TROUBLESHOOTING BALL BEARINGS TBB001
Ball bearings are not designed to wear out. Unlike mechanical seals they have no wearable surfaces or components. The life of a bearing should be determined by fatigue and is referred to as the L10 life of a bearing
To understand the term “fatigue” we will conduct an experiment:
- Straighten out a standard paper clip.
- Flex it a little and then let it go. You will 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 rubber).
- 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 the material, the amount of stress on the material, and the number of 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 ring).
- Each ball carries a portion of the stress as the ball roll under the load. It is this stress that can eventually fatigue the metal parts.
When a pump is operating at its best efficiency point (BEP) the only loads the bearing has to carry are:
- The weight of the rotating assembly.
- The stress caused by the interference fit on the shaft.
- Any bearing pre load specified by the manufacturer.
Most bearings become overloaded because of:
- The wrong interference fit between the bearing and the shaft (the shaft was out of tolerance).
- The pump is not aligned to its driver.
- Bent shafts.
- An unbalanced rotating element.
- Pushing the bearing too far up a tapered sleeve.
- Operating the pump off of its best efficiency point (BEP.).
- 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 stress.
- Water hammer.
- Axial thrust.
- The bearing housing is sometimes out of round.
- 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 applications.
- A bad bearing was supplied. This is becoming more of a problem with the increase in counterfeit parts we are finding in industry.
This overloading will cause excessive heat to be generated and heat is another common cause of premature bearing failure. Heat 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. Bearings should be hand packed, a grease gun should not be used.
- The shaft material is conducting heat from the pumpage back to the bearing housing. This is a problem with 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 or inoperative.
- A loss of barrier or buffer fluid between dual seals, causing a temperature rise that conducts heat back to the bearings.
- 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. These seals can add as much as 38°C (100°F) to the shaft temperature
- A failed cooling quench in an API (American Petroleum Institute) seal gland.
A leading ball 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 bearings.
Probably the 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 housing:
- A flinger ring to deflect packing or seal leakage away from the bearings. A silly arrangement at best.
- 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 heat. Most commercially available pumps do not have enough oil capacity. Two liters should be a minimum, with the oil level half way through the bottom ball when the pump is at rest.
- The use of “so called” sealed bearings. You can call them any thing you want, but the seals are not very effective, especially against 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 less than desirable statically.
The moisture that is getting into the bearings comes from multiple sources:
- Packing leakage flows back to the bearing area.
- Because of packing leakage a water hose is used to wash down the area. This washing splashes on to the bearing case also.
- Aspiration. This means that moist air enters through the lip or labyrinth seals when the bearing case cools down. The cooler atmospheric air is replacing the heated air that went out the bearing casing vent
- A seal quench gland that often has steam, condensate or cooling water leaking out and directed at the radial bearing.
The moisture causes several problems:
- Water aids in the depletion of antioxidants, but it also cripples or diminishes the performance of many other additives. These include AW, EP, rust inhibitors, dispersants, detergents and demulsifying agents.
- Water can hydrolyze some additives, agglomerate others or simply wash them out of the working fluid into puddles on sump floors.
- Sulfur-phosphorous EP additives in the presence of water can transform into sulfuric and phosphoric acids, increasing an oil’s acid number (AN).
Aeration and Foam.
- Water lowers an oil’s interfacial tension (IFT), which can cripple its air-handling ability, leading to aeration and foam. It takes only about 1,000 ppm water to turn your bearing sump into a bubble bath.
- Air can weaken oil films, increase heat, induce oxidation, cause cavitation and interfere with oil flow; all catastrophic to the bearing.
- Aeration and foam can also incapacitate the effectiveness of oil slingers/flingers, ring oilers and collar oilers.
- Rust requires water. Even soluble water can contribute to rust formation.
- Higher temperatures and water gives acids their greatest corrosive potential.
- Etched and pitted surfaces from corrosion on bearing raceways and rolling elements disrupt the formation of critical elastohydrodynamic oil films that give bearing lubricants film strength to control contact fatigue and wear.
- Static etching and fretting are also accelerated by free water.
- Often called hydrogen embrittlement or blistering.
- The sources of the hydrogen can be water, but also electrolysis and corrosion (aided by water).
