MIL-DTL-6162C
3.4.5.13.7 Brush life. The machines shall have a minimum brush life as follows:
a.
Generators shall have a minimum brush life as follows:
(1) Class A generators - 1000 hours from sea level to 35,000 feet, and 600 hours from 35,000 to
50,000 feet.
(2) Class B and Class C generators - 750 hours from sea level to 50,000 feet, and 500 hours from
50,000 to 65,000 feet.
b. Starter-generators shall meet the above requirements for generator brush life plus a minimum of 1200
engine starts.
3.4.5.13.8 Brush stop. Design of brush and holder shall be such that the commutator will not be subjected to
mechanical damage if the brushes are not replaced when their wearing depth is exhausted. Additionally, means shall
be provided to prevent brushes from being thrown out of the holders as a result of severe shock or from being
displaced to such a position that they may bind and not return to normal operating position. The devices used shall
not unduly complicate brush replacement.
3.4.5.14 Bearings. The armature or rotor of the machine shall be designed to be supported on both ends by
bearings conforming to the reliability and performance requirements contained herein. The manufacturer shall
furnish bearing specifications and data with machines submitted for qualification.
3.4.6 Cooling.
3.4.6.1 Blast cooling. Type I and Type III. (See 4.5.11).
3.4.6.1.1 Blast cooling inlet tube. Air cooled machines shall include provisions for entrance of air to provide
cooling in addition to that already provided by integral means. Radial inlet tubes, when specified, shall be positional
to four (4) approximately equally spaced positions (preferably adjustable to any position).
3.4.6.1.2 Amount of blast cooling. The quantity of cooling air for Type I and Type III generators is based on
the total head at the entrance of the cooling duct and the duct loss. Total head shall be 7 ± 0.5 inches of water for
Class A machines and 11 ± 0.5 inches of water for Class B and C machines. Rated mass airflow for Type I and
Type III machines operating at rated output at sea level are determined from Figure 6. Type I machines shall operate
with an air duct which produces a duct loss of one (1) inch of water at rated air flow and 1.69 inches of water at 130
percent of rated mass air flow. Type III machine shall operate from a duct, which produces a duct loss of 0.5 inches
of water at, rated airflow and 0.85 inches at 130 percent air flow. During altitude operation, a head of 7.0 ± 0.5 or
11.0 ± 0.5 inches of water, as applicable, shall be maintained at the cooling duct inlet.
3.4.6.1.3 Duct loss simulation. Hydrodynamic head losses in the cooling air duct shall be simulated by an
adjustable restriction within the cooling duct. Head measurements shall be made with a pitot tube located in the
center of the duct and in a region of essentially laminar flow. The pitot tube shall be located in a length of straight
smooth duct. There shall be no obstruction or other causes of turbulence for at least 20-cooling duct diameters up
stream from the pitot tube and 10-diameters down stream. During self-cooled Type II and Type III operation, the
duct loss shall be measured as the difference between atmospheric pressure and the pitot tube head as indicated by a
water manometer. During blast air cooling, the duct loss shall be measured as the difference between manometer
readings above and below the adjustable restriction.
3.4.6.2 Self-cooling - Type II. The machine shall be self-cooled by internal means which provide sufficient
cooling to permit full rated generator output at minimum speed regulation at sea level at 71° C (160° F) and at
continuous revolutions per minute (RPM) at 15,000 feet altitude with cooling air at 0° C (32° F). When a cooling
air duct is used, the duct loss shall be the same as that specified for Type III machines, except that no cooling tube
inlet head will be provided during altitude operation unless otherwise specified in the associated specification sheet.
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