Improper O-ring design, it will accelerate the damage, loss of sealing performance. Experiments show that, as the various parts of the sealing device designed, simply increasing the pressure, and will not damage the O-ring. Under high pressure, high temperature working conditions, mainly due to O-ring damage is permanent deformation and distortion of the O-ring O-ring seal material is extruded into the gap caused by the bite of an O-ring gap occurs when the phenomenon of movement .

 

1, permanent deformation

Since synthetic rubber O-ring seals used belong to a viscoelastic material, the amount of pressing and rebound blocked the ability of the initial setting after a long period of use, it will produce permanent deformation gradually lose eventually leak. Permanent deformation and elasticity are the main reason for the disappearance of the O-ring sealing properties are lost, the following is the main reason for causing permanent deformation.

 

1) compression ratio and the amount of stretch relationship with the permanent deformation

Production of rubber O-rings used in various formulations, in a compressed state will have a compression stress relaxation phenomenon, this time, the compressive stress decreases the growth over time. Use longer, the greater the compression ratio and the amount of stretch, by rubber stress relaxation and stress generated greater decline, resulting in lack of elastomeric O-ring, loss of sealing capability. Therefore, under the allowed conditions of use, trying to reduce the compression ratio is desirable. Increased cross-sectional dimensions of the O-ring is to reduce the compression ratio of the easiest way, but it will bring increased structure size.

 

It should be noted that people in the calculation of the compression ratio, often overlooked section height reduced O-ring in the assembly caused by stretching. Change the O-ring cross-sectional area is inversely proportional to the change and its perimeter. At the same time, due to the tension of the cross-sectional shape of the O-ring will change, on the performance of its height reduced. In addition, the surface tension of the outer surface of the O-ring becomes more level, and that cross-sectional height decreases slightly. It is also a manifestation of the O-ring compression stress relaxation.

 

The degree of deformation of the O-ring cross section, but also on the O-ring material hardness. In the case of the same amount of stretch, large hardness O-ring cross section height is reduced more, from this point of view, we should be in accordance with the conditions of use try to use low hardness material. Under the influence of fluid pressure and tension of the rubber O-ring material will gradually plastically deformed, its section height will be reduced accordingly, so that the final loss of sealing ability.

 

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

Operating temperature is another important factor affecting the O-ring of permanent deformation. High temperature will accelerate the aging of the rubber material. The higher the temperature, the compression set O-ring will be. When the permanent deformation is greater than 40%, O-ring seal will lose the ability to leak. The initial stress due to compressive deformation in the rubber O-ring is formed, will be with the relaxation process and temperature O-ring decline gradually reduce the role that disappear. O-ring temperature of minus work, which may be due to initial compression temperature decreases drastically reduced or disappear completely. In the case of -50 ~ -60 ℃, low-temperature resistance of the rubber material will completely lose the initial stress; low temperature even if the rubber material, the initial stress at this time is not more than 25% 20 ℃ initial stress. This is because the initial amount of compression depends on the O-ring coefficient of linear expansion. Therefore, the selection of initial compression, we must ensure due process and the relaxation of stress caused by temperature drop after drop still sufficient sealing capacity.

 

O-ring temperature of minus work, should pay special attention to recovery index and index of deformation of rubber materials.

 

In summary, the design should try to ensure that the O-ring having a suitable operating temperature, or use high resistance, low temperature O-ring material, in order to extend the service life.

 

3) Working medium pressure and permanent deformation

Pressure of the working medium is a major factor causing permanent deformation of the O-ring. Working pressures of modern hydraulic equipment are increasing. Long high pressure causes the O-ring permanent deformation. Therefore, the design should be the choice of appropriate pressure rubber material according to the working pressure. The higher operating pressure, high pressure and hardness properties of the materials used should also be higher.

 

In order to improve the pressure resistance of O-ring material, increase the flexibility of the material (particularly increased elastic at low temperatures), lowering the compression permanent deformation of the material, the general need to improve the formulation material, adding plasticizers. However, O-ring seal with a plasticizer, a long soak in the working medium, the plasticizer will gradually be absorbed by the working medium, resulting in volume contraction O-ring, the O-ring may even produce negative compression (i.e. a gap between the O-ring and the seal surface). Thus, in calculating the amount of compression of the O-ring and mold design, should be fully taken into account the amount of shrinkage. Still maintain the necessary size should be pressed out of the O-ring soak for 5 to 10 days and nights after working medium.

 

Compressed O-ring material permanent deformation rate and temperature. When the deformation rate of 40% or more, that there will be leaks, so some rubber heat resistance limit of: NBR 70 ℃, EPDM 100 ℃, Viton 140 ℃. Therefore States permanent deformation O-rings made provisions. Chinese standard rubber O-ring material change in the size of the table at different temperatures. O-rings of the same material at the same temperature, a large cross-sectional diameter of the O-ring compression permanent deformation lower rate.

