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
General
Design and Gland Dimensions
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Static Seal Design STATIC SEALS are divided into two primary categories: Radial
Static type and Face or Flange type. RADIAL SEALS require the presence of a diametral clearance gap for installation. FACE OR FLANGE type seals have no clearance gap, but consist of a groove cut into one flange with a flat mating flange bolted together to give a surface-to-surface contact. |
There are two types of static radial
designs:
MALE
or PLUG SEAL design is based on
the following factors (refer to Fig. 20). Bore Diameter (a) |
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Mechanical SQUEEZE for the gland is determined by the bore diameter
and the groove diameter in a plug or male type seal (Figure 20). The
formula for determining the groove diameter when the bore and gland depth are
know is: B min. = A max. minus 2 F
max. SQUEEZE is measured from the bottom of the groove to the mating surface and in the case of radial seals includes the clearance gap. The following formula is used to determine the actual gland depth with tolerances: |
Max. Gland Depth = Max.
groove diameter minus TOTAL DIAMETRAL CLEARANCE is the difference between the bore diameter and the plug diameter dimensions (Fig. 21). Tolerances of the bore and plug diameters determine the maximum and minimum diametral clearance gap. These values divided by two will give the radial maximum and minimum clearance gaps. (Fig. 23). |
The PLUG DIAMETER (H) must be figured
using the minimum clearance gap as given in Figure 20 and the maximum
clearance gap as shown in Figure 22. The plug diameter can also be
calculated by subtracting the minimum gap from the minimum bore giving the
maximum plug diameter. The minimum plug diameter can be calculated by
subtracting the maximum gap (see Figure 22) from the maximum bore
dimension. By using this method, it allows the use of softer compounds without
the possibilities of encountering extrusions.
STATIC FEMALE OR TUBE seals (Figure 24) are based upon the
following:
Throat (Bore) Dimensions
(E)
Tube Dimensions (c)
Groove Dimension (D)
Groove Width (G)
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Maximum Radial Clearance Gap to
Prevent Extrusion |
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Maximum Pressure PSI |
"O" Ring Hardness (Shore
A) |
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50 |
60 |
70 |
80 |
90 |
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100 1,000 |
.008" .005" .001" |
.009" .003" |
.010" .005" |
.013" .008" |
.016" .010" |
Figure 22
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Maximum Radial Clearance Gap to Prevent
Extrusion Clearances apply to dynamic O-Rings with
no backup washers for .139 inch (3.53mm) O-Ring cross-sections and up.
Smaller cross-sections do not give effective moving seal life, are less
resistant to extrusion, and may be critical in sealing high frequency dynamic
motion. Mechanical SQUEEZE for this type of seal is determined by the groove
diameter dimension (D) and the tube diameter dimension (C). When these tube
dimensions and the gland depth dimensions are known, then the groove diameter
can be determined by the following formula: D max. = C min. plus 2F
max. |
SQUEEZE is measured from the bottom of the groove to the
mating surface and includes the clearance gap. Use the following formula for
determining the actual gland depth with tolerance. Max. Gland Depth = Max. groove diameter
minus min. tube diameter divided by 2 Min. Gland Depth = Min. groove diameter minus max. tube diameter divided by 2 |
TOTAL DIAMETRAL CLEARANCE
The difference between the throat diameter and the
tube diameter.. Tolerances of the throat diameter and the tube diameter
determine the maximum and minimum total diametral clearance gap. These values
divided by 2 will give the radial maximum and minimum gaps.
The THROAT (BORE) DIAMETER must be
calculated using the minimum clearance gap given in Figure 21 and the
maximum clearance gap as shown in Figure 23.
The throat (bore) diameter can be calculated
by adding the minimum clearance gap (Fig. 20) to the maximum tube (rod)
dimension. This will give the minimum throat dimension. The maximum throat
dimension can be obtained by adding the maximum gap (Fig. 22) to the
minimum type (rod) dimension.
By using this method, it allows the use of softer compounds without the possibilities of extrusion.
STATIC FACE or FLANGE seals are much easier to design than
static radial seals since in this type of design there is no extrusion gap.
There are fewer design steps and tolerances are easier to control. There are
two types of face seals: those designed for internal pressure and those
designed for external pressure.
INTERNAL PRESSURE design is based upon the groove OD to limit the side
movement in the groove due to pressure direction. Figure 25 shows a
diagram for a face seal designed for internal pressure.
Grooved OD (outside diameter Y)
Groove Depth
Groove Width
This type of design is based upon groove OD
to limit the side movement of the O-Ring in the groove due to pressure
direction. Since the groove width is larger than the cross section of the
O-Ring, then the design should be made for the O-Ring to have a snug fit on the
grooved OD in order to limit O-Ring movement.
EXTERNAL PRESSURE or VACUUM design parameters are based upon
the following:
Groove I.D. (inside diameter) (x)
Groove Depth
Groove Width
This design is based upon the groove I.D. to
limit side movement in the groove due to pressure direction (Fig. 26)
Due to the absence of any extrusion gap,
these seal designs are capable of withstanding very high pressures without
encountering extrusion problems.
