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Hydraulic oil is used in hydraulic transmission systems as a working medium, serving to transmit, convert, and control energy. It also performs functions such as lubricating, preventing corrosion and rust, and cooling the various components within the hydraulic system. Seals in hydraulic systems serve to prevent fluid from leaking between mating surfaces, maintain pressure, and ensure the transmission or conversion of energy. Most of the sealing materials used domestically and internationally today are polymer elastomers; under certain special conditions, plastics and various metals are also utilized. But regardless of the type of material, it should possess the following properties: 1. Certain mechanical and physical properties: such as tensile strength, strength at a fixed elongation, and elongation rate ； 2. It has a certain degree of elasticity, appropriate hardness, and minimal permanent compression deformation ； 3. Suitable for the working medium, and does not easily suffer from swelling, decomposition, or hardening ； 4. Wear-resistant, with certain tear resistance ； 5. It possesses aging resistance to high and low temperatures. However, no sealing material possesses all of the aforementioned properties; it is necessary to select an appropriate sealing material based on the working environment, such as temperature, pressure, the medium involved, and the mode of movement, and to formulate the material composition accordingly to meet specific requirements. Alternatively, a composite or combined structure using two or more materials can be employed to leverage their respective advantages and achieve a more comprehensive outcome. Formation of the sealing effect: Dynamic seals are divided into non-contact seals and contact seals. Non-contact sealing mainly refers to various mechanical seals, such as graphite packing rings, floating ring seals, etc ； Both rubber-plastic composite seals and rubber-plastic combined seals belong to contact seals; they achieve a sealing effect by blocking the leakage paths through the pre-compression force applied within the sealing chamber. Seals used in hydraulic systems are mostly static seals (face seals), reciprocating motion seals (piston and piston rod seals), and rotary seals. Factors affecting the sealing performance: the selection of the sealing structure and oil film formation, pressure, temperature, material compatibility, as well as the material, hardness, geometric shape, and surface finish of the working surfaces in contact in dynamic seals. I. Compatibility of sealing materials with hydraulic oil: One source of particle contamination in hydraulic oil is the \"debris\" or \"wear particles\" resulting from incompatibility between the sealing material and the hydraulic oil. The \"debris\" resulting from the damage to the seal due to \"swelling,\" along with the unbound inorganic substances and filler reinforcement materials that are \"extracted,\" cause the seal to be damaged and cease to function. At the same time, this leads to contamination of the oil, resulting in deterioration of the hydraulic fluid and its loss of effectiveness. Vane pumps are widely used in hydraulic systems; when the operating pressure exceeds 6.9 MPa, wear becomes a significant issue, which is why anti-wear agents are added to the hydraulic oil ； To accommodate hydraulic systems in the vicinity of high-temperature heat sources and open flames, flame-retardant phosphate esters, water-ethylene glycol hydraulic fluids, oil-in-water and water-in-oil emulsions are used. There are also a wide variety of hydraulic oils designed for various applications, incorporating various types of composite additives such as antioxidants and rust inhibitors that arise from the need for specific \"uses\". For example, among the types of anti-wear hydraulic oil additives, the zinc-free (ash-free) type of anti-wear hydraulic oil additive is one that uses hydrocarbon sulfides, phosphates, phosphites, etc., as well as S-P-N extreme pressure anti-wear agents containing all three elements of sulfur, phosphorus, and nitrogen. In extreme pressure industrial gear oils, P-S type extreme pressure additives are also the main ones. The reason for \"swelling\" or \"extraction\" of seals is the various chemical elements contained in the hydraulic oil additives and their concentrations; based on the principle of \"like dissolves like,\" these elements have different effects on different seal materials, which in turn determines the resistance of those seal materials to the medium. For example, the concentration of polar phosphorus (P) in Shell Omala 320 gear oil and Shell Omala 460 gear oil is around 300 ppm. As a result, nitrile rubber, due to the presence of acrylonitrile groups, is polar and possesses excellent oil resistance; however, it is not suitable for use in environments with these types of oils. With the continuous development of various types of hydraulic oils, various anti-wear and extreme pressure additives, metal deactivators, demulsifiers, and antifoam additives designed to improve the properties of these oils, it is necessary to verify their impact on the materials used in seals through experiments. II. Evaluation of the oil resistance of sealing materials: To assess the oil resistance of rubber-based seals, standard test oils are generally used; these materials are immersed under the temperature conditions and for the duration specified in the relevant test standards. Their performance is evaluated by comparing the test values before and after immersion, such as changes in material hardness, rate of change in tensile strength, rate of change in elongation at break, rate of volume change, and compression set. No1 standard oil is a \"low-volume increase oil\"; this mixture of mineral oils consists of solvent-extracted components, chemically treated paraffin residues, and neutral oils ； Standard Oil No. 3 is a \"high volume increase oil\" composed of two lubricant components obtained through vacuum distillation of selected naphthenic crude products. Different applications have different requirements for the performance metrics of materials. III. The impact of hydraulic oil on the performance of seals: Seal theory suggests that there is a complete lubrication film between a dynamic flexible seal and its mating surface. Under normal conditions, it is this lubricating film that helps achieve sealing and prolongs the lifespan of the seal. In hydraulic systems, the sealing performance of seals is influenced by various factors related to the hydraulic fluid itself, such as the compatibility between the fluid and the seal material, the viscosity of the oil, how its viscosity changes with temperature, the speed of reciprocating (rotating) motion, pressure, as well as the material, hardness, geometric shape, and surface finish of the shafts, cylinder bodies, or piston rods. Additionally, the structure of the seals also affects the formation of the lubrication film and its properties, all of which influence the sealing effect. Generally speaking, forming a continuous lubricating film can achieve an ideal sealing effect. The Japanese company NOK was the first in the world to use the latest image processing technology to explain the sealing principle of oil seals. It is believed that the sealing mechanism of oil seals (rotating dynamic seals) consists of two aspects: lubrication properties and sealing principles. Lubrication properties: The frictional properties of an oil seal are determined by the viscosity of the fluid and the sliding speed. The relative sliding surfaces between the oil seal and the shaft move in a lubricated state with a separated oil film, which helps to keep frictional resistance low and wear minimal. Sealing principle: The flow of oil on the sliding contact surface of the oil seal is a cycle that moves from the atmospheric side to the oil side, and then back from the oil side to the atmospheric side. The sliding surface is well lubricated, which prevents wear from progressing and thus eliminates leakage. When the system’s operating speed is too high, it hinders the formation of a continuous lubricating film, leading to an increase in frictional heat. If this heat exceeds the temperature tolerance of the sealing material, it results in damage to the seal. When the pressure is too high, it not only affects the formation of the oil film but also exerts a \"squishing\" effect on rubber and plastic seals; this can generally be mitigated by using retaining rings. Alternatively, choosing a PTFE composite seal structure with a low friction coefficient and good self-lubricating properties can help improve lubrication between the frictioning surfaces; it is suitable for sealing cylinder pistons and piston rods in applications involving high and low speed reciprocating motion as well as high-pressure systems. Selecting seals and sealing materials appropriately based on the system’s operating conditions and the medium environment, as well as installing, using, and maintaining them correctly, are key to achieving effective sealing performance, reliability, and a long service life.