Plain bearings, bushings, and rod ends also require care in order to maintain precision throughout their life.

Miniature ball bearings, whether constructed from engineered metals, ceramics, or synthetic materials, are extremely precise devices. In many cases, their geometric tolerances are measured in millionths of an inch. Such exactness is often required, especially in aircraft, telecommunications products, and medical instrumentation.

Maintaining ultra-high levels of precision in bearings is an ongoing challenge over the life of the machine in which bearings are used. So sensitive are today’s bearings that they are more likely to fail from poor handling techniques or faulty lubrication than from actual use.

Foreign particulate matter is particularly troublesome. Not only does it shorten bearing (and lubricant) life, it also increases noise, friction, and heat, and causes balls to skid. Thus, keeping the external surfaces and contiguous components (i.e. shafts and housings) clean is a must.

Contamination on bearing faces, mating shoulders, and shafts poses another risk. Even a small amount can make a bearing pair unparallel. This causes balls to run in a non-circumferential plane within the raceways, causing ball speed variations, ball skidding, and binding between the races. These phenomena increase friction. The resulting heat causes the lubrication to oxidize, resulting in premature bearing failure.

Following some simple guidelines, however, can help limit bearing contamination.

Keep work areas clean: No food, drink, or smoking should be allowed in bearing handling areas. It is also recommended that external garb, such as smocks, caps, and shoe covers be worn. They should be made of a suitable lint-free plastic yarn to prevent contamination. In addition, no cosmetics or lotions should be used.

Analysis of a turbine bearing from a Chinese made dental handpiece shows the effects of imprecision due to improper (careless) bearing handling and maintenance.
Select figure to enlarge.

Bearings should never be handled with bare hands. Technicians and engineers should wear talc-free finger cots or surgical gloves. In addition, the work area should be clean and the entire environment and surrounding area clear of any source of magnetism. The use of laminar flow workstations is strongly recommended.

Bearings should be kept in their original packaging until they are ready for installation: Ball bearings must never be left exposed to the environment in uncovered containers. They should be maintained in a dry place as they are susceptible to corrosion; even moisture and acid from a person’s hands can cause a bearing to corrode.

Don’t clean housings and shafts with common shop rags or cotton swabs because they can contaminate anything they touch: Clean room wipes or swabs should be used at all times. Even high-pressure air guns can cause dust and other contaminants to enter bearings.

Bearing mounting tooling should be maintained properly: Bearing work tools must be kept clean and burr free. Tools should be inspected properly to ensure that the bearings are mounted squarely on the shaft and into the housing.

Bearings should never be subjected to shock or impact load: If a bearing is dropped on the floor or hammered into place, it can easily become “brinelled,” with dents in raceways.

When press fitting a bearing, force should be applied only to the ring being fitted. For example, if press fitting onto a shaft, force should be applied to the face of the inner ring only. Under no circumstances should force be applied in a way that allows transmission from one bearing ring to the other through the balls.

Friction and heat

Overheating will also cause a bearing to fail. The most common reasons for overheating, are contamination, misalignment, excessive loads, excessive speed, excessive ambient temperature, and a chemical or mechanical breakdown of lubricant.

Continue on page 2