Bearing Failure: Causes and Cures
Precision ball bearings are designed to have a long and useful Iife. Assuming the application is correct to begin with, maximizing longevity means bearings must be properly installed, lubricated and maintained. Poor operating environments, particularly moist or contaminated areas and improper handling practices invite premature bearing failure.
When a bearing does fail, it is important to determine the exact cause so appropriate adjustments can be made. Examination of the failure mode often reveals the true cause of failure. This procedure is complicated by the fact that one failure mode may initiate another. For example, corrosion in a ball race leaves rust-an abrasive-which can cause wear, resulting in loss of preload or an increase in radial clearance. The wear debris can, in a grease-lubricated bearing, impede lubrication, resulting in lubrication failure and subsequent overheating.
This guide will assist in properly identifying and analyzing 12 primary causes of bearing failure. Each characteristic is described in detail and is shown by a colored photograph. Flaws, in most cases, are readily apparent. In some cases, the imperfections may be virtually invisible to the naked eye. Specific remedies for each situation are also suggested.
1- Excessive Loads:
Excessive loads usually cause premature fatigue. Tight fits, brinelling and improper preloading can also bring about early fatigue failure. This type of failure looks the same as normal fatigue, although heavy ball wear paths, evidence of overheating and more widespread spalling (fatigue area) are usually evident with shortened life.
The solution is to reduce the load or redesign using a bearing with greater capacity.
2- Overheating:
Symptoms are discoloration of the rings, balls and cages from gold to blue. Temperatures in excess of 400 oF 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 raise 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.
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 pressing a bearing onto a shaft by applying force to outer ring.
Install bearing by applying force only to the ring being press-fitted, i.e., do not push the outer ring to force inner ring onto shaft.
4- False Brinelling:
False brinelling-elliptical wear marks in axial direction at each ball position with a bright finish and sharp demarcation, often surrounded by a ring of brown debris-indicates excessive external vibration. A small relative motion between balls and raceway occurs in non-rotating ball bearing that is subject to external vibration. When the bearing isn’t turning, an oil film cannot be formed to prevent raceway wear. Wear debris oxidizes and accelerates the wear process.
Correct by isolating bearings from external vibration and using greases containing antiwear additives such as molybdenum disulfide when bearings only oscillate or reverse rapidly as in actuator motors.
5- Normal Fatigue Failure:
Fatigue failure-usually referred to a spalling-is the fracture of the running surfaces and subsequent removal of small, discrete 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.
6- Reverse Loading:
Angular contact bearings are designed to accept an axial load 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 a grooved wear band caused by the ball riding over the outer edge of 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.
Contamination is one of the leading causes of bearing failure. Contamination symptoms are denting of the bearings raceways and balls resulting in high vibration and wear.
Contaminants include airborne dust, dirt or any abrasive substance that finds its way into bearing. Principal sources are dirt tools, contaminated work areas, dirty hands and foreign matters in the 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.
Discolored (blue/brown) ball tracks and balls are symptoms of lubricant failure. Excessive wear of balls, ring and cages will follow resulting in overheating and subsequent catastrophic failure.
Ball bearings depend on the continuous presence of a very thin-millionth of an inch-film of the 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.
Users should use most suitable lubricant type and quantity. Any steps taken to correct improper fit, control preload better and cool the shafts and housing will reduce bearing temperature and improve lubricants life.
9- Corrosion:
Red/brown areas on the 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 also helpful.
10- Misalignment:
Misalignment can be detected on the raceway of nonrotating ring by a ball wear path that is not parallel to raceway edges. If misalignment exceeds 0.001 in./in you can expect abnormal temperature rise in the bearing and/or housing and heavy wear in the cage ball-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 shafts and housing for runout of shoulders and bearing seats; use single point-turned or ground threads on nonhardened shafts and ground threads only on hardened shafts; and using precision grade locknuts.
11- 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, faces) will wear heat, causing noise and runout problems.
12-Tight Fits:
A heavy ball wear in the bottom of raceway around the entire circumference of the inner ring and outer ring indicates a tight fit.
Where interference fits exceed the radial 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.