- There is evidence that water is attracted to microscopic fatigue cracks in balls and rollers by capillary forces. Once in contact with the free metal within the fissure, the water breaks down and liberates atomic hydrogen. This causes further crack propagation and fracture. High tensile-strength steels are at greatest risk.
- Sulfur from additives, mineral oils and environmental hydrogen sulfide may accelerate the progress of the fracture. Risk is posed by both soluble and free water.
- Many bearings have only a limited volume of lubricant and, therefore, very little antioxidant.
- High temperatures flanked by metal particles and water can consume the antioxidants rapidly and rid the lubricant from the needed oxidative protective environment.
- The negative consequences of oil oxidation are numerous but include corrosion, sludge, varnish and impaired oil flow.
Oil Flow Restrictions.
- Water is highly polar, and as such, can attract oil impurities that are also polar (oxides, dead additives, particles, carbon fines and resin) to form sludge balls and emulsions.
- These amorphous suspensions can enter critical oil ways, glands and orifices that feed bearings of lubricating oil. When the sludge impedes oil flow, the bearing suffers a starvation condition and failure is imminent.
- Additionally, filters are short-lived in oil systems loaded with suspended sludge. In subfreezing conditions, free water can form ice crystals which can interfere with oil flow as well.
Compromised Film Strength.
- Rolling element bearings depend on an oil’s viscosity to create a critical clearance under load.
- If the loads are too great, speeds are too low or the viscosity is too thin, then the fatigue life of the bearing is shortened.
- When small globules of water are pulled into the load zone the clearance is often lost, resulting in bumping or rubbing of the opposing surfaces (rolling element and raceway).
- Lubricants normally get stiff under load (referred to as their pressure-viscosity coefficient) which is needed to bear the working load (often greater than 500,000 psi).
- However, water’s viscosity is only one centistoke and this viscosity remains virtually unchanged, regardless of the load exerted. It is not good at bearing high-pressure loads. This results in collapsed film strength followed by fatigue cracks, pits and spalls.
- Water can also flash or explode into superheated steam in bearing load zones, which can sharply disrupt oil films and potentially fracture surfaces.
- Water is a known promoter of microorganisms such as fungi and bacteria.
- Over time, these can form thick biomass suspensions that can plug filters and interfere with oil flow.
- Microbial contamination is also corrosive.
Wash down hoses.
- When grease is contaminated with water, it can soften and flow out of the bearing.
- Water sprays can also wash the grease directly from the bearing, depending on the grease thickener and conditions.
What’s the answer to this water problem?
- Replace grease or lip seals with labyrinth seals or positive face seals.
- Try not to aim wash down hoses at the bearing case and bearing housing vent.
We get solids into the bearing 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 material.
- Abrasive particles leach out of the bearing housing casting.
- Often solid particles were already contaminating the grease or oil we are using for the lubricant.
- Solids were introduced into the system during the assembly process because of a lack of cleanliness.
- Airborne particles penetrate the bearing seals.
- Particles worn off of the grease or lip seals penetrate into the bearings.
How to keep solids and moisture out of the bearing housing.
- Seal the metal 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 apply this coating be careful about using some of the new high detergent oils. They might be powerful enough to remove this protective coating.
- 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 lost.
- Clean the oil in the bearing casing by installing a simple oil circulating and filtering system or change the oil frequently.
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.
Much of the blame for these conditions is caused by an increase in counterfeit bearings appearing on the market. If you adopt the above suggestions you should not have to be changing your bearings as frequently as you are now.
ANALYZING THE INDIVIDUAL PARTS OF THE BEARING
In a properly operating bearing, the raceways and rolling elements will become dull in appearance. This dullness is not an indication of wear and has no affect on the life of the bearing. These dull surfaces form the visible paths that I will be referring to in the following paragraphs so their appearance and location is important in analyzing any type of bearing failure.
When we install a bearing into a piece of rotating equipment the general rule is to have the interference fit on the race that is rotating and, therefore, carrying the load. Almost all centrifugal pumps, motors, and a high percentage of other types of rotating equipment have the bearings installed with the inner race an interference fit and rotating with the shaft. The outer race remains stationary or in a fixed position.
