posted 20 April 2001 01:43 PM               

The AIAG FMEA manual designates (both on page 13 for DFMEA and page 35 for PFMEA) that the severity rating can be modified for specific parts/products as long as the group agrees on the criteria.

?????

posted 20 April 2001 02:24 PM              

All along, through my posts I have been saying that a high severity ranking (which, by definition, is the ranking of the severity of the effect of the potential failure-I think I got ripped once for looking at it in this way)is an indication that specific attention should be given to reducing the occurrence and detection rankings, therefore the overall RPN.

I think I referred to this as "Low hanging fruit." And, I would suggest that all the horns and secondary chutes actually, on a system level, do just that. They reduce the occurrence of the mechanism of the potential failure, and provide a means of detection of a failure. So, the redundant systems are a fantastic way to lower an RPN with design changes. But, if I remain cognizant that these items only will come into use if the main system fails, I must also be prepared to address what happens if the back-up systems (ABS, stability control, horns, back-up chutes, and the like) fail. And in the vast majority of cases, the severity of effect of the failure (again, this is the definition of DFMEA severity)of the parent system is inherited by the child, or back-up system.

If I don't, at least, consider what could POTENTIALLY happen if all the wheels fall off,(btw, does anybody remember when I said this was hypothetical? Sometimes exaggeration helps make the point. eg. I'll bet the guy who designed the hydraulics on that plane never dreamed that the main and two back-up systems could be wiped out in one event, either. But, it happened.) I haven't really assessed the seriousness of the effect of all the failure modes. Again, I admit that the wheels falling off was an unlikely HYPOTHETICAL (I am sure I said that before. Didn't I??)

As for System FMEA/ Design FMEA goes, I can't help but look at designs in terms of systems. And the AIAG FMEA reference manual specifically states that, "End items, along with every related system, subassembly and component should be evaluated." So, I don't think I am off base by taking a systems approach.

Perhaps we must agree to disagree on whether or not a severity ranking for the effect of a potential failure (the definition of DFMEA Severity)can be effectively reduced by a design change and get on with our lives.

I will stick to focusing on designs that will favorable impact occerrence and detection, esecially on the high severity items.

I thank all of you for your patience with me on this topic.

posted 20 April 2001 02:40 PM               

Don't say good-bye. Join the forum and post your opinions, we all need a wake-up call now and again.

posted 20 April 2001 04:31 PM               

TRUE
"No suppose I design a smartchute that has built in diagnostics that emits a loud audible alarm telling me it is not going to open...It still doesn't open, but warns me that I am about to die, therefore making it a 9 (failure occurs with warning.)"

TRUE but the potential failure effect of the of the alarm becomes 10.

"If I take it a step further, and add a smaller backup chute that deploys, that allows me to land without dying, I can make a case for the severity being a 7 (Item operable, but at a reduced level of performance. Customer dissatisfied)."

TRUE, but the potential failure effect of the backup chute becomes 10; and so on through out the design phase.
The original potential failure effect was "the chute does not open" The ranking will never change but will only be carried over to each design change. IMHO

"The question of action should be based on the RPN, not severity only. I think your ISO organization is wrong if they are trying to force you to lower the severity just because it is a 9 or 10. We often have 9's and 10's on our DFMEA's because many of our products effect safe vehicle operation or a government regulation."

posted 20 April 2001 04:45 PM              

Thank you!

I knew somebody out there had to understand what I was trying to say.

I agree wholeheartedly with the line you refer to from the manual.

And I think that comes through from my posts.

posted 20 April 2001 05:23 PM               

quote:

---

Originally ...

Refer to the AIAG FMEA manual pg. 21step 18 the last underlined sentence in the paragraph..........[/B]

---

I gracefully ask, what is your point?

The FMEA manual says we can define our own criteria and what do you consider high severity?

I think it is company specific and based upon customer requirements!

Does underlining a sentence that starts with "In General......." make a difference?

I will not go into the other subjects such as "shall", "should", "note", "as applicable" etc..............

ASD...

posted 23 April 2001 09:20 AM            

Yes, the OEM's get a little antsy when 9's and 10's are in FMEA's despite the fact many of them are driven by the system they own. (I wonder what the Firestone FMEA had in it?) If you look closely at my original post, I did say..."If the severity is high, we at least think about any changes that might be made." The actual verbiage in our internal procedure is "...Action plans will be considered and initiated, where possible, for the following FMEA conditions: 1) the top (highest) 3 RPNs, 2) a severity number greater than 8...). Why does it say "...where possible..."? I still stand on my claim that real action is based on RPN. What is a design team to do if they have a S=9,O=1,D=1 making the resultant RPN=9? My answer?...nothing, unless there is some easy and inexpensive change that can be made. My company's engineering resources will be better spent working on the other issues or designing the next product. In general I agree with the underlined statement on page 21, section 18. This doesn't mean I force my design organization into the redesign loop just because I have a 9 or 10.

As for the analysis of my scenarios, I agree that a failure of the main chute AND the alarm constitute a 10. I suppose one could make some argument as to the failure of the alarm itself being a 10, but I would put it further down on the scale. Most FMEA's don't take into account 2nd, 3rd, and 4th level failures in determining severities. While that would be nice, for complex products, it becomes an impractical exercise and none of us would want to pay the cost in sticker price for each part of a vehicle to take the FMEA to that level.

In the end, if I have a 10 and my design is so robust that my occurrence is a 1 and my design controls are so thorough as to be a 1, I move on and really worry about those items that have S=7,O=5,D=6,RPN=210 or 9 and 10 severities that do not have low occurence and detection ratings.

posted 25 April 2001 10:06 AM               

Hopefully there is data (PPM, Scrap, Rework etc...) from past or current projects that can be used in the determiniation of your ranking of the occurance.

