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Asking the correct questions for FMEA

JAG Engineering LLC


An article appeared in an engineering forum entitled Why Designs Fail. As I thought about the article and the examples presented, the Titanic, Tacoma Bridge, etc., I realized mitigating the consequences of a failure are more important than preventing the failure.

One of the subjects I did not learn in the classroom was Failure Mode and Effect Analysis (FMEA). I do not intend to cover the subject of FMEA but would like to emphasize one small portion. FMEA was introduced to me on my first job in the late 70's. One aspect of human nature needs to be over come when applying FMEA. Problems we do not expect to happen due to low probability, we tend to dismiss with no contingency if it occurs. FMEA forces you to separate probability from consequences.

During my FMEA training I was told to ask what happens if a particular failure were to occur, without arguing over the probability, which is addressed separately.  Most homes have smoke detectors and many have fire extinguishers. These items provide early warning and a method to stop or at least slow the progression of a fire until help arrives. Few homes have precautions for an asteroid strike though the results will be far worst with little mitigation possible.  

A different state of mind comes into play when you accept the sinking of a ship as a possibility. Expecting to foresee all the possible ways it could happen may be impossible. How a ship could fill with water is irrelevant once it happens.

If the Titanic designers had asked themselves what would happen if the entire ship flooded, not worrying about the how, lives would have been saved. Perhaps the compartments would have been 100% isolated from each other or more lifeboats provided.



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Let's consider your example of the actual fact that my home has no provisions at all to mitigate an asteroid strike. As you noted, I do have fire extinguishers and smoke detectors. What should I do? Should I invest time, effort, and money making provisions for the asteroid strike? My home would be a terrible mess if the asteroid were to strike, most likely along with my small city. What should I do? Guide me, O wise JAG!


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FMEA sets priorities for addressing potential problems. There would be no need to set priorities if all issues could be addressed. FMEA starts with questions that separate probability from consequences. Next, the user assigns a number (1-4, 1-5, 1-10) to each answer. The numbers for each answer to a particular failure/problem are multiplied together. The issue with the highest value is the first priority for action.

For the average person the asteroid would likely be at the end of the list, with the daily need for food, power, shelter being much higher on the list. For food, the scenarios could be the stores are empty, or we have no money, or we cannot get to the store etc. Most of us would never reach the end of the list but would address the items that have a combination of high probability and high negative consequence. Bill Gates likely could afford a precaution for everything on the average persons list. You and I would likely not. When you have yachts, jets, mansions, servants, people assisting with the daily chores of life, the list would be bigger and the last items may not be cost effective even for Bill to address.  A servant makes off with one of Bill’s yachts. An annoyance to Bill, but not life threatening to him and an unlikely event for those with neither a yacht nor servants.

Here is an example from many years ago. I worked on differentials for a front wheel drive car. A possible  problem was a differential assembled without one or more gears. As far a severity, this would be a 5 on a 1-5 scale with 5 being the worse case. Then a series of questions were asked relating to detection of the missing gear: What is the likelihood of the differential getting into a transaxle, likelihood of the transaxle being placed in a car, likelihood of the car making it to the dealer, likelihood of the car making it to the customer. Knowing the operations, the likelihood of a missing gear was assigned a 5, but a 1 for the differential making it to the customer.

Now run though the same scenario for a missing roll-pin used to secure the differential pinion shaft, which the pinion gears mount on. The likelihood of the pin not being installed may be as great or greater than the gear. Moreover, the likelihood of it progressing further through the system is also higher. In fact, one service report I investigated was for the car model in question. The pinion shaft dislodged as the customer was backing out of his driveway. Very loud noise and destroyed transaxle. Had this happened at highway speeds it could have been deadly. It was not a missing roll pin but a sheared pin. Cause was speculative.

So comparing the two different problems, the missing gear will likely be found early due to normal operations. So no added precautions are needed. For the roll pin steps could be taken. The pin could be plated a bright color to make visual inspection easier. A physical inspection step could be added to the existing process. The possibilities are numerous.

With respect to asteroid protection, likely low on the priority list for you and I. Mr. Gates may have a panic room to protect against home invasions. Make it deep enough and strong enough and you have a shelter against asteroids.  During the cold war, regular people build bomb shelters. Here is a work sheet. http://www.npd-solutions.com/fmea.html

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