Basic knowledge of seals

Thread Content

1. Function and classification of seals In hydraulic and pneumatic transmission systems and their components, the purpose of installing sealing devices and elements is to prevent the leakage of the working medium as well as the intrusion of external dust and foreign particles. The component installed in a sealing device to perform sealing functions is called a seal. In the working fluids of hydraulic and pneumatic systems, as they flow or remain temporarily within the chambers of the systems and components, changes in pressure, intermittent conditions, viscosity, and other factors can cause a small amount of the working fluid to escape beyond the boundaries of those chambers. This escape from high-pressure areas to low-pressure areas or to the outside environment is referred to as leakage. Leakages are divided into two categories: internal leaks and external leaks. An internal leak refers to the leakage of the working medium from the high-pressure chamber to the low-pressure chamber within the system or component ； External leakage refers to the leakage from inside the system or components to the outside. The volume of working medium that leaks per unit time is called the leakage rate. In hydraulic transmission systems, internal leakage causes a sharp decline in the system’s volumetric efficiency, preventing it from reaching the required operating pressure and thus preventing the equipment from functioning properly ； External leakage leads to waste of the working medium and environmental pollution, and may even cause equipment malfunction and personal accidents. 2. Classification of seals. The function of seals is to prevent leakage. There are mainly two reasons for the leakage: one is the presence of gaps on the sealing surface ； Second, there is a large pressure difference on both sides of the sealing area. Eliminating or reducing either factor can prevent or minimize leakage. Therefore, the common methods of sealing include: 1) Sealing the gap at the joint surface ； 2) Cut off the leakage path ； 3) Increase the resistance in the leakage path ； 4) Install a working element to apply pressure on the leaking medium, in order to counteract or balance the pressure difference in the leakage path. Based on whether there is relative motion between the sealed mating surfaces during equipment operation, seals can be divided into two main categories: static seals and dynamic seals. Furthermore, it can be further subdivided based on the manufacturing material of the seal, its structural form, and the sealing mechanism. 3. Requirements for seal material: The seal material must meet the requirements for sealing performance. Due to the differences in the sealed working medium and equipment operating conditions, seal materials should possess varying degrees of adaptability. General requirements for sealing materials: 1) The material should be dense to prevent leakage of the working medium ； 2) Good adaptability and stability to the working medium ； 3) It has appropriate mechanical strength and hardness, and is less affected by the working medium ； 4) Good compressibility and resilience, with minimal permanent deformation ； 5) Good temperature adaptability; does not soften or decompose at high temperatures ； Does not harden or crack at low temperatures ； 6) Low friction coefficient, good wear resistance ； 7) It has excellent corrosion resistance, allowing it to operate for extended periods in the working medium, with minimal changes in volume and hardness ； 8) Good softness and elasticity of the sealing surface iron box ； 9) It has good ozone resistance and aging resistance, as well as a long service life ； 10) Good processability and low cost. 4. Common rubber sealing materials: The common rubber sealing materials are mainly synthetic rubbers. There are many types of synthetic rubber, and their properties vary from one another. Therefore, when selecting them, in addition to ensuring that they meet the aforementioned usage requirements, it is also necessary to make the proper choice based on the characteristics and application ranges of different types of adhesives, as well as considering the operating conditions of the seals. 5. Common synthetic resin sealing materials: Among common synthetic resins, polytetrafluoroethylene resin is the most widely used. By incorporating different filler materials into polytetrafluoroethylene, its overall physicochemical properties can be improved, thereby expanding its range of applications. Therefore, polytetrafluoroethylene resin sealing materials can be used for sealing working media such as petroleum-based hydraulic oils, water-oil emulsions, water-ethylene glycol-based hydraulic fluids, and phospholipid-based hydraulic fluids. 6. Common metal sealing materials: Metal sealing materials are mainly used for static sealing