PFMEA Severity - What is Process FMEA Severity estimation based on?

prototyper said:

The whole point of a PFMEA is to focus attention on the most important functional areas of the product.
If a poor weld would result in a wheel falling off a car then the consequences of failure are far more important than something cosmetic, like scratched paintwork. The weld may have a specification, but a specification won’t stop you making bad parts.
The reason for having a severity rating is to ensure that features which are of a safety critical nature are properly controlled.
The severity ratings must, therefore, consider implications to the end user as well as downstream processes.

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Very good. In fact, if one reads the Severity table (in the PFMEA section of the AIAG FMEA book, 3rd ed.), it does a very good job of describing possible severity levels to both the assembly plants and the end users. A mandatory read if you are going to use FMEAs.

You can also reference the Ford FMEA handbook version 4.1 which was released in February 2004.
The guidelines for PFMEA severity include both the effect to the customer, "Very high severity ranking when a potential failure mode effects safe vehicle operation and/or involves noncompliance with goverment regulation without warning", as well as the effect to downstream assembly/manufacturing operations.

Helmut Jilling said:

Very good. In fact, if one reads the Severity table (in the PFMEA section of the AIAG FMEA book, 3rd ed.), it does a very good job of describing possible severity levels to both the assembly plants and the end users. A mandatory read if you are going to use FMEAs.

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Sure, I agree that the AIAG FMEA 3rd edition book does have severity levels listed but they are pretty tough with the exception of a rating of 9 or 10 which are for safety.

Let's take rating 8 for an example.

"vehicle/item inoperable (loss of primary function)" or in the plant "0r 100% of the product may have to be scrapped, or vehicle/item repaired in repair department with a repair time greater than one hour"

That is pretty easy if a car does not start - 8 but in the plant it becomes fuzzy. If we are missing a hole and the parts will not assemble, we have to come up with a disposition of 100% scrap?? More than 1 hour in the repair department? It could be less than an hour depending upon when the nonconformity was caught. Maybe we have parts mixed with some have all the holes and some with a hole missing.

How is this? Doesn't work (won't assemble) - 8 and don't worry about the disposition
7 - works but needs big hammer or car starts but sputters down the road
6 - works but needs smaller hammer.
4 - visually pretty ugly - everyone sees it
3 - visually bad - less severe than ugly

The AIAG standard is recommended but not manditory. I would use it to come up with something a bit easier in the plant.

The AIAG and Ford listed severity levels are purely guidelines to aid the selection of an appropriate severity rating. Tha manuals could not possibly cover all eventuality.
At the end of the day, a common sense approach is needed and as long as you can justify your selection as conforming to the guidelines, you won't get many arguements.
Remember that PFMEA is a tool to help you to predict failure and improve your controls. Be realistic, based on your knowledge of the fit and function of the product and your customer processes (internal and external).

Re: Severity estimation in a PFMEA is a bit confusing

Jim Wynne said:

My own opinion is that process FMEAs should be concentrated on processes, not products. There will be disagreement in this regard, but to me, it's fairly simple. If a customer has done a DFMEA, and the output of that process is reflected in the product specifications, and the product is manufactured in accordance with the specifications, everyone should be happy, all else being as it should be. This means that I, as the producer of the product, should be able to concentrate on designing a process that will produce product that meets the specifications, and I shouldn't need to concern myself with what might blow up in end use if the product doesn't conform--that's the designer's responsibility.

So to answer your question, I think that the PFMEA Severity factor should be based on the risks of the producer, not the customer.

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Dear sir,


YOU HAVE BROUGHT A BEAUTIFUL ANGLE. I AGREE WITH YOUR STATEMENT YET IN PRACTICE I CONSIDER BOTH PRODUCT & PROCESS & FINAL SEV. I TAKE AS MAX. OF TWO

Hi,
I am new here today. I have gone thru some of the discussion regarding DFMEA & PFMEA.
I am doing a DFMEA & PFMEA for a Throttle Body (butterfly valve) for Automotive/engine. I appreciate if anyone has any example of this.
Thanks,

uptick said:

Hi,
I am new here today. I have gone thru some of the discussion regarding DFMEA & PFMEA.
I am doing a DFMEA & PFMEA for a Throttle Body (butterfly valve) for Automotive/engine. I appreciate if anyone has any example of this.
Thanks,

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Welcome to The Cove Forums!

Forum courtesy suggests to not duplicate posts in other threads.

Your other post is addressed.

