Rolling element bearing lubrication

Rolling element (or anti-friction) bearings need to be lubricated to prevent inter-metallic contact between the balls or rollers, raceways and cages. The lubricant however also has the addi­tional function of protecting the bearing against corrosion or other sources of environmental wear.

Bearings may be lubricated with grease, oil or in rare cases with a solid. The best operating temperature for a bearing is ob­tained when the minimum of lubricant necessary to ensure reli­able operation is provided. However the lubricants become contaminated in service and must therefore be replenished or changed from time to time. The choice of lubricant depends on the operating temperature range, environmental conditions and rotational speed.

As previously noted, rolling element bearings are used for the great majority of fan applications. Wherever possible grease is used for lubrication as it is more easily retained in the bearings no matter what the inclination. It also helps to seal the housing against outside impurities such as dust and water. Lubricating greases are thickened mineral oils or synthetic fluids. Their consistency depends on the quantity and type of thickening agents included. Consistency, miscibility, operating tempera­ture range and rust inhibiting properties are the important properties of a good lubricant.

The lubrication interval is dependent on bearing size, rotational speed, operating temperature and grease type. Figure 10.28 is applicable to bearing temperatures around 70°C. Below this temperature the intervals are likely to increase, but above this temperature they will reduce considerably. Reference should be made to the fan and/or bearing manufacturer for further information.

For small ball bearings, especially those used in electric mo­tors, the lubricating interval may be longer than their service life. They may then be fitted with shields or seals and are sealed for life.

The amount of grease needed for a charge can be obtained from the formula

G = K D L Equ 10.1

Where:

G = grease quantity (g)

K = constant 0.005

D = bearing outside diameter (mm)

L = bearing axial length (mm)

The means of relubrication will depend on the frequency neces­sary. Where convenient, the housing caps can be removed and fresh grease can then be packed between the rolling elements. If more frequent relubrication is necessary, grease nipples may be fitted to the bearing housings and a grease gun used.

In all cases, too much grease will lead to overheating and main­tenance staff must be encouraged not to lubricate every time they pass the fan. High-speed fans however, often require fre­quent greasing. There is then a danger that the used grease will collect in the bottom of the bearing housings. In this case grease escape valves should be fitted. These enable excess grease to be discharged. They permit greasing to be carried out without having to stop the fan.