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The final four steps of the Design FMEA are where the assessment happens. These steps do the original analysis and manage the mitigation of high risk failures.

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Quick reference

Design FMEA Analysis Steps

The last four steps of the Design FMEA process are the ones where the analysis is accomplished.  These are the steps that determine the technical design risk.

When to use

Whenever a Design FMEA is conducted, the analysis steps must be completed.  After the first three steps are done, the remaining four steps can be completed.  These steps must be completed in the order indicated.

Instructions

The heart of the Design FMEA is the analysis and these steps provide direction for how to do that analysis.  These steps complete each of the columns on the Design FMEA form.

Step 4. Identify functions and failure modes

Use the Block Diagram from Step 3 to list all the functions for each part or component that are included in this Design FMEA.  Each function should be on a separate line.  For each function, list all applicable failure modes.  There should be one failure mode on each line of the Design FMEA form.  If a function has multiple failure modes, add additional lines below the line listing the function.

Step 5. Analyse failures

For each failure mode, describe the effect of that failure on product performance.  Then score the severity of that effect.  Next list the possible causes of that failure mode.  If there is more than one cause, list each cause on a separate line.  Now score the probability of occurrence of that cause for this product in its intended application.  Finally, describe the design control process based upon the product development procedure that would uncover this failure mode in this design.  Score the detection capability.

Step 6. Implement mitigation plan

The three scores from Step 5 are multiplied together to get a Risk Priority Number or RPN.  When this value exceeds the organizational threshold, a mitigation action should be taken. Typical actions are:

  • Design change to lower the severity (safety covers, or less volatile materials)
  • Design change to lower occurrence (more robust technology or design)
  • Development process change to improve detection (additional testing or analysis)

After a mitigation is completed, the failure mode is rescored.  Depending upon the design change, new failure modes may have been introduced and these should also be analysed.

Step 7.  Document analysis

The Design FMEA is placed under revision control and maintained as part of the design documentation.  It should be revised whenever there is a change to the design, a change to functions the product is intended to perform, or if new information becomes available concerning the probability of occurrence of failure modes.

