MIL-DTL-55668D
4.7.10 Bending flex. The test specimen shall be a finished straight cord (or convenient length thereof), or
a finished retractile cord, as applicable. The cord-flexing machine shall contain a rotating actuating arm
whose normal zero position is vertical. The arm shall rotate about a horizontal axis, the outer end
traversing a circular arc in a vertical plane. The arm shall rotate 90 degrees from the zero position to a
position 90 degrees from the zero position in the opposite direction and then reverse its direction and
rotate back to zero position, thus completing a cycle. The arm shall be rotated at approximately 14±1
cycles per minute. Two adjustable-position mandrels, each approximately .300 inches (7.62 millimeters)
in diameter, shall be mounted in the machine with their axes perpendicular to the plane of rotation of the
actuating arm. The plane through the axes of the two mandrels shall be horizontal and shall pass
through the axis of rotation of the actuating arm. The mandrels shall be placed so that the axis of rotation
lies midway between the mandrels. With the fixture positioned vertically, the specimen shall be clamped
2 inches above the helix, so that the straight end protrudes above the clamping device and passes
between the mandrels with some slight clearance. The conductors and shields shall be wired in series
and connected to the electrical test apparatus as indicated by the schematic on figure 1. (A cord identical
to the one being tested is also wired in series, and connected as the reference leg of the Wheatstone
Bridge). A 5-pound weight shall be attached to the 10th convolution of the helix (counting from the
clamped end down). If the helix has less than 10 turns, the weight shall be clamped to the lower straight
end, as near as possible to the helix. The test weight shall be restricted from twisting (to simulate service
conditions) by a slack anchor attached to the weight. The electrical test circuit is energized and meter
null adjustments made with the variable resistor. A mechanical counter shall be used to record the
number of cycles. The meter relay shall be set to trip at a fluctuation of 50 milliohms (25 milliohms from
the null point) in the resistance of the specimen, terminating the test. Tests on tinsel wire conductors
shall be discontinued, if no failure occurs, at the number of cycles specified in table IV. Tests on stranded
wire conductors shall be run until failure, and the number of cycles recorded for comparison with the
requirements in table IV.
4.7.11 Stretch cycling. The test specimen shall be the finished cord, or, for very long cords, at least an 8
inch helical section. The resistance (Ri) of each conductor shall be measured prior to cycling. The helix
shall then be extended to 6 times its retracted length and allowed to retract. This stretch cycle shall be
repeated 60,000 times at a rate of not more than 40 cycles per minute. At the completion of 60,000
cycles, twenty five (25) additional cycles shall be run; during this period, the conductor resistance shall be
monitored continuously and recorded.
(a) The percent variation in resistance during these cycles shall be calculated as follows:
Rv
Percent variation
100
R
Where R = The conductor resistance in ohms at the start of the final cycling.
Rv = Difference between highest and lowest resistance values of the
conductor during the cycling.
(b) The percent increase in conductor resistance due to cycling shall be calculated as
follows:
Rm Ri
Percent increase
100
Ri
Where Ri = The conductor resistance in ohms before cycling,
Rm = Maximum resistance of the conductor during cycling or at
completion of cycling, whichever is higher.
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