Improper design and use of O-rings will accelerate its damage and lose its sealing performance. Experiments show that if the various parts of the sealing device are designed reasonably, simply increasing the pressure will not cause the O-ring to be damaged. Under high pressure and high temperature working conditions, the main reason for the damage of O-rings is the permanent deformation of the O-ring material and the gap bite caused by the O-ring being squeezed into the sealing gap. The first-level O-ring is twisted during movement.

Permanent deformation

Since the synthetic rubber material used for O-ring sealing rings is a viscoelastic material, the initial set compression amount and rebound blocking ability will produce permanent deformation and gradually lose after long-term use, and eventually leak. Permanent deformation and disappearance of elasticity are the main reasons for the loss of sealing performance of O-rings. The following are the main reasons for permanent deformation.

1). Relationship between compression rate and stretching amount and permanent deformation

The various formulas of rubber 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 stretching amount, the greater the stress drop caused by rubber stress relaxation, resulting in insufficient elasticity of the O-ring and loss of sealing ability. Therefore, it is advisable to try to reduce the compression rate under the permitted conditions of use. Increasing the cross-sectional size of the O-ring is the simplest 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 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 O-ring cross section also depends on the hardness of the O-ring material. Under the same stretching amount, the cross-sectional height of the O-ring with high hardness is also reduced more. From this point of view, low hardness materials should be selected as much as possible according to the conditions of use. Under the action of liquid pressure and tension, the O-ring made of rubber material will gradually undergo plastic deformation, and its cross-sectional height will be reduced accordingly, so that it will eventually lose its sealing ability.

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

Operation temperature is another important factor affecting the permanent deformation of O-rings. High temperature will accelerate the aging of rubber materials. The higher the working temperature, the greater the compression permanent deformation 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 with the relaxation process of the O-ring and the effect of temperature drop.

For O-rings working at sub-zero temperatures, their initial compression may decrease or disappear completely due to the sharp drop in temperature. At -50~-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. For O-rings working at sub-zero temperatures, special attention should be paid to the recovery index and deformation index of the rubber material.

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

3). Medium working pressure 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 day by day. Long-term high pressure will cause permanent deformation of the O-ring. Therefore, when designing, appropriate pressure-resistant rubber materials should be selected according to the working pressure. 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 increase the elasticity of the material at low temperatures), and reduce the compression permanent deformation of the material, it is generally necessary to improve the material formula and add plasticizers. However, if the O-ring with plasticizer is immersed in the working medium for a long time, the plasticizer will gradually be absorbed by the working medium, resulting in the shrinkage of the O-ring volume, and may even cause negative compression of the O-ring (that is, a gap 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 considered. The pressed O-ring should be able to maintain the necessary size after being soaked in the working medium for 5 to 10 days and nights.

The compression permanent deformation rate of the O-ring material is related to the temperature. When the deformation rate is 40% or more, leakage will occur, so the heat resistance limits of several rubber materials are: 70℃ for nitrile rubber, 100℃ for EPDM rubber, and 140℃ for fluororubber. Therefore, various countries have made regulations on the permanent deformation of O-rings. The size changes of O-rings made of Chinese standard rubber materials at different temperatures are shown in the table. For O-rings of the same material, at the same temperature, the compression permanent deformation rate of O-rings with large cross-sectional diameters is lower.

The situation in oil 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 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.

Gap bite

The sealed parts have poor geometric accuracy (including roundness, ovality, cylindricity, coaxiality, etc.), non-concentricity between parts, and expansion of the inner diameter under high pressure, which will cause the expansion of the sealing gap and the aggravation of the gap extrusion phenomenon. 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 serious the gap extrusion phenomenon of the O-ring.

The measure to prevent gap bite is to strictly control the hardness and sealing gap of the O-ring. Select sealing materials with appropriate hardness to control the gap. The hardness range of commonly used O-rings is HS60~90. Low hardness is used for low pressure, and high hardness is used for high pressure. Using an appropriate sealing ring protection ring is an effective way to prevent the O-ring from being squeezed into the gap.

Twist phenomenon

Twist refers to the phenomenon that the O-ring twists along the circumferential direction. Twist phenomenon generally occurs in the dynamic sealing state.

If the O-ring is properly assembled and used under appropriate conditions, it is generally not easy to roll or twist in reciprocating motion, because the contact area between the O-ring and the groove is larger than the friction contact area on the sliding surface, and the resistance of the O-ring itself can prevent twisting. The distribution of friction also tends to keep the O-ring stationary in its groove, because static friction is greater than 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 twisting damage, the most important of which are uneven clearance between the piston, piston rod and cylinder, excessive eccentricity, uneven cross-sectional diameter of the O-ring, etc., which causes the O-ring to be subjected to uneven friction in more than one week, and some parts of the O-ring are rubbed too much and twisted. Usually, O-rings with smaller cross-sectional dimensions are prone to uneven friction and twisting (this is why the cross-sectional diameter of the O-ring for movement is larger than that of the O-ring for fixed use).

