O-Ring: Characteristics, Function, Parameters, Application, Material Selection

O-ring is a kind of rubber sealing ring with a circular cross-section.

Because its cross-section is O-type, it is called O-type rubber sealing ring, also known as O-ring.

It began to appear in the mid-19th century, when it was used as a sealing element of steam engine cylinders.

Because of its low price, simple manufacture, reliable function and simple installation requirements, O-ring is the most common mechanical design for sealing.

O-Ring: Characteristics, Function, Parameters, Application, Material Selection 1

The O-ring bears a pressure of tens of megapascals (kilopounds).

O-rings can be used in static applications or in dynamic applications where there is relative motion between components, such as rotating pump shafts and hydraulic cylinder pistons.

1. Overview

1.1 Characteristics of O-ring

O-ring is a small cross-section ring-shaped sealing element, and the common cross-section is circular.

The main material is synthetic molding compound, which is the most commonly used seal in hydraulic engineering.

It is mainly used for static seal and sliding seal.

Compared with other seals, it has the following characteristics:

a. Good sealing and long service life;

b. A single circle can seal in both directions;

c. Good adaptability to oil, temperature and pressure;

d. Small dynamic friction resistance;

e. Small size, light weight and low cost;

f. The structure of the sealing part is simple and easy to disassemble;

g. It can be used as static seal or dynamic seal;

h. The size and groove have been standardized, which is convenient for selection and outsourcing.

Its disadvantage is that when the dynamic seal is started, the friction resistance is large, about 3 ~ 4 times of the dynamic friction, and it is easy to be squeezed into the barrier under high pressure.

1.2 Representation

1GB/T3452.1-1982 expression method

Inner diameter d1 × Wire diameter d2

For example:

1) O-ring 20 × 2.4 GB3452.1-82

20 means the inner diameter of O-ring is 20mm;

2.4 the sectional diameter of the O-ring is 2.4mm;

Gb3452.1 represents the standard number;

82 represents the year when the standard was published.

2)24002000 GB3452.1-82

2400 represents that the section diameter of the O-ring is 2.4mm;

0200 means the inner diameter of O-ring is 20mm.

O-Ring: Characteristics, Function, Parameters, Application, Material Selection 2

2. Representation of GB/T3452.1-2005

Such as

(1) O-ring 7.5 × 1.8G GB/T3452.1,

7.5- inner diameter

1.8-section diameter

G-Series

(G-universal O-ring)
(A-O-ring for Aerospace)

(2)A 0 × 0 × 7 × 5XG GB/T3452.1

A-O ring wire diameter 1.80mm  

B-O ring wire diameter 2.65mm

C-O ring wire diameter 3.55mm

D-O ring wire diameter 5.30mm

E-O ring wire diameter 7.30mm

2. Working state of O-ring seal

2.1 Function of O-ring for static seal

The O-ring is an extrusion seal.

The basic working principle of the extrusion seal is to rely on the elastic deformation of the seal, causing contact pressure on the sealing contact surface.

If the contact pressure is greater than the internal pressure of the sealed medium, there will be no leakage, otherwise there will be leakage.

The process of changing the contact state of the O-ring by the medium itself to realize sealing is called “self sealing”.

O-Ring: Characteristics, Function, Parameters, Application, Material Selection 3

Q-ring pre sealing

Self sealing effect:

Due to the pre sealing effect, the O-ring is in close contact with the sealed smooth surface and the bottom surface of the groove.

In this way, when the fluid enters the groove through the gap, it can only act on one side of the O-ring.

When the fluid pressure is high, push the O-ring to the other side surface of the groove and squeeze it into a D shape, and transfer the pressure to the contact surface.

The self sealing of O-rings is limited.

When the internal pressure is too high, the phenomenon of “rubber extrusion” of O-rings will appear.

That is, because there is a gap at the sealing part, the O-ring under high pressure will produce stress concentration at the gap.

When the stress reaches O, the rubber will be squeezed out.

