BEARING LUBRICATION IN CENTRIFUGAL PUMPS. B021
Theoretically, ball bearings rotating at a constant speed require no lubrication at all. However, pump speed is never constant because the conditions that affect the speed, change constantly. Not only do we get changing load conditions but we also experience slight variations in voltage, and these variations cause the bearing balls to lead or lag the speed of the shaft because of inertia..
This leading and lagging causes a small amount of sliding and hence the need for lubrication.
Another limitation is that the hardest of bearing materials suffers small amounts of deformation any time it carries a load, upsetting perfect point contact and adding another sliding action to the problem.
Most bearings do not come close to obtaining their L10 life. It turns out that most bearings fail prematurely for two main reasons:
- Contamination of the bearing oil, usually from water or moisture.
- High heat usually as a result of over 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 sources:
- Packing leakage.
- Water hoses used to wash down the area because of packing leakage.
- Aspiration or moisture in the air entering the bearing case especially when the pump is stopped.
In another section of this CD I talk about the bearing seals you can use to keep this moisture out of your bearing case, so lets address the second reasons bearings fail……… high 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 water and this would make you hot and tired 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.
The problem with grease and oils 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 circulating 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. They further state that the life of grease and oil is cut in half for each 10°C rise in temperature. That means that at 100°C (212°F) oil and grease has 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
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. (350 to 400 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”.
The 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 period of time.
The most common synthetic oils are:
- Diesters that are usable from -60°C to 120°C.
- Silicone oils that are usable from -70°C to 200°C.
- 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. Their oxidation stability is good and they have recorded service lives ten times longer than those of corresponding mineral oils. Their gravity is more than one 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 from -70°C to 200°C.
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 as they say.
- Anti-foaming additives prevent foaming that would reduce the load carrying capability of the lubricant. They cause the bubbles to burst when they hit the surface of the lubricant.
- 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 disulphide improve the lubricating qualities. The particles are about 2 micron in size and adhere to the metal surfaces.
Be sure the bearing has an interference fit on the shaft with no knurled surfaces or polymers used to build up the shaft to the proper dimension. We need this interference fit to conduct heat away from the bearing and into the shaft.
Some metal bellows seal salesman tell their customers that the bellows seal doesn’t 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 and allowing it to remove heat from the bearings and the bearing lubrication.