Improper design and use of O-rings can accelerate their damage and loss of 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 damage to the O-ring. Under high pressure and high temperature working conditions, the main cause of O-ring damage 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 O-ring appears distorted during movement.
1. Permanent deformation
Due to the fact that the synthetic rubber material used for the O-ring seal is a viscoelastic material, the initial set compression amount and rebound blocking ability will gradually lose permanent deformation after long-term use, ultimately leading to leakage. Permanent deformation and loss 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) The relationship between compression rate, stretching amount and permanent deformation
The various formulations of rubber used in the production of O-rings will experience compression stress relaxation under compression, where the compression stress decreases with time. The longer the use time, the greater the compression rate and stretching amount, the greater the stress drop caused by the relaxation of rubber stress, 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 allowable usage conditions. Increasing the cross-sectional size of the O-ring is the simplest way to reduce compression, but this will result in an increase in structural size.
It should be noted that when calculating the compression ratio, people often overlook 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 effect of tension, the cross-sectional shape of the O-ring will also change, manifested as a decrease in its height. In addition, under the action of surface tension, the outer surface of the O-ring becomes flatter, indicating a slight decrease in cross-sectional height. This is also a manifestation of the compression stress relaxation of the O-ring seal.
The degree of deformation of the O-ring section also depends on the hardness of the O-ring material. Under the same stretching amount, the section height of O-rings with high hardness also decreases significantly. From this point of view, materials with low hardness should be selected as much as possible according to the usage conditions. Under the action of liquid pressure and tension, the O-ring of rubber material will gradually undergo plastic deformation, and its cross-sectional height will correspondingly decrease, ultimately losing its sealing ability.
2) The relationship between temperature and the relaxation process of O-ring
The use temperature is another important factor affecting the permanent deformation of the O-ring. High temperatures can 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 exceeds 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 and temperature decrease of the O-ring. The initial compression of O-rings operating at temperatures below zero may decrease or completely disappear due to a sharp decrease in temperature. At temperatures ranging from -50 to -60 ℃, rubber materials that are not resistant to low temperatures 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, it is necessary to ensure that there is still sufficient sealing capacity after the stress decreases due to the relaxation process and temperature drop.
For O-rings operating at temperatures below zero, special attention should be paid to the recovery index and deformation index of rubber materials.
In summary, the design should try to ensure that the O-ring has a suitable working temperature, or choose O-ring materials that are 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 action can cause permanent deformation of the O-ring. Therefore, appropriate pressure resistant rubber materials should be selected according to the working pressure during 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 their elasticity (especially at low temperatures), and reduce their compression permanent deformation, it is generally necessary to improve the material formula and add plasticizers. However, O-ring seals with plasticizers can be soaked in the working medium for a long time, and the plasticizer will gradually be absorbed by the working medium, leading to volume shrinkage of the O-ring seal and even negative compression (i.e., gaps between the O-ring seal and the surface of the sealed part). Therefore, when calculating the compression amount of the O-ring and designing the mold, these shrinkage amounts should be fully considered. The compressed O-ring should be soaked in the working medium for 5-10 days and nights to maintain the necessary size.
The compression permanent deformation rate of O-ring material is related to temperature. When the deformation rate is 40% or greater, leakage occurs, so the heat resistance limits of several rubber materials are: nitrile rubber 70 ℃, EPDM rubber 100 ℃, and fluorine rubber 140 ℃. Therefore, countries have established regulations for 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. O-rings of the same material have a lower compression set rate for O-rings with larger cross-sectional diameters at the same temperature.
The situation is different in oil. Due to the fact that the O-ring is not in contact with oxygen at this time, the aforementioned adverse reactions are greatly reduced. In addition, it usually causes a certain degree of 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.
2. Interstitial bite
The sealed parts have poor geometric accuracy (including roundness, ellipticity, cylindricity, coaxiality, etc.), non concentricity between parts, and internal diameter expansion under high pressure, all of which can cause the expansion of sealing gaps and exacerbation of gap extrusion phenomenon. The hardness of the O-ring also has a significant impact 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 severe the gap squeezing phenomenon of the O-ring.
The measure to prevent gap bite is to strictly control the hardness and sealing gap of the O-ring seal. 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, while high hardness is used for high pressure.
The use of appropriate sealing rings to protect the retaining ring is an effective method to prevent the O-ring from being squeezed into the gap.
3. Distortion phenomenon
Twisting refers to the phenomenon of O-ring twisting along the circumference, which 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 under reciprocating motion, because the contact area between the O-ring and the groove is greater than the friction contact area on the sliding surface, and the resistance of the O-ring itself can originally prevent twisting. The distribution of friction force 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 that of the sliding surface.
There are many reasons for twisting and damage, among which the main one is due to uneven clearance, excessive eccentricity, and uneven diameter of the O-ring section between the piston, piston rod, and cylinder barrel. Due to the uneven friction force on the O-ring during one cycle, some parts of the O-ring have excessive friction, resulting in twisting. Usually, O-rings with smaller cross-sectional dimensions are prone to uneven friction. Causing distortion (the reason is that the cross-sectional diameter of the O-ring used for movement is larger than that of the O-ring used for fixation.)
