Estimated reading time: 15 minute
The sealed part is between the two coupling surfaces that need to be sealed. Usually, according to the relative movement of the coupling surface when the machine is running, the seal can be divided into two categories: dynamic seal and static seal. According to the manufacturing materials, installation methods, and structural forms of the seals, they are further divided into different sub-categories.
Point 1: Compatibility of Seals And Working Medium
The selection of the seal should be based on the working medium used in the system. If the selection is inappropriate, it will cause systematic seal failure.
Therefore, attention should be paid. For example, nitrile rubber seals are not compatible with phosphate ester media; polyurethane rubber seals are only compatible with petroleum-based and mineral-based oils, and are incompatible with water-glycol and phosphate ester media; silicone rubber seals are compatible with water -Glycol medium is not compatible; Fluorine rubber and PTFE seals are compatible with the above working media.
Point 2: Where to Use an O-shaped Rubber Sealing Ring
- O-shaped rubber sealing ring (GB/T 3452.1-1982, GB/T 1235-1976) has the advantages of simple structure, good sealing performance, small friction, small groove size, easy manufacturing, etc., so it is currently used in hydraulic and pneumatic components widely used.
- Where to use O-shaped rubber sealing ring. O-shaped rubber sealing ring (referred to as O-ring) is used for sealing, fixed sealing, and flange sealing of reciprocating motion, rotary motion, and spiral motion in hydraulic and pneumatic components. It can be used for inner diameter sealing and outer diameter sealing.
- An O-ring is a rubber ring with a circular cross-section, as shown in the figure.
- Working pressure: 0～32MPa, temperature: 35～200℃, working medium: general mineral, hydraulic oil, lubricating oil, water, and compressed air.
- compared with other sealing rings, it has the following main advantages.
1. Good airtightness and long life.
2. The amount is small, and a single ring can seal both directions.
3. The dynamic friction resistance is small.
4. Good adaptability to oil, temperature, and pressure. e. Small size, lightweight, and low cost.
5. The sealing part has a simple structure, small space occupation, and convenient assembly and disassembly.
O-ring seals can be used as dynamic seals and static seals, but when O-rings act as dynamic seals, the starting frictional resistance is relatively high.
Larger and shorter life span.
- Compared with lip-shaped seals, O-rings have a poorer life; the accuracy of the mechanical parts of the sealing device needs to be very good; when used, if the standing time is too long, the starting resistance will increase; in addition, when the speed is very slow, it will cause the phenomenon of “creeping”, and its use speed range is 0.005~0.3m/s.
- The slip ring combined with O-shaped (or rectangular) sealing ring (“Omega” shape) can improve the shortcomings of the above-mentioned O-shaped sealing ring, but the installation is more inconvenient. The use speed of this kind of sealing device can reach 1m/s, and the service life is longer than that of the O-ring seal. In this combination ring, the use of a rectangular ring has a longer life than an O-ring, and the sealing performance is also better.
Point 3: Gap Extrusion Problem of O-ring
The sealing retaining ring is installed in the sealing groove to prevent the gap “extrusion” of the O-ring.
1. Application occasions of retaining ring
- Under normal circumstances, for dynamic seals, when the working pressure exceeds 10MPa, if the O-ring is subjected to medium pressure on one side, a retaining ring is used on the low-pressure side; if it is subjected to medium pressure on both sides, each Use a retaining ring on each side. For static seals, when the pressure exceeds 32MPa, consider the use of retaining rings. The setting of the retaining ring is shown in the figure.
- When the pressure is pulsating (the size and direction of the pressure often change), a retaining ring should generally be used to prevent accelerated wear of the O-ring.
- O-rings for single-acting pistons or plungers are subjected to unidirectional pressure. When the piston moves against the direction of the pressure, it will cause “extrusion”. It is best to use retaining rings. However, when the retaining ring is used, it will increase the friction resistance of the sealing device, which should be considered in the application.
2. Type of retaining ring
The general working pressure is 25MPa. Not suitable for rotation and spiral motion
The working pressure can reach 70MPa, which is used in the integral installation groove
The working pressure can reach 70MPa, used in the combined installation groove
3. The material of the retaining ring
Generally, the material of the retaining ring is PTFE and leather, and nylon 6 and nylon 1010 can also be used.
The temperature range of the leather retaining ring is -70～+80℃, and the friction force generated under high pressure is larger than that of the PTFE retaining ring. Suitable leather for retaining rings is mainly chromic acid leather with good heat and oil resistance.
In ultra-high pressure applications (for example, above 140MPa), the gap is required to be as small as possible, and a retaining ring made of soft metal such as babbitt alloy should be used.