Stijloor.

I strongly disagree with Jim!!! If a wheel can fall off the car, it's not only caused by design. It can be an improper use of tool, no control in place, etc. That's why the purpose of an PFMEA is to ensure safety and quality to the user, but also to peolple working further on the line. From the PFMEA, you have to create control to reduce failure.

Amouchicou said:

I strongly disagree with Jim!!! If a wheel can fall off the car, it's not only caused by design. It can be an improper use of tool, no control in place, etc. That's why the purpose of an PFMEA is to ensure safety and quality to the user, but also to peolple working further on the line. From the PFMEA, you have to create control to reduce failure.

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If the wheel falls off of the car due to a manufacturing defect unrelated to design (i.e., the product didn't meet specifications), then what I said earlier (The whole point of a PFMEA is to understand [and mitigate, as far as possible] the risks in the production process) is correct, no? Keep in mind that one of the prime risks in production processes is production of nonconforming material.

Hi all,

I'm new to this site, and forums in general, but feel lucky to have come across this wealth of information. I've been involved with and facilitated FMEAs in the past, and my current employer has me looking at their pFMEA methods to extract more value and to streamline the process (pFMEA). I need to make this tool practical and valuable for the product/process development efforts I'm involved in.

This thread hits on an item that continues to nag me as I try to clarify and standardize certain aspects of our pFMEAs. I'd appreciate any help to get to a workable solution. I hope you'll forgive my wordiness here to try to express my complete thoughts.

It seems that failure mode definition is very key in getting the pFMEA started in the right direction. For a dFMEA, I think failure mode definition is more clear than for a pFMEA. A pFMEA has a potential dual path for FM definition: 1) a symptom/defect on the product or 2), failure of a process step to perform its intended function.

If I choose #1 (a symptom/defect on the product), I seem to be more in line with how most pFMEAs are done (is this true)?

Possible Benefits of #1 Approach:
> Failure mode generation seems more straight forward (if requirements are well defined) basing them on the ways in which the built product can vary from the product requirements - dimensional, visual, design..
> FM at the product makes Effect and SEV easier to tie to critical design characteristics and customer impacts.
> Causes in the process can be expressed at a higher level to help with efficiency of the pFMEA, but still identify areas of concern that may require further root cause investigation.

Potential Problems with #1:
> It seems to me, this approach makes identifying internal effects and SEV meaningless. Since the severity assessment is supposed to be based on the failure, without consideration of frequency or probability of occurance, I can't see how to tie a non-conforming product into a quantifyable internal impact. For instance, often I've seen generic process impacts of scrap, downtime, rework in these types of FMEAs. Sure, but in order to assign a SEV ranking to these, I need to understand the nature of the cause, or failure in the process, to determine how much - scrap, downtime, etc.
> I saw in an earlier post, and agree, that working from a product based symptom to a process cause can miss failure potentials in the process because the focus is not on the process based requirements/functions, but on the product based requirements.

#2 FM defined as failure of a process step to perform its intended function.

Possible Benefits:
> This method focuses on the process requirements (such as - hold energy at XmJ +/- YmJ; clamp part; spin part at Xrpm, etc), which need to stem from the product requirements, and can capture more ways in which the process can fail by focusing on how the process requirements could not be met.
> The causes for these FMs are closer to, or are, the root causes of the problems, getting the team closer to proper mitigation action.
> The internal effects can be assessed at the failure mode stage of the analysis because the nature of the process failure is more clear. Also, the external effect can still be assessed. Note: I think a tie to the external is always needed to put failures into context so appropriate actions and priorities can be set. All product requirements are not equal - some can kill if not met, others make the product less pretty.


Possible Problems:
> This seems to me a deeper level FMEA. We have moved down one notch in the cause-effect; or effect-cause in this case, hierarchy by choosing process failure - which is the cause of the product defects - as our FM. A deeper FMEA requires more up front time, better understanding of product/process relationships, better defined steps and functions, more specific process SME involvement - maybe not bad things? But this may not be as familiar and will take more time - which management won't like, (but I can sell it if it's the right thing to do).

Lastly, I've seen FMEAs where both #1 and #2 types of failure modes are used which would be very confusing - one FM is a defect and another FM can cause the same defect.


I hope you can see how this has been bothering me, and that I've given it some significant thought. I believe that I need to choose one path here to help avoid some of the pitfalls and get a more functional system Please send your thoughts and recommendations. Thank You!!