Rotary Seals - Dilanda.it

Radial Oil Seal
Metal case
The principal function of the metal case is to give rigidity
and strength to the seal. It must not normally be exposed
to axial loads. A special design is required to enable the
case to withstand axial loads.
The case is normally made of cold rolled steel sheet AISI
1008, DIN 1624. Environmental conditions may dictate
other materials, such as brass or stainless steel AISI 304,
DIN 1.4301.
Garter spring
Function
When rubber is exposed to heat, load or chemical action, it
will gradually lose its original properties. The rubber is then
said to have aged. The original radial force exerted by the
sealing element will then diminish. The function of the
garter spring is therefore to maintain the radial force.
Experiments have shown that the radial force must vary
with the size and type of seal. Experiments have also clearly
indicated the significance of maintaining changes in the
radial force within narrow limits during the service life of
the seal. Extensive investigations in the laboratory have
formed the basis for defining the radial force.
The garter spring is close wound and carries an initial
tension. The total force exerted by the spring thus consists
of the force required to overcome the initial tension and
the force due to the spring rate. The use of a garter spring
with initial tension provides the following advantages:
- as the sealing element wears, the total radial force
attributable to the initial tension will not change.
Initial Force
%
100
75
50
Figure 8
Change of the
spring force
due to wear
of the sealing
element
stabilized spring
unstabilized spring
50 100 150
time h
The change in the initial tension in stabilized
and not-stabilized garter springs.
Force
before heat treatment
after heat treatment
spring
rate
initial
tension
- by eliminating some of the initial tension by heat
treatment, it can be adjusted to achieve the required
radial force for the actual shaft diameter.
- the heat treatment of the spring takes place at a
temperature above the operating thermal level of the
seal, thus ensuring that the spring force will be stabilized.
This procedure eliminates the risk that the original spring
force will change during service.
Figure 9
“Wear”
Spring force versus elongation
Elongation
Figures 8 and 9 show the change in the initial tension in
stabilized and not-stabilised garter springs.
Material
Spring steel SAE 1074, DIN 17223 is normally employed. If
resistance to corrosion is required stainless steel AISI 304,
DIN 1.4301 is used. Garter springs of bronze or similar
materials are not recommended, since they tend to fatigue
after a long service life or as a result of exposure to high
temperatures. In special cases, the garter spring can be
protected against fouling by means of a thin rubber hose.
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Edition April 2006
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