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MIL-G-5514G Use of D-rinq packings.  D-ring packings are intended for use as rod
seals only in landing gear shock struts.  The D-ring packing is designed
primarily for use in lieu of the U-ring packings in landing gear installations
where spiral failure of the O-ring packing is a problem. It may be used
without backup or with one or two backup rings, depending on the pressure,
deflection, and other requirements of the specific utilization. For the
diametrical dimensions of the installation of D-rings, the dimensions
established in Table I for O-rings are recommended.  For groove width
(dimensions "G"), the dimensions of Table II are recommended. The MS28772
D-rings in their full range of sizes are equivalent in ID and OD dimensions to
the MS28775 o-rings in sizes -335 through -460.
3.2.5 surface finishes of glands.  The following surface finishes shall be
used in units containing O-ring packings, unless performance or qualification
tests indicate that other surface finishes are satisfactory These finishes
are indicated as surface roughness as defined in ANSI/ASME B46.1-85.
3.2.6 o-ring groove shape.  Rectangular type groove shapes, following the
general design criteria of Table I, are preferred.  The grooves may have up to
5 degrees slope on the sides to facilitate machining; the radius in the bottom
corners of the groove must be a compromise between strength requirements, type
of nonextrusion device, and adequate groove volume. When TFE rings are used,
the radius must be a minimum consistent with installation and performance
requirements.  The width of the groove shall be consistent with the
performance requirements of the gland and the type of nonextrusion device
used.  Consideration s should be given to cross-sectional squeeze and volumetric
swell of the O-ring owing to oil immersion and temperature.  The recommended
groove width dimensions are shown tin Table I.  Narrower grooves make it more
difficult to remove the O-ring for inspection and replacement and increase the
possibility of nicking and scratching the edge of the groove during removal of
the O-ring. The outer corner of the groove must be smooth with the corner
broken slightly to prevent sharpness.  Too large a corner radius will
contribute toward local failure.  This effect will be greater with increased
operating pressures and temperatures.
3.2.7 Redundant seals.  Two seals in separate seal grooves may be used in
tandem with or without venting the space between the seals.  The two seal
unvented combination is restricted to applications where one side of the
tandem seal combination is vented to the atmosphere.  This application must
allow for increase in breakout and running friction of dynamic seals.  The
first stage of an unvented tandem seal combination must be resistant to
extension in the upstream direction.  The use of two or more seals in the same
cavity is not permitted.
3.2.8 Breathinq.  The piston head gland of actuating cylinders and similar
components in which the gland is confined by a lightweight cylindrical member,
which breathes diametrically as pressure is applies, may have a total
diametrical working clearance greater than that shown by the clearance columns
of Table I in Type I systems.  Experience with actuators up to 5-1/2 inch bore
diameter has indicated that the diametrical breathing, owing to pressure
application only, should not exceed approximately 0.0020 inch per inch of bore
diameter at the midpoint of the cylinder barrel with the piston bottomed at
one end of the cylinder.  The breathing should actually be less than 0.20020
inch per inch of bore diameter in the transverse plane immediately adjacent to
the normal actuating cylinder piston head seal, owing to decreasing cylinder

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