Concluding remarks

Abstract

The study of ‘reliability’ in this book has been aimed at general mechanical equipment. The intention has been to present a theoretical background to the subject and build on that to more general aspects. The reliability of electronic and electrical equipment has been specifically excluded, but because that field has been so fully covered and because so much progress has been made, and because the achievements in that field have often been held up as an example to mechanical engineers, comparisons between the two topics have been made whenever appropriate. Thus it is hoped that the reader will have become aware of substantial differences in application, though the underlying theory is exactly the same. There can be little doubt that mechanical reliability is a much more complicated subject. This arises partly from the much greater diversity of mechanical products, and from the far greater range of operating conditions they can be expected to encounter: but it is also in a large way due to the preponderance of wear-out phenomena. Wear-out is a complicated subject, markedly non-linear, and totally inadequately understood and therefore modelled. Although a lot of good work has been done in this field, it has concentrated on the initial stages of the process, to reduce wear and hence prolong life: the life of intrinsic reliability in the phraseology of this book. For firm believers in intrinsic reliability this is the right priority. It does mean, however, that very little is known of the final stages and what controls the wear-out failure pattern. This field is one of almost total empiricism. Yet it is surprising how often estimates based on empirical information and sound practical experience can be used to great practical purpose and advantage, particularly when those making the estimates fully appreciate all the basic mechanisms and factors involved and how far to push empiricism. As a consequence we find that mechanical reliability is very high indeed — far higher than many of its detractors would have us believe, especially when not deliberately sacrificed for performance and economy. Much equipment, properly used, goes on for ever and ever, and is still durable when out-moded. Maybe we have not made great advances in mechanical reliability during the last decade or so — but then, we did not start from the thermionic valve!