MIL-PRF-39015 Resistors, Variable, Wirewound, \(Lead Screw Actuated\), Nonestablished Reliability, And Established Reliability, General Specification ForThis specification covers the general requirements for nonestablished reliability \(non-ER\) and established reliability \(ER\), lead screw actuated; wirewound variable resistors with a contact bearing uniformly over the surface of the entire resistive element, wound linearly, when positioned by a multiturn lead screw actuator. These resistors are capable of full load operation \(when the maximum resistance is engaged\), at a maximum temperature of 85OC and are capable for continuous operation, when properly derated, at maximum temperature of 150OC \(see 3.6\), with a resistance tolerance of ??5 percent. The ER resistors have life failure rate \(FR\) levels ranging from 1.0 percent to 0.001 percent per 1,000 hours \(see 1.2.1.4\). These FR levels are established at a 60 percent confidence level on the basis of life tests.

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MIL-PRF-39015E

4.8.23 Rotational life (see 3.29).

4.8.23.1 Mounting. Resistors shall be mounted in such a manner as to allow electrical connections to

be made to the terminals and concurrent contact arm actuation of each "pair" of resistors. The resistors

shall be ganged in pairs, and each pair shall be connected in series, as shown in figure 4, so that a

nominally constant current flows through the resistors, irrespective of the contact arm position during the

turning of the operating shaft.

FIGURE 4. Rotational life test circuit.

4.8.23.2 Procedure. Total resistance shall be measured as specified in 4.8.4.1. A dc potential,

equivalent to that required to dissipate rated wattage across the entire resistive element of resistors

having the same nominal total resistance as those under test, shall then be applied as shown in figure 4.

The lead screw actuators shall be continuously cycled through 90 percent to 100 percent of the actual

effective electrical travel, at the rated one cycle for 2.5 minutes, for a total of 200 cycles. A cycle shall

consist of travel through 90 percent to 100 percent of actual effective electrical travel and return to the

starting point. At no time during this test shall the contact arm be allowed to idle at either end of the

travel. After rotation, total resistance shall be measured as specified in 4.8.4.1. Resistors shall then be

examined for evidence of mechanical damage.

4.8.24 Terminal strength (see 3.30).

4.8.24.1 Pull (applicable to all terminal types). Resistors shall be tested in accordance with

method 211 of MIL-STD-202. The following detail and exception shall apply:

a. Test condition A: Applied force 2 pounds. Resistors clamped by the resistor body, force applied

to each lead individually.

b. Measurement after test: Resistors shall be examined for evidence of mechanical damage, and

tested for electrical continuity.