posted 16 January 2001 05:41 PM               

In a previous life we even had a helpful SQA that gave us RPN numbers of our competitors (no names involved) to benchmark against.

I would like to hear from other forum members if they have any specific supplier limits they could share.

ASD...

posted 17 January 2001 09:17 AM               

From the FMEA manual;
"The risk oriority number is the product of severity (S), Occurrence (O), and detection (D). RPN = (S) x(OD) x (D)
This value should be used to rank order the concerns in the process (e.g., in Pareto fashion)
The RPN will be between "1" and "1,000". For higher RPN,s (higher than 1000) the team must undertake efforts to reduce this calculated risk through corrective action(s). In general practice, regardless of the resultant RPN( S x O x D ), special attention should be given when severity (page 35) is high (7,8,9,10)"
Note: expressions in parenthesis is my input.

posted 17 January 2001 12:41 PM               

On the other hand, unwisely dispensing resources to countermeasure a low rating could prove equally bad.

Keep in mind that the FMEA is a LIVING DOCUMENT. This means it must be revisited from time to time. As such, you might find that an RPN for an item that was low because of a false expectation proves to be high. The reverse is also true. Still, as the process or system changes, the RPN could toggle back and forth.

So when do you react? Well, what is a manageable risk? Keep in mind that the organization for the most part is the AUTHORITY on the process or design. You make the decision as to what is a high or low risk (please pay attention to regulatory requirement, ie Essential Requirements which govern your actions). Being the authority on the product/process, the experts, and having knowledge of your manageable resources, you pick your own battles.

Another not so scientific method, but perhaps better than arbitrarily established RPNs, is to use the 80/20 principle. Use Pareto analysis to establish your priorities and address the "vital few" as your resources permit. Use Corrective/Preventive Actions. As you periodically do your review the FMEA, update your RPNS,reprioritize, and repeat the process.

Just another opinion.

Regards,

posted 19 January 2001 04:53 PM            

1. Any RPN >= 100 must have an action plan to reduce.

2. If no RPN's are over 100, the top 3 RPN's must be considered. Why considered and not an action plan?...because if my top RPN is 20, I will probably get more value out of my engineers elsewhere.

3. Any severity of 9 or 10 must be evaluated. Again, the resultant RPN could be low if occurrence and detectability are 1, but because a 9 or 10 can affect safe vehicle operation or impact a govt. regulation, they deserve special attention.

posted 15 February 2001 03:30 PM               

I have facilitated a number of FMEA meetings. I have noticed if there is a arbitrary cut off value, all of the RPNâs magically and conveniently fall below that value. Now, it make take a bit of time and group justification to accomplish this, but by the end of the day they have convinced themselves of it.

If the RPNâs are on a pareto chart, I have found the group to achieve more realistic RPNâs and make the FMEA process more productive.

Anybody else experience this?

[This message has been edited by AJLenarz (edited 15 February 2001).]

posted 15 February 2001 05:33 PM               

Also agree and have been through it in depth.

Once a limit is set, it seems to be the mind set that human nature will direct you to set your "goals" below the acceptable limit.

Although goals need to be set, once continuous improvement is in place those goals have to be reset downward to continue the improvement process.

I think that internal goals, such as "PPM" must be set lower than any customer expectation.

I believe and have read/heard the mantra "Zero Defects".

Work this entire process into the actual cost of making a good product and we can all see that doing it right in the first place is a good premis, but that finding the faults in the process will lead to real, quantifiable results.

MHO

posted 22 February 2001 09:26 AM               

The Ford system of :

1. Quality History Analysis (FMA),
2. 'P' Diagram,
3. DFMEA (a. Block Diagram, b. Function Trees, c. DFMEA),
4. DVP,
5. PFMEA,
6. Control Plan

helps supplier and OEM engineers to look at what went wrong in the past and try to fix it for the future. An FMEA must consist of a cross-functional team and also include design, quality and manufacturing representatives.

posted 16 March 2001 05:44 AM              

My opinion is that the RPN indication doesnot make sure that your process is safe-guarded against potential failure modes with the controls that you identify. There is an inherent methodology called Process review (Omnex wrote this metholodogy) that can be practiced for the same. For eg., consider the following option:
S=5*0=7* D=5 = 175. We come to the conclusion that an action has to be taken.
But consider the following:
S=9 * O=5 * D=1
This will give an RPN of 45, but when the problem occurs, it will have a higher impact ont he customer. Hence the analysis tool has to be changed. What we are discussing will not work if you are targetting improvement.
For more info visit www.omnex.com or email me at akumar@omnex.com

posted 06 April 2001 04:05 AM               

If a FMEA is prepared taking into consideration the above the resulting RPN will have a close link with the Process and the Failure Modes.

