How To Choose O-ring
May 23, 2020
How to choose O-ring
The well-known O-ring is the most commonly used seal in sealing. However, due to improper selection, groove design, processing and assembly, it often causes oil leakage failures, which can be described as small and bad things.
The standards for the size of O-rings in our national standards are GB1235-76, GB3452.1-88 and GB3452.1-92, of which the cross-sectional diameter (also called wire diameter) of O-rings specified in GB1235-76 is 1.9, 2.4, 3.1, 3.5, 5.7, etc., O-rings use the "outer diameter x wire diameter" marking method, generally called the old national standard; GB3452.1-82 is basically the same as GB3452.1-92, generally called the new national standard The cross-sectional diameters of O-rings specified by it are 1.8, 2.65, 3.55, 5.3, etc. The new national standard adopts international practice, and the O-rings adopt the marking method of "inner diameter x wire diameter"
When choosing an O-ring, the new national standard should be used as much as possible. The use of wire diameters of 2.65 and 3.55 is the most common. When the outer diameter is greater than about 30 and the structure size allows, the O-ring with a diameter of 3.55 is used as much as possible to achieve Greater compression and contact area.
The materials of the O-ring are mainly NBR and FKM, which are more suitable for high temperature conditions.
In addition to materials, one of the most important indicators of O-rings is their hardness, which is generally expressed in Shore hardness, ranging from about 60 to 90. The larger the value, the higher the hardness. Under the same pressure, the higher the hardness, the stronger the ability to prevent 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 nitrile rubber, and the Shore hardness is generally around 70, which is difficult to meet the requirements of a considerable part of the working conditions.
The groove design of the O-ring seal has relevant national standards, mainly divided into three types: dynamic seal, axial static seal and radial static seal. The O-rings of flanges, box covers, etc. are axially compressed to facilitate assembly and ensure sealing performance. In addition, the groove size standard in China's national standards has been significantly lagging behind. For example, the outer diameter of the O-ring seal groove under internal pressure in the national standard is equal to the outer diameter of the O-ring seal, while the O-ring groove of the PARKER flange joint The outer diameter of the groove is smaller than the outer diameter of the O-ring. When designing, appropriate reference should be made to foreign standards.
Do not use flat sealant on the sealing surface of the O-ring, you can apply a small amount of butter to the O-ring to prevent it from falling off. During maintenance, the specifications of the O-ring should be determined according to the groove size, not the thicker the wire diameter, the better.
