Overview And Sealing Principle Of O-rings
Mar 25, 2022
Overview and sealing principle of O-rings
A rubber ring called an O-ring is a circular shape of a circular rubber ring. O-rings are used in the most widely sealed hydraulic and pneumatic systems. O-rings have good sealing performance and can be used for static sealing as well as dynamic sealing; they can not only be used alone but also can be combined with many basic components of the sealing device. It has a wide range of uses and, if the material is chosen correctly, can accommodate a wide variety of media and sports conditions.
The O-ring is an extrusion-type seal. 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 the contact pressure. The contact pressure is greater than the pressure of the sealing medium, and no leakage occurs, and vice versa.
1. Compression ratio is the amount of stretch
O-rings are typical squeeze seals. The compression ratio and stretching amount of the O-ring diameter are the main design of the seal, and the sealing performance and service life are very important. The O-ring has a good sealing effect, which mainly depends on the correct matching of the size of the O-ring and the size of the groove to form a reasonable amount of sealing and compression.
2. Compression ratio
The compression ratio 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 and the sealing surface (groove depth), that is, the height of the cross-section of the O-ring after compression (mm)
When choosing an O-ring compression ratio, the following three aspects should be considered:
There should be sufficient sealing contact area;
3. The friction force is as small as possible;
4. Try to avoid permanent deformation.
From these factors, it is not difficult to find contradictions between them. Compression ratios can be larger contact pressures, but excessive compression ratios will undoubtedly increase sliding friction and permanent shape. If the compression ratio is too small, it may be due to the leakage caused by the partial loss of compression due to the sealing error of the coaxial groove and the O-ring error that does not meet the requirements. Therefore, when choosing an O-ring compression rate, we must weigh various factors. Generally, the compression ratio of static seals is greater than that of dynamic seals, but the limit value should be less than 25%, otherwise, the compressive stress will be significantly loosened and excessive permanent deformation will occur, especially under high-temperature conditions.
The selection of O-ring compression ratio should consider the conditions of use, static seal or dynamic seal; static seal can be divided into the radial seal and axial seal; radial seal (or cylindrical static seal) leakage gap is radial gap, axial seal (or plane static seal) leakage clearance is axial clearance. The axial seal is based on the pressure medium on the inner or outer diameter of the O-ring and the internal pressure and external pressure. The internal pressure increases and stretches, and the external pressure reduces the initial O-ring stretch. In different forms of static seal, the sealing medium is different in the direction of the O-ring, so the preload design is also different. Dynamic seals are used to differentiate between reciprocating seals and rotary seals.
1. Static sealing: cylindrical static sealing device and reciprocating sealing device, generally take W = 10% ~ 15%; plane static sealing device W = 15% ~ 30%.
2. For dynamic seals, it can be divided into three cases; reciprocating motion generally takes W=10% to 15%. The Joule heating effect must be considered when selecting the compression rate of the rotary motion seal. 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%. In order to reduce frictional resistance, O-rings for low-friction motion generally choose a smaller compression rate, namely W=5%-8%. In addition, the expansion of rubber materials caused by the medium and temperature must be considered. Usually, outside the given compression deformation, the maximum allowable expansion rate is 15%. Exceeding this range indicates that the material selection is not suitable, and O-rings of other materials should be used, or the given compression deformation rate should be corrected.
3. Stretch amount
After the O-ring is installed in the sealing groove, it generally has a certain amount of stretching. Like the compression rate, the amount of tension has a great influence on the sealing performance and service life of the O-ring. A large amount of tension will not only make it difficult to install the O-ring, but also reduce the compression rate due to the change of the cross-sectional diameter d0, resulting in leakage.
