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|  MIL-H-8891A
Swivel joints - Swivel joints may be used where rela-
3.10.23
tive motion exists between two points (see 3.10.15). Life test data shall
be submitted to the procuring activity for approval. Where lines or
fittings are used to drive swivel joints, they shall be adequately sup-
ported and shall be of sufficient strength to insure a satisfactory
operating installation.
Vents - All vent openings or fluid exposed to breath-
3.10.24
ing action through vents shall be protected by filters. For reservoir,
reference paragraph 3.10.20.6.
Tubing -
Tubing materials - Tubing shall be either corrosion-
3.11.1
resistant steel conforming to MIL-T-6845 or MIL-T-8504. Higher strength-
to-weight ratio steel or titanium tubing may be used with the approval of
the procuring activity. Tubing mounted on shock struts shall be corrosion-
resistant steel. The minimum wall thickness in any alloy or tube size
shall be not less than .020 inch.
Tube sizinq - Tubing size is a function of several
3.11.2
parameters. One of the more important parameters is pressure drop.
Generally they should be sized to permit approximately 107. of system
operating pressure drop in the total pressure transmission system, i.e.,
pump to actuator, and a 107. drop for the return system at the minimum
rated fluid temperature, Unless otherwise specified the rated tempera-
ture is -20F. This pressure drop analysis shall utilize the data listed
in AIR 1362 "Physical Properties of Hydraulic Fluids". Another important
parameter is fluid velocity which results in pressure surges and spikes
and is discussed in paragraph 3.5.3 "Fluid velocity limitations". Addi-
tional parameters are located with respect to the: (1) pump pressure line,
this high energy line is sensitive to resonance conditions developed from
pump ripple, structural vibrations and engine vibrations, and (2) structural
defections between connecting end points, and (3) specific local applica-
tions.
Tubing fabrication -
3.11.3
Tubi ng bends - Bends shall be uniform and shall be in
3.11.3.1
accordance with MS33611.
Tubing and fitting identification - All hydraulic oil
3.11.3.2
lines shall be permanently marked in accordance with MIL-STD-1247. (For
l pplications in systems where environmental temperatures do not pemit the
use of markings in accordance with MIL-STD-1247, the contractor shall pre-
pare an identification specification and submit it for approval to the pro-
curing activity.) A sufficient number of hydraulic lines shall be marked
in conspicuous locations throughout the vehicle in order that each run of
line may be traced. This marking shall indicate the unit operated and the
direction of flow, such as "Landing gear up", "Flaps down", etc. These
markings shall be repeated as often as necessary, particularly on lines
28
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