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Figure 4 now shows the action of
this properly installed O-Ring when pressure is applied.
Since both the inner and outer walls are in firm contact
with the O-Ring material, the pressure tends to force it
along its groove. Engineered to deform, the rubber
compound flows up to the passage, completely sealing it
against leakage. The higher the pressure trying to leak
past, the tighter the seal that is thus formed. Upon
release of the pressure, the resiliency of the rubber
compound results in the O-Ring returning to its natural
round form, undamaged and ready for similar cycles.
By this fundamental explanation, the criteria of design
are clearly visible. The initial "diametral squeeze" is
vitally important. An initial "diametral squeeze" of 10%
results in a flat sealing surface of about 40 to 45% of
the initial cross-section area of the O-Ring AT ZERO
PRESSURE.
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Thus, at zero or very low pressures, the natural
resiliency of the rubber compound provides the seal. It
follows that very low pressure sealing may be improved
by increased "diametral squeeze" (but note that such
increased squeeze may adversely affect dynamic sealing
at higher pressures).
The "diametral squeeze" induces a frictional force
between the O-Ring and the walls of the sealed passage
that tend to hold the O-Ring in "neutral" position.
Until the forces applied are sufficient to either
overcome the frictional force or deform the rubber
compound, the O-Ring will retain its initially deformed
shape and will seal purely by diametral pressure.
The diametral squeeze applied to the
constant volume of the O-Ring material will produce an
increase in length of rubber across the groove.
Expansion or swell of the rubber compound in the fluid
or from heat will further increase this length of
squeezed rubber. |
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