Front wheel bearing experts generally catalog bearing failure into individual areas which include: wear, fatigue, corrosion, brinelling, fractures, and retainer breakage. Most bearing failure occurs because of wear, rust, and fatigue.
Before diagnosing any bearing for the cause of failure, it should be cleaned thoroughly in a safe solvent and allowed to air dry. It is usually not a good idea to blow the assembly dry with compressed air.
Using the accompanying illustrations as references, let's itemize each bearing failure problem.
Lubricant is contaminated with tiny bits of abrasive material. Finish on rollers and outer cup is dulled but in a uniform pattern. Minor wear of this type is not cause for bearing replacement.
Wear Advanced wear affects the roller cage, which does call for bearing replacement.
Can be caused by breakdown of the lubricant or an overloaded condition. Draw a file across discolored areas. If file "bites," surface has lost its heat-treated hardness and bearing replacement is necessary.
Metal smearing at ends of roller ruins bearing assembly. Can be caused by improper adjustment or poor lubrication, possibly due to inferior grade of lubricant
Usually due to mishandling. Even d slight bend such as shown can ruin a bearing assembly. Always cover a metal workbench with heavy cardboard or similar soft material when working with bearings.
Dents in cup caused by impact from rollers. Usually occurring when bearing is not rotating, such as that which can be caused by a collision or other accident.
Bits of metal can flake off (spall) bearing surfaces as assembly is in extended service. Metallic particles can cause further damage. A thorough cleaning of the entire area is required before a new assembly is installed.
Gray or grayish black bearings indicate that there has been some erosion of surface metal of the cup at the same spacings as the rollers. Dirty lubricant is suspected but bearing assembly might be reusable if etching is slight.
Overheating (overloading) related to unwanted slippage between parts can cause metal smear. A loose fit, poor lubricant, or excessive loads can lead to smearing, and a smeared bearing must be replaced.
Ends of rollers are worn, probably due to contaminated lubricant. Bearing probably should be replaced if not, do not preload beyond specifications during bearing adjustment.
Surface low spots or pits on cups or roller are generally due to contaminated lubricant. Very minor indentations can often be tolerated, but all lubricant must be removed from bearing and associated parts before bearing can be reused or new bearing installed.
Check bearing assembly for magnetism which, if present, can cause tiny metallic bits to lodge hidden in the cage. If magnetized, demagnetize before considering the assembly clean enough for reuse.
Small burrs or foreign material that is lodged between bearing and outer cup indicates that the assembly was in service with cage and roller cocked with respect to outer cup. Bearing assembly probably requires replacement.
Cracked Inner Race
Can happen when too much force is used to seat those bearings that are press fit, or if wrong tools or rough handling were used in service.
Corrosion due to slight relative motion between parts without adequate lubrication. Parts replacement is called for.
Caused by breakdown of lubricant. If stain can be removed with solvent or light polish, a complete cleaning and repacking is all that is probably required to put assembly back in service.