O-ring failure causes and prevention measures


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O-ring design, improper use will accelerate the damage, loss of sealing performance. The experiments show, such as the rational design of each part of the sealing device, simply increase the pressure does not result in the destruction of the O-ring. Under high pressure, high temperature working conditions, O-

Ring damage mainly due to the permanent deformation of the material of the O-ring and O-ring squeeze into the sealing gap caused by the gap bitten by an O-ring distortions in the exercise.

1, the permanent deformation

Gradual loss of the O-ring seals used in synthetic rubber materials are viscoelastic materials, so the initial set of compression capacity and resilience to clogging by the use of long, permanent deformation, and eventually leak. Permanent deformation and elastic

The disappearance of the O-ring main reason for loss of sealing performance, the following is the main reason for the permanent deformation.

1) compression rate and stretch the amount of permanent deformation

Production of rubber O-ring used a variety of formulations, in the compressed state will have a compression stress relaxation at this time, the compressive stress decreases with the growth of the time. Use the longer, the greater the compression ratio and the amount of stretch from rubber stress loose

Relaxation and stress drop, the greater, so that O ring flexibility, loss of sealing ability. Therefore, allow the use of conditions to try to reduce the compression ratio is desirable. Increase in cross-section dimensions of the O-ring is the most simple way to reduce the compression ratio, not

Over this increase in the size of the structure.

It should be noted that people often overlooked in the calculation of compression ratio, the O-ring in the assembly under tension caused by the height of cross section decreases. Changes in the O-ring cross-sectional area is inversely proportional to its perimeter changes. At the same time, due to the role of tension, O-

The cross-section shape of the circle will change the performance of its height decreases. In addition, the effect of surface tension, the outer surface of the O-ring becomes more flat, that is section height decreases slightly. This is also a manifestation of the O-ring compression stress relaxation.

The degree of deformation of the O-ring cross section, but also depends on the hardness of the O-ring material. Stretch of the same case, the hardness of the O-ring cross-section height decreases more, from this point of view, should try to use the low hardness of the material in accordance with the conditions of use

. Under the action of liquid pressure and tension, the rubber O-ring material plastic deformation, its section height will be reduced accordingly, so that the final loss of sealing ability.

2) temperature and the O-ring of the relaxation process of relationship

The use of temperature is another important factor in the permanent deformation of the O-ring. The high temperature will accelerate the aging of the rubber material. Higher operating temperature, the O-ring compression is greater. When permanent deformation greater than 40%, O-ring to lose the seal can

Force and leak. The initial stress value, the formation of compressive deformation in the rubber material of the O-ring with the O-ring, the relaxation process and the temperature dropped gradually reduced even disappeared. Temperature minus the work of O-ring, the initial compression may

Due to a sharp reduction in temperature decrease or completely disappear. In the case of -50 to -60 C, the cold intolerance of the rubber material will completely lose the initial stress; even if the low-temperature-resistant rubber material, the initial stress not greater than 20 C when the initial stress

25%. This is because the O-ring, the initial amount of compression depends on the coefficient of linear expansion. Therefore, the initial amount of compression is selected, you must ensure that there is still sufficient sealing capability after the stress drop due to the relaxation process and the temperature dropped.

Temperature minus the work of O-ring, should pay particular attention to the recovery index, and deformation index of the rubber material.

In summary, the design should try to ensure the appropriate operating temperature for the O-rings, or selection of resistance to high and low temperature O-ring material to extend the service life.

3) media pressure and permanent deformation

The pressure of the working medium is to cause a permanent deformation of the main factors of the O-ring. Modern hydraulic equipment, pressure is increasing. Long high pressure will make the O-ring permanent deformation. Therefore, the design shall be based on the choice of appropriate work pressure

The pressure rubber material. Higher working pressure, material hardness and high voltage performance should be the higher.

In order to improve the pressure resistance of the O-ring material, increasing the flexibility of the material (in particular, to increase the elasticity of the material at low temperatures), reducing the compression of the material, the general need to improve the formula of the material, adding plasticizers. However, with plasticizer

O-ring, a long soak in the working medium, the plasticizer will be gradually absorbed by the working medium, resulting in the contraction of the O-ring volume, and may even produce negative compression (ie, in the O-ring and O-ring seals the surface between

Gap). Therefore, in the calculation of the O-ring compression mold design should take full account of these shrinkage. Suppression of the O-ring should be made to still maintain the necessary size of 5 to 10 days and nights immersed in the working medium.

