O-ring seal is a typical extrusion type seal. The compression ratio and tension of the cross-sectional diameter of the O-ring are the main contents of sealing design, which are of great significance for sealing performance and service life. The O-ring is generally installed in the sealing groove to provide a sealing effect. The good sealing effect of O-ring largely depends on the correct matching of O-ring size and groove size, forming a reasonable compression and stretching amount of the sealing ring. When designing and processing sealing devices, if the compression of the O-ring is too small, it will cause leakage; Excessive compression can cause stress relaxation in the O-ring rubber and lead to leakage. Similarly, excessive stretching during O-ring operation can accelerate aging and lead to leakage. The standards of various countries around the world have strict regulations on this.

1) Compression rate

The compression rate W is commonly expressed as follows:

W=(do h)/do%

In the formula, do - 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, 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:

a. There should be sufficient sealing contact area;

b. Try to minimize the friction force;

Try to avoid deformation as much as possible.

It is not difficult to see from the above factors that there are contradictions between them. A high compression ratio can result in high contact pressure, but excessive compression undoubtedly increases sliding friction and deformation. If the compression ratio is too small, it may be due to the coaxiality error and O-ring error of the sealing groove not meeting the requirements, resulting in the disappearance of some compression and causing leakage. Therefore, when choosing the compression ratio of the O-ring, multiple factors need to be considered. Generally, the compression rate of static seals is greater than that of dynamic seals, but its extreme value should be less than 30% (related to rubber materials), otherwise the compression stress will be significantly relaxed, resulting in excessive deformation, especially in high temperature conditions.

The selection of compression ratio W for O-ring seals should consider the usage conditions, static or dynamic seals; Static sealing can be divided into radial sealing and axial sealing; The leakage gap of radial seal (or cylindrical static seal) is the radial gap, and the leakage gap of axial seal (or planar static seal) is the axial gap. Axial sealing is divided into two situations based on whether the pressure medium acts on the inner or outer diameter of the O-ring: internal pressure and external pressure. The increase in internal pressure leads to tension, while the external pressure reduces the initial tension of the O-ring The different forms of static seals mentioned above have different directions of force exerted by the sealing medium on the O-ring, so the pre pressure design is also different. For dynamic seals, it is necessary to distinguish whether they are reciprocating or rotary seals.

Design principles for O-ring seals in 2023

1. Static sealing: Like reciprocating sealing devices, cylindrical static sealing devices generally take W=10%~15%; Flat sealing device

Set W=15%~30%.

2. For dynamic sealing, it can be divided into three situations:

a. The reciprocating motion seal generally takes W=10%~15%.

b. When selecting the compression ratio for rotary motion seals, the Joule heat effect must be considered. Generally speaking, the inner diameter of the O-ring used for rotary motion is 3% to 5% larger than the shaft diameter, and the compression ratio of the outer diameter is W=3% to 8%.

C. O-rings are used for low friction movements. In order to reduce friction resistance, a smaller compression rate is generally selected, which is W=5%~8%. In addition, the expansion of rubber materials caused by the material and temperature bow should also be considered. Generally, the maximum allowable expansion rate is 15% beyond the given compression deformation. If this range is exceeded, it indicates that the material selection is not appropriate. O-rings made of other materials should be used instead, or the given compression deformation rate should be corrected. The specific value of compression deformation is generally formulated or recommended by countries based on their own usage experience.

2) Stretching amount

After being installed in the sealing groove, the O-ring generally has a certain amount of stretching. Unlike compression ratio, the amount of stretching also has a significant impact on the sealing performance and service life of 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 do, resulting in leakage. The stretching amount a can be expressed as follows:

A=(d+do)/(d1+do)

In the formula

D - shaft diameter (mm);

D1- Inner diameter of O-ring (mm);

Do - cross-sectional diameter of O-ring (mm).

3) Contact width

After the O-ring is installed into the sealing groove, its cross-section undergoes compression deformation. The width after deformation and its contact width with the shaft are related to the sealing performance and service life of the O-ring, and too small a value will affect the sealing performance; Excessive friction increases friction, generates frictional heat, and affects the lifespan of the O-ring.

The width BO (mm) of the deformed O-ring is related to the compression rate W and cross-sectional diameter dO of the O-ring, and can be calculated using the following equation

BO={1/(1-W) -0.6W} dO (W takes 10% to 40%)

The width b (mm) of the contact surface between the O-ring and the shaft also depends on W and dO:

B=(4W2+0.34W+0.31) dO (W takes 10%~40%)

For O-ring seals with high friction limitations, such as pneumatic seals and hydraulic servo control component seals, friction can be estimated based on this.