MIL-DTL-55668D
4.7.7.3 Retraction after static vertical loading. At room temperature, the finished cord shall be secured at
one end and hung vertically with a weight attached to the lower end, sufficient to extend the helical
portion of cord to 250±1 percent of its original horizontal retracted length. After 48 hours, the weight shall
be removed and the cord placed on a horizontal surface. This surface shall be tapped to reduce frictional
restraint. Thirty minutes after being placed on the horizontal surface, the helical portion of the cord shall
be measured to determine percent retraction of original length (see table IV). Percent retraction shall be
calculated as follows:
Retracted length after extension
Percent Retraction
100
Original retracted length
4.7.7.4 Low temperature (-55 C) extension and retraction. A 6-inch (152 millimeter) helical portion of a
completed cord with all coils contiguous shall be placed in a horizontal position within an environmental
test chamber maintained at -551 C, fastened at one end, and conditioned for not less than 20 hours
prior to testing. If the helical portion of the finished cord is less than 6 inches (152 millimeters) long, the
complete cord shall be used and measurements made of the helical portion. At the conditioning
temperature, a force shall be applied to the free end of the cord sufficient to extend the helical portion four
times its retracted length within 5 seconds. The maximum force shall be as specified in table IV. Upon
release of the force and while still at -55 C, within 30 seconds after release, the helical portion of the
specimen shall retract to not more than the percent of its original length as specified in table IV. Frictional
restraint shall be minimized by tapping the surface on which the cord rests. Upon visual examination at
room temperature, the jacket, insulation, and conductors in both extended and retracted states shall be
free of cracks and other defects.
4.7.8 Cold bend. Specimens shall be taken from the straight portions of the cords. The specimens from
any one cord shall be divided, one part for checking the finished cord as a whole (jacketed cord) and the
other for checking insulation, as specified in table IV. The part selected for checking insulation shall have
the insulated conductors removed for test; shielding, if any, shall be removed. Each insulated conductor
shall be tested. The test specimens shall be attached to mandrels of diameters equal to the diameters of
the specimens, and shall be suspended vertically with lower ends weighted sufficiently to keep the
specimens taut and to permit bending them without handling. The mandrels and specimens shall be
placed for at least 24 hours in a cold chamber maintained at -551 C. While at this temperature, the
specimens shall be bent for five close turns around the mandrels at the rate of one turn per second. After
removal from the cold chamber, the specimens shall be examined under 3X magnification for cracks,
breaks, or other faults. Jacketed specimens shall be stripped and the insulation of the conductors
examined.
4.7.9 Low temperature rapid extension. The test specimen shall be a completed cord if the helix is not
greater than 9-inches long, or portion thereof including one straight end and at least 8 inches of the
helical section. The specimen, with all coils contiguous, shall be placed in a horizontal position within a
cold chamber and conditioned for 7 days at -551 C. While at -55 C, the specimen shall be secured at
one end so that the cord hangs vertically, with a straight section at the bottom. A 1-pound (minimum)
weight shall be attached to the free straight end and then allowed to fall. The distance of fall shall be
sufficient to extend the helical portion of the cord to four times its retracted length. After removal from the
test chamber, the cord shall be examined under 3X magnification for cracks or other defects in the jacket
and conductor insulation (see table IV).
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