Compression of the O-ring material to permanent deformation rate and temperature. When the deformation rate of 40% or greater, that is, there will be leakage, heat resistance of the boundaries of the several rubber material: NBR 70 , 100 C EPDM, Viton 140 . Therefore

Countries makes provision for the permanent deformation of the O-ring. Dimensional changes of the standard rubber O-ring at different temperatures are shown in Table. O-ring of the same material, at the same temperature, the cross-section large diameter O-ring compression permanent deformation rate is low.

In the oil situation is different. The O-ring in contact with oxygen, so the above adverse reactions are greatly reduced. Combined they usually cause the rubber expansion due to temperature variations caused by the compression rate will be offset. Therefore

, In oil, the heat resistance is greatly improved. Nitrile rubber, for example, its operating temperature up to 120 C or higher.

2, the gap bites

The existence of geometric accuracy (roundness, ovality, cylindricity, concentricity, etc.) bad sealed parts, the parts between the different heart and under high pressure inner diameter swell phenomenon, will lead to the expansion of the sealing gap and gap extrusion the intensification of the phenomenon

. Also have a significant impact on the hardness of the O-ring gap extrusion phenomenon. The higher the pressure of liquids or gases, the smaller O-ring material hardness, the gap of the O-ring extrusion phenomenon is more serious.

Measures to prevent the gap bites, the hardness of the O-ring and seal the gap to be strictly controlled. Selection of the appropriate hardness sealing material control gap. The hardness range of commonly used O-ring HS60 ~ 90. The low hardness for low voltage, high hardness

Those used for high pressure.

Equipped with the appropriate seals to protect the retainer ring, is an effective way to prevent O-ring squeeze into the gap.

3, the distortions

Distortions refers to the phenomenon of the O-ring torsion along the circumference, the distortions that generally occurs in the state of dynamic seal.

O-ring assembly proper, and appropriate conditions of use are generally not easy to produce reciprocating motion state rolling, or distorted, because the contact area of ​​the O-ring groove is greater than the friction in the sliding surface contact area, and O ring itself

Resistance ability of the original will be able to prevent distortions. The distribution of friction tends to keep the O-ring in its groove, stationary, static friction greater than sliding friction, and the groove surface roughness is generally not as good as the roughness of the sliding surface.

Cause distortions injury for many reasons, foremost of which is uneven due to the clearance of the piston, piston rod and cylinder eccentricity is too large, the O-ring cross-section diameter is uneven caused, resulting in O-ring friction by more than a week is uneven, the O-shaped

Some part of the circle of friction is too large, distorted. Typically, the smaller section size O-ring can easily lead to friction and uneven. Cause distortions in the (sports O-ring cross-section diameter of the O-ring large is the truth.)

In addition, because the seals coaxial deviation, seal height are not equal and non-uniform cross-section diameter of O-ring phenomenon, may make it part of the O-ring compressed large, the other part is too small or not compressed. When trench existence of eccentric

Coaxial deviation is greater than the O-ring compression seal will be completely ineffective. Seals the concentricity deviation of the other harmful effects to make the O-ring compression uneven along the circumference. There is also due to the O-ring cross-section diameter, material hardness, lubricant film thickness

Of inequality, as well as sealing the shaft surface roughness and other factors, led to a part of the O-ring slide along the work surface, the other part is rolling, resulting in distortion of the O-ring. Movement with O-ring is easy to distort the damage, which is a sealed device

Important reason for damage and leaks. Therefore improve the processing precision seals and reducing the eccentricity is to ensure that the O-ring with reliable sealing and an important factor in life.

Install seals should not be in a distorted state. If the installation has been twisted, distorted the damage will soon occur. At work, the distortions in the O-ring cut off, resulting in a large number of oil spills, and cut off the O-ring mingled hydraulic system

Other parts of the system, causing a major accident.

In order to prevent distortions of the O-ring damage, note the following points should be designed

1) O-ring installation groove concentricity size, should be easy to process and does not produce distortions in the two aspects to consider.

