Figure 2-1: Graphical Representation of balanced and unbalanced projects.
Reliability engineering provides the theoretical and practical tools whereby the probability and capability of parts, components, equipment, products and systems to perform their required functions for desired periods of time without failure, in specified environments and with a desired confidence, can be specified, designed in, predicted, tested and demonstrated. [19]
This section includes the following subsections:
Reliability engineering assessment is based on the results of testing from in-house (or contracted) labs and data pertaining to the performance results of the product in the field. The data produced by these sources are utilized to accurately measure and improve the reliability of the products being produced. This is particularly important as market concerns drive a constant push for cost reduction. However, one must be able to keep a perspective on "the big picture" instead of merely looking for the quick fix. It is often the temptation to cut corners and save initial costs by using cheaper parts or cutting testing programs. Unfortunately, cheaper parts are usually less reliable and inadequate testing programs can allow products with undiscovered flaws to get out into the field. A quick savings in the short term by the use of cheaper components or small test sample sizes will usually result in higher long-term costs in the form of warranty costs or loss of customer confidence. The proper balance must be struck between reliability, customer satisfaction, time to market, sales and features. Figure 2-1 illustrates this concept. The polygon on the left represents a properly balanced project. The polygon on the right represents a project in which reliability and customer satisfaction have been sacrificed for the sake of sales and time to market.
Figure 2-1: Graphical Representation of balanced and unbalanced projects.
Through proper testing and analysis in the in-house testing labs, as well as collection of adequate and meaningful data on a product's performance in the field, the reliability of any product can be measured, tracked and improved, leading to a balanced organization with a financially healthy outlook for the future. (Note: More information on reliability planning and developing a reliability program in the framework of an organization's business goals can be found at http://www.weibull.com/articles/relintro/index.htm.)
For a company to succeed in today's highly competitive and technologically complex environment, it is essential that it knows the reliability of its product and is able to control it in order to produce products at an optimum reliability level. This yields the minimum life-cycle cost for the user and minimizes the manufacturer's costs of such a product without compromising the product's reliability and quality. [19]
Our growing dependence on technology requires that the products that make up our daily lives successfully work for the desired or designed-in period of time. It is not sufficient that a product works for time shorter than its mission duration, but at the same time there is no need to design a product to operate much past its intended life, since this would impose additional costs on the manufacturer. In today's complex world where many important operations are performed with automated equipment, we are dependent on the successful operation of these equipment (i.e. their reliability) and, if they fail, on their quick restoration to function (i.e. their maintainability). [19]
Product failures have varying effects, ranging from those that cause minor nuisances, such as the failure of a television's remote control (which can become a major nuisance, if not a catastrophe, depending on the football schedule of the day), to catastrophic failures involving loss of life and property, such as an aircraft accident. Reliability engineering was born out of the necessity to avoid such catastrophic events and, with them, the unnecessary loss of life and property. It is not surprising that Boeing was one of the first commercial companies to embrace and implement reliability engineering, the success of which can be seen in the safety of today's commercial air travel.
Today, reliability engineering can and should be applied to many products. The previous example of the failed remote control does not have any major life and death consequences to the consumer. However, it may pose a life and death risk to a non-biological entity: the company that produced it. Today's consumer is more intelligent and product-aware than the consumer of years past. The modern consumer will no longer tolerate products that do not perform in a reliable fashion, or as promised or advertised. Customer dissatisfaction with a product's reliability can have disastrous financial consequences to the manufacturer. Statistics show that when a customer is satisfied with a product he might tell eight other people; however, a dissatisfied customer will tell 22 people, on average.
The critical applications with which many modern products are entrusted make their reliability a factor of paramount importance. For example, the failure of a computer component will have more negative consequences today than it did twenty years ago. This is because twenty years ago the technology was relatively new and not very widespread, and one most likely had backup paper copies somewhere. Now, as computers are often the sole medium in which many clerical and computational functions are performed, the failure of a computer component will have a much greater effect.
Reliability engineering covers all aspects of a product's life, from its conception, subsequent design and production processes, through its practical use lifetime, with maintenance support and availability. Reliability engineering covers:
Reliability.
Maintainability.
Availability.
All three of these areas can be numerically quantified with the use of reliability engineering principles and life data analysis. (The combination of these three areas introduces a new term, as defined in ISO-9000-4, Dependability.)
Most products (as well as humans) exhibit failure characteristics as shown in the bathtub curve of Figure 2-2. (Do note, however, that this figure is somewhat idealized.)
Figure 2-2: An idealized reliability bathtub curve, with the three major
life regions: early, useful and wearout.
