Rotary seal

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Rotary seals I. Rotating shaft lip seals (oil seals) Rotating shaft lip seals are commonly known as oil seals. It is a dynamic sealing element installed between the rotating shaft and the stationary parts, used to prevent lubricant leakage and stop external dust and debris from entering the equipment. The operating pressure for ordinary oil seals is less than 0.5 MPa, while the operating pressure for pressure-resistant oil seals can reach 1–1.2 MPa. The operating temperature range for oil seals is as follows: those made of nitrile rubber operate at -40~+120°C, those made of fluororubber at -25~+200°C, and those made of acrylate rubber at -20~+150°C. The operating linear speed of oil seals is generally less than 15 m/s. Classification of oil seals: Based on the rotational linear speed of the shaft, they can be divided into low-speed oil seals (less than 6 m/s) and high-speed oil seals (greater than 6 m/s). Based on the pressure that the oil seal can withstand, they can be classified as normal-pressure oil seals and pressure-resistant oil seals. Classified by the structure and sealing principle of oil seals, they can be divided into standard oil seals and dynamic return oil seals. Classified by the material of the components that make up the oil seal, they can be further divided into skeleton-type oil seals and skeleton-free oil seals ； There are spring-type oil seals and springless oil seals. Generally, the requirements for oil seals are: 1) Simple structure, easy to manufacture and install/remove ； 2) The dimensions of the installation chamber are compact, making it easy to process ； 3) It has good sealing performance and can maintain its sealing effect for a long time ； 4) It has good adaptability to shaft oscillation and eccentricity. 1. Sealing principle: The installation of the oil seal on the rotating shaft is as shown in the figure below. In its free state, the inner diameter of the oil seal is smaller than the shaft diameter, resulting in a certain amount of interference. In this way, once the oil is sealed onto the shaft, a certain radial force acts on the shaft even in the absence of a spring. To ensure the reliability of the seal, a spring is installed above the oil seal lip. It relies on the clamping force of the spring on the shaft to overcome the gaps caused by vibration and play while the shaft is rotating, ensuring that the lip of the oil seal remains in close contact with the surface of the shaft at all times. The key to maintaining the sealing performance of an oil seal is the oil film between the lip and the shaft surface. Therefore, the sealing principle of oil seals is based on the lubrication theory of sliding bearings. During the dynamic rotation of the shaft, the thickness of the oil film also changes continuously, with the variation generally ranging from 20% to 50%. The reason for the variation in oil film thickness is the continuous fluctuation of the surface tension that maintains the oil film. The surface tension of the oil film is related to factors such as viscosity and velocity. It is generally believed that leaks are likely to occur after the oil film forms ； If the oil film is too thin, it will lead to dry friction. The ideal situation is to maintain a “critical oil film thickness,” that is, to always be in a state of critical lubrication. The formation and maintenance of the \"critical oil film thickness\" are directly related to the magnitude and distribution of the radial force exerted by the oil seal on the shaft. The ideal situation is to use the smallest possible radial force to achieve the sharpest \"peak\" pressure distribution, thereby obtaining the best sealing effect. 2. Selection of oil seals: When choosing the material for an oil seal, it is necessary to consider the material’s suitability for the working medium, operating temperature, and the conditions under which the seal’s lip rotates at high speeds on the shaft. Generally, when an oil seal is in operation, if the temperature of its lip is 20~50°C higher than the temperature of the working medium, this should be taken seriously. The operating pressure for atmospheric oil seals generally does not exceed 0.05 MPa. When the operating pressure exceeds 0.05 MPa, a pressure-resistant oil seal should be used. Currently, the operating pressure of pressure-resistant oil seals abroad can reach 10 MPa. There are many forms and types of oil seals, which can be selected by referring to relevant design manuals based on operating conditions and usage requirements. When installing the oil seal, in order to achieve an appropriate initial radial force, it is necessary to ensure the proper interference between the lip and the rotating shaft; the value of this interference is shown in the figure below. II. Rotating Shaft Ring Seal The rotating shaft ring seal consists of a sealing ring made of high-grade Turcn material, along with an elastic O-ring that applies force, as shown in the figure below. To meet the requirements of different operating conditions at high and low speeds, one or two annular grooves are fabricated on the surface of the oil seal, depending on the size of the ring’s cross-section. This groove serves the following function: it improves the sealing effect by increasing the specific pressure of the sealing surface ； A lubrication chamber is formed, thereby reducing friction. To ensure that the medium pressure can act on the O-ring, several radial grooves are provided on the side of the gland to support the O-ring. The back side of the collar is designed with a concave curve to increase the contact area and prevent the collar from rotating along with the shaft. Rotating grooved rings have the following characteristics: 1) Both internal sealing and external sealing are applicable ； 2) It has a lubrication chamber, resulting in low friction and no sticking phenomenon; it does not crawl during startup ； 3) Good wear resistance and dimensional stability. The rotary gland sealing device is used for dynamic sealing at shafts, rods, pins, rotary joints, and other components that undergo rotational motion. It can withstand pressure from both sides or alternating pressure. Working pressure: not more than 30 MPa at 1 m/s, or not more than 20 MPa at 2 m/s ； Operating temperature: -54~+200°C (depending on the material of the O-ring) ； Working media: petroleum-based hydraulic oil, flame-retardant fluids, environmentally friendly and safe hydraulic oils (bio-oils), water, air, etc. III. Rotating Variseal seal – The rotating Variseal seal is a one-way sealing element used at rotating shafts. It consists of a U-shaped seal made of Turcon material and a V-shaped stainless steel spring, as shown in the figure below: There is a flange on the outside of the seal, which prevents it from rotating within the groove ； Its short and thick sealing lip reduces friction, extends service life, and provides good sealing performance even in working media with high viscosity. At low and zero pressure, the initial sealing force is generated by the spring. When the system pressure increases, the main sealing force is generated by the system pressure. Therefore, a good seal can be ensured from zero pressure to high pressure. Since the materials of the sealing ring and spring exhibit good compatibility with the working medium, they can be widely used in industries such as chemistry, pharmaceuticals, and food, in addition to hydraulic systems. If the cavity at the spring is filled with elastic silicone to prevent contaminants from entering the seal and to enable disinfection, it becomes a \"clean-type\" seal. This structure can also be used in applications involving sludge, slurry, or gels, preventing gravel from entering the seal cavity and interfering with the operation of the spring. Examples of the application of rotary Variseal seals are shown in the figure below: The operating pressure of the seal: not more than 15 MPa under dynamic load or not more than 25 MPa under static load ； Working speed: not more than 2 m/s (for rotational motion) or not more than 10 m/s (for reciprocating motion) ； Operating temperature: -100~+200℃ ； Working medium: various liquids, chemicals, and gases. The advantages of the rotating Variseal seal ring are: 1) It can be used for rotary, reciprocating motion seals as well as static seals ； 2) No crawling phenomenon during precise control ； 3) It provides excellent scraping performance ； 4) It has good wear resistance and dimensional stability ； 5) Capable of withstanding sudden temperature changes ； 6) It can be disinfected, and it does not contaminate foods, medications, etc.