O-ring is a kind of rubber ring with circular section.
O-ring is the most widely used seal in hydraulic and pneumatic systems.
O-ring has good sealing performance, which can be used not only in static sealing, but also in reciprocating sealing. It can not only be used alone, but also is a basic part of many combined sealing devices.
Its application range is very wide. If the material is selected properly, it can meet the requirements of various motion conditions. The working pressure can be from 1.333 × 105Pa vacuum to 400MPa high pressure; the temperature range can be from - 60 ℃ to 200 ℃.
Compared with other sealing types, O-ring has the following characteristics:
1) the structure size is small and easy to install and disassemble
2) both static and dynamic seals can be used, and there is almost no leakage when they are used as static seals
3) single O-ring is used, with bidirectional sealing function
4) the dynamic friction resistance is small
5) low price
The O-ring seal is a kind of extruded seal. The basic working principle of extruded seal is that the elastic deformation of the seal results in the contact pressure on the sealing contact surface. If the contact pressure is greater than the internal pressure of the sealed medium, no leakage will occur, otherwise, leakage will occur. When it is used for static seal and dynamic seal, the reason and calculation method of contact pressure of sealing contact surface are different, which should be explained separately
1. Sealing principle for static sealing
O-ring is widely used in static seal. If it is designed and used correctly, the O-ring can achieve absolute sealing without leakage in the static sealing
After the O-ring is installed into the seal groove, its section bears the contact compression stress and 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 the chamber is filled with the pressure medium, under the action of the medium pressure, the O-ring displaces and moves to the low pressure side, at the same time, its elastic deformation increases step by step, filling and sealing the gap δ. At this time, the contact pressure of the coupling surface of the seat used for sealing pair rises to PM:
Pm=Po+Pp
Where PP -- contact pressure transmitted to contact surface through O-ring (0.1MPa)
Pp=K. P
K - pressure transfer coefficient, k = 1 for rubber O-ring
P -- pressure of sealed liquid (0.1MPa)
Thus greatly increasing the sealing effect. As K ≥ 1 in general, PM > P. Therefore, as long as the initial pressure of the O-ring exists, the absolute sealing without leakage can be realized. The pressure of the medium itself changes the contact state of the O-ring to realize the sealing property, which is called self sealing function
Theoretically, even if the compression deformation is zero, it can be sealed under oil pressure, but in fact, the O-ring may be eccentric when it is installed. Therefore, after the O-ring is installed into the sealing groove, its section is generally subject to 7% - 30% compression deformation. The static seal takes the larger compression rate value, and the dynamic seal takes the smaller compression rate value. This is because the synthetic rubber is compressed at low temperature, so the pre compression shrinkage of the static seal O-ring should be considered to compensate its low temperature shrinkage
2. Sealing principle for reciprocating movement
Reciprocating seal is one of the most common sealing requirements in hydraulic and pneumatic components and systems. Reciprocating seals are used in power cylinder piston and cylinder block, piston intervention cylinder head and all kinds 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 the fluid. When used as reciprocating seal, the pre sealing effect and self sealing effect of O-ring are the same as that of static seal, and the O-ring has the ability of automatic compensation after wear due to its own elastic force. However, when the liquid medium is sealed, the situation is more complicated than that of the static seal due to the action of the rod moving 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 orderly arranged on the metal surface, forming a strong boundary layer oil film between the sliding surface and the seal, and producing great adhesion to the sliding surface. The liquid film always exists between the seal and the reciprocating moving surface, it also plays a sealing role, and it is very important for the lubrication of the moving sealing surface. But it's harmful to the leak. However, when the reciprocating shaft is pulled out, the liquid film on the shaft will be pulled out together with the shaft. Due to the "wipe" effect of the seal, when the reciprocating shaft retracts, the liquid film will be blocked outside by the sealing element. With the increase of reciprocating stroke times, the more liquid is blocked outside, and finally oil drops are formed, which is the leakage of reciprocating sealing device. Because the viscosity of hydraulic oil decreases with the increase of temperature, and the thickness of oil film decreases accordingly, when the hydraulic equipment starts at low temperature, the leakage at the beginning of the movement is relatively large. With the increase of temperature caused by various losses during the movement, the leakage has a trend of gradually decreasing
As a reciprocating seal, O-ring has compact structure and small size, which can reduce the price of components. Mainly used in:
1) in low-pressure hydraulic components, it is generally limited to short stroke and medium pressure of about 10MPa
2) in hydraulic spool valves with small diameter, short stroke and medium pressure
3) in pneumatic slide valve and pneumatic cylinder
4) as the elastomer in the combined reciprocating sealing device
O-ring as reciprocating
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