At this time, although the Q-ring can temporarily maintain the seal, it has actually been damaged.

Therefore, strict selection is required.

O-Ring: Characteristics, Function, Parameters, Application, Material Selection 4

2.2 Function of O-ring for dynamic seal

In dynamic seal, the pre sealing effect and self sealing effect of O-ring are the same as that of static seal.

However, the situation is more complicated because it is easy to bring fluid between the O-ring and the rod when the rod moves.

In operation, assume that the left side of the O-ring acts on the medium pressure P1 (as shown in fig. a).

If the contact part between the O-ring and the rod is enlarged (Fig. b), its contact surface is actually four convex, and not every point is in contact with the metal surface.

Due to the self sealing effect, the contact pressure generated by the O-ring on the rod is greater than P1 and is sealed.

However, when the rod starts to move to the right, the medium attached to the rod is brought to the mold slit (Fig. c).

Due to the hydrodynamic effect, the pressure of this part of the medium is greater than P1. When it is greater than the contact force of the O-ring on the rod, the medium will squeeze into the first groove of the O-ring (Fig. d).

When the rod continues to move to the right, the medium will continue to enter the next groove, and the medium will leak along the direction of the rod movement.

When the rod moves to the left, it is not easy to leak because the driving direction is opposite to the pressure direction of the rod.

The leakage increases with the viscosity of the medium and the moving speed of the rod, and is also closely related to the size and working pressure of the O-ring.

Leakage of rubber O-ring in reciprocating motion

O-Ring: Characteristics, Function, Parameters, Application, Material Selection 5

2.3 Sealing form of O-ring

1. According to the relative motion state of the seal and the sealed device, it can be divided into:

Static seal, reciprocating seal, rotating seal and switch seal.

2. According to the compression amount (tightness) of the O-ring compression seal fit in the rectangular groove, it can be divided into:

There are five basic seal fits: compression, sleeve tightening, hydraulic, pneumatic and rotating, as well as squeeze seal fits in the chamfer groove of the end face.

In addition, there are two special sealing methods, sliding seal and floating seal.

3. According to the structure of the sealed parts, it can be divided into:

End seal refers to axial seal, angular seal (chamfer groove seal on hole end surface, chamfer groove seal on shaft end surface), cylindrical seal refers to radial seal (cylindrical inner diameter seal (piston rod seal), cylindrical outer diameter seal (piston seal)), conical seal and spherical seal.

3. Design and application of O-ring

3.1 Service parameters of O-ring

3.1.1 Compression ratio

The compression ratio W is usually expressed by the following formula:

w = (d2-h) /d2 × 100%

Where

d2 — section diameter of O-ring in free state (mm);

h — the distance between the groove bottom of the O-ring and the sealed surface (groove depth), that is, the section height of the O-ring after compression (mm).

When selecting the compression ratio of O-ring, the following three aspects should be considered:

1. There should be enough sealing contact area;

2. The friction shall be as small as possible;

3. Try to avoid permanent deformation.

The selection of O-ring compression ratio w should consider the service conditions, static seal or dynamic seal:

Static seal can be divided into radial seal and axial seal:

The leakage clearance of radial seal is radial clearance, and that of axial seal is axial clearance.

The axial seal can be divided into internal pressure and external pressure according to whether the pressure medium acts on the inner diameter or outer diameter of the O-ring.

The internal pressure increases the tension, and the external pressure decreases the initial tension of the O-ring.

For the above different forms of static seals, the action direction of the sealing medium on the O-ring is different, so the pre pressure design is also different.

For dynamic seals, it is necessary to distinguish between reciprocating seals and rotary seals.

1. Static seal: the cylindrical static seal device is the same as the reciprocating seal device, which is generally taken as W=-10%~15%; the plane static seal device is taken as W=-15%~30%.

2. For dynamic seal, it can be divided into three cases: reciprocating motion generally takes W=10%~-15%.

Joule heat effect must be considered when selecting the compression rate of rotary motion seal. Generally speaking, the inner diameter of O-ring used for rotary motion is 3%-5% larger than the shaft diameter, and the compression rate of outer diameter is w=-3%-8%.

