Bending endurance MIL-DTL-24643C

The ability of completed cable to withstand repeated smooth and continuous, reversing bending motion while subjected to a specified temperature shall be determined.

4.8.3.1 Specimen. The specimen shall consist of a 30±6, -0 inch length of completed cable. Specimen
conductor ends shall be electrically interconnected such that a single series electrical circuit is formed by the
conductors. The two end conductors of this series circuit shall be connected as specified in 4.8.3.3.
4.8.3.2 Special apparatus. Apparatus shall include the following:
a. Flexing machine. A device for suspending the specimen vertically and for automatically bending the upper
portion of the specimen alternately left and right by 90±5, -0 degrees from vertical over rollers as shown on figure 5
and as further described below.

(1) The specimen shall be suspended and bent by means of a rigid, motor driven arm, which initially shall
be vertical, which shall be horizontally pivoted about its lower end, and which shall incorporate a clamp at its upper
end for securing the upper end of the specimen jacket. The distance between the upper end of the specimen jacket
and the nearest edge of this clamp shall be not less than 1 inch. When its motor is turned on, the arm shall move at
the rate of not more than 14 nor less than 12 cycles per minute, where a cycle is 180±10 degrees of clockwise arm
travel plus 180±10, -0 degrees of counterclockwise arm travel (360±20, -0 degrees total travel). The specimen
clamp shall be positioned such that when the arm is vertical (straight up), the longitudinal axis of the specimen shall
intersect the pivoting axis. The lower edge of the clamp shall be 8½±½ inches above a horizontal line drawn across
the tops of the upper rollers.

(2) The specimen shall be bent over two solid steel cylindrical rollers (the upper rollers as shown on figure
5) of the diameter specified. Unless otherwise specified in the applicable specification sheet: for cables over 1 inch
in diameter, the roller shall have a diameter four times the diameter of the specimen; for cables with a diameter
between 0.75 and 1 inch, the roller diameter shall be 3.5 times the maximum diameter of the cable; and for cables
with a diameter less than 0.75 inch roller size shall be three times the maximum diameter of the cable. Both rollers
shall have a smooth surface finish on their curving surfaces, shall be pivoted about their axes and capable of free
rotation at all times. They shall be positioned such that their axes are horizontal and parallel and share a common
horizontal centerline. The tops of the rollers shall be below the horizontal centerline of the flexing machine arm
pivoting axis by a distance equal to one half of the maximum specified overall cable diameter ±10 percent. The
rollers shall be located equidistantly on opposite sides of the vertical centerline passing through the flexing machine
arm-pivoting axis. They shall be spaced apart such that their nearest edges are ǩ± 1
32 inch from the specimen passing

between them when the flexing machine arm is vertical.
(3) Two cylindrical rollers (the lower rollers as shown on figure 5), of not less than Ǫ inch diameter shall
be provided to restrain lateral motion of the specimen. Both rollers shall be pivoted about their axes and capable of
free rotation at all times. They shall be positioned such that their axes are horizontal centerline. The axes of these
rollers shall be 7±1 inch below the axes of the rollers over which the specimen is bent (the upper rollers as shown on
figure 5). The lower rollers shall be located equidistantly on opposite sides of the vertical centerline passing through
the flexing machine arm pivoting axis. They shall be spaced apart such that their nearest edges are ǩ±1
32 inch from

the specimen passing between them when the flexing machine arm is vertical.
(4) A weight shall be provided for applying tension to the specimen. This weight shall be attached to the
specimen by means of a second clamp. The lower end of the specimen jacket shall be a distance of not less than 1
inch below the nearest edge of this clamp. The weight shall be chosen such that the:
Specimen tension produced by the clamp plus the weight in pounds = 10 (3.14159)d 2 ± (3.14159)d 2 /2
Where: d = specified maximum overall cable diameter, in inches.
The clamp and weight shall be attached such that the specimen tension is applied vertically downward along the
specimen axis

(5) Both the specimen clamp on the flexing machine arm and the specimen clamp supporting the
tensioning weight shall apply radial compression to the specimen that neither the specimen nor any of its internal
components will slip in the clamping area during the test.
(6) An automatic counter shall be provided to total the number of bending cycles performed on the
specimen during a test.
(7) The specimen shall not come into contact with any piece of machinery at any time during the test,
except for the upper and lower rollers, the two specimen clamps and the circuit continuity monitor.

b. Circuit continuity monitor. Circuit continuity monitor is a means for continuously monitoring the electrical
continuity of the series through connected specimen conductors, which shall automatically stop the flexing machine
arm (as by removing motor power) when a specimen conductor breaks.
c. Test chambers. Test chambers are chambers for maintaining the specified air temperature within ±2 °C for
accommodating the flexing machine with the specimen attached and for supporting the requirements as specified in
4.8.3.3. The chamber air temperature shall be measured in the immediate vicinity of the upper roller of the flexing
machine. The chamber shall incorporate a viewing port for observing the specimen under test.
d. Voltage withstand test apparatus. Voltage withstand test apparatus is as specified in 4.9.8.2.

4.9.8.2 Special apparatus. Apparatus shall include the following:
a. An adjustable voltage source, which shall be in accordance with either of the following, as appropriate (see
3.1).
(1) An AC voltage source which shall produce a single-phase, approximately sinusoidal voltage under all
test conditions, which shall have a frequency of not more than 100 Hz, and which can be adjusted to the specified
rms magnitude (see 3.1). This source shall have the capacity to provide an effective power of not less than 5 kVA,
and shall incorporate an overload relay which shall reduce the source voltage to zero whenever an applied load (that
is, the specimen, see 4.9.8.3) attempts to draw electrical current to exceed this effective power capacity.
(2) A source of DC voltage which can be adjusted to the specified magnitude (see 3.1), and which shall
have the capacity to provide an electrical current of not less than ¼ A. This source shall incorporate an overload
relay which shall reduce the source voltage to zero whenever an applied load (that is, the specimen, see 4.9.8.3)
attempts to draw electrical current to exceed this current capacity.
b. When required, a water tank, which shall be filled with water and shall be of sufficient size to contain the
specimen, shall be used as specified in 4.9.8.3.f and 4.9.8.3.g (may not be required, see 4.9.8.3.f and 4.9.8.3.g).