Main failure causes of O-ring seals and their prevention measures

O-ring design, improper use will accelerate its damage and loss of sealing performance. Experiments show that if the design of each part of the sealing device is reasonable, simply increasing the pressure will not cause the destruction of the O-ring. Under high pressure and high temperature working conditions, the main cause of O-ring failure is the permanent deformation of the O-ring material and the gap between the O-ring caused by the O-ring being squeezed into the sealing gap. .
1. Permanent deformation Because the synthetic rubber material used for the O-ring seal is a viscoelastic material, the initial set compression amount and rebound clogging ability will be permanently deformed and gradually lost after a long period of use. leakage. Permanent deformation and loss of elasticity are the main reasons for the loss of sealing performance of O-rings. The following are the main causes of permanent deformation.
1) Relationship between compressibility and elongation and permanent deformation The rubber of various formulations used for making O-rings will produce compressive stress relaxation under compression, and at this time, the compressive stress decreases with time. The longer the use time, the greater the compression ratio and the amount of stretching, the greater the stress drop caused by the relaxation of the rubber stress, so that the O-ring is insufficiently elastic and loses the sealing ability. Therefore, it is desirable to try to reduce the compression ratio under the allowed use conditions. Increasing the cross-sectional dimension of the O-ring is the easiest way to reduce the compression ratio, but this will result in an increase in the size of the structure.
It should be noted that when calculating the compression ratio, it is often overlooked that the height of the section caused by the O-ring being stretched during assembly is reduced. 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 tensile force, the cross-sectional shape of the O-ring will also change, which is represented by its height reduction. In addition, under the surface tension, the outer surface of the O-ring becomes flatter, that is, the height of the section is slightly reduced. This is also a manifestation of the compression stress relaxation of the O-ring.
The degree of deformation of the O-ring section also depends on the hardness of the O-ring material. In the case where the amount of stretching is the same, the height of the section of the O-ring having a large hardness is also greatly reduced. From this point of view, a material having a low hardness should be selected as much as possible according to the use conditions. Under the action of liquid pressure and tension, the O-ring of the rubber material will gradually plastically deform, and the height of the section will be correspondingly reduced, so that the sealing ability is finally lost.
2) Relationship between temperature and O-ring relaxation process The use temperature is another important factor affecting the permanent deformation of the O-ring. High temperatures 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 as the O-ring relaxation process and temperature decrease. An O-ring whose temperature is operating at zero may have an initial compression that may decrease or disappear completely due to a sharp drop in temperature. In the case of -50 to -60 ° C, the low temperature resistant rubber material will completely lose the initial stress; even if the low temperature resistant rubber material, 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 the initial compression amount is selected, it is necessary to ensure sufficient sealing ability after the stress drop due to the relaxation process and temperature drop.
For O-rings whose temperature is below zero, special attention should be paid to the recovery index and deformation index of the rubber material.
In summary, the design should ensure that the O-ring has a suitable working temperature, or use high-temperature, low-temperature O-ring materials to extend the service life.
3) Medium working pressure and permanent deformation The working medium pressure is the main factor causing permanent deformation of the O-ring. The working pressure of modern hydraulic equipment is increasing. Long-term high pressure will cause permanent deformation of the O-ring. Therefore, the design should be based on the working pressure to select the appropriate pressure rubber material. The higher the working pressure, the higher the hardness and high pressure resistance of the materials used.
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 temperatures), and reduce the compression set of the material, it is generally necessary to improve the formulation of the material and add a plasticizer. However, O-rings with plasticizers, soaked in the working medium for a long time, plasticizers will gradually be absorbed by the working medium, resulting in shrinkage of the O-ring seal, and may even cause negative compression of the O-ring (ie A gap occurs between the O-ring and the surface of the seal. Therefore, when calculating the amount of compression of the O-ring and designing the mold, these shrinkage amounts should be fully considered. The pressed O-ring should be immersed in the working medium for 5 to 10 days and nights to maintain the necessary dimensions.