For O-rings used in low friction sports, in order to reduce friction resistance, generally select a small compression ratio, that is W=5%-8%. In addition, consider the expansion of rubber materials caused by medium and temperature.

Generally, in addition to the given compression deformation, the allowable maximum expansion rate is 15%.

Exceeding this range indicates that the material selection is inappropriate, and the O-ring of other materials should be used, or the given compression deformation rate should be corrected.

3.1.2 Stretching amount

After the O-ring is installed into the sealing groove, it usually has a certain amount of tension.

Like the compression ratio, the amount of tension also has a great impact on the sealing performance and service life of the O-ring.

A large amount of tension will not only make it difficult to install the O-ring, but also reduce the compression ratio due to the change of the section diameter d2, resulting in leakage.

The stretching amount a can be expressed by the following formula:

a = (d+d2) / (d1+d2)

Where

d — shaft diameter (mm); d1 — inner diameter of O-ring (mm).

The value range of stretching amount is 1% – 5%.

As table 1 gives the recommended value of the stretching amount of O-ring, the stretching amount of O-ring can be selected and limited according to the size of shaft diameter.

Table I limits of compression ratio and stretching amount of O-ring

Sealing formSealing mediumStretching amount a (%)Compression ratio w (%)
Static sealHydraulic oil1.03~1.0415~25
Air<1.0115~25
Reciprocating motionHydraulic oil1.0212~17
Air<1.010.95~112~173~8
Rotational motionHydraulic oil0.95~13~8

3.2 Installation groove of O-ring

The compression amount of the O-ring is mainly guaranteed by the structure and size of the installation groove.

Commonly used groove shapes include rectangle and triangle. Generally, triangle is only used for some fixed seals.

Due to the different amount of compression, the grooves of static seal, reciprocating seal and optional motion seal are similar in shape, but their sizes are different.

3.2.1 Slot width

Slot width is mainly considered from the following three aspects:

1) It must be greater than the maximum diameter of the O-ring after compression deformation;

2) The expansion and medium swelling of O-ring caused by motion heating must be considered;

3) It must be ensured that there is a certain space in the groove during reciprocating motion to make the O-ring roll freely.

It is generally believed that the cross-sectional area of the O-ring should occupy 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.

Note:

The narrow trough will increase the friction resistance during movement, and the wear of O-ring will increase, which is vulnerable;

The groove is too wide, which increases the swimming range of the O-ring and is easy to wear, and under the pulsating pressure of the static seal, the O-ring may also produce pulsating swimming and abnormal wear.

In addition, when the internal pressure is very high, the retaining ring must be used, and the groove width should be increased accordingly.

3.2.2 Groove depth

Groove depth is the key dimension for O-ring to work well, which mainly depends on the compression deformation of O-ring.

This deformation amount is composed of the compression deformation amount (A1) at the inner diameter of the O-ring and the compression deformation amount (A2) at the outer diameter of the O-ring.

When a1=a2, the cross section of the O-ring coincides with the center of the groove cross section, and the two desired circles are equal, indicating that the O-ring is not stretched during installation;

When a1>a2, the circumference of the section center of the O-ring is smaller than that of the groove center, indicating that the O-ring is in the groove in a stretched state;

When a1<a2, the section perimeter of the O-ring is greater than the center perimeter of the groove section.

At this time, the O-ring is used by circumferential compression. During disassembly, the O-ring will bounce.

When designing the groove depth, first determine the use mode of the O-ring, and then select a reasonable compression deformation rate (see Table 2 for details).

In addition, the swelling of the material to the medium, the swelling of the material itself and other related factors should also be considered, and many design factors should be considered.

However, the state has given relevant standards for the structure of grooves.

