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  • 1. Using RCM, design-out is applicable under certain specific conditions, namely1. There is a failure mode for which we cannot find a maintenance task to mitigate againstthe consequence of failure. Usually (but not always) such failure modes are hidden andaffect Safety/Environment.2. When there is a wear-in failure mode (infant mortality)redesign may be economical.Note that redesign is not merely that of hardware or sofyware; it can be of people(training)or procedures (revision).Rennie, designing out a failure mode invariably introduces a new failure mode. For example,you can add a dashboard indicator light to tell you if your car brake light has failed (hiddenfailure). If the indicator light comes on, you can replace the brake light bulb (as Steveexplained), BUT you have a new (hidden) failure mode of the indicator bulb itself.Remember that as long as degradation takes place, you cannot eliminate maintenance.Every time we modify equipment, i.e. implement design-out, we add a new layer of risks.Sometimes the solution is worse than the problem! In high hazard industries ALLmodification go through a strict change control procedure. A full blown HAZOP is oftenrequired. Hence,one has to think carefully before undertaking redesign. This is the reasonfor the high hurdle, of the kind Steve explained ( requiring startling results).Josh, top performers use a risk ranking of all modifications with e.g. a risk matrix. They alsoimpose hurdles to prevent people proposing design-out solutions willy-nilly.To sum up, design-out is a valid and applicable strategy, to be used when certain conditionsare met. They are not magic wands to be waved about.[edit]Uses Development of system requirements that minimize the likelihood of failures. Development of methods to design and test systems to ensure that the failures have been eliminated. Evaluation of the requirements of the customer to ensure that those do not give rise to potential failures. Identification of certain design characteristics that contribute to failures, and minimize or eliminate those effects. Tracking and managing potential risks in the design. This helps avoid the same failures in future projects. Ensuring that any failure that could occur will not injure the customer or seriously impact a system. To produce world class quality products[edit]Advantages Improve the quality, reliability and safety of a product/process Improve company image and competitiveness Increase user satisfaction
  • 2. Reduce system development time and cost Collect information to reduce future failures, capture engineering knowledge Reduce the potential for warranty concerns Early identification and elimination of potential failure modes Emphasise problem prevention Minimise late changes and associated cost Catalyst for teamwork and idea exchange between functions Reduce the possibility of same kind of failure in future Reduce impact on company profit margin Improve production yield[edit]LimitationsSince FMEA is effectively dependent on the members of the committee which examines product failures,it is limited by their experience of previous failures. If a failure mode cannot be identified, then externalhelp is needed from consultants who are aware of the many different types of product failure.FMEA is thus part of a larger system of quality control, where documentation is vital to implementation.General texts and detailed publications are available in forensic engineering and failure analysis. It is ageneral requirement of many specific national and international standards that FMEA is used in [source needed]evaluating product integrity .If used as a top-down tool, FMEA may only identify major failure modes in a system. Fault tree analysis(FTA) is better suited for "top-down" analysis. When used as a "bottom-up" tool FMEA can augment orcomplement FTA and identify many more causes and failure modes resulting in top-level symptoms. It isnot able to discover complex failure modes involving multiple failures within a subsystem, or to report [citationexpected failure intervals of particular failure modes up to the upper level subsystem or system.needed]Additionally, the multiplication of the severity, occurrence and detection rankings may result in rankreversals, where a less serious failure mode receives a higher RPN than a more serious failure [26]mode. The reason for this is that the rankings are ordinal scalenumbers, and multiplication is notdefined for ordinal numbers. The ordinal rankings only say that one ranking is better or worse thananother, but not by how much. For instance, a ranking of "2" may not be twice as severe as a ranking of"1," or an "8" may not be twice as severe as a "4," but multiplication treats them as though they are.See Level of measurement for further discussion.quote:FMEA can apply an RPNumber, which consists of a probability, consequence and detectionscale (all 0-10). This does not forms an FMECA.Each scale is 1-10, not 0-10. In an FMEA, we have to identify the failure mode AND itsConsequence. If we are doing RCM, we add our estimate of probability. That gives us therisk of failure. An FMEA does NOT neeed an RPN.quote:
