. It takes a lot of energy to get through the same temperature water and this
would make you hot and fatigued instead of cool and refreshed
It's the same thing with lubrication. Too high a lubrication level and the bearing will
consume energy as it plows through the lubricant. This energy will show up as heat added
to the lubricant causing it to first lose its viscosity and then the lubricant will begin to
form varnish and coke as it gets hotter. Varnish and coke are another name for solids.
The problem with grease and oil lubricants is their low specific heat and their poor
conductivity. Some of the synthetics are better, but they have a temperature limit that is
still too low for many pumping applications. It is for this same reason that we do not
recommend putting any type oil between dual seals if we can avoid it.
The SKF bearing company claims that uncontaminated grease and oil has a useful life of
thirty years at 30°C (86°F) They further state that the life of grease and oil is cut in half
for each 10°C (18°F) rise in temperature. That means that at 100°C (212°F) oil and
grease have a useful life of only 90 days. Here are your lubrication options:
Grease packed
Grease is hard to change because the usual method is to pump grease into a grease
fitting and let the new grease push out the old grease. This method guarantees the
bearing will be over lubricated.
The only proper way to grease a bearing is to hand pack it full, but not the cavity
where it is located. As the bearing heats up some of the grease will leak into the
cavity reducing the amount of lubrication in the bearing.
Oil mist is the preferred method if you can solve the fugitive emissions problem.
Oil mist can provide a positive pressure inside the bearing to keep out
contaminants.
It takes 5000 to 6000 psi (340 to 405 bar) to mist 30-weight oil and that pressure
is not available in your pump. Mixing the oil with air presents a problem because
of venting hydrocarbons to the atmosphere.
If you find the bearing lubricant is getting too hot, most pumps have a facility for cooling
the oil in the bearing case. Never attempt to cool a bearing by cooling the outer case.
Steel will expand or contract at the rate of about 0.001 of an inch, per inch, per 100
degree Fahrenheit. (0.001 mm/mm/ 50°C).
In other words if you cool the bearing case it will contract or shrink and increase the load
on the bearing. The rule is "cool the oil, never the bearing".
Lubricants are made from various oils and additives. The three most popular oils are:
Be sure the bearing inner race has an interference fit on the shaft, with no knurled
surfaces, shims, or polymers used to build the shaft up to the proper tolerance. We need
this fit to conduct heat away from the bearing and into the shaft.
Some metal bellows salesman tell their customers that their bellows seal does not need
cooling and recommend that the customer shut off the stuffing box cooling jacket to save
either water or steam.
They either forget, or do not know that this stuffing box cooling is also cooling the shaft,
allowing it to conduct heat away from the bearings.