1. Lubricant effect Lubrication is very necessary for the normal operation of the bearing. The main functions of lubricants are as follows:
- Reduce friction between sliding surfaces inside bearing components
- Reduce or prevent direct metal contact between rolling parts and raceways.
- Protect metal surfaces from corrosion
- Play a certain sealing effect on external contaminants
- Provide heat conduction (when lubricating oil is used)
2. Lubricant type There are two basic types of bearing lubricants: lubricating oil and grease.
The lubricating oil can be a natural mineral oil with additives to prevent rust and oxidation, or it can be a synthetic oil. The base of synthetic oil is usually polyolefin olefin (PAO), polyenol (PAG) and esters. Although the two look similar, they cannot be interchanged due to their different nature. Among them, mineral oil is the more common one of these two oils.
The grease is usually composed of about 85% of mineral or synthetic oil, and the rest of the grease is composed of thickeners. It is the presence of this thickening agent that gives the impression that grease is a more viscous oil. However, it is the oil in the grease that really plays the role of lubrication, and its viscosity is usually lower than that of lubricating oil.
Generally speaking, grease should fill about one-third to half of the internal space, although this varies depending on the bearing structure and internal space. It must be remembered that excessive amounts of grease will generate heat when agitated, which can modify, deteriorate or soften the grease. When the bearing is running at low speed, it can be filled to more than two-thirds.
3. Advantages and disadvantages of lubricants
Each type of lubricant has its own advantages and disadvantages, which need to be selected according to the nature of the application. The main advantages and disadvantages of the two basic types of lubricants are shown in the following table:
Lubricant
| advantage
| shortcoming
|
lubricating oil
| Easy to disperse, splash to lubricate other parts, low resistance, easy to discharge and replace. More suitable for high temperature.
| Possible leaks (environmental issues) leading to reduced lubricating oil
|
grease
| Keep it in place, it is not easy to leak, and the seal can be improved.
| It takes effort to clean and replenish the grease. High temperature grease is expensive.
|
4. Important Index of Lubricant-Viscosity
For bearing lubricants, the most important performance is its viscosity. Viscosity is a measure of friction or resistance to flow in a fluid. If it is oil, the viscosity value will be oil viscosity. If it is a grease, it usually indicates "base oil viscosity" or other similar terms. Viscosity values are usually expressed in cSt @ 40° C and cSt @ 100°C values. The difference in viscosity at different temperatures is the viscosity index (VI). The viscosity of the oil is related to the thickness of the oil film it can form. This thickness is crucial for the separation of the rolling and sliding elements in the bearing. Generally, as the temperature increases, the viscosity value of the oil drops very quickly. Therefore, it is very important to choose a lubricant that can provide the smallest acceptable viscosity at the bearing operating temperature.
