Causes Of Main Failure Of Liquid O-Rings And Its Preventive Measures

The improper design and use of liquid o-rings will accelerate the damage of o-rings and lose its sealing performance. The experimental results show that if all parts of the sealing device are properly designed and the pressure is simply increased, the damage of the o-rings can not be caused. Under high pressure and high temperature working conditions, the main reasons for the damage of o-ring are the permanent deformation of o-rings material and the bite of o-ring in the gap caused by o-rings being squeezed into the gap.

1. Permanent deformation of liquid o-rings material

Since the synthetic rubber material used in the o-ring is a viscoelastic material, the initial set of compaction and rebound plugging capacity after a long time of use, will produce permanent deformation and gradually lose, and eventually leakage. Permanent deformation and elastic loss is the main reason for o-ring seal performance loss, the following is the main reason for o-rings material permanent deformation.

1) the relationship between compression ratio and stretching amount and the permanent deformation of o-rings material, the compression stress relaxation will occur in all kinds of rubber formulations used for o-rings under the compression condition, and the compression stress decreases with the increase of time. The longer the service time, the greater the compression ratio and the tensile strength, the greater the stress drop caused by the stress relaxation of the rubber, resulting in insufficient elasticity of the o-ring seal and loss of sealing ability. Therefore, it is advisable to try to reduce the compression ratio under the allowable conditions of use. Increasing the cross-section size of the o-ring is the easiest way to reduce the compression ratio, but this will lead to an increase in structural size.

It should be noted that in the calculation of the compression ratio, it is often overlooked that the o-type seal ring in the assembly of tension caused by the reduction of cross-section height. The change of section area of o-ring is inversely proportional to the change of its circumference. At the same time, due to the role of tension, the cross-section shape of the o-ring will also change, which is manifested in the reduction of its height. In addition, under the action of surface tension, the outer surface of the o-ring becomes flatter, that is, the cross-section height decreases slightly. This is also o-type sealing ring compression stress relaxation of a performance.

The degree of cross-section deformation of liquid o-rings also depends on the hardness of o-rings material. In the case of the same tensile strength, the hardness of the o-rings, the height of its cross-section is also reduced more, from this point of view, should be in accordance with the use of low-hardness materials. Under the action of liquid pressure and tension, the rubber o-rings will gradually plastic deformation, o-ring section height will decrease accordingly, and finally lose sealing ability.

2) the relationship between temperature and relaxation of o-rings

The operating temperature is another important factor that affects the permanent deformation of o-rings. High temperature will accelerate the aging of rubber material. The higher the working temperature, the greater the compression permanent deformation of the o-ring. When the permanent deformation is more than 40% , the o-ring will lose its sealing ability and leak. The initial stress formed in the rubber material of the o-rings due to compression deformation will gradually decrease and disappear with the relaxation process of the o-rings and the effect of temperature decrease. The initial compression of the o-ring may decrease or disappear completely due to the drastic decrease of the temperature of the o-rings. At-50 ~-60 °C, the initial stress of the rubber material which is not resistant to low temperature will lose completely, and even if the rubber material is resistant to low temperature, the initial stress will not be more than 25% of the initial stress at 20 °C. This is because the initial compression of the o-ring depends on the Coefficient of linear expansion. Therefore, when selecting the initial compression, it is necessary to ensure that there is enough sealing capacity after stress reduction due to relaxation process and temperature decrease.