O-ring, referred to as O-ring, is a rubber ring with a circular cross-section. The O-ring is the most widely used seal in hydraulic and pneumatic systems.
O-ring has good sealing property, can be used for static sealing, can also be used for reciprocating sealing; Not only can it be used alone, but is also the basic component of many combined sealing devices. It has a wide range of applications. If the material is selected properly, it can meet the requirements of various movement conditions, and the working pressure can be from 1.333×10^5Pa vacuum to 400MPa high pressure; The temperature range is from -60℃ to 200℃.
Compared with other sealing types, the O-ring has the following characteristics:
1) Small structure size, convenient installation, and disassembly.
2) Static and dynamic seals can be used, and there is almost no leakage when used as static seals.
3) The use of a single O-ring provides, two-way sealing effect.
4) The dynamic friction resistance is small.
5) Low price.
O-ring is an extrusion seal; the basic working principle of an extrusion seal is to rely on the elastic deformation of the seal, causing contact pressure on the sealing contact surface. The contact pressure is greater than the internal pressure of the sealed medium, and there is no leakage, and vice versa. When used for static sealing and dynamic sealing, the cause and calculation method of the contact pressure of the sealing contact surface are different, and need to be explained separately.
1. The sealing principle for static sealing
The O-ring is the most widely used in static sealing. If designed and used correctly, the O-ring can achieve absolute sealing without leakage in the static seal.
After the O-ring is loaded into the sealing groove, its section is subjected to contact compression stress, which produces elastic deformation.
A certain initial contact pressure Po is generated on the contact surface. Even if there is no medium pressure or the pressure is very small, the O-ring can be sealed by its own elastic force; When a pressurized medium is filled into the cavity, under the action of the medium pressure, the O-shaped sealing ring dislocates and moves to the low pressure side, while its elastic deformation further increases, and the filling and sealing gap δ. At this time, the contact pressure at the coupling surface of the sealing pair rises to Pm:
Pm=Po+Pp
In the formula, Pp -- contact pressure (0.1MPa) transmitted to the contact surface through the O-ring
Pp=K*P
K - pressure transfer coefficient, for rubber O-ring, K=1;
P -- The pressure of the sealed liquid (0.1MPa).
Thus, greatly increases the sealing effect. Since generally K≥1, Pm>P. It can be seen that as long as there is initial pressure in the O-ring, absolute sealing without leakage can be achieved. This depends on the pressure of the medium itself to change the contact state of the O-ring to achieve the nature of sealing, which is called self-sealing.
In theory, even if the compression deformation is zero, it can be sealed under oil pressure, but in practice, the O-ring may be eccentric when installed. Therefore, after the O-ring is loaded into the sealing groove, its cross-section is generally subjected to 7%-30% compression deformation. The static seal takes a larger compression rate value, and the dynamic seal takes a smaller compression rate value. This is because synthetic rubber is compressed at low temperatures, so the pre-compression amount of the static seal O-ring should be considered to compensate for its low temperature shrinkage.
2. The sealing principle for reciprocating motion sealing
In hydraulic rotation, pneumatic components and systems, reciprocating sealing is one of the most common sealing requirements. Reciprocating seals are used on power cylinder pistons and cylinder blocks, piston intervention cylinder heads, and all types of slide valves. The gap is formed by a cylindrical rod and a cylindrical hole, and the rod moves axially in the cylindrical hole. The sealing action limits the axial leakage of fluid.
When used as a reciprocating motion seal, the pre-sealing effect and self-sealing effect of the O-ring are the same as that of the static seal, and due to the elasticity of the O-ring itself, it can automatically compensate after wear. However, when the liquid medium is sealed, the situation is more complicated than the static seal due to the rod movement speed, liquid pressure, and viscosity.
When the liquid is under pressure, the liquid molecules interact with the metal surface, and the polar molecules contained in the oil are closely and neatly arranged on the metal surface, forming a strong boundary layer oil film between the slip surface and the seal, and producing great adhesion to the slip surface. The liquid film always exists between the seal and the reciprocating movement surface; it also plays a certain sealing role, and the lubrication of the moving sealing surface is very important.
