MIL-DTL-24643C
4.8.21.2 Special apparatus. Apparatus shall include the following:
a. A twisting machine for suspending the specimen vertically, and for automatically twisting the upper
specimen end alternately clockwise and counterclockwise by 180+10, -0 degrees with respect to the lower specimen
end as described below.
(1) The specimen shall be suspended and repeatedly twisted from one extremity by means of a clamp,
which shall be fastened to a motor driven, rotating and reversing mechanism (such as a pinion connected to a rack,
which in turn is made to reciprocate by an arm from a motor). The upper specimen end shall be twisted about its
axis at a rate of not more than 14, nor less than 12 cycles per minute. Each cycle shall produce 180+10, -0 degrees
of clockwise axial specimen rotation plus 180+10, -0 degrees of counterclockwise axial specimen rotation (360+20,
-0 degrees total rotation). The upper end of the specimen jacket shall be a distance of not less than 1 inch above the
nearest edge of the clamp.
(2) A weight shall be provided for applying tension to the specimen. This weight shall be attached to the
lower specimen extremity by means of a second clamp, which shall be constrained from rotating but which shall be
free to move in the vertical direction at all times during the test. The weight shall be chosen such that the:
Tension from the second clamp plus the weight in pounds = 10(3.14159)d2 ± (3.14159)d2/2
Where:
d
=
Specified maximum overall cable diameter, in inches (see 3.1).
This second clamp shall be separated from the rotating clamp by a distance which shall not exceed the sum
of 6 inches plus 10 times the specified maximum overall cable diameter (see 3.1). The second clamp and weight
shall be constructed such that tension is applied vertically downward along the specimen axis. The lower end of the
specimen jacket shall be a distance of not less than 1 inch below the nearest edge of a clamp.
(3) Both the rotating clamp and the tensioning weight clamp shall apply radial compression to the
specimen that neither the specimen nor any of its internal components will slip in the clamping area.
(4) An automatic counter shall be provided to total the number of twisting cycles performed on the
specimen during a test.
(5) The specimen shall not come into contact with any piece of machinery, except for the rotating clamp,
the tensioning weight clamp and the circuit continuity monitor [see 5(b)] at any time during the test.
b. Circuit continuity monitor (a means for continuously monitoring the electrical continuity of the series
connected specimen conductors) shall automatically stop the motion of the rotary clamp as by removing motor
power when a specimen conductor breaks.
c. A test chamber for maintaining the specified air temperature (see 3.1) within ±2 °C, for accommodating the
twisting machine with specimen attached and for supporting the requirements as specified in 4.8.21.3. The chamber
air temperature shall be measured in the immediate vicinity of the specimen, approximately midway between the
two specimen clamps of the twisting machine. The test chamber shall have a viewing port for observing the
specimen under test.
d.
Voltage withstand test apparatus shall be as specified in 4.9.8.2
4.8.21.3 Procedure. The twisting machine shall be installed in the test chamber, and the specimen shall be
installed in the twisting machine. The circuit continuity monitor shall be connected between the two end conductors
of the single series circuit within the specimen. The test chamber doors shall then be closed, and the air temperature
within the chamber shall be brought to the specified value (see 3.1) within ±2 °C. After the air temperature within
the chamber has been maintained at this value for a period of not less than 1 hour, the automatic counter shall be set
initially to zero and the twisting endurance test begun by turning on the motor driven, rotating clamp. The specified
number of twisting cycles (see 3.1) shall be performed, unless specimen failure (see 4.8.21.4) occurs prior to the
completion of this number of cycles. The test may then be terminated prematurely. At all times during the test, the
temperature within the chamber shall be maintained as specified (see 3.1) within ±2 °C and the test chamber doors
shall remain closed. Following the test, the specimen shall be removed from the twisting machine and the test
chamber.
4.8.21.4 Observation. Either of the following shall constitute specimen failure, except that specimen damage
within 2 inches of the location where either specimen clamp had been fastened shall not constitute specimen failure,
but shall require a complete retest using a different specimen from the same length of cable:
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