MIL-DTL-24640C
(c) Specimens with a conductor cross section of not more than 94,999 circular mils nor less than
22,000 circular mils: 750±35 grams.
(3) The final insulation thickness (t) shall be defined as ½ of the difference between the final specimen
diameter, measured as specified and the previously measured specimen conductor diameter.
4.7.9.4 Observation. The specimen shall be considered to have failed the test if the percentage heat distortion is
greater than that specified in the applicable specification sheet.
4.7.10 Permanence of printing (conductor insulation). This test shall determine the ability of printed
information on insulated conductors to remain legible in the presence of repeated abrasion.
4.7.10.1 Specimen. Each specimen shall consist of a single insulated conductor which shall have sufficient
length for use in the test as specified in 4.7.10.3. Specimens shall be removed from completed cable as follows:
a. Cables containing four or fewer conductors: one specimen shall be removed from each insulated
conductor.
b. Cables containing more than four but fewer than 21 conductors: one specimen shall be removed from each
of four different insulated conductors (a total of four specimens).
c. Cables containing 21 or more conductors: The number of specimens removed shall be equal to the square
root (rounded, if necessary, to the nearest whole number) or the total number of conductors. Each specimen shall be
removed from a different insulated conductor.
4.7.10.2 Special apparatus. Apparatus shall include an abrading machine, which shall secure a specimen
horizontally between two fixed supports, and which shall abrade the specimen printed identification by means of a
motor-driven, reciprocating tape consisting of number 50-2/20 unbleached cotton braid. One possible arrangement
for the abrading machine is shown on figure 7. The braided cotton tape shall lie flat on the specimen surface, shall
remain perpendicular to the specimen axis, and shall abrade an unchanging area of the specimen periphery, over an
arc of not less than 135 degrees, at all times during the test. One end of the tape shall be reciprocated at a rate of
28+3 cycles per minute, such that the tape is drawn across the specimen for a distance of 10+1, -0 inches in each
direction (20 inches minimum total excursion during cycle). The opposite end of the tape shall be maintained under
tension by a freely suspended ½+1/64, -0 pound weight. Each side of the tape shall be used to perform not more than
500 cycles; the tape shall then be discarded and replaced. The abrading machine shall incorporate an automatic
counter to total the number of abrasion cycles to which the specimen is subjected during the test.
4.7.10.3 Procedure. Each specimen shall be wiped with a clean, dry cloth to remove any lubricant or dirt. One
specimen shall then be selected and mounted between the two fixed supports of the abrading machine. Mounting
shall be accomplished such that the specimen shall be as taut as practicable, and such that the braided cotton tape
shall repeatedly abrade the same portion of the specimen printed identification at all times during the test. The
automatic counter shall be set initially to zero. The abrading machine motor shall then be turned on. Allowing the
braided cotton tape to reciprocate and abrade the specimen printed identification either until the printed
identification is no longer legible in its abraded region, or until not fewer than the specified number of abrasive
cycles (see 3.1) have been completed, whichever occurs first. The number of abrasive cycles completed shall then
be noted, and the remaining specimens then tested in an identical manner.
4.7.10.4 Observation. Specimen failure shall be construed if the median value of the number of abrasive cycles
completed on each specimen is less than that specified (see 3.1).
4.7.11 Permanence of printing (jacket). This test shall determine the ability of printed information of jacketing
material to remain legible in the presence of repeated abrasion.
4.7.11.1 Specimen. The specimen shall consist of a piece of completed cable which shall have sufficient length
for use in the test as specified in 4.7.11.3.
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