Sealing Principle Of O-ring
An O-ring, also known as an O-ring, is a rubber ring with a circular cross-section. It is the most widely used seal in hydraulic and pneumatic systems. O-rings offer excellent sealing properties and can be used for both static and reciprocating seals. They can be used independently and are a fundamental component of many modular sealing systems. They have a wide range of applications. If the material is properly selected, they can meet the requirements of various operating conditions. Operating pressures range from a vacuum of 1.333 × 10⁵Pa to a high pressure of 400 MPa, and temperatures extend from -60°C to 200°C.
Compared to other seal types, O-rings have the following advantages:
1) Small size and easy assembly and disassembly.
2) Can be used for both static and dynamic sealing, with virtually no leakage when used as a static seal.
3) A single O-ring provides bidirectional sealing.
4) Low dynamic friction.
The O-ring is a type of extrusion seal. Its basic operating principle relies on elastic deformation of the seal element, creating contact pressure on the sealing surface. If the contact pressure exceeds the internal pressure of the sealed medium, leakage will occur; otherwise, leakage will occur. The causes and calculation methods of contact pressure on the sealing surface differ for static and dynamic seals and require separate explanations.
1. Sealing Principle for Static Seals
O-rings are the most widely used in static seals. If designed and used correctly, O-rings can achieve a leak-free, absolute seal.
After an O-ring is installed in a sealing groove, its cross-section undergoes contact compression stress, causing elastic deformation. This generates a certain initial contact pressure Po on the contact surface. Even with no or very low pressure, the O-ring maintains a seal due to its own elastic force. When pressurized medium enters the chamber, the O-ring shifts toward the lower pressure side under the influence of the medium pressure, further increasing its elastic deformation to fill and close the gap δ. At this point, the contact pressure on the mating surfaces of the sealing pair rises to Pm:
Pm=Po+Pp
Where Pp is the contact pressure transmitted to the contact surface through the O-ring (0.1 MPa).
Pp=K·P
K is the pressure transmission coefficient, with K=1 for rubber O-rings;
P is the pressure of the sealed fluid (0.1 MPa).
This greatly enhances the sealing effect. Since K is generally ≥ 1, Pm>P. Therefore, as long as there is initial pressure on the O-ring, it can achieve a leak-free, absolute seal. This property of the O-ring, which relies on the pressure of the medium itself to change the contact state of the O-ring and achieve a seal, is called self-sealing.
Theoretically, even if the compression deformation is zero, it can still seal under oil pressure. However, in practice, O-rings may be eccentric during installation. Therefore, after the O-ring is installed in the sealing groove, its cross-section generally experiences a compression deformation of 7%-30%. A higher compression ratio is used for static seals, while a lower compression ratio is used for dynamic seals. This is because synthetic rubber compresses at low temperatures, so the pre-compression of static O-rings should account for its low-temperature shrinkage.
2. Sealing Principles for Reciprocating Motion Seals
Reciprocating motion seals are a common sealing requirement in hydraulic and pneumatic components and systems. Reciprocating motion seals are used in power cylinder pistons and cylinder bodies, piston-to-cylinder interposition and cylinder heads, and various types of sliding valves. A gap is formed between a cylindrical rod and a cylindrical bore, within which the rod moves axially. The seal restricts axial leakage of the fluid. When used as a reciprocating motion seal, the O-ring's pre-sealing and self-sealing properties are similar to those of static seals. Furthermore, due to its inherent elasticity, the O-ring can automatically compensate for wear. However, when sealing liquid media, the situation is more complex than with static seals due to the influence of rod speed, liquid pressure, and viscosity.
When liquids are under pressure, the liquid molecules interact with the metal surface. The polar molecules in the oil align tightly and evenly on the metal surface, forming a strong boundary film between the sliding surface and the seal, which exerts strong adhesion to the sliding surface. This liquid film always exists between the seal and the reciprocating surface, providing a certain degree of sealing and crucial for lubricating the moving sealing surface. However, it is detrimental to leakage. When the reciprocating shaft is pulled outward, the liquid film on the shaft is pulled along with it. Due to the "wiping" action of the seal, when the reciprocating shaft retracts, this liquid film is retained outside by the sealing element. As the number of reciprocating strokes increases, more liquid is retained outside, eventually forming oil droplets, which represent leakage in reciprocating seals. Because the viscosity of hydraulic oil decreases with increasing temperature, the oil film thickness decreases accordingly. Therefore, when hydraulic equipment is started at low temperatures, leakage is greater at the beginning of the movement. As the temperature rises due to various losses during movement, the leakage tends to gradually decrease.
O-rings, as reciprocating seals, are compact and small in size, and are primarily used in:
1) Low-pressure hydraulic components, generally limited to short strokes and medium pressures around 10 MPa.
2) Small-diameter, short-stroke, medium-pressure hydraulic spool valves.
3) Pneumatic spool valves and cylinders.
4) As elastomers in combined reciprocating seals.
O-rings are best suited as reciprocating seals for small diameters, short strokes, and low-to-medium pressures, such as in reciprocating components like pneumatic cylinders and spool valves. In hydraulic components, the use of O-rings as primary dynamic seals is generally limited to short strokes and medium-to-low pressures around 10 MPa. O-rings are not suitable for very low-speed reciprocating seals or as the sole seal for high-pressure reciprocating applications. This is primarily due to the high friction in these conditions, which can lead to premature seal failure. In any application, the seal must be used according to its rated data or capacity and properly assembled to achieve satisfactory performance.
3. Rotary Seals
Oil seals and mechanical seals are commonly used for rotary seals. However, oil seals operate at lower pressures and are larger, more complex, and less manufacturable than O-rings. While mechanical seals can operate at high pressures (40 MPa), high speeds (50 m/s), and high temperatures (400°C), their more complex and bulky structure and high cost make them suitable only for heavy machinery in the petroleum and chemical industries.
The primary problem with O-rings for rotary applications is Joule heating. This frictional heat generated at the contact point between the high-speed rotating shaft and the O-ring causes the temperature of these contact points to rise continuously, severely deforming the rubber material and causing changes in compression and elongation. This heat also accelerates the aging of the sealing material, reducing the service life of the O-ring. It also destroys the sealing oil film, causing oil breakage and accelerating seal wear.
Based on the above situation, extensive and in-depth research has been conducted both domestically and internationally on O-rings for rotary motion in recent years. To avoid Joule heating, the key lies in correctly selecting O-ring structural parameters based on the rubber's properties, primarily the O-ring's elongation and compression ratio. Experimental studies have shown that O-rings for rotary motion should be designed with an inner diameter equal to or slightly larger than the rotating shaft diameter, typically 3% to 5% larger. During installation, the O-ring is compressed from the inner diameter inward, and the cross-sectional compression is designed to be minimal, typically around 5%. Furthermore, sealing materials with minimal thermal impact are used whenever possible, and proper consideration is given to heat dissipation at the O-ring installation site. This significantly improves the performance of O-rings, enabling their application in sealing rotating shafts with speeds up to 4 m/s.
Recently, heat-resistant fluororubber and wear-resistant polyurethane rubber have emerged, and with a deeper understanding of the Joule heating effect in rubber components, solutions have been developed to address this issue, leading to the design of new O-ring sealing structures that better suit high-speed, high-pressure rotary motion.
O-rings are widely used in rotary motion sealing devices due to their small size, simple structure, low cost, good process performance and wide range of applications.