MIL-DTL-28778A
Mounting and terminal screws and mounting inserts. Screw threads shall be class 2A or 2B,
3.5.1
as applicable, in accordance with FED-STD-H28. External screw threads, class 2 fit, shall, after receiving a finish, be
capable of accepting a nut of class 2B fit and internal screw threads, class 2 fit, shall, after receiving a finish, be
capable of accepting a screw of class 2A fit with maximum installation torque in accordance with the following:
Screw Size
Torque
(pound-inches)
4-40 - - - - - - - - - - - -
3
6-32 - - - - - - - - - - - -
5
8-32 - - - - - - - - - - - -
6
10-32 - - - - - - - - - - -
8
¼-20- - - - - - - - - - - -
8
¼-28- - - - - - - - - - - -
8
5/16-18- - - - - - - - - -
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Nuts shall run down to within two threads of mounting surface.
Screw terminals. External screw terminals shall be supplied with two nuts, two flat washers,
3.5.2
and one lock washer. For cased units, the height of the terminal assembly shall be the distance from the free end of
the screw to the terminal mounting surface. The type of terminal, size of screw thread, and the exposed length of
threads ± 1/16 inch shall be as specified (see 6.2)
3.5.3
Internal wire leads. Internal wire leads shall be attached to the coils and other internal
components and terminals or case by soldering, welding, brazing, or other method (that is, load-sweating of nylon-
coated wires) in such a manner as to provide adequate electrical connection and mechanical strength. Where soft
solder is used to provide the electrical connection, wire leads shall be anchored mechanically.
3.5.4
Core and coil mounting. Cores and coils shall be secured rigidly to prevent any permanent
change in the relative position of the parts. The means of securing the core and coil to the devices for mounting the
transformer in the equipment (that is, studs, lugs, inserts, brackets and so forth) shall not depend on soft solder alone
for mechanical strength, nor shall the transmission of the mechanical load of the core to the mounting device depend
only on soft solder. The core shall be grounded to the case, or shall be electrically accessible.
3.5.5
Paint color. The color of the paint shall be light gray, semi-gloss, formula No. 111, as
specified in MIL-DTL-15090. Unless otherwise specified, the manufacturer shall omit paint from the mounting area
surface (see 6.2).
3.5.6
Potting, filling, or encapsulating material. The amount and coverage of potting, filling, or
encapsulating material used shall be essentially the same for all units of a specific design. Potting, filling, or
encapsulating material shall not flow from the case of the transformer during any of the applicable tests.
3.5.7 Mechanical construction. The mechanical construction of the transformer shall be as shown
in figure 1.
3.6 Electrical characteristics. When transformers are tested as specified in 4.6.2 through 4.6.2.7 the
electrical characteristics and tolerances shall be as follows:
3.6.1 Primary terminals. The terminals shall be stud type and shall be grouped to minimize
misconnections. Each phase of the primary shall have 2 windings for series or parallel operation. Primary terminals
shall be numbered as specified in table I.
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