 

In the oil situation is different. Since this time the O-ring is not in contact with oxygen, so these adverse reactions is greatly reduced. Coupled compound and usually causes a certain degree of expansion, so the compression set caused by temperature will be canceled. Therefore, in the oil heat greatly improved. In nitrile rubber, for example, its operating temperature up to 120 ℃ or higher.

 

2, the gap bites

Sealed parts there are geometric accuracy (including roundness, ovality, cylindricity, concentricity, etc.) bad, and different heart under high pressure between the inner diameter of the part swell phenomenon will cause the gap to expand and seal extrusion gap exacerbate the phenomenon. Hardness of the O-ring also has a significant impact on the extrusion gap phenomenon. The higher the pressure of liquid or gas, the smaller O-ring material hardness, the more serious gap O-ring extrusion phenomenon.

 

Gap measures to prevent bites is to be stricter controls on O-ring seal gap and hardness. Selection of suitable sealing material hardness control clearance. Commonly used hardness range of O-ring is HS60 ~ 90. Low hardness are used for low pressure, high hardness are used for hypertension.

 

Equipped with a suitable seal protection ring, O-ring is an effective method is to squeeze into the gap prevented.

 

3, distortions

Twist refers to O-ring in the circumferential direction of the torsion phenomenon, distortions generally occurs in dynamic sealing state.

 

O-ring if fitted properly and use proper conditions, are generally not easily back and forth in a reciprocating motion generated rolling, or distorted, because the contact area between the O-ring groove and the contact area is greater than the friction on the sliding surface, and O-ring itself can stop the original ability to resist twisting. Friction distribution tends to keep O-ring stationary in their trenches, because static friction is greater than sliding friction and roughness of the surface of the trench is generally not as good as the sliding surface roughness.

 

Many causes distortion of injury, most notably due to the clearance between the piston rod and the cylinder uneven, eccentric too large, cross-sectional diameter of the O-ring caused by uneven due to the friction caused by the O-ring in more than a week by the force uneven, some parts of the friction O-ring is too large, distorted. Typically, the cross-sectional dimensions smaller O-ring, prone to uneven friction. Distorting (motion O-ring is larger than the cross-sectional diameter of the fixing O-ring is the truth.)

 

In addition, since the sealing groove there concentricity deviation seal highly unequal and O-ring cross-section diameter uneven phenomenon, may make a portion of the O-ring compression is too large, the other part is too small or are not compressed. When the trench that is when the presence of the eccentric coaxial deviation is greater than the amount of compression of the O-ring seal will be completely ineffective. Seal groove concentricity deviation great harm another is to make the O-ring in the circumferential compression uneven. There is also the influence of uneven O-ring cross-section diameter, material hardness, lubricant film thickness and surface roughness of the shaft seal and other factors, led to the work surface along a portion of the O-ring sliding, rolling and partly occur, resulting in twist O-ring. Motion easily distorted due to damaged O-ring, which is an important reason for the sealing device is damaged and leaking. Thereby increasing the sealing groove machining precision and reduce bias is to ensure that the O-ring seal and a reliable life important factors.

 

Installation should seal it in twisted state. If the installation was distorted, twisted damage will happen soon. In operation, the O-ring distortions will cut off, resulting in a large number of oil spills, and cut off the O-ring will be mingled other parts of the hydraulic system, causing a major accident.

 

In order to prevent distortion of the O-ring damage, the design should pay attention to the following points

 

1) O-ring mounting groove concentricity size, convenience and should not produce distortions in both processing considerations.

2) O-ring section size should be uniform and should be fully apply oil or grease seals installed at each site. Sometimes you can use the felt saturated with lubricating oil refueling device.

3) increase the cross-sectional diameter of the O-rings, dynamic seal with O-ring cross-sectional diameter generally be greater than the static sealing O-ring; In addition, O-rings should be used to avoid the large-diameter piston seal.

4) at low pressure also distorting damage, can be used to protect seal rings.

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

6) low coefficient of friction material O-ring.

7) Available difficult to produce distortions in place of the O-ring seals.

 

4, abrasive wear phenomena

When the sealing gap have relative motion, the working environment of dust and sand and the like are adhered to the surface of the piston rod, and with the reciprocating motion of the piston rod and the film is brought together with the cylinder, become invasive O-ring surface abrasive, accelerated wear O-ring, so that the loss of its tightness. To prevent this from happening, in the extended shaft end reciprocating sealing device must use dustproof.