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O - ring use, installation and fault analysis

O - ring use, installation and fault analysis

2020-06-08

1. Use of O-rings

O - ring is widely used in a variety of hydraulic and pneumatic parts pipe joints, cylinder surface and flange surface junction.For the O-ring used in the process of movement, when the working pressure is greater than 10MPa, such as unidirectional compression, a retaining ring is set on the other side of the o-ring compression direction;In case of bidirectional compression, a retaining ring shall be placed on each side of the O-ring.To reduce friction, wedge retainers can also be used.When the pressurized liquid ACTS from the left side, the right ring is pushed up and the left ring does not come into contact with the sealed surface, thus reducing friction.In general, the use of the retainer will increase the friction of the sealing device, and the wedge retainer is very important to reduce the friction.For fixed O-rings, reefer rings are also required when the working pressure is greater than 32MPa. 

After the o-ring is used, the working pressure can be greatly increased.Static seal pressure can be increased to 200~700MPa;Dynamic seal pressure can also be increased to 40MPa.The retainer also helps keep the O-ring well lubricated. 

When using o-rings as reciprocating seals, attention must be paid to the failure of the seal due to the full-tone rolling torsion and increased friction due to adhesion.O-rings, if properly assembled and used, are generally less likely to roll or twist in reciprocating motion because the contact area between the O-ring and the sealed groove is greater than the frictional contact area on the sliding surface, and the resistance of the O-ring itself prevents distortion in the first place.At the same time, the distribution of friction tends to keep the O-ring stationary in its groove because static friction is greater than sliding friction, and the sealed groove surface is generally less smooth than the sliding surface. 

O-rings are generally used in low pressure pneumatic systems below 0.7mpa and have higher requirements for sealing.In pneumatic systems, care must be taken to lubricate the O-ring. 

Usually, sliding surfaces are electroplated to improve corrosion resistance and wear resistance.The material of the coating has a certain effect on the service life of the O-ring. 

The service life of O-ring is directly related to many factors such as service condition, assembly quality, material of moving parts, machining accuracy, lubrication condition and dustproof measures.Therefore, the exact service life of O-ring can only be judged according to the specific working conditions.The o-ring used for dynamic sealing is a wearing part. In any application of reciprocating movement sealing, it should be used according to the rated data or capacity of the sealing part and should be properly assembled so as to obtain satisfactory performance. 

Sealing with O-ring seals should try to make the direction of pressure and friction opposite, if the two forces are in the same direction, the o-ring extrusion trend is more obvious, and may reduce the effective pressure rating.If the groove design cannot be set in such a way that the pressure direction is opposite to the friction direction, a retaining ring should be used under the pressure of 7~10MPa.Self-priming hydraulic pumps or pipelines with instantaneous flow rate greater than 10m/s can produce negative pressure (vacuum) phenomenon. In this case, the working state is negative pressure sealing.Negative pressure is different from positive pressure seal safety.And in contrast to the positive pressure leakage phenomenon, the negative pressure is suction.So their seal is the opposite.If you do not pay attention and mistakenly use the positive pressure for the negative pressure, it is possible to suck in the O-ring, causing air into the hydraulic system, causing the tank to "boil" phenomenon.For seals that may produce negative pressure or vacuum, protective grooves should be used to prevent o-rings from being sucked into the orifice. 

2. Installation of O-ring

The installation quality of O-ring has an important influence on its sealing and service life.Leakage problems are often caused by poor installation.The o-ring is not allowed to be scratched and improperly installed, and the O-ring is not allowed to be distorted in the installation process.Before assembly, seal groove and seal coupling surface must be cleaned strictly;At the same time, apply grease to the surface to be passed in the Assembly of O-ring. 

In order to prevent the O-ring from being cut or scratched by sharp edges such as sharp Angle and thread during installation, an inlet Angle of 15 °~30° shall be left at the shaft end and hole end of installation.When the O-ring needs to pass through the external thread, a special thin-walled metal guide sleeve should be used to cover the external thread. 

In addition, to prevent leakage or use of discarded O-rings.And also pay attention to the installation direction of the baffle ring. 

