What makes a part reliable? When asked, many people would say a reliable part is one purchased according to a certain source control document and bought from an approved vendor. Unfortunately, these two qualifications are not always guarantees or reliability. The following case illustrates this problem. A clock purchased according to PD 4600008, procured from an approved vendor for use in the ground support equipment of a missile system was subjected to qualification tests as part of the reliability program. These tests consisted of high and low temperature, mechanical shock, temperature shock, vibration, and humidity.
A failure analysis revealed revealed that lubricants in the clock's mechanism from humidity. A second approved vendor was selected. His clocks failed the high-temperature test. In the process, the dial hands and numerals turned black, making readings impossible from a distance of 2 ft. A third approved vendor's clocks passed all the tests except mechanical shock, which cracked two of the caes. Ironically , the fourth approved vendor's clocks, through less expensive, passed all the tests.