Bolt loading or clamping force should be
sufficient to prevent any gap between mating flanges and to ensure proper
squeeze. Flanges, especially in large application, should be sufficiently flat
to prevent any extrusion gap and ensure proper squeeze.
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Dynamic seals require additional design factors including
lighter squeeze to help eliminate friction, wear, and heat buildup. FRICTION - the higher the durometer hardness (above 70
Shore A) the greater the friction. This is due to the fact that the
compressive force, at the same squeeze, is greater than with softer
compounds. Compound-swell decreases with hardness and
may increase the friction. The lower the operating temperature, the harder
the seal becomes which can also increase friction. However, thermal
contraction of the seal material, which reduces effective squeeze, may offset
any increased friction caused by an increase in hardness. Breakout friction is the force necessary to start the unit moving. It is dependent upon the length of time between cycles. It also depends on the surface finish of the metal, the rubber hardness, squeeze, and other friction-affecting factors. Utilizing softer O-Ring compounds can reduce breakout friction. |
Running Friction depends on two factors: (1) The force exerted on
the ring's rubbing surface by the compression force of the squeeze. This depends
on the hardness of the ring, the percent of the squeeze, and the length of
the rubbing surface; and (2) the force of the system's pressure against and
tending to distort the ring into a "D" shape. This depends on
friction per square inch of groove area times the area of each particular
size ring. There are many variables that make it
difficult to accurately predict the total friction for a particular case. The
best way to be sure that dynamic O-Rings do not have excessive friction is by
experimentation. Decreasing squeeze, surface finishes, cross-section, and
using external lubrication will help. GROOVE FINISH - Straight-sided grooves are best to prevent
extrusion or nibbling, but 5 degree sloping sides are easier to machine and
are suitable for pressures up to 1500 psi. The rubbing surfaces should be 8 to 16 RMS without longitudinal or circumferential scratches. The best surfaces are honed, burnished, or hard chrome-plate. Finishes below 5 RMS wipe too clean for good moving seal life. |
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OSCILLATING SEALS - The inner or outer member of the assembly moves
in an arc relative to the other, rotating one of the members in relation to
the O-Ring. Sealing can be complicated by the fact that the O-Ring can snake
or bunch in the gland. The relative motion of the moving part to the fixed
part tries to carry its O-Ring contact with it, distorting the rubber
circumferentially. Many oscillating seals utilize standard reciprocating
design parameters or slight modifications to reduce friction. ROTARY SEALS - Rotary seals are where an inner or outer member
of the sealing assembly revolves around the shaft axis in only one direction.
The direction may be reversed. Should there be multiple brief arcs of motion,
the designer should refer to parameters for oscillating seals.
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RECIPROCATING SEALS - These seals are designed much like static radial
seals except for dynamic applications the squeeze is less likely to reduce
friction and heat buildup. There are two types of reciprocating seals - male
or piston gland design (Fig. 27) and female or rod gland design (Fig.
28). MALE or PISTON RECIPROCATING SEALS - The design parameters for this type of seal are
as follows: Bore dimension (A) FEMALE or ROD RECIPROCATING SEALS - The design parameters include the following: Bore dimension (e)
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Anti-Extrusion Devices Billions of O-Rings are used as basic
seals for pressures below 1500 psi. Where large clearance gaps or higher
pressures are necessary, O-Rings are assisted by leather, Teflon*, hard
rubber, backup or other close clearance anti-extrusion devices to prevent
extrusion and nibbling. Teflon* Back-up Washers Teflon* back-up washers have been used
since 1952 to fill a need for better chemical and heat resistance. They
reduce friction because small particles of anti-frictional material coat the
rubbing surface. The spiral cut, which was the earliest,
costs less and is the most popular. It is Government approved for -65° to
225°F for 3000 psi. The single turn scarf-cut is approved at -65°F to 275°F
for 3000 psi and offers somewhat better extrusion protection. Both should be
installed without overlays at the gap. They may be clipped in the field to
fit non-standard sizes. They must be installed carefully to prevent shearing
during assembly. The solid no-cut type generally requires two-piece glands although, in larger sizes, they may be fold-fitted with care into a rod groove and then sized to correct for distortion. This type may be used to 400°F and 5000 psi. |
Hard Rubber Back-up Washers Hard rubber back-ups, having a concave
side, are available in many sizes corresponding to standard AS568 O-Ring
sizes.
The curvature of the backup helps maintain
the O-Ring shape under the pressure. Rubber backups are easy to install since
they can be stretched into a groove. O-Rings cannot be damaged by a split or
sharp surface since these back-ups are of a continuous molded surface. Back-ups are available in standard nitrile, ethylene propylene, and fluorocarbon. |
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GAP CHART Back-up Rings are installed on the
downstream side of an O-Ring to help prevent premature failure and leakage.
If the sealing ring cannot resist increasingly higher pressure, part of the
seal will be forced into the diametrical gap. Back-up rings are used to block
the clearance gap and provide support for the seal. Applications that fall on the right side of the curve require use of back-up rings. |
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