MIL-PRF-22E
3. REQUIREMENTS.
3.1 Associated specifications. The individual part requirements shall be as specified herein and in accordance
with the applicable associated specifications. In the event of a conflict between requirements of this specification and
the associated specifications, the latter shall govern (see 6.2).
3.2 Qualification. Resistors furnished under this specification shall be products which are qualified for listing on
3.3 Materials. Materials shall be used which will enable the resistors to meet the performance requirements of this
specification. Acceptance or approval of any constituent material shall not be construed as a guaranty of the
acceptance of the finished product.
3.3.1 Fungus. All external materials shall be nonnutrient to fungus growth or shall be treated to retard fungus
growth. The manufacturers shall certify that all external materials are fungus resistant or shall perform the test
specified in 4.7.16. There shall be no evidence of fungus growth on the external surface as a result of the test.
3.3.2 Tin plated finishes. The us of pure tin, as an underplate or final finish is prohibited both internally and
externally. Tin content of resistor components and solder shall not exceed 97 percent, by mass. Tin shall be alloyed
with a minimum of 3 percent lead, by mass (see 6.8).
3.4 Interface and physical dimensions. The resistors shall meet the interface and physical dimensions specified
(see 3.1).
3.4.1 Windings. The resistance element shall consist of a winding of resistance wire or ribbon (see 3.4.1.1) on a
form which shall not char or break down at any combination of temperatures and loads specified herein. The
resistance element shall be wound in such a manner to preclude the possibility of shorting the turns. Joints, welds
and bonds shall be held to an absolute minimum, the wire shall be securely set on the form so as to avoid lateral
displacement or bunching under action of the contact arm. The insulating coating between the wires shall be below
the contact surface of the winding.
3.4.1.1 Resistance wire. The resistance wire shall possess a uniform cross sectional area of conductor. The
cross sectional area of the wire shall be the maximum consistent with other requirements of this specification.
3.4.2 Contact arm assembly. Contact pressure on the resistance element shall be maintained by uniform positive
pressure and shall permit smooth electrical and mechanical control of the resistor over the entire range of continuity
travel (applicable to resistors with stops). The moving contact shall have continuous electrical contact with its
terminal throughout the entire mechanical travel and shall be insulated from the operating shaft, bushing, and case.
3.4.3 Operating shaft. All operating shafts shall be of corrosion resistant metal.
3.4.3.1 Size. The diameter and length of the operating shaft shall be as specified (see 3.1 and tables II and III.)
3.4.3.2 Flatted shaft (when applicable). Flatted shafts shall have their flatted surface within ±3O of being
diametrically opposite the contact arm. The flatted surface with the exception specified in footnote 1/ of table III shall
have a minimum length of 0.625 inch for resistors with a high power rating of 150 watts or less and a minimum length
of 1.5 inches for resistors of higher power ratings, or it shall be flatted to within 0.156 inch of the mounting bushing (or
mounting surface if no bushing present), whichever length is shorter. Thickness of the flatted portion shall be as
specified (see 3.1).
3.4.3.3 Slotted shaft (when applicable). The dimensions shall be as specified. In no case shall the slot be deep
enough to interfere with shaft retainers. The slot shall be so positioned that the centerline is within ±10O of the
centerline of the contact arm (see 3.1).
3.4.4 Stops. Stops employed to limit the mechanical travel of the contact arm assembly may be part of, but shall
not complete, any electrical circuit. In no case shall the mechanical stop contact the electrical pick off portion of the
contact assembly.
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