The sealing principle of O-ring seals

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The sealing principle of O-ring seals: An O-ring seal, also simply referred to as an O-ring, is a rubber ring with a circular cross-section. The O-ring is the most widely used type of seal in hydraulic and pneumatic systems. O-rings provide excellent sealing properties and can be used for both static sealing and sealing in reciprocating movements; they can be used alone as well as serving as a fundamental component in many combined sealing systems. It has a wide range of applications, and with proper material selection, it can meet the requirements of various operating conditions. The operating pressure can range from a vacuum of 1.333×105 Pa to high pressures of 400 MPa; the temperature range can be from -60°C to 200°C. 　　Compared to other sealing types, O-rings have the following advantages: 1) They have a small size, making them easy to install and remove. 　　2) Both static and dynamic seals can be used; when used as a static seal, it has almost no leakage. 　　3) A single O-ring is used, providing dual-direction sealing. 　　4) The dynamic frictional resistance is low. 　　5) Low price. 　　An O-ring is a type of compression seal. The basic working principle of compression seals is that the seal element undergoes elastic deformation, thereby creating a contact pressure at the sealing surface. If this contact pressure is greater than the internal pressure of the medium to be sealed, no leakage occurs; otherwise, leakage takes place. When used for static sealing and dynamic sealing, the causes and calculation methods of the contact pressure at the sealing contact surface differ, and these need to be explained separately. 　　1. Sealing principle for use in static seals O-rings are the most widely used in static seals. If designed and used correctly, O-rings can achieve an airtight seal with no leaks in static sealing applications. 　　After the O-ring is inserted into the sealing groove, its cross-section undergoes elastic deformation due to the compressive stress from contact. A certain initial contact pressure Po is applied to the contact surface. Even in the absence of medium pressure or when the pressure is very low, the O-ring can achieve sealing due to its own elastic force; when a pressurized medium is introduced into the chamber, the pressure of this medium causes the O-ring to shift towards the low-pressure side, and its elastic deformation increases further, thereby filling and sealing the gap δ. At this point, the contact pressure acting on the coupling surface of the sealing pair rises to Pm:
Pm = Po + Pp
Where Pp is the contact pressure transmitted to the contact surface via the O-ring (0.1 MPa).
Pp = K·P
K is the pressure transmission coefficient; for rubber O-rings, K = 1.
P is the pressure of the liquid being sealed (0.1 MPa). 　　Thus, **the sealing effect is improved. Since generally K≥1, Pm>P. It can be seen that as long as there is an initial pressure in the O-ring, an airtight absolute seal can be achieved. This property, in which the contact condition of the O-ring is altered by the pressure of the medium itself to achieve sealing, is known as self-sealing. 　　Theoretically, sealing can be achieved under oil pressure even if the compression deformation is zero, but in practice, the O-ring may be misaligned during installation. Therefore, after the O-ring is inserted into the sealing groove, its cross-section generally undergoes a compression deformation of 7% to 30%. Larger compression ratio values are used for static seals, while smaller compression ratio values are used for dynamic seals. This is because synthetic rubber contracts at low temperatures; therefore, the pre-compression amount of the static seal O-ring should take into account its contraction at low temperatures in order to compensate for it. 　　2. Sealing principle for use in reciprocating motion sealing In hydraulic and pneumatic components and systems, reciprocating motion sealing is one of the most common sealing requirements. Reciprocating motion seals are used on the piston of the power cylinder and the cylinder body, the piston interacting with the cylinder head, as well as various slide valves. The gap is formed by a cylindrical rod and a cylindrical hole, with the rod moving axially within the cylindrical hole. The sealing action prevents axial leakage of the fluid. When used for reciprocating motion sealing, the pre-sealing effect and self-sealing capability of the O-ring are the same as those in static sealing, and thanks to the elasticity of the O-ring itself, it has the ability to compensate automatically for wear. However, when using a liquid medium for sealing, the situation is more complex than with static sealing due to the effects of the rod’s movement speed, the pressure of the liquid, and its viscosity. 　　