The Main Reasons For Failure Of O-ring Seals And Their Preventive Measures

Improper design and use of the O-ring will accelerate the damage of the O-ring 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 the working conditions of high pressure and high temperature, the main reasons for the damage of the O-ring seal are 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 material

Since the synthetic rubber material used for the O-ring is a viscoelastic material, the initially set pressing 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 for the loss of sealing performance of O-rings. The following are the main reasons for permanent deformation of O-ring materials.

1) The relationship between the compression ratio and the tensile amount and the permanent deformation of the O-ring material The rubber of various formulations used to make the O-ring will produce a compressive stress relaxation phenomenon in the compressed state. At this time, the compressive stress changes with time. increase and decrease. The longer the use time, the greater the compression ratio and the greater the tensile amount, the greater the stress drop caused by the relaxation of the rubber stress, so that the O-ring has insufficient elasticity and loses its sealing ability. Therefore, it is advisable to try to reduce the compression ratio under the allowable usage conditions. Increasing the cross-sectional size of the O-ring is the easiest way to reduce the compression ratio, but this will increase the size of the structure.

It should be noted that when calculating the compressibility, the reduction of the section height caused by the tension of the O-ring during assembly is often ignored. The change of the cross-sectional area of the O-ring is inversely proportional to the change of its circumference. At the same time, due to the action of tension, the cross-sectional shape of the O-ring will also change, which means that its height is reduced. 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 O-ring compressive stress relaxation.

The degree of deformation of the O-ring section also depends on the hardness of the material of the O-ring. In the case of the same amount of stretching, the O-ring with high hardness also reduces the cross-sectional height. From this point of view, the material with low hardness should be selected as far as possible according to the conditions of use. Under the action of liquid pressure and tension, the O-ring of the rubber material will gradually undergo plastic deformation, and the height of the cross-section of the O-ring will decrease accordingly, so that the sealing ability is finally lost.

2) The relationship between temperature and O-ring relaxation process

The operating temperature is another important factor that affects the permanent deformation of the O-ring. High temperature will 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 loses its sealing ability and leaks. 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 subzero temperatures, the initial compression of the O-rings may decrease or disappear completely due to a sharp drop in temperature. In the case of -50 to -60 °C, the rubber material that is not resistant to low temperature will completely lose the initial stress; even if the rubber material is resistant to low temperature, 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 coefficient of linear expansion. Therefore, when selecting the initial compression amount, it is necessary to ensure that there is still sufficient sealing capacity after the stress drop due to the relaxation process and temperature drop.