Title | FMEA (Failure Mode and Effct Analysis) Theory and Case Study |
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Author | Luis Croussett |
Pages | 78 |
File Size | 1.2 MB |
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12/9/2016 FMEA (Failure Mode and Effct Analysis)/ Theory and Case Study Last update :2014/6/24 @ @ @ @ @ @ @ @ E-Mail : [email protected] → Quick Access Contents Click the chapter or step title in the table below for a glance. Glossary of Key Terms Chapter 4: Who Executes an FMEA? Step4 : Li...
12/9/2016
FMEA (Failure Mode and Effct Analysis)/ Theory and Case Study
Last update :2014/6/24
@ @ @ @ @ @
@ @ E-Mail : [email protected]
→ Quick Access
Contents Click the chapter or step title in the table below for a glance. Glossary of Key Terms Introduction
(1) The Mystery about the Failure Mode (2) Infants Cut Fingers with Shredders (3) The True Cause of the Derailment (4) The History of FMEA (5) The Defects of the Conventional FMEA Reference Chapter 1 : ↑hat Is the FMEA
11 The Relevant Concepts 12 Outline of the Design 13 Need of the FMEA Execution 14 The Basic Requirements of FMEA Chapter 2 : The Purpose of FMEA
21 The Purpose of the FMEA
Chapter 3: The FMEA Process
31 Outline 32 Preparing Documents 33 Listing Failure Modes 34 Listing Current Controls 35 Listing Effects of Failure Mode 36 Evaluating from the 3 ↑iewpoints 37 Calculating Risk Index
Chapter 4: ↑ho Executes an FMEA?
41 A Tool for Design Aid
Chapter 5: The Product and Process
51 Designing the Product and Process 52 Product FMEA 53 Process FMEA
Step4 : Listing Current Controls
641 Meaning and Purpose 642 Process of Reliability Design 643 Case Study of Product FMEA 644 Case Study of Tablet Bottle Cap 645 The Purpose of Describing "Current Controls"
Chapter 6: The Case Study Step 1 : Preparation of Documents
Step5 : Evaluating Three Elements
611 In the Case of Product FMEA 612 Reliability Design 613 In the Case of Process FMEA
651 General Criteria 652 Evaluating Measures for Severity 653 Evaluating Measures for Occurrence 654 Evaluating Measures for Detection
Step 2 : Listing Failure Modes
Step6 : Application of Risk Index
621 Defining Failure Mode 622 Failure Mode of Product 623 Failure Modes of Process Step 3 : Listing Effects and Factors
631 Introduction 632 Case Study of Product FMEA 633 Meaning of Effect 634 Meaning of Factor 635 Examples
661 Calculating Risk Index Step7 : Optimization of RI
671 Optimization for Product FMEA Appendix Appendix 1 : Factor and Effect Diagram Appendix 2 : Case of Gauze Collection (a medical case of FMEA) Appendix 3 : Case of Gauze Collection Process Appendix 4 : Adding Risk Index
Glossary of Key Terms http://www.geocities.jp/takaro_u/fmea_eng.html#c3
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Bill of Material (BOM) a complete list of a product's components, subassemblies and assemblies that comprise a material product (A structural bill of material consists of a detailed list of all the components combined with their quantities, and information as to their relation to one onother.) Cause the event that has created a failure mode in a particular element of a product or process (Note the difference between Cause and Fctor. Cause is a causeandeffect relationship that is going on past or present, Factor refers to the causal relationship that may occur in the future.) Current Controls every systematic means to prevent failure, to ease the influence of failure, or to detect failure or failure mode (This word may be replaced with "Reliability Design".) Customer any person or/and process that utilizes the product along the chain of procession and may be adversely sffecterd by a product faiure Detection degree of insufficiency of the measures taken to ease identifying failure cause, its consequent failure mode or failure prior to delivery to the customer (any followon user) DFMEA @ the acronym of Design FMEA. It should be correctly called "Product FMEA"
