Sealing Theory - (Home)
Static vs. Dynamic Sealing
Leakage Control
Lip vs. Squeeze Seals
Effects of Lip Geometries
Friction
Pressure Effects and Extrusion
Seal Wear
Seal Stability
Surface Speed
Compression Set
Influence of Temperature
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| O-Rings,
seals and bearings for all applications |
Compression Set
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| Compression set is the inability of a
seal to return to its original shape after being compressed.
As defined by ASTM, it is the percent of deflection by
which the seal fails to recover after a specific deflection,
time and temperature. Compression set is calculated using
the following equation: |
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Compression set reduces sealing forces,
resulting in poor low pressure sealability. It takes
place primarily because of excessive exposure to a high
temperature. A material’s upper end temperature
limit may give an indication of its compression set
resistance. Although compression set always reduces
the seal’s dimensions, chemical swell or shrinkage
can either positively or negatively impact the final
geometry of the seal. If material shrinkage occurs due
to the system fluid, the deflection of the seal will
decrease, accelerating leakage. If chemical swell is
present, it can negate or offset the negative effects
of compression set. While it is true that swelling can
offset compression set, extreme fluid incompatibility
can break down the polymer’s chemical structure
and cause the material to be reformed in its compressed
state.
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Lip wear is also a dimensional
loss, but is not related to compression set. Dimensional
loss due to lip wear will increase the final compression
set value.
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The seal shown in Figure 2-9 exhibits
nearly 100% compression set with minimal wear. Note
how the lips flare out very little.
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