But it's bad for leakage. However, when the reciprocating shaft is pulled out, the liquid film on the shaft is pulled out together with the shaft. Due to the wiping action of the seal, when the reciprocating shaft is retracted, the liquid film is blocked by the sealing element to stay outside. With the increase in the number of reciprocating strokes, more liquid is blocked outside, and finally, the formation of oil droplets, which is the leakage of the reciprocating sealing device. Because the viscosity of hydraulic oil decreases with the increase of temperature, and the thickness of the oil film decreases accordingly, the leakage at the beginning of the movement is larger when the hydraulic equipment is started at a low temperature, and the leakage tends to decrease gradually as the temperature rises due to various losses during the movement. O-ring as a reciprocating seal, compact structure, and small size can reduce the price of components.
Mainly used in:
1) In low-pressure hydraulic components, it is generally limited to a short stroke and medium pressure of about 10MPa.
2) Small diameter, short stroke, and medium-pressure hydraulic slide valve.
3) Pneumatic slide valve and pneumatic cylinder.
4) As an elastomer in a combined reciprocating sealing device.
O-ring as a reciprocating seal is most suitable for small diameter, short stroke, medium and low pressure applications, pneumatic cylinder, pneumatic slide valve, and other reciprocating components. In hydraulic components, the O-ring is used as the main dynamic seal, which is generally limited to short stroke and medium and low pressure of about 10MPa.
O-rings are not suitable for use as very low-speed reciprocating seals and as high-pressure reciprocating seals alone. This is mainly because the friction under this condition is large, which will lead to premature seal failure. In any type of application, it is necessary to use according to the rated data or capacity of the seal, and to assemble properly to obtain satisfactory performance.
3. Seal for rotating motion
In rotary motion seals, oil is usually used
Seal and mechanical seal. However, the pressure of the oil seal is low, and compared with the O-ring, it is too large and complex, and the process is poor. Although mechanical seals can be used for high pressure (40MPa), high speed (50m/s), and high temperature (400℃), the structure is more complex, huge, and the cost is high, and it is only suitable for some heavy machinery and equipment, such as the petroleum and chemical industry.
The main problem of O-rings for rotating motion is the Joule thermal effect. Joule thermal effect produces friction heat at the contact between the high-speed rotating shaft and the O-ring, and the generated heat makes the temperature of these contact parts rise continuously, and the rubber material is seriously deformed by heat, and the compression and elongation change. Heating also accelerates the aging of sealing materials and reduces the service life of O-rings; it damages the sealing oil film, resulting in the oil cut-off phenomenon, and accelerates the wear of the seal.
Based on the above situation, O-rings have been extensively and deeply studied in recent years. To avoid the Joule thermal effect, the key is to correctly select the structural parameters of the design O-ring according to the performance of the rubber, mainly the tensile strength and compression rate of the O-ring. According to the experiment, the O-ring is designed to have an inner diameter equal to or slightly larger than the diameter of the rotating shaft, generally 3%~5%. When installing the O-ring, it is compressed from the inner radial direction, and the compression of the section is also designed to be smaller, generally about 5%. In addition, try to use sealing materials that are less affected by heat, and fully consider the heat dissipation problem at the O-ring installation. In this way, the working condition of the O-ring is greatly improved, and it can be applied to the sealing of the rotating shaft with a maximum speed of 4m/s.
In recent years, there have been heat-resistant fluorine rubber and wear-resistant polyurethane rubber, and the Joule thermal effect of rubber components has a better understood. And to address the solution to this problem, a new O-ring sealing structure has been designed, so that the O-ring can be better applied and high-speed, high-pressure rotating motions.
Because of its small size, simple structure, low cost, good process performance, and wide application range, the O-ring is widely used in rotary sealing devices.
