Overview and sealing principle of O-ring
O-ring rubber seal is referred to as O-ring, which is a rubber ring with a circular cross-section. O-ring is the most widely used seal in hydraulic and pneumatic systems. O-ring has good sealing performance and can be used for both static and dynamic sealing; it can not only be used alone, but also is a basic component of many combined sealing devices. Its application range is very wide. If the material is properly selected, it can meet the requirements of various media and various motion conditions.
O-ring is an extrusion seal. The basic working principle of extrusion seal is to rely on the elastic deformation of the seal to create contact pressure on the sealing contact surface. If the contact pressure is greater than the internal pressure of the sealed medium, no leakage will occur, otherwise leakage will occur.
(II) Compression rate and stretching amount
O-ring is a typical extrusion seal. The compression rate and stretching amount of the cross-sectional diameter of the O-ring are the main contents of the seal design, which are of great significance to the sealing performance and service life. The good sealing effect of the O-ring depends largely on the correct matching of the O-ring size and the groove size, forming a reasonable compression and stretching amount of the sealing ring.
2.1. Compression rate
The compression rate W is usually expressed by the following formula:
W=(d0-h)/d0 ×100%
Where d0-----the cross-sectional diameter of the O-ring in the free state (mm);
h------the distance between the bottom of the O-ring groove and the sealed surface (groove depth), that is, the cross-sectional height of the O-ring after compression (mm)
When selecting the compression rate of the O-ring, the following three aspects should be considered:
1. There must be enough sealing contact area;
2. The friction force should be as small as possible;
3. Try to avoid permanent deformation.
It is not difficult to find from the above factors that they are contradictory to each other. A large compression rate can obtain a large contact pressure, but an excessively large compression rate will undoubtedly increase the sliding friction and permanent deformation. If the compression rate is too small, leakage may be caused by the disappearance of part of the compression amount due to the coaxiality error of the sealing groove and the error of the O-ring not meeting the requirements. Therefore, when selecting the compression rate of the O-ring, various factors should be weighed. Generally, the compression rate of static seals is greater than that of dynamic seals, but its extreme value should be less than 25%, otherwise the compression stress will be significantly relaxed, which will cause excessive permanent deformation, especially serious in high temperature conditions.
The selection of the compression rate W of the O-ring should take into account the use conditions, static seals or dynamic seals; static seals can be divided into radial seals and axial seals; the leakage gap of radial seals (or cylindrical static seals) is the radial gap, and the leakage gap of axial seals (or plane static seals) is the axial gap. Axial seals are divided into two situations: internal pressure and external pressure according to whether the pressure medium acts on the inner diameter or outer diameter of the O-ring. The internal pressure increases the stretching, and the external pressure reduces the initial stretching of the O-ring. For the above-mentioned different forms of static seals, the direction of the sealing medium acting on the O-ring is different, so the pre-pressure design is also different. For dynamic seals, it is necessary to distinguish whether it is a reciprocating motion seal or a rotary motion seal.
1. Static seal: The cylindrical static seal device is the same as the reciprocating seal device, and W=10%~15% is generally used; the plane static seal device takes W=15%~30%.
2. For dynamic seals, it can be divided into three cases; reciprocating motion generally takes W=10%~15%. When selecting the compression rate for rotary motion seals, the Joule heat effect must be considered. Generally speaking, the inner diameter of the O-ring for rotary motion is 3%-5% larger than the shaft diameter, and the compression rate of the outer diameter is W=3%-8%. For O-rings used in low-friction motion, in order to reduce friction resistance, a smaller compression rate, that is, W=5%-8%, is generally selected. In addition, the expansion of rubber materials caused by the medium and temperature must also be considered. Usually, outside the given compression deformation, the maximum allowable expansion rate is 15%. Exceeding this range indicates that the material selection is inappropriate, and other materials O-rings should be used instead, or the given compression deformation rate should be corrected.
2.2 Stretching amount
O-rings generally have a certain amount of stretching after being installed in the sealing groove. Like the compression rate, the amount of stretching also has a great impact on the sealing performance and service life of the O-ring. A large amount of stretching will not only make the O-ring difficult to install, but also reduce the compression rate due to the change in the cross-sectional diameter d0, which will cause leakage. The stretching amount a can be expressed by the following formula:
α=(d+d0)/(d1+d0)
Where d-----shaft diameter (mm); d1----O-ring inner diameter (mm).
The range of the stretching amount is 1%-5%. The table gives the recommended value of the O-ring stretching amount. According to the size of the shaft diameter, the stretching amount of the O-ring can be selected according to the table. The preferred range of the compression rate and stretching amount of the O-ring