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application is from one side only, is desired in all cases to facilitate
standardization of groove dimensions and service procedures.  Where it is
self-evident, however, that pressure can be applied from one direction only.
and space limitations to provide for two rings create a hardship, a single
backup ring may be used; this ring to be placed on the side of the O-ring away
from the pressure.  When the pressure differential across a packing is
unidirectional, only one backup ring need be installed.  This backup ring
shall be on the low-pressure side.  The groove width dimensions shown in Table
I for one backup ring may be used, as applicable. Continuous turn TFE backup rings.  A continuous TFE ring may be used
for new designs.  Installation of the small sizes of this ring into the gland
will probably require use of split or multiple piece adapters. Continuous turn
backup rings shall be in accordance with the uncut sizes of MS2828774.  For Air
Force use, continuous turn backup rings shall be in accordance with MS27595,
or the uncut sizes of MS28774. Spiral TFE backup rings.  Great care must be exercised in the
installation of spiral TFE backup rings.  If the groove radius are large for
structural reasons, the spirals may be sheared upon installation. Therefore,
it is important that when installing spiral TFE backup rings that the mating
part be rotated in the proper direction, so that the spiral will tend to wrap
itself deeper into the groove and will not be sheared off when the parts are
assembled.  Spiral TFE backup rings shall be in accordance with MS28782 and
MS28783. Sinqle-turn TFE backup rings.  This ring may be used for new designs.
It is the simplest to install and does not require any special installation
procedures.  Single turn backup rings shall be in accordance with MS28774 and
3.5 Static seals.
3.5.1 Type I and Type II systems.  All nonmoving packings (static seals) used
for sealing of fluid pressures shall be standard-approved static seals. These
static seals shall not be compressed into threads or against other irregular
or rough surfaces which would cut or otherwise damage them. Recommenended static
seal glands are shown in Table I.  The groove fillet radius and the fatigue
requirements should receive very careful consideration.
3.5.2  Type III systems.  Static seals and backup rings for use in type III
systems shall be designed in such manner as to fulfill the performance and
qualification test requirements of the individual hydraulic component.
3.5.3 O-ring static seals.  O-ring packing conforming to the sizes shown in
MS28775, MIL-P-83461/1, and MIL-P-87175/l for -013 through -028, -117 through
-149, and -223 through - 247 are intended for use as static seals.  All of the
detail requirements specified for O-ring packing will apply to the use of
o-ring static seals, except as noted in the following paragraphs.
3.5.4 Pressure limitations and use of O-ring static seals.

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