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

4.8.19.2 Procedure. Total resistance shall be measured as specified in 4.8.4.1. The resistors shall be

placed in a chamber at room temperature. The temperature shall be gradually decreased to -55oC +0oC,

-5oC within a period of not less than one and one-half hours. For conformance inspection only, and at

the option of the manufacturer, the resistors may be placed in the chamber when the chamber is already

at the extreme low temperature. After one hour of stabilization at this temperature, setting stability shall

be measured as specified in 4.8.11.1. Full rated continuous working voltage (see 3.1 and 3.7) shall be

applied for 45 minutes. The resistors may be loaded individually or in parallel. Fifteen minutes +5

minutes, -0 minutes after removal of voltage, setting stability shall be measured as specified in 4.8.11.1.

The temperature in chamber shall be gradually increased to room temperature within a period of not more

than 8 hours. The resistors shall be removed from the chamber and maintained at a temperature of 25oC

�5oC for a period of approximately 24 hours. Total resistance shall be measured as specified in 4.8.4.1.

Resistors shall then be examined for evidence of mechanical damage.

4.8.20 Low temperature storage (for qualification only) (see 3.26).

4.8.20.1 Mounting. Resistors shall be mounted by their normal mounting means and in such a position

with respect to the air stream that the mounting offers substantially no obstruction to the flow of air across

and around the resistors.

4.8.20.2 Procedure. Total resistance shall be measured as specified in 4.8.4.1. Within one hour after

this measurement, the resistor shall be placed in a cold chamber at a temperature of -65�C �2�C for a

period of 72 hours +8 hours, -0 hours. The resistors shall then be removed from the chamber and

maintained at a temperature of 25�C �5�C until thermal stabilization is achieved. Total resistance shall

be measured as specified in 4.8.4.1. Resistors shall be examined for evidence of mechanical damage.

4.8.21 High temperature exposure (see 3.27).

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

be made to the terminals.

4.8.21.2 Procedure. Total resistance and setting stability shall then be measured as specified in

4.8.4.1 and 4.8.11.1, respectively. The resistors shall then be exposed to an ambient temperature of

150oC +5oC, -0oC for a period of 1,000 hours �8 hours. Not less than 2 hours after the end of the

exposure period, setting stability and total resistance shall be measured as specified in 4.8.11.1 and

4.8.4.1, respectively. Dielectric withstanding voltage (at atmospheric pressure), and insulation resistance

shall be measured as specified in 4.8.8.1 and 4.8.9, respectively. Resistors shall be examined for

evidence of mechanical damage.

4.8.22 Integrity of shaft (see 3.28).

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

be made to the terminals.

4.8.22.2 Pull force. A force of 5 pounds shall be applied along the axis of the operating shaft away

from the body of the resistor. The force shall be maintained for a minimum of 1 minute.

4.8.22.3 Perpendicular. A force of 2 pounds shall be applied in a direction perpendicular to the axis of

the operating shaft for a minimum of 1 minute.

4.8.22.4 Examination after test. Resistors shall be examined for evidence of shaft breakage.