Purposes and methods of lubrication have important effects on the function of the bearing. Whether the lubricant and the method are suitable or not shall influence the bearing life.
The reasons for lubrication:
1) Reducing the friction and wear by lubricating every part of the bearing.
2) Minimizing the heat generated inside the bearing caused by friction and or other reasons.
3) Forming an oil film in the rolling contact surface in order to elongate the bearing life.
4) Preventing the bearing from rusting and dust contamination.
The lubricating methods include oil lubrication and grease lubrication. The comparisons of these two methods are given in Table1.
Talbe1 Comparisons between oil lubrication and grease lubrication
Sealing device
More complicate, requiring maintenance
Lubricating function
Very good
Rotational speed
Low to medium speed
Higher speed applications
Change of lubricant
Life of lubricant
Very short
Cooling effects
Possible (using forced oil circulation)
Elimination of inclusion
Grease lubrication
Grease put inside the bearing can last a comparatively long time without replenishment, and the sealing device is very simple. Therefore it is extensively applied. There are two methods for grease lubrication; one is to put the grease inside the sealed bearings in advance, the other is to fill when inside the housing with a certain amount of grease and refill it or change the grease inside at intervals. Moreover, for machine with several bearings requiring lubrication, the method of centralized greasing through pipes connecting the places to be lubricated is adopted.
1) Amount of lubricating grease
The amount of lubricating grease to be filled in the housing depends on the structure and volume of the housing. Normally it is preferred to fill the volume by 1/3 to 1/2. If too much grease is filled, the grease may turn bad, age or soften due to the heat caused by rotation. But for bearings with low rotation speed, sometimes 2/3 to the whole volume shall be filled with grease in order to avoid the intrusion of foreign matters.

2) The replenishment and change of grease
The replenishment and change of grease is closely related to the lubricating method. No matter which method id applied, the grease must be clean and care be taken to avoid intrusion of dirt. The grease to be replenished should be of the same brand. Try to ensure that the replenished grease has entered into the inside space of the bearing.
Oil lubrication: is applied to high-speed and heat-resistant bearings and is ineffective for reducing vibration and lowering noise. Mostly it is used in cases where the grease lubrication is not suitable. Oil lubrication has the following methods:
(1) Oil bath lubrication           (2) Oil drip lubrication      (3) Splash lubrication
(4) Oil circulating lubrication    (5) Oil jet lubrication      (6) Oil mist lubrication
(7) Oil air lubrication