What is Reliability Design
Consumer products, industrial machinery, and military equipment are intently evaluated for reliability of performance and life expectancy. Although the “military” and particular industrial users (for example, power plants both fossil fuel and nuclear fuel) have always followed some sort of reliability programs, consumer products have of late received the widest attention and publicity. One of the most important foundations for product reliability is its design, and it is apparent that the designer should at least be acquainted with some of the guidelines. Thomas Sabo Rings
The article entitled “A Manual of Reliability” offers the following definition of reliability: “Reliability is the probability that a device will perform without failure a specific function under given conditions for a given period of time” . From this definition, we see that a thorough and in-depth analysis of reliability will involve statistics and probability theory.
All products, systems, assemblies, components and parts exhibit different failure rates over their service lives. Although the shape of the curve varies, most exhibit a low failure rate during most of their useful lives and higher failure rates at the beginning and end of their useful lives. The curve is usually shaped like a bathtub as is shown in Fig. 5. 1. Infant mortality of manufactured parts occurs because a certain percentage, however small, of seemingly identical parts are defective. If those parts are included in a system, the system will fail early in its service life. Product warranties are usually designed to reduce customer losses due to infant mortality. Parts wear out due to friction, overload, plastic deformation, fatigue, changes in composition do to excessive heat, corrosion, fouling, abuse, etc.
The reliability of a part or system is a function of many factors. Reliability is related to failure rate by equation (5-1).
R = 1-F (5-1)
Where R is reliability, 0<.R<1; F is failure rate, 0<1.
Part and system reliability is affected by manual machining and assembly operations. Human reliability in performance is increased by proper training, low emotional stress level, proper attention to human factors in machine design, comfortable environment, etc. Human reliability in machining is increased by decreasing part complexity and increasing tolerances. Human reliability in assembly is increased by decreasing the complexity of the assembly process. Adequate design for manufacturing and assembly should keep reliability high.
The design function of engineering should include an examination of reliability and should seek to provide adequate reliability in a part or system commensurate with its use. When the safety of people is concerned, product reliability with respect to potential injury producing failure must be very high. Human health and safety cannot be compromised for the sake of profit.
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