MIL-PRF-18953C(SH)
TABLE IV. Test on medium weight shock machine.
Hammer drop
Blow
Group
Anvil travel
Operating
Fan
No.
No.
(inches)
condition
orientation
1
I
3
(Based on
weight, see
2
II
3
Secured
No.1
MIL-S-901)
3
III
1.5
Operating
No. 1
4
I
3
Operating
No 2A
5
II
3
Secured
No. 2A
6
III
1.5
Operating
No. 2A
7
I
3
Operating
No. 2B
8
II
3
Secured
No. 2B
9
III
1.5
Operating
No. 2B
NOTE: If two speed motor, the fan shall be operated at fast speed.
4.8.2 Vibration test. Types A and L fan-motor units shall be subjected to Type I environmental vibration tests
as specified in MIL-STD-167-1. The vibration test shall be conducted as specified in 4.10. Any unit that fails to
meet any requirements shall be rejected.
4.8.3 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 ¼ 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 this suspension test, rigid brackets shall be
bolted to the fan end flanges, and these brackets shall be elastically mounted on a rigid, level floor. Another alternative
is that 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
more than 35 inches, and the link size of the chains shall be not less than ¼ inch nor more than inch. With either
method, the internally excited vibrational displacement (single amplitude of vibration) shall be measured in mils as
follows: four axial measurements 90 degrees apart on the face of one end flange, one horizontal measurement
perpendicular to the axis on the edge of each flange, and one vertical measurement on the top edge of each flange. The
maximum amplitude of vibration shall be the maximum single reading, and not an average. The operating frequency at
which the maximum amplitude occurs shall be measured. Exceeding vibration limit specified in 3.13.3 is cause for
rejection.
4.8.4 Performance. Performance tests shall be conducted in accordance with AMCA 210. Any of the alternate
test stands and associated instrumentation may be used. The total pressure or the volume or both not being within
the specified limits is cause for rejection.
4.8.4.1 Stall and recovery. Test results from AMCA 210 meet the requirements of 3.12.1.2 and 3.12.2.2.
a.
Draw a vertical line through the highest point on the continually rising portion of the fan total pressure
(TP) curve. The point at the highest pressure shall be established by a test point. If a fan is represented
as having constantly rising pressure characteristics from free delivery to cut-off, it shall be the
manufacturer's responsibility to provide such characteristics with sufficient test points.
b.
Locate a point on the vertical line defined above at a pressure 1-inch wg less than the highest point.
c.
Draw a system resistance line through the point defined in b above and intersecting the TP curve.
15
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