2) O-ring cross section size should be uniform, and each installation should be fully smear of oil or grease in a sealed area. Sometimes can be soaked in lubricant vinyl lap refueling device.

3) increase the cross-section diameter of the O-ring, dynamic sealing O-ring cross-section diameter should generally be greater than the static sealing O-ring; In addition, O-ring should be avoided as a large-diameter piston seal.

4) at low pressure distorting injury, you can use the ring to protect the retaining ring.

5) reduce the surface roughness of the cylinder and the piston rod.

6) low coefficient of friction O-ring material.

7) can be difficult to produce distortions in the ring instead of the O-ring.

4, the abrasive wear phenomenon

When sealing the gap with the relative movement of the working environment of dust and sand adhere to the surface of the piston rod, and with the reciprocating motion of the piston rod and the film into the tank, invade the O-ring surface abrasive to accelerate the O-

Circle wear, resulting in the loss of tightness. In order to avoid this from happening, the Department must use the dust ring stretch in reciprocating seals the outer shaft end.

5, the sliding surface of the O-ring

The roughness of the sliding surface is a direct factor of the surface friction and wear of the O-ring. Generally speaking, the smooth surface friction and wear, the sliding surface roughness values ​​are often low (Ra0.2 ~ 0.050m). However, the experiments show that

The low surface roughness (Ra lower than 0.050m) and give the friction and wear an adverse impact. This is because the tiny surface uneven, you can maintain the necessary lubricant film. To select the proper surface requirements.

The sliding surface of the material life of the O-ring is also affected. The greater the hardness of the sliding surface material, the higher the wear resistance, the greater ability to maintain a smooth, longer life expectancy of the O-ring. This is also the hydraulic cylinder piston rod surface chrome

. Similarly with the same roughness of copper can be explained, made of aluminum alloy sliding surface is more serious than the surface of the steel sliding friction and wear on the seals, low hardness, compression seals as high hardness, compression the amount of the ring and durable


6, friction, and the application of the O-ring

Seals, friction and wear are important factors for the O-ring is damaged. The degree of wear depends on the size of the friction. When the liquid pressure is small, the size of the O-ring friction depends on its pre-compression. When work liquid bearing

By pressure, friction along with the increase in work pressure increases. In the case of working pressure less than 20MPa, the approximate linear relationship. Pressure is greater than 20MPa, the O-ring with the metal surface contact area increases gradually with increasing pressure

Slow, friction increase correspondingly slow. Under normal circumstances, the life of the O-ring with the rise of the liquid pressure will approximate the square of the decrease.

Increase of the friction between rotating or reciprocating motion axis and the O-ring to produce large amounts of frictional heat. As the majority of O-rings are made of rubber, the poor thermal conductivity. Therefore, the frictional heat caused by the aging of rubber, resulting in O-ring

Effectiveness and undermine its sealing performance. Friction can also cause surface damage of the O-ring, so reducing the amount of compression. Serious friction will soon cause the surface of the O-ring damage, loss of tightness. Pneumatic reciprocating motion by the seal, the frictional heat can also cause adhesion

Resulting in further increase of friction.

Movement sealed in slow motion, friction is caused by crawling a factor, affecting the performance of components and systems. So the seal for the movement, friction is one of the important properties. An evaluation of the coefficient of friction is the friction characteristics

Index, coefficient of friction of synthetic rubber, sealed in a state of motion, usually in the working fluid or lubricant to participate in the mixed lubrication, the friction coefficient is generally below 0.1.

The friction force depends largely on the seal surface hardness and surface roughness.

7, the Joule heating

Joule heating of the rubber material is in a stretched rubber when exposed to heat shrinkage phenomenon. Install the O-ring seals in order to make it does not produce a rocky ledge, used as the seal of the reciprocating motion, does not produce distortions in general

So that it is in some degree of tension state. However, if this installation method for the rotational motion, it will produce bad results. Already confining the rotation axis O-ring contraction due to frictional heat generated by the rotary motion, and thus make this

Confining force increases, so, generate frictional heat contraction confining force increases generate frictional heat ..., and so forth cycle, greatly contributed to the aging and wear of the rubber.



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