(1) Overview and sealing principle of O-ring
The rubber ring called O-ring is a circular shape of a circular rubber ring. O-ring is the most widely used hydraulic and pneumatic system for sealing. O-ring has good sealing performance, can be used for static sealing and dynamic sealing; not only can it be used alone, but also can be combined with many basic components of the sealing device. It has a wide range of applications. If the material is selected correctly, it can meet various media and sports conditions.
O-ring is an extrusion seal. The basic working principle of extrusion seal is that it relies 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 there is no leakage, and vice versa.
(2) Compression ratio and current tension
O-ring is a typical extrusion seal. Compression ratio and tension of O-ring diameter are the main design of seals. Sealing performance and service life are very important. O-ring has good sealing effect, which mainly depends on the correct matching of O-ring size and groove size to form a reasonable seal and compression.
2.1. Compression Rate
Compression ratio W is usually expressed by the following formula:
Among them, the cross section diameter (mm) of d0 - - - O ring in free state;
The distance between H - - - O groove bottom and sealing surface (groove depth), that is, the cross section height (mm) of O-ring after compression.
When choosing compression ratio of O-ring, the following three aspects should be considered:
Adequate sealing contact area should be provided.
2. The friction force is as small as possible.
3. Avoid permanent deformation as far as possible.
From these factors, it is not difficult to find contradictions between them. Compression ratio can be a high contact pressure, but excessive compression ratio will undoubtedly increase sliding friction and permanent shape. The low compression ratio may be due to the leakage caused by the sealing error of coaxial grooves and O-ring errors that do not meet the requirements, and part of the loss of compression. Therefore, when choosing O-ring compression rate, we must weigh various factors. Generally, the compression ratio of static seal is greater than that of dynamic seal, but its limit value should be less than 25%. Otherwise, the compressive stress will be significantly loosened and excessive permanent deformation will occur, especially at high temperature.
The selection of O-ring compression ratio should consider the use conditions, static seal or dynamic seal; static seal can be divided into radial seal and axial seal; leakage clearance of radial seal (or cylindrical static seal) is radial clearance, and leakage clearance of axial seal (or plane static seal) is axial clearance. Axial seal according to the pressure medium on the O-ring inner diameter or outer diameter, as well as internal pressure and external pressure, internal pressure increases tension, external pressure reduces the initial O-ring tension. Different static seals have different sealing media in the direction of O-ring, so the preloading design is different. Dynamic seal is to distinguish reciprocating motion seal from rotating motion seal.
1. Static seal: Cylindrical static seal device and reciprocating seal device, generally take W = 10% [15%] and plane static seal device W = 15% [30%].
2. For dynamic seals, it can be divided into three cases; the reciprocating motion is usually W=10%-15%. Joule heat effect must be taken into account when choosing compression ratio for rotary motion seal. Generally speaking, the inner diameter of O-ring for rotary motion is 3%-5% larger than that of shaft diameter, and the compression ratio of outer diameter W=3%-8%. In order to reduce frictional resistance, O-rings for low frictional motion are usually chosen with a smaller compression ratio, i.e. W=5%-8%. In addition, the expansion of rubber materials caused by medium and temperature should be taken into account. Usually, the maximum allowable expansion rate is 15% in addition to the given compression deformation, which indicates that the material is not suitable for selection. The O-ring of other materials should be used or the given compression deformation rate should be revised.
2.2 Tensile Quantity
The O-ring usually has a certain amount of tension after it is loaded into the sealing groove. Like the compression ratio, the tension has a great influence on the sealing performance and service life of O-rings. The large amount of stretching will not only make it difficult to install O-ring, but also reduce the compression rate due to the change of cross-section diameter d0, which will lead to leakage.
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