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Reliability in Design

Published by the American Society of Agricultural and Biological Engineers, St. Joseph, Michigan

Citation:  Reliability in Design. ASAE Distinguished Lecture No. 13, pp. 1-27. Winter Meeting of the American Society of Agricultural Engineers, 14 December 1988, Chicago, Illinois  913C0888.(doi:)
Authors:   Ray A. Brandt and William W. Brixius
Keywords:   The Importance of Reliability, Definition of Reliability, Importance to Customer, Importance to Manufacturer, Development of Product Reliability, Product Definition, Reliability Goals and Apportionment, Design Analysis, Failure Mode and Effect Analysis (FMEA), Multifunctional Design Reviews, Testing, Laboratory and Stress Tests, Complete Machine Tests, Test Time Requirements, Correlation Factors, Reliability Growth and Status Reporting, Growth Curves, Reliability Calculations, Tools and Procedures, Weibull Analysis, Theory, Median Ranks, Suspended Items, Accelerated Tests, Sudden Death Testing, Sequential Analysis, Reliability Projections Using Redesign Credits, Bayes Method, Risk Analysis, Warranty Data Analysis, Reliability of a Part, Reliability of a Component, Reliability of a Complete Machine, Failure Index, Sources of Failure, Reliability in Production, Early Problem Reporting, Quality Audit, Customer Surveys, Post-Warranty Information, Corrective Action

Good reliability is critically important in all products. The customer's expectation for quality and reliability is provided by more and more manufacturer's of various products. Competition and changing customer expectations are continually increasing the acceptable level of reliability that must be provided.

In simple terms reliability depends upon:

good design

capable manufacture (including procurement) and

continuing elimination of problems.

The design process occurs first and involves making many critical decisions so it largely determines the reliability level of a product. Therefore, it is important to concentrate efforts for achieving good reliability at the design and development phase of the product cycle.

The efforts worthy of being incorporated are included in reliability engineering methodology. Of course, good design and development engineering practice are important. What reliability technology has brought to product design and development is the quantification and management of the level of reliability along with some methods that guide and improve the level.

There are now good methods available to assess the reliability levels of existing product as well as to estimate and project the levels for new product before routine manufacture. In very general terms quantifying and managing the reliability of a new product involves:

definition of the product

determining the level of reliability needed (expected) by the customer

stating this needed level in quantitative terms that can be verified by measurement through testing and/or analytical methods

establish special program efforts to develop and prove this level

measure the results and compare them to the objective

provide correction for any problems that preclude meeting the objective

feed forward to the next design project changes in the development system that will improve management or reliability.

These generally stated activities are described in more specific terms in this report. They fit together in what is best thought of as an evolutionary process. Fig. 1 shows more specific efforts and where they relate with significant design-development efforts as the product phase advances. This figure shows the product phase in the inner ring, the reliability engineering activity in the outer ring, and the concept of feedback with the arrows.

None of the "reliability engineering" efforts have to be done by a reliability or quality engineer. In some organizations they may be sponsored or done by other functions. It is merely important that they are done. Efforts such as product definition is generally done by marketing and product engineers. It is mentioned as a reliability engineering activity because it requires the reliability engineer's involvement and often his sponsorship. There are other activities that are similarly related.

The earlier program activities are the most important. Prevention of problems is more efficient than detecting and correcting problems. Of course, problem detection through tests and analytical methods and subsequent correction of these problems is important too. After all they are a portion of the continuing elimination of problems mentioned above. Up-front or prevention efforts, however, are emphasized.

Much of the reliability methodology now available was originally developed for electronic systems. Many of these methods are applicable to mechanical systems. Other methodology has been developed or evolved to fit mechanical systems. This paper stresses those methods that apply to mechanical system and component development.

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