NOTES on OIL LUBRICATION


Choosing the proper lubricant is one of the most important factors in achieving long bearing life. Lubricants prevent contact between rolling elements and raceways in a bearing and protect against wear and overheating. This article focuses on a common bearing lubricant- oil.

Oil lubrication is generally used when high speed or operating temperatures preclude the use of grease, when frictional or applied heat has to be transferred away from the bearing, or when adjacent machine parts are lubricated by oil.

Content and Viscosity.

Solvent-refined mineral oils are the optimal lubricants for ball and roller bearings. Oils containing additives for improved properties such as extreme pressure behavior or oxidation resistance can be employed in special cases. Synthetic oils, however, are used in extreme temperature conditions to lubricate rolling bearings.

Choose an oil with a viscosity that is adequate for the job. An oil's viscosity varies with the operating temperature, decreasing as the temperature rises. In order for a sufficient film of oil to form between bearing rolling elements, the oil must retain a minimum viscosity at the operating temperature. Oils with a high viscosity index of at least 85 are recommended.

The viscosity needed for adequate lubrication can vary with bearing design. For example, tapered roller and spherical roller bearings normally have a higher operating temperature than ball and cylindrical roller bearings under the same conditions and require oil of a slightly higher viscosity.

Methods of Oil Lubrication.

The most simple method is the oil bath, in which the bottom portion of the bearing housing is filled with oil. As the bearing rotates, the rolling elements collect the oil and distribute it within the bearing, before allowing it to drain back into the bottom of the housing. When the bearing is stationary, the oil level should be just below the center of the lowest ball or roller.

When operating at higher speeds, temperatures rise and oil oxidation increases. Oil circulation systems can be used to reduce the need for frequent oil changes in high-speed bearing applications. Oil passes through the bearing, then is filtered and often cooled before being returned to the bearing.

At extremely high speeds, oil circulating inside a bearing can begin to churn, causing resistance to bearing rotation. In these situations, an oil jet is recommended. The oil jet directs oil under high pressure at the side of the bearing. Small droplets penetrate the turbulent air around the bearing and get through to lubricate the rolling elements.

Oil mist lubrication is often recommended for bearings running at high speed in continuous operation applications. Under this method, a mixture of air and atomized oil is supplied to the bearing housing under pressure. It is important for the bearings to be wetted before they are put into actual operation to ensure that they don't run dry at start-up. For that reason, the oil mist system should be actuated several minutes before putting the equipment into actual operation, or a wet sump should be used. Oil mist is very effective in reducing a bearing's operating torque. The system provides a continuous supply of clean, fresh oil, and the air flow prevents excessive oil from accumulating in the bearing. Since the air under pressure in the housing escapes through the housing enclosures or vents, the system is also effective at preventing the entrance of moisture and contaminants. Care should be taken to prevent excessive mist from escaping into the shop environment.

Another technique is called oil spot lubrication. Here, small quantities of oil are delivered to individual bearings by compressed air and injected via a nozzle. This minimum quantity of oil enables bearings to operate at lower temperatures or at higher speeds than other lubrication methods. The oil spot technique is often used for grinding spindles and similar applications.

Changing Oil.

Oil change frequency depends mainly on operating conditions and oil quantity. With an oil bath, change oil once a year- provided there is no contamination and the operating temperature does not exceed 122F (50C). For operating temperatures around 240F, change oil every three months or consider using a synthetic oil.

For circulating systems, oil change frequency depends on how often the oil is circulated and whether it is cooled. You can determine the best frequency by carefully checking the oil for signs of contamination.

Changing oil is not necessary in oil spot lubrication, since oil circulates through the bearing only once and is not reused.