Considering your example 2 even thought the RPN is only 45, Still action is required since the Severity is 9 which is very high which can affect the safety of the user & can be against the govt. regulations.

posted 07 April 2001 02:13 PM               

I can't see why, severity is supposed to be determined by the customer and hypothetically out of your control. (internal or external customer doesn't matter)

As a supplier I would worry more about occurrence & detection than severity and RPN. O & D are within my control. This is not to say that RPN is discounted, if your O & D are high the actions would be commensurate with the severity level.

MHO

posted 09 April 2001 08:20 AM               

quote:

---

Originally posted by Paul Alexander:
On the Severity issue - if it is high - could you not suggest a design change to lessen its impact?

Only a thought.......

---

You sure could, it always helps to improve youself and help the customer!

ASD...

posted 17 April 2001 01:44 PM              

For example, suppose the design in question is for a parachute. The severity of a failure to open would have to be a 10. How do you lower that severity?? A secondary parachute? That doesn't really lessen the severity, though. It just shifts it to the "n'th" redundant chute.

The fact of the matter is that severity of failure is not a function of the design. However, controlling the OCCURENCE and DETECTION RPN's is definitely facilitated by design alternatives, ie. design a chute with a 99.9999% successful deployment rating, and throw in a couple of redundant back-up chutes with equal success ratings, and your occurrence is extremely low.

But if the chutes don't open......

The severity rating cannot change. At least, I don't understand how.

posted 17 April 2001 01:52 PM               

quote:

---

Originally posted by Help Me:

I would suggest that there is NO way to lower a severity rating with a design change.

---

posted 17 April 2001 02:06 PM               

Device failure mode: Chute does not open.
Device failure effect: Probable death.
Design change: Add small extra parachute so if main chute fails, the small chute can be deployed.

New failure effect: Probable broken leg bones.

********************************

Example of 'severity of failure' change as a result of a design change.

posted 17 April 2001 02:23 PM              

If you get to the point where this chute is required and it, too, doesn't deploy, how have you reduced the overall severity? I don't think you have.

Sure, you have reduced the severity of the main chute not deploying (Assuming that the secondary chute deploys). If anything, the addition of the second chute has reduced the occurrence of the catastrophic failure (my earlier contention). But, the same severity exists no matter how many redundant chutes fail. Though, the occurrence will hopefully improve (eventually, one of the dang chutes will probably deploy).

I guess I am still missing something.

posted 17 April 2001 06:01 PM               

Now I like living. I like my customers to live to, so I design a safety razor. Pretty slick, but I can still cut myself. At present, my design will cut, but it would take great effort to kill myself. Severity: 9

This is a pretty extreme example, but with some time, we could all come up with things in our lives/work that if the design is changed, will result in a lower severity rating.

Regards,

posted 17 April 2001 07:05 PM               

Assuming a design FMEA, the DFMEA is about a product. A parachute. In the 'old days' it was nothing more than a half a bag with strings. As the design evolved different things happened. But the point is you take possible failures (e.g.: chute does not deploy) and rate them. Yes - if the main chute doesn't deploy and the small chute doesn't deploy you're still up s__t creek. But the probably of both failing is small compared to only 1 not deploying.

And, we're rating each failure mode individually. If you decide you want to assess what will happen if multiple failures occurr simultaneously you're going to be getting into a more complicated analysis.

->If you get to the point where this chute is required and
->it, too, doesn't deploy, how have you reduced the overall
->severity? I don't think you have.

The second chute is like adding a safety to a gun. It's not specifically required (or wasn't long ago). It's an enhancement of the gun design.

You should stop thinking about each possible individual failure mode as a catastrophic failure which causes the whole device (the product as a whole) to fail. This can happen but not always.

posted 17 April 2001 07:55 PM               

Severity applies to Effect and there can be multiple Modes which have their own Severity which can be internal or external customers.

