Reliability Prediction

Abstract

Whilst it is component failure rate that is measured the reliability of complete equipment and systems is the ultimate concern of the designer and customer. Reliability prediction is the process of calculating the anticipated system reliability from assumed component failure rates. It provides a quantitative measure of how close a design comes to meeting the reliability objective and also permits comparisons between alternative design proposals. The simplest type of prediction involves little more than a parts count. Individual stress levels are not considered and an average failure rate for each component type is multiplied by the number involved. The overall total failure rate is used to calculate the system MTBF or reliability. It will be seen in section 15.3 that this simple addition of failure rates takes no account of redundancy and therefore gives a worst case prediction. It was mentioned in section 7.5 that failure rate data usually refers to random failures (flat portion of the bathtub). As a result ‘parts count’ reliability predictions involve constant failure rates and the summing of failure rates is permissible. This is not always the case and the exceptions to this procedure will be clearly explained in this chapter. As the design details become firmer more sophisticated predictions can be attempted taking account of failure modes, redundancy of parts and modules, stresses and environment and the quality and screening of components. Some examples of typical failure rate data are given in Appendix 3 and are expressed in terms of 10⁻⁹ per hour.