The compression set rate of the O-ring material is temperature dependent. When the deformation rate is 40% or more, leakage occurs, so the heat resistance limits of several rubber compounds are: 70 ° C for nitrile rubber, 100 ° C for EPDM, and 140 ° C for fluororubber. Therefore, countries have defined the permanent deformation of O-rings. The dimensional changes of O-rings of Chinese standard rubber materials at different temperatures are shown in the table. O-rings of the same material, at the same temperature, O-rings with large cross-sectional diameters have a lower compression set ratio.
The situation in oil is different. Since the O-ring is not in contact with oxygen at this time, the above-mentioned adverse reaction is greatly reduced. In addition, it usually causes a certain expansion of the rubber, so the compression set rate due to temperature will be offset. Therefore, the heat resistance in the oil is greatly improved. Taking nitrile rubber as an example, its working temperature can reach 120 ° C or higher.
2, gap bite sealed parts have geometric accuracy (including roundness, ovality, cylindricity, coaxiality, etc.), the difference between the parts and the expansion of the inner diameter under high pressure, etc., will cause the seal gap The expansion and the phenomenon of gap extrusion are intensifying. The hardness of the O-ring also has a significant effect on the gap extrusion phenomenon. The higher the pressure of the liquid or gas, the smaller the hardness of the O-ring material, and the more the gap extrusion phenomenon of the O-ring is.
The measure to prevent the gap bite is to strictly control the hardness and sealing gap of the O-ring. Use a sealing material with a suitable hardness to control the gap. The hardness of commonly used O-rings ranges from HS60 to 90. Low hardness is used for low pressure, high hardness is used for high pressure.
The use of a suitable seal to protect the retaining ring is an effective way to prevent the O-ring from being squeezed into the gap.
3. Distortion phenomenon Distortion refers to the phenomenon that the O-ring is twisted in the circumferential direction, and the distortion phenomenon generally occurs in the dynamic sealing state.
If the O-ring is properly assembled and used properly, it is generally not easy to cause rolling or twisting in the reciprocating state because the O-ring has a larger contact area with the groove than the frictional contact area on the sliding surface. The resistance of the O-ring itself can prevent distortion. The distribution of friction also tends to keep the O-ring stationary in its grooves because the static friction is greater than the sliding friction and the roughness of the groove surface is generally not as good as the roughness of the sliding surface.
There are many reasons for the distortion damage, the most important of which is due to the unevenness of the gap between the piston, the piston rod and the cylinder barrel, the excessive eccentricity, the uneven diameter of the O-ring section, etc., due to the friction caused by the O-ring in a week. The force is not uniform, and some parts of the O-ring are excessively rubbed and twisted. Generally, O-rings having a small cross-sectional size are prone to frictional unevenness. This causes distortion (the O-ring for motion is larger than the diameter of the section of the fixed O-ring.)
In addition, due to the coaxiality deviation of the sealing groove, the unequal sealing height and the uneven diameter of the O-ring cross-section, it is possible that a part of the O-ring is compressed too much, and the other part is too small or uncompressed. When the groove is eccentric, that is, the coaxial deviation is greater than the compression of the O-ring, the seal will completely fail. Another disadvantage of the large deviation of the sealing groove from the coaxiality is that the O-ring is unevenly compressed circumferentially. In addition, due to the influence of the O-ring cross-sectional diameter, the material hardness, the thickness of the lubricating oil film, and the surface roughness of the sealing shaft, a part of the O-ring slides along the working surface, and the other part rolls, resulting in The twist of the O-ring. Sports O-rings are easily damaged by twisting, which is an important cause of damage and leakage of the seal. Therefore, improving the precision of the sealing groove and reducing the eccentricity are important factors for ensuring the O-ring has a reliable seal and life.
Installing the seal should not be that it is twisted. If it is twisted during installation, the distortion damage will happen quickly. In the work, the distortion will cut off the O-ring, resulting in a large amount of oil leakage, and the cut O-ring will be mixed into other parts of the hydraulic system, causing a major accident.
In order to prevent the twisting damage of the O-ring, the following points should be noted in the design: 1) The concentricity of the O-ring mounting groove should be considered from the aspects of convenient processing and no distortion.
2) The O-ring section size should be uniform, and the lubricant or grease should be fully applied to the seal at each installation. Sometimes it is possible to use a felt-type refueling device that is saturated with lubricating oil.
