Basic Knowledge of Wear and Lubrication

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Wear is the phenomenon in which the material from the surface layer is continuously lost as the surfaces of objects in contact move relative to each other. Based on the length of time the wear is delayed, it can be divided into natural wear and accidental wear. The causes of wear due to accidents include the following factors: defects in the machine’s design, poor quality of the materials used for components, inadequate manufacturing and processing of parts, incorrect assembly or installation of components or the machine itself, violation of the machine’s safety and lubrication procedures, poor quality of repairs or repairs that were not carried out properly, as well as other accidental reasons. Under normal circumstances, when natural wear reaches its limit and is not repaired in time, it becomes the main cause of accident-related wear. II. Lubrication 1. Functions of lubricants (1) Cooling and heat dissipation function. It prevents the temperature of components from rising due to heat generated by friction, which could lead to increased adhesive wear and corrosion wear, as well as accidents such as the damage of rubber seals or bearing shells. (2) Sealing and protection function. Lubricating grease can effectively protect against the erosion caused by moisture, oxygen, and harmful substances in humid air; it also prevents the leakage of internal substances. Furthermore, lubricating grease stops water, dust, and impurities from reaching the friction surfaces. (3) Dirt-washing function. When the friction pair moves, abrasive particles are generated, and external impurities and dust can also exacerbate wear on the friction surfaces. Forced liquid circulation lubrication can carry away these abrasive particles, thereby reducing or preventing wear. (4) Reduce friction and wear. Since the lubrication film can reduce the friction factor between two moving parts, it reduces wear and degradation of the components. It also serves to provide damping and vibration absorption, thereby extending the lifespan of the equipment, reducing power consumption, and improving its operational performance. 2. Types of lubricants: Lubricants can be classified into four categories based on their state: liquids (lubricating oils), semisolids (greases), solids, and gaseous lubricants. (1) Lubricating oil: Also known as light oil, its physicochemical properties include good fluidity ； Low freezing point ； Appropriate viscosity and viscosity coefficient ； Good load tolerance ； Good oiliness, compressibility resistance, and wear resistance ； Good corrosion resistance and rust prevention ； It has a certain degree of refinement ； Low ash, residual carbon, and acid value ； Good thermal stability, not easily volatile or flammable, with a high ignition point and flash point ； Good antioxidant safety, resistant to aging and deterioration ； Good water repellency and anti-emulsification properties ； It should have a certain degree of antifoaming property. Principles for selecting lubricating oil: a. To reduce energy consumption, lubricants with low viscosity should be preferred, provided that this ensures the safe operation of the machine’s friction components. b. Lubricants with low viscosity should be used for friction parts operating under high-speed, high-load conditions, while lubricants with high viscosity should be used for friction parts operating under low-speed, high-load conditions. c. When the ambient temperature is low, lubricants with low viscosity should be used; conversely, lubricants with high viscosity should be used ； Lubricants with a high flash point should be used under high-temperature conditions ； Lubricants with a low freezing point should be used under low-temperature conditions. d. Shock, vibration, as well as reciprocating and intermittent motions are detrimental to the formation of an oil film; therefore, lubricants with higher viscosity, or greases or solid lubricants, should be used to ensure reliable lubrication. e. Lubricants with low viscosity should be used for friction pairs with a small clearance, as well as for working surfaces with high surface finish precision. f. Under mechanical cycling conditions, lubricants with lower viscosity should be used; when refilling the lubricant intermittently, those with slightly higher viscosity are preferable ； For vertical lubrication surfaces, as well as exposed gears, chains, wire ropes, etc., lubricants with a higher viscosity should be used. g. If no lubricant of the appropriate grade is available, a lubricant of a similar grade can be used as a substitute or blended with another type of lubricant. When using a substitute, it should be one with a viscosity slightly higher than the specified value; when blending lubricants, it is best to avoid combining those with different properties, different manufacturers, or those containing additives. (2) Grease Grease, also known as dry oil, is a semi-solid lubricant. It is a plastic lubricant with a colloidal structure, formed by mixing lubricating liquids such as mineral oil with thickeners. Principles for selecting grease: a. Choose grease with a high penetration value under high-speed and light-load conditions ； Lubricants with a low penetration value should be selected under conditions of shock, vibration, and intermittent operation. b. In winter or under low-temperature conditions, greases formulated with oils having a low freezing point and low viscosity should be used ； In summer or under high-temperature conditions, grease with a high drip point should be selected. c. When the fit clearance is large and the surface is rough, use a lubricant with a low penetration; otherwise, use a lubricant with a high penetration. d. Environmental conditions: Calcium-based greases are generally recommended in humid conditions, while sodium-based greases are usually preferred in high-temperature conditions. (3) Solid lubricants: There are many substances that can be used as solid lubricants, including metals, metal compounds, inorganic materials, and organic materials. Among them, molybdenum disulfide and graphite are the most commonly used lubricants. Molybdenum disulfide lubricant is a powder with a lead-gray to black luster, characterized by excellent lubricating properties, adhesion, heat resistance, wear resistance under compressive loads, as well as oxidation and chemical corrosion resistance. It provides superior lubrication performance for equipment operating in conditions of high speed, high temperature, high load, low temperature, or exposure to chemical corrosives. The molybdenum disulfide lubricants currently available include powders, aqueous solutions, oils, ointments, greases, and solid film-forming agents such as crayons. The “five fixes” for equipment lubrication: fixed points, fixed quality, fixed quantity, fixed intervals, and designated personnel. Use lubricant quality monitoring techniques: Identify key equipment and those that consume large amounts of oil, determine the parameters to be tested for oil quality, such as kinematic viscosity, moisture content, acid value, water-soluble acids and bases, and mechanical impurities. Establish reasonable criteria for oil replacement, set specific targets, and conduct regular sampling and testing analyses (usually every 3 months). Based on these results, decide whether an oil change is necessary. For new equipment and important precision equipment, it is also possible to extend or shorten the testing interval by relying on visual inspection.