Ordinary O-ring in the ultra-high pressure seal

 

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Abstract: The ultra-high pressure seal is a very difficult problem in the industry. Demanding products and for pressure vessels and other security, in addition to design features, but also pay attention to safety. In this paper, for example, the "bomb-resistant ultra-high pressure automatic detection system for practical ultra-high pressure seal difficult problem, a new program to use ordinary rubber O ring to solve the problem. Focused on the O-ring structure size in the design, material selection, the trench to determine the size and pay attention to the problem. The test results show that this method is simple, practical, efficient, and also to broaden the use of the common O-ring.
Keywords: high pressure; seal; pressure vessels; rubber
0 Introduction
    In the hydraulic system and its components, design, placement of seals and sealing elements of the role is to prevent the intrusion of leakage of the working media and external dust and foreign bodies. Therefore, the choice of seals and sealing effect of a direct impact on the performance of the products.
    Quality testing of pressure vessels, the hydrostatic test is a key process for military production, detection of bomb body cavity with the external presence of a microporous, so as not to projectile impact by shock waves in the bore and bore deep-fried. Used to test the design of the hydraulic press it is necessary to solve the problem of positioning of the tested missile body, but also to prevent the increase water pressure in the process shot the top up, thus affecting the measurement results and cause an accident.
    Is to design a hydraulic test machine, its technical requirements: The water pressure should be adjustable within the control range of 10 ~ 46 MPa, and stable setpoint accuracy of 0.5 MPa; bomb body bore diameter was 34 mm, maximum 130 mm projectile minimum height is 196 mm, the maximum is 560 mm, designed to take into account enough of the trip, the relevant parts to take into account can change the dwell time is 10 s, accuracy 0.5 s; the oil pressure to ensure that high-pressure underwater can compress the elastomer; system should be able to detect the leakage occurred in any part of the missile body, especially micro-osmosis; test the efficiency of the 1500 ~ 2000 p/6h.
    It can be seen from the above-mentioned technical parameters, the hydraulic part of the seal belongs to a ultra-high pressure (pressure classification shown in Table 3) Seal. For this problem, put forward new ideas to solve with ordinary rubber O ring. This article focuses on the design process, the O-shaped rubber ring structure size, choice of materials, trench size to determine and pay attention to the problem.
A rubber seal materials, the applicable conditions and parameter selection
1.1 rubber seal materials requirements of the applicable conditions
1) The material requirements
    Rubber seals seal is achieved by the installation of pre-pressure and the ring deformation at work by the pressure of the working medium. Sealing performance will directly affect the hydraulic components work on the ring material the following requirements: have a good stability in the dynamic oil, small volume change (not easy expansion and contraction) and difficult to dissolve, soften and hardened; compression resilience in the power of oil, the permanent deformation is small; have adequate mechanical strength, mechanical strength changes affected by the power of oil; good heat resistance, cold resistance and vibration absorption performance; good wear resistance, coefficient of friction small; dense material leakage is small; contact with the metal does not work with each other, such as adhesion, corrosion, etc.; easy to manufacture, high dimensional accuracy and cheaper prices. Commonly used material is nitrile rubber, and its specific characteristics and applications are listed in Table 1 [1] as shown.
 
