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O-ring deformation of the reasons?

Release time:

2019-05-13 12:30

Source:

Causes of O-ring deformation
 
O-ring seal is the most used sealing form in fluid power system. Because the O-ring has the advantages of simple structure, good sealing performance, small friction, small groove size, easy to manufacture, etc., it is widely used in hydraulic and pneumatic components.
 
The working mechanism of the O-ring seal is to install the O-ring seal in the sealing groove, deform it through the pressure of oil (or gas), squeeze it to one side of the groove, and seal the sealed contact surface. The liquid flow (or air flow) cannot pass through. This seal has a good sealing effect in the fluid power system. However, if the O-ring seal is not designed and used properly, it is easy to cause deformation of the sealing ring.
 
1. Minimize the roughness of the surface of the cylinder barrel and the piston rod;
 
2, can be used to wear similar sealing ring sealing ring instead of work;
 
3. The size of the end face is designed evenly, and the lubricating oil should be fully applied to the sealing part every time it is installed;
 
4, the installation groove concentric precision condition, should be convenient processing and does not produce distortion phenomenon two aspects to consider;
 
5. Increase the cross-sectional diameter of the seal. The cross-sectional diameter of the dynamic seal ring is generally larger than the static seal ring specifications. At the same time, the seal ring should be avoided as a seal for large-diameter pistons;
 
6. The sealing ring is made of low friction material, and the sealing ring can be used to protect the role of the retaining ring.
 
O-ring is the most widely used in all sealing products, the production process is relatively simple, the application of more materials. The O-ring seal is a typical sealing effect produced by extrusion and deformation. It produces contact pressure with the sealing surface during installation. When the pressure is greater than the pressure of the sealing medium, the sealing effect is produced, and when it is less than, leakage will occur.
 
The causes of permanent deformation of the o-ring are as follows:
(1) O-ring compression and tension factors. Due to the different formulations of O-ring rubber, the compression and tension of O-ring produced by different enterprises are also different. Products in the state of long-term compression will produce compression stress relaxation imagination, this phenomenon will gradually expand over time, the longer the amount of compression and tension will be smaller, so that the lack of elasticity lead to leakage phenomenon, the most direct change method is to increase the cross-sectional size of the product, but also lead to the increase in the structure of the product.
 
(2) The relationship between temperature and the relaxation process of O-ring The temperature is very important for the relaxation degree of O-ring. No matter what kind of rubber material, it will accelerate its aging speed at high temperature. When the temperature in the environment is higher, the deformation of gas compression will be greater. When the deformation of the product exceeds 40%, the O-ring will slowly lose its elastic ability and cause leakage. When installing the O-ring, there will be initial stress, which will slowly disappear after long-term relaxation and gradual decrease of temperature. In some cases, it may disappear with the sharp decrease of temperature. Even if the rubber material is resistant to low temperature, it will not be greater than 25% of the stress generated at 20 degrees. Therefore, the initial stress is set when installing the O-ring, the temperature factor in the working environment should be fully considered.

(3) the influence of medium working pressure on the deformation of o-ring. Compared with the above two factors, the pressure of the medium has a greater influence on the deformation of the O-ring, which is the most common situation that causes the deformation of the O-ring in all working conditions. With the development of modern hydraulic equipment, the pressure of the hydraulic medium is increasing. The O-ring will cause permanent deformation of the O-ring when working in this high-pressure environment for a long time, which is irreversible, therefore, different materials should be selected for different working pressures, and relatively pressure-resistant rubber materials should be selected. Correspondingly, in order to cope with higher working pressures, the hardening degree of the pressure-resistant material sealing ring will also increase. The pressure of the working medium is the main factor that causes the permanent deformation of the O-ring. The working pressure of modern hydraulic equipment is increasing.
Prolonged high pressure will cause permanent deformation of the O-ring. Therefore, the design should be based on the working pressure selection of appropriate pressure rubber materials. The higher the working pressure, the higher the hardness and high pressure resistance of the material used.
 
Since the synthetic rubber material used in the silicone seal ring is a viscoelastic material, the initial set of compression and rebound blocking ability will produce permanent deformation and gradually lose after long-term use, and eventually leak. Permanent deformation and elastic loss are the main reasons for the loss of sealing performance of O-rings.
 
