Causes of Major Failures of O-rings and Preventive Measures


O-ring design and improper use will accelerate the dama […]

O-ring design and improper use will accelerate the damage of O-ring and lose the 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 cause of O-ring damage is the permanent deformation of the O-ring material and the gap bite caused by the O-ring being squeezed into the sealing gap. The first-level O-ring is moving. Distortion occurs.
Permanent deformation of O-ring seal material
Because the synthetic rubber material used for O-rings is a viscoelastic material, the initial set pressure and rebound blocking capacity will be permanently deformed and gradually lost after a long period of use, and eventually leak. Permanent deformation and loss of elasticity are the main reasons for the loss of sealing performance of O-rings. The following are the main causes of permanent deformation of O-rings. Zh
1) The relationship between the compression rate and the amount of stretch and the permanent deformation of the O-ring seal material. The rubber of various formulas used in the production of O-ring seals will cause compressive stress relaxation under compression. Growth and decrease. The longer the use time, the greater the compression rate and the amount of stretching, the greater the stress drop caused by rubber stress relaxation, resulting in insufficient elasticity of the O-ring and loss of sealing ability. Therefore, it is advisable to try to reduce the compression ratio under the allowed usage conditions. Increasing the cross-sectional size of the O-ring is the easiest way to reduce the compression rate, but this will increase the structural size.
It should be noted that when calculating the compression ratio, the reduction of the cross-section height caused by stretching of the O-ring during assembly is often ignored. The change in the cross-sectional area of ​​an O-ring is inversely proportional to the change in its perimeter. At the same time, the cross-sectional shape of the O-ring will also change due to the pulling force, which manifests itself as a reduction in height. In addition, under the action of surface tension, the outer surface of the O-ring becomes flatter, that is, the cross-sectional height is slightly reduced. This is also a manifestation of the compressive stress relaxation of the O-ring.
The degree of deformation of the O-ring seal section also depends on the hardness of the O-ring seal material. In the case of the same amount of stretch, the hardness of the O-ring with a large hardness also decreases the cross-section height. From this point of view, the material with low hardness should be selected according to the use conditions. Under the effect of liquid pressure and tension, the O-ring of the rubber material will also gradually plastically deform, and the cross-section height of the O-ring will be reduced accordingly, so that the sealing ability will be lost. Zh
2) Relationship between temperature and relaxation process of O-ring
The use temperature is another important factor affecting the permanent deformation of the O-ring. High temperatures can accelerate the aging of rubber materials. The higher the operating temperature, the greater the compression set of the O-ring. When the permanent deformation is greater than 40%, the O-ring will lose its sealing ability and leak. The initial stress value formed in the rubber material of the O-ring due to compression deformation will gradually decrease and disappear with the relaxation process of the O-ring and the effect of temperature drop. For O-rings operating at temperatures below zero, the initial compression of the O-rings may decrease or disappear completely due to a sharp decrease in temperature. In the case of -50 to -60 ° C, non-low temperature resistant rubber materials will completely lose the initial stress; even at low temperature resistant rubber materials, the initial stress at this time will not be greater than 25% of the initial stress at 20 ° C. This is because the initial compression of the O-ring depends on the linear expansion coefficient. Therefore, when selecting the initial compression amount, it is necessary to ensure that there is still sufficient sealing ability after the stress reduction due to the relaxation process and temperature drop.