Main failure causes and prevention measures of the

The main failure causes and prevention measures of O-ring

improper design and use of O-ring will accelerate its damage and lose its sealing performance. The experiment shows that if the design of each part of the sealing device is reasonable, simply increasing the pressure will not cause the damage of the O-ring. Under the working conditions of high pressure and high temperature, the main reasons for the destruction 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. The primary O-ring is distorted during movement

1. Permanent deformation

as the synthetic rubber material used for O-ring sealing ring is a viscoelastic material, the compression amount and rebound blocking ability set at the initial stage will produce permanent deformation and gradually lose after long-term use, and eventually leak. Permanent deformation and the 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 ratio, tensile amount and permanent deformation

various formulations of rubber used to make O-rings will produce compression stress relaxation under compression. At this time, the compression stress will decrease with the increase of time. The longer the service time is, the greater the compression ratio and tensile amount are, the greater the stress drop caused by the stress relaxation of the rubber is, so that the elasticity of the O-ring is insufficient and the sealing ability is lost. Therefore, it is advisable to try to reduce the compression ratio under the allowable service conditions. Increasing the section size of O-ring is the simplest way to reduce the compression ratio, but it will increase the structural size

it should be noted that when calculating the compression ratio, people often ignore the reduction of section height caused by the tension of O-ring during assembly. The change of the cross-sectional area of the O-ring is inversely proportional to the change of its circumference. At the same time, due to the effect of tension, the cross-sectional shape of the O-ring will also change, which is manifested in the reduction of its height. In addition, under the action of surface tension, the outer surface of the O-ring becomes flatter, that is, the section height decreases slightly. This is also a manifestation of the compression stress relaxation of the O-ring

the degree of cross-section deformation of O-ring also depends on the hardness of O-ring material. In the case of the same amount of stretching, the section height of the O-ring with high hardness is also reduced more. From this point of view, the material with low hardness should be selected as far as possible according to the service conditions. Under the action of liquid pressure and tension, the O-ring of rubber material will gradually undergo plastic deformation, and its section height will be reduced accordingly, so that it will eventually lose its sealing ability

2) relationship between temperature and O-ring relaxation process

service temperature is another important factor affecting the permanent deformation of O-ring. High temperature will 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 seal. 1. The leakage occurs because the working seal capacity of standards and regulations is strengthened. 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 O-rings operating at a temperature below zero may decrease or disappear completely due to a sharp decrease in temperature. At - 50 ~ - 60 ℃, rubber materials that are not resistant to low temperature will completely lose their initial stress; Even for low temperature resistant rubber materials, 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 coefficient of linear expansion. Therefore, when selecting the initial compression amount, we must ensure that there is sufficient sealing capacity after the stress drop caused by the relaxation process and temperature drop

for O-rings with temperatures below zero, special attention should be paid to the recovery index and deformation index of rubber materials

to sum up, the design should try to ensure that the O-ring has a suitable working temperature, or choose high and low temperature resistant O-ring materials to prolong its service life

3) working pressure and permanent deformation of medium

the pressure of working medium is the main factor causing permanent deformation of O-ring. The working pressure of modern hydraulic equipment is increasing. Long term high pressure will permanently deform the O-ring. Therefore, appropriate pressure resistant rubber materials should be selected according to the working pressure in the design. 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 O-ring materials, increase the elasticity of materials (especially at low temperature) and reduce the compression set of materials, it is generally necessary to improve the formula of materials and add plasticizers. However, the O-ring with plasticizer is soaked in the working medium for a long time, and the plasticizer will be used by the mirror 10 fixed by the pin to observe whether the sample is transparent in order to judge whether the computer-controlled impact test machine breaks down at the moment of hammering. Whether the test is gradually absorbed by the working medium, resulting in the volume shrinkage of the O-ring, and even the negative compression of the O-ring (that is, there is a gap between the O-ring and the surface of the sealed part). Therefore, these shrinkage should be fully considered when calculating the compression of O-ring and designing the mold. The pressed O-ring should be able to maintain the necessary size after soaking in the working medium for 5 ~ 10 days and nights

the compression set rate of O-ring material is related to temperature. When the deformation rate is 40% or more, leakage will occur, so the limit of heat resistance of several compounds is to ensure that the testing machine can operate continuously for a long time: nitrile rubber 70 ℃, ethylene propylene diene monomer 100 ℃, fluororubber 140 ℃. Therefore, the permanent deformation of the O-ring is regulated by various countries. 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 set rate of O-rings with large section diameter is low

the situation in oil is different. Since the O-ring does not contact with oxygen at this time, the above adverse reactions are greatly reduced. In addition, it usually causes a certain expansion of the rubber, 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