When thinking occurance don't consider effects or controls, think of causes.

posted 25 April 2001 11:01 AM              

However, the first time inspection is the DETECTION, isn't it? Therefore, if the first time inspection is robust, the overall RPN will be driven down. So, in and of itself, the occurrence of 1 in 20 is not necessarily a bad thing. But it is a huge red flag to make sure that BOTH your detection is robust, AND make sure that your severity is reasonable. If you have a high severity and a 1 in 20 occurrence, I would think that the process may need additional tweaking.

posted 25 April 2001 02:30 PM            

JRS

posted 25 April 2001 03:53 PM               

I think we are arguing semantics here. The FMEA manual states ãThe possible failure rates are based on the number of failures which are anticipated during the process executionä. In my own opinion the setup process is part of the overall global ãprocess executionä.

Look at the broad picture as Al suggests. Base the occurrence value off historical performance data such as PPM, scrap or fallout from past or current projects.

posted 25 April 2001 03:53 PM              

There are some assumptions being made by everyone regarding Laura's example. And, I think we maybe aren't making the same assumptions.

So, help me out Al and J.R. In your interpretation of the scenerio, what would be the process weakness, and what is the process control?

I guess I am looking at it as if the set-up is the weakness, and the inspection is the control. And I think of inspection as a type 2 control. The inspection doesn't really change the occurrence of a wrong set-up. It does, however, detect it.

It almost seems like you are considering the inpection as a type 1 control. Maybe that is acceptable.

Just different ways of looking at it, I suppose. I trust that you will let me know if I am off base.

posted 25 April 2001 06:02 PM               

1: Process Set-Up Is A Process Funtion.

2: Oversized Diameter would be a Potential Failure Mode

3: Unable to process through next operation would be the Potential Effect Of The Failure.

4: Lack of set-up instructions would be the Potential Cause Of The Failure.

5: For Current Process Controls, setup instructions would be a Prevention Control, and 1st artical inspection would be a Detection Control.

The Occurance value would be derived based on #4 from above, the "Potential" occurance of the cause.

This is an over-simplified example.

Severity --- Potential Effect
Occurance --- Potantial Cause
Detection --- Current Controls (Prevention & Detection)

In Laura's example I don't see a reference to the cause of the failed set-up. There might be multiple causes which would call for multiple occurance ratings.

MHO

posted 25 April 2001 10:17 PM               

My thoughts aligned with ASD - however the company did not want to be as specific as "would not assemble" as the effect. They preferred "parts out of spec" - because they are a machine shop and didn't necessarily know the detail of the effect. So each dimension wasn't specifically identified as out of spec - the entire operation was grouped as "parts out of spec." The customer rep was actually present, but also didn't know all of the effects, and the group really wanted to focus on eliminating the cause.

MHO? - the occurrance criteria being used should be established when you start out (we didn't do that). That the PFMEA manual is intended to be in terms of parts (rejects, PPM) and that it needs to be "changed" by company procedures if the processes are set-up dependent. I started out thinking 1/100,000 but changed to 1/20 - with the number off the table being the same - just modifying the criteria.

posted 26 April 2001 12:24 PM               

Your statement:

MHO? - the occurrance criteria being used should be established when you start out.....

You have it in the proverbial nutshell!!!!

Thought Process!!!!!!!!!!!!!

posted 02 May 2001 01:26 PM               

quote:

---

Originally posted by FMEA best:

yOU CAN NOT CHANGE THE SEVERITY OF DYING ONLY THE OCCURANCE OF IT HAPPENING OR THE DETECTION OF THIS THE WARNING SHUTE IS A DETECTION METHOD SO CHANGES THE DETECTION SCORE AND NOT THE SEVERITY. tHE SMART CHUTE IS WHERE YOU ARE REDUCING THE OCCURANCE OF SOMEONE DYING SO AGAIN YOU CAN NOT CHANGE THE SEVERITY OF SOMEONE DYING BUT YOU CAN REDUCE THE OCCURRANCE OR INCREASE THE DETECTION BEFORE JUMPING.

---

Just because you can cite a case where you believe the severity cannot be modified does not extend to imply, much less prove, that in no case is this possible.

To those of you who still feel that you can never modify a severity rating through a design change, I suggest you write a paper and contact the ASQC and other related organizations. Every FMEA book I have read in the last 15+ years states this basic premise. I believe if you can prove your theory you will be the first to do so and will probably go down in 'quality history'. Your revelation will be a turning point and will profit you greatly. You will cause authors to rewrite their books and training companies will change their courses. As an example, you will force the AIAG to change their statements on pages 21 and 43 of their FMEA manual.

posted 03 May 2001 07:59 AM               

The topic was fun for a while, but as you say don't beat the dead horse here. Take it to somebody who cares.

posted 13 May 2001 04:42 PM               

Please see the discussion in the FMEA manual.

D ranking relates to the ability of your detection system, it doesn't matter if it to detect the cause or the failure mode.

So to my understanding you can use the same ranking table for the cause as well as the failure.

Good Luck
MSAFAI

posted 13 May 2001 05:33 PM               

quote:

---

Originally posted by MSAFAI:
Dear Ibrahm,

Please see the discussion in the FMEA manual.

D ranking relates to the ability of your detection system, it doesn't matter if it to detect the cause or the failure mode.

So to my understanding you can use the same ranking table for the cause as well as the failure.

Good Luck
MSAFAI

---

Be careful, the detection rating assumes the failure has already occurred.

Detection ratings apply to how well your current process controls will detect the failure.

I can't see where the Severity, Occurance, and Detection ratings can be interchanged.

MHO