The Elsmar Cove Forum How to Calculate RPN in Design FMEA (DFMEA) vs. Process FMEA (PFMEA)
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#1
19th March 2008, 05:29 AM
 nsraj Shy Poster (1 to 5 Posts)   Registration Date: Oct 2007 Posts: 3 Thanks Given to Others: 1 Thanked 0 Times in 0 Posts Karma Power: 27 Karma: 10
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

#2
19th March 2008, 08:16 AM
 Valeri Appreciated Member   Registration Date: Nov 2002 Location: Indiana Age: 63 Posts: 264 Thanks Given to Others: 59 Thanked 102 Times in 60 Posts Karma Power: 75 Karma: 1195
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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#3
19th March 2008, 08:43 AM
 Jim Wynne Cross Forum Moderator   Registration Date: Jan 2005 Location: Southeast Wisconsin Posts: 13,886 Thanks Given to Others: 1,982 Thanked 5,834 Times in 3,730 Posts Blog Entries: 2 Karma Power: 1509 Karma: 41632
Re: How to Calculate RPN in Design FEMA (DFMEA) vs. Process FMEA (PFMEA)

Quote:
 In Reply to Parent Post by nsraj 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.

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#4
19th March 2008, 09:45 AM
 nsraj Shy Poster (1 to 5 Posts)   Registration Date: Oct 2007 Posts: 3 Thanks Given to Others: 1 Thanked 0 Times in 0 Posts Karma Power: 27 Karma: 10
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???
#5
19th March 2008, 09:50 AM
 Jim Wynne Cross Forum Moderator   Registration Date: Jan 2005 Location: Southeast Wisconsin Posts: 13,886 Thanks Given to Others: 1,982 Thanked 5,834 Times in 3,730 Posts Blog Entries: 2 Karma Power: 1509 Karma: 41632
Re: How to Calculate RPN in Design FMEA (DFMEA) vs. Process FMEA (PFMEA)

Quote:
 In Reply to Parent Post by nsraj 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.

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It is difficult to get a man to understand something, when his salary depends upon his not understanding it. --Upton Sinclair
 Thanks to Jim Wynne for your informative Post and/or Attachment!
#6
12th November 2012, 09:52 PM
 parallel Shy Poster (1 to 5 Posts)   Registration Date: Nov 2012 Posts: 3 Thanks Given to Others: 0 Thanked 0 Times in 0 Posts Karma Power: 6 Karma: 10
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.

#7
13th November 2012, 12:24 AM
 Chennaiite Never-say-die   Registration Date: Mar 2009 Posts: 487 Thanks Given to Others: 159 Thanked 172 Times in 125 Posts Blog Entries: 5 Karma Power: 71 Karma: 1409
Re: How to Calculate RPN in Design FMEA (DFMEA) vs. Process FMEA (PFMEA)

Quote:
 In Reply to Parent Post by parallel 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.

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#8
13th November 2012, 08:21 PM
 parallel Shy Poster (1 to 5 Posts)   Registration Date: Nov 2012 Posts: 3 Thanks Given to Others: 0 Thanked 0 Times in 0 Posts Karma Power: 6 Karma: 10
Re: How to Calculate RPN in Design FMEA (DFMEA) vs. Process FMEA (PFMEA)

Thank you. However my doubt still remains.

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