（1） Overview and sealing principle of O-ring
The O-ring rubber ring is a circular shape of a circular rubber ring. O-ring is a widely used hydraulic and pneumatic system for sealing. The O-ring has good sealing performance, which can be used for both static and dynamic sealing; Not only can it be used alone, but many basic components of the sealing device can also be combined. It has a wide range of applications, and if the materials are selected correctly, it can meet various media and sports conditions.
The O-ring is an extrusion type seal, and the basic working principle of the extrusion type seal is to rely on the elastic deformation of the seal. The contact surface is caused by contact pressure, and the contact pressure is greater than the pressure of the sealing medium, so there is no leakage, and vice versa.
（2） Compression rate and stretching amount
The O-ring is a typical extruded seal. The compression ratio and stretching amount of the O-ring diameter are the main design of seals, and sealing performance and service life are important. The O-ring has a good sealing effect, which mainly depends on the correct matching of the O-ring size and groove size, forming a reasonable sealing and compression amount.
2.1 Compression rate
The compression ratio W is usually represented by the following equation:
W=(d0 h)/d0 × 100%
Where d0 refers to the cross-sectional diameter of the O-ring in the free state (mm);
H - Distance between the bottom of the O-ring groove and the sealing surface (groove depth), i.e. the cross-sectional height of the O-ring after compression (mm)
When selecting the compression ratio of the O-ring, the following three aspects should be considered:
There should be sufficient sealing contact area;
2. Keep the friction force as small as possible;
3. Try to avoid permanent deformation.
From these factors, it is not difficult to find contradictions between them. The compression ratio can be a large contact pressure, but an excessive compression ratio undoubtedly increases sliding friction and permanent shape. The compression ratio is too small, which may be due to the sealing error and O-ring error of the coaxial groove not meeting the requirements, resulting in leakage caused by partial loss of compression. Therefore, when choosing the compression ratio of the O-ring, we need to weigh various factors. Generally, the compression rate of static seals is greater than that of dynamic seals, but its limit value should be less than 25%, otherwise the compressive stress will significantly loosen and excessive permanent deformation will occur, especially under high temperature conditions.
The selection of O-ring compression ratio should consider the usage conditions, such as static sealing or dynamic sealing; Static sealing can be divided into radial sealing and axial sealing; The leakage clearance of radial seals (or cylindrical static seals) is radial clearance, while the leakage clearance of axial seals (or planar static seals) is axial clearance. Axial sealing is based on the influence of pressure medium on the inner or outer diameter of the O-ring, as well as internal and external pressures. The internal pressure increases and stretches, while the external pressure decreases and the initial O-ring stretches. Different forms of static sealing have different sealing media in the direction of the O-ring, so the preloading design is also different. Dynamic sealing is used to distinguish between reciprocating motion sealing and rotary motion sealing.
1. Static sealing: cylindrical static sealing devices and reciprocating sealing devices, generally W=10% to 15%; Flat static sealing device W=15% to 30%.
For dynamic sealing, it can be divided into three situations; The reciprocating motion is generally taken as W=10% to 15%. When selecting the compression rate for rotary motion seals, it is necessary to consider the Joule heat effect. Generally speaking, the inner diameter of the O-ring used for rotary motion is 3% -5% larger than the shaft diameter, and the compression rate of the outer diameter is W=3% -8%. For low friction sports, O-rings are generally selected with a smaller compression ratio of W=5% -8% in order to reduce frictional resistance. In addition, the expansion of rubber materials caused by medium and temperature should also be considered. Usually, beyond the given compression deformation, the allowable expansion rate does not exceed 15%. Exceeding this range indicates that the material selection is not appropriate, and O-rings of other materials should be used instead, or the given compression deformation rate should be corrected.
2.2 Stretching amount
After being installed in the sealing groove, the O-ring generally has a certain amount of stretching. Like the compression rate, the amount of stretching also has a significant impact on the sealing performance and service life of the O-ring. A large amount of stretching not only makes it difficult to install the O-ring, but also reduces the compression rate due to changes in the cross-sectional diameter d0, leading to leakage.