This curve is plotted with the product life on the x-axis and with the failure rate on the y-axis. The life can be in minutes, hours, years, cycles, actuations or any other quantifiable unit of time or use. The failure rate is given as failures among surviving units per time unit. As can be seen from this plot, many products will begin their lives with a higher failure rate (which can be due to manufacturing defects, poor workmanship, poor quality control of incoming parts, etc.) and exhibit a decreasing failure rate. The failure rate then usually stabilizes to an approximately constant rate in the useful life region, where the failures observed are chance failures. As the products experience more use and wear, the failure rate begins to rise as the population begins to experience failures related to wear-out. In the case of human mortality, the mortality rate (failure rate), is higher during the first year or so of life, then drops to a low constant level during our teens and early adult life and then rises as we progress in years.
Looking at this particular bathtub curve, it should be fairly obvious that it would be best to ship a product at the beginning of the useful life region, rather than right off the production line; thus preventing the customer from experiencing early failures. This practice is what is commonly referred to as burn-in, and is frequently performed for electronic components. The determination of the correct burn-in time requires the use of reliability methodologies, as well as optimization of costs involved (i.e. costs of early failures vs. the cost of burn-in), to determine the optimum failure rate at shipment.
Figure 2-3 shows the product reliability on the x-axis and the producer's cost on the y-axis.
Figure 2-3: Total product cost vs. product reliability.
If the producer increases the reliability of his product, he will increase the cost of the design and/or production of the product. However, a low production and design cost does not imply a low overall product cost. The overall product cost should not be calculated as merely the cost of the product when it leaves the shipping dock, but as the total cost of the product through its lifetime. This includes warranty and replacement costs for defective products, costs incurred by loss of customers due to defective products, loss of subsequent sales, etc. By increasing product reliability, one may increase the initial product costs, but decrease the support costs. An optimum minimal total product cost can be determined and implemented by calculating the optimum reliability for such a product. Figure 2-3 depicts such a scenario. The total product cost is the sum of the production and design costs as well as the other post-shipment costs. It can be seen that at an optimum reliability level, the total product cost is at a minimum. The optimum reliability level is the one that coincides with the minimum total cost over the entire lifetime of the product.
The following list presents useful information that can be obtained with the implementation of a sound reliability program:
Optimum burn-in time or breaking-in period.
Optimum warranty period and estimated warranty costs.
Optimum preventive replacement time for components in a repairable system.
Spare parts requirements and production rate, resulting in improved inventory control through correct prediction of spare parts requirements.
Better information about the types of failures experienced by parts and systems that aid design, research and development efforts to minimize these failures.
Establishment of which failures occur at what time in the life of a product and better preparation to cope with them.
Studies of the effects of age, mission duration and application and operation stress levels on reliability.
A basis for comparing two or more designs and choosing the best design from the reliability point of view.
Evaluation of the amount of redundancy present in the design.
Estimations of the required redundancy to achieve the specified reliability.
Guidance regarding corrective action decisions to minimize failures and reduce maintenance and repair times, which will eliminate overdesign as well as underdesign.
Help provide guidelines for quality control practices.
Optimization of the reliability goal that should be designed into products and systems for minimum total cost to own, operate and maintain for their lifetime.
The ability to conduct trade-off studies among parameters such as reliability, maintainability, availability, cost, weight, volume, operability, serviceability and safety to obtain the optimum design.
Reduction of warranty costs or, for the same cost, increase in the length and the coverage of warranty.
Establishment of guidelines for evaluating suppliers from the point of view of their product reliability.
Promotion of sales on the basis of reliability indexes and metrics through sales and marketing departments.
Increase of customer satisfaction and an increase of sales as a result of customer satisfaction.
Increase of profits or, for the same profit, provision of even more reliable products and systems.
Promotion of positive image and company reputation.
The typical manufacturer does not really know how satisfactorily its products are functioning. This is usually due to a lack of a reliability-wise viable failure reporting system. It is important to have a useful analysis, interpretation and feedback system in all company areas that deal with the product from its birth to its death.
If the manufacturer's products are functioning truly satisfactorily, it might be because they are unnecessarily over-designed, hence they are not designed optimally. Consequently, the products may be costing more than necessary and lowering profits.
Products are becoming more complex yearly, with the addition of more components and features to match competitors' products. This means that products with currently acceptable reliabilities need to be monitored constantly as the addition of features and components may degrade the product's overall reliability.
If the manufacturer does not design its products with reliability and quality in mind, SOMEONE ELSE WILL.
See Also:
Reliability Engineering
Go to weibull.com
Go to ReliaSoft.com
©1996-2006. ReliaSoft Corporation. ALL RIGHTS RESERVED.