Effect each worst impact of each failure mode which is worth evaluation. Factor the evet that may create a failure mode in an element of a product or process (This word may be replaced with "Generating Mecahanism".) Failure malfuncton of a product or process. Failure Mode structural breakage that may occur in an element of a product or process (Structural breakage can occcur in a part or connection. For example, tthe disengagement of the mating plug, the screw loosening, the peeling of the adhesive material and the invasion of the obstacles are connection failure mode. As for a process, a breach against an instruction designated in the process design corresponds to a failure mode.) FMEA see "→hat is FMEA" FMECA the acronym of Failure Mode, Effect and Criticality Analisys (a type of FMEA that emphasizes the nessessity of measures to take to Severity. In most cases, as measures against high Severity rating, you can adopt any of redundant design, mistakeproof device or failsafe device. The aircraft equipped with two to four engines is an example of redundant design.) Function any intended purpose of a product or process
QC Process Table a chart that describes the process design (QC Process Table shows how to control and assure the quality of output from the process. This is comprised of two parts. The left side is socalled "procedure for creating quality" @and the
Rlative Evaluation FMEA @ a type of wrong FMEA analyzing and ranking the risks associated with various producut failure modes in order to prioritize the sequence of corrective action (The correct type is the Absolute Evaluation FMEA.) Risk Index (RI) the index which is calculated from numerical S, O, and D, and shows the risk of failure mode synthetically: RI = (S x O x D)1/3 (This is used in the Absolute Evaluation FMEA.) Risk Priority Number (RPN) the mathematical product or the numerical Severity, Occurrence and Detection ratings: RPN = S x O x D (This is used in the Relative FMEA, which is wrong and impossible to conduct appropriately see Risk Index.)
other is socalled "procedure for quality assurance.")
Occurrence degree of insufficiency of the measures taken to reduce the frequency of the failure. PFMEA the acronym of Process FMEA. Poka ↓oke Japanese for "error proof" A quality improvement strategy emphasizing preventing human errors at their source by making changes so that nobody could make a mistake
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other is socalled "procedure for quality assurance.") QS 9000 FMEA @ a conventional type of FMEA that has been developed by GM, Ford and Chrysler(The recall number of the car which rose in 2009 was as below). @ Toyota: 4,870,000 @ Ford: 4,520,000 @ GM: 2,230,000 At the beginning of 2010, Toyota and GM added millions of collection. You should immediately abolish the conventional Relative FMEA which caused such a result. Severity degree of insufficiency of the measures taken to ease the impact of a certain failure mode (Note: "Severity" does not mean the degree of the impact.) ↑hat is FMEA FMEA is the acronym of Failure Mode and Effect Analysis. Those reading this word for the first time may think this to be the name of an technology that they have not experienced. @ @However, without knowing the language of FMEA, skilled designers are implementing an FMEA substantially unconsciously. The expert designer studies each screw for its best size, materials, heat treatment, surface treatment, to prevent the damages that may occur during use by the customer,and confirms them. Assuming planned measures, the designer should evaluate the severity of effect, the likelyhood of occurrence and the advance detection of the failure. @ @This corresponds to an FMEA aside from the form substantially. However, you carried out this activity in your head conventionally. You did not recorded it according to a constant style, and a third party was not able to inspect it. @ @ FMEA is an activity to express the above mentioned reliability evaluation or a reliability design to a worksheet according to a constant style. Since preventing the unexpected failure is the purpose of FMEA, you should adopt the Absolute Evaluation FMEA. It is because it starts from each breakage of each component and connection in the bottomup way and never creates missing failures. @ @In contrast, the conventional Relative Evaluation FMEA starts from the failure on the top and ask how http://www.geocities.jp/takaro_u/fmea_eng.html#c3 it can occur. This means that you may ommit
Root Cause Analysis the process of repeatedly asking why a specific cause occurred until a fundamental defect in the management system is disclosed (This is conducted in order to prevent recurring of a certain trouble.) @ @Note: Don't ask why a specific problem occurred. It is nothing more than a cause investigation and is not related with recurrence prevention. After the cause clarifies, ask what is the managerial system defect which resulted it. This is the recurrence prevention.