2. Gap bite

the sealed parts have poor geometric accuracy (including roundness, ovality, cylindricity, coaxiality, etc.), non concentricity between parts, and inner diameter swell under high pressure, which will cause the expansion of sealing gap and the intensification of gap extrusion. The hardness of O-ring also has an obvious influence 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 squeeze of the O-ring

the measure to prevent gap bite is to strictly control the hardness and sealing clearance of O-ring. Select the sealing material with appropriate hardness to control the gap. The hardness range of the commonly used O-ring is hs60 ~ 90. Low hardness is used for low pressure, and high hardness is used for high pressure

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it is an effective method to prevent the O-ring from being squeezed into the gap by using an appropriate sealing ring to protect the retaining ring

3. Twisting phenomenon

twisting refers to the phenomenon that the O-ring twists along the circumferential direction. The twisting 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 the reciprocating state, 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 distortion. The distribution of friction also tends to keep the O-ring stationary in its groove, because the static friction is greater than the sliding friction, and the roughness of the groove surface is generally less than that of the sliding surface

there are many reasons for distortion damage, the most important of which is due to uneven clearance between piston, piston rod and cylinder barrel, excessive eccentricity, uneven cross-section diameter of O-ring, etc. due to uneven friction of O-ring in one week, some parts of O-ring have excessive friction, resulting in distortion. Generally, the O-ring with small section size is prone to uneven friction. Cause distortion (this is the reason why the section diameter of the sports O-ring is larger than that of the fixed O-ring.)

in addition, due to the coaxiality deviation of the sealing groove, the unequal sealing height and the uneven cross-sectional diameter of the O-ring, one part of the O-ring may be compressed too much, and the other part may be 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 large coaxiality deviation of sealing groove is uneven compression of O-ring 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, some of the O-ring slides along the working surface, while the other part rolls, resulting in the distortion of the O-ring. The O-ring for sports is easy to be damaged due to distortion, which is an important reason for the damage and leakage of the sealing device. Therefore, improving the processing precision of the sealing groove and reducing the eccentricity are important factors to ensure the reliable sealing and service life of the O-ring

the sealing ring should not be installed in a twisted state. If it is twisted during installation, the distortion damage will occur soon. During work, the distortion will cut off the O-ring, resulting in a large amount of oil leakage, and the cut-off O-ring will mix 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 O-ring installation groove should be considered from the two aspects of convenient processing and no distortion

2) the section size of O-ring should be uniform, and the sealing part should be fully coated with lubricating oil or grease during each installation. Sometimes, felt ring oiling device saturated with lubricating oil can also be used

3) increase the cross-sectional diameter of the O-ring, and the cross-sectional diameter of the O-ring for dynamic seal should generally be larger than that of the O-ring for static seal; In addition, O-rings should be avoided as seals for large diameter pistons

4) when distortion damage also occurs under low pressure, the sealing ring can be used to protect the retaining ring

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

6) use materials with low friction coefficient to make O-ring seals

7) the O-ring can be replaced by a sealing ring that is not easy to produce distortion

4. Abrasive wear phenomenon

when the gap of the seal has relative movement, the dust and sand in the working environment are adhered to the surface of the piston rod, and with the reciprocating motion of the piston rod, they are brought into the cylinder together with the oil film, becoming abrasive particles invading the surface of the O-ring, accelerating the wear of the O-ring, so that it loses its tightness. In order to avoid this situation, dust prevention must be used at the extended shaft end of the reciprocating sealing device