PFMEA (Process FMEA) issue about lower RPN - AIAG 3rd edition page 55

[LI Yan]

Hi, I have a question about FMEA manual(AIAG, 3rd edition), in P55, as following
"To reduce the probability of occurrence, process and/or
design revisions are required. An action-oriented study
of the process using statistical methods could be implemented
with an ongoing feedback of information to the
appropriate operations for continuous improvement and
defect prevention.
• Only a design and/or process revision can bring about a
reduction in the severity ranking."

1. Can somebody explain why "design" and "process" in different sequence to lower "S" and "O"?
2. Can only "process revision" lower "S"? I have heard from somebody, said no.

[D.Scott]

In Reply to Parent Post by LI Yan

Hi, I have a question about FMEA manual(AIAG, 3rd edition), in P55, as following
"To reduce the probability of occurrence, process and/or
design revisions are required. An action-oriented study
of the process using statistical methods could be implemented
with an ongoing feedback of information to the
appropriate operations for continuous improvement and
defect prevention.
• Only a design and/or process revision can bring about a
reduction in the severity ranking."

1. Can somebody explain why "design" and "process" in different sequence to lower "S" and "O"?
2. Can only "process revision" lower "S"? I have heard from somebody, said no.

1) I think the easiest explanation comes by way of looking at the process of packing a parachute. The severity assigned to a parachute failure to open would be 10 because failure would result in some one's death. The only way to change the severity would be to redesign the parachute so it wouldn't matter if it didn't open. Nothing you do in your packing process will effect the severity. You can, by improving the process, lower the number of times the chute is packed wrong. This would reduce the "O" (occurrence) RPN. Detection ("D") can also be improved in your packing process and reduce the RPN.

2) I don't like to say "only" but in most cases the severity ("S") has to be reduced through re-design of the product, not the process. I suppose it is possible for a process revision to lower the severity RPN but I sure can't think of an example.

I hope this addresses your question.

Dave

[LI Yan]

Thank Dave's quick answer. I also cannot imagine what examples can demonstrate a process revision can lower severity. But the reference manual mentions "Only a design and/or process revision can bring about a
reduction in the severity ranking." . That mearns both design and process revision could reduce S alone, not MUST be coperated to use?
I also want clearify a Engilish grammar:
Can expression "A and/ or B" disassemble to "A+B" or "A alone" or "B alone"?

[harry]

In Reply to Parent Post by LI Yan

I also want clearify a Engilish grammar:
Can expression "A and/ or B" disassemble to "A+B" or "A alone" or "B alone"?

Yes, you are right.

[LI Yan]

This question puzzle me several years. Years ago, I attended a training presented by a TS training company (so well known ) the trainer explained this grammar like this: "A and/or B" = "A+B"; or "A alone". Has any body know it?

[LI Yan]

For this case, "only Design and /or process revision can lower S" means "design and process revision can lower S" or "only design revision can lower S", but process revision cannot lower S. If so, very reasonable!

But precondition(grammar) is not correct, right?

[Bill Ryan - 2007]

I think we may be getting hung up on the Failure Mode being Product related. In that case, I agree that the "only" way to reduce the Severity ranking is through a design change.

If the Failure Mode is Process related (a parameter of some sort), then it seems a change made to the process could reduce the ranking. For instance: if my process has a high severity for hold time in the machine and I performed a DOE which showed I could change a less critical parameter which would lessen the impact of hold time, I would then reduce the severity ranking for that Failure Mode.

Does that help any????

[LI Yan]

Maybe you are right. But still I cannot got whole picture of your example. I also think you maybe lowered the ocurence ranking, not severity ranking. Your detailed example will be appreciated.