Point 4: O-ring Installation Contraindications
The installation quality of the O-ring has an important impact on the sealing performance and service life. During the installation process, the O-ring should not be scratched or improperly installed.
- The shaft end and shoulder through which the O-shaped ring is installed must be chamfered or rounded, and the fillet radius should be at least equal to the diameter of the O-shaped ring section. There should be no burrs, rust, or corrosion on the metal surface.
- When installing the sealing ring, if you need to pass the external thread and the undercut groove, you should put a special sleeve on each of them. In the design, the diameter of the external thread and the undercut can also be smaller than the inner diameter of the sealing ring. If you need to pass through the internal thread surface and orifice, the internal diameter of the internal thread and the orifice can be made larger than the outer diameter of the sealing ring, or the chamfer can be processed. When the O-ring is installed in the cylinder or the inner hole, the orifice The edges should be chamfered 10°～20°.
- If the O-ring needs to pass through the internal horizontal hole, the orifice should be inverted into the shape shown in Figure 6-26, where the diameter D is not less than the actual outer diameter of the O-ring, and the slope of the groove is generally α=120 °～140°.
- When the O-ring needs to pass through the external thread, use the metal guide bush as shown in the figure.
- When the O-ring is installed in a stretched state, to reduce the assembly resistance, grease or working oil should be applied to the assembly passing part. If you need to slide a long distance on the shaft to be placed in the groove, the surface of the shaft must have a low roughness and be coated with lubricant. For O-shaped rings with small cross-sections and large diameters, after they are installed in the grooves, the cross-section deformed by extension should be restored to a circular shape before being assembled into the cylinder.
- It should be noted that the sealing of negative pressure and positive pressure is completely different, as shown in the figure. For example, a self-priming hydraulic pump or a pipeline with an instantaneous flow rate greater than 10m/s can produce negative pressure (vacuum). If the figure is used for negative pressure by mistake, the O-ring may be sucked in, causing air to enter the hydraulic pressure. The danger of the system. When installing, the lip end of the sealing ring should face the direction of the liquid pressure.
- During installation, avoid excessive stretching of the sealing ring to avoid plastic deformation.
- When installing the sealing ring, prevent the inclusion of iron filings, sand, cotton yarn, and other debris.
- The O-shaped sealing ring should have a retaining ring when it is used for high pressure. Due to the sealing effect of the O-ring, when the O-ring is subjected to partial pressure on one side, the sealing ring will be pressed to the low-pressure side. Because the mating surface has a certain gap, the O-ring will be pressed into the gap on the low-pressure side. , With the movement of the O-ring, it is easy to damage the O-ring by cutting. At this time, a retaining ring should be installed on the low-pressure side.
- The compression of the O-ring for dynamic sealing should be less than that of the static sealing O-ring. For static sealing structure, there is no relative movement between the O-ring and the sealed part, and a larger amount of compression can be selected; and in the dynamic seal, due to the relative movement of the sealed part and the O-ring, the friction will increase when the compression is large. Larger, the mechanical efficiency is reduced, and the heat generated by friction will deteriorate the working condition of the O-ring and affect the service life.
Point 5: The Use of a Y-shaped Sealing Ring
- Y-shaped rubber sealing ring (HG4-335-66), the cross-section is Y-shaped, belongs to the lip-shaped sealing ring, it is one of the better ones.
- Good sealing, low friction resistance, starting friction resistance have little relationship with the length of stopping time and internal pressure (when the internal pressure exceeds a certain value), and the operation is stable. Good stability, suitable for the sealing of hydraulic cylinders with large cylinder diameter, long-stroke, high speed (up to 0.5m/s), and high pressure (up to 32MPa). The pressure resistance is better, the service life is longer, and the application pressure range is wider.
- The sealing device has a simple structure, can be sealed in a single ring, and the processing cost is low.
- Easy to install, no special tools are needed.
- Applicable conditions: pressure: ≤20MPa; temperature: -30~+80℃; medium: liquid, gas.
- Instructions for use: Under normal circumstances, the Y-shaped seal ring can be directly installed in the groove without the support ring, which can play a sealing role. However, in the place where the pressure fluctuates greatly and the sliding speed is high, the support ring should be used to Fix the sealing ring. To add working pressure to the inner and outer lips of the sealing ring at the same time, to open the lips, several holes must be made in the support ring. The main dimensions of the support ring are shown in the figure.
- The Y-shaped sealing ring has longer service life than the O-shaped sealing ring. Operating speed range: 0.01~0.6m/s when using nitrile rubber; 0.01~1m/s when using polyurethane rubber; 0.05~0.3m/s when using fluorine rubber. Polyurethane rubber is the best for sealing performance, life span, and use pressure limit when the support ring is not used. The installation groove of the Y-shaped sealing ring can be designed as an integral groove.
