Today I cut open a NSK wheel bearing that lasted 250,00 miles and show you spalling that occured on the inner, outer, and ball bearings in side the bearing.
The top bearing “killers,” or causes of damage and premature end of bearing life, are inadequate lubrication, contamination, overload, and improper handling and installation. They affect cylindrical, spherical, needle, tapered, and ball bearings. When they strike, they force companies to pay for repairing and replacing both the bearing and adjacent components that get damaged such as housings and shafts.
Inadequate lubrication is the major bearing killer. It can be broken down into eight basic categories: overfilling, underfilling, incorrect specification, mixing or incompatibility, incorrect lubrication and intervals, deteriorated grease or oil, water contamination, and particulate contamination.
OEMs and bearing suppliers typically specify the right lubricant amount, type, grade, supply system, replenishment cycle, viscosity and additives for bearings. They can also help select lubricants for bearings based on history, loading, speeds, sealing, service conditions, and expected life.
Mixing different grades and brands of greases or oils can be disastrous. Lubricants must be compatible to function properly over time. Because of significant chemical differences in thickening agents, base oils, additives, and other ingredients, some lubricants should never be mixed with others.
High operating temperatures can be caused by high ambient or process temperatures and by severe operating conditions that increase rolling-contact friction. Conversely, low ambient temperatures also challenge lubricants, thickening them and making them flow improperly. Check with bearing suppliers for the acceptable temperature range.
Contamination by foreign particles also creates trouble for bearings. Even small amounts of dust, dirt, or debris can disrupt the oil film, damaging the surface of the bearing. Water is particularly damaging to lubricant. As little as 1% water in grease or oil can significantly shorten bearing life.
Sand, fine metal from grinding or machining, and fine metal or carbides from gears can wear or lap rolling elements and races. Abrasive wear also affects other machine elements and accelerates seal wear, degrading performance. Some reasons why fine particles accumulate include improper cleaning of housings and parts, and ineffective filtration or improper filter maintenance.
Hard particles rolling through bearings can pit and bruise the rolling elements and races. Bruising raises metal around the dents, which act as surface-stress risers to cause premature spalling and shorten bearing life. Metal chips or large pieces of dirt remaining after machining due to improper cleaning can initiate early fatigue damage. The internal debris can come from worn gears, splines, seals, clutches, brakes, and joints, as well as improperly cleaned housings and damaged or spalled components.
Moisture and water contamination leads to etching or corrosion in bearings. The surface finishes on races and rolling elements makes them susceptible to corrosion if not adequately protected. Etching is often caused by condensate collecting in the bearing due to temperature changes or improper storage. Equipment left idle for long periods is also susceptible to corrosion, which can quickly lead to severe spalling.
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