 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.
Static
and dynamic applications
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.
Dimensioning
O-Ring
dimensional selection depends on the type of application, compounds,
surface finish, etc..
Deformation or squeeze 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.
Glands,
squeeze and extension
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 1.
Table 1
Gland dimensions
C/S
d2
(mm) |
Pressure |
No
Back-up |
One
Back-up |
Two
Back-up |
Radius |
Stat
|
Dynam.
|
Pneum.
|
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 an 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.
Back-up
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, spiral. Usually they are made of virgin PTFE, Polyurethane
or other suitable resins.
Surface
finish
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 2 reports
recommended surface finishes for glands and mating surfaces.
Table 2
Gland surface finish
| Surface |
Application |
Pressure |
Roughness
Ra µm |
| 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 |
Relative
speeds
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.
Assembly
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
- 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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