Parker O-Ring Handbook.pdf

Recommended dimensions for fl oating pneumatic piston seal
glands are tabulated in Design Chart 5-3 and Design Table 5-3
at the end of this section. The “fl oating” feature of this design
is the virtual lack of squeeze on the O-ring cross-section.
Sealing is accomplished by the peripheral squeeze applied
to the outside diameter of the O-ring as it is assembled into
the bore, and air pressure moving the ring into facial contact
with the wall of the groove.
When this principle is understood, it will be seen that when the
direction of pressurized air is reversed, a puff of air escapes
between the inside diameter of the O-ring and the bottom
of the groove during the small fraction of a second it takes
the O-ring to move to the other side of the gland. This is the
primary reason for the slight increase in leakage mentioned
for this design.
The fl oating seal will not trap pressure between two
O-rings in separate grooves unless considerable rubber swell
is encountered.
Five or six O-rings are used in adjoining fl oating seal glands.
This design has been used for some hot water and steam
applications as a method of increasing O-ring life. The full
effect of the hot steam is brought to bear on the inner rings
and a lesser amount on the outer rings. Consequently, the seal
is effective long after a single O-ring would have failed.
For the design of pneumatic reciprocating rod seals, use
Design Chart 5-2 and Design Table 5-2. This is the cross section
squeeze design used for hydraulic piston and rod seals.
Floating seals are not recommended for pneumatic rods, as
they would require stretching the O-ring, causing early aging.
Furthermore, since pneumatically actuated shafts often
move rapidly, a stretched O-ring in this situation would be
subject to the Gow-Joule effect described in the rotary seal
discussion. For static pneumatic seal designs, use Design
Chart 4-1 and Design Table 4-1.
5.26 Uni-Directional Gland
This design modifi cation utilizes a uni-directional fl oating
seal groove and more than one O-ring (see Figure 5-20). The
addition of drilled holes in the grooves causes each O-ring
to seal in one direction only, preventing a pressure trap of
non-compressible liquid between the O-rings. When using
this design, the gland dimensions given in Design Table 5-3
are suggested and the holes should be drilled into the pressure
side of the outside grooves on the piston and the inside
Figure 5-20: Use of Multiple O-Rings in a Floating O-Ring
Design
Parker O-Ring Handbook
of the rod glands. As many individual seals as required may
be used as long as each groove is vented. The O-rings on
either end, alternately seal and release trapped pressure.
The three piston O-rings and the two rod O-rings are never
all sealing at the same time. The vents are not necessary in
pneumatic designs.
5.27 Rotary Seal
An O-ring has proved to be a practical rotary shaft seal in
many applications. With the correct design, Parker O-Ring
rotary seal compound N1090-85, will provide satisfactory
service at surface speeds up to 1500 feet per minute.
The design conditions are most critical for rotary seals, as
would be expected. Relatively high durometer compounds,
close control of tolerances, and minimum cross section
are required.
Rotary seals usually should not be used at temperatures
below -40°C (-40°F) even though fl exibility to -54°C
(-65°F) is claimed, since thermal shrinkage and loss of resilience
tend to cause loss of contact with the shaft. In some
cases, initial leakage of frozen seals may be tolerable until
heat build-up occurs in higher speed shafts. Spring loading
may be helpful in some situations.
High-speed shafts of soft metal should be avoided since they
will normally wear more rapidly than the rubber, opening
the clearance and allowing leakage. Hardened steel shafts in
the range of 55 Rockwell are desirable, but not mandatory.
Attention to clearances, side thrust, and end-play are critical
in designing effective rotary O-ring seals.
Whenever it can be avoided, an O-ring should not be installed
in a gland that holds it in more than a minimum of tensional
stress. This principle is especially important to consider when
designing for an O-ring rotary shaft seal. Most elastomers
when heated in the stressed, or stretched condition will contract.
This is of practical importance in a rotary seal because
it results in a tendency for the O-ring to seize the high-speed
rotating shaft. This phenomenon, known as the Gow-Joule
effect, occurs only if the rubber is under tensile stress.
The friction between the O-ring and the rotating shaft creates
heat. When it is installed in more than a minimum of tensional
stress, the O-ring tends to contract when heated and seize the
high speed rotating shaft. This contraction causes more friction
which in turn causes more heat and the process becomes
self-perpetuating, until the O-ring is destroyed.
Even at low surface speeds, where heating is not a problem,
a stretched O-ring tends to rotate with the shaft and leak. For
speeds below 200 feet per minute, the squeeze recommended
in Design Chart 5-2 may be used. However, the shaft diameter
should be no larger than the free state ID of the O-ring.
Shaft seal applications where the O-ring is installed in a
groove in the shaft are not recommended if the shaft rotates.
This is due to the centrifugal action which causes the O-ring
to rotate and rub on all surfaces which generally causes early
seal leakage or failure.
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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