Definition
: -
An O-Ring is a doughnut-shaped object,
or torus. The opposite sides of an
O-Ring are squeezed between the walls
of the cavity or "gland"
into which the O-Ring is installed.
The resulting zero clearance within
the gland provides an effective seal,
blocking the flow of liquids or gases
through the gland's internal passage.
An O-Ring can be defined by its dimensions
(I.D. inner diameter i.e. hole and
C.S. cross section), durometer (Shore
A hardness), and material composition.
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Dimensional
Considerations :-
Inside Diameter :
To provide an effective seal, the
O-Ring's inside diameter (I.D.) must
be smaller than the piston groove
diameter, so that the O-Ring is slightly
stretched, fitting snugly in the groove.
This stretch should be between 1%-4%
with 2% as the ideal in most applications.
A stretch greater than 4% is not recommended.
The stress due to excessive stressing
of the O-Ring will cause accelerated
aging, cross section reduction and
ultimately result in premature seal
failure..
Calculate the O-Ring I.D. according
to the following formula:
| O-Ring
I.D. = |
Groove
Diameter |
| |
% of Stretch
Desired |
Cross Section :
When calculating the cross section
(C.S.) of an O-Ring, you need to consider
the size of the gland to be filled
as well as the amount of squeeze needed
to create a good seal.
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Material
Considerations :-
After determining the O-Ring size,
the right material is to be selected.
The list of materials and their properties
can be viewed in the General Elastomer
Properties Section. The compatiblity
of the material with various chemicals
can be viewed on the Chemical Compatiblity
Section. |
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Factors
:-
Before purchasing a seal, the following
factors are to be taken into consideration.
Chemical Attack
Its important to study the reactivity
of the O-Ring material to the chemicals
its exposed to. Specific elastomers
degrade on exposure to certain chemicals.
Therefore, the first step while designign
a seal is to match the application's
chemicals with the O-Ring material.
Temperature
The range of temperature to which
the O-ring is exposed to is an important
factor when considering the efficiency
of the Seal. Its important to measure
temperature in the immediate O-Ring
environment, not just the system temperature.
The time of exposure to any high temperature
must also be considered. It may involve
short bursts, long bursts, sustained
levels. To view the temperature resistance
of various Sealing Materials view
the General Elastomer Properties section.
Friction
There are two types of friction, both
of which are important considerations
in dynamic (moving) applications.
When part movement is intermittent,
the effects of BREAKOUT FRICTION can
cause excessively high pressures to
develop. This pressure can tear portions
of the seal that adhered to the gland
wall causing seal failure.
In continuously moving applications,
excessive O-Ring RUNNING FRICTION
can cause heat to build up within
the O-Ring material itself. This causes
swelling, which causes more heat to
develop, and eventually results in
seal extrusion and failure. This phenomenon
is known as the "Joule"
effect.
Pressure
Presence of High Pressure on the Seal
can hamper its ability to seal. Therefore,
while selecting the right kind of
sealing material, its important to
keep in mind the pressure under which
the O-rings would perform.
However, low pressure can be a problem
as well. If the system pressure is
below 100 psi, it is classified as
low pressure. Because system pressure
is not great enough to "activate"
the seal, the design must rely solely
on the resiliency of the elastomer
to retain its original profile under
compression. Over time, the elastomer
will not resist compression as much
and take a compression "set",
resulting in possible seal failure.
However, by proper component design
which may include lowering the seal
durometer or cross section, maximum
seal utility is achieved. By lowering
the durometer and/or cross section,
the force required to compress a given
cross section decreases, resulting
in an effective seal at lower applied
force per unit length of seal.
Durometer
Durometer (Shore A) is a measurement
of the hardness of an elastomeric
compound. The numerical ratings for
hardness run from lower numbered (less
than 70) softer materials to higher
numbered (greater than 70) harder
materials, noting that fluorocarbon
has a base rating of 75. This classification
system is designed to work within
a ±5 point range. All materials
are not available in all hardnesses.
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Summary
:-
For PERFECT functioning of a seal
and INCREASED usage life, a number
of factors are to be considered. These
include the size, squeeze, stretch,
chemical compatibility, and the ability
to resist pressure, temperature, and
friction. For more information on
any of these points, please contact
our Customer Service Executive. |
