Bearing contamination

Contaminants exist in solid and liquid forms and are one of the principle causes of bearing failure. They can be introduced:

· During mechanism assembly.

· From an outside source during the service life of a mechanism.

· Internally during operation.

During assembly

Most bearings are treated with a rust preventive before they are packaged. Some are even pre-lubricated. If new bearings are handled in a clean work area, they normally do not require cleaning.

However, contaminants can be transferred to new bearings from dirty hands or tools used to apply grease. Moisture and dust particles also can enter bearings, often causing premature failure.

Outside sources

The primary purpose of the seal is to keep grease in the system. Defective seals not only allow grease to escape from the system, but also allow dirt and rust-forming moisture to enter the bearing area. It is as important to maintain good seals as it is to keep bearings clean during assembly.

Breathers and fill openings are areas where contaminants can enter a system. These areas should be cleaned periodically.

Internally generated

Constant rubbing of metal parts, no matter how well lubricated, results in metallic debris which can enter the bearing and cause damage from abrasive wear. Oxidized lubricants produce sludge and corrosive chemicals that also contribute to bearing wear.

Results of contamination

Excessive bearing wear or failure is the end product of bearing contamination. Abrasive wear damage and corrosive damage are types of damage resulting from contaminated bearings. Abrasive bearing wear is usually caused by dirt; corrosive bearing wear by moisture or rust in a bearing.

Cleaning used bearings

Dirt and oxidized lubricants must be eliminated from used bearings if they are to be reinstalled. Clean bearings that have gummy lubricants in a solvent, then wipe with a lint-free cloth to remove liquid. For maximum protection, rotate bearings slowly in a light oil to make sure all parts of the bearing are covered with a protective film of oil.

For bearings that are relatively clean, dip in light oil heated to between 180 degrees and 200 degrees F (82 to 83 degrees C).

Use a paint brush to clean the bearing interior around the ball or rollers. Compressed air is sometimes used to clean bearings. Do not spin bearings by compressed air. If the bearing is dirty, high-speed rotation can cause damage because of abrasive wear. Slow rotation by hand is recommended.

Inspection procedure

After cleaning used bearings, give them a thorough visual inspection. Replace the bearing if the nonremovable seals or shields are damaged or worn. Replace defective replaceable seals.

Check the bearing exterior for discoloration, cracks, dents, or damaged separators, balls, or rollers. For inner surface and rolling element inspection, use a small flashlight or reflected light.

Illus. PS8503

Straight bearing – rotate outer race slowly; inspect bearings for damage

Illus. PS8504

Thrust bearing – rotate by light hand pressure

If there are visible signs of wear and damage, hold the bearing and rotate the outer race slowly -- do not spin it.

Clean the bearing again if sticking or clicking is detected. Replace the bearing if the sticking or clicking still exists after cleaning.

Last modified: 14Nov02.