[Weekly Topic] July 5-12, 2011

The purpose of forced lubrication is to improve lubrication conditions, reduce friction and deformation resistance as well as tool and power consumption, enhance the deformation process, and improve product quality.

Self-lubrication features high load-bearing capacity, impact resistance, high-temperature tolerance, and strong self-lubricating ability. It is particularly suitable for applications with heavy loads, low speeds, and reciprocating or oscillating movements where it is difficult to apply lubrication and form an oil film. It is also resistant to water washout as well as erosion and scouring by other acidic substances. A forced lubrication system, including a pump body, a liquid storage tank, an inlet pipe, an outlet pipe, a liquid replenishment pipeline, etc. It can break the air pockets formed by the localized vaporization of the lubricant due to heat generated by operation, thereby improving the lubrication conditions ； By increasing the pressure of the lubricant, forced lubrication can be achieved. 　

Reply 1# sxtblo 1. Hydrodynamic lubrication: Lubricating oil is brought to the friction surfaces through the rotation of the shaft journals in the bearings. Oil pressure is generated due to the viscosity of the lubricating oil and the hydrodynamic effects resulting from the wedge-shaped gaps between the components of the bearings; this creates a load-bearing oil film, which is what is known as hydrodynamic lubrication. The assumption underlying the hydrodynamic lubrication theory is that the lubricant has constant viscosity; in other words, the viscosity of the lubricating oil remains unchanged with pressure at a given temperature ； Secondly, it is assumed that the surfaces in relative frictional motion are rigid, that is, their elastic deformation is not considered under load and oil film pressure. Under the aforementioned assumptions, for general non-overloaded sliding bearings (with a contact pressure of 15 MPa), these assumptions are close to the actual conditions. However, when the surface contact pressure in rolling bearings and gears increases to 400–1500 MPa, the aforementioned assumptions no longer correspond to the actual conditions. At this point, the deformation of the friction surfaces can be several times the thickness of the oil film. Moreover, taking into account both the elastic deformation of the lubricated metal friction surfaces and the change in lubricant viscosity with pressure makes it more practical to study and calculate the patterns of oil film formation, as well as its thickness, the shape of its cross-section, and the pressure distribution within it. This type of lubrication is known as elastohydrodynamic lubrication. 　　2. Hydrostatic lubrication: Through a high-pressure hydraulic supply system, lubricating oil at a certain pressure is forced into the gaps between the friction surfaces of moving parts, via throttle dampers (such as in the gaps of hydrostatic sliding bearings, flat hydrostatic sliding guides, hydrostatic screws, etc.). Before any movement begins, the frictioning surfaces are separated by high-pressure oil, which forces the formation of an oil film; this ensures that the moving parts remain in a state of liquid lubrication even under certain working loads. This type of lubrication is known as hydrostatic lubrication. 　　3. Hydrodynamic and hydrostatic lubrication 　　With the advancement of science and technology, new types of bearings that utilize hydrodynamic and hydrostatic lubrication have emerged in industrial production in recent years. The hydrostatic and hydrodynamic combined bearing takes full advantage of the strengths of both hydrodynamic bearings and hydrostatic bearings, while overcoming their respective weaknesses. Main working principle: During startup or braking, hydrostatic liquid lubrication is employed by forcing high-pressure lubricating oil into the bearing load-bearing area, thereby lifting the shaft and ensuring liquid lubrication conditions. This prevents friction and wear that would occur due to direct contact between the metal surfaces in contact (the shaft journal surface and the bearing surface) when a dynamic oil film cannot be formed as a result of speed changes during startup or braking. When the bearing pair reaches stable operation at full speed, the hydrostatic oil supply system can be stopped, and a hydrodynamic oil film can be formed by using hydrodynamic lubrication oil, thereby maintaining the condition of liquid lubrication for the shaft journals within the bearings. 　　Theoretically, such a method completely eliminates semi-liquid lubrication and boundary lubrication throughout the entire process of starting, operating, braking, and reversing direction of the bearing pair, resulting in liquid lubrication. Therefore, the friction coefficient is very low; it is sufficient to overcome the frictional resistance between molecules within the liquid caused by the viscosity of the lubricant. Furthermore, the friction surfaces are completely separated by a static oil film and a dynamic oil film; therefore, under normal conditions, almost no wear occurs, which **significantly extends the service life of the bearings and reduces energy consumption. 　　4. Boundary lubrication (i.e., boundary friction) Boundary lubrication is the critical state that lies between internal friction between lubricant molecules on the friction surfaces (i.e., fluid lubrication) and direct contact between the friction surfaces. At this time, there is an adsorbed film on the friction interface, with a thickness of approximately 0.1 μm, which provides certain lubricating properties. We call this thin film the boundary film. The lubrication performance of the boundary film mainly depends on the properties of the friction surfaces ； It depends on the structure of the boundary film formed on the metal friction surfaces by the oily additives and extreme pressure additives in the lubricant, and has little to do with the viscosity of the lubricant. 　　5. Extreme pressure lubrication Extreme pressure lubrication is a special case of boundary lubrication. It occurs when the frictioning surfaces are under heavy loads (or high contact stresses), at high speeds, and in high temperatures. The extreme pressure additives present in the lubricant react with the metal friction surfaces to form a chemical reaction film that separates the two surfaces. This film helps to reduce the friction coefficient, slow down wear (or prevent severe wear resulting from direct contact between the metal surfaces), thereby serving the purpose of lubrication. This phenomenon is known as extreme pressure lubrication. 　　6. Solid lubrication 　Lubricants that consist of solid powdered materials placed between the friction surfaces can also provide excellent lubrication effects. There is a solid lubricant between the two friction surfaces; its shear resistance is very low, and even a slight external force will cause slipping between the molecules. In this way, the external friction between the two friction surfaces is transformed into internal friction between the molecules of the solid lubricant. There are two necessary conditions for solid lubrication. First, the molecules of the solid lubricant must have a low shear strength, so that sliding can occur easily ； Secondly, the solid lubricant must have a strong affinity for the friction surfaces; during friction, it should maintain a layer of itself on these surfaces at all times, and this layer of solid lubricant must not corrode the friction surfaces. Generally, it adheres mechanically to the metal surface, but chemical bonding can also occur. There are many solid substances with the above properties, such as graphite, molybdenum disulfide, and talcum powder. 　　For non-layered solid lubricants or soft metals, it is their low shear force that enables lubrication, allowing them to adhere to the friction surfaces and form a lubricating film. The lubrication mechanism of the already formed solid lubricant film can be approximated by the boundary lubrication mechanism to explain its lubricating effect. 　　7. Self-lubrication 　　For the several types of lubrication mentioned above, a lubricant needs to be applied between the friction surfaces during frictional movement. Self-lubrication, on the other hand, involves mixing solid lubricant powders with lubricating properties with other solid materials and then compressing and sintering them into a finished product, or impregnating porous materials with solid lubricants ； Or, a solid lubricant can be directly compressed into a material to serve as the friction surface. In this way, throughout the entire friction process, no lubricant needs to be added, yet good lubrication is still achieved. The mechanisms of self-lubrication include solid lubrication, boundary lubrication, or a combination of both. For example, compressor piston rings, bearing shells, and shaft sleeves made of polytetrafluoroethylene are self-lubricating; therefore, in such components, they can maintain good lubrication without the need for any additional lubricants.