We have so far in this book really been concerned with the component, the building brick on which everything else depends as it were, though we have found it necessary to relate the component to the system in connection with maintenance and to some extent in connection with design. We must now tackle the vexed question of system or equipment reliability. The problem is the assessment of the multi-component system reliability given that of the individual maintained components. This is usually required for the selection of the most cost-effective system, or for redesigning the system for higher reliability. It is generally accepted that this is feasible for electronic systems, and even if the accuracy of the assessment is not very high, it is believed that alternative competing systems can be placed in the correct order of merit. This greatly assists decision making at the feasibility and early design stages. It is sometimes claimed that similar techniques can be applied to mechanical systems but, since it has been observed on numerous occasions in this book that there are significant differences between mechanical and electronic behaviour, it would be unwise to assume that electronic practice can be carried over to mechanical systems. We shall therefore examine mechanical system reliability assessment from first principles, but pay attention to those techniques which have proved successful in the electronic world.
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