PFMEA rating - Occurence Rating and Detection Rating - Some clarification

Chennaiite

Never-say-die
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#1
Whether the Occurence rating and detection rating are given for the Potential failure mode or for the potential cause??
Thanks.
 
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D

David DeLong

#2
Re: PFMEA rating - Some clarification

Whether the Occurence rating and detection rating are given for the Potential failure mode or for the potential cause??
Thanks.
Yes, this does get confusing and the AIAG manual doesn't help too much.

If we have one potential cause, then we rate the potential failure mode rather than the cause.

If we have 3 potential causes to one failure mode, here is what I would suggest.

Rate the potential failure mode first from 1 - 10 on occurring. Let's us say it is a 6. Now rate the causes independently but one must make sure that they end up with an average of 6. Let's say potential cause #1 is a 8, #2 is a 6 while #3 is a 4. What do we have? An average of 6.

If we rate the potential failure mode a 4, we would could have cause #2 a 6, cause #2 a 4 while cause #3 is a 4.

I have seen so many times when one would rate a cause quite high but the potential failure mode really had a low rating and your company could end up spending a lot of $$ correcting a cause that contributes to a potential failure mode that would rarely happen.

As far as the detection rating, it does relate to catching potential causes (per AIAG FMEA 4th edition) but there may be certain cases where is must be applied to a cause also. It does depend.

Hope this helps a bit.
 

Miner

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#3
Interesting take on that Dave. It is worth deeper consideration.

My practice has been to rate Occurrence on the Failure Cause. This is why the OCC column is immediately after the Failure Cause column. As the SEV column is immediately after the Failure Effects column.

Detection may be rated for either the failure mode or the failure cause, whichever your detection system is set up to detect. If the detection system will detect both the mode and the cause, use the lower of the 2 ratings. Detecting the failure cause is considered more effective than detecting the failure mode because in the latter case, a failure has already occurred, while in the former, a failure may not yet have occurred and may still be prevented.
 

Chennaiite

Never-say-die
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#4
My practice has been to rate Occurrence on the Failure Cause.
If occurrence rating is provided to Potential cause, practically is it not difficult to justify the rating with back-up data? Normally, I guess data for potential cause is difficult to track. Or do we have any other method of providing genuine rating?
Thanks.
 

Miner

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Staff member
Admin
#5
This depends on whether you identify multiple potential causes. A single potential cause would have the same occurrence as the failure mode. If you have two (or three) potential causes, try to determine the relative frequency of one cause versus the other. For example, one cause happens twice as often as the other, so you allocate the occurrence of the failure mode to the causes.
 

Jim Wynne

Staff member
Admin
#6
This depends on whether you identify multiple potential causes. A single potential cause would have the same occurrence as the failure mode. If you have two (or three) potential causes, try to determine the relative frequency of one cause versus the other. For example, one cause happens twice as often as the other, so you allocate the occurrence of the failure mode to the causes.
It becomes a little clearer if we (A) think of failure modes as potential failures of the process and not defects in the product and (B) rate the probability of potential occurrences of causes of process failures. For example, in a machining process, excessive tool wear will result in nonconforming product. "Excessive tool wear" should be seen as a process failure mode, not a cause. The cause might be failure to adhere to a tool-change schedule. The "occurrence" factor should be applied to the likelihood that tools won't be changed when they should be.

If there are several potential causes, each should be given its own occurrence rating, and the RPN should be calculated separately for each, using the same Severity and Detection ratings for each.
 
A

AndyJP - 2012

#7
Basically everything goes back to the failure mode.
Severity - how bad could that failure mode be?
Occurenec - what are the odds on that failure mode being caused by the cause
Effect - with that cause what is the effect of the failure mode
Detection - how will you find that failure mode caused by the cause and how likely is it to find it.
For prevention look at how it will prevent the failure mode.
 
J

jefnik3201028

#8
Hello everyone,
Can anyone give an advise on how to really rate DETECTION?
The rating table clearly mentioned the word "Failure Mode Detection" so does it mean we need to rate based on how well we can detect the failure mode and disregard the effect of the prevention control?

We had trainings for FMEA before we get the TS16949 certification and another one just a few hours ago....I ended up arguing with the trainer because the previous trainer said just rate based on the effectiveness of the detection control...the latest table clearly show the word failure mode and error (cause) in each rating so can we refer to that table?

Additional concern is how do you control the FMEA documents? do we treat it as "controlled" document and follow the standard document change procedures?

Regards,
 

Jim Wynne

Staff member
Admin
#9
Hello everyone,
Can anyone give an advise on how to really rate DETECTION?
The rating table clearly mentioned the word "Failure Mode Detection" so does it mean we need to rate based on how well we can detect the failure mode and disregard the effect of the prevention control?

We had trainings for FMEA before we get the TS16949 certification and another one just a few hours ago....I ended up arguing with the trainer because the previous trainer said just rate based on the effectiveness of the detection control...the latest table clearly show the word failure mode and error (cause) in each rating so can we refer to that table?
The AIAG FMEA manual, 4th Edition says, on page 99, that to determine the detection factor,:
Assume the failure has occurred and then assess the capabilities of all "Current Process Controls" to prevent shipment of the part having this failure mode. Do not automatically presume that the detection rating is low because the occurrence is low, but do assess the ability of the process controls to detect low frequency modes or prevent them from going further in the process.
Additional concern is how do you control the FMEA documents? do we treat it as "controlled" document and follow the standard document change procedures?
That's the way it's usually done.
 
A

amaga

#10
Is there a more simplistic way to do the scoring like what are the keywords to look for to be able to make a scoring. This is more on the detection controls like if the word is related to sensor or alarms, what is the detection rating. If it is a chart control, what is the rating.
 
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