China's national standard on O-ring size standards are GB1235-76, GB34452.1-88 and GB34452.1-92, of which GB1235-76 stipulated O-ring section diameter (also known as wire diameter) has 1.9, 2.4, 3.1, 3.5, 5.7, O-ring using "outer diameter × wire diameter" marking method, Generally known as the old national standard; Gb34452.1-82 and GB34452.1-92 are basically the same, generally known as the new national standard, it specifies the O-ring section diameter of 1.8, 2.65, 3.55, 5.3, etc., the new national standard adopts international practice, O-ring using the "inner diameter × line diameter" marking method.
When choosing O-rings, first of all, the new national standard should be selected as far as possible, the line diameter 2.65 and 3.55 are the most common use, in the case of outer diameter greater than 30 or so, the structural size allows, try to choose the line diameter 3.55 O-rings, in order to achieve greater compression and contact area.
O-ring materials are mainly butadiene rubber NBR and fluorine rubber FKM, fluorine rubber is more suitable for high temperature conditions.
In addition to the material, an important indicator of the O-ring is its hardness, generally expressed by Shore hardness, ranging from 60 to about 90, the larger the value, the higher the hardness. Under the same pressure, the higher the hardness, the stronger the ability to resist extrusion (deformation or even tearing). Therefore, O-rings with higher hardness should be selected.
However, the common O-rings on the market are made of butyl cyanide rubber, Shore hardness is generally about 70, it is difficult to meet the requirements of a considerable part of the working conditions.
The groove design of O-ring has relevant national standards, mainly three kinds of dynamic seal, axial static seal and radial static seal. The O-rings of flanges and case covers are axially pressed to facilitate assembly and ensure sealing performance. In addition, the groove size standard in China's national standard has been significantly backward, such as the O-ring groove outside diameter under internal pressure in the national standard is equal to the O-ring outside diameter, and the O-ring groove outside diameter of PARKER flange joint is less than the O-ring outside diameter. The design should make appropriate reference to foreign standards.
The sealing surface with O-ring shall not use flat sealant, and a small amount of butter can be applied to the O-ring to make it not fall off. During maintenance, the O-ring specifications should be determined according to the trench size, not the thicker the line diameter, the better.