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Each of the functions identified in the block diagram are susceptible to failures. These failures are identified and recorded on the FMEA.

Quick reference

DFMEA Failure Modes

Once the functions for the system or device that is being analyzed have been determined, the failures of each of those functions can be identified.  The functions and failures then initiate the Design FMEA form.

When to use

Every Design FMEA requires the failure modes for the functions to be identified in order to be scored.  Capturing these failure modes is Step 4 in the Design FMEA process.

Instructions

Using the functions that were identified in the block diagram, product specification and interface matrix, brainstorm with the FMEA team the failure modes associated with each function.  Use the failure types discussed in previous lessons to assist in identifying failures.  Generally, the intermittent failure and over-processing failure are not useful for the Design FMEA.  There is not enough history with the new product to identify intermittent problems and the over-processing failure is focused on the work done in the operational processes.

Every function will have at least one failure and many will have multiple failure modes.  Each failure mode may manifest itself in one or more failure types.  For instance, a failure mode may lead to a complete failure, a partial failure, or degraded performance depending upon the magnitude of the actual failure involved.  The Function-Failure matrix is an excellent tool to help brainstorm failures.  Your organization may have other tools and techniques or you may just record the failures that are identified in a brainstorming session.  I have found this technique helps to both stimulate the process and document the comments.

With your list of functions and failures, you are now ready to initiate the Design FMEA form.  These items make up the first two columns of the form.  If a function has multiple failure modes, each failure mode is a separate line on the FMEA form.  Every function should have at least one failure mode associated with it.  However, there is no maximum number of failure modes, you should record all that are relevant to that function.

Hints & tips

  • Don’t jump to effects yet; just record the function and how that function could fail to operate as expected.  The next step will discuss effects.
  • Don’t combine different failure modes onto one line;  each failure mode will likely have a different score and should be analyzed by itself.
  • If the same failure mode shows up with multiple functions, you can either combine all those into one failure mode or continue to treat them separately.  A caution when combining is that you need to be sure that you consider the impact to all the functions when evaluating severity and detection. The absence of some functions may be much more severe than others.  For that reason, I often will keep them both, each associated with the different functions.
  • The Function-Failure matrix is a tool to help brainstorming.  Don’t get hung up in the format.  At the end of the analysis what we are keeping and documenting is the Design FMEA, not the Function-Failure matrix.
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  • 00:04 Hello, I'm Ray Sheen.
  • 00:06 Well, we determined the functions, so now it's time to list the failure modes.
  • 00:11 This lesson will build on a previous lesson that discussed types of failures.
  • 00:16 We start with the orignal list of failures and
  • 00:19 six of those failure types occur frequently in the Design FMEA analyses.
  • 00:24 These six are complete failures, partial failures, excessive function,
  • 00:28 degraded function, too early, too late, and unwanted functions.
  • 00:34 Every part or component in a design has at least one function.
  • 00:37 Otherwise, it would not be in the design and most parts or components
  • 00:42 have multiple functions and every function has at least one failure mode.
  • 00:46 There's something that could block, prohibit,
  • 00:49 or undermine the ability of that function to be completed successfully.
  • 00:55 So let's look at this matrix known as the function failure mode matrix.
  • 00:59 The first column is the function or action that we found in a previous lesson and
  • 01:04 the next column, the Item, is the object of that function.
  • 01:07 We now have a complete function description.
  • 01:10 At this point, you brainstorm what types of failures could occur with the function.
  • 01:15 You can use the next six columns as brainstorming aids.
  • 01:19 These columns are the failure types that are most commonly found in a Design FMEA.
  • 01:24 You may be saying to yourself,
  • 01:25 why do you need to indicate more than one failure type for each failure mode.
  • 01:29 Isn't one enough to make sure you're analyzing it?
  • 01:32 Well, yes, one is enough.
  • 01:33 But knowing that there are multiple failure types associated with the failure
  • 01:36 mode will be helpful when analyzing the failure severity and
  • 01:40 detection, which we will do in the next few lessons.
  • 01:43 A failure that can occur in many ways is more likely to be severe and
  • 01:47 also that failure mode can be more difficult to detect.
  • 01:51 If your detection is only looking for complete failures but
  • 01:55 the failure mode may manifest itself as the graded performance, you might miss it.
  • 01:59 Let’s work through a few of these functions and failures.
  • 02:02 The first line is the function Store and what we are storing is Ink.
  • 02:06 Well, of course, failing to contain or
  • 02:09 store the ink can only occur in one of two ways.
  • 02:12 A failure of the ink tube or a failure of the ball.
  • 02:15 We're classifying both of those as being potentially complete failures,
  • 02:19 partial failures, or degraded function.
  • 02:21 Another function is to dispense the ink.
  • 02:24 Keep in mind that this is done by the ball rolling across the column of ink
  • 02:28 in the tube and then the ink which now covers the ball
  • 02:31 is deposited on the paper as the ball rolls across the paper.
  • 02:35 We see that there are four possible failure modes.
  • 02:38 Clogged at the tip of the Ink column, dried out at the tip of the Ink column,
  • 02:41 drips before writing, and blots while writing.
  • 02:45 Clogged at the tip is a complete failure or
  • 02:47 a partial failure, depending on the amount of clogging.
  • 02:50 The dried out at the tip is a degraded function or
  • 02:53 too late dispensing of the ink until the ball if fully lubricated again.
  • 02:58 Drips before writing or too early are unwanted function
  • 03:01 just depends on how much is dripping and where it is dripping.
  • 03:04 And finally, blots while writing are an excessive function.
  • 03:07 Take a look at the rest of the matrix and see how this approach works.
  • 03:11 We'll be working with the functions of the ball on the next slide.
  • 03:16 Let's transfer the information to the Design FMEA form.
  • 03:20 I will walk you through this form.
  • 03:21 In the upper-left corner, you can see the system is the pen,
  • 03:25 the subsystem is the refill tube.
  • 03:26 And the actual part that we'll be analyzing on this form is the ball.
  • 03:30 In the upper right column, you can see that we're on a revision A.
  • 03:34 It's June of 2017 and there are four of us on the Design FMEA team.
  • 03:40 Now since this is the ball that we are analyzing,
  • 03:42 we'll only include functions of the ball.
  • 03:45 Three of these are to maintain a point, resist abrasion, and roll smoothly.
  • 03:50 There are more functions but that's all that we will show in this example.
  • 03:54 We could be doing this analysis at the subsystem level instead of at
  • 03:58 the part level.
  • 03:59 In that case, we would have the functions of the ink tube assembly,
  • 04:02 which is the refill tube, the ball, the ink, and the front spring.
  • 04:06 Now, for each function, we list all the possible failures of that function.
  • 04:11 Notice that each failure gets its own line.
  • 04:14 That's because we are evaluating the failure modes, not the functions.
  • 04:18 So for the function maintain point, we have failure modes of ball falls out,
  • 04:23 ball stops rolling, and ball corrodes.
  • 04:27 The function resist abrasion has a failure of wearing a flat spot.
  • 04:31 And for the function of the ball rolling smoothly,
  • 04:34 the failure is ball skips while rolling.
  • 04:37 There is also a failure of ball stops rolling associated with that function but
  • 04:41 we've already listed that failure and that item, so we don't need to list it again.
  • 04:47 Well, we have the FMEA form well underway.
  • 04:50 Now that the failures are identified, it's time to begin scoring them.

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