MIL-PRF-18953C(SH)
3.11.1.3 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.
3.11.1.4 Bearings. Motors shall be equipped with sealed grease-filled bearings, Type 120 or equivalent, to
provide proper bearing lubrication in all fan orientations to insure maximum bearing life. Unless otherwise
specified (see 6.2), bearings shall be in accordance with ANSI/ABMA precision classification of ABEC 1 in
accordance with ANSI/ABMA STDs 4, 9, 13, 20, and ANSI/ABMA/ISO 3290. Special bearings (ABEC 5) for
quiet operations when specified (see 6.2) shall be in accordance with ANSI/ABMA STDs 4, 9, 13, 20,
ANSI/ABMA/ISO 3290, and MIL-B-17931.
3.11.1.5 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.
3.11.1.6 Power leads and grounding conductor (for Types L and X-L fans). Power source leads and a
grounding conductor shall be provided and shall extend at least 5 feet beyond fan casing. A separate grounding
connection shall be established between the motor frame and the fan casing.
3.11.1.7 Test connection for explosion-proof motors. A single, drilled, tapped, and plugged opening shall be
provided in the front bracket of each explosion-proof motor for determining enclosure air leakage rate.
3.11.1.8 Spraytight enclosure. A spraytight enclosure shall be in accordance with IEEE STD 45 for watertight
enclosures.
3.11.2 Conduit box. For Types A and X-A fans, a conduit box shall be mounted on the outside of the fan
casing in line with motor lead entrance. The box shall permit connection from either side or from one end. Boxes
shall have the same degree of enclosure as motors with which they are used.
3.11.3 Conduits. For Types A and X-A fans, the lead wires from the conduit box to the motor shall be enclosed
in a watertight conduit or race. An effective electrical ground shall be provided from the motor frame to the fan
casing and the conduit box.
3.12 Fan performance.
3.12.1 Types A and X-A vaneaxial fans.
3.12.1.1 Aerodynamic performance. The design point for each size vaneaxial fan shall be the volume-pressure
point as specified in 1.2.2. The volume shall be the volume at the fan inlet and the pressure shall be the total
pressure (Pt) in water gauge (wg) at the fan discharge. The volumetric flow rate shall not exceed a range of -2% to
+5% of the applicable base curve on Figure 4, as measured on a system line. The upper and lower bounds of this
stable operation region are as indicated in Figure 3. Throughout this region, the fan shall exhibit stable operation.
3.12.1.2 Aerodynamic stall. The stall and recovery region is established along the TP curve from the highest
TP to point "c" in Figure 3. Point "c" shall occur at volume capacities less than or equal to the fan volume-pressure
3.12.2 Types L and X-L tubeaxial fans.
3.12.2.1 Aerodynamic performance. The design point for each size tubeaxial fan shall be the volume-pressure
point as specified in 1.2.2. The volume shall be the volume at the fan inlet and the pressure shall be the total
pressure (Pt) in water gauge (wg) at the fan discharge. The Pt shall rise continually from free delivery through the
design point to a point determined by the intersection of the fan performance curve and the system resistance line
passing through a point at 85% of the design volume and the design pressure as specified in 1.2.2. The volumetric
flow rate shall not exceed a range of -2% to +5% of the applicable base curve on Figure 5, as measured on a system
resistance line. The upper and lower bounds of this stable operation region are as indicated in Figure 3. Throughout
this region, the fan shall exhibit stable operation.
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