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How to Calculate RPN in Design FMEA (DFMEA) vs. Process FMEA (PFMEA)

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fmea - failure modes and effects analysis, fmea rankings, rpn (fmea risk priority number)
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  Post Number #1  
Old 19th March 2008, 06:29 AM
nsraj

 
 
Total Posts: 3
Please Help! How to Calculate RPN in Design FMEA (DFMEA) vs. Process FMEA (PFMEA)

Hi,

In FMEA normally we'll calcualte the RPN as Sev x Occ x Det,

but recently I came to know that in DFMEA the RPN calculation method is slightly difference. i.e.

First Calulate RPN for Failure Mode (RPN FM = Sev (Max) x Occ X Det(FM))

and then RPN for causes,

but in calculating RPN for causes, I was told the formula as

RPN C = Sev (max) x Occ x Det (Min (FM, Causes)

Example

Assuming
for Failure mode RPN FM = 5 X 5X 3 = 125

for cause 1 suppose if i get Sev = 5, Occ=5 and Det = 4,

I normally calculate as 5 x 5 x 4,

but i was told to calculate as 5 x 5 x 3(Det of FM)

is there any reference to this method???


nsr

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  Post Number #2  
Old 19th March 2008, 09:16 AM
Valeri's Avatar
Valeri

 
 
Total Posts: 265
Re: How to Calculate RPN in Design FEMA (DFMEA) vs. Process FMEA (PFMEA)

Good morning,

I'm not quite sure where you found/heard that calculation for RPN but it should be the same (S)x(O)x(D)=RPN (severity x occurrence x detection = risk priority number) no matter if it's a DFMEA or PFMEA. Then again, maybe I'm not understanding the question
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  Post Number #3  
Old 19th March 2008, 09:43 AM
Jim Wynne's Avatar
Jim Wynne

 
 
Total Posts: 14,077
Re: How to Calculate RPN in Design FEMA (DFMEA) vs. Process FMEA (PFMEA)

Quote:
In Reply to Parent Post by nsraj View Post

Hi,

In FMEA normally we'll calcualte the RPN as Sev x Occ x Det,

but recently I came to know that in DFMEA the RPN calculation method is slightly difference. i.e.

First Calulate RPN for Failure Mode (RPN FM = Sev (Max) x Occ X Det(FM))

and then RPN for causes,

but in calculating RPN for causes, I was told the formula as

RPN C = Sev (max) x Occ x Det (Min (FM, Causes)

Example

Assuming
for Failure mode RPN FM = 5 X 5X 3 = 125

for cause 1 suppose if i get Sev = 5, Occ=5 and Det = 4,

I normally calculate as 5 x 5 x 4,

but i was told to calculate as 5 x 5 x 3(Det of FM)

is there any reference to this method???


nsr
I've never seen anything like it, and it seems to be a needless overcomplication of an already dubious practice.
  Post Number #4  
Old 19th March 2008, 10:45 AM
nsraj

 
 
Total Posts: 3
Re: How to Calculate RPN in Design FMEA (DFMEA) vs. Process FMEA (PFMEA)

Hi Jim

Thanx for the reply, me too thought the same, but wanna know if realy anything is there like that or not. If anybody come across such info pls pass it on

and Valeri,

let me put it in other way, do we need to calculate RPN for "Failure mode" Separately and RPN for its "causes" separately???
  Post Number #5  
Old 19th March 2008, 10:50 AM
Jim Wynne's Avatar
Jim Wynne

 
 
Total Posts: 14,077
Re: How to Calculate RPN in Design FMEA (DFMEA) vs. Process FMEA (PFMEA)

Quote:
In Reply to Parent Post by nsraj View Post

Hi Jim

Thanx for the reply, me too thought the same, but wanna know if realy anything is there like that or not. If anybody come across such info pls pass it on

and Valeri,

let me put it in other way, do we need to calculate RPN for "Failure mode" Separately and RPN for its "causes" separately???
You said that you "came to know" this method--from where? Unless you have a customer that requires it or you can find some utility in it in your own application, I would just stick to the SxOxD method and be done with it.
Thanks to Jim Wynne for your informative Post and/or Attachment!
  Post Number #6  
Old 12th November 2012, 10:52 PM
parallel

 
 
Total Posts: 3
Re: How to Calculate RPN in Design FMEA (DFMEA) vs. Process FMEA (PFMEA)

Hi

I have a slightly different doubt about RPN:

RPN=S x O x D

If S is more which is bad, is given maximum rating.
If O is more which is bad, is given maximum rating.
If D is more, which is good (prevents failure from reaching the customer), should we not give minimum rating?

This way priority will have balanced scale.

I know my thought does not tally with our conventional method of calculating RPN.

Please advice.
  Post Number #7  
Old 13th November 2012, 01:24 AM
Chennaiite's Avatar
Chennaiite

 
 
Total Posts: 559
Re: How to Calculate RPN in Design FMEA (DFMEA) vs. Process FMEA (PFMEA)

Quote:
In Reply to Parent Post by parallel View Post

Hi

I have a slightly different doubt about RPN:

RPN=S x O x D

If S is more which is bad, is given maximum rating.
If O is more which is bad, is given maximum rating.
If D is more, which is good (prevents failure from reaching the customer), should we not give minimum rating?

This way priority will have balanced scale.

I know my thought does not tally with our conventional method of calculating RPN.

Please advice.
Without getting into the semantics, please refer whichever rating criteria you have. I am sure it says 'lower the better' for Detection rating as well.
  Post Number #8  
Old 13th November 2012, 09:21 PM
parallel

 
 
Total Posts: 3
Re: How to Calculate RPN in Design FMEA (DFMEA) vs. Process FMEA (PFMEA)

Thank you. However my doubt still remains.
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