PTFE (polytetrafluoroethylene) composite material is a new type of composite material which is composed of two or more different properties or different phase materials containing PTFE, which are blended, copolymerized or combined to have better performance than the constituent materials. Through material compounding, it not only exerts the excellent characteristics of PTFE, but also overcomes the shortcomings of cold flow creep and non-wear resistance of PTFE, so that it has antifriction, wear resistance, self-lubrication, corrosion resistance, heat resistance, aging resistance and resistance. Compression, creep resistance, dimensional stability and low coefficient of linear expansion, and improved thermal conductivity and hardness, reducing costs. This new type of composite material has been used at home and abroad to make seals for special working conditions, such as high pressure, high temperature, high speed, high vacuum, flammable and explosive, toxic, highly corrosive medium, etc. These seals are widely used in petrochemical industry. In the chemical fertilizer, pesticide, aerospace, aerospace and nuclear industries, it is often used as a seal for reciprocating axial movement, such as the sealing of pistons and piston rods. It can be used for dry friction and oil-free lubrication, and oil lubrication can be carried out when conditions permit.
In order to enhance the elasticity of the PTFE composite seal, it must be combined with an elastomer, a compression ring, a convex seat, a concave seat, and various forms of metal springs to form an elastically actuated seal. The common use of zui is metal spring actuation, which is to place a spiral flat spring or round wire spring in an annular space made by machining or molding in a sealing ring to ensure high elasticity and wear of the seal. Automatic compensation to create an excellent dynamic seal.
2.1 J-shaped and L-shaped PTFE seals J and L-shaped seals are commonly used to add graphite, glass fiber, bronze, molybdenum disulfide and various metal oxides to improve and enhance the properties of PTFE. It is characterized by low friction and can withstand dry friction; the wear rate of the lip and shaft surface is low; the temperature is applicable from -50 to 200 ° C; oil and corrosion resistance; and excellent sealing. For example, an L-shaped sealing ring made of PTFE-added glass fiber or the like is used for hydraulic and pneumatic pistons. If 15% glass fiber is added, the coefficient of thermal expansion can be reduced by 1/2. The L-shaped seal can be used as a one-way or two-way piston seal.
The J-ring seal acts as a seal for the piston rod and is typically mounted on the housing as a fixed seal for sealing the reciprocating shaft. The bottom of the sealing ring is clamped in the recess of the housing and supported by the gland. The lip end is always facing the positive pressure side, and the inner diameter of the lip is in contact with the shaft so that the seal can be achieved by the self-tightening principle, and the lip can move freely along the shaft surface.
2.2 U-ring seal U-shaped seal made of PTFE as the base material and then added with filler, can be installed on the moving piston, and can be sealed as a fixed seal on the casing (Fig. 29.6-4), without Need to adjust the packing gland.
The U-shaped seal is a balanced seal that seals both inside and outside diameter and is equipped with a support ring to protect the seal (Figure 29.6-4a) from being crushed or twisted under pressure. The U-shaped ring is installed in the sealing groove, and the gap between the pressing ring and the lip is 1.6~3mm, that is, at least 25% of the thickness of the lip, so that the lip does not touch the pressing ring, so that the system fluid can enter freely, so that the self-tightening principle can be utilized. Sealed. The transverse holes in the vertical web of the pressure ring ensure a pressure balance within the U-shaped opening. Figure b shows the use of a rubber O-ring to increase the static and dynamic contact forces to improve the initial sealing effect.
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Figure 29.6-4 Installation of U-ring seal
a) U-shaped seal diagram with piston movement; b) U-shaped seal diagram with O-ring
2.3 V-ring seals V-rings can be made by adding glass powder, graphite or molybdenum disulfide to PTFE. The V-rings are mounted in groups of multiple rings with a convex seat and a concave seat (Fig. 29.6-5). This will improve the sealing effect and service life. Figure c shows the use of a wave spring to increase the automatic contact force, thereby preloading and automatically compensating for the wear of the V-ring.
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Table 29.6-5 V-rings and components
a) V-ring assembly, b) convex and concave seats; c) V-rings and components with wave springs
The inner contour of the cross section of the lip is a curve, and the outer contour line is a straight line, so that two adjacent loops are in point contact for higher flexibility. The curve of the lip causes the sealed seals to form a single line contact, forcing the seals to flex with a small friction of Zui against the 45° bevel of the adjacent loop.
The function of the concave seat is to prevent the seal from deforming when the pressure rises. The convex seat helps the lip to function automatically and produces a reliable seal when subjected to pressure.
Suitable spring sizes are such that the average load on the circumferential length of the seal is 18 to 27 N/cm. The number of V-rings in each packing seal assembly is 3-5.
2.4 Other sealing rings Figure 29.6-6 shows that fluorocarbon sealing is made of PTFE with graphite, molybdenum disulfide and other mixtures. The wear resistance of this material is hundreds of times that of pure PTFE. The sporting life is longer than the silicone rubber seal. The temperature range is -40 to 200 ° C, and the sealing pressure is 70 MPa.
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Figure 29.6-6 Fluorocarbon seal for reciprocating shaft
This seal is used for the sealing of the reciprocating shaft. The seals are fitted with various mechanical springs (available in stainless steel) with round and V-shaped cross sections, and are available in open PTFE composites. The seal must be compressed during installation so that an initial seal can be created without pressure. Because of its self-sealing mechanism, sealing effectiveness increases with increasing pressure. The seal should be inserted so that the opening of the C-sleeve always faces the pressure side of the system to form a self-tightening seal. The open spring material should be selected according to the corrosive nature of the fluid. When a metal spring is used, an O-ring elastomer can be used instead.
The ring groove sealing anti-PTFE composite material is made up of various small grooved plates and combined with other elastic extrusion type seals to form a ring groove seal, see Figure 29.6-7, PTFE composite material plate sealing contact surface With a small zigzag groove. These circumferential small grooves form a number of individual compartments like a labyrinth, causing a pressure drop in each of the grooves, thereby dividing the total pressure into a series of pressure differences created by the ring grooves, which are stored with a small amount of lubricating fluid. The reservoir keeps the sealing element constantly running on the liquid film, reducing friction and wear.
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Figure 29.6-7 Small groove type seal structure
