An O-ring is a type of rubber sealing ring with a circular cross section. Its name comes from its O-shaped cross section and is commonly called an O-ring.
The O-ring was first introduced in the mid-19th century as a sealing element for steam engine cylinders. Today, it is widely used due to its affordable price, ease of manufacturing, reliable performance, and simple installation requirements. As a result, the O-ring is the most commonly used design for mechanical sealing.
The O-ring can withstand high pressure, measured in tens of megapascals (kilo pounds). It can be used in static and dynamic applications where components move relative to each other, such as rotary pump shafts and hydraulic cylinder pistons.
1. Overview
1.1 Features of the sealing ring
An O-ring is a small ring-shaped sealing element that typically has a circular cross-section. The main material used in its manufacture is synthetic molding compound, making it the most used type of seal in hydraulic engineering. It is mainly used for static and sliding seals.
Compared with other seals, the O-ring has several advantages, including:
The. Effective sealing and long service life
B. The ability to seal in both directions with a single ring
w. Good compatibility with oil, temperature and pressure
d. Low resistance to dynamic friction
It is. Small size, light weight and low cost
f. A simple and easily dismantled sealing structure
g. The ability to be used as a static or dynamic seal
H. Standardized size and groove, making it convenient for selection and supply
One of the disadvantages of the O-ring is that, when used as a dynamic seal, it has a high resistance to friction, which is about 3 to 4 times greater than its dynamic friction. Furthermore, it is prone to being squeezed into the barrier under high pressure.
1.2 Representation
1GB / T Expression Method 3452.1-1982
Inner diameter d1 × Wire diameter d2
For example:
- Sealing ring 20 × 2.4 GB3452.1-82
The “20” indicates that the inner diameter of the O-ring is 20mm.
The “2.4” refers to the cross-sectional diameter of the O-ring, which is 2.4 mm.
“GB3452.1” is the default number.
“82” represents the year the standard was published.
- 24002000 GB3452.1-82
The “2400” represents the cross-sectional diameter of the O-ring, which is 2.4 mm.
The “0200” indicates that the inner diameter of the O-ring is 20mm.
As in the first example, “GB3452.1” is the standard number and “82” represents the year the standard was published.
2. Representation of GB / T 3452.1-2005
For example:
(1) Sealing ring 7.5 × 1.8 G GB/T3452.1
The “7.5” indicates the inner diameter of the O-ring.
The “1.8” refers to the cross-sectional diameter of the O-ring.
The “G” series refers to the “universal O-ring”. There are other series, such as “A” which stands for “O-ring for Aerospace”.
(2) One 0 × 0 × 7 × 5XG GB/T3452.1
“A” series refers to O-ring wire diameter of 1.80mm. There are other series with different wire diameters, such as:
- “B” for 2.65mm seal ring wire diameter
- “C” for 3.55mm seal ring wire diameter
- “D” for 5.30mm seal ring wire diameter
- “E” for 7.30mm seal ring wire diameter
2. Operating status of the sealing ring
2.1 Function of O-ring for static seal
O-ring is a type of extrusion seal. The basic principle of an extrusion seal is that it relies on elastic deformation of the seal to create contact pressure on the sealing surface. If this contact pressure is greater than the internal pressure of the sealed medium, there will be no leakage, otherwise, leakage will occur. The process in which the medium itself changes the contact state of the O-ring to achieve sealing is called “self-sealing”.
Q-ring pre-seal
Self-sealing effect:
Due to the pre-sealing effect, the O-ring is in close contact with both the sealed smooth surface and the bottom of the groove. As a result, when fluid enters the groove through a gap, it only acts on one side of the O-ring. When the fluid pressure is high, it pushes the seal ring to the other side of the groove and compresses it into a D-shape, transferring pressure to the mating surface.
However, the self-sealing ability of O-rings is limited. When the internal pressure is too high, the O-ring may experience “rubber extrusion”. This occurs when there is a gap at the sealing point and the high pressure causes stress concentration in the gap. When the tension reaches a certain level, the rubber will be squeezed. Although the O-ring may temporarily maintain the seal, it has actually been damaged. Therefore, it is important to carefully select the appropriate O-ring for the application.
2.2 Function of O-ring for dynamic sealing
In dynamic seals, the pre-sealing and self-sealing effects of the O-ring are similar to those of static seals. However, the situation is more complicated in dynamic seals due to the potential for fluid to be introduced between the O-ring and stem during movement.
When the rod is in operation, if the left side of the O-ring is acted upon by the medium pressure P1 (as shown in Figure a), the contact pressure generated by the O-ring on the rod is greater than P1 due to the self-sealing effect, ensuring a seal.
