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

4.8.10 Torque (see 3.16).

4.8.10.1 Operating (see 3.16.1). The maximum operating torque required to move the contact arm on

the resistive element shall be determined at approximately 10 percent, 50 percent, and 90 percent of the

actual effective electrical travel by the torque wrench or by any other method acceptable to the

government.

4.8.10.2 Clutch (see 3.16.2). The contact arm shall be adjusted to each extreme limit of mechanical

travel, and sufficient torque shall be applied to the lead screw actuator to permit the contact arm to idle for

25 complete mechanical turns of the lead screw actuator. During idle, an electrical indicating device

connected between the contact arm terminal and the adjacent end terminal shall be observed for

electrical continuity. After idle, the lead screw actuator shall be rotated in the opposite direction and the

indicating device observed to determine if the contact arm reversed direction without ohmmeter

discontinuity.

4.8.11 Thermal shock (see 3.17). Resistors shall be tested in accordance with method 107 of

MIL-STD-202. The following details and exceptions shall apply:

a. Test condition: B.

b. Measurement before cycling: Total resistance and setting stability shall be measured as specified

in 4.8.4.1 and 4.8.11.1, respectively.

c. Measurements after cycling: Setting stability, total resistance, and continuity shall be measured as

specified in 4.8.11.1, 4.8.4.1, and 4.8.6, respectively.

d. Examination after test: Resistors shall be examined for evidence of mechanical damage.

4.8.11.1 Setting stability. The contact arm shall be set at approximately 40 percent of the actual

effective electrical travel. An adequate dc test potential shall be applied between end terminals. The

voltage between the end terminals, and the voltage between one end terminal and the contact arm, shall

be measured and applied to the following formula:

Setting stability = E1 x 100

(in percent)

where:

E1 = Voltage across one end terminal and the contact arm terminal.

E2 = Voltage across the end terminals.

The difference between the initial measurement made before the environmental test and the

measurement made after the test indicates the setting stability in percent.

4.8.12 Solderability (applicable to terminals P, W, X, and Y) (see 3.18). Resistors shall be tested in

accordance with method 208 of MIL-STD-202. The 3 pin terminals of each resistor shall be tested.