In addition, due to the coaxiality deviation of the sealing groove, unequal sealing height and uneven diameter of the O-ring section, part of the O-ring may be compressed too much, while the other part is too small or not compressed. When the groove is eccentric, that is, the coaxial deviation is greater than the compression of the O-ring, the seal will fail completely. Another harm of large coaxiality deviation of the sealing groove is that the O-ring is compressed unevenly along the circumference.

In addition, due to the uneven cross-sectional diameter, material hardness, lubricating oil film thickness of the O-ring and the surface roughness of the sealing shaft, part of the O-ring slides along the working surface, while the other part rolls, causing the O-ring to twist. Sports O-rings are easily damaged by twisting, which is an important reason for damage and leakage of sealing devices. Therefore, improving the machining precision of the sealing groove and reducing eccentricity are important factors to ensure reliable sealing and life of the O-ring.

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The seal ring should not be installed in a twisted state. If it is twisted during installation, twisting damage will occur quickly. During work, the twisting phenomenon will cut off the O-ring, causing a large amount of oil leakage, and the cut O-ring will be mixed with other parts of the hydraulic system, causing major accidents.

In order to prevent the twisting damage of the O-ring, the following points should be noted during design:

1). The concentricity of the O-ring installation groove should be considered from the two aspects of convenient processing and no twisting.

2). The cross-sectional size of the O-ring should be uniform, and lubricating oil or grease should be fully applied to the sealing part during each installation. Sometimes a felt ring type oiling device soaked in lubricating oil can also be used.

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 not be used as a seal for a large-diameter piston.

4). When distortion damage occurs under low pressure, a seal ring can be used to protect the retaining ring.

5). Reduce the surface roughness of the cylinder barrel and piston rod.

6). Use materials with low friction coefficient to make O-rings.

7). O-rings that are not prone to distortion can be used instead of seals.

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 piston rod reciprocates, becoming abrasive particles that invade the surface of the O-ring, accelerating the wear of the O-ring, and causing it to lose its sealing performance. In order to avoid this situation, a dust ring must be used at the extended shaft end of the reciprocating seal device.

The influence 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 smoother the surface, the less friction and wear there will be, so the roughness value of the sliding surface is often very low (Ra0.2~0.050μm). However, tests have shown that too low surface roughness (Ra below 0.050μm) will have an adverse effect on friction and wear. This is because the tiny surface unevenness can maintain the necessary lubricating oil film. Therefore, it is necessary to select appropriate surface requirements.

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The material of the sliding surface also affects the life of the O-ring. The harder 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 chrome plating on the surface of the hydraulic cylinder piston rod. The same reason can explain why the friction and wear of the sliding surface made of copper and aluminum alloy with the same roughness on the sealing ring is more serious than that of the steel sliding surface, and the sealing ring with low hardness and large compression is not as durable as the sealing ring with high hardness and small compression.

Friction and O-ring application

In dynamic sealing devices, friction and wear are important factors affecting the damage of O-rings. The degree of wear mainly depends on the magnitude of friction. When the liquid pressure is small, the magnitude of the friction of the O-ring depends on its pre-compression. When the working liquid is under pressure, the friction increases with the increase of working pressure. When the working pressure is less than 20MPa, it is approximately linear. When the pressure is greater than 20MPa, as the pressure increases, the increase in the contact area between the O-ring and the metal surface gradually slows down, and the increase in friction is also slow. Under normal circumstances, the service life of the O-ring will decrease approximately in a square relationship with the increase in liquid pressure.

The increase in friction generates a large amount of friction heat between the rotating or reciprocating shaft and the O-ring. Since most O-rings are made of rubber, the thermal conductivity is very poor. Therefore, friction heat will cause aging of rubber, leading to the failure of O-rings and destroying their sealing performance. Friction will also cause surface damage of O-rings, reducing the compression amount. Severe friction will quickly cause surface damage of O-rings and lose their sealing properties. When used as a seal for pneumatic reciprocating motion, friction heat will also cause adhesion, causing the friction force to increase further.

When the motion seal moves at a low speed, friction resistance is still a factor that causes creeping, affecting the working performance of components and systems. Therefore, for motion seals, friction is one of the important properties. The friction coefficient is an evaluation index of friction characteristics. The friction coefficient of synthetic rubber is relatively large. Since the seal is usually in a mixed lubrication state with the participation of working oil or lubricant when in motion, the friction coefficient is generally below 0.1. The magnitude of friction depends to a large extent on the surface hardness and surface roughness of the sealed component.

Joule heat effect

The Joule heat effect of rubber materials refers to the phenomenon that rubber in a stretched state shrinks when heated. When installing the O-ring, in order to prevent it from moving in the sealing groove and not twisting when used as a reciprocating seal, it is generally stretched to a certain extent. However, if this installation method is used for rotational motion, it will produce undesirable results. The O-ring, which is already tightly clamped on the rotating shaft, shrinks due to the frictional heat generated by the rotational motion, which in turn increases the clamping force. In this way, frictional heat is generated → shrinkage → increased clamping force → frictional heat is generated → ..., and this cycle is repeated, which greatly promotes the aging and wear of the rubber.