However, when the rod starts to move to the right, the medium attached to the rod is taken into the space between the O-ring and the rod (Figure b). Due to the hydrodynamic effect, the pressure of this part of the medium is greater than P1 and may exceed the contact force of the O-ring on the rod, causing the medium to squeeze into the first groove of the O-ring (Figure c). As the rod continues to move to the right, the medium will continue to enter the next groove, resulting in leakage in the direction of rod movement.
Leakage is less likely to occur when the rod moves to the left because the direction of conduction is opposite to the direction of rod pressure. The probability of leakage increases with the viscosity of the medium and the speed of the rod movement, in addition to being closely related to the size and working pressure of the O-ring.
2.3 S shape O-ring seal
- O-ring seals can be categorized based on the relative movement between the seal and the sealed device:
- Static seals
- Alternative stamps
- Rotary Seals
- Switch Seals
- The compression (tightening) of the O-ring compression seal adjustment in the rectangular groove can be divided into five basic seal adjustments:
- Compression fit
- Sleeve tightness adjustment
- Hydraulic adjustment
- Pneumatic adjustment
- Swivel adjustment
In addition, there is a compression seal fit in the end face chamfer groove, as well as two special sealing methods:
- Slip Seal
- Floating Seal
- The sealed parts structure can be used to categorize O-ring seals into the following types:
- End seals, which include thrust seals and angle seals (such as chamfered groove seals on the end surface of a hole or shaft)
- Cylindrical seals, which include radial seals (such as inner diameter cylinder seals for piston rods and outer diameter cylinder seals for pistons)
- Conical seals
- Spherical seals.
3. O-ring design and application
3.1 S O-ring service parameters
3.1.1 C compression ratio
The compression ratio (W) of an O-ring is expressed as:
W = (d2 – h) / d2 × 100%
Where:
d2 – The diameter of the cross section of the O-ring in its free state (mm)
h – The distance between the bottom of the O-ring groove and the sealed surface (groove depth), which is the height of the O-ring cross-section after compression (mm).
When choosing the compression ratio of an O-ring, it is important to consider the following factors:
- Adequate sealing contact area
- Minimal friction
- Avoid permanent deformation
The selection of the compression ratio (W) must also take into account the service conditions and whether it is a static or dynamic seal.
Static seals can be divided into radial seals and axial seals. Radial seals have radial clearances and axial seals have axial clearances.
Axial seals can be divided into internal pressure seals and external pressure seals depending on whether the pressure medium acts on the inner diameter or the outer diameter of the O-ring. Internal pressure increases the tension, while external pressure decreases the initial tension of the O-ring.
For these different forms of static seals, the direction of the sealing medium in the O-ring is different, so the prepressure design is also different.
For dynamic seals, it is important to distinguish between reciprocating seals and rotary seals.
- Static seal: The cylindrical static seal device is similar to the alternative seal device and typically has a compression ratio of -10% to 15%. The flat static sealing device has a compression ratio of -15% to 30%.
- For dynamic seals, it can be divided into three cases: Reciprocating motion typically has a compression ratio of 10% to -15%.
When selecting the compression ratio for rotary motion seals, it is necessary to consider the effect of Joule heat. Generally, the inner diameter of the O-ring used for rotary movement is 3% to 5% larger than the shaft diameter, and the compression ratio of the outer diameter is -3% to 8%.
For o-rings used in low-friction applications, a small compression ratio of 5% to 8% is typically selected to reduce frictional resistance. It is also important to consider the expansion of rubber materials due to the medium and temperature.
Typically, the maximum allowable expansion rate is 15% in addition to the given compression strain. If this range is exceeded, it indicates that the material selection is inadequate and a different material for the O-ring must be used or the compression strain rate must be corrected.
3.1.2 S amount of stretching
After the O-ring is installed in the sealing groove, it normally has a certain level of tension. This tension, as well as the compression ratio, greatly affects the sealing performance and service life of the O-ring. Excessive tension makes O-ring installation difficult and reduces the compression ratio, causing leaks.
The amount of stretching can be calculated using the following formula:
a = (d + d2) / (d1 + d2)
Where:
d – shaft diameter (mm) d1 – inner diameter of the O-ring (mm)
The recommended range for amount of stretch is 1% to 5%. Table 1 provides the recommended elongation value for o-rings, and the elongation value can be selected and limited based on the shaft diameter size.
Table I Limits of O-Ring Compression Ratio and Stretch Amount
| Sealing way | Sealing medium | Amount of stretching a (%) | Compression ratio w (%) |
| Static seal | Hydraulic oil | 1.03~1.04 | 15~25 |
| Air | <1.01 | 15~25 | |
| Alternative movement | Hydraulic oil | 1.02 | 12~17 |
| Air | <1,010.95~1 | 12~173~8 | |
| Rotational movement | Hydraulic oil | 0.95~1 | 3~8 |
3.2 EU O-ring installation groove
The compression of an O-ring is primarily determined by the design and dimensions of the installation groove.
