1. O-ring design:
Improper use will accelerate the damage of O-rings and lose their sealing performance. Experiments show that if the design of each part of the sealing device is reasonable, simple pressure will not cause damage to the O-ring. Under pressure and high temperature working conditions, the main reason for the damage of O-rings is the deformation of O-ring materials and the gap bite caused by the O-ring being squeezed into the sealing gap. The first-level O-ring twists during movement.
2. Deformation of O-ring materials:
Since the synthetic rubber material used for O-rings is a viscoelastic material, the initial set compression amount and rebound blocking ability will be deformed and gradually lost after long-term use, resulting in leakage.
Deformation and disappearance of elastic force are the main reasons for the loss of sealing performance of O-rings. The following are the main reasons for the deformation of O-ring materials:
1) The relationship between compression rate and stretching and deformation of O-ring materials: Rubbers of various formulas used to make O-rings will produce compression stress relaxation under compression. At this time, the compression stress decreases with the increase of time.
The longer the use time, the greater the compression rate and the amount of stretching, the greater the stress drop caused by the relaxation of rubber stress, so that the O-ring is not elastic enough and loses its sealing ability.
Therefore, under the permitted conditions of use, it is advisable to try to reduce the compression rate. Increasing the cross-sectional size of the O-ring is a simple way to reduce the compression rate, but this will increase the structural size. It should be noted that when calculating the compression rate, people often ignore the reduction in the cross-sectional height caused by the stretching of the O-ring during assembly.
The change in the cross-sectional area of the O-ring is inversely proportional to the change in its circumference. At the same time, due to the action of tension, the cross-sectional shape of the O-ring will also change, which is manifested as a decrease in its 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 relaxation of the compression stress of the O-ring.
The degree of deformation of the cross-sectional area of the O-ring also depends on the hardness of the material of the O-ring. Under the same stretching amount, the cross-sectional height of the O-ring with high hardness is also reduced. From this point of view, low hardness materials should be selected as much as possible according to the use conditions. Under the action of liquid pressure and tension, the O-ring made of rubber material will gradually undergo plastic deformation, and the cross-sectional height of the O-ring will decrease accordingly, resulting in loss of sealing ability.
2) The relationship between temperature and the relaxation process of the O-ring: The operating temperature is another important factor affecting the deformation of the O-ring. High temperature will accelerate the aging of rubber materials. The higher the working temperature, the greater the compression deformation of the O-ring.
When the 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 working at sub-zero temperatures, the initial compression of the O-ring may decrease or disappear completely due to the sharp drop in temperature.
At -50 to -60℃, rubber materials that are not resistant to low temperatures will completely lose their initial stress; even for rubber materials that are resistant to low temperatures, 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 linear expansion coefficient. Therefore, when selecting the initial compression, it is necessary to ensure that there is still sufficient sealing ability after the stress decreases due to the relaxation process and temperature drop.