MIL-DTL-917F(SH)
3.2.18.3 Stray field source. DC equipment and DC circuits in or associated with the equipment shall be
designed in accordance with DOD-STD-2143 for a Class 3 magnetic field source. Additional guidance on proper
design of DC equipment and DC circuits in or associated with the equipment is provided in MIL-HDBK-802.
3.2.19 Noise reduction. Control of noise generated by equipment shall be accomplished by proper design of
the equipment and of those parts which are inherently a source of airborne and structureborne noise, including
dynamic balancing of rotating equipment. The use of exterior soundproof enclosures to meet noise requirements is
prohibited, except where specified by the individual equipment specification or the contract (see 6.2).
3.2.19.1 Airborne noise. When specified in the individual equipment specification or in the contract (see 6.2),
the equipment shall be tested for airborne noise in accordance with MIL-STD-1474. Acceptable levels shall be as
specified in the individual equipment specification or in the contract (see 6.2).
3.2.19.2 Structureborne noise. When specified in the individual equipment specification or in the contract
(see 6.2), the equipment shall be tested for structureborne noise in accordance with MIL-STD-740-2. Acceptable
levels shall be as specified in the individual equipment specification or in the contract (see 6.2).
3.2.19.3 Dynamic balancing. For rotating equipment to be used on submarines, or when specified in the
equipment specification (see 6.2), provision for dynamic balancing shall be included as specified in 3.2.20.
3.2.20 Provisions for dynamic balancing. For rotating equipment to be used on submarines, each rotor shall be
provided with at least two accessible balancing rings or discs. On double armature machines whose maximum
operating speed exceeds the first critical speed of the lateral flexibility of the shaft, three balancing rings shall be
provided. One shall be between the armatures. The design of the rings shall have the following features:
a. It shall be possible to add weights of various sizes to the balancing rings in a plane perpendicular or parallel
to the shaft axis, either in a continuous groove or 12 or more evenly spaced positions.
b.
Means shall be provided to lock the weights in place against centrifugal force and vibration.
c. It shall be possible to place or remove any weight without disassembly of large covers or disturbing the
bearing alignment and with minimum danger of loss of small parts inside the machine.
d. It shall be possible to add in a space of 30 degrees in each plane, a total of 10 W/n ounce-inches of balance
weights in increments of W/n or less.
Where:
W = the weight of the rotor in pounds.
n = the maximum operating speed of rotation in revolutions per minute (r/min).
e. Where balance is to be achieved by the use of tapped holes and movable screws, each hole shall be
permanently numbered. Where balance is to be achieved by the use of a continuous groove, the angular locations
shall be permanently marked at intervals of not more than 10 degrees, and the markings shall be permanently
numbered at intervals of not more than 30 degrees. All markings and numbering shall be readily observable through
access openings.
f. The balancing planes shall be located as close to the rotating mass as feasible, while allowing accessibility
for balance correction.
g. Balance weight locations shall be accurately located both angularly and radially. The bottoms of radially
tapped holes shall be accurately located. Angular dimensions shall be held within ±1 degree and all linear
dimensions within ±1/64 inch.
h. In order that refinement of balance after installation may be accomplished without overcrowding the
balancing rings, factory balancing shall be generally accomplished by removal or addition of weight at points other
than the balancing rings.
3.2.21 Leakage current. Leakage current through a human body (see 6.5.5) shall be in accordance with
MIL-STD-1399-300. This applies even where EMI suppression devices are permitted and used.
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