MIL-PRF-24751A(SH)
in the wall separating the chamber from the reverberant room, and to ensure that no noise contributions are
transmitted from the chamber into the reverberant test room.
4.6.3.5 Airborne noise detected by ear. Fan-motor units not undergoing airborne noise testing described in
4.6.3.1 through 4.6.3.4 shall be operated at their design points. Any unusual airborne noise or any airborne noise
thought to be excessive shall be corrected if the cause is apparent. If the cause is not apparent and the condition is
not corrected, the fan-motor unit shall be tested in accordance with 4.6.3, and sound power levels shall be computed
to determine conformance with sound power level requirements specified in Tables II and III.
4.6.3.6 Airborne noise test facility. The facility wherein measurements of fan-motor unit airborne noise are
performed shall be registered in accordance with AMCA 111 as being a qualified facility in which to perform octave
band sound measurements in accordance with AMCA 300.
4.6.4 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 ⅜ 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 each 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 3.150.2 cause for
rejection.
4.6.5 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 from 4 Hz
up to and including 33 Hz at the table amplitude specified herein. The vibration test shall be conducted as specified
in 4.6.12. Any unit, which fails to meet any requirements, shall be rejected.
4.6.6 Casing tightness. When this test is required, the requirements as well as the test conditions shall be
specified (see 6.2). If conditions are not specified, the test shall be conducted as follows: Blind flanges (one with air
supply and gauge connections) shall be bolted to gasketed fan flanges. Air at a pressure of 50 percent higher than
the maximum pressure capability of the fan shall be supplied to the casing interior. The pressure drop in 10 minutes
shall not exceed 5 percent of the test pressure.
4.6.7 Speed. The fan-motor unit shall be operated at free delivery to determine whether the speed conforms
approximately to design speed at rated voltage.
4.6.8 High-impact shock. The fan-motor unit shall be shock tested on the medium weight machines as
specified for type A of MIL-S-901. The high-impact shock test shall be conducted after the tests specified in 4.6.1,
4.6.2, 4.6.3, 4.6.4, 4.6.5, 4.6.6, 4.6.7, 4.6.9, 4.6.10, and 4.6.11. After conducting the high-impact shock test, tests
specified in 4.7.1 and 4.7.2 shall be again conducted without correction of damages, which 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 I, and aerodynamic stall with capacities not to occur greater than 90 percent of the fan
design or rated ft3/min. Evidence of fragmentation or missile effect of part, deformation that will cause active
interference between parts, or failure to operate shall be cause for rejection. In this test, bolts shall be used in each
of the flange boltholes. Resilient mountings 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.
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