In addition, due to the coaxiality deviation of the sealing groove, unequal sealing height, and uneven cross-sectional diameter of the O-ring, some parts of the O-ring may be compressed too much, while the other parts may be too small or not compressed. When there is eccentricity in the groove, i.e. the coaxial deviation is greater than the compression amount of the O-ring, the seal will completely fail. Another disadvantage of the large coaxiality deviation of the sealing groove is that it causes uneven compression of the O-ring along the circumference. In addition, due to uneven cross-sectional diameter, material hardness, lubricating oil film thickness, and surface roughness of the sealing shaft, some parts of the O-ring slide along the surface of the workpiece, while the other part rolls, causing distortion of the O-ring. Movement can easily cause damage to the ring due to twisting, which is an important reason for damage and leakage of the sealing device. Therefore, improving the machining precision of the sealing groove and reducing eccentricity are important factors to ensure the reliable sealing performance and lifespan of the O-ring.
The installation of the sealing ring should not be in a twisted state. If it is twisted during installation, twisting damage will occur quickly. In work, the distortion phenomenon can cut off the O-ring, causing a large amount of oil leakage, and the cut O-ring can mix with other parts of the hydraulic system, causing major accidents.
To prevent twisting and 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 two aspects: easy processing and no distortion.
2) The cross-sectional size of the O-ring should be uniform, and the sealing area should be fully coated with lubricating oil or grease during each installation. Sometimes a felt ring type refueling device soaked in 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 used for dynamic sealing should generally be greater than that of the O-ring used for static sealing; In addition, O-rings should be avoided as seals for large diameter pistons.
4) When twisting damage occurs under low pressure, sealing rings 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-ring seals.
7) Sealing rings that are not prone to twisting can be used instead of O-rings.
4. Abrasive wear phenomenon
When the sealed gap has relative motion, dust and sand particles in the working environment are adhered to the surface of the piston rod, and are brought into the cylinder with the reciprocating motion of the piston rod and the oil film, becoming abrasive particles that invade the surface of the O-ring seal, accelerating the wear of the O-ring and causing it to lose its sealing performance. To avoid this situation, a dust ring must be used at the extended shaft end of the reciprocating sealing device.
5. The influence of sliding surface on O-ring
The roughness of the sliding surface is a direct factor affecting the friction and wear of the O-ring surface. Generally speaking, smooth surfaces have less friction and wear, so the roughness values of sliding surfaces are often very low (Ra0.2-. 050 μ m) . However, experiments have shown that the surface roughness is too low (Ra below 0.050 μ m) It will also have adverse effects on friction and wear. This is because the small surface unevenness can maintain the necessary lubricating oil film. Therefore, appropriate surface requirements should be selected.
The material of the sliding surface also affects the lifespan of the O-ring. The higher the hardness, wear resistance, and ability to maintain smoothness of the sliding surface material, the longer the lifespan of the O-ring. This is also an important reason for the chrome plating on the surface of the hydraulic cylinder piston rod. Similarly, it can be explained that sliding surfaces made of copper and aluminum alloys with the same roughness have more severe friction and wear on the sealing ring than steel sliding surfaces. Sealing rings with low hardness and large compression are not as durable as those with high hardness and small compression.
6. Friction and the Application of O-ring
In dynamic sealing devices, friction and wear are important influencing factors for O-ring damage. The degree of wear mainly depends on the magnitude of friction. When the liquid pressure is small, the magnitude of the friction force of the O-ring depends on its pre compression shrinkage. When the working fluid is under pressure, the frictional force increases with the increase of working pressure. When the working pressure is less than 20MPa, the relationship 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 force also correspondingly slows down. Under normal circumstances, the service life of the O-ring will approximately decrease in a square relationship with the increase of liquid pressure.
The increase in friction creates a large amount of frictional heat between the rotating or reciprocating shaft and the O-ring seal. Due to the fact that most O-rings are made of rubber, their thermal conductivity is extremely poor. Therefore, frictional heat can cause rubber aging, leading to the effectiveness of the O-ring and damaging its sealing performance. Friction can also cause surface damage to the O-ring, reducing the amount of compression. Severe friction can quickly cause surface damage and loss of sealing of the O-ring. When using seals for pneumatic reciprocating motion, frictional heat can also cause adhesion, causing further increase in friction. During low-speed movement, friction resistance is still a factor that causes crawling and affects the working performance of components and systems. So for motion seals, friction is one of the important properties. The friction coefficient is an evaluation indicator of friction characteristics, and synthetic rubber has a large friction coefficient. When the seal is in motion, it is usually in a mixed lubrication state with the participation of working oil or lubricant, and the friction coefficient is generally below 0.1.
The magnitude of friction force largely depends on the surface hardness and roughness of the sealed component.
7. Joule heating effect
The Joule heat effect of rubber materials refers to the phenomenon of rubber in a stretched state contracting when exposed to heat. When installing an O-ring, in order to prevent it from moving in the sealing groove and to prevent twisting when used as a reciprocating seal, it is generally kept in a certain degree of tension. But if this installation method is used for rotational motion, it will produce adverse results. The O-ring, which was already tightly clamped on the rotating shaft, contracts due to the frictional heat generated by the rotational motion, thereby increasing the clamping force. This results in frictional heat → contraction → increased clamping force → frictional heat →... This repeated cycle greatly promotes the aging and wear of the rubber.
