An Introduction To Ball and Roller Bearing Failure Causes And prevention
Introduction
If the life of a bearing is compared with that of a human being, fatigue failure can be consideredto be a "natural death", a normal occurrence: having been subjected to the loads and stresses
for which it was designed, the bearing must be replaced within a time conforming to its specification.
There is no failure, but rather fatigue.
However, life of a bearing may be shortened suddenly. An event, often of external origin, causes
damage.This is the type of problem that like a disease, abnormal wear of a bearing can develop slowly or suddenly. It also exhibits symptoms, more or less easy to detect.
the main failure cause is the inappropriate lubrication of the bearing rolling elements (approximately 80% of the cases), followed by inadequate bearing selection (10%), improper mounting (5%), indirect failure (4%), and material defects and manufacturing errors (less than 1%).
Classification of bearing failure causes
Failures can be classified in 3 major categories:
1• Failures due to environmental factors: improper or negligent mounting, dirty working conditions,
inadequate lubrication, over-loading, vibrations, excessive speed, overheating, pollution...
2• Failures due to a misapplication: inadequate size or type of bearing, or improper installation
process.
3• Failures due to the quality of the bearing itself: improper steel or defect in its structure, problems of internal geometry, cage quality, seals, etc.
4• Failures can occur following the use of inappropriate fitting tools.
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| FIG 1 |
Causes and origins of bearing environmental failures
1- EXCESSIVE LOAD
Excessive loads usually cause premature fatigue (figure 2).it fits, and improper preloading can also ring about early fatigue failure, . This type of failure looks the same as normal fatigue. although heavy ball wear paths, evidence of overheating and a more widespread spalling (fatigue area) are usually evident with shortened life. The solution is to reduce the load or redesign using bearing with greater capacity.
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| FIG 2 |
2- OVERHEATING
Symptoms are discoloration of the rings, balls, and cages from gold to blue. Temperatures in excess of 400°F can anneal the ring and ball materials. The resulting loss in hardness reduces the bearing capacity causing early failure. In extreme cases, balls and rings will deform. The temperature rise can also degrade or destroy lubricant Common culprits are heavy electrical heat loads, inadequate heat paths. and insufficient cooling or lubrication when loads and speeds are excessive. Thermal or overload controls, adequate heat paths, and supplemental cooling are effective cures.
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| FIG 3 |
3- TRUE BRINELLING
Brinelling occurs when loads exceed the elastic limit of the ring material. Brinell marks show as indentations in the raceways which increase bearing vibration (noise]. Severe brinell marks can cause premature fatigue failure. Any static overload or severe impact can cause brinelling. Examples include: using hammers to remove or install bearings, dropping or striking assembled equipment, and a bearing onto a shaft by applying force to the outer ring. Install bearings by applying force only to the ring being pressfitted, i.e., do not push the outer ring
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| FIG 4 |
4- NORMAL FATIGUE FAILURE
Fatigue failure-usually referred to as spalling-is the fracture of the running surfaces and subsequent removal of small. discrete particles of material. Spalling can occur on the inner ring, outer ring, or balls. This type of failure is progressive and once initiated will spread as a result of further operation. It will always be accompanied by a marked increase in vibration, indicating an abnormality. The remedy is to replace the bearing or consider redesigning to use a bearing having a greater calculated fatigue life.
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| FIG 5 |
5- REVERSE LOADING
Angular bearing contact are designed to accept an axial load are in one direction only. When loaded in the opposite direction, the elliptical contact area on the outer ring is truncated by the low shoulder on that side of the outer ring. The result is excessive stress and an increase in temperature, followed by increased vibration and early failure. Failure mode is very similar to that of heavy interference (tight) fits. The balls will show grooved wear band caused by the ball riding over the outer edge of the raceway . Corrective action is to simply install the bearing correctly . Angular contact bearings must be installed with the resultant thrust on the wide face-which is marked " thrust"-of the outer ring and the opposite face of the inner ring
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| FIG 6 |
6- CONTAMINATION
Contamination is one of the leading causes of bearing failure. Contamination symptoms are denting of the bearing raceways and balls resulting in high vibration and wear. Contaminants include airborne dust, dirt or any abrasive substance that finds its way into the bearing. Principal sources are dirty tools, contaminated work areas, dirty hands and foreign matter in lubricants or cleaning solutions. Clean work areas, tools, fixtures and hands help reduce contamination failures. Keep grinding operations away from bearing assembly areas and keep bearings in their original packaging until you are ready to install them. Seals are critical-damaged or inoperative seals cannot protect bearings from contamination
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| FIG 7 |
7- LUBRICANT FAILURE
Discolored (blue/brown) ball tracks and balls are symptoms of lubricant failure. Excessive wear of balls, ring. and cages will follow, resulting in overheating subsequent catastrophic failure. . Ball bearings depend on the continuous presence-of a very thin-millionths of an inch-film of lubricant between balls and races, and between the cage, bearing rings, and balls. Failures are typically caused by restricted lubricant flow or excessive temperatures that degrade the lubricant's properties.
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| FIG 8 |
8- CORROSION
Red/brown areas on balls, raceways, cages, or bands of ball bearings are symptoms of corrosion. This condition results from exposing bearings to corrosive fluids or a corrosive atmosphere. The usual result is increased vibration followed by wear, with subsequent increase in radial clearance or loss of preload. In extreme cases. corrosion can initiate early fatigue failures. Correct by diverting corrosive fluids away from bearing areas and use integrally sealed bearings whenever possible. If the environment is particularly hostile, the use of external seals in addition to integral seals should be considered. The use of stainless steel bearings is also helpful.
