What factors should be considered when matching friction pairs?
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Friction pairs generally use a combination of one soft material and one hard material. Currently, carbon-graphite (hereinafter referred to as graphite) is commonly used for soft rings, while there are many different materials available for hard rings. A hard-to-hard pairing is only chosen in special cases, such as when the medium contains solid particles, there is crystal precipitation, or in highly viscous fluids. When a soft and a hard pair is used, the width of the end face of the hard ring is greater than that of the soft ring; this restriction does not apply to hard-to-hard materials. To reduce leakage and the possibility of mechanical impurities entering the sealed surface, an internal flow-type sealing structure should be used as much as possible. Materials with good thermal conductivity are chosen for the moving ring to dissipate more heat and reduce the temperature of the sealed surface. To ensure the overall strength and stiffness of the sealing ring, its wall thickness must not be too thin. For materials with good thermal conductivity, increasing the ring thickness facilitates heat dissipation ； For materials with poor thermal conductivity, the thickness can be smaller. For sealing rings under high stress, they need to be redesigned from a mechanical perspective, and reinforcement measures should be taken if necessary. The ring should have an axially symmetric shape to reduce its asymmetric mechanical and thermal deformation. When compensation is provided by the moving ring, such as with domestic 104 and B104 types of seals (Figures 13 and 14), the clearance between the moving ring and the shaft (shaft sleeve) should be between 0.40 and 0.60 mm, in order to prevent the moving ring from getting stuck during operation and thus losing its compensatory function ； The inner diameter of the static ring should be 1~2 mm larger than that of the shaft (shaft sleeve) to prevent friction between them. The width of the sealed end face is an important parameter that needs to be considered comprehensively.
Several factors need to be considered when selecting materials for friction pairs: 1. Operating conditions: It is necessary to choose an appropriate material combination based on the properties of the working medium (such as whether it is corrosive or contains solid particles), temperature, pressure, and sliding speed. 2. Material hardness: Generally, pairing soft/hard materials enables better adaptation to the irregularities and wear on the sealing surface, thereby extending its service life. Hard-to-hard material pairing is used in special cases, such as when there are solid particles or crystalline deposits in the medium. 3. Width of the sealed end face: The width of the end face of the hard ring is generally greater than that of the soft ring; however, there is no such restriction for hard-to-hard material pairings. Furthermore, the width of the sealed end face also needs to take into account factors such as sealing performance and heat dissipation capacity. 4. Structural design: An internal flow-type seal can reduce leakage and the likelihood of mechanical impurities entering the sealing surface. 5. Thermal conductivity of the material: To dissipate more heat and reduce the temperature of the sealed end face, a material with good thermal conductivity should be chosen for the moving ring. 6. Wall thickness of the ring: It is necessary to ensure the overall strength and stiffness of the sealing ring; for materials with good thermal conductivity, the ring thickness can be increased to facilitate heat dissipation ； For materials with poor thermal conductivity, the wall thickness can be appropriately reduced. 7. Applied pressure: For sealing rings subjected to high pressure, they need to be designed from a mechanical perspective, and reinforcement measures should be taken if necessary. 8. Geometric shape: The shape of the ring should remain axially symmetric in order to reduce mechanical and thermal deformation. 9. Compensating capacity: A certain gap should be maintained between the moving ring and the shaft to prevent jamming during operation ； The inner diameter of the stationary ring should be slightly larger than the diameter of the shaft to prevent friction. In summary, when selecting materials for friction pairs, it is necessary to consider not only individual factors but also various requirements and constraints in order to achieve the best sealing performance and the longest service life. .