Bearing engineers often say that “bearings are the heart of machines”, in which case lubrication can be compared to “the blood of machines”. There is an interesting saying that the entire mechanical industry floats on a few micrometers of oil film.
These expressions highlight the importance of lubrication for bearings and, in fact, for all mechanical equipment.
From the mechanics of lubrication, we know that the ideal lubrication state is when the friction surfaces are well separated by a lubricant, eliminating any direct contact between them. For bearings, this means using an oil film to separate the rolling elements and the raceway.
If lubrication is poor or insufficient, the rolling elements and bearing race cannot be separated properly, resulting in direct metal-to-metal contact, which can lead to surface degradation. Engineers are generally well aware of the problems caused by insufficient lubrication.
On the other hand, lubrication also involves a question of “degree”. As the common saying goes, “excess is as bad as deficiency”. In fact, over-lubrication can have significant negative impacts on bearings.
Typically, there are two main aspects of over-lubrication: first, excessive lubricant viscosity (excessively high viscosity); and second, excessive amount of lubricant.
Excessive selection and treatment of lubricant viscosity
According to the lubrication mechanism, the viscosity of the base oil directly affects the formation of the lubricating film. Specifically, under the same rotational speed and load, a higher viscosity of the base oil facilitates the formation of an oil film.
As a result, many people can easily conclude that the higher the viscosity of the base oil, the better. However, with an increase in the viscosity of the lubricant, the resistance to movement of the rolling element on the raceway surface also increases, which leads to an increase in the torque resistance of the bearing.
In situations that require specific bearing resistance torque, this may cause the bearing rotation to become less flexible. For example, some air conditioning manufacturers require the engine's low-temperature starting torque to be within a certain range.
When the bearing is at a low temperature, the decrease in temperature causes the viscosity of the lubricant to increase, leading to an increase in the bearing's starting torque and consequently resulting in starting problems.
On the other hand, the resistance torque caused by the excessive viscosity of the lubricant will end up being dissipated as heat during the rotation of the bearing. Therefore, an excessively high viscosity of the lubricant can also lead to an increase in bearing temperature.
In practical engineering, there have been cases where excessive grease viscosity has caused the lubricant to clump, preventing it from effectively entering between the rolling element and the raceway to form an oil film.
A suitable indicator for evaluating whether the viscosity of bearing grease is appropriate is the kappa number k. At working temperature, when the kappa number k is greater than 4, it indicates an excessively high viscosity of the bearing grease.
Therefore, when choosing lubricants, it is essential to check the kappa number in relation to the working temperature. If the bearing experiences lubrication-related heating, the kappa number should also be checked to ensure correct bearing grease selection.
Excessive lubrication
After bearing installation, a sufficient amount of grease must be filled in to ensure the formation of an oil film through optimal grease distribution between the rolling elements and the raceway during operation.
In fact, a certain amount of grease is required for optimal lubrication. During bearing operation, a layer of lubricant is distributed over the surface of the rolling elements and the raceway, forming a certain cyclic flow of lubricant.
However, the space between the rolling elements and the raceway is not filled entirely with grease. This observation is in line with common knowledge; During bearing installation, the bearing may become filled with grease.
When the amount of grease is adequate, after some time of operation and after inspection, it can be seen that the grease inside the bearing is expelled, leaving only a layer on the surface of the bearing.
This indicates that during bearing operation the grease distributed between the rolling elements is “squeezed”.
This “squeezing” is an approximate description, but it shows that during bearing operation, the rolling elements agitate the grease. If grease continually fills the inside of the bearing, this agitation also leads to an increase in the bearing's rotational torque, resulting in the bearing overheating.
The basic requirement for bearing grease filling is as follows: during the initial lubrication, the entire inside of the bearing must be filled, and 30%-50% of the grease must be filled into the bearing chamber that does not contain the bearing.
After the bearing is put into operation, it is crucial to ensure the normal discharge path of the bearing grease. When there is excess grease inside the bearing, the excess will be expelled when the bearing operates.
During supplemental lubrication, the appropriate amount is 0.5% of D*B, where D is the bearing diameter and B is the bearing width.
Whether it is initial or additional lubrication, it is recommended not to exceed the above-mentioned amount as this may result in excessive lubrication of the bearing.
