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The groove must always be made sufficiently long to
allow for the maximum expansion of the rubber, otherwise
very high stresses will be set up on the installation.
Normally the groove length will permit the O-ring to
slide or to roll a small amount within the groove.
When break-out force is applied, the O-Ring will slide
or roll in the direction of force applied until it
contacts the end of the groove. From this point on,
further pressure or force can only result in deformation
of the O-Ring into tighter contact with the inner wall,
the groove end and the outer wall.
The O-Ring will initially deform into a "D" shape. This
normal deformation will increase the surface contact
area to 70 to 80 percent of the initial cross-section.
Thus, the contact area of sealing under pressure is
roughly twice the area of contact of the original
zero-pressure seal resulting from diametral squeeze. It
will be apparent from the foregoing that the O-Ring will
seal in either pressure direction.
Figure 5 shows the extreme case of deformation
just before failure.
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Note that a small portion of the rubber material has
been forced into the small clearance beyond the groove.
Assuming the rubber has reached its
limit of flow under pressure, further increase of force
will result in failure by shear or extrusion as shown in
Figure 6. The clearance allowed will bear a
direct relation to the force causing failure.
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