from McNally Institute
As discussed in a another technical paper on this web site, bearings have no wearable surfaces, they are instead designed to fatigue after many hours of service.
In a properly operating bearing the race ways 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:
The radial load is rotating with the shaft, This is caused by an unbalanced rotating assembly or a bent shaft.
- The inner ring appearance. The load acts all of the time at the same place in the race way. 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 race way and will extend around slightly less the half the race way circumference.
- The outer ring appearance. The path will extend around the entire race way. It will be uniform in width and if the load is only radial, it will be in the center of the race way.
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 race way, uniform in width and visible around the entire circumference of the race way.
- 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 race way, if the load is only radial.
The radial load is multidirectional . Cavitation, too tight an interference fit, preloading, or cooling a bearing outside diameter are all common causes of this problem.
- The inner ring appearance. All around the race way, widest where the load was the greatest.
- The outer ring appearance. All around the race way, 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 race way 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 trouble shooter.
- 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 forms 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 bearings are being installd next to the area where the mechanic is grinding a new edge on his lawnmower blade.
- The bearing lubricant could have been over heated during the installation process.
- Rust coming off the inside of the casting.
- Silica leaching out of the casting
- Particles of material flaking off of the protective coating put on the inside of the housing to prevent rust.
- Airborne – through the bearing seals or housing vent.
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 piece 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 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 transfers 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”.
- The pump was located too close to another piece of equipment that was vibrating. This can be a big problem during storage.
- 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 Duriron”.
- The bearing was installed by pressing 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.
- The pump was used as a ground for a welding rig.
Look for flaking or spalling of the metal race way. 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 (B.E.P.).
- 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 design.
- 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 power end case 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 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 being used to wash down the area.
- Normal aspiration as the pump cooled down, and the moisture ladened atmosphere entered the bearing case.
- Steam or water from a seal quench gland. This is a common problem with the A.P.I. 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 good 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.