3.2.3 Selection and design of grooves

1. Installation form of groove

O-Ring: Characteristics, Function, Parameters, Application, Material Selection 6
O-Ring: Characteristics, Function, Parameters, Application, Material Selection 7

Explain:

1) In general, in order to prevent the O-ring from being damaged by being squeezed into the gap, when the liquid working pressure exceeds 10MPa, fix the seal.

When the liquid pressure exceeds 32MPa, add a sealing ring (as shown in Fig. c).

The number of the ring depends on the pressure of the O-ring.

2) When the external pressure of the axial seal is pressed, pay attention to add the boss at the diameter d8 to prevent the O-ring from entering the pipeline.

Table II radial groove size of O-ring

O-ring section diameter d2

1.80

2.65

3.55

5.30

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

Groove depth t

Piston rod seal, (for calculation d3)

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

Static seal

1.38

2.07

2.74

4.19

5.67

Piston rod seal, (for calculation d6)

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

Groove bottom fillet radius r1

0.2-0.4

0.4-0.8

0.8-1.2

Groove fillet radius r2

0.1-0.3

Maximum diameter of piston rod seal groove bottom d3max=d4+2t, d4 piston rod diameter

The minimum diameter of piston rod seal groove bottom d6min=d5max+2t, d5max piston rod maximum diameter.

China has formulated standards for the groove size series of O-rings.

See Table 3 for details.

Table III groove size and compression for sealing

0-ring section dimension tolerance

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

Groove 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

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

Groove 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

6.0

7.0

7.8

11.5

17.2

r≤

0.4

0.7

0.8

Note:
h refers to the height of the groove; b represents the width of the trench; r refers to the chamfer of the groove.

3. O-ring groove processing requirements

In order to avoid leakage caused by scratches and improper installation, O-rings have certain requirements for the accuracy of grooves and related components during installation.

First of all, the scream passing through during installation needs to be blunt or rounded, and the inner hole passing through should be chamfered by 10~20 °.

Secondly, pay attention to the surface accuracy on the installation path of the O-ring.

The shaft must have a low roughness value and be coated with lubricant if necessary.

See Table IV for the requirements of installation groove and matching surface accuracy.

Table IV surface finish of mating parts of O-shaped rubber seal groove

surface

Applications

Pressure condition.

Surface finish

Bottom and sides of trench

Tight seal

Non alternating and non pulse,

R.3.2um

Alternating or pulse,

R.1.6um

Dynamic seal,

Non alternating and non pulse.

Mating surface

Tight seal

Non alternating and non pulse.

R.1.6um.

Alternating or pulse,

R.0.8um

Dynamic seal

 

R0.4 μ m

3.3 Material selection of O-ring

The material selection of O-ring mainly considers the following points:

1) The working state of the O-ring refers to whether the O-ring is used for static seal, dynamic seal or sliding seal;

2) The working state of the machine refers to whether the machine is working continuously or intermittently, and considering the length of each interruption, whether there is impact on the sealing part;

3) The working medium is gas or liquid, and its physical and chemical properties shall be considered;

4) Working pressure, pressure magnitude, fluctuation amplitude and frequency, as well as the instantaneous maximum pressure, etc;

5) Working temperature includes instantaneous temperature and alternating temperature of cold and hot;

6) Price and source.

Generally speaking, nitrile rubber 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, copolyazol rubber for cold resistance and oil resistance, etc.

See Table 5 for the scope of application of various adhesives.

Table V specification for use of O-ring sealing materials

Material Science

Applicable media

Service temperature / ℃

Remarks

For sports

Static use

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

butadiene styrene rubber

Alkali, animal and vegetable oil, 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

silicon 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 in moving parts

Chlorosulfonated polyethylene

High temperature oil, oxygen, ozone

100

-10~150

Avoid using in moving parts

Polyurethane rubber

Water, oil

60

-30~80

Wear resistant, but avoid high-speed use

Fluororubber

Hot oil steam air, inorganic acid

150

-20~200

 

teflon

Acids, bases, various solvents

 

-100~260

Not applicable to moving parts

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