If need be for internal Severity, we can, and should define our own evaluation and ranking criteria.

Back to real life, Severity is defined by the customer and can be influenced by our suggestions to improve their processes. We all realize that during a PFMEA we have to "assume" that the DFMEA is viable.

MHO and waiting for more responses!

posted 18 April 2001 08:58 AM              

It is quite possible that the designer/manufacturer of the safety chute is not the same party that is designing/manufacturing the safety chute (remember, the chute example is hypothetical).

Since this forum is heavily automotive, I will try to use an automotive example:

The function of the wheels on the car are to provide directional stability. If a wheel falls off because the one and only lug nut included in the design falls off, the result is loss of vehicle control, possible injury/death.

Through analysis it is determined that three lug nuts are sufficient to retain the wheel in any real world situation. Being a very safety concious engineer (btw, I might want to drive one of these things) I decide that if three is sufficient, I will add three more to my design. This is kind of like replacing the razor blade on a stick into a safety razor (whchh, btw doesn't eliminate the potential for cutting the jugular. It just reduces the occurrence to nil).

Now I have 6 lug nuts in my design.

DOES THIS REDUCE THE SEVERITY OF THE FAILURE MODE: LOSING A WHEEL? Absolutely not. If the wheel falls off, I still have the potential of injury/death. Yes, this is another catastrophic event. But, by definition, that is what a severity 9-10 is.

Now, is the 6 lug wheel going to fall off? Very unlikely. But that doesn't change the severity the failure effect should it happen.

Isn't that what a design FMEA does? Take into consideration the:
Severity X Occurrence X Detection = RPN

From a design standpoint I have to appreciate a high severity. That high severity makes me pay particular attention to occurrence and detection factors.

The essence of a Design FMEA is, as it should be, to "What if" the design to death.

posted 18 April 2001 10:02 AM               

You also have to look at the wording of your DFMEA. If the potential failure mode is lug nut failure, the potential effect will probably not be that the whole wheel will fall off if one fails (if you have 6 lug nuts). If you have only 1 lug nut then the potential effect of the failure mode would undoubtedly be that the wheel will fall off. So - by changing the design you have also changed the potential effect(s) of the failure.

So - with 1 lug nut the potential effect of the failure of a lug / stud is very serious. With a re-design to include 6 lug nuts total, the potential effect of a lug / stud failure isn't very significant. If you now take it to the extreme and say "...I want to address the issue of all studs failing simultaneously..." you have to put that in as a line item in and of its self as a potential failure mode.

Part of the mis-conception here may be from the fact that in a process FMEA the ONLY way to reduce severity is through a design change. This is typically true of Design FMEAs as well.

Another possible source of confusion here is that it is NOT necessarily true that every severity rating CAN be reduced. For SOME failure modes there is very little you can do to reduce severity of the potential effects. If your DFMEA line item for a Potential Failure Mode reads 'Failure of all lugs / studs simultaneously ' there is not much you can do to address severity.

Then again, with all the new electronics coming into play, this, too, may be reduced in the future (if it is not already) by recognition of the loss of traction, car body position, etc. So - you may, through a design change where a computer helps maintain control, in fact reduce the severity of all lugs / studs failing at the same time. The reduction in severity may be small, but it is there. It may be that loosing a tire in and of its self, for any reason, is not so much a problem because the computer helps maintain lateral stability.

BTW - I have had tires fall off of a car I was driving twice. Once with while pulling a loaded horse trailer a rear drive wheel of my van fell off (someone stole 3 of 5 lugs off each wheel the night before but I didn;t notice them missing) and once a front tire on a car I had (it had mags and like with the van someone decided they wanted my lugnuts late at night). Neither time did I loose control nor did I really feel I had lost control. Luckily I was on an expressway both times (and I won't begin to get into the flats I've had in my life).

To go to the extreme, you could put in a DFMEA a line item for a Potential Failure Mode of all lugs / studs on all wheels simultaneously failing with the Potential Effect being all wheels fall off of the car at the same time.

