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Applications (O-Rings) |
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 There 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.
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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.
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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. |
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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
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-
-
-
1.50
-
-
1.55
1.70
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-
-
-
4.16
-
-
4.00
4.20
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-
-
-
1.50
-
-
1.55
1.70
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-
-
-
6.16
-
-
5.40
5.60
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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
-
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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.
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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. |
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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 |
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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.
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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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