Bearing Basics

Bearings Don't Like Particles

The performance of ball bearings can be critically affected by minute particles. Particles as small as 0.005mm can cause problems. To prevent such problems, we use a class 1,000 clean room throughout our bearing assembly operation. "Keeping bearings clean" is the most important rule of bearing use. Avoid exposing bearings to any environment where particles may be present. Once inside a bearing, such particles can cause raceway scratches, abrasion and shortened life. They can also generate acoustic noise and vibration.

We use shields or seals to keep particles out. But there is still a gap between the shield and inner ring of shields and non-contact seals. This gap may permit particle entry. So it is important to observe the following procedures carefully.

1. Keep your bearing handling area as clean as possible.

2. Do not remove the bearings from their packaging until just before use. If you move the bearings to a container, be sure the container is clean. Its lid should be kept closed and it should be cleaned every day to prevent particle accumulation.

3. Never use a bearing that has been dropped. It may be brinelled (race track dented). In use, a brinelled bearing will generate a high level of acoustic noise.

4. Before applying adhesive to a bearing, use a cloth dampened with an alcohol agent to clean oily materials such as anti-corrosion oil from the inner and outer rings. Do not saturate the cloth excessively with the cleaning agent. The liquid agent itself could leak into the bearing, carrying particles with it. Always wring out the cloth before use.

5. When applying a lubricant to the outer circumference of a bearing, take care to make sure the lubricant is not contaminated.

6. Do not handle bearings in a place where they could be directly exposed to outside air. A seasonal wind might bring in dust and other particles.

Bearings Rust Easily

Since bearings are metallic products they rust easily. So their treatment requires certain precautions.

1. When handling bearings, use finger caps, pincers or gloves that do not generate cotton fibers.

2. When you must use naked fingers, first make sure they are clean and free from perspiration and dust. Then apply a quality mineral oil to the fingertips before touching the bearings. Do not use hand cream, as it may induce rust.

3. If a shaft is dirty on the surface, rust may gather between the shaft and the bearing after they are fitted. So it is important to make certain the shaft is free from finger prints and perspiration.

4. When handling bearings choose a place that is dry. Always place the bearing boxes on a shelf or pallet. Avoid placing the boxes directly on the floor or other locations where moisture and dirt may be present.

5. Avoid storing bearings near air conditioners and direct sunlight. Bearings may rust when placed near an air conditioner outlet or any place where wind or sunlight can enter directly.

6. A great temperature variance may cause condensation to form on bearings. In colder climates allow the bearings to reach room temperature before unpacking them.

7. There have been cases of bearings gathering rust overnight when sulfuric acid mist fell around them. This occurred when heating stoves were turned off at night, causing the combustion gas to turn into sulfuric acid. Bearings can rust when chloro-fluorocarbon gas from cleaning equipment is overheated resolved and converted to chlorine. We recommend storing bearings in centrally heated and properly ventilated environment.

8. Varnish applied to a motor winding may also cause bearings to rust when the acid generated by varnish gas is absorbed into the grease of the bearing. Be sure to test for this condition and be aware that changes in the process such as drying time changes may affect this condition.

How to Mount Bearings

When bearings are mounted incorrectly the balls will cause brinelling on the raceway and undermine bearing performance and life. Brinelling (dents) as small as 0.1micron in depth will have an adverse effect on acoustic noise levels as well as causing increased torque.

Several general rules apply to mounting bearings. When assembling a bearing on its shaft or in a housing, it is critical that no force be applied to the balls. When mounting a bearing to a shaft always press the inner ring. When mounting into a housing press the outer ring. Never apply force to the outer ring when mounting to a shaft or to the inner ring when mounting into a housing. Never apply a shock load in either case.

1. When manually fitting a shaft into a bearing through its bore do not force the shaft because it may cause brinelling to the bearing.

2. After gluing a bearing to a housing using a guide through the bearing bore take the guide out very carefully or it may cause brinelling to the bearing.

3. When motors are being assembled bearings may be attracted by the magnet and hit something causing brinelling. To avoid this, hold the motor shaft in your palm and cautiously insert the rotor. For automated assembly, use an air cylinder and steadily operate the assembly. Bearings are easily affected by shock forces. In the case of NMB P/N L-940ZZ, for example, brinelling would begin when a 100 gram weight was dropped 4mm away from the bearing.

4. Brinelling would also occur when bearings are automatically press-fitted to a rotor shaft if the shaft and bearing bore are not kept accurately in line through their bore.

Fitting Effects

Shaft out of Round
The main purpose of an interference fit either between the inner ring of a bearing and a shaft or between the outer ring and a housing is to prevent slippage. When slippage or creep occurs it prevents the bearings from operating properly due to abnormal friction and heat generation and possible entry of abrasive particles into the bearing.

Inner or Outer Ring Race Out of Round after Press Fitting
The cross-section of a miniature bearing's inner and outer rings is so thin that the configuration of the shaft and housing is directly mirrored by the bearing rings. In cases where the shaft or housing is out of round or if the shaft or housing are scratched or contain foreign particles, the adjacent ring will reflect that condition. This will cause the bearing to become noisy, its life to be shortened and other negative performance characteristics to occur.

Misalignment
Misalignment, or the tilting of one ring with respect to the other, may also have a negative effect on performance and life of bearings. If the races are misaligned, the balls track in a non-circumferential path. Misalignment of more than ¼ degree can lead to early failure. The amount of misalignment allowable in a given bearing is determined by its radial play and track curvature value.