3. Storage of O-rings

The O-ring is a wearing part, so it needs to be properly kept.O - ring safekeeping has a great impact on its service life.The o-ring shall be kept as follows:

1) Avoid direct sunlight, moisture and air circulation.Because these are the factors that accelerate the aging of o-ring materials.The suitable temperature for storing O-ring is 0~20℃, and the suitable air humidity is above 70%. 

2) The O-ring must be stored away from 1m of heating equipment and should not be placed in the room with acid or alkali. 

3) When the O-ring is stored, it is not allowed to be pressed, so as not to cause permanent deformation of compression. 

4) The date of manufacture and manufacture of polythene plastic bags with O-rings must be recorded for use in order of precedence.The storage period of o-ring made of rubber material is usually 2~5 years. 

4, O ring failure and solutions

O - ring seal easy to appear fault, reasons and solutions. 

Improper design and use of O-ring will accelerate its damage and loss of sealing performance.The experimental results show that the o-ring will not be damaged if the sealing device is designed reasonably and the pressure is simply increased.Under the working conditions of high pressure and high temperature, the main reasons for the failure of O-ring are the permanent deformation of o-ring material and the gap bite caused by the O-ring being squeezed into the sealing gap. 

1. Permanent deformation

As the synthetic rubber material used for o-ring sealing ring is viscoelastic material, the initial set amount of compression and resilience plugging ability will gradually lose permanent deformation and eventually leak.Permanent deformation and elastic disappearance are the main reasons for the loss of sealing performance of O-ring. The following are the main reasons for permanent deformation. 

1) The relationship between compression ratio and tensile quantity and permanent deformation

The rubber of various formulations used to make O-ring will produce compressive stress relaxation phenomenon in the compression state, at this time, the compression stress decreases with the increase of time.The longer the service time, the greater the compression ratio and the greater the amount of tension, the greater the drop in stress caused by rubber stress relaxation, so that the O-ring is not elastic enough, and lose the sealing ability.Therefore, it is advisable to try to reduce the compression rate under permitted conditions of use.Increasing the section size of the O-ring is the easiest way to reduce the compression ratio, but it will increase the size of the structure. 

It should be noted that when calculating the compression ratio, the reduction of section height caused by stretching of O-ring during assembly is often ignored.The change in area of o-ring section is inversely proportional to the change in circumference.At the same time, due to the action of tension, the shape of the section of the O-ring will also change, which is shown as the decrease of its height.In addition, under the action of surface tension, the outer surface of the O-ring becomes flatter, that is, the cross section height decreases slightly.This is also the O - ring compression stress relaxation of a performance. 

The degree of cross section deformation of O-ring also depends on the hardness of the O-ring material.In the case of the same tensile amount, the o-ring with high hardness also reduces the height of its section more. From this point of view, the material with low hardness should be selected as far as possible according to the conditions of use.Under the action of liquid pressure and tension, the rubber material O - ring will gradually produce plastic deformation, the height of the section will be reduced accordingly, so that the final loss of sealing ability. 

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

Temperature is another important factor affecting the permanent deformation of the O-ring.High temperature will accelerate the aging of rubber materials.The higher the working temperature is, the greater the compression permanent deformation of the O-ring will be.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.The initial compression of an O-ring operating at sub-zero temperatures may decrease or completely disappear due to a sharp drop in temperature.Under the condition of -50~-60℃, the rubber material which is not resistant to low temperature will completely lose the initial stress.Even for rubber materials resistant to low temperature, the initial stress at this point 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 amount, it is necessary to ensure that there is still sufficient sealing capacity after the stress drop due to the relaxation process and temperature drop. 

Special attention should be paid to the recovery index and deformation index of rubber materials for o-rings operating at sub-zero temperatures.To sum up, in the design should try to ensure that the O ring has the appropriate working temperature, or choose high and low temperature resistance of O ring materials, in order to extend the service life. 

3) Medium working pressure and permanent deformation

The pressure of working medium is the main factor that causes the permanent deformation of O-ring.The working pressure of modern hydraulic equipment is increasing day by day.The o-ring will be permanently deformed by prolonged high pressure.Therefore, the design should be based on the appropriate pressure - resistant rubber material.The higher the working pressure is, the higher the hardness and high pressure resistance of the materials used should be. 

