Harm and treatment of bearing over-lubrication

2022-12-27

Bearing engineers often say that "bearing is the heart of machinery". If so, lubrication can be compared to "blood of machinery". It is interesting to say that the whole machinery industry is suspended on a few microns of oil film. These descriptions illustrate the importance of lubrication for bearings and even the whole mechanical equipment.
Excessive selection and treatment of lubricating viscosity
According to the lubrication mechanism, the viscosity of lubricating base oil directly affects the formation of lubricating oil film. Specifically, the high viscosity of lubricating base oil is conducive to the formation of oil film at the same speed and load. Therefore, it is not difficult for many people to conclude that the higher the viscosity of lubricating base oil, the better. In fact, with the increase of lubricating viscosity, the motion resistance of the rolling body on the raceway surface also increases, which will lead to the increase of the resistance torque of the bearing operation.
For some occasions that require the bearing resistance torque, the bearing will not rotate flexibly. For example, some air conditioning manufacturers require the low temperature starting torque of the motor to be within a certain range. When the bearing is at low temperature, the viscosity of the lubricant will rise due to the temperature reduction, which will lead to the increase of the starting torque of the bearing, thus causing the starting problem.
On the other hand, the resistance torque caused by the high viscosity of the lubricant will eventually dissipate in the form of heat during the rotation of the bearing, so the high viscosity of the lubricant will also cause the temperature of the bearing to rise. In engineering practice, there have even been cases where the grease agglomerates due to the high consistency of the grease, resulting in the grease can not effectively enter between the rolling body and the raceway to form an oil film.
The indicator to determine whether the viscosity of bearing grease is appropriate is kappa coefficient k. At operating temperature, when kappa coefficient k is greater than 4, it is obvious that the viscosity of bearing grease is too high. Therefore, the Kappa coefficient should be checked against the operating temperature when selecting lubrication. When the bearing has heat related to lubrication, it is also necessary to check the Kappa coefficient to ensure the correct selection of bearing grease.
Excessive lubrication
After the bearing is installed, it needs to be filled with enough lubricating grease to ensure that there is a good distribution of lubricating grease between the rolling body and the raceway to form an oil film when the bearing is running. There is indeed a certain quantity requirement for the filling of lubricating grease. A good lubricant distribution is that when the bearing is running, there is a layer of lubricant distribution on the surface of the bearing rolling body and raceway, forming a certain lubricant circulation flow. In this case, the rolling body and raceway are not filled with grease. This is consistent with the common sense seen in daily life. When the bearing is installed, it may be filled with grease inside the bearing. When the grease is properly filled, it will be found that the grease inside the bearing will be squeezed out after being opened for a period of time, and only a layer of grease will be distributed on the bearing surface. It can be seen that the grease distributed between the rolling bodies will be "extruded" during the operation of the bearing. This "extrusion" is only a general adjective, but it can be seen from this process that when the bearing is running, the rolling element has a certain stirring effect on the lubricating grease. If the lubricating grease is always filled inside the bearing, this stirring also causes the increase of the running resistance torque of the bearing, and then causes the bearing to heat.
The basic requirements for the filling of bearing grease are: when the bearing is lubricated for the first time, the inside of the bearing shall be filled completely, and the space in the bearing room excluding the bearing shall be filled with 30% - 50% grease.
After the bearing is put into operation, it is necessary to ensure the normal grease drainage path of the bearing. When there is too much grease inside the bearing, the excess grease will be discharged with the operation of the bearing.
When replenishing lubrication, the appropriate amount is 0.5% D * B. Where D is the outer diameter of the bearing and B is the width of the bearing.
Whether it is initial lubrication or supplementary lubrication, it is recommended not to exceed the above amount, otherwise it may cause excessive lubrication of the bearing.

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