how an o-ring functions
\ physical
properties of o-ring compounds \ types of seals \ general design and gland
dimensions \ standard
o-ring sizes and dimensions \ compound
information
Types
of Seals
STATIC
SEAL
A
static seal is one in which the metal parts do not move with relation to each
other. This is a gasket-type seal, further divided as follows:
TYPES:
AXIAL
SQUEEZE: The squeeze is on the top
and bottom of the ring like a flat gasket. Bolting the flanges together makes
the seal.
RADIAL
SQUEEZE: The squeeze is between the
ID and OD of the ring like that of a piston or piston rods. The radial
clearance gap influences the pressure that can be sealed.
DYNAMIC
SEAL
A
seal in which one or more parts moves in relation to the O-Ring is a dynamic
seal, further divided as follows
TYPES:
RECIPROCATING
SEAL: Where the rod or piston moves
back and forth through or with the O-Ring.
ROTARY
SEAL: Where a shaft rotates with
relation to the O-Ring.
OSCILLATING
SEAL: Where a shaft turns and
returns with relation to the O-Ring (one that combines oscillating and
reciprocation motion is classed as an oscillating-reciprocating seal). Sealing
a rising valve spindle is an example.
STATIC SEALS
Seals that see no relative motion in operation
include plug and tube seals, flange seals, and boss type fittings.
RADICALLY SQUEEZED STATIC SEAL:
Figure 7 illustrates radial static seals. One is a radically
squeezed O-Ring in a screwed-on cylinder end cap. It needs low radial clearance
gap or "back-up" rings for high pressures. The groove dimensions and
the radial clearance gap should be the same as for reciprocating seals. Chamfer
the cylinder barrel and use a heavy cup grease to hold the ring in place during
assembly.
Another is a radial seal in the end cap of a
power cylinder. The O-Ring snaps over the male member to save cost and space, a
metal snap ring holds the assembly in place. The squeeze at low pressure and
the deformation of the cross-section under pressure makes the seal tighter and
tighter.
The third shows an O-Ring sealed tie-rod
held cylinder end cap. The tie-rods and bolts are not stressed as with flat
gaskets. They hold only the force exerted by the fluid pressure against the
caps. Combine this with O-Ring piston and piston rod seals to make an
inexpensive, easily assembled unit.
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AXIAL OR FACE
STATIC SEALS Figure 8 shows an O-Ring flange gasket. It seals tight to over 100,000 psi. There is no bolt strain as in compression gaskets. Nuts are tightened only enough to makes flanges stay in contact. The pressure does the rest. There are many varieties of this kind of seal using full or 1/2 counter-bored grooves. Lower tensile bolting is used. No special care is required...no pipe dope need be used...there’s nothing to shred off...there is no line construction, no maintenance, and no problem of human error.
Figure 9 shows an end cap application. It is simple, economical, easy to install, saves space and weight.
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DYNAMIC SEALS All seals in which one or more parts moves in relation to the other parts are classed as dynamic seals, further divided as follows: RECIPROCATING
SEALS: A reciprocating seal is one in which a member moves axially in relation to a fixed member and returns. This means that one member will move in relation to the O-Ring. Unless very exaggerated, any oscillating motion involved in a reciprocating seal is not important. Refer to the example in Figure 10.
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Liquid Flange Seal Groove Dimensions |
|
O-Ring Series |
O-Ring Width |
Gland Depth |
Gland Width |
Minimum Squeeze |
|
|
W |
F |
G |
|
|
004 thru 050 |
.070 ± .003 |
.047 ± .003 |
.095 ± .003 |
.017 |
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ROTARY SEALS: A rotary seal is one in which one member rotates
continuously about its axis in relation to a fixed member. As in preceding
cases, one member will move in relation to the O-Ring
Unlike reciprocating seals, rotary seals are in contact with the same rubbing surface. Lubrication and heat build-up become a problem., O-Rings can successfully be used in rotary applications up to about 600 fpm contact speed when properly designed. |
OSCILLATING SEALS: An oscillating seal is one
in which a member turns about its axis in relation to a fixed member and
returns. One member will turn in relation to the O-Ring. Any axial motion,
such as might be caused by a spiral thread, does not change the
classification of this seal, if minor. (A seal that combines extensive
oscillating and reciprocating motion would be classed as an
oscillating-reciprocating seal). An oscillating seal is not necessarily
limited to 360 degrees or less - a seal involving several turns of a valve
spindle is still classified as an oscillating seal. Refer to the example in Figure
12.
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