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  Applications (O-Rings)
   
  pic_o-ringsThere are several definitions regarding the various types of sealing applications:

- axial static
- radial static

- reciprocating dynamic
- revolving dynamic
- composite dynamic.

Dynamic sealing have continuous or intermittent motion.

   
 
  To obtain a proper sealing effect, the O-Ring is always deformed (compression), but it is essential to consider some basic elements:
  • type of sealing
  • seal housing
  • fluid to be contained and its concentration
  • temperature and pressure of fluid (sudden pressure surge if foreseeable)
  • for dynamic sealing, type of motions.

In addition, the compression set of the elastomer must be considered. The deformation to be set on the elastomeric elements to assure the proper sealing must be compatible with the compression set of the specific elastomer, especially with thin cross section O-Rings.

   
 
  O-Ring dimensional selection depends on the type of application, compounds, surface finish, etc..
Deformation or sqeeze given to an O-Ring is the result of various parameters employed by the specifier. Squeeze must be higher for static applications. The squeeze in dynamic seal applications should be kept to a minimum due to the friction and higher temperatures generated. Seal lubrication is very important to limit friction and for helping heat exchange thus reducing temperature build-up. Standard sizes of O-Rings corresponding to several international standards can be found later in our Catalog.
   
 
  For proper O-Ring gland dimensions, it is imperative to consider 2 basic points:
  • a) coefficient of thermal expansion of elastomers is more or less 10 times the coefficient of steel;
  • b) swelling (volume change) due to fluid contact can be much higher than 10%.

Gland dimensions for O-Rings, utilizing considerations previously outlined are listed in Table 3.

Table 3
Gland dimensions

C/S d2
(mm)
Pressure No Back-Up One Back-Up Two Back-Up Radius
static
Dynamic
Pneumatic
P
(mm)
L
(mm)
P
(mm)
L1
(mm)
P
(mm)
L2
(mm)
r1
(mm)
1.00
1.50
1.60
1.78-1.80
1.78-1.80
1.90
1.90
2.00
x
x
x
x

x

x



x


x
x




x

0.70
1.14
1.21
1.49
1.58
1.40
1.55
1.68

1.40
2.40
2.40
2.50
2.50
2.60
2.60
2.80

-
-
-
1.50
-
-
1.55
1.70

-
-
-
4.16
-
-
4.00
4.20

-
-
-
1.50
-
-
1.55
1.70

-
-
-
6.16
-
-
5.40
5.60

0.20
0.30
0.30
0.40
0.40
0.40
0.40
0.40

2.40
2.40
2.40
2.50
2.50
2.62-2.65
2.62-2.65
2.70


x

x
x

x
x


x

x

x

x


x

x
1.80
2.16
1.90
2.12
2.28
2.25
2.36
2.30
3.30
3.30
3.20
3.50
3.50
3.25
3.25
3.60
2.05
-
-
2.15
-
2.28
-
2.30
4.70
-
-
4.90
-
4.65
-
5.00
2.05
-
-
2.15
-
2.28
-
2.30
6.10
-
-
6.30
-
6.50
-
6.40
0.50
0.50
0.50
0.50
0.50
0.60
0.60
0.60
2.95
3.00
3.00
3.00
3.10
3.50
3.53-3.55
3.53-3.55
x


x
x
x
x
x
x


x
x
x


x




x
2.20
2.58
2.72
2.45
2.50
2.65
2.70
3.21
3.90
4.10
4.10
3.80
4.10
4.60
4.80
4.40
2.50
2.60
-
-
2.70
3.05
3.10
-
5.30
5.50
-
-
5.50
6.00
6.20
-
2.50
2.60
-
-
2.70
3.05
3.10
-
5.30
6.90
-
-
6.90
7.40
7.60
-
0.60
0.60
0.60
0.60
0.60
0.60
0.80
0.80
3.60
4.00
4.50
5.00
5.00
5.30-5.33
5.30-5.33
5.70
x
x
x
x

