Emphasizing Severity more than Occurrence and Detection vs. Overall RPN

S

someshvar

Jim,

To an extent, I would agree with you.The example you just gave of an Automobile Airbag will have same severity rating for risk of injury in pre an post airbag scenario, isn't it?

Moreover when we talk of DFMEA, we might give example you just gave even though I totally don't agree with you.

But what in case of a process FMEA?
__________________

Hardik
 

Jim Wynne

Leader
Admin
Jim,

To an extent, I would agree with you.The example you just gave of an Automobile Airbag will have same severity rating for risk of injury in pre an post airbag scenario, isn't it?

It depends on how the potential failure mode is defined. If it's defined as "serious injury due to violent contact with the steering apparatus," then introduction of the airbag will lessen the severity, no?

Moreover when we talk of DFMEA, we might give example you just gave even though I totally don't agree with you.

But what in case of a process FMEA?
Again, it depends on how failure modes are characterized. If you're talking about failures in the process, as opposed to manifestations of process failures in the product (part defects) then risk may be mitigated by a change in the design of the process. This is one reason that I recommend using process failures as failure modes.
 
R

ralphsulser

Has anyone heard about the auto industry (or specific auto manufacturers) placing emphasis on the Severity score and not the overall Risk Priority Number (severity x occurrence x detection)?

It was suggested to our organization to determine action plans based on severity only and not the overall RPN.

Thank you,

Dirk

Yes, I have been informed to place more emphasis on Severity and Occurence by a big 4 OEM SQE.
We have not needed to do an FMEA since, but will be soon, plus using their requirements. There have been mixed comments in the past about RPN numbers min or max. We will find out more when the info is prepared for review.
 

Al Rosen

Leader
Super Moderator
Can anyone tell me how would you reduce severity for a said failure mode?

As per best of my knowledge even if you improve the design, you can reduce occurrence and not severity.

I am looking forward to serious discussion as to how would you reduce the severity.

Thanks and regards,
Hardik Someshvar
I agree, the severity of a failure wouldn't change, but the frequency of occurrence could be reduced or the frequency of detection increased.
 

Miner

Forum Moderator
Leader
Admin
You can change a design to eliminate the failure mode entirely, or by adding a backup.

An example would be a bonded metal to rubber engine mount. Most modern designs include a mechanical interlock such that if the rubber fails (e.g., tears, delaminates from metal, etc.) the metal components keep the engine from dropping.

The severity changes from engine falls to pavement to harsh vibration and noise. A clear drop in severity.

After reading this a second time, I was not clear on how this related to a Process FMEA.

The process is the application of an adhesive to a metal substrate to bond rubber to metal. The potential failure mode is for the bond to fail. The potential effect of the failure is for the engine to drop, which is a high severity.

A change in the process cannot reduce this severity. However, changing the design of the part to add a mechanical interlock will eliminate the effect of engine dropping. A new effect is created, which is metal to metal contact without rubber damping resulting in noise and vibration. The severity of noise and vibration is lower than the previous severity.
 

Al Rosen

Leader
Super Moderator
After reading this a second time, I was not clear on how this related to a Process FMEA.

The process is the application of an adhesive to a metal substrate to bond rubber to metal. The potential failure mode is for the bond to fail. The potential effect of the failure is for the engine to drop, which is a high severity.

A change in the process cannot reduce this severity. However, changing the design of the part to add a mechanical interlock will eliminate the effect of engine dropping. A new effect is created, which is metal to metal contact without rubber damping resulting in noise and vibration. The severity of noise and vibration is lower than the previous severity.
The way I see it, the probability of the engine dropping was reduced by adding the mechanical interlock, and the severity remains the same if the engine drops to the ground.
 

Jim Wynne

Leader
Admin
The way I see it, the probability of the engine dropping was reduced by adding the mechanical interlock, and the severity remains the same if the engine drops to the ground.
I agree in this instance, but like I said earlier, a lot depends on how the failure mode (and effect) are characterized.
 
G

Geoff Withnell

It depends on how the potential failure mode is defined. If it's defined as "serious injury due to violent contact with the steering apparatus," then introduction of the airbag will lessen the severity, no?
Actually, serious injury due to violent contact with the steering apparatus is NOT a failure mode. It is a failure effect. The failure mode that the airbag is in place to mitigate is "frontal impact while seat belt is not fastened", which has the effect above. Oddly enough, the airbag increases the severity of the failure of "frontal impact while seat belt is fastened", since it provides no protection the seat belt does not, and may actually cause injury.

Geoff Withnell
 

Jim Wynne

Leader
Admin
It depends on how the potential failure mode is defined. If it's defined as "serious injury due to violent contact with the steering apparatus," then introduction of the airbag will lessen the severity, no?
Actually, serious injury due to violent contact with the steering apparatus is NOT a failure mode. It is a failure effect. The failure mode that the airbag is in place to mitigate is "frontal impact while seat belt is not fastened", which has the effect above. Oddly enough, the airbag increases the severity of the failure of "frontal impact while seat belt is fastened", since it provides no protection the seat belt does not, and may actually cause injury.

Geoff Withnell

Once again, how we characterize modes, effects and causes will influence how severity is rated and whether or not the severity factor can be reduced. In another thread, someone asked,
Severity relates to consequence of error/failure. How can a design change change this?
My response was,
If you're filling the balloons at a children's birthday party with methane, the consequences of popping a balloon using a match can be made less severe by using helium.:cool:
 
M

Mrs. PPAP MKM

We have considered both the customer and the end user when evaluating severity. In our process FMEA we consider the effect on the part that we manufacture as well as the effect on the vehicle. It has proved to be very useful to us and our customers really like it.
 
Top Bottom