In the following paragraphs I will be discussing various load conditions and the resultant appearance of the raceways and rotating elements in this type of an installation Now lets have a look at the ball bearing parts and do some troubleshooting:
The radial load is rotating with the shaft. An unbalanced rotating assembly or a bent shaft causes this.
- The inner ring appearance. The load acts all of the time at the same place in the raceway. Here the path pattern is at its widest, tapering off at the ends. If the load is only radial, the pattern will be in the center of the raceway and will extend around slightly less the half the raceway circumference.
- The outer ring appearance. The path will extend around the entire raceway. It will be uniform in width and if the load is only radial, it will be in the center of the raceway.
The radial load is unidirectional. This is what we would expect to find with a properly operating piece of equipment. If the equipment is operating off of its best efficiency point, is misaligned, or if there is excessive pipe strain the pattern will be the same; only more pronounced.
- The inner ring appearance. The path will be in the center of the raceway, uniform in width and visible around the entire circumference of the raceway.
- The outer ring appearance. The pattern will be widest at the load point and tapering towards the ends. If the fit and clearances are normal the pattern will extend around to slightly less than one half of the raceway. It will be located in the center of the raceway, if the load is only radial.
The radial load is multidirectional. Cavitation, too tight an interference fit, pre loading, or cooling a bearing outside diameter are all common causes of this problem.
- The inner ring appearance. All around the raceway, widest where the load was the greatest.
- The outer ring appearance. All around the raceway, widest where the load was the greatest.
The axial load is unidirectional. This is the normal condition of all end suction centrifugal pumps.
- Both the inner and outer rings. The pattern will extend around both raceways and is displaced axially from the center. A centrifugal pump thrusts towards the thrust bearing until it reaches 65% of its efficiency and then it thrusts towards the volute or wet end during normal operation.
An oval compression of the outer ring. Caused by an out of round housing.
- The inner ring appearance. The path extends around the entire ring and is uniform in width.
- The outer ring appearance. Two wider paths where the ring was distorted to the oval shape.
The inner ring was misaligned. Normally happens during the installation process.
- The inner ring appearance. The pattern extends around the entire ring and is uniform in appearance.
- The outer ring appearance. The ball path will be oval, extending from one side of the raceway to the other, and wider in two diametrically opposite sections.
Now that we know what some typical wear paths look like, we will inspect the only two things that are visible to the trained troubleshooter.
- Evidence of rubbing.
- Evidence of corrosion and damage.
Look for damage caused by solid particles. These particles will be rolled into the race ways and can:
- Score, or cause small indentations in the precision races and rolling elements.
- Interfere with the transfer of heat within the tight tolerances, causing discoloration, thermal expansion, seizing etc. The particles come from:
- Varnish and coke that form where the lubricant overheated.
- Parts of the ball cage that have broken loose due to a lack of lubrication. Brass cage parts will turn the lubricant green.
- Pieces from a failed grease or lip seal.
- A contaminated lubricant.
- Lack of cleanliness during the installation process.
- The bearing lubricant could have been over heated during the installation process.
- Rust coming off the inside of the casting.
- Silica or other minerals leaching out of the bearing housing casting.
- Particles of material flaking off of the protective coating put on the inside of the housing to prevent rust.
- Airborne – through the seals or the vent.on top of the bearing housing.
Look for lack of lubrication that can eventually cause the bearing to seize:
- You will see mirror like surfaces on the metal parts that look like the part was lapped
- The metal will become discolored and soften as it anneals. Annealing can occur any time the temperature exceeds 300°F (150°C):
- Straw yellow 600° F. 315° C.
- Brown 700° F. 370° C.
- Blue 800° F. 425° C.
- Black 900° F. 480° C.
- If a pre- lubricated bearing was heated by immersing it in a hot oil bath (200°F or 100°C), the hot oil will wash out the grease and leave the bearing with little to no lubrication.
- Many pre-lubricated bearings actually have no lubricant at all installed. Check yours to be sure. Bearing quality is a serious maintenance problem.
- A clogged oil level gauge can give a false reading of lubrication level.
- If the bearing case has no expansion chamber installed, a build up of internal pressure as the bearing case comes up to temperature can blow out of the seals. At shut down moisture laden air will return to the case through the same seals.
- A poorly designed labyrinth seal can pump hot oil out of the bearing case. The lubricating oil level should be at the middle of the lower bearing ball when the pump is at rest.