Rectangular and triangular grooves are the most commonly used shapes, with triangular grooves only used for specific fixed seals.
The groove shapes for static seals, alternative seals, and dynamic seals can be similar, but their sizes vary to accommodate different compression requirements.
3.2.1 S lot width
Slot width is considered from the following three perspectives:
- It must be larger than the maximum diameter of the O-ring after compression deformation.
- The impact of motion-induced heating on O-ring expansion and swelling must be taken into consideration.
- Adequate space must be provided in the groove to allow the seal ring to roll freely during reciprocating motion.
It is generally recommended that the cross-sectional area of the O-ring occupies at least 85% of the rectangular cross-sectional area. In many cases, the groove width is 1.5 times the cross-sectional diameter of the O-ring.
It is important to note that a narrow groove will increase friction and cause greater wear on the O-ring. On the other hand, if the groove is too wide, it will increase the O-ring's range of motion and make it more susceptible to wear. Furthermore, under static seals with pulsating pressure, the O-ring may exhibit pulsating movement and abnormal wear.
In high pressure situations, a retaining ring must be used and the groove width must be increased accordingly.
3.2.2 G roof depth
The depth of the groove is a crucial factor for the proper functioning of the O-ring. It mainly depends on the compression deformation of the O-ring.
This deformation is composed of the compressive deformation (A1) in the internal diameter of the O-ring and the compressive deformation (A2) in the external diameter of the O-ring.
When A1 = A2, the cross-section of the O-ring coincides with the center of the cross-section of the groove and the two circles are equal, indicating that the O-ring is not stretched during installation.
When A1>A2, the circumference of the center of the O-ring section is smaller than that of the center of the groove, indicating that the O-ring is installed in a stretched state.
When A1
When designing the groove depth, the intended use of the O-ring should be considered first, followed by the selection of a reasonable compression strain rate. The swelling of the material in the middle, the swelling of the material itself and other related factors must also be taken into consideration.
However, there are relevant standards provided by the state for the structure of the grooves.
3.2.3 Selection and design of grooves
1. Groove installation way
Explain:
- To prevent the O-ring from being damaged when compressed into a gap, it is generally recommended to fix the seal when the working pressure of the liquid exceeds 10MPa. If the liquid pressure exceeds 32MPa, a sealing ring must be added (as shown in Fig. c). The number of rings depends on the O-ring pressure.
- When there is external pressure applied to the axial seal, it is important to add a boss on diameter d8 to prevent the O-ring from entering the pipeline.
Table II O-ring radial groove size
| Seal ring section diameter d 2 | 1.80 | 2.65 | 3.55 | 5:30 p.m. | 7:00 | ||
| trench width | Pneumatic seal | 2.2 | 3.4 | 4.6 | 6.9 | 9.3 | |
| Hydraulic dynamic seal or static seal | b +0.25 | 2.4 | 3.6 | 4.8 | 7.1 | 9.59.5 | |
| b1 +0.25 | 3.8 | 5.0 | 6.2 | 9.0 | 12.3 | ||
| b2 +0.25 | 5.2 | 6.4 | 7.6 | 10.9 | 15.1 | ||
| T-slot depth | Piston rod seal, (for d3 calculation) | Hydraulic dynamic seal | 1.42 | 2.16 | 2.96 | 4.48 | 5.95 |
| Pneumatic seal | 1.46 | 2.23 | 3.03 | 4.65 | 6:20 am | ||
| Static seal | 1.38 | 2.07 | 2.74 | 4.19 | 5.67 | ||
| Piston rod seal, (for d6 calculation) | Hydraulic dynamic seal | 1.47 | 2.24 | 3.07 | 4.66 | 6.16 | |
| Pneumatic seal | 1.57 | 2.37 | 3.24 | 4.86 | 6.43 | ||
| Static seal | 1.42 | 2.15 | 2.85 | 4.36 | 5.89 | ||
| Minimum chamfer length Zmin | 1.1 | 1.5 | 1.8 | 2.7 | 3.6 | ||
| Bottom fillet radius of slot r1 | 0.2-0.4 | 0.4-0.8 | 0.8-1.2 | ||||
| Slot r2 fillet radius | 0.1-0.3 | ||||||
| Maximum diameter of the bottom of the piston rod sealing groove d 3max. =d 4 +2t, d 4 piston rod diameter | |||||||
| The minimum diameter of the bottom of the piston rod sealing groove d 6 minutes =d 5max. +2t, d 5max. maximum piston rod diameter. | |||||||
China has set standards for the series of groove size sealing rings. Details can be found in Table 3.
