O-rings are the most common mechanical design for sealing because of their cheap price, simple manufacture, reliable function, and simple installation requirements. Although O-rings are cheap, in some specific environments, frequent replacement will increase the maintenance cost of the machine and affect the normal operation of the equipment. Therefore, it is necessary to understand the characteristics of O-rings.
At -50 to -60°C, rubber materials that are not resistant to low temperatures will completely lose their initial stress; even for 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 coefficient of linear expansion.
Improper design and use of the O-ring will accelerate its damage and lose its sealing performance.
Experiments have shown that if the design of each part of the sealing ring 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 cause of O-ring damage is the permanent deformation of the O-ring material, the gap bite caused by the O-ring being squeezed into the sealing gap, and the distortion of the O-ring during movement.
Permanent deformation Since the synthetic rubber material used for the O-ring sealing ring is a viscoelastic material, the initially set compression amount and rebound blocking capacity will be permanently deformed and gradually lost after long-term use, eventually resulting in leakage.
Permanent deformation and loss of elastic force are the main reasons why O-rings lose their sealing performance. The following are the main reasons for permanent deformation.
The relationship between compression ratio and stretching amount and permanent deformation
The various formulations of rubber used to make O-rings will produce compressive stress relaxation in the compressed state. At this time, the compressive stress will decrease with the increase of time. The longer the service time, the greater the compression rate and stretching, the greater the stress drop caused by the relaxation of rubber stress, so that the O-ring is insufficiently elastic and loses its sealing ability. Therefore, it is advisable to try to reduce the compression ratio under the allowed conditions of use.
Increasing the cross-sectional size of the O-ring is the easiest way to reduce the compression ratio, but this will increase the structural size. It should be noted that when people calculate the compression ratio, they often ignore the reduction in section height caused by the O-ring being stretched during assembly. The change in the cross-sectional area of an O-ring is inversely proportional to the change in its circumference. At the same time, due to the effect of tension, the cross-sectional shape of the O-ring will also change, which is manifested as a decrease in its height. In addition, due to surface tension, the outer surface of the O-ring becomes flatter, i.e. the height of the section decreases slightly. This is also a manifestation of O-ring compression stress relaxation.
The degree of deformation of the O-ring section also depends on the hardness of the O-ring material. In the case of the same stretching amount, the O-ring with high hardness will also have a greater reduction in cross-sectional height. From this point of view, materials with low hardness should be selected as much as possible according to the conditions of use. Under the action of liquid pressure and tension, the rubber O-ring will gradually undergo plastic deformation, and its cross-sectional height will decrease accordingly, so that it will eventually lose its sealing ability.
The relationship between temperature and O-ring relaxation process
The service temperature is another important factor affecting the permanent deformation of the O-ring. High temperatures 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 even disappear with the relaxation process and temperature drop of the O-ring. For O-rings that work at sub-zero temperatures, their initial compression may be reduced or completely disappeared due to a sharp drop in temperature. At -50 to -60°C, rubber materials that are not resistant to low temperatures will completely lose their initial stress; even for 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 coefficient of linear expansion. Therefore, when selecting the initial compression, it is necessary to ensure that there is still sufficient sealing ability after the stress drops due to the relaxation process and temperature drop. For O-rings that work at subzero temperatures, special attention should be paid to the recovery index and deformation index of the rubber material.
To sum up, the design should try to ensure that the O-ring has a suitable working temperature, or choose high- and low-temperature-resistant O-ring materials to prolong the service life.
Medium working pressure and permanent deformation The pressure of the working medium is the main factor causing the permanent deformation of the O-ring.
The working pressure of modern hydraulic equipment is increasing day by day. Long-term high pressure will cause permanent deformation of the O-ring. Therefore, appropriate pressure-resistant rubber materials should be selected according to the working pressure during design. The higher the working pressure, the higher the hardness and high pressure resistance of the material used should be. In order to improve the pressure resistance of the O-ring material, increase the elasticity of the material (especially increase the elasticity of the material at low temperature), and reduce the compression set of the material, it is generally necessary to improve the formula of the material and add a plasticizer. However, if the O-ring with plasticizer is immersed in the working medium for a long time, the plasticizer will be gradually absorbed by the working medium, causing the volume of the O-ring to shrink, and may even cause negative compression of the O-ring (ie A gap occurs between the O-ring and the surface of the sealed part).
Therefore, when calculating the compression of the O-ring and designing the mold, these shrinkages should be fully taken into account. The pressed O-ring should maintain the necessary size after soaking in the working medium for 5-10 days and nights.
The compression set of O-ring materials is temperature dependent. When the deformation rate is 40% or greater, leakage will occur, so the heat resistance limits of several rubber materials are: nitrile rubber 70 ° C, EPDM rubber 100 ° C, fluorine rubber 140 ° C. Therefore, countries have made regulations on the permanent deformation of O-rings.
For O-rings of the same material, at the same temperature, the O-ring with a larger cross-sectional diameter has a lower compression set. In oil the situation is different. Since the O-ring is not in contact with oxygen at this time, the above-mentioned adverse reactions are greatly reduced. In addition, it usually causes a certain expansion of the rubber compound, so the compression set rate caused by temperature will be offset. Therefore, the heat resistance in oil is greatly improved. Taking nitrile rubber as an example, its working temperature can reach 120°C or higher.