- Be sure the pump has been leveled prior to alignment to insure the correct lubrication height.
Look for smearing of the metal. When two non-lubricated surfaces slide against each other, under load, the material can transfer from one surface to the other.
- The metal melts and then re-hardens causing localized stress that can produce cracks in the metal.
- The load was too light for the speed. Centrifugal force threw the balls out.
- The outer race will smear on the outside diameter if it slides during operation due to an improper “slip fit”. This slipping can also cause fretting corrosion as the protective oxide film is worn away from the metal surface.
Look for evidence of static vibration. You will see indents in the raceway that could be either shiny or rusted in the bottom. The frequency of the vibration has no affect, but greater energy causes greater damage. Roller bearings are more susceptible to this type of damage because the balls in a ball bearing can roll in many directions. Rollers, how ever, can roll in only one direction. Movement in the other directions takes the form of “sliding”.
There are multiple causes of static vibration that include:
- The pump was located too close to another piece of equipment that was vibrating. This can be a big problem during storage or with standby pumps
- The shaft was not locked during shipment.
- In addition to vibration, equally spaced indents can be caused by:
- An induction heater was used during assembly, causing false brinelling.
- The bearing was installed using an arbor press on the wrong race.
- The bearing was driven too far up a tapered shaft.
Look for electric current damage. It will show up on both the races and the rolling element. The bottom of the depression will be dark in color.
- This happens when the pump was used as an electrical ground for a welding rig.
Look for flaking or spalling of the metal raceway. Since there is nothing in a bearing to wear out, flaking or spalling is a sign of normal fatigue. Overloading however, can cause premature fatigue. Look for the following causes of bearing overloading:
- The bearing housing is out of round.
- The shaft is over size.
- The bearing was driven up too far on a tapered shaft.
- Misalignment between the pump and its driver.
- The rotating assembly is out of balance.
- The shaft is bent.
- The pump is operating too far off of its best efficiency point (BEP).
- Pipe strain.
- Water hammer in the lines.
- The bearing had a quality problem to start with.
- Shaft thermal expansion.
- The bearing housing is being cooled, causing the outer race to shrink, increasing the load.
- Excessive axial thrust.
- Pulley driven designs.
- Hydrogen embrittlement of the metal caused by moisture entering the lubricant.
- Pumping a high specific gravity fluid such as sulfuric acid can almost double the radial load.
Overloading is often accompanied by a change in appearance of the lubricant. You will see varnish or coke as the lubricant is subjected to this high heat.
In addition to overloading there are additional sources of heat that can destroy the lubricant:
- Soak temperatures through the shaft. This can be a big problem in either hot oil or hot water applications.
- Over lubrication of the bearing.
- Plugged oil return holes.
- Constant oil cups at the wrong level.
- Insufficient clearance in labyrinth seals.
- The oil gage breather hole is blocked and showing the wrong lubrication level.
- Bent lock washer prongs can rub against the bearing race.
- Grease or lip seals are too tight on the shaft.
- The pump stuffing box cooling jacket was shut off and drained when the metal bellows seal was installed in a high temperature oil application.
- Someone is cooling the pump’s power end casing causing the bearing outer race to shrink.
- Friction with the seal cage.
- Sliding friction caused by small changes in the shaft speed. Inertia keeps the balls moving as the shaft slows down.
- The stuffing box packing has been over tightened.
Look for cracks in the bearing metal.
- The bearing was driven too far up a tapered shaft.
- Any type of flaking or smearing can cause a fracture notch that will lead to cracking.
Look for signs of corrosion.
- Moisture is in the lubricant. It came from:
- Packing or seal leakage.
- A water hose is being used to wash down the area.
- Normal aspiration as the pump cooled down, and the moisture laden atmosphere entered the bearing case.
- Steam or water dripping from a seal quench gland. This is a common problem with the API (American Petroleum Institute) gland that is commonly used in oil refineries.
- Regardless of the protective coating put on the bearing races, (cadmium, chromium, zinc, etc.) the rolling elements are almost always fabricated from 52100 bearing steel, and it rusts.
The major bearing companies do a great job of providing the literature and photographs that you need to do effective comparison troubleshooting. Check with your bearing supplier for the availability of this information.