MIL-PRF-19004B
missile effect of parts, deformation that will cause active interference
between parts, or failure to operate shall be cause for rejection. Resilient
mounting shall not be used unless specified (see 6.2). Fan-motor units that
have passed this test may be used as production units after replacement of
motor bearings.
4.7.1.1 Shock on lightweight machine. The motor and fan support shall be
mounted as shown on Figure 4c of MIL-S-901. The test shall consist of nine
blows with the unit operating at maximum speed and nine blows with the unit
not engaged.
4.7.1.2 Shock on medium weight machine. The test of a fan-motor unit shall
consist of nine blows indicated in Table VI. The required mountings for
required tests in Table VI are as follows: (Designations for rotation and
discharge of centrifugal fans are in accordance with AMCA 2406-66).
1.
CWTH or CCTH, motor and impeller shaft horizontal.
2A. CWTH or CCTH, motor and impeller shaft horizontal, mounting base
inclined 30°.
2B. CWUB or CCOB, base, motor and impeller shaft inclined 30°, motor on
top.
TABLE
VI.
Test on medium weight shock machine.
Blow
Group
HAMMER
Anvil
Operating
Fan
No.
No.
drop
travel (in)
Condition
Orientation
1
I
(Based on
3
Secured
No. 1
2
II
weight, see
3
Operating
No. 1
3
III
MIL-S-901)
1.5
Secured
No. 1
4
I
3
Operating
No. 2A
5
II
3
Secured
No. 2A
6
III
1.5
Operating
No. 2A
7
I
3
Secured
No. 2B
8
II
3
Operating
No. 2B
9
III
1.5
Secured
No. 2B
Upon completion of the shock test, post-shock test inspection, and tests
specified in Table V, the fan-motor unit shall be returned to the contractor
for examination and further testing.
4.7.2 Vibration test. Fan-motor units shall be subjected to type I
environmental vibration tests as specified in MIL-STD-167-1. The exploratory
vibration test specified in MIL-STD-167-1 shall include frequencies up to 33
Hz at the table amplitude specified herein. The vibration test shall be
conducted as specified in 4.11. Any unit that fails to meet any requirement
shall be rejected.
4.7.3 Balance test. In this test, the fan-motor unit shall be operated at
maximum speed and free delivery, with shaft horizontal. It shall be suspended
from a stand by means of two elastic vibration cords, one secured to each end
flange. The cords shall have a natural frequency of less than 1/4 of the
minimum rotational frequency of the unit in Hz. The static deflection shall be
within the limits of MIL-STD-167-1 for elastic mounting elements, and this
shall be the basis for determining cord size for each unit tested. As an
alternative to the suspension test, the fan-motor unit shall be suspended from
an electric hoist by means of two chains, one secured to the end flanges of
the fan-motor unit (that is, the ends of the chain secured to opposite
flanges); one secured to the electric hoist and to the center of the chain
secured to the end flanges. The length of the chains shall be not less than 15
inches nor greater than 35 inches, and the link size of the chains shall be
not less than 1/4 inch nor greater than 3/8 inch. With either method, the
internally excited vibrational displacement (single amplitude mils, peak)
shall be measured as follows: four axial measurements 90 degree apart on the
face of each end flange, one horizontal measurement perpendicular to the axis
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