- The performance and service life of the combined or compound lip-shaped sealing ring are better than that of the Y-shaped sealing ring.
- Y-shaped and V-shaped sealing rings have a certain directionality to pressure. Therefore, when used for the piston seal of a double-acting cylinder, two sets of sealing rings must be used back to back.
Point 6: Use of V-shaped And Combined Lip-shaped Seals
- The V-shaped sealing ring is a kind of lip-shaped sealing ring, which is mainly used for reciprocating sealing of piston and piston rod.
- According to different working pressures, the corresponding number of V-shaped sealing rings can be used to overlap, and the pressing force can be adjusted to obtain the best sealing effect. However, the frictional resistance and the structural size of the sealing device are larger, and the more the number of overlapping V-shaped rings, the more serious. The number of V-rings is large, the sealing performance is improved, but the friction resistance is increased.
- When the piston is under eccentric load or moving under eccentric state, it can still seal well. When the V-shaped sealing ring cannot be installed from the axial direction, it can be installed with incisions. As long as the incisions are staggered during installation, the sealing effect will not be affected. However, it is inconvenient for maintenance and replacement.
- According to different materials, there are V-shaped rubber or fabric rubber seals and plastic seals. However, the performance of the plastic V-ring is poor and it is used less.
- The relationship between the material of the V-ring and the working pressure: pure rubber, up to 32MPa; fabric rubber, up to 50MPa; leather, up to 100MPa.
- Requirements for the roughness of the sealed moving surface: when using rubber sealing rings: Ra≤0.4um, Rz≤1.6um; when using interlaced rubber sealing rings: Ra≤0.8um, Rz≤3.2um.
- Applicable conditions:
- V-shaped clip fabric rubber sealing ring (HG4-337-66), pressure: ≤50MPa; temperature: -40～+80℃; medium: liquid.
- V-shaped plastic sealing ring, pressure: ≤32MPa; temperature-10～＋60℃; medium: water and oil.
- B-type V-clamp fabric rubber sealing ring can be used to seal the piston rod. When the pressure is less than 10MPa, a group of three is sufficient to ensure tightness.
- The sealing performance of the V-shaped sealing ring is worse than that of the Y-shaped sealing ring, but the reliability is better, especially the V-shaped sealing ring made of fabric rubber, which has a long life. Operating speed range: 0.02～0.3m/s when using nitrile rubber; 0.005～0.5m/s when using fabric rubber.
- Installation taboos.
1. To increase the service life of the V-shaped sealing ring and reduce the friction resistance, lubricating oil should be forced to be supplied to the sealing part.
2. The entrance of the sealing installation groove should be chamfered or rounded; it is best to leave a shoulder when designing, so that special tools can be placed and installed correctly.
3. During installation, the support ring should be placed on the side that bears the pressure of the medium. When the V-clamp fabric rubber seal ring cannot be axial.
4. When installing people, or when the specifications can’t meet the needs and the adjacent one-size seal ring is selected, it can be installed and used by notching (but pure the rubber sealing ring cannot be installed through an incision). The direction of the incision is 45°from the lips of the sealing ring to the bottom. When installed, the cuts of the adjacent seal rings must be staggered by 90°(for example, when the number of seal rings is 2, they are staggered by 180°). When the sealing ring is installed, the liquid pressure must be able to open it. When using the flange gland, to make the sealing ring tight, 2~3 pieces of adjusting gaskets can be added between the glands, each with a thickness of 1mm, and the material can be 08 steel.
5. It is not advisable to use threaded end glands to compress seal packing such as V-rings.
6. When installing the V-ring compression device, the screw cap can not be used for compression. The reason is that the thread is easy to causes damage to the V-shaped ring during the installation process, and the operation is laborious when pressing the V-shaped ring.
7. Adjustment of the pressing force of the V-shaped sealing ring. Since the V-shaped sealing ring is gradually deformed and worn in use, it needs to be adjusted frequently.
8. Pressing force. The adjustment method is generally adjusted by adding an adjusting washer or using a nut and also using a spring for adjustment. The spring and the sealed cavity should have a good fit and be concentric. Conical springs are used when the diameter is less than 60mm, and if the diameter is larger, multiple small springs are used, the number is at least 3 or more, and they must be evenly distributed on the circumference.
9. To improve the overall performance of the V-ring, it is necessary to use the softness of the pure rubber V-ring and the fabric rubber V.
Because of the rigidity of the ring, they are often used alternately.
In addition to Y-shaped and V-shaped sealing rings, lip-shaped sealing rings also have U-shaped, L-shaped, and J-shaped sealing rings. The common disadvantage is the structure is more complex, the volume is large, and the friction resistance is large, therefore, the application is less.