Design and use of O-ring (1)


Improper design and use of O-ring will accelerate its d […]

Improper design and use of O-ring will accelerate its damage and lose its sealing performance. Experiments show that if the design of each part of the sealing device is reasonable, simply increasing the pressure will not cause damage to the O-ring. Under high-pressure and high-temperature working conditions, the main reasons for O-ring damage are the permanent deformation of the O-ring material and the gap bite caused by the O-ring being squeezed into the sealing gap, and the O-ring is twisted during movement.

Permanent deformation
Since the synthetic rubber material used for the O-ring seal is a viscoelastic material, the initial compression amount and rebound blocking ability will be permanently deformed and gradually lost after long-term use, and eventually leakage will occur. Permanent deformation and loss of elasticity are the main reasons why O-rings lose sealing performance. The following are the main reasons for permanent deformation.
1) The relationship between compression rate and stretch and permanent deformation
2) The relationship between temperature and O-ring relaxation process
3) Medium working pressure and permanent deformation
The compression set rate of O-ring material is related to temperature. When the deformation rate is 40% or greater, leakage will occur. Therefore, the heat resistance limits of several rubber compounds are: nitrile rubber at 70°C, EPDM rubber at 100°C, and fluoro rubber at 140°C. Therefore, various countries have made regulations on the permanent deformation of O-rings. The dimensional changes of O-rings of Chinese standard rubber materials at different temperatures are shown in the table. For O-rings of the same material, at the same temperature, O-rings with a larger cross-sectional diameter have a lower compression set rate. The situation is different in oil. Since the O-ring is not in contact with oxygen at this time, it usually occurs in a dynamic sealing state. If the O-ring is properly assembled and used under proper conditions, it is generally not easy to roll or twist under reciprocating motion, because the contact area between the O-ring and the groove is larger than the frictional contact area on the sliding surface, and the O-ring The resistance of the circle itself can prevent distortion. The distribution of friction also tends to keep the O-ring stationary in its groove, because static friction is greater than sliding friction, and the roughness of the groove surface is generally not as good as the roughness of the sliding surface.

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