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| FIG 9 |
9- MISALIGNMENT
Misalignment can be detected on the race way of non rotating ring by a wear ball path that is not parallel to the raceway edges . If misalignment exceeds 0.001 in/in you can expect an abnormal temperature rise in the bearing and / or housing and heavy wear in the cage pockets.
The most prevalent causes of misalignment are : bent shafts , burrs ,or dirt on shaft or housing shoulders ,shaft threads that are not square with shaft seats, and locking nuts with faces that are not square to the thread axis. The maximum allowable misalignment varies greatly with different applications, decreasing, for example, with speed. Appropriate corrective action includes: inspecting shah and housings for run-out of shoulders and bearing seats; use of single point-turned or ground threads on non Hardened shaft and ground threads only on hardened shafts .and using precision grade lock nuts.
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| FIG 10 |
10- LOOSE FITS
Loose fits can cause relative motion between mating parts. If the relative motion between mating parts is slight but continuous, fretting occurs. Fretting is the generation of fine metal particles which oxidize. leaving a distinctive brown color. This material is abrasive and will aggravate the looseness. If the looseness is enough to allow considerable movement of the inner or outer ring, the mounting surfaces (bores, outer diameters, h] will wear and heat (see figure 11 ), causing noise and runout problems.
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| FIG 11 |
11- TIGHT FITS
A heavy ball wear path in the bottom of the raceway around the entire circumference of the inner ring and outer ring indicates a tight fit. Where interference fits exceed the radii clearance at operating temperature, the balls will become excessively loaded. This will result in a rapid temperature rise accompanied by high torque. Continued operation can lead to rapid wear and fatigue. Corrective action includes a decrease in total interference better matching of bearings to shafts and housings-taking into consideration the differences in materials and operating temperatures. Increased radial clearance will also increase bearing life under the above conditions.
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| FIG 12 |
A bearing rotating under functional conditions will exhibit certain "normal" operating levels of temperature, noise and vibrations. Exact levels will vary, as they are dependent on several factors: load, speed, lubrication, type of bearing...These levels should be considered as signals or warnings when they exceed those established for the "normal" or steady state conditions.
Deviations from the "norms" established under your operating conditions should signal the need
for implementation of your preventive maintenance program or at least a close check to determine the cause of the deviation. These signals are:
Deviations from the "norms" established under your operating conditions should signal the need
for implementation of your preventive maintenance program or at least a close check to determine the cause of the deviation. These signals are:
1. Vibrations
These can be detected by hand or with electronic equipment (frequency or amplitude analyzer). This
type of equipment can be used to alert the operator or to stop the machine.
2. Noises
Some abnormal noises can be heard immediately, such as those due to rolling element indentations
because of improper mounting; others are progressive. Noise is usually an indication of incipient failure and varies in intensity and frequency with the extent of the damage. Conversely the damage due to unbalanced loads is generally inaudible since their frequency is identical to the rotating assembly frequency.
3. Temperature rise Any operating bearing is subjected to a temperature rise above the ambient. This rise depends on many factors and reaches a certain level considered as normal, for a particular mounting. Any rise beyond that normal level is an indication of failure.
4. Increase of the friction torque Any rotation system (shaft, wheel, pulley...) presents a resistant torque, even when mounted on bearings. Any torque increase is indicative of an
alteration in the bearing. Generally, an increase of the friction torque generates a temperature rise.
Interpretation of external indications
1. Vibrations
Spalling
Abrasive wear - foreign particles
Corrosion
Unbalanced loads due to bearing wear
Excessive clearance
Excessive loose fit of a ring...
2. Noises
Rolling element indentations
Spalling
False brinelling (indentations caused by vibrations)
Foreign particles
Corrosion
Elimination of the internal clearance due to excessive
press fit
Cage or rolling element failure...
3. Temperature rise
Excess or lack of lubricant
Elimination of the internal clearance
Axial and radial overload, unintentional or due to
improper mounting
Excessive speed...
4. Abnormal friction torque
Cage distortion
Lubricant deterioration
Seal damage or displacement
Elimination of radial play
Do's and Don'ts for Preventing Ball and Roller Bearing Failures
1- Select the type of bearing, the lubricant and the schedules for its application carefully in accordance with the recommendations of the manufacturer as applied to the working conditions such as speed, axial and radial loads, temperature, etc.
2- During service trials, monitor the condition of the lubricant and the active surfaces. In the event of appearance of any signs of impending failure investigate fully and make modifications until satisfactory performance is achieved.
3- Ensure correct fitting after every overhaul.
4- Ensure correct lubrication at all times by specifying intervals for replenishment and for replacement of lubricant. These may be based on manufacturers' recommendations, but should be verified for suitability under actual working conditions by periodical examination of lubricant condition.
5- Monitor active surfaces and shock-pulse measurements.
6- Investigate each case of bearing failure to determine the root causes and then eliminate them.
7- Some of the common causes of failures are:
Incorrect selection of bearing type
Absence of lubricant
Wrong lubricant
Defective lubricant
Inadequate lubricant
Excess lubricant
Contaminated lubricant
Race too loose
Race too tight
Axial and angular position error
Slipping of outer race
Damage during assembly
Seal failure
8-Provide clean rooms for inspection and assembly of bearings and storage of lubricants.
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