The lubricating oil film is one of the important properties to protect the internal components of the machine and reduce wear. The strength of the oil film mainly depends on the base oil and additives used in the lubricating oil. This article will take you to take a look at the importance of lubricant oil film strength and the main factors that affect the effect of the oil film.
When it comes to lubrication, what do you think of? It should first produce a thick film to separate the base oil on the two metal surfaces, because the role of lubricating oil is to avoid surface contact between metals. Therefore, under this demand, the oil must be able to provide the ability to separate the friction surface, which requires three supporting factors-relative speed, base oil viscosity and load. These three factors are also affected by temperature, pollution and other factors. When the thickness of the oil film balances these factors, that is, a viscous fluid film is generated by the relative speed to completely separate the two friction surfaces, and the pressure generated by the fluid film balances the external load, it is called hydrodynamic lubrication.
In applications with rolling contact (negligible relative sliding motion), even with a large local pressure point, it may affect the thickness of the oil film between the metal surfaces. In fact, these pressure points also play an important role. The relationship between pressure and viscosity of the base oil allows the viscosity of the oil to temporarily increase due to higher pressure. This is called elastohydrodynamic lubrication. Although the oil film will be very thin, it can still produce a complete oil film separation.
In practice, the ideal state of the machine surface is to achieve complete separation, and the film thickness is to provide the best protection for reducing friction and wear. But if there are no conditions to meet these oil film thicknesses, such as when the relative flow rate is insufficient, the viscosity is insufficient, or the load is too large, what will happen? In fact, the design and operating parameters of most machines allow for insufficient speed, such as when starting, stopping or changing direction. When the temperature is too high, the viscosity will decrease, and excessive pollution will also cause the abrasive particles in the oil film gap to contact.
Non-lubricated surface interaction
If you observe the mechanical contact surfaces at the molecular level under a microscope, you will find that even if they are processed very smoothly, they are actually relatively rough. It's like astronauts looking at the earth from a distant space, the earth is a perfectly smooth sphere, and people standing on the surface of the earth see the earth as full of high and low mountains and valleys.
This is because when two metal surfaces are in contact, the actual contact area will be significantly lower than the apparent contact area. From the "microscopic mountain" under the microscope, these contact surfaces are all the highest points of unevenness, and the low rough surface has a lower contact rate. These rough surfaces will undergo elastic deformation due to the corresponding shear strength of the metal. Therefore, the initial contact point first produces elastic deformation, and then more contact points will be connected, and the actual contact area will increase with the increase of the load intensity.
These friction and wear control additive films on the metal surface reduce the shear strength at the contact point. The low-shear strength film is "sacrificed" in physical interactions to protect the surface from adhesion, abrasive particles, and fatigue wear. These sub-micron films have liquid to solid properties as they get closer to the metal surface. Although base oil is the material of choice for hydrodynamic and elastohydrodynamic lubrication to protect machine surfaces, boundary conditions still exist. Therefore, in order not to be restricted by boundary conditions, a suitable additive formulation with friction and wear control properties should be used to blend the lubricant, so as to ensure the oil film strength proportional to the mechanical interaction within a reasonable limit.
