The FMEA Mini-Series: Using an FMEA vs. SxO for Prioritizing

[Help Me]

OK. I'll bite. (I almost made it a full 24 hour without breaking my self imposed moratorium on posting).

How does the severity of a wheel falling off ever change?

IF it falls off, it falls off.

Just as an aside, a few years ago, there was a jet that crashed. It was determined upon investigation that what had happened was that an engine blade came loose and cut through the primary and the two or three back-up hydraulic lines meant to provide control to the tail sufaces used to control the flight of the plane.

Now, I am sure that you could say that the original severity ranking for the effect of the main hydraulic line failing was decreased by the addition of the secondary and tertiary hydraulic lines. But, in the big picture, shouldn't the severity ranking of last redundant hydraulic line have inherited the high severity ranking for effect of potential failure: Loss of control surface articulation, if you will.
This real world, actually happened, example, I think, illustrates my contention that you don't really reduce the severity ranking with a design. Whether it is a safety razor, or a space shuttle.

Yes, I think in terms of systems. I think it is a bit naive to not think in terms of systems. I really believe that the low hanging fruit and the biggest bang for the buck(pardon the gratuitous usage of cliche) is in focusing on design changes that will reduce the occurence and detection rankings.

Obviously my focus on a system FMEA is incorrect as no responses have echoed my stance.

Many thanks for your tireless (not intended as a pun related to the lugnut issue) efforts to explain the logic to me.

 

[Kevin Mader]

Create a new design where you don't need any wheels. Then they can't fall off.

 

[Al Dyer]

Let's take a different direction:

I start with acute pancreatitis. What is the mortality rate? <10% (death)

I increases to chronic pancreatitis. What is the mortality rate? >50? (death)

I increase to pancreatic cancer. What is the mortality rate? >99? (death)

Can I reduce the severity by acting upon O & D by redesigning my life style?

Yes

ASD...

[This message has been edited by Al Dyer (edited 19 April 2001).]

 

[Help Me]

AHA!!!

The pancreas example is a good one to explain my position.

And you have included all the information that I need to explain myself!

Firstly, the severity rating for all three cases is identical:

Acute-P, chronic-P, and P-cancer have identical severity ratings. The rating is 10 because the POTENTIAL effect is death. And the potential effect is death regardless of how good the detection is.

In other words, if the patient dies, he/she is just as dead if the pancreatic condition (take your pick which one)was detected or not.

Your model also illustrates nicely that the occurrence of the effect is very different for the different conditions. As you have stated, the Ocurrence of potential effect are, <10%, >50%, >99%, respectively. So, let's give them O's of 1, 6 and 10, respectively. Now, I am going to say from this that my detection had better be good, near 100% for all three cases due to the high severity. I want to have a detection rating no higher than 1 for any of the potential failure modes (acute P, chronic P, P cancer).

What this does is effectively control my RPN.
Results:

RPN Acute=10
RPN Chronic=60
RPN Cancer=100

But, as you pointed out, Al, the severity of all three remain at 10 (the potential effect of failure:death).

HM

 

[Sam]

Ok guys, I Think "Help Me" has us backed up into a corner. I do believe he is corect in his assumptions.
From the book; Severity of effect 10
"Very high severity ranking when a potential failure mode affects safe vehicle operation and/or involves noncompliance with a government regulation without warning"

Having read and re-read this statement several times I agree with "Help Me", There is nothing one can do to lessen the severity ranking. You can design a vehicle, as someone mentioned, without wheels, but you haven't lessened the ranking you have eliminated it; and just replaced it with another ranking.
You can add dual wheels, but that only reduces the severity ranking on one wheel.

IMHO the answer is a careful review and analysis of the potential effects of failure before selecting a ranking.

 

[Sam]

Ok guys, I Think "Help Me" has us backed up into a corner. I do believe he is corect in his assumptions.
From the book; Severity of effect 10
"Very high severity ranking when a potential failure mode affects safe vehicle operation and/or involves noncompliance with a government regulation without warning"

Having read and re-read this statement several times I agree with "Help Me", There is nothing one can do to lessen the severity ranking. You can design a vehicle, as someone mentioned, without wheels, but you haven't lessened the ranking you have eliminated it; and just replaced it with another ranking.
You can add dual wheels, but that only reduces the severity ranking on one wheel.

IMHO the answer is a careful review and analysis of the potential effects of failure before selecting a ranking.

 

[Marc]

->Ok guys, I Think "Help Me" has us backed up into a corner.
->I do believe he is corect in his assumptions.

You may be backed into a corner but I'm not. You and Help Me are missing a couple of important concepts.

If you, for example, redesign the car with no functional wheels (let's say you use an air cushion) where previously the severity of a wheel falling off was 10 (possible death), the severity of a wheel falling off now becomes zero. If the car doesn't depend upon wheels any more (a design change), if you have a wheel (maybe as an ornanment) and it falls off its severity is now 4 or less.

In reality the re-design would effect the removal of the FMEA line item for a Potential Failure Mode for a wheel falling off, or, if a wheel is kept for 'looks', the Potential Effect would now probably be 4 or less - customer dissatisfaction.

->You can design a vehicle, as someone mentioned, without
->wheels, but you haven't lessened the ranking you have
->eliminated it;

It is a given that some design changes will eliminate a potential failure mode all together.

->You can add dual wheels, but that only reduces the
->severity ranking on one wheel.

That was what was originally being looked at. The line item for the Potential Failure Mode was Wheel falls off. From another point of view. You have a line item for a wheel falling off. You re-design the vehicle to have 2 rear wheels, two centre wheels and 2 front wheels. This design change will lessen the severity of any individual wheel falling off.

 

[Help Me]

Yes,
You can lessen the severity of any one wheel falling off by adding more wheels. But, you still have to look at the POTENTIAL EFFECT of the redundant wheels falling off. Whether they fall off simultaneously, or one at a time without any detectable warning to the operator. Again, the high severity ranking will be lurking in some other part of the design. You can't get rid of it. At best, you can find a wall to throw it over so that the engineer of another system has to cintend with a high severity.

Replace the wheels with a cushion of air. Fine, the severity fo losing an ornamental wheel is relatively minor. But, you have also changed the "Item/Function". Once the function of the wheel changes, you have to go back and look at the sytem/component that replaced the original wheels' function. In this case, the air cushion system, or whatever. So throw it over the wall to the Air Cushion Engineer. Loss of air pressure severity ranking on his DFMEA HAS to be the same as the original ranking for losing a wheel.

 

[Marc]

->The severity applies to the EFFECT only. The effect of a
->given failure will not change unless you change the design
->of the system or part. In the parachute example, if the
->chute doesn't open, you probably die and therefore it is a
->10 (failure occurs without warning). 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.) 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).
->
->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. If the severity is
->high, we at least think about any changes that might be
->made. Often times, we have no control on what the vehicle
->does when our parts fail...this is determined by the car
->companies and we all know they are infinitely wise in
->areas of quality and safety. If changes are not feasible,
->we then focus on occurrence and detection (See my post
->much, much earlier in this string on what our procedure
->is) to bring the RPN into an acceptable level.

Well said and concise. I wish I had said that!

 

[Al Dyer]

Just as an aside,

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.

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ASD...