Selection Of O-ring And Matters Needing Attention

The well-known O-ring is one of the most commonly used seals in sealing. However, due to improper selection, groove design, processing and assembly, oil leakage is often caused, which can be described as a small and a big event.

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 known as wire diameter) of the O-ring specified in GB1235-76 is 1.9, 2.4, 3.1, 3.5, 5.7, etc., the O-ring adopts the marking method of "outer diameter × wire diameter", which is generally called the old national standard; GB3452.1-82 is basically the same as GB3452.1-92, and is generally called the new national standard , it stipulates that the cross-sectional diameter of the O-ring seal is 1.8, 2.65, 3.55, 5.3, etc. The new national standard adopts international practice, and the O-ring seal adopts the marking method of "inner diameter × wire diameter".

When choosing an O-ring, the new national standard should be selected first. The wire diameters of 2.65 and 3.55 are the most common. When the outer diameter is greater than about 30 and the structure size allows, try to choose an O-ring with a wire diameter of 3.55 to achieve Greater compression and contact area.

The materials of O-ring are mainly nitrile nitrile rubber NBR and fluororubber FKM, fluororubber is more suitable for high temperature conditions.

In addition to the material, an important indicator of the O-ring is its hardness, which is generally expressed by Shore hardness, ranging from 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 nitrile rubber, and the Shore hardness is generally around 70, which is difficult to meet the requirements of a considerable number of working conditions.

The groove design of the O-ring has relevant national standards, mainly divided into three types: split 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 my country's national standards has been significantly backward. For example, the outer diameter of the O-ring 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.

Flat sealant shall not be used on the sealing surface with O-ring, and a small amount of butter can be applied to the O-ring to prevent it from falling off. During maintenance, the specification of the O-ring seal should be determined according to the size of the groove, not the thicker the wire diameter, the better.