MIL-PRF-24755B(SH)
4.7.6 Speed. The fan-motor unit shall be operated at each speed at free delivery to determine whether the speed
conforms approximately to design speed at rated voltage.
4.7.7 High-impact shock. The fan-motor unit shall be shock tested as specified for Grade A shock of MIL-S-
901. The high-impact shock test shall be conducted after the tests specified in 4.7.1, 4.7.2, 4.7.3, 4.7.4, 4.7.4.1,
4.7.5, 4.7.6, 4.7.8, and 4.7.9. After conducting the high-impact shock test, tests specified in 4.7.1and 4.7.2 shall be
again conducted without correction of damages that may have occurred during shock test. Shock testing in
accordance with these requirements shall demonstrate that the fan-motor unit performs its function with acceptable
changes in performance and aerodynamic stall. Shock test acceptance shall be contingent upon the ability of the
equipment after shock testing to satisfy performance within ±5 percent of the volume indicated in table II, and
aerodynamic stall with capacities not to occur greater than 90 percent of the fan design or rated ft3/min. In this test,
bolts shall be used in each of the flange bolt holes. Resilient mounting shall not be used unless specified (see 6.2).
If resilient mountings or flexible mounting elements (vibration or shock) are specified for use, classification of
shock testing shall be determined in accordance with MIL-S-901 criteria and the appropriate medium weight or
heavy weight shock test will be determined and conducted. Evidence of any of the following shall be cause for
rejection:
a.
Fragmentation or missile effect of part.
b. Deformation that will cause active interference between parts resulting in permanent degradation of fan
performance in excess of the allowable degradation noted above.
c. Failure to operate. Fan-motor units that have passed this test may be used as production units after
replacement of motor bearings.
4.7.7.1 Medium weight shock test. The test of a fan-motor unit shall consist of the nine blows specified in
table V. Auxiliary channels shall be used to bolt the mounting brackets or plate to standard fixtures. The required
fan orientation mounting for tests required in table V are as follows:
a. Fan horizontal (motor shaft horizontal), steel mounting brackets attached to both fan flanges for mounting
to test platform. This is fan orientation No. 1.
b. Fan vertical, fan impeller down (motor shaft vertical, impeller on bottom). Fan flange bolted to a steel
plate for mounting to test platform. This is fan orientation No. 2A.
c. Fan inclined at 30 degrees, fan impeller up (motor shaft inclined 30 degrees from the vertical, impeller on
top). Fan flange bolted to a steel plate for mounting to test platform. This is fan orientation No. 2B.
TABLE V. Medium weight shock test.
Anvil Travel
Operating
Fan
Blow No.
Group No.
Hammer Drop
(inches)
Condition
Orientation
1
I
3
Secured
No. 1
2
II
3
Operating
No. 1
3
III
1.5
Secured
No. 1
4
I
3
Operating
No. 2A
(Based on
weight, see
5
II
3
Secured
No. 2A
MIL-S-901)
6
III
1.5
Operating
No. 2A
7
I
3
Secured
No. 2B
8
II
3
Operating
No. 2B
1.5
Secured
No. 2B
9
III
NOTE:
1.
Upon completion of the shock test, post-shock test inspection, and specified test, the fan-motor unit shall be
returned to the contractor for examination and further testing.
13
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