Medium working pressure and permanent deformation

Update:19-10-2018
Summary:

The pressure of the working medium is the main cause of […]

The pressure of the working medium is the main cause of permanent deformation of the O-ring. The working pressure of modern hydraulic equipment is increasing. Long-term high pressure will cause permanent deformation of the O-ring. Therefore, the design should be based on the working pressure to select the appropriate pressure rubber material. The higher the working pressure, the higher the hardness and high pressure resistance of the materials used.

In order to improve the pressure resistance of the rubber O ring material, increase the elasticity of the material (especially increase the elasticity of the material at low temperatures), and reduce the compression set of the material, it is generally necessary to improve the formulation of the material and add a plasticizer. However, O-rings with plasticizers, soaked in the working medium for a long time, plasticizers will gradually be absorbed by the working medium, resulting in shrinkage of the O-ring seal, and may even cause negative compression of the O-ring (ie A gap occurs between the O-ring and the surface of the seal. Therefore, when calculating the amount of compression of the O-ring and designing the mold, these shrinkage amounts should be fully considered. The pressed O-ring should be immersed in the working medium for 5 to 10 days and nights to maintain the necessary dimensions.

The compression set rate of the O-ring material is temperature dependent. When the deformation rate is 40% or more, leakage occurs, so the heat resistance limits of several rubber compounds are: 70 ° C for nitrile rubber, 100 ° C for EPDM, and 140 ° C for fluororubber. Therefore, countries have defined 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. O-rings of the same material, at the same temperature, O-rings with large cross-sectional diameters have a lower compression set ratio.

The situation in oil is different. Since the O-ring is not in contact with oxygen at this time, the above-mentioned adverse reaction is greatly reduced. In addition, it usually causes a certain expansion of the rubber, so the compression set rate due to temperature will be offset. Therefore, the heat resistance in the oil is greatly improved. Taking nitrile rubber as an example, its working temperature can reach 120 ° C or higher.