MIL-DTL-83729C
3.4 Materials. Materials shall be as specified herein. However, when a definite material is not specified, the
material used shall be of such quality as to ensure that the chopper meets all requirements of this specification.
Materials used externally shall be fungus inert, self-extinguising and shall not support combustion, nor give off
noxious gases in harmful quantities. Materials used internally shall not give off gases in quantities sufficient to cause
explosion of sealed enclosures, cause contamination of the contacts or other parts of the chopper that will adversely
affect life or reliability, or form current-carrying tracks when subjected to any of the tests specified herein. Cotton-
filled or wood-flour-filled materials shall not be used. Ceramic used for external surfaces shall be glazed. The use of
silicone or silicone compounds for any purpose is prohibited. The selection of materials shall be such as to provide
maximum shelf life. Acceptance of approval of any constituent material shall not be construed as a guaranty of the
acceptance of the finished product.
3.4.1 Metals. Metals shall be of a corrosion-resistant type or shall be plated or treated to resist corrosion. The
use of mercury or mercury compounds is prohibited. The use of magnesium or magnesium alloys is prohibited (not
applicable to contact systems).
3.4.1.1 Plated finishes.
(a)
Pure tin plating is prohibited internally and externally. Tin-lead finish is acceptable, provided that
the minimum lead content is 3 percent.
(b)
Pure zinc plating is prohibited internally and externally.
(c)
Pure cadmium plating is prohibited internally and externally.
3.4.2 Dissimilar metals (not applicable to hermetically sealed choppers). When dissimilar metals are used in
intimate contact with each other, protection against electrolysis and corrosion shall be provided. The use of dissimilar
metals in contact, which tends toward active electrolytic corrosion (particularly brass, copper, or steel used in contact
with aluminum or aluminum alloy), is not acceptable. However, metal spraying or metal plating of dissimilar base
metals to provide similar or suitable abutting surfaces is permitted. Dissimilar metals are defined in 6.4. In hermetic
seals, the 0.25 V difference between the header material and the housing material is not acceptable.
3.5 Interface and construction. Choppers shall be of the design, construction, physical dimensions, and weight
specified (see 3.1 and 6.2.2). Choppers shall be so constructed as to insure proper operation when mounted in any
3.5.1 Enclosure. Enclosures shall be hermetically sealed (see 6.5) unless otherwise specified (see 3.1 and 6.2.2).
Enclosures may also be sealed or enclosed.
3.5.2 Coil (drive). The coils shall be evenly would and insulated to meet the performance requirements specified
herein. The coils shall be completely insulated from the frame and other grounded parts, and unless otherwise
specified (see 3.1 and 6.2.2), coil leads shall not be internally grounded. Coils shall be securely anchored to the
frame or other stationary parts in a manner to prevent coil movement or strain on the coil leads.
3.5.3 Contact arrangement. Choppers shall have contact arrangements as specified (see 3.1 and 6.2.2).
3.5.4 Threaded parts. Unless otherwise specified (see 3.1 and 6.2.2), all threaded parts shall be in accordance
with FED-STD-H28. Wherever possible, unified screw threads shall be used.
3.5.5 Shielding. The chopper coil shall be shielded and such shielding shall provide effective shielding between
the coil and contacts. Shielding connections shall be compatible with the specified base connections (see 3.1 and
6.2.2).
3.5.6.1 Plug-in terminations. Plug-in terminations shall conform to the arrangements and dimensions necessary
for proper mating with the applicable connectors or sockets covered by SAE-AS50151, MIL-DTL-28748, or MIL-DTL-
12883. The mounting arrangement of the chopper and its corresponding socket shall be so designed that the entire
weight of the chopper will be supported and the stability of its mounting will be provided by an auxiliary mounting
means other than the electrical terminals of the socket.
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