MIL-DTL-81963C
4.9.4.2 Moment of inertia by trifilar suspension. The rotor of the servocomponent shall be placed in a
plate (circular disc or equilateral triangle) so that the axis of rotation of the rotor is perpendicular to the
plate and in the exact center of the plate. The weight of the plate shall be approximately equal to the
weight of the rotor. Three threads of equal length shall be connected to a flat, level mounting surface and
the plate shall be attached to the threads. The threads shall be equidistant and parallel to the axis of
rotation. The distance between the mounting surface and the plate shall be greater than twice the radius
from the axis of rotation to the threads (see figure 5). The plate shall be protected from air currents and
extraneous vibrations, rotated and the time period measured as the plate oscillates through a small angle
about the axis of rotation with and without using the following formula:
22
2
2
2
Wr t + Wbr (t tb )
I =
4L
4L
where:
2
I = rotor moment of inertia in gram-centimeters
W = weight of rotor in grams
Wb = weight of plate in grams
t = time period of oscillation of the plate with rotor in seconds
tb = time period of oscillation of plate without rotor in seconds
L = distance from the plate to the mounting surface in centimeters
r = radius from the axis of rotation to the threads in centimeters
4.9.5 Breakaway torque.
4.9.5.1 Mechanical breakaway torque. With the applicable weight of figure 6 attached to the dial of
figure 7 (corresponding to the developed torque specified in the applicable documents and the
specification sheet that cite MIL-DTL-81963 as a direct reference) and with the dial mounted rigidly on the
shaft and hanging free at the start of the test, the housing of the servocomponent shall be rotated through
at least three revolutions in each direction at a constant rate between 4 and 6 rpm, in accordance with
3.6.5.1. The servocomponent shall fail the test if the dial turns one revolution.
4.9.5.2 Electrical breakaway torque. The servocomponent shall be brought to the stabilized operating
temperature condition (as specified in the applicable general specification). With the shaft in any initial
angular position, voltage applied to the control winding shall be increased slowly from zero. The voltage
at which the shaft starts to rotate continuously shall be noted. The test shall be performed three times for
each direction of rotation (a total of six measurements). The highest voltage observed shall be recorded
as the electrical breakaway torque, and shall meet the requirements of 3.6.5.2.
4.9.6 Dielectric withstanding voltage. The dielectric withstanding voltage test shall be performed in
accordance with method 301 of MIL-STD-202 and the requirements of 3.6.6. The applicable test
potentials shall be applied between those points designated. Any dielectric withstanding voltage test that
is repeated on the same servocomponent shall be performed with the test potential reduced to 80 percent
of the specified value of the initial test. The test potential shall be raised slowly (minimum time 3
seconds) from zero to the specified value. The peak value of the test potential throughout this test shall
not exceed 1.5 times the specified rms test potential. The test equipment employed shall be such as to
differentiate between winding leakage current and surge discharge current. Following this test, the
insulation resistance test shall be performed in accordance with 4.9.7.
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