x

x
x
x
x
x

x

x




x

x
2.80
3.46
3.50
4.45
4.65
4.72
4.85
4.60
4.80
5.40
5.80
6.70
6.70
6.45
6.45
7.20
3.15
3.51
4.00
4.46
-
4.75
-
5.00
6.20
6.80
7.50
8.50
-
7.66
-
8.90
3.15
3.51
4.00
4.46
-
4.75
-
5.00
7.60
8.20
9.20
10.30
-
10.26
-
10.60
0.80
0.80
0.80
0.80
0.80
1.20
1.20
1.20
6.00
6.00
6.99-7.00
6.99-7.00
8.40
8.40
8.40
x

x



x
x

x

x

x

x

x
5.06
5.30
6.05
6.22
7.65
7.85
7.35
7.60
7.40
8.75
8.75
11.10
11.10
9.10
5.12
-
6.10
-
7.68
-
-
9.40
-
10.12
-
13.30
-
-
5.12
-
6.10
-
7.68
-
-
11.20
-
13.55
-
15.50
-
-
1.20
1.20
1.50
1.50
1.50
1.50
1.50

The design of a sealing system must consider a peculiar property of elastomers known as the "Joule effect".
Any elastomer under tensile stress, will shrink whenever temperature increases (Joule effect).
When a O-Ring is assembled on a shaft under tension, it tends to contract during an increase in temperature, generating consequently, higher friction and increasing the temperature. The result is a total and rather quick destruction of the O-Ring. Therefore, it is always advisable to assemble O-Rings under compression instead of tension. In such case, an increase of temperature can only decrease the compression, relieving the strain in the assembly.

   
 
  Anti-extrusion Back-Up rings function as a containment for the elastomer when subject to high pressure, preventing seal extrusion through the clearance existing in the couplings. When pressure is applied from both directions, Back-Up rings are mounted on each side of the O-Ring. Back-Up rings are produced in standard dimensions in 3 styles: solid, single-turn, and spiral. Usually they are made of virgin PTFE, Polyurethane or other suitable resins.
   
 
  The finish of mating surfaces in contact with O-Rings is critical for heat build-up and premature wear and tear.

"break-out friction" can be as high as 3 times the dynamic friction (with lubricant). The former can yield sticking even in the presence of squeeze, lubrication, pressure and temperature existing in the system. Material hardness is recommended to be as near as possible to 70 IRHD, except when different values are required for the application.

Table 4 reports recommended surface finishes for glands and mating surfaces.

Table 4
Gland surface finish

Surface Applicatione Pressure Roughness
Ra um
Housing static diam. Static non pulsating 1,6
pulsating 0,8
Dynamic all types 0,8
Mating surfaces with O-Ring Static non pulsating 0,8
pulsating 0,5
Dynamic all types 0,5
   
 
  It is difficult to recommend limits of relative speed in dynamic seals due to the wide range of fluids and working conditions. A reference speed limit can be 0.5 m/sec with particularly well finished surfaces and slightly widened tolerances of gland dimensions utilizing O-Rings with IRHD 80 and system pressure of 5 bar maximum.

With higher pressure, speed must be drastically reduced.

   
 
  The function and duration of an O-Ring is dependent upon the assembly procedure and care. It is advisable to comply with the following points:
  • Dimensioning of glands should conform to values listed at dimensioning and glands section
  • Avoid sharp edges and rough metallic surfaces.
    During assembly of O-Rings, it is imperative to use suitable installation tools to make the seals slide, without damage over threads, grooves or holes. The stretch of O-Rings during assembly must be kept at a minimum, again using a suitable tool
  • O-Rings must be clean, in good shape and slightly lubricated with a compatible lubricant for the elastomer.
  • Assembly glands and cavities must be cleaned and greased with care
  • during assembly, O-Rings already positioned must be protected by suitable means to avoid damage from other components to be assembled (see point b)
  • small dimension and high hardness O-Rings should be preferably positioned into composite glands to avoid excessive tensioning and deformations of the elastomer.

When an O-Ring is to be installed by automated assembly, it is advisable to use internally lubricated compounds which are specially formulated to provide lower friction. Special design considerations regarding tolerances, concentricity, etc. should be specified.

   
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