Compression rate of rubber O-ring

The compression rate W of rubber O-ring is usually expressed as follows:
W= (d0-h) /d0 x 100%
Where, the section diameter of d0----- o-ring in 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 section 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 should be sufficient sealing contact area;
2. As little friction as possible;
3. Try to avoid permanent deformation.
From the above factors, it is not difficult to find that there are contradictions between them. Large compression rate can obtain large contact pressure, but too large compression rate will undoubtedly increase the sliding friction and permanent shape. However, if the compression rate is too small, the leakage may be caused by the coaxiality error of the sealing groove and the o-ring error not meeting the requirements, and the partial compression amount will be lost. Therefore, when choosing the compression rate of the O-ring, it is necessary to weigh various factors. Generally, the compression rate of static seal is greater than that of dynamic seal, but its extreme value should be less than 25%, otherwise the compression stress is obviously relaxed, which will produce excessive permanent deformation, especially in high temperature conditions.
The selection of the compression rate W of the O-ring should consider the conditions of use, static seal or dynamic seal; Static seal can be divided into radial seal and axial seal; The leakage gap of the radial seal (or cylindrical static seal) is the radial gap, and the leakage gap of the axial seal (or planar static seal) is the axial gap. The axial seal is divided into internal pressure and external pressure according to the pressure medium acting on the inner diameter or the outer diameter of the O-ring. The internal pressure increases the tension, and the external pressure reduces the initial tension of the O-ring. For the above different forms of static sealing, the direction of action 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 between reciprocating motion seals or rotary motion seals.
1. Static seal: cylindrical static seal device and reciprocating sealing device, generally take W=10% ~ 15%; Plane static sealing device W=15% ~ 30%.
2. For dynamic sealing, it can be divided into three situations; The reciprocating motion is generally taken W=10% ~ 15%. The Joule thermal effect must be considered when selecting the compression rate of the rotary motion seal. In general, 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 W=3%-8%. Low friction movement with O-ring, in order to reduce friction resistance, generally choose a small compression rate, that is, W=5%-8%, in addition, but also take into account the expansion of rubber materials caused by the medium and temperature. Usually outside the given compression deformation, the maximum allowable expansion rate is 15%, more than this range indicates that the material selection is not suitable, should be changed to other materials O-rings, or the given compression deformation rate to be corrected.