MIL-PRF-24755B(SH)
3.16 Motor mounting. Motors shall be face mounted. Mounting shall be designed to dampen rotor vibration.
3.17 Fan impellers. Impellers shall be designed to prevent retention of water when the fan is installed
horizontally, vertically with the impeller up, and vertically with the impeller down. The total impeller shall have at
least a safety factor of 8 under normal operation, based on the ultimate tensile strength of the material. Impeller
factor of safety analysis shall include impeller balancing features. The hub of the impeller shall not distort or the fit
loosen on the drive shaft during the life of the equipment.
3.17.1 Nose piece. If a nose piece is required, it shall be match marked with the impeller or attached to it to
ensure reinstallation in the same position relative to the impeller.
3.17.2 Balancing. The impeller shall be statically and dynamically balanced.
3.18 Electric equipment.
3.18.1 Motors. In consideration of the cooling effect of the air stream, motors for vaneaxial fans need not
conform to referenced motor specifications in regard to horsepower rating of a standard frame sizes. Motors shall
have sufficient torque to start the fans at 90 percent of rated voltage. The motors shall conform to the following:
a.
60 Hertz, AC, integral: horsepower.
b.
Service: Navy H.I. shock qualified (Type A).
c.
Ambient temperature: +149 °F (65 °C).
d.
Voltage, phase, and frequency: 440 volts, 3-phase, 60 Hertz.
e.
Power source quality: In accordance with MIL-STD-1399-300.
f.
Duty: Continuous-air over.
g.
Enclosure: Spraytight.
Horsepower: In accordance with figure 1, IEEE 45, and NEMA MG 1.
h.
i.
Revolutions per minute (r/min): 3,600 (synchronous) constant speed.
j.
Type: Squirrel cage induction, with not less than 80 percent power factor full load.
k.
Mounting: Face mounted.
l. Bearings: The motor shall be designed so that the bearing temperature rise at the outer surface of the
bearing shall not exceed 86 °F (30 °C) in 149 °F (65 °C) ambient. For quiet bearing, temperature rise shall not
exceed 77 °F (25 °C) in 149 °F (65 °C) ambient as measured on the external surface of the outer ring.
m. Insulation required: Class F or better, sealed insulation system with Class B temperature rise, as described
in MIL-DTL-17060.
n. Random wound sealed insulation systems: Each random wound sealed insulation system motor shall have
a stainless steel nameplate attached near the regular nameplate stating, "Stator winding insulated to provide a sealed
insulation system. CAUTION: If rewinding is required do not use burnout oven for stripping. Cut off one end turn,
heat winding by radiant heat and pull windings from slot." An additional octagonal nameplate shall be attached near
the regular nameplate starting at the top using two lines "DO NOT OVERHAUL", in the center using two lines
"HIGH RELIABILITY", and the bottom using two lines "SEE MOTOR DRAWING".
o.
Not for submarine service.
p. Motor structureborne noise levels shall not exceed the levels required in MIL-DTL-17060, or lower when
specified by auxiliary manufacturer.
q.
Shock test: MIL-S-901, Type A, for H.I. shock applications.
r.
Air gap: In accordance with NEMA MG 1.
s.
Airborne noise levels: In accordance with NEMA MG 1.
t.
Locked rotor current: In accordance with NEMA MG 1.
u. Torque: The locked rotor torque, pullup torque, and breakdown torque of motors shall be at least 70
percent, 70 percent, and 200 percent, respectively, of full load torque.
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