MIL-PRF-24751A(SH)
3.13.1.2 Speed-torque characteristics. The speed-torque characteristics shall be coordinated with the combined
inertia of the rotating assembly, including the fan impeller. It shall be possible to accelerate the fan unit from
standstill to high or low speed at 90 percent of the rated voltage when the current rating of the controller overload
protective device does not exceed the motor full load current.
3.13.1.3 Motor thermostats. A thermostatic protective device shall be installed in each of the motor phases
wired such that any of them, when wired in connection with the motor starter control circuit, will cause motor
shutdown if damaging heat buildup is encountered.
3.13.1.4 Bearings. Motors shall be equipped with sealed grease-filled bearings. Bearings shall be in
accordance with ANSI/ABMA precision classification of ABEC-5 in accordance with ANSI/ABMA STDs 4, 9, 13,
20, and ANSI/ABMA/ISO 3290. Means shall be provided to ensure that grease is always available to the bearing
and to prevent the leakage of oil or grease along the shaft. Special bearings for quiet operations when specified (see
6.2) shall be in accordance with ANSI/AFBMA 4, 9, 13, 20, ANSI/ABMA/ISO 3290, and MIL-B-17931.
3.13.1.5 Bearing temperature rise. Bearing temperature rise not to exceed 35 °C in 65 °C ambient as measured
on the outer ring. For quiet bearings, temperature rise not to exceed 25 °C in 65 °C ambient as measured on the outer
ring.
3.14 Fan performance.
3.14.1 Volume and pressure. The design volume-pressure point for each size fan shall be as specified in
Table I. The total pressure in inches of water shall rise continually from free delivery to a value at least as high as
the total pressure at the stated volume as shown in Table I and throughout this range of stable performance the
volume in ft3/min shall be within 5 percent of the volume on the applicable curve of Figure 1.
3.14.2 Aerodynamic stall. Effective stall shall not occur at capacities greater than 85 percent of the fan design
or rated volume (ft3/min) (see 4.5.2).
3.14.3 Compressor surge. The fan assemblies shall withstand a gas turbine surge condition defined below:
a.
Cooling Air Fan for Propulsion Gas Turbines - Single pressure pulse of approximately square wave
form with a pulse magnitude of 3 lb/in2 positive pressure and a duration of 20 milliseconds.
b.
Cooling Air Fan for Ship's Service Gas Turbine Generator (SSGTG) Sets - The requirements of
3.14.3a above shall apply.
3.14.4 Surge stress. Surge stress, when combined with normal operating stress, shall not exceed 80 percent of
the material static yield stress. Testing is not required.
3.14.5 Differential pressure. The fan assembly, when installed in the ship's ducting system, shall withstand a
maximum differential pressure of 20 inches H2O between the outside and the inside of the fan. The lower pressure
shall be inside the fan.
3.14.6 Self-generated vibration. All parts of the fan equipment shall be free of self-induced vibration in
accordance with the type II requirements of MIL-STD-167-1 for both steady-state and transient conditions.
Dynamic balancing and tolerance control shall be employed over the complete operational range to ensure smooth
operation of all rotating parts.
3.15 Environmental conditions.
3.15.1 High impact shock resistance. Unless otherwise specified (see 6.2), the fan-motor units shall meet the
shock requirements of MIL-S-901 for Grade A, class 1 equipment.
3.15.2 Vibration and balance. Fan-motor units shall be designed such that no damage will occur or malfunction
be caused by the environmental vibrations specified in MIL-STD-167-1. Fan-motor units shall be in static and
dynamic balance. The vibration amplitude shall not exceed the limits for type II bearing vibration as specified in
MIL-STD-167-1.
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