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:
- The bearing material
- The load on the bearing. (stress)
- The number of load cycles the bearing material is exposed to (strain).
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:
- Contamination of the bearing lubricant by water or moisture.
- High heat often caused by too much lubrication.
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.
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 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 is easy to install and change.
- Be sure you have an oil level indicator on your pump.
- Be sure the pump is level. Many pumps have been aligned without checking to see if they were level.
- The oil level should be half way through the bottom ball when the pump is at rest.
- Unfortunately you cannot use oil lubrication on a vertical installation.
- Some mechanical applications use bearings of different diameters. This makes it impossible to maintain a correct oil level. Vertical applications have the same problem.
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:
- Mineral oils, pure and refined.
- Synthetic oils for higher temperatures.
- Animal and vegetable oils that are not normally used for bearing lubrication because of the risk of acid formation after a short operating period.
The most common synthetic oils are:
- Diesters that are usable to 120°C. (250°F)
- Silicone oils that are usable to 200°C.(395°F)
- Fluorinated oils have good oxidation stability but are so expensive most lubricating companies do not use them.
- Polyglycols are good for bearings over 90°C (195°F) Their oxidation stability is good and they have recorded service lives ten times longer than those of corresponding mineral oils. Their specific gravity is more than 1.0 so water floats on top of them.
- Synthetic hydrocarbons have the advantage of a viscosity that is reasonably independent of temperature. They can be used to 200°C ( 395°F)
Lubricants are supplied with various additives to increase their performance:
- Anti-oxidants improve the oxidation stability of the lubricant by 10 to 150 times, decreasing corrosion and preventing the oil from becoming more viscous.
- Corrosion protective additives do just they say.
- Anti-foaming additives prevent foaming that would reduce the load carrying capability of the lubricant. They cause the foam bubbles to burst when they hit the surface.
- Film stiffeners reduce wear through metallic contact. They form a surface layer with a surface tension greater than the lubricant.
- Additives with a polar effect cause the molecules to take up an orientation perpendicular to the metal surfaces. They reduce friction at temperatures up to a maximum of approximately 100°C (212°F)
- Organic zinc compounds have an anti-wear affect. They prevent direct contact between the ball and the races.
- Active EP additives form a chemical combination with the bearing metal reducing friction.
- Solid additives such as molybdenum disulfide improve the lubricating qualities. The particles are about 2 micron in size and adhere to the metal surfaces.
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.