The metalworking industry is known for its unceasing improvements in production processes due to the ever-rising demand. Faster machines and automatic metalworking processes require a wide variety of lubricants to sustain high performance in high speeds. Inefficient lubrication can lead to various problems like overheating and contaminants entering the system. Lubrication mechanisms in metalworking are tactful because the speed, load involved, workpiece interface and geometry of the metalworking tool, resist the existence of a lubricant film. In this article, we are going to discuss some lubricating mechanisms which are prevalent in the metalworking industry.
Also known as hydrodynamic lubrication, the thick-film lubrication is a full-fluid film which separates both the surfaces to be lubricated by continuously flowing in between them. The breadth of the fluid film is approximately ten times the magnitude of roughness of the two surfaces. This film can be developed by wedge effect from the presence of a viscous lubricant at the interface of the two surfaces, or by entrapping the lubricant (hydrostatically). This is why bulk properties rather than surface effects are important in this type of lubrication.
Thick-film lubrication is normally used when the loads are light and speed is high. This results in a very low coefficient of friction that ranges from 0.001 to 0.002. Only a third material (a foreign body in the system) is capable of generating wear. This type of lubrication only occurs in die-work piece interfaces in isolated regions.
This type of lubrication is called quasi-hydrodynamic regime or thin film lubrication, it occurs when the film thickness in the thick-film lubrication is reduced. There are various methods for reducing the film thickness, like by – decreasing the viscosity, decreasing the sliding speed or increasing the load. In this type of lubrication, the load is partly supported by the fluid film in hydrodynamic pockets present in the surface roughness of the metal and partly by the metal-to-metal contact of the working tool and the work piece.
This thickness of the film is usually less than three times the surface roughness; hence the coefficient of friction goes as high as 0.4, which considerably increases the forces, power consumption and wear generation. Lubrication occurs from two main sources -
Lubricant entrapped in the surface roughness of the metals and from the hydrodynamic pockets between two metal surfaces; this is why the surface roughness of 15 microns is commonly recommended.
Boundary lubrication occurs on the surface of the metals by the use of molecular or chemical adhesion; a thin layer of oils, fatty acids, fatty oils or soaps are used as lubricants. Their reactivity on materials such as stainless steel and titanium is very low, but they can still form quickly on clean surfaces. To enhance such lubrication, it is better to have boundary lubrication on the surface of the metalworking tool instead of lubricating the workpiece.
Unlike full-fluid-film lubrication, the chemical properties of the lubricant with regards to the surface of the metal rather than the bulk properties of the lubricant are important. Viscosity plays a secondary role in boundary lubrication. The coefficient of friction stays between 0.1 and 0.4; it depends primarily on the thickness and strength of the boundary film. The main source of wear generation in this type of lubrication is through desorption due to excessive heat or through friction generated in the process of metalworking. The effectiveness of boundary lubrication completely depends on the strength and adherence of the film; as soon as the protective boundary layer is destroyed, wear and friction increases.
This type of lubrication requires activating of the metal surface by irreversible chemical reactions. These reactions involve sulphur, phosphorus and chloride and they form salts on the mating surfaces of the metals. The surfaces that are activated reduce or prevent wielding of larger magnitudes at the interface even when contact pressure between workpiece and metal working tool is high. This is why the lubrication is termed as “Extreme pressure”. Furthermore, due to low sheer strength, friction is also reduced in such surfaces.
An increase in temperature can break down the lubricating film. The temperature, at which the film will break down, depends on the additives of the Extreme pressure lubricant such as chlorine and sulphur; and the properties of the metal surfaces. The wear and friction increases only after the breakdown of the lubricating film in this type of lubrication as well but the protective films of chlorides and sulphates form again relatively easier on clean new surfaces. Water, humidity, Air and Oxygen are important to consider in Extreme Pressure Lubrication.
These were some of the Major Lubricating mechanisms involved in Metalworking. Note that the primary concern of any lubrication program for metal working is reducing friction and wear of the metalworking process. Carefully evaluate the requirements of your system to find the best-suited method of lubrication and use high-performance lubricants from reputed companies for preventing unexpected maintenance breaks.