  • 3. The RPNumber can be applied to rate the status quo of a failure mode(what is the scorewhen no maintenance is performed on this part) and to predict what the influence of theproposed (maintenance)action will be by rating the three aspects again.The RPN is an estimate of the risk posed by each failure mode AND how easy it is recognizeits existence. That determines what maintenance action is required. It says nothing aboutthe status and is not a measure of performance. The RPN will remain what it is irrespectiveof whether we act on the required maintenance task or not, as long as our estimate of the 3factors remains the same.quote:FMECA is a method using a criticality rating formed by probability * consequence (both scale0-10) to rate the criticality of a failure mode.. No, FMECA = FMEA + RPN, scale is 1-10 for each of 3 elements.quote:Clearly, the criticality rating provides a more precise view on the criticality of a failuremode, because only the two most important factors are used for the failure mode rating.. There is a problem of nomenclature and definition. In FMECA, the RPN is termed"criticality", while elsewhere, we understand Risk, which has only two elements, as"criticality". No doubt this causes confusion.quote:This means that if I only use the factors probability and consequence to make the criticalityrating, I have an FMECA. And if I use an extra factor to rate a failure mode, nameddetection, I have an FMEA?No, you need only the failure mode defined and its consquence to have an FMEA. If you addthe RPN, you have FMECA.quote:Is this the only large difference?You have reversed the two, but yes, that is the main difference.quote:And which reasons do you know to use one of the two methods?It all depends on your end objective; do you want to improve your design, identifymaintenance requirements? A version of FMEA can help the designer, another (Functional)FMEA can help identify maintenance requirements, when using the RCM logic charts, andFMECA can help find maintenance requirements for certain kinds of systems where there arelots of man-machine interfaces.Qualitative Versus QuantitativeFMEA provides only qualitative information, whereas FMECA also provides limitedquantitative information, or information capable of being measured. FMEA is widely usedin industry as a "what if" process It is used by NASA as part of its flight assurance program
  • 4. for spacecraft. FMECA attaches a level of criticality to failure modes; it is used by the U.S.Army to assess mission critical equipment and systems.ExtensionFMECA is effectively an extension of FMEA. In order to perform FMECA, analysts mustperform FMEA followed by critical analysis (CA). FMEA identifies failure modes of aproduct or process and their effects, while CA ranks those failure modes in order ofimportance, according to failure rate and severity of failure.Critical AnalysisCA does not add information to FMEA. What it does, in fact, is limit the scope of FMECA tothe failure modes identified by FMEA as requiring reliability centered maintenance (RCM)Criticality AnalysisThe MIL-STD-1629A document describes two types of criticality analysis: quantitative andqualitative. To use the quantitative criticality analysis method, the analysis team must: Define the reliability/unreliability for each item, at a given operating time. Identify the portion of the items unreliability that can be attributed to each potential failure mode. Rate the probability of loss (or severity) that will result from each failure mode that may occur. Calculate the criticality for each potential failure mode by obtaining the product of the three factors: Mode Criticality = Item Unreliability x Mode Ratio of Unreliability x Probability of Loss Calculate the criticality for each item by obtaining the sum of the criticalities for each failure mode that has been identified for the item. Item Criticality = SUM of Mode CriticalitiesTo use the qualitative criticality analysis method to evaluate risk and prioritize correctiveactions, the analysis team must: Rate the severity of the potential effects of failure. Rate the likelihood of occurrence for each potential failure mode. Compare failure modes via a Criticality Matrix, which identifies severity on the horizontal axis and occurrence on the vertical axis.Applications and BenefitsThe FMEA / FMECA analysis procedure is a tool that has been adapted in many differentways for many different purposes. It can contribute to improved designs for products andprocesses, resulting in higher reliability, better quality, increased safety, enhanced customersatisfaction and reduced costs. The tool can also be used to establish and optimizemaintenance plans for repairable systems and/or contribute to control plans and otherquality assurance procedures. It provides a knowledge base of failure mode and correctiveaction information that can be used as a resource in future troubleshooting efforts and as atraining tool for new engineers. In addition, an FMEA or FMECA is often required to complywith safety and quality requirements, such as ISO 9001, QS 9000, ISO/TS 16949, Six
