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.
Scale a Radial ball bearings
Scale b Cylindrical roller bearings, needle roller bearing
Scale c Spherical roller bearings, taper roller bearings, thrust ball bearings
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
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.
Proprietary grease dispensers may also be fitted to the bearing housings which ensure that the correct amount of grease is dispensed at the appropriate time interval.
Oil lubrication is used not only for most sleeve bearings, but also for rolling element bearings when the rotational speed is above the allowable limit for grease. It may be essential where high operating temperatures make grease unsuitable.
The simplest method of oil lubrication is by use of an oil bath, but increasing speed raises the bearing temperature, and leads to oxidation of the oil. To avoid frequent lubricant changes the oil may be filtered and externally cooled before being recirculated by means of a pump. Oiljetsormistmay be necessary to ensure that the lubricant reaches the parts where frictional heat is generated.
Solvent-refined mineral oils are normally used for oil-lubricated fan bearings. Additives to improve lubricant film strength or oxidation resistance are only required in extreme circumstances. Viscosity is one of the most important properties of a lubricating oil and the requisite value must be maintained at the operating temperature. It is unwise therefore to change the oil characteristics without reference to the fan and bearing manufacturers.
Posted in Fans Ventilation A Practical Guide