One. The temperature affects the permanent deformation of the silicone seal ring. The use temperature is another important factor that affects the permanent deformation of the silicone seal ring. High temperature will accelerate the aging of rubber materials. The higher the working temperature, the greater the compression set of the silicone seal 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 silicone seal ring due to compression deformation will gradually decrease and disappear with the action of the relaxation process and temperature drop of the silicone seal ring. The initial compression of the O-ring working at a temperature below zero may be reduced or completely disappeared due to a sharp decrease 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 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 silicone seal depends on the linear expansion coefficient. 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 the temperature drop. For silicone seals that work at sub-zero temperature, special attention should be paid to the recovery index and deformation index of rubber materials. In summary, the design should try to ensure that the silicone sealing ring has a suitable working temperature, or use high and low temperature resistant silicone sealing ring material to extend the service life.
 
II. Compression rate and tensile effect of permanent deformation of the various formulations of rubber used in the production of silicone seals, in the compressed state will produce compressive stress relaxation phenomenon, at this time, the compressive stress decreases with the growth of time. The longer the use time, the greater the compression rate and the greater the amount of tension, the greater the stress drop caused by the relaxation of the rubber stress, so that the O-ring seal is not elastic enough and loses its sealing ability.
 
Therefore, it is desirable to try to reduce the compression rate under the allowable use conditions. Increasing the cross-sectional size of the O-ring is the simplest way to reduce the compression rate, but this will increase the size of the structure. It should be noted that when calculating the compression rate, people often ignore the reduction of the cross-sectional height caused by the stretching of the silicone seal ring during assembly. The change of the cross-sectional area of the silicone seal ring is inversely proportional to the change of its circumference.
 
At the same time, due to the pulling force, the cross-sectional shape of the silicone seal ring will also change, which is reflected in the reduction of its height. In addition, under the action of surface tension, the outer surface of the silicone seal ring becomes flatter, that is, the cross-sectional height is slightly reduced. This is also a manifestation of the compression stress relaxation of the silicone seal ring. The degree of deformation of the silicone seal ring section also depends on the hardness of the silicone seal ring material. In the case of the same amount of stretching, the hardness of the silicone seal, the cross-sectional height is also reduced more, from this point of view, should be in accordance with the conditions of use as far as possible to choose low hardness material. Under the action of liquid pressure and tension, the silicone sealing ring of rubber material will gradually undergo plastic deformation, and its cross-sectional height will be reduced accordingly, so that the sealing ability will be lost in the end.
Three. The pressure of the working medium causes the permanent deformation of the silicone sealing ring. The pressure of the working medium is the main factor causing the permanent deformation of the silicone sealing ring. The working pressure of modern hydraulic equipment is increasing. Long time high pressure will make the silicone sealing ring permanent deformation. Therefore, the design should be based on the working pressure selection of appropriate pressure 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 silicone sealing ring material and increase the elasticity of the material (especially to increase the elasticity of the material at low temperature and reduce the compression set of the material, it is generally necessary to improve the formulation of the material and add plasticizers. However, the silicone sealing ring with plasticizer is soaked in the working medium for a long time, and the plasticizer will be gradually absorbed by the working medium, resulting in the volume contraction of the silicone sealing ring, and even the negative compression of the silicone sealing ring (I. e., a gap occurs between the surface of the silicone sealing ring and the sealed part).
 
Therefore, these contractions should be fully taken into account when calculating the compression of the silicone seal ring and designing the mold. The pressed silicone sealing ring should be kept to the necessary size after soaking in the working medium for 5 to 10 days and nights. The compression permanent deformation rate of silicone seal ring material is related to temperature.
 
When the deformation rate is 40% or more, leakage will occur, so the heat resistance limits of several compounds are: nitrile rubber 70 ℃, EPDM 100 ℃, fluorine rubber 140 ℃. Therefore, countries have made provisions for the permanent deformation of silicone seals. The dimensional changes of O-rings made of Chinese standard rubber materials at different temperatures are shown in the table. The silicone sealing ring of the same material, at the same temperature, the silicone sealing ring with a large cross-sectional diameter has a lower compression set.
 
The situation is different in oil. Since the silicone seal ring is not in contact with oxygen at this time, the above adverse reactions are greatly reduced. In addition, it usually causes a certain expansion of the rubber material, so the compression permanent deformation 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 ℃ or higher.
                   

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