Table III Groove size and compression for sealing
| Ring Section Dimensional Tolerance 0 | 1.9±0.08 | 2.4±0.08 | 3.1±0.10 | 3.5±0.10 | 5.7±0.15 | 8.6±0.16 | |||
| Axial fixed seal | Compression amount | 0.60~0.40 | 0.70~0.504 | 0.85~0.55 | 0.90~0.65 | 1.3~0.9 | 1.6~1.0 | ||
| Slot Size | H | 1.3~1.5 | 1.7~1.9 | 2.25~2.55 | 2.60~2.85 | 4.40~4.80 | 7:00~2:60 | ||
| B | 2.50 | 3.20 | 4.2 | 4.70 | 7:50 am | 11.2 | |||
| r≤ | 0.40 | 0.7 | 0.80 | ||||||
| For sports | Compression amount | 0.47~0.28 | 0.47~0.27 | 0.54~0.30 | 0.60~0.324 | 0.85~0.45 | 1.06~0.68 | ||
| Slot Size | H | 1.43~1.62 | 1.93~2.13 | 2.65~2.80 | 2.90~3.18 | 4.85~5.25 | 7.54~7.92 | ||
| B | Without retaining ring | 2.5 | 3.2 | 4.2 | 4.70 | 7.5 | 11.2 | ||
| Add a retaining ring | 3.9 | 4.4 | 5.2 | 6.0 | 9.0 | 13.2 | |||
| Add two retaining rings | 5:40 am | 6.0 | 7.0 | 7.8 | 11.5 | 17.2 | |||
| r≤ | 0.4 | 0.7 | 0.8 | ||||||
|
Observation: h refers to the height of the groove; b represents the width of the trench; r refers to the chamfer of the groove. |
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3. O-ring groove processing requirements
To prevent leaks due to scratches and improper installation, there are certain requirements for the accuracy of grooves and related components when installing O-rings.
First, the edges passing through during installation must be blunt or rounded, and the inner hole passing through must be chamfered at an angle of 10 to 20 degrees.
Secondly, the surface accuracy along the O-ring installation path must be carefully considered. The shaft must have a low roughness value and be lubricated if necessary.
Requirements for installation groove and corresponding surface accuracy can be found in Table IV.
Table IV Surface finish of mating parts of O-shaped rubber seal groove
| surface | Forms | Pressure condition. | Surface Finish |
| Bottom and sides of the trench | Airtight seal | Non-alternating and pulseless, | R.3.2um |
| Alternating or pulse, | R.1.6um | ||
| Dynamic seal, | Non-alternating and pulseless. | ||
| Coupling surface | Airtight seal | Non-alternating and pulseless. | R.1.6um. |
| Alternating or pulse, | R.0.8um | ||
| Dynamic seal | R0.4μm |
3.3M O -ring material selection
O-ring material selection takes the following factors into consideration:
- The operating state of the O-ring, such as whether it is used for static sealing, dynamic sealing, or sliding sealing.
- The operational state of the machine, including whether it operates continuously or intermittently, and the duration of each interruption and its impact on the sealing component.
- The working medium, whether gaseous or liquid, and its physical and chemical properties.
- The working pressure, including pressure magnitude, fluctuation amplitude, frequency and maximum instantaneous pressure.
- The working temperature, including the instantaneous temperature and the alternating temperature of hot and cold.
- Cost and availability.
Typically, nitrile rubber is used for oil resistance, chloroprene rubber for weather resistance and ozone resistance, acrylate rubber or chlorine rubber for heat resistance, polyurethane rubber for high pressure resistance and wear resistance, and rubber of copolyazole for cold resistance and oil resistance.
The scope of application of various adhesives can be found in Table 5.
Table V Specification for use of O-ring sealing materials
| Materials science | Applicable media | Service temperature/℃ | Comments | |
| For sports | Static usage | |||
| Nitrile rubber | Mineral oil, gasoline, benzene | 80 | -30~120 | |
| Neoprene | Air, water, oxygen | 80 | -40~120 | Precautions for sports |
| butyl rubber | Animal and vegetable oil, weak acid, alkali | 80 | -30~110 | Large permanent deformation, not suitable for mineral oil |
| styrene butadiene rubber | Alkalis, animal and vegetable oils, air, water | 80 | -30~100 | Not applicable to mineral oil |
| Natural rubber | Water, weak acid, weak base | 60 | -30~90 | Not applicable to mineral oil |
| Silicone rubber | High and low temperature oil, mineral oil, animal and vegetable oil, oxygen, weak acid, weak base | -60~260 | -60~260 | Not suitable for steam, avoid using on moving parts |
| Chlorosulfonated polyethylene | High temperature oil, oxygen, ozone | 100 | -10~150 | Avoid using on moving parts |
| Polyurethane Rubber | Water, oil | 60 | -30~80 | Wear-resistant, but avoid high-speed use |
| Fluorinated rubber | Hot oil vapor, inorganic acid | 150 | -20~200 | |
| teflon | Acids, bases, various solvents | -100~260 | Not applicable to moving parts | |