Hints & tips

  • These steps are the focus of the Design FMEA.  They will likely take the most time and effort to complete.
  • The goal is to identify technical risk, so list all functions and failure modes, not just a minimal set in order to say you did a Design FMEA.  I was reviewing the development process for an organization one time and saw a Design FMEA for a complex system with dozens of components and numerous functions that only listed three failure modes, all of which were rated as very low and not requiring mitigation.  Yet, the reason I was reviewing the procedure was because that product (and several others) were experiencing major production and field failure problems.  The Design FMEA they completed was worthless since it overlooked 90% of the failure modes.  Use the Design FMEA for what it was intended – to identify technical risk during the design process, so that risk can be mitigated to an acceptable level.
  • Include Desing FMEA revisions in the change control process for design documentation.  Make it a required submittal before any design change can be approved.
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  • 00:04 Hi I'm Ray Sheen.
  • 00:06 Well now that we've completed the preparatory steps,
  • 00:09 let's consider the four steps of the Design FMEA process that
  • 00:12 are associated with actually doing the analysis.
  • 00:15 We talked about the first three preparation steps in an earlier lesson.
  • 00:21 The next step is to take all the functions from the block diagram and
  • 00:25 list those on the Design FMEA form.
  • 00:28 Then for each of these functions, we'll list the possible failure modes.
  • 00:32 Now it's time for analysis.
  • 00:34 Describe the effect of the failure mode and rate its severity,
  • 00:37 the cause of the failure mode and rate its probability of occurrence, and
  • 00:40 the mechanism for detecting the failure mode during the design process.
  • 00:45 And rate the detection.
  • 00:46 These three scores will give us an RPN value.
  • 00:49 If the RPN value is above your organization's threshold,
  • 00:53 then take an action to mitigate the design or improve your design control.
  • 00:57 Update the scores and ratings, and
  • 00:59 then maintain the Design FMEA form in your design documentation for the product.
  • 01:05 Let's look at each of these steps in a little more detail.
  • 01:08 I will start with step four, functions and failures.
  • 01:11 Transfer the functions from the block diagram on to the Design FMEA form.
  • 01:17 Make sure you list all the functions of all the components on the part or
  • 01:21 sub system that you're analyzing and then all the functions for
  • 01:25 each of those components.
  • 01:26 If you already know some functions will be critical to a product performance,
  • 01:30 be sure they are included.
  • 01:32 Now, for each function, list the failure modes.
  • 01:35 Many functions will have multiple failure modes, and
  • 01:37 each failure mode is on a separate line in the form.
  • 01:40 Of course, you should start with known failures that have occurred,
  • 01:44 but then brainstorm other possibilities.
  • 01:47 Make sure you think about all eight failure types, not just complete failures.
  • 01:51 Include partial, unwanted failures, intermittent failures,
  • 01:54 degraded part failures, or excessive performance, over-processing.
  • 01:59 Now, it's time to do the actual analysis.
  • 02:02 For each failure mode, list the effect of that failure on the product performance.
  • 02:07 We'll be scoring the effect.
  • 02:09 Make sure you include failures that incur with infrequent processes,
  • 02:13 such as a startup or shutdown, or maintenance or calibration process.
  • 02:18 The effect is the impact of the failure on the product or the user.
  • 02:22 Each effect will be evaluated for severity.
  • 02:25 We have an entire lesson devoted to this scoring.
  • 02:28 For each effect,
  • 02:29 you will also need to list all the causes of that type of failure.
  • 02:33 The causes will be scored based upon the probability of occurrence.
  • 02:37 Look at the history of other products and
  • 02:39 the design maturity when considering this score.
  • 02:42 Finally, list the method that's used in the development process,
  • 02:46 the design control activity that checks for that type of failure or cause, and
  • 02:50 then score that appropriately.
  • 02:53 One more comment about the scoring, I normally score across the form,
  • 02:57 not down the form.
  • 02:59 By that I mean, when I identify a failure, I square the severity, probability of
  • 03:03 occurrence and detection for that failure before I go on to the next one.
  • 03:07 We'll do things differently here in training but that's what I do in practice.
  • 03:12 Step six is to conduct mitigation if required.
  • 03:15 The product of the three scores is the risk priority number.
  • 03:19 If that value exceeds your organization's threshold,
  • 03:22 you'll need to do a mitigation or get approval from senior
  • 03:25 management to continue with the design that has a known weakness.
  • 03:29 If you do a mitigation, then re-score the failure mode, keeping in mind that
  • 03:33 the mitigation may eliminate one failure, but may introduce others.
  • 03:37 One way to mitigate is to change the design so that the failure mode effect
  • 03:41 is less severe, possibly provide safety covers or warning lights.
  • 03:45 Another is to reduce the likelihood of the failure occurrence
  • 03:48 by changing the design to increase reliability.
  • 03:51 You could add redundant systems or use proven technology.
  • 03:56 The third mitigation approach is to change your design and development process so
  • 03:59 that it is able to do a better job of detecting this type of failure.
  • 04:03 The obvious implication is that if you find the failure in development,
  • 04:07 you will improve the design to eliminate that failure.
  • 04:11 The final step is to document the analysis.
  • 04:14 Of course the first thing is to complete the Design FMEA form.
  • 04:18 If the RPN was above the threshold, do the mitigation action and
  • 04:22 then re-score the system, but
  • 04:24 be sure you only do this after the mitigation is actually implemented.
  • 04:28 There are no points for good intentions that never turned into actions.
  • 04:32 In most cases, the form is electronic so the finished form is actually a file.
  • 04:37 This file should then be placed under document control and
  • 04:40 treated as part of the design documentation.
  • 04:43 Whenever design change occurs, the Design FMEA must be updated.
  • 04:48 Of course if you found some key insights that would have applicability to other
  • 04:51 products, be sure to share those through your PMO or other mechanism for
  • 04:56 sharing best practices.
  • 04:58 Let other teams leverage the good work that you've done.
  • 05:01 The analysis steps of the Design FMEA are where the insights
  • 05:05 into technical design risk are revealed.
  • 05:08 Take them seriously if you want to manage your risk.

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