![]() ![]() Over-riding all of these aspects, though, is the management of the reliability engineering effort. Unfortunately not all reliability training, literature and practice reflect this reality. Mathematical and statistical methods can make valuable contributions in appropriate circumstances, but practical engineering must take precedence in determining the causes of problems and their solutions. Therefore the role of mathematical and statistical methods in reliability engineering is limited, and appreciation of the uncertainty is important in order to minimise the chances of performing inappropriate analysis and of generating misleading results. In practice the uncertainty is often in orders of magnitude. However, because of the high levels of uncertainty involved these can seldom be applied with the kind of precision and credibility that engineers are accustomed to when dealing with most other problems. The basic methods are described in Chapter 2, to provide an introduction for some of the applications described subsequently. Mathematical and statistical methods can be used for quantifying reliability (prediction, measurement) and for analysing reliability data. The primary skills are nothing more than good engineering knowledge and experience, so reliability engineering is the first and foremost the application of good engineering, in the widest sense, during design, development, manufacture and service. It is also necessary to have knowledge of the methods that can be used for analysing designs and data. The primary skills that are required, therefore, are the ability to understand and anticipate the possible causes of failures, and knowledge of how to prevent them. The reason for the priority emphasis is that it is by far the most effective way of working, in terms of minimizing costs and generating reliable products. To apply methods for estimating the likely reliability of new designs, and for analysing reliability data. To determine ways of coping with failures that do occur, if their causes have not been corrected. To identify and correct the causes of failures that do occur, despite the efforts to prevent them. To apply engineering knowledge and specialist techniques to prevent or to reduce the likelihood or frequency of failures. The objectives of reliability engineering, in the order of priority, are:ġ. Reliability can also be expressed as the number of failures over a period.ĭurability is a particular aspect of reliability, related to the ability of an item to withstand the effects of time (or of distance travelled, operating cycles, etc.) dependent mechanisms such as fatigue, wear, corrosion, electrical parameter change, etc.ĭurability is usually expressed as a minimum time before the occurrence of wearout failures. The probability that an item will perform a required function without failure under stated conditions for a stated period of time. This results in the usual engineering definition of reliability as: Whether an item will work for a particular period is a question which can be answered as a probability. Reliability is therefore an aspect of engineering uncertainty. Whether failures occur or not and their times to occurrence, can seldom be forecast accurately. This distinction marks the difference between traditional quality control and reliability engineering. On the other hand, reliability is usually concerned with failures in the time domain. ![]() The product either passes a given test or it fails. The inspectors' concept is not time-dependent. We therefore come to the need for a time-based concept of quality. ![]() In any case, the manufacturer will also probably incur a loss of reputation, possibly affecting future business. Outside the warranty period, only the customer suffers. If it fails often, the manufacturer will suffer high warranty costs, and the customers will suffer inconvenience. Even within a warranty period, the customer usually has no grounds for further action if the product fails once, twice or several times, provided that the manufacturer repairs the product as promised each time. However, this approach provides no measure of quality over a period of time, particularly outside a warranty period. This simple approach is often coupled with a warranty, or the customer may have some protection in law, so that he may claim redress for failures occurring within a stated or reasonable time. The customer, having accepted the product, accepts that it might fail at some future time. The simplest, purely producer-oriented or inspectors' view of reliability is that in which a product is assessed against a specification or set of attributes, and when passed is delivered to the customer. ![]()
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