3) Increase the cross-sectional diameter of the O-ring. The cross-sectional diameter of the O-ring for dynamic sealing should generally be larger than that of the O-ring for static sealing. In addition, the O-ring should be avoided as a seal for large-diameter pistons.
4) When twisting damage occurs at low pressure, the retaining ring can be used to protect the retaining ring.
5) Reduce the surface roughness of the cylinder and piston rod.
6) O-rings are made of materials with low friction coefficient.
7) Replace the O-ring with a seal that is less prone to distortion.
4. Abrasive wear phenomenon When the sealed gap has relative motion, dust and sand in the working environment are adhered to the surface of the piston rod, and are brought into the cylinder together with the oil film as the reciprocating motion of the piston rod becomes intrusion. Abrasive particles on the surface of the O-ring accelerate the wear of the O-ring so that it loses its seal. In order to avoid this, a dust seal must be used at the outrigger end of the reciprocating seal.
5, the impact of the sliding surface on the O-ring The roughness of the sliding surface is a direct factor affecting the friction and wear of the O-ring surface. Generally speaking, the surface smoothness and wear are small, so the roughness of the sliding surface is often low (Ra 0.2 ~ 0.050 μm). However, tests have shown that surface roughness is too low (Ra below 0.050 μm) and adversely affects friction and wear. This is because the minute surface is uneven and the necessary lubricating oil film can be maintained. Therefore, choose the appropriate surface requirements.
The material of the sliding surface also has an effect on the life of the O-ring. The greater the hardness of the sliding surface material, the higher the wear resistance, the stronger the ability to maintain smoothness, and the longer the life of the O-ring. This is also an important reason for the chrome plating on the piston rod surface of hydraulic cylinders. Similarly, it can be explained that the sliding surface made of copper and aluminum alloy with the same roughness is more serious than the friction and wear of the sealing surface of the steel sliding surface, and the sealing ring with low hardness and large compression is not as high hardness and small compression. The amount of seal is durable.
6. Friction and O-ring Application In dynamic sealing devices, friction and wear are important factors influencing the damage of O-rings. The degree of wear depends mainly on the amount of friction. When the liquid pressure is small, the amount of friction of the O-ring depends on its pre-compression amount. When the working fluid is subjected to pressure, the frictional force increases as the working pressure increases. In the case where the working pressure is less than 20 MPa, it is approximately linear. When the pressure is greater than 20 MPa, as the pressure increases, the contact area between the O-ring and the metal surface increases gradually, and the frictional force increases accordingly. Under normal conditions, the service life of the O-ring will decrease with a square relationship as the liquid pressure increases.
The increase in friction causes a large amount of frictional heat to be generated between the rotating or reciprocating shaft and the O-ring. Since most O-rings are made of rubber, the thermal conductivity is extremely poor. Therefore, the frictional heat causes the rubber to age, causing the O-ring to be effective and destroying its sealing performance. Friction can also cause damage to the O-ring surface, reducing the amount of compression. Severe friction can quickly cause damage to the surface of the O-ring and lose its seal. When used for pneumatic reciprocating seals, frictional heat can also cause sticking, resulting in a further increase in friction.
When the sports seal is used at low speeds, the frictional resistance is also a factor that causes creep and affects the performance of the components and systems. Therefore, friction is one of the important properties for sports seals. The friction coefficient is an evaluation index of friction characteristics. The friction coefficient of synthetic rubber is large. Because the seal is in the moving state, it is usually in the mixed lubrication state in which the working oil or lubricant participates, and the friction coefficient is generally below 0.1.
The amount of friction depends to a large extent on the surface hardness and surface roughness of the seal being sealed.
7. Joule heating effect The Joule heating effect of rubber material refers to the phenomenon that the rubber in the stretched state shrinks when heated. When the O-ring is installed, in order to prevent it from being swayed in the sealing groove, when it is used as a reciprocating seal, no distortion occurs, and it is generally stretched to some extent. However, if this mounting method is used for a rotary motion, undesirable results are produced. The O-ring seal that has been tightly hung on the rotating shaft shrinks due to the frictional heat generated by the rotational motion, thereby increasing the tightening force, thus generating frictional heat → shrinking → tightening force is increased → friction is generated Heat → ..., so repeated cycles, greatly promoted the aging and wear of rubber.