Parker O-Ring Handbook.pdf

for this service. See Section II, Basic O-Ring Elastomers, for
more information on rotary seal compounds.
Figure 5-21 shows two methods of “spring loading” the
hard rotary seal. Either of these should only be used when
absolutely necessary to obtain the desired seal.
See Table 5-6 for a rotary seal design example.
5.28 Oscillating Seal
In this guide, two types of oscillating seals are considered:
1. Faucet or valve stems are excellent examples of assemblies
that can be simplifi ed by the use of an O-ring seal. Compression
type or multiple-lip packing can be eliminated, reducing
space requirements and eliminating the need for adjusting or
take-up devices. For applications of this type, if the speed is
under 200 feet per minute, use Design Table 5-2 for selecting
O-ring sizes and gland dimensions.
2. Constantly oscillating shafts, such as those used on timing and
metering devices, can be sealed satisfactorily with O-rings. If
the motion is continuous for long periods of time, use Design
Table 5-4 for O-ring sizes and gland dimensions.
5.29 Seat Seals
A properly designed check or poppet type valve, with an
O-ring on the seat, will give an exceptionally long, non-leaking
service. Many designers and engineers make the costly
mistake of trying to use a conventional groove (square or
rectangular) design to hold the O-ring.
With this type of groove, “blow-out’’ will normally occur
when the valve is unseated.
“Blow-out” is a type of seal failure caused by the action of
the pressure in the system on the side of the O-ring, forcing it
out of the groove into some other part of the valve or system.
“Blow-out” usually occurs at differential pressures above 5.5
Bar (80 psi). The exact pressure will depend on the gas or
fl uid, valve design and the physical properties of the O-ring
when a non-retaining or conventional type groove is used.
It should be kept in mind that blow-out is similar to extrusion,
but that it occurs at considerably lower pressures.
Figure 5-22 shows an O-ring on the seat of a check valve in a
conventional groove. The seal is satisfactory as long as the valve is
not opened at or near the pressure necessary to cause blow-out.
O-Ring
Pressure
Figure 5-22: Valve Seat Seal, Standard Groove
Parker O-Ring Handbook
Figure 5-23 illustrates a valve opening above “blow-out”
pressure. As the valve opens, the space between the two faces
becomes increasingly larger. The pressure opening the valve
is also acting on the O-ring, causing it to continue to seal the
opening until it is stretched completely out of the groove and
is blown out or forced into another part of the system.
Gases such as air, LPG, CO2, etc. enter or permeate the
O-ring. With release of pressure, the gas inside the O-ring
can cause the seal to “balloon” or swell momentarily. (The
amount depends on the pressure.) The ballooning effect that
can occur at very low pressure usually pops the O-ring out of
the groove the same as blow-out. “Ballooning” and “blow-out”
often combine to cause valve seal failure. Another term often
used to describe this phenomenon is “explosive decompression.”
O-ring blow-out may be prevented by using a groove
design which encloses more than 180° of the O-ring cross
section or by venting the groove. Typical methods used are
shown in Figure 5-24. If a rectangular groove must be used,
alter the dimensions as follows:
Groove depth — 0.015 to 0.025 less than
O-ring cross section diameter.
Groove width — 1.00 to 1.10 times the
O-ring cross section diameter.
Groove side angle — 0°, if possible.
O-Ring
Pressure
Figure 5-23: O-Ring Blow-Out, Standard Groove
O-Ring
Figure 5-24: Groove Designs to Prevent Blow-Out
O-Ring
Pressure
Parker Hannifi n Corporation • O-Ring Division
2360 Palumbo Drive, Lexington, KY 40509
Phone: (859) 269-2351 Fax: (859) 335-5128
www.parkerorings.com
Dynamic O-Ring Sealing
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