Many engineers often believe that the more lubrication added to a bearing, the better, given the common understanding of its importance. This practice is prevalent in many engineering applications.
It is easy to understand the significant impact that insufficient lubrication can have on bearing operations. However, people rarely contemplate the effects of excessive lubrication. Is more lubrication always beneficial for bearings?
To answer these questions, let's discuss the functions of lubrication.
First, we must understand why we add lubrication to bearings.
1. The function of lubrication
Reducing friction and wear
A bearing is a mechanical part with relative movement between its internal components during operation. These moving parts are typically made of steel and lubricant is filled into the gaps between these parts (rolling elements, inner ring, outer ring, cage). Once operational, a film of lubricating oil forms on the component surfaces.
This film reduces or eliminates direct and hard contact between metals, thus reducing friction between the internal parts of the bearing. This not only prevents wear, but also reduces the heat generated.
Cooling and Heat Dissipation
Whether using grease or oil for lubrication, the heat generated by rolling or sliding friction is dissipated by conduction, which serves to cool and reduce the temperature.
Rust and corrosion prevention
The lubricant used in bearings also has excellent anti-rust properties, which can prevent bearings from rusting.
From the characteristics mentioned, we understand that lubrication is an indispensable part of the bearing's operation. Furthermore, when calculating bearing life, lubrication is a crucial factor in the equation.
Having understood the function of bearing lubrication, we pay close attention to adding grease, often preferring to “add too much” rather than risk “adding too little”. This is because, based on practical experience, engineers feel that adding a little extra provides an extra layer of security, which can't be wrong.
However, reality is often not the case.
2. Insufficient lubrication…
We can easily understand the condition of insufficient lubrication, where the metallic components of the bearings are not fully segregated. There is still metal-to-metal contact, causing wear that increases the temperature and can even generate wear debris inside the bearings.
But what about excessive lubrication? What impacts does this have?
3. Excessive lubrication…
When bearings operate, a lot of friction is generated internally, including 1) rolling friction and 2) sliding friction. According to modern bearing friction theory, we must also consider 3) seal friction and 4) grease friction.
“Wait, does grease cause friction too?”
Yes, that is indeed the case.
In bearings, friction includes the four aspects mentioned above, represented by the following formula:
M = M n +M SF +M seal +M drag
Where:
- M: Total internal friction torque of the bearing
- M n : Rolling friction torque
- M SF : Sliding friction torque
- M seal : Built-in sealing friction torque (for starter bearings, the built-in sealing friction torque is 0)
- M drag : Loss of lubrication drag
When grease is added to the inside of bearings, excess grease can cause increased lubricant agitation. This, in turn, increases the frictional torque of the grease, resulting in an overall increase in friction.
The result is not only a decrease in grease cooling performance, but also an increase in overall operating temperature. Additionally, adding too much grease can put extra pressure on the seals, causing leakage incidents.
So, with the advancement of modern performance in bearing manufacturing, our concern is not insufficient lubrication, but rather over lubrication. It is vital to maintain an adequate amount of grease, including initial lubrication and relubrication.