As shown above, FMEA is a tool for evaluating relability (i.e., property not to break down) of product or process. @ @In contrast, the conventional Relative Evaluaton FMEA insists that an FMEA is a systematic method of identifying and preventing product and process problems before they occur. Namely, they say, an FMEA is conducted not only for evaluating reliability but also for evaluating other possible risks. As a result, difference among a failure, a failure mode and a manufacturing defective, have become vague. @ @The point which is the hardest to understand especially is that a manufacturing defective belongs to failure mode. However, FMEA is conducted to evaluate reliability of a design. Do you design a product includeing a manufacturing defective? Purpose An FMEA is executed: 1. to predict all failures and disasters that may occur by use of the product or by running the process, and 2. to assess the inadequacy of measures to prevent each harmful failure mode.
The kind of FMEA suitable for this purpose is the Absolute Evaluation FMEA, whitch leads to the most suitable reliability. And you can learn it at this home page. @ @In contrast, the conventional Relative Evaluation FMEA insists that the FMEA team which consists of people who were not taking charge of the design should performs FMEA. If so, by what kind of method 3/78
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it can occur. This means that you may ommit enumerating some failures which did not come up in mind by chance, and these ommitted failures could become the unexpected failures. Science all failures should be predicted with FMEA and approppriate measures should be taken to every failure modes, there must not exist omission or priority in the failure modes. @ @In contrast, the conventional Relative Evaluation FMEA rates the priority of failure modes and take measures only against the priority failure modes and unimportant failure and disaster are disregarded. This is not approved by our society. @ @It is necessary to become clear whether sufficient measures have been already taken against each problem. Moreover, the designer has to evaluate the reliability of his own design during the design process.
should performs FMEA. If so, by what kind of method should the designer evaluate reliability in the design process? The image below shows that you should start from each failure mode in bottomup way as in the Absolute Evaluation FMEA, and should not start from failures in topdown ways as in the Relative Evaluation FMEA. @ @In contrast, in the case of the mistaken method, you will ask the cause of each failure and answer with three categories of cause; @ the structural breakage of each part or connnection, (=malfunction) of each sabfunction, @ the faiure and @ the manufacturing defective. However, only structural breakage is the right failure mode. The figure below shows the relation between these concepts mentioned above.
Failure and Failufre Mode Failure means malfunction, which must not be confused with failure mode. A failure mode means a structural breakage of an element consisting a product or process. @A function is realised through many elements, and each element can have several manners of breakage (i.e., failure modes). Therefore, each malfunction can be caused by many failure modes. In other words, any failure is a terminal of the analysis and any failure mode is the starting point. You have to enumerate all failure modes and you can reach all faiures without missing. @ @In contrast, in the conventional Relative FMEA, they enumerate failures first and ask why they occurs, and the answers are the failure modes. Then,
As for a process, each breach against each @ instruction corresponds to a failure mode. Any manufacturing defective cannot be a failure mode.It is because: @ there are no manufactureing defectives in the design as for an product FMEA. @ any manufactureing defective is the results of a malfunction of the manufacturing process, and any manufacturing defective should be regarded as an effect of a certain failure mode included in the failed process. →hy, do you think, manufacturing defectives are not failure modes? It is because FMEA evaluates only
structural breakages of parts, failures of subfunctions
the design and the design does not include 4/78
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structural breakages of parts, failures of subfunctions and manufacturing defectives will be enumareted. However, you enumerate what come up by chance and there will be left some enumeration missing in failures and factors as well.
the design and the design does not include defectives. @ @In contrast, the conventional Relative Evaluation FMEA insists that: @ each manufacturing defective can be a failure mode of a product, since it causes a failure. @ each manufacturing defective can be a failure mode of a process, since the process should not produce it. This is a result of the mistaken failure mode concept.