2) the applicable conditions
    O-shaped rubber seals for hydraulic and pneumatic components in reciprocating motion to rotary motion and spiral movement of the seal and stationary seal and flange sealing, etc., can be used for the inner diameter of seal can also be used outside diameter seal.
Working pressure: 0 ~ 32 MPa, temperature: -35 ~ +200 C, the working medium: general mineral hydraulic oil, lubricating oil, water and compressed air.
    Rubber O ring has a simple structure, good sealing performance, friction is small, the trench size is small, easy to manufacture, so widely used in hydraulic and pneumatic components.
1.2 O-shaped rubber ring parameter selection
    Static sealing parts used seals are basically the O-ring. Small O-ring, but it is a precision rubber products, under the complex conditions of use, has good dimensional stability and performance to maintain its own characteristics. In the design selection of the appropriate choice of materials and sizes, according to the conditions of use, and take reasonable installation and maintenance measures, in order to achieve satisfactory sealing effect. Select the O-ring should take into account the following points.
1) compression ratio
    O-ring is installed in a trench with the compression elastic contact pressure to achieve sealing. Therefore, the compression ratio is the main design parameters of its size directly affect the seal performance and life. Compression ratio and the seal is not good, easy to flip the role of fluid pressure, undercut; compression ratio, the frictional resistance increases, the contact pressure increases, and prone to fracturing, deformation, twist, assembly difficulties. A reasonable choice of compression ratio is very important. Compression ratio is calculated as follows:
= [(d-h) / d] 100%
Where d --- O-ring cross section diameter (mm)
            h --- seal groove depth (mm)
    The compression ratio is generally selected within the range of 7% to 30%. Static seal to choose a large value, the dynamic seal selected smaller value in Table 2.
Installation groove machining accuracy is an important factor affecting the compression ratio. Groove dimensions have been standardized, but non-standard design or processing too deep or too shallow and cooperate with each piece seal contact surface of the geometric accuracy of ultra-poor, will cause the O-ring to reach even after installation or compression volume.
2) The amount of stretch and seal gap
    O-ring diameter is generally smaller than the groove bottom diameter, so that the O-ring is stretched, to maintain a certain amount of stretch. The plane should not make the static seal O-ring is stretched, but should be in a free state. Stretch the amount of the size of the O-ring life. Too large, not only assembly difficulties, and narrow so that the compression rate and cause a leak due to the cross section diameter. The stationary seal the amount of stretching of the shaft diameter from 3% to 4% in general, the reciprocating movement of 2% to 3%, the rotary motion sealing O-ring should not be a stretch, its inner diameter than the shaft diameter of about 5%.
 
    In actual use, the O-ring often undercut phenomenon. This is mainly due to the sealing gap is too large to squeeze into the gap in the role of fluid pressure, the O-ring. The so-called seal gap refers to the mutual cooperation between the gap. Under normal circumstances, the high pressure seal gap should be smaller than the low-pressure, and the rubber hardness should increase. When the pressure exceeds 10 MPa, in order to prevent the ring was to squeeze into the gap, often set in the groove compression side of the Teflon retaining ring. If it is bi-directional force, each side set up a retaining ring. Seal gap size and the O-ring cross-section diameter, rubber hardness. At different pressure levels, different cross-section diameter and hardness of the seal clearance allowed values ​​can refer to GBl235-76 provided for the data to determine.
3) the width of the groove, form and roughness
    Install the O-ring structure slot in many forms, such as rectangles, triangles, Y-shaped, dovetail-shaped, semi-circular, oblique bedforms, etc., can be selected according to the different conditions of use appropriate forms of the groove, you can not - none in terms of . The most groove is rectangular, its processing is simple, but easily lead to the phenomenon of ring undercut, reversing.
    Rectangular groove width can not be too large and too small. The big ring in the fluid pressure, the effect of friction prone to torsion; small amount of compression is nowhere to be content, will produce racking and tear. Under normal circumstances, the groove width is 1.3 to 1.5 times the diameter of the ring cross section. Static seal compression, width, whichever is greater, reciprocating motion to take a smaller value. The rotational movement of the sealing groove width should be 1.0 to 1.1 times of the cross-section diameter.
    Spouse material and surface roughness of the contact with the O-ring directly affect their frictional resistance, sealing performance and service life of sliding contact with the O-ring material to be hard, wear, surface roughness Ra value of 0.32 ~ 0.63m. Table of the trench roughness Ra is generally not greater than 1.6 ~ 3.2m.
2 O-shaped rubber ring in the mold design
2.1 tooling design background
     The task of this project is to design a military is used to detect whether the shells casings crack automatic real-time detection system. The principle is that the casings clamped on the table, to the cartridge case into a 35 MPa high-pressure water, allowed to dwell for some time, high-precision pressure sensor to detect water pressure changes in casings. If the water pressure exceeds a certain value, then the cartridge case there are cracks or leaks, in order to determine the casings for scrap. The entire device has two molds, one is used to compress the cartridge case is not channeling hydraulic high-pressure water is injected into the casings internal mold clamping, and the other is used to place the cartridge case and play a role in positioning the mold, the mold is related to to the high-pressure water sealing problems. The following highlights the selection of the structure of the two kinds of mold and seals.
2.1.1 The structure of the mold design and size to determine the
    Figure 1 shows that mold the central 32 of taper, but not with the cartridge case is similar to the arc, which is taking into account the interchangeability sake. Arc affected by the processing conditions and technologies, processing is consistent with the cartridge case is difficult, and the model of the cartridge case, the arc can not be with similar types of casings are consistent, while the tapered bore can solve the above problem, a similar model interchangeability of good.
 
