RELIABILITY ENGINEERING

RELIABILITY ENGINEERING

Kartik Gupta2K13/PE/016INTRODUCTION Reliability engineeringisengineeringthat emphasizesdependabilityin thelifecycle managementof aproduct. Dependability, or reliability, describes the ability of a system or component to function under stated conditions for a specified period oftime.

Reliability is theoreticallydefined as theprobabilityof success (Reliability=1-Probability of Failure), as the frequency of failures, or in terms ofavailability, as a probability derived from reliability and maintainability. "Reliability is, after all, engineering in its most practical form."

OBJECTIVES OF RELIABILITY ENGINEERING

The objectives of reliability engineering, in the order of priority, are: To apply engineering knowledge and specialist techniques to prevent or to reduce the likelihood or frequency of failures.To identify and correct the causes of failures that do occur, despite the efforts to prevent them.To determine ways of coping with failures that do occur, if their causes have not been corrected.To apply methods for estimating the likely reliability of new designs, and for analysing reliability data.

A basic life project cycle

WHY IS RELIABILITY IMPORTANT?

ReputationCustomer SatisfactionWarranty CostsRepeat BusinessCost AnalysisCustomer RequirementsCompetitive Advantage

REASONS FOR FAILURE

The load and strength of an item may be generally known, however there will always be an element of uncertainty. The actual strength values of any population of components will vary; there will be some that are relatively strong, others that are relatively weak, but most will be of nearly average strength. Similarly there will be some loads greater than others but mostly they will be average. Figure 1, below shows the load strength relationship with no overlaps.

However if, as shown in figure 2, there is an overlap of the two distributions then failures will occur. There therefore needs to be a safety margin to ensure that there is no overlap of these distributions.

MEASURING RELIABILITY

REQUIREMENTS

Many customers will produce a statement of the reliability requirements that is included in the specification of the product. This statement should include the following:

The definition of failure related to the products function and should cover all failure modes relevant to the function

A full description of the environments in which the product will be stored, transported, operated and maintained

A statement of the reliability requirement

THE BATH TUB CURVE

The bath-tub curve is a representation of the reliability performance of components or non repaired itemsThe infant mortality or early failures portion shows that the population will initially experience a high hazard function that starts to decrease.After the initial phase when the weak components have been weeded out and mistakes corrected, the remaining population reaches a relatively constant hazard function period, known as the useful life periodThe final portion of the bath-tub curve is called the wear-out phase, this is when the hazard function increases with time

LIFE DISTRIBUTIONS

If you take a large number of measurements you can draw a histogram to show the how the measurements vary. A more useful diagram, for continuous data, is the probability density function. The y axis is the percentage measured in a range(shown on the x-axis) rather than the frequency as in a histogram. If you reduce the ranges(or intervals) then the histogram becomes a curve which describes the distribution of the measurements or values. This distribution is the probability density function or PDF.

Survival function is given by

RELIABILITY PREDICTION

Assuming all the parts in a system are independently exponentially distributed, i.e. one part does not cause the other to fail then the overall system failure rate can be calculated using the series system model shown above. For example, the failure rate of a printed circuit board is the sum of the failure rates of each of the components.

RELIABILITY TESTING

The purpose of reliability testing is to discover potential problems with the design as early as possible and, ultimately, provide confidence that the system meets its reliability requirements.

Reliability testing may be performed at several levels and there are different types of testing. Complex systems may be tested at component, circuit board, unit, assembly, subsystem and system levels

Reliability testing on the basis of no. of failures to the total time of operation

Difference between Quality and Reliability So while this product may have a reliable design, its quality is unacceptable because of the manufacturing process. Just like a chain is only as strong as its weakest link, a highly reliable product is only as good as the inherent reliability of the product and the quality of the manufacturing process.

Six SigmaSix Sigma at many organizations simply means a measure of quality that strives for near perfection. Six Sigma is a disciplined, data-driven approach and methodology for eliminating defects (driving toward six standard deviations between the mean and the nearest specification limit) in any process from manufacturing to transactional and from product to service.Thestatistical representationof Six Sigma describes quantitatively how a process is performing. To achieve Six Sigma, a process must not produce more than 3.4 defects per million opportunities. A Six Sigma defect is defined as anything outside of customer specifications. A Six Sigma opportunity is then the total quantity of chances for a defect.

CONCLUSION

Ultimately the aim for reliability engineering is to maximize reliability during service life by measurement & control of manufacturing, quality / screening, optimized design & build process to improve intrinsic reliability, assure no systematic faults present in product and to provide sufficient margin to meet life requirements.

Thus all these factors lead to customer satisfaction which is of utmost importance to every manufacturer.