In order to improve the compressive resistance of O-ring materials, increase the elasticity of materials (especially increase the elasticity of materials at low temperature) and reduce the compression permanent deformation of materials, it is generally necessary to improve the material formula and add plasticizer.However, with a plasticizer O sealing ring, a long time in the working medium immersion, plasticizer will gradually be absorbed by the working medium, resulting in the volume contraction of O sealing ring, and may even make O sealing ring negative compression (that is, in O sealing ring and sealed surface gap).Therefore, in the calculation of O - ring compression and die design, should be fully taken into account these shrinkage.Should make the pressed O - ring in the working medium immersion 5~10 days and nights after still can maintain the necessary size. 

The compression permanent deformation rate of O-ring materials is related to temperature.When the deformation rate of 40% or more, that is, there will be leakage, so the limit of heat resistance of several kinds of rubber: NBR 70℃, EPdm 100℃, fluororubber 140℃.Therefore, the permanent deformation of the O-ring has been stipulated in various countries.The size change of O-ring of Chinese standard rubber material at different temperature is shown in the table.The compression permanent deformation rate of the o-ring with large section diameter is lower at the same temperature. 

It's a different story in oil.Since the O-ring is not in contact with oxygen at this time, the above adverse reactions are greatly reduced.In addition, it usually causes some expansion of the adhesive, so the compression permanent deformation rate due to temperature will be offset.Therefore, the heat resistance in oil is greatly improved.Nitrile butadiene rubber, for example, can work at 120℃ or higher. 

2. Gap bite

The parts are sealed with poor geometric accuracy (including roundness, ellipticity, cylindricity, coaxiality, etc.), different centers between parts and the phenomenon of inner diameter expansion under high pressure, which will lead to the expansion of the sealing clearance and the intensification of the clearance extrusion phenomenon.The hardness of O-ring also has obvious influence on the clearance extrusion.The higher the pressure of liquid or gas, the less the hardness of O-ring material, the more serious the o-ring clearance extrusion phenomenon. 

The measure to prevent gap bite is to strictly control the hardness and sealing clearance of O - ring.Choose suitable hardness sealing material to control the clearance.The hardness range of commonly used O-rings is HS60~90.Low hardness for low pressure, high hardness for high pressure.It is an effective way to prevent the O-ring from being squeezed into the clearance by using proper sealing ring to protect the retainer ring. 

3. Distortion

Distortion refers to the circumferential torsion of The O-ring, which generally occurs in the state of dynamic sealing. 

If properly assembled and used, o-rings are generally less likely to roll or twist in reciprocating motion because the contact area between the O-ring and the groove is greater than the frictional contact area on the sliding surface, and the resistance of the O-ring itself prevents distortion in the first place.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 groove surface is generally less rough than the sliding surface. 

There are many reasons for distortion damage, the most important is due to the piston, piston rod and cylinder clearance uneven, too large eccentricity, O ring section diameter is not uniform, due to cause O ring in a week of friction uneven, O ring some parts of the friction too large, distortion.In general, the section size of small O - ring, easy to produce uneven friction.Cause distortion (this is why the diameter of the o-ring for motion is larger than that of the o-ring for fixation.) 

In addition, due to the existence of sealing groove coaxiality deviation, sealing height is not equal and the o-ring section diameter is not uniform and other phenomena, may make the O-ring part of the compression is too large, 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 completely fail.Another disadvantage of misaligned seal grooves is uneven compression of o-ring seals along the circumference.In addition, due to the impact of factors such as section diameter, material hardness, oil film thickness and seal shaft surface roughness, one part of the O-ring slides along the working surface and the other part rolls, thus causing the Distortion of the O-ring.The o-ring for motion is easily damaged by distortion, which is an important cause of damage and leakage of the sealing device.Therefore, improving the machining precision of the sealing groove and reducing the eccentricity are important factors to ensure the reliable sealing and life of the O-ring.If it is distorted during installation, distortion damage can occur very quickly.During operation, the distortion can cut off the O-rings, causing a large amount of oil to leak, and the cut O-rings can get mixed up with other parts of the hydraulic system, causing major accidents. 

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

1) The concentricity of the o-ring installation groove shall be considered from the two aspects of easy processing and no distortion. 

2) The o-ring section size shall be uniform and the sealing part shall be fully applied with lubricating oil or grease during each installation.Sometimes a felt-coil filling device impregnated with lubricating oil may be used. 


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