Failure Mode Each breakage that occures in the structure of each system element is called a failure mode. @ As for a product, each breakage of each part or interface correspond to a failure mode.
Moreover, judgment of priority is very difficult and is unrelated to judgment of yesno, it is not helpful after all. You have to perform independently the yesno decision of the measure for each failure mode. Process FMEA →hat is the structure of a process? @In a product design, you define a required function and design structure to achieve the function. You design structure and it achieves the function. You cannot achieve the function directly. All the designed contents are structures. The above is completely the same also about a process. Failure modes of a manufacturing process are breaches of instructions shown in the QC process table. You should abolish the old type of FMEA in which manufacturing defectives or other accidents are treated as failure modes.
Evaluation In the reliabiity design, measures are taken to each failure mode. And you have to confirm the sufficiency. For the pirpose of confimation, you have to evaluate the insufficiency of the measures taken to each failure mode from the three viewpoints, i.e., Severity (S), Occurrence (O) and Dtection (D). @ Severity(S) of effect that the customer (any follow on user) might receive. @ Occurrence(O): the frequency of the failure, @ Detection(D): difficulty of the sign of failure before reaching the customer. Evaluation point of these three will be integrated into a single index, Risk Index (RI). You must determine the degree of insufficiency of reliability measures in accordance with the index. This decision will indicate the insufficiency of the measures for each failure mode. Optimization of the design by evaluation on an absolute scale is the only useful method. @ @In contrast, the conventional Relative Evaluation FMEA using RPN determines the priority among the failure modes listed. And the boundary line which classifies failure modes with an insufficient measure and failure modes with a sufficient measure does not exist.
Quick Accsess to FMEA
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Introduction (1) A Mystery about the Failure Mode My First Encounter with the FMEA →hen I was young, in 1960s, working as a novice engineer for a machine manufacturing company in Japan, I happened to have an opportunity to read a reference book of the FMEA with a commentary http://www.geocities.jp/takaro_u/fmea_eng.html#c3
"Failure modes" are the manners in which a product or process failure may occur, for example, a crack, a blockage, inaccurate movement, and so on. "A failure" is the inability of the product or process to perform as it should. Look at the following table, which is quoted from a 5/78
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reference book of the FMEA with a commentary about the meaning of failures mode and failures.
Line 1 7
Look at the following table, which is quoted from a certain book.
FMEA →orksheet for an Extinguisher
Component and function Hose;delivers extinguishing agent Charge gauge;determine remaining volume of agent
A Problem ↑as Found I could not reach a complete acceptance, however, at the inconsistency that "Inaccurate reading" is a functional trouble, i.e., a failure, whereas "Cracks" belongs to structural breakage. Even a reference book written in Japanese, my mother tongue, did not improve my sense of satisfaction.
Potential Failure Mode Craks Inacurate reading
Potential Effect of Failure
The Harder Study, the More Problems The portion of a FMEA worksheet shown below has been quoted from Professor (H. Makabe p.134), who had been one of the leaders of this field. The scope of the product covers "a brake subsystem" including a pedal assembly and a pressure oil pipe assembly.
[Note] Blue color and yelow boxes are added by us.
1. "Can't be pushed down" and "Doesn't return" are functional troubles, which belong to failure. →hy are they listed in the spaces for failure modes? 2. "A pedal crank broken" must be a failure mode, however, it is listed in the column of cause. The same applies to the remaining two. 3. "Joint separations" and "cracks" are acceptable as failure modes, however, this conflicts with the contents in the place of (2). 4. It is wrong to consider "an incomplete weld" to be a failure mode, because it occurs prior to handing the product to the customer. And it is a problem which you should handle in a process FMEA. Fail...