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The main purposes of bearing lubrication are to prevent metallic contact between the rolling elements, raceways, and cage, and also to prevent the bearing from corrosion and wear. Additional functions are sealing and cooling of the bearing. Bearings may be lubricated with grease or oil-in special cases with a solid lubricant-the choice of which depends primarily on the temperature range, operating speeds, and loading conditions of the bearings concerned. All lubricants must be changed from time to time because their properties deteriorate as a result of aging and contamination of the lubricants. The limited speeds for both grease and oil lubrication are given in the bearing tables.

Grease Lubrication

Grease lubrication is generally used for bearings operating under normal conditions. Grease has certain advantages over oil because it's easily retained in housing as well as its sealing effect against the entry of moisture and outside impurities. In general, the bearing should only be 1/3 to 1/2 partly filled with grease because over filling will cause rapid temperature rise, particularly if speeds are high.

Oil Lubrication

Oil lubrication is generally used when high speeds or operating temperatures are beyond the effective range of greases, or when it is so designed that heat developed in the bearing must be tranferred away through a lubricating oil circulation system. Viscosity of the lubricating oil is the main factor to be considered to suit the diverse applications with respect to speed, temperature, and loading conditions. Oil lubrication is effective; however, its oil feeding and sealing devices must be provided. Some of them are recommended as follows:

1. Oil Bath Lubrication

This is the simplest method and is only suitable for slow speeds. The oil is picked up by the rotating bearing elements. The oil levels should be slightly below the center of the lowest ball or roller.

2. Dripping Oil Lubrication

This method is suitable for the application where a small quantity of lubricating oil is constantly fed into the bearing without interruption. The excessive amount of oil dripped may cause a rise in the temperature of the bearing.

3. Circulating Oil Lubrication

For high speed bearing application, an oil circulation system can be used. The lubricating oil after passing through the bearing is filtered, sometimes cooled, and then pumped back to the bearing. This method of lubrication is the common practice of spindle design for high speed precision machine tools.

4. Oil Jet Lubrication

Oil jet provides a very effective lubricating method for high speed applications. It is important to ensure that sufficient amount of oil will reach the bearing components and will be able to dissipate the heat generated by friction. The velocity of the oil jet, usually 15 M/S, must enable some of the oil to penetrate through the turbulent air membrane surrounding the bearing. The position of the oil jet should be placed between the inner race and cage of the bearing.

5. Oil Mist Lubrication

This method is often used for high speed applications, such as grinding spindles. The oil mist is produced in an atomizer. Dry compressed air, after filtered, is used in the oil lubricator. The oil is then introduced into the bearings. The air current will also serve to cool the bearing, and its slightly higher pressure in the housing will also prevent impurities from entering. The small quantity of oil can be regulated so that the lubricant friction is practically negligible.

Solid Lubrication

Sometimes it is found that the addition of a small amount of solid lubricant, such as MoS2, into the grease will greatly improve the lubricating properties. In some special cases where very high temperature or high vacuum prevails, solid lubrication will be the solution.