Subject: Ball bearing lubrication in centrifugal pumps 14-1.
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's usually measured in years. This fatigue or L10 life is determined from:
Duriron (Flowserve) 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's causing the premature failure? Is it a manufacturing, installation, maintenance, or operation problem?
It turns out that bearings fail for two main reasons:
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:
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.
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:
Oil is easy to install and change.
Oil mist is the preferred method if you can solve the fugitive emissions problem.
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:
The most common synthetic oils are:
Lubricants are supplied with various additives to increase their performance:
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.
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