8.
9.
10.
11.
Low distortion of structure
Small space requirement
Reliability
Effectiveness over wide pressure and
temperature ranges
As stated previously, O-rings are used in both
static (as gaskets) and dynamic (as packing)
applications. An O-ring will almost always be the
most satisfactory choice of seals in static
applications if the fluids, temperatures, pressure,
and geometry permit.
Standard O-ring packings are not specifically
designed to be used as rotary seals. When
infrequent rotary motion or low peripheral
velocity is involved standard O-ring packings may
be used, provided consistent surface finishes over
the entire gland are used and eccentricities are
accurately controlled. O-rings cannot compensate
for out-of-round or eccentrically rotating shafts.
As rotary seals, O-rings perform satisfactorily
in two application areas:
1. In low-speed applications where the surface
speed of the shaft does not exceed 200 ft/min
2. In high-speed moderate-pressure appli-
cations, between 50 and 800 psi
The use of low-friction extrusion-resistant
devices is helpful in prolonging the life and
improving the performance of O-rings used as
rotary seals.
O-rings are often used as reciprocating seals
in hydraulic and pneumatic systems. While best
suited for short-stroke, relatively small diameter
applications, O-rings have been used successfully
in long-stroke, large diameter applications.
Glands for O-rings used as reciprocating seals are
usually designed according to MIL-G-5514 to
provide a squeeze that varies from 8 to 10 percent
minimum and 13.5 to 16 percent maximum. A
squeeze of 20 percent is allowed on O-rings with
a cross section of 0.070-inch or less. In some
reciprocating pneumatic applications, a floating
O-ring design may simultaneously reduce friction
and wear by maintaining no squeeze by the gland
on the O-ring. When air pressure enters the
cylinder, the air pressure flattens the O-ring,
causing sufficient squeeze to seal during the
stroke. If the return stroke does not use pneumatic
power, the O-ring returns to its round cross
section, minimizing drag and wear on the return
stroke.
Identification
As a maintenance person or supervisor
working with fluid power systems, you must be
able to positively identify, inspect, and install the
correct size and type of O-ring to ensure the best
possible service. These tasks can be difficult since
part numbers cannot be put directly on the seals
and because of the continual introduction of new
types of seals and obsolescence of others. (Naval
Ships’ Technical Manual, chapter 078, contains
a table that cross-references obsolete and current
O-ring specifications for ship applications.)
O-rings are packaged in individually sealed
envelopes. O-ring seals manufactured to govern-
ment specifications are marked according to the
requirements of the specific military specification
and standard. The required marking for each
package is as follows:
1.
2.
3.
4.
5.
6.
7.
8.
9.
National stock number (NSN)
Nomenclature
Military part number
Material specification
Manufacturer’s
Manufacturer’s
Manufacturer’s
name
compound number
batch number
Contract number
Cure date
NOTE: Keep preformed packings in their
original envelopes, which provide preservation,
protection, identification, and cure date.
When you select an O-ring for installation,
carefully observe the information on the package.
If you cannot positively identify an O-ring,
discard it. The part number on the sealed
package provides the most reliable and complete
identification.
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