MIL-PRF-24751A(SH)
4.6.8.1 Shock on medium weight machines. The test of a fan-motor unit shall consist of the nine blows indicated
in Table V. The required fan orientation mountings for tests required in Table V are as follows:
1.
Fan horizontal (motor shaft horizontal), steel mounting brackets attached to both fan flanges for
mounting to test platform.
2A. Fan vertical, fan impeller down (motor shaft vertical, impeller on bottom). Fan flange bolted to a steel
plate for mounting to test platform.
2B. 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.
Auxiliary channels shall be used as necessary to bolt the mounting brackets or plate to standard fixtures.
TABLE V. Test on medium weight shock machine.
Group
Hammer
Anvil travel
Operating
Fan
Blow No.
No.
drop
(inches)
condition
orientation
(Based on
1
I
3
Secured
No. 1
weight, see
2
II
3
Operating
No. 1
MIL-S-901)
3
III
1.5
Secured
No. 1
4
IV
3
Operating
No. 2A
5
V
3
Secured
No. 2A
6
VI
4.5
Operating
No. 2A
7
VII
3
Secured
No. 2B
8
VIII
3
Operating
No. 2B
1.5
Secured
No. 2B
9
IX
4.6.9 Motor temperature. Motor temperature rise shall be measured as specified in the applicable motor
specification. In each test, the motor shall be installed in the fan, and a duct shall be attached to the discharge. Full
load shall be obtained by means of terminal throttle on the test duct. Since neither the fan nor a test stand will
normally be available at the motor plant, the heat test may be conducted at the plant of the fan manufacturer.
4.6.10 Bearing temperature. Bearing temperature shall be measured with a fan mounted in vertical with
impeller on bottom. Full load shall be obtained by means of a terminal throttle on attached test duct. Bearing outer
ring temperature shall not exceed requirements of 3.13.1.5.
4.6.11 Blade tip clearance measurement. The fan blades shall be in a stationary position for the entire
measurement. Inspect visually to locate an apparent maximum clearance (radial clearance) from the blade tip to the
inside of fan casing. Measure this apparent maximum clearance and move the measurement to three different locations
at 90, 180, and 270 degrees relative to the apparent maximum clearance. Compare the four measurements to determine
the true maximum blade clearance. This maximum clearance shall not exceed ¼ of 1 percent of the casing inside
diameter.
4.6.12 Test schedule. The schedule for testing the fan shall be performed in the following order:
a.
Balance (4.6.4).
b.
Casing tightness (4.6.6).
c.
Blade tip clearance measurement (4.6.11).
d.
Performance (4.6.1), aerodynamic stall (4.6.2), speed (4.6.7), motor heat (4.6.9), and bearing
temperature (4.6.10). These tests shall be conducted concurrently.
e.
Airborne noise (4.6.3).
f.
Vibration test (4.6.5). The correction of damages, which may have occurred during the vibration tests,
shall not be performed prior to the test specified in 4.6.8.
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