    In addition, in the middle of the tapered bore open a slot for placing the ring. The role of the ring is to ensure that mold and measured the airframe the mouth with good contact, and provide a reliable seal to prevent high pressure water overflow detection process affect the measurement results or even cause an accident.
2.1.2 The face seal and radial seal
    Pressure load to the sealing force by an increasing trend called self-tight seal. Since the tight degree of geometric balance reflects the area of ​​fluid pressure on the sealing area of ​​the ratio K High-pressure static sealing geometric balance is generally greater than 3, the geometric balance of dynamic seal is generally 0.5 to 1.5.
    The balance of the sealing pressure face seal design needs to be considered one of the most important issue. This is because on the one hand, the end face seal of the fluid film load should be low enough to prevent the generation of high fever and high wear; the other hand, must be sealed to keep in touch or maintain close the gap in any conditions.
     Face Seal common seal: copper pad, the metal skeleton of compound pad, seal ring, bowl seal, diaphragm, bellows. Copper pad is easy to deform and the deformation can not be recovered in the course of their work must be frequently changed, otherwise it is impossible to seal. The metal skeleton of the composite pad model, if you do not have the necessary model will have to build the high cost.
In addition, can be seen from Table 3 to 35 MPa pressure should belong to the ultra-high pressure range. It is not appropriate due to the high sealing pressure face seal.
 
Due to higher processing accuracy of the DUT, the shape of the arc-shaped structure, more suitable for the use of radial seal, concrete mold structure shown in Figure 1.
2.1.3 ring selection
    DUT between the die and apparently static seal, but due to work pressure, a slight accident, it will cause excessive leakage, thus affecting the measurement results. In addition, the lethality of 35 MPa ultra-high pressure water, can cause personal injury. Therefore, the choice of seals.
     Because it is a static high-pressure seal, to take the O-ring compression rate of 20%, calculated by the formula (1) required for O-ring cross-section diameter of 2.65 mm, and then considering the amount of stretch and seal gap, and ultimately select the material for the high hardness nitrile rubber, marked as: 65 2.65GB3452.1-82.
    It is worth noting: Because it is tapered bore, groove dimensions design in addition to standard in accordance with the GB3452.3-88, also take into account the size than the O-ring groove on the edge of small diameter, under the edge size than the inner diameter of the O-ring, so as to meet the design requirements.
2.2 The results
   Table 4 Experimental data measured by high-precision sensor, and after the conversion. The tests proved that the design meets the requirements.
 
3 Conclusion
The ultra-high pressure seal forms and seals a wide range, we propose to use ordinary rubber O rings to solve the problem of ultra-high pressure seal is only an attempt. The results showed that seal better. Examples that the rubber O rings is a very broad use of seals. In short, the rubber O rings and other sealing applications, there are many worthy of study

 

 

 
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