Bottom line is you miss the step where your design change (1 nut to 6) changes the effect of the failure.

posted 18 April 2001 11:28 AM              

The point I have been trying to make (unsuccessfully) is that the severity of the POTENTIAL effect of a failure cannot be reduced with a design change to that system in question. The anecdotal evidence of maintaining control of one vehicle after losing a wheel is, indeed, good news! Though, a less skilled driver, in different conditions, may not have been so lucky. But, maybe I made a mistake by choosing a bad HYPOTHETICAL (I wish I could make that word boldface and about 14 font sizes larger). So, I will refrain from trying to use any aditional hypotheticals.
I also agree that the switch to 6 lugnuts would drive new line items. One of which would have to be all 6 lugnuts failing simultaneously. High severity/extremely low occurrence (where have I heard this before?) Likewise, for the secondary, tertiary, nth chute. Severity for the main chute not deploying reduces because the high severity ranking trickles down to the nth chute. Though, eventually, you would run out of altitude before you run out of chutes, I suppose(HYYYYYYPOTHHHHETTTTTTTTTTICALLLLLLLLL).
The bottom line is that our ISO team where I work have picked up on the verbage in the FMEA manual that says that severity rankings can only be reduced by design changes. Therefore, in their minds, any severity ranking of 9-10 is not acceptable and must be reduced by way of design changes.

It is encouraging that you have stated that not all severity rankings can be reduced through design change. Unfortunately, they do not share your interpretation. In their minds, if any severity rankings can be addressed (Lowered) with a design change, then, all severity rankings can.

I know I am going to tick you all off again. But, I still think in the design changes you have suggested in your responses to my hypothetical cases only serve to shift the high severity rankings to another component/system.

posted 18 April 2001 11:55 AM               

On 17 April 2001 you said:

-> I would suggest that there is NO way to
->lower a severity rating with a design change.

If you approach it from the point of view of a system FMEA (as you now are) and your line item for the potential failure is a wheel falling off (due to whatever reason), you can change the design so that the severity of the effect of the failure (the wheel falling off) can be reduced. I can think of lots of ways - some of which would be pretty far fetched, but none the less...

posted 19 April 2001 09:43 AM              

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.

posted 19 April 2001 06:14 PM               

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...

posted 20 April 2001 09:22 AM              

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).

posted 20 April 2001 09:38 AM               

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.

posted 20 April 2001 09:39 AM               

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.

posted 20 April 2001 10:04 AM               

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.

posted 20 April 2001 11:44 AM              

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.

posted 20 April 2001 12:41 PM               

If you will stick to the original line item, the design change will have reduced the severity of that failure mode or, as often happens, eliminated the specific potential failure effect. Your arguement is based upon trying to keep the potential effect of the potential failure.

You keep saying you still have to address the effect of the potential failure.
->But, you still have to look at the POTENTIAL EFFECT of the
->redundant wheels falling off.

Now you're talking a higher level. You're changing the subject.

One effect is for a given wheel to fall off. You want to now say, well, all the wheels might fall off so you have to address that. Well, maybe, maybe not. But let's say you you do want to address all wheels falling off at the same time. That is more of a system FMEA than a design or process FMEA. None the less, we'll say it's the line item you're concerned with.

Potential Failure Mode = All wheels fall off at the same time.
Potential Failure Effects = Car uncontrollable, possible death.

If I redesign the car to ride on an air cushion, the severity of all wheels falling off approaches zero.

->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.

Yes - sometimes there are tradeoffs. Sometimes not.

->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".

So? Is that somehow illegal? Please explain why that does that not count for reducing the severity of the effect of a wheel falling off. So what if you assign it to another system which can better handle the problem.

-> Whether they fall off
->simultaneously, or one at a time without any detectable
->warning to the operator.
This is not a valid arguement because that's not how FMEAs evolve.

I have never, by the way, seen a top level automotive FMEA (or lower level FMEA, whether design, system, or process) which has as a Potential Failure Mode of "Every wheel falls off at the same time". It doesn't happen in real life. One wheel - yes. Maybe even 2. But 3? Or 4?

posted 20 April 2001 12:52 PM            

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).

posted 20 April 2001 12:56 PM               

You posted:

...Every month you hear of another stability function being added to cars. All wheel drive, ABS, electronic stability control systems - all of these reduce the severity of a wheel falling off. And they are all design changes.

Thank you, severity can be reduced!!!!!!!