Rolling bearings must be adequately lubricated to operate reliably. The main purposes of bearing lubrication are to reduce friction and wear indside the bearings that may cause premature failure. The effects of bearing lubrication may be briefly explained as follow:
- Bearing lubrication can reduce friction and wear
Direct metallic contact between the bearing rings, rolling elements and cage, which are the basic components of a bearing, is prevented by an oil film which reduces the friction and wear i the contact areas. - Bearing lubrication can extend fatigue life
The rolling fatigue life of bearings depends greatly upon the viscosity and film thickness between the rolling contact surfaces. A heavy film thickness prolongs to fatigue life, but it is shortened if the viscosity of the oil is too low so the film thickness is insufficient. - Dissipation of frictional heat and cooling
Circulation lubrication may be used to carry away frictional heat or heat or heat transferred from the outside to prevent the bearing from overheating and the oil from deteriorating. - Others
Adequate lubrication also helps to prevent foreign material from entering the bearings and guards against corrosion or rusting.
How lubrication relates to other selection criteria
Lubrication selection and lubricant properties greatly influence the operating temperature, which in turn influences:
- whether you should use grease or oil
- the relubrication interval required for grease
- whether oil lubrication is necessary, because circulating oil can be used to remove heat
Bearing lubrication methods
Bearing lubrication methods for bearings can be roughly divided into grease and oil lubrication. Satisfactory bearing performance can be achieved by adopting lubricating method which is most suitable for particular application and operation condition.
In general, oil lubrication offers superior lubrication; however, grease lubrication allows a simpler structure around the bearings. A comparison of grease and oil lubrication is given in table below.
| Item | Grease lubrication | Oil lubrication |
|---|---|---|
| Housing structure and sealing method | Simple | May be complex, Careful maintenance required. |
| Cooling effect | Poor | |
| Fluidity | Poor | Good |
| Full lubricant replacement | Sometimes difficult | Easy |
| Removal of foreign matter | Removal of particles from grease is impossible. | Easy |
| High speed rotation | Poor | Good |
| External contamination due to leakage | Surroundings seldom contaminated by leakage. | Ofer leaks without proper countermeasures. Not suitable if external contamination must be avoided. |
The main reasons to choose grease are:
- Cost-effectiveness
- Simplicity - grease is easily retained in the bearing and housing, thus requiring less complicated sealing arrangements compared with those for oil lubrication
The main exceptions to choosing grease are in applications where:
- Operating conditions require a grease relubrication interval that is unacceptably short
- Lubricating oil must be used for other purposes (such as in gearboxes)
- Heat removal via circulating oil is required
- Purging or removing used grease becomes cumbersome or expensive to handle