Redundant Component DFMEA Severity & Occurrance

  • Thread starter Disciple of Quality
  • Start date
D

Disciple of Quality

#1
What is the proper SEV & OCC to assess on a component DFMEA, for a component that is redundant at the subsystem level?

Consider a subsystem DFMEA that reflects a high SEV effect of a failure caused by failure of all of the redundant components. It seems reasonable to base the OCC rating of the cause of the subsystem failure on the combined likelihood of failure of all of the redundant components, which would be substantially less than the likelihood of failure of any one of the redundant components.

Since the subsystem relies on each component to back up all of the others, it seems reasonable to characterize the component requirements accordingly; Each component is therefore required to perform a certain function in the absence of all of the others. For this reason, it seems reasonable to cascade the same high SEV from the subsystem DFMEA to the component DFMEA.

The problem is the OCC rating on the component DFMEA. This OCC rating is supposed to be based on the likelihood of the cause of the component failure, which could be substantially higher than the likelihood of the cause of the subsystem failure. It seems that this would tend to drive recommended actions on the component DFMEA, potentially leading to an over-designed component, while at the subsystem level it appears that no recommended actions might be needed.

An example would be a subsystem consisting of five lug nuts (redundant components), a wheel, & a hub assembly (where the hub assembly includes five threaded studs). Consider the subsystem function, "Support the weight of the vehicle at one of 4 corners".

Assumptions
1) The wheel will be adequately retained if any one lug nut is functioning (for simplicity)
2) On the corresponding component DFMEA, the lug nut FM, "Failure to retain wheel to hub assembly" has as it's only PC, "Lug nut threads strip due to USL of the specified lug nut torque range in excess of thread strength" (for simplicity)
3) P(Lug nut threads strip due to USL of the specified lug nut torque range in excess of thread strength) = 10% (just to put some numbers to the example)
It follows from assumptions 2 & 3 that P(Failure of any 1 lug nut to retain wheel to hub assembly) = 10%.

Subsystem DFMEA:
FM: Does not support the weight of the vehicle at one corner
EFF: Corner of vehicle drops to the ground while under way, leading to loss of control & injury or death, without warning
SEV: 10
PC: All 5 lug nuts fail to retain wheel to hub assembly
OCC: 1; P(All 5 lug nuts fail to retain wheel to hub assembly) = 0.01^5 = 1E-8

Component DFMEA:
The lug nut function is "Retain wheel to hub assembly"
FM: Does not retain wheel to hub assembly
SEV: 10
PC: Lug nut threads strip due to USL of the specified lug nut torque range in excess of thread strength
OCC: 7; P(Lug nut threads strip due to USL of the specified lug nut torque range in excess of thread strength) = 0.01

According to the subsystem DFMEA, the design is probably adequate based on OCC = 1. But the component DFMEA seems to suggest that a recommended action is called for, based on SEV = 10 & OCC = 7.

Did I properly assess SEV & OCC in the subsystem & component DFMEAs in the example above? If so, then how should the high SEV / high OCC item be addressed on the component DFMEA? If not, then how should SEV & OCC have been assessed & why?

Thanks
 
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B

Bill Ryan - 2007

#2
Welcome to the Cove :bigwave:

I'm afraid I'm not going to be much help to you as I don't have "hands on" experience with DFMEAs. I'm sure some other Covers do and they will, hopefully, chime in. I'm also not the strongest in probability statistics but the OCC ranking of 1 in your subsystem DFMEA just doesn't seem right with a component failure rate of 10%.
 
D

Disciple of Quality

#3
Thanks

The OCC of 1 in the subsystem DFMEA comes from the assumption that all 5 lug nuts must fail in order for the system function to fail. If even one lug nut continues without failing, then the wheel will be retained. Since there are 5 lug nuts, then I think the likelihood of all 5 failing is 1E-8; Therefore OCC = 1.

Admittedly, it is not realistic that one lug nut would suffice, but I hope this assumption will simplify my query, and I think any guidance based on it can be easily extrapolated to more realistic cases, (i.e. wherein more than 1 lug nut would be required to retain the wheel).
 
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