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material flexure at this point.  Breathing will usually be less in small
diameter low-pressure cylinders owing to manufacturing considerations. In
large diameter cylinders, or units which have large values of diametrical
breathing, tests will have to be made to the satisfaction of the procuring
activity to ensure adequate life of the seal and gland.
3.3 Nonstandard hydraulic Packings.
3.3.1 Use of nonstandard hydraulic packings. Type I and Type II systems.  When a satisfactory installation using
standard packings is not possible owing to performance requirements,
nonstandard packings such as TFE cap rings and non-circular cross section
elastomeric shapes may be used, subject to approval by the procuring activity,
and provided the unit satisfactorily completes qualification tests.
3.3.2 Design considerations.
On nonstandard packing and gland
designs, consideration shall be given to the design application and the
following points shall be noted:  Surface finish, extreme temperature,
sealing, low-pressure and high-pressure leakage, air inclusion as a result of
servo operation without fluid pressure, etc.
3.4 Nonextrusion devices.
Application of nonextrusion devices (backup rings).  Type I and Type II systems.  Where required, to permit a component to
conform to performance or qualification tests, nonextrusion devices within the
packing gland may be used. At lower pressures, nonextrusion
devices will prolong the normal wearing life of the O-ring and, at higher
pressures, non-extrusion devices permit greater diametrical clearances between
mating parts.  Unless otherwise approved by the procurihg activity, the
applicable standard packings and backup devices shall be used.  Type III systems.  Packings and backup rins for use in Type III
system shall be designed in such manner as to fulfill the performance and
qualification test requirements of the individual hydraulic component.
purchase order, the contractor may use his own or any other facilities
3.4.2  Glands for packings and gaskets with nonextrusion devices.  Glands for
combination of packings with backup rings and for gaskets with backup rings
shall be designed to the same considerationsas for nominal rectangular
grooves, great importance, since a groove which is too wide may permit the
backup rings to roll and thus become ineffective as a nonextrusion device.
Angles in excess of 5 degrees on the side of the groove may produce the same
result.  A groove which is too narrow may cause high friction and extreme
difficulty on installation.  The data shown in Table I have been found to be
acceptable and desirable.  Glands for TFE nonextrusion devices must have
corner radii consistent with the performance and cross-sectional requirements
of the TFE backup ring; otherwise difficult installation and operation
problems may result.
3.4.3 Installation of nonextrusion rims.  The use of two backup rings in
each gland, one on either side of the O-ring seal, even though the pressure

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