When the liquid is under pressure, the liquid molecules interact with the metal surface; the \"polar molecules\" present in the oil arrange themselves tightly and orderly on the metal surface, forming a strong boundary layer of oil film between the sliding surface and the seal, which in turn exerts a great adhesive force on the sliding surface. This liquid film is always present between the seal and the reciprocating surface; it also serves a sealing function and is extremely important for lubricating the moving seal surface. But it is harmful for leaks. But when the axially moving shaft is pulled outward, the liquid film on the shaft is drawn out along with it; due to the \"wiping\" effect of the seal, when the axially moving shaft retracts, this liquid film is retained outside by the sealing element. As the number of reciprocating movements increases, more liquid gets trapped outside, eventually forming oil droplets; this is what causes leakage in reciprocating-type sealing devices. Since the viscosity of hydraulic oil decreases as temperature rises, the thickness of the oil film also decreases accordingly. Therefore, when hydraulic equipment is started in low temperatures, leakage is relatively high at the beginning of operation; as the temperature rises due to various losses during operation, the leakage tends to decrease gradually. 　　As a reciprocating seal, the O-ring has a compact structure and small size, which can reduce the cost of components. It is mainly used in: 1) low-pressure hydraulic components, generally limited to short strokes and moderate pressures of around 10 MPa. 　　2) In hydraulic spool valves with small diameter, short stroke, and medium pressure. 　　3) In pneumatic slide valves and pneumatic cylinders. 　　4) As an elastomer in a modular reciprocating seal device. 　　As a reciprocating seal, the O-ring is most suitable for applications with small diameters, short strokes, and medium to low pressures, such as in reciprocating components like pneumatic cylinders and pneumatic slide valves. In hydraulic components, where O-rings are used as the primary dynamic seal, this approach is generally limited to short strokes and medium to low pressures around 10 MPa. O-rings are not suitable for use as reciprocating seals at very low speeds, nor as standalone high-pressure reciprocating seals. This is mainly because friction is high under such conditions, leading to premature failure of the seal. In any application, it must be used in accordance with the rated data or capabilities of the seal, and properly installed to achieve satisfactory performance. 　　3. Seals for rotational motion In rotary motion sealing, oil seals and mechanical seals are commonly used. However, oil seals operate at lower pressures, and compared to O-rings, they are larger and more complex, with poorer manufacturability. Although mechanical seals can be used in high-pressure (40 MPa), high-speed (50 m/s) and high-temperature (400°C) environments, they have a more complex and larger structure as well as higher costs; therefore they are only suitable for some heavy machinery used in industries such as petroleum and chemicals. 　　The main problem associated with the use of O-rings in rotational motion is the Joule heating effect. The Joule heating effect generates frictional heat at the point of contact between the high-speed rotating shaft and the O-ring; the heat produced causes the temperature of these contact areas to rise continuously, resulting in severe deformation of the rubber material, as well as changes in its compression and elongation amounts. Heat also accelerates the aging of sealing materials, reducing the service life of O-rings; it damages the sealing oil film, leading to oil leakage and accelerating the wear of the seals. 　　Based on the above circumstances, extensive and in-depth research has been conducted in recent years on O-rings for rotational motion both domestically and internationally. To avoid the Joule heating effect, it is crucial to correctly select the design parameters of the O-ring based on the properties of the rubber, primarily the stretching amount and compression ratio of the O-ring. Based on experiments, the rotary motion O-ring is designed with an inner diameter that is equal to or slightly larger than the diameter of the rotating shaft, typically by 3% to 5%. When installing the O-ring, it is compressed from the inside, and the degree of compression of its cross-section is also kept low, usually at around 5%. Furthermore, seal materials that are less affected by heat should be used as much as possible, with full consideration given to heat dissipation at the O-ring installation site. This significantly improves the performance of the O-ring, allowing it to be used for sealing rotating shafts with speeds up to 4 m/s. 　　In recent years, heat-resistant fluororubber and wear-resistant polyurethane rubber have been developed. There has also been a deeper understanding of the Joule heating effect associated with the operation of rubber components, and solutions to this issue have been sought through the design of new O-ring sealing structures, enabling O-rings to be used more effectively in high-speed, high-pressure rotary applications. 　　O-rings are being widely adopted in rotary motion sealing devices due to their small size, simple structure, low cost, good processability, and wide range of applications.