Main causes of failure of O-ring seals-Permanent deformation

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 distorted.

As the O-ring seal with synthetic rubber material is a viscoelastic material, so the initial set of compression and rebound plugging capacity by a long time of use, will produce permanent deformation and gradually lost, and eventually leak. Permanent deformation and elasticity loss is the main reason for the O-ring to lose sealing performance, the following are the main reasons for permanent deformation.

1). The relationship between compression rate and tensile volume and permanent deformation

O-rings used in the production of various formulations of rubber, in the compression state will produce compression stress relaxation phenomenon, at this time, the compression stress with the growth of time and reduce. The longer the use of time, the greater the compression rate and the amount of stretching, the greater the stress generated by the rubber stress relaxation, resulting in the O-ring elasticity is insufficient, the loss of sealing capacity. Therefore, it is desirable to try to reduce the compression rate under the permitted conditions of use. Increasing the cross-sectional size of the O-ring is the easiest way to reduce the compression rate, although this will result in an increase in the size of the structure.

It should be noted that when calculating the compression rate, people often ignore the reduction in cross-sectional height caused by the O-ring being stretched during assembly. the change in cross-sectional area of the O-ring is inversely proportional to the change in its circumference. At the same time, the cross-sectional shape of the O-ring also changes due to tension, which is manifested in the reduction of its height. In addition, the outer surface of the O-ring becomes flatter under surface tension, i.e. the height of the cross-section is slightly reduced. This is also a manifestation of the relaxation of the compressive stress in the O-ring.

The degree of deformation of the O-ring cross-section also depends on the hardness of the O-ring material. In the case of the same amount of stretching, the hardness of the O-ring, its cross-sectional height is also reduced more, from this point of view, should be used in accordance with the conditions of use as far as possible to choose a low hardness material. In the role of liquid pressure and tension, rubber material O-ring will also gradually plastic deformation, its cross-sectional height will be correspondingly reduced, so that the final loss of sealing capacity.

2). Relationship between temperature and O-ring tensioning process

Operating temperature is another important factor affecting the permanent deformation of O-rings. High temperatures accelerate the ageing of rubber materials. The higher the operating temperature, the greater the permanent deformation of the O-ring in compression. When the permanent deformation is greater than 40%, the O-ring loses its sealing ability and leakage occurs. The initial stress value formed in the rubber material of the O-ring due to compression deformation will gradually decrease and disappear with the O-ring's tensioning process and temperature drop.

O-rings that work at temperatures below zero, their initial compression may be reduced or disappear completely due to the sharp drop in temperature. In the case of -50 ~ -60 ℃, the rubber material that is not resistant to low temperature will completely lose the initial stress; even if the rubber material that is resistant to low temperature, the initial stress at this time will not be greater than 25% of the initial stress at 20 ℃. 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 has dropped due to the Chi-Tension process and the temperature drop. For O-Rings working at temperatures below zero, special attention should be paid to the recovery index and deformation index of the rubber material.

In summary, in the design should try to ensure that the O-ring has a suitable working temperature, or choose high and low temperature resistant O-ring material, in order to extend the service life.

3). Working pressure of the medium and permanent deformation

The pressure of the working medium is the main factor causing permanent deformation of the O-ring. The working pressure of modern hydraulic equipment is increasing. The high pressure over a long period of time can cause permanent deformation of the O-ring. Therefore, the design should be based on the working pressure of the appropriate pressure-resistant rubber materials. The higher the working pressure, the higher the hardness and high pressure resistance of the material used.

In order to improve the pressure resistance of the O-ring material, increase the elasticity of the material (especially to increase the elasticity of the material at low temperatures), reduce the material compression permanent deformation, generally need to improve the material formula, adding plasticizers. However, O-seal rings with plasticisers are immersed in the working medium for a long time and the plasticisers are gradually absorbed by the working medium, resulting in a shrinkage of the O-seal volume and possibly even a negative compression of the O-seal (i.e. a gap between the O-seal and the surface of the seal being made). Therefore, these shrinkage volumes should be fully taken into account when calculating the O-ring compression volume and carrying out the mould design. The pressed O-ring should be made to maintain the necessary dimensions even after 5 to 10 days and nights of immersion in the working medium.

The rate of permanent deformation of the O-ring material in compression is temperature dependent. When the deformation rate is 40% or greater, that is, there will be leakage, so the heat resistance of several rubber materials boundaries: nitrile rubber 70 ℃, EPDM rubber 100 ℃, fluorine rubber 140 ℃. Therefore, the permanent deformation of the O-ring is stipulated in various countries. Chinese standard rubber material O-ring in different temperatures, the size change is shown in the table. The same material O-ring, in the same temperature, the cross-sectional diameter of the O-ring compression permanent deformation rate is lower.

The situation in oil is different. As the O-ring is not in contact with oxygen at this time, the above-mentioned adverse reactions are greatly reduced. In addition, this usually causes a certain expansion of the rubber, so that the temperature-induced permanent deformation in compression is offset. As a result, the heat resistance in oil is greatly improved. NBR, for example, can operate at temperatures of 120℃ or higher.

The above are the causes of permanent deformation and failure of O-rings and their preventive measures. Do you know other reasons can cause O-rings to fail?

Comments and follow are welcome, and I will introduce other reasons in subsequent articles. If you have more ideas, welcome email me by [email protected]