The lubricating oil film formed by the lubricating grease applied between the metal surfaces in contact with each other can bear the load from the axial and radial directions. The lubricating oil film has the characteristics of bearing the load, which is called the extreme pressure of the grease.
As the load increases, the thickness of the oil film between the metal surfaces becomes thinner. When the load increases to a certain extent, the continuous oil film is destroyed on the top of the metal surface, and the lubrication state in which direct contact between some metal surfaces is called boundary lubrication .
In boundary lubrication, when only a moderate load is applied to the metal surface, and when there is an additive that is adsorbed on the metal surface or violently wears against the metal surface, the additive is called a wear-resistant additive.
When a high load is applied to the metal surface, a large number of metal surfaces are in direct contact and a large amount of heat is generated, and the film formed by the anti-wear agent is also destroyed, which cannot protect the metal surface. For example, the additives chemically react with the metal surface to form a chemical reaction film, which plays a lubricating effect, prevents damage to the metal surface and prevents welding. Generally, this most severe boundary lubrication is called extreme pressure lubrication, and this additive is called extreme pressure additive.
Grease used under severe conditions adds extreme pressure agents to improve extreme pressure. At present, a four-ball testing machine is generally used to determine the oil film strength of the grease.
