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Knowledge of O-rings

Release time:

2021-09-15

O-rings are typical extruded seals. The compression ratio and elongation of the O-ring section diameter 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 size of the O-ring and the groove size to form a reasonable amount of compression and tension of the sealing ring.

The compression ratio W is usually expressed by the following formula:

W=(d0-h)/d0 ×100%

In the formula, 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 surface to be sealed (groove depth), that is, 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:

1. There must be enough sealing contact area;

2. Friction as small as possible;

3. Try to avoid permanent deformation.

From the above factors, it is not difficult to find that they are contradictory to each other. A large compression ratio can obtain a large contact pressure, but an excessive compression ratio will undoubtedly increase the sliding friction and permanent deformation. If the compression ratio is too small, it may cause leakage due to the coaxiality error of the sealing groove and the O-ring error that do not meet the requirements and disappearing part of the compression amount. Therefore, when choosing the compression ratio of the O-ring, there are various factors to be weighed. Generally, the compression ratio of static seals is greater than that of dynamic seals, but the extreme value should be less than 25%, otherwise the compressive stress will be significantly relaxed, resulting in excessive permanent deformation, especially in high temperature conditions.

The selection of the compression rate W of the O-ring seal should consider the conditions of use, static seal or dynamic seal; static seals can be divided into radial seals and axial seals; the leakage gap of radial seals (or cylindrical static seals) is Radial clearance, leakage clearance of axial seal (or plane static seal) is axial clearance. According to the pressure medium acting on the inner diameter or outer diameter of the O-ring, the axial seal is divided into two situations: internal pressure and external pressure. The internal pressure increases and the external pressure reduces the initial stretching of the O-ring. For the above-mentioned different forms of static seal, the action direction of the sealing medium 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 seal or a rotary seal.

1. Static seal: The cylindrical static seal is the same as the reciprocating seal, and generally takes W=10% to 15%; the plane static seal takes W=15% to 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 stretch. 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. The amount of stretching a can be expressed by the following formula:

α=(d+d0)/(d1+d0)

In the formula, d-----shaft diameter (mm);

d1----O-ring inner diameter (mm)

The range of stretching amount is 1%-5%. As shown in the table, the recommended value of the stretching amount of the O-ring is given. According to the size of the shaft diameter, the stretching amount of the O-ring can be selected according to the table.