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The sealing principle of O-ring

Release time:

2019-05-28 12:30

Source:

O-ringThe sealing principle
 
O-ring is a rubber ring with a circular cross-section. O-ring is the most widely used seal in hydraulic and pneumatic systems. O-ring has good sealing, can be used for static seal, can also be used for reciprocating motion seal; not only can be used alone, but also is the basic component of many combined sealing device. Its application range is very wide, if the material is properly selected, 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) small structure size, easy assembly and disassembly. 2) Both static and dynamic seals can be used, and there is almost no leakage when used as a static seal. 3) the use of a single piece of O-ring, a two-way sealing effect. 4) The dynamic friction resistance is small. 5) The price is cheap.
 
O-ring seal is a kind of extrusion type seal. The basic working principle is to rely on the elastic deformation of the seal to cause contact pressure on the seal 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 causes and calculation methods of contact pressure on the sealing contact surface are different, which should be explained separately. 1. The sealing principle for static sealing is the most widely used O-ring in static sealing. If the design and use of the correct, O-ring in the static seal can be achieved without leakage of the absolute seal.
After the O-ring is installed in the sealing 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 cavity is filled with a pressurized medium, under the action of the medium pressure, the O-ring occurs Displacement, moving to the low pressure side, while its elastic deformation is further increased, filling and closing the gap δ. At this time, the contact pressure on the coupling surface used for sealing the auxiliary coupling surface rises to Pm: Pm = Po Pp, where Pp-contact pressure transmitted to the contact surface via o-ring (0.1MPa) Pp = k p k-pressure transmission coefficient, K = 1 for rubber o-ring; P-pressure of sealed liquid (0.1MPa).
 
This greatly increases the sealing effect. Since K ≥ 1 in general, Pm>P. It can be seen that as long as the O-ring has an initial pressure, it can achieve an absolute seal without leakage. This depends on the pressure of the medium itself to change the contact state of the O-ring to achieve the nature of the seal, called the self-sealing effect.
 
In theory, even if the compression deformation is zero, it can be sealed under oil pressure, but in fact, the O-ring may be eccentric when installed. Therefore, after the O-ring is installed in the sealing groove, its section is generally subjected to 7%-30% compression deformation. The static seal takes the larger compression ratio value, and the dynamic seal takes the smaller compression ratio value. This is because synthetic rubber is compressed at low temperatures, so the pre-compression 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 dynamic sealing is one of the most common sealing requirements.
 
Power cylinder piston and cylinder block, piston intervention cylinder head and all kinds of slide valves are used in reciprocating motion seal. The gap is formed by a cylindrical rod and a cylindrical bore in which the rod moves axially. The sealing action limits axial leakage of fluid. When used as a reciprocating seal, the pre-sealing effect and self-sealing effect of the O-ring are the same as those of the static seal, and due to the elastic force of the O-ring itself, it has the ability to automatically compensate after wear. However, due to the liquid medium seal, due to the rod movement speed, liquid pressure, viscosity, the situation is more complex than static seal.
 
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 sliding surface and the seal, and producing great adhesion to the sliding surface. The liquid film always exists between the seal and the reciprocating surface, it also plays a certain sealing role, and the lubrication of the moving sealing surface is very important. But it is harmful to the 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" effect of the seal, when the reciprocating shaft retracts, the liquid film is blocked by the sealing element. Outside. As the number of reciprocating strokes increases, the more liquid is retained outside, and finally oil droplets are formed, which is the leakage of the reciprocating sealing device.
 
Since the viscosity of hydraulic oil decreases with the increase of temperature, and the thickness of the oil film decreases accordingly, when the hydraulic equipment starts at low temperature, the leakage at the beginning of the movement is large, and the temperature increases due to various losses during the movement., The leakage has a tendency to gradually decrease. O-ring as a reciprocating seal, compact structure, small size, can reduce the price of components.

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