installations where moving parts do not come in
contact with the seal. Normally, the type of gasket
used is an O-ring. In some cases it might be the same
seal that is used as a packing in other installations, or
it may be one that is manufactured only for use as a
gasket.
In hydraulic systems where the operating
temperature ranges from -65°F to +160°F, the
AN6290, MS28778, AN6230B-1 through -25,
MS28775-013 through -028, -117 through -149, and
-223 through -247 O-rings are intended for use as
gaskets. In systems where temperature limits range
from -65°F to +275°F, MS28778 and designated sizes
of MS28775 O-rings are used as gaskets. Normally,
O-rings designated as MS28778 should be used only
in connections with straight thread tube fittings, such
as boss fittings and end caps of check valves, etc.
Identification
Figure 2-44.—O-ring package identification.
O-rings are manufactured according to military
specifications and are identified from the technical
information printed on the O-ring package. See
figure 2-44. The size of O-rings cannot be positively
identified by visual examination without the use of
special equipment. For this reason, O-rings are made
available in individual, hermetically sealed envelopes
labeled with all the necessary pertinent data.
NOTE: Colored dots, dashes, and stripes or
combinations of dots and dashes on the
surface of the O-ring are no longer used for
identification of O-rings. O-rings still found
with these color identification markings are
NOT to be used in naval aircraft hydraulic
systems or components and should be
depleted from stock.
Figure 2-44 shows the information printed on
O-ring packages that is essential to determine the
intended use, qualifications, and age limitations. The
manufacturer’s cure date is one of the more important
printed items listed on the package. This cure date is
denoted in quarters. For example, the cure date 2Q82
indicates that the O-ring was manufactured during the
second quarter of 1982. Synthetic rubber parts
manufactured during any given quarter are not
considered one quarter old until the end of the
succeeding quarter.
Most O-ring age limitation is
determined by this cure date, anticipated service life,
and replacement schedule.
Age limitation of synthetic rubber O-rings is
based on the fact that the material deteriorates with
age. O-ring age is computed from the cure date. The
term cure date is used in conjunction with
replacement kits, which contain O-rings, parts, and
hardware for shop repair of various components.
O-ring cure dates also provide bases for O-ring
replacement schedules, which are determined by
O-ring service life. The service life (estimated time
of trouble-free service) of O-rings also depends upon
such conditions as use, exposure to certain elements,
both natural and imposed, and subjection to physical
stress. Operational conditions imposed on O-rings in
one component may necessitate O-ring replacement
more frequently than replacement of identical O-rings
in other components. It is necessary to adhere to the
recommended replacement schedule for each
individual component. The age of O-rings in a spare
part is determined from the assembly date recorded on
the service or identification plate and/or on the
exterior of the container. All O-rings over 24 months
old should be replaced or, if nearing their age limit (24
months), should not be used for replacement.
Storage
Proper storage practices must be observed to
prevent deformation and deterioration of rubber
O-rings. Most synthetic rubbers are not damaged by
several years of storage under ideal conditions.
However, most synthetic rubbers deteriorate when
exposed to heat, light, oil, grease, fuels, solvents,
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