  • 5. Sigma, FDA Good Manufacturing Practices (GMPs), Process Safety Management Act (PSM),etc.ReliaSofts Xfmea software facilitates analysis, data management and reporting for failuremode and effects analysis (FMEA) and failure modes, effects and criticality analysis(FMECA). The software supports all major standards (AIAG FMEA-3, J1739, ARP5580, MIL-STD-1629A, etc.) and provides extensive customization capabilities for analysis andreporting, allowing you to configure the software to meet your organizations specificanalysis and reporting procedures for all types of FMEA / FMECA.Advantages and disadvantagesStrengths of FMECA include its comprehensiveness, the systematic establishment of relationshipsbetween failure causes and effects, and its ability to point out individual failure modes for corrective actionin design. Weaknesses include the extensive labor required, the large number of trivial cases considered,and inability to deal with multiple-failure scenarios or unplanned cross-system effects such as sneakcircuits.According to an FAA research report for commercial space transportation, Failure Modes, effects, and Criticality Analysis is an excellent hazard analysis and risk assessment tool, but it suffers from other limitations. This alternative does not consider combined failures or typically include software and human interaction considerations. It also usually provides an optimistic estimate of reliability. Therefore, FMECA should be used in conjunction [18] with other analytical tools when developing reliability estimates.The Pareto priority index (PPI), so named because of its connection with the Pareto principle, which isin turn named after the economist Vilfredo Pareto, can be used to prioritize several (quality improvement)projects. It is especially used in the surroundings ofsix sigma projects. It has first been established [citation needed]by AT&T.The PPI is calculated as follows: A high PPI suggests a high project priority.The bathtub curve is widely used in reliability engineering. It describes a particular form of the hazardfunction which comprises three parts: The first part is a decreasing failure rate, known as early failures. The second part is a constant failure rate, known as randomfailures. The third part is an increasing failure rate, known as wear-out failures.The name is derived from the cross-sectional shape of a bathtub.The bathtub curve is generated by mapping the rate of early "infant mortality" failures when firstintroduced, the rate of random failures with constant failure rate during its "useful life", and finally the rateof "wear out" failures as the product exceeds its design lifetime.
  • 6. In less technical terms, in the early life of a product adhering to the bathtub curve, the failure rate is highbut rapidly decreasing as defective products are identified and discarded, and early sources of potentialfailure such as handling and installation error are surmounted. In the mid-life of a product—generally,once it reaches consumers—the failure rate is low and constant. In the late life of the product, the failurerate increases, as age and wear take their toll on the product. Many consumer products strongly reflectthe bathtub curve, such as computer processors.While the bathtub curve is useful, not every product or system follows a bathtub curve hazard function, forexample if units are retired or have decreased use during or before the onset of the wear-out period, theywill show fewer failures per unit calendar time (not per unit use time) than the bathtub curve.The term "Military Specification" is often used to describe systems in which the infant mortality section ofthe bathtub curve has beenburned out or removed. This is done mainly for life critical or system criticalapplications as it greatly reduces the possibility of the system failing early in its life. Manufacturers will dothis at some cost generally by means similar to accelerated stress testing.In reliability engineering, the cumulative distribution function corresponding to a bathtub curve may beanalysed using a Weibull chart.[edit]Critics: Invalid concept for modern complex systemsSome investigations in the aerospace and other industries have discovered that most failures do notcomply with the bathtub curve. It is argued that the bathtub curve is an old concept and should not beused as a stand alone guide to reliability. Most interesting in these investigations was the conclusion that [citation needed]wear-out issues in complex systems only count for about 4% of all failures (refer toReliabilitycentered maintenance (RCM); Boeing 747 - MSG2 and MSG3 investigations). According to "The RCMapproach" about 6 different types of failure rate curves can be distinguished. It is also remarkable that thehighest contribution to failures appear to be failures that have a constant failure rate character. Thismainly counts for complex systems, being highly integrated.