Ball bearings are very precise mechanisms that, in many cases, have geometrical tolerances measured in millionths of an inch. When handling, inspecting, or mounting bearings, one should treat them as precision instruments. Poor handling techniques cause most bearing failures.
Foreign particulate matter entering a bearing shortens its life, and provides one or all of the following unsatisfactory operating conditions:
- Increased noise
- Increased friction
- Increased heat
- Shortened lubricant life
- Ball skidding.
Keeping external surfaces and contiguous components (i.e., shafts and housings) contamination-free is also important. Contamination on bearing faces or mating shoulder of the housings or shafts will cause a bearing pair to be out of parallel. This will cause the balls to run in a noncircumferential plane within the raceways, causing ball speed variations, ball skidding, and binding between the races. These phenomena increase friction, which produces heat. Heat causes the lubrication to oxidize, resulting in premature bearing failure.
Follow these 12 rules when handling bearings to eliminate contaminants and damage:
- Never handle bearings with bare hands. Talc-free finger cots or talc-free surgical gloves should be worn.
- Work areas must be clean, clean, clean (no food or smoking should be allowed).
- Keep bearings in their original packaging until the moment they are ready to be installed.
- Do not clean housings and shafts with shop rags or cotton swabs, as these items usually will contribute to contamination. Clean room wipes or swabs should be used.
- The use of laminar flow workstations is strongly recommended.
- Keep bearing work tools clean and burr free.
- Keep bearings clear of any source of magnetism.
- Keep bearings in a dry environment, as they are susceptible to corrosion. Moisture and acid from hands can cause a bearing to corrode.
- Bearing mounting tools should assure that the bearings are mounted squarely on the shaft and into the housing.
- Never subject bearings to shock or impact load. Normally, a bearing will become Brinelled (dents in the raceways) if dropped from the workstation to the floor, or hammered into place.
- When press fitting a bearing, apply force only to the ring that is being fitted. For example, if press fitting a bearing onto a shaft, apply force to the face of the inner ring only. Under no circumstances should force be applied in a manner such that it is transmitted from one bearing ring to the other through the balls.
- When handling miniature bearings, external garb (caps, shoe covers and smocks) made of a lint-free suitable plastic yarn should be worn. The use of cosmetics/face powders is not allowed. Bearings should be stored in their original containers. Never leave them exposed to the environment in uncovered containers.
As an example of how greatly contaminants affect the life of a bearing, AST Bearings asked some customers to run accelerated life tests on their products using bearings that are lubricated with a factory grease lubricant, such as those supplied by Chevron, Shell, or ExxonMobil, to name a few. Then additional testing was conducted with new bearings lubricated with the same type of grease, only ultrafiltered through a 3 micron membrane. This process assures that the largest particulate matter is 34 microns dia or less.
The results showed that product life for bearings with ultrafiltered grease was 2-3 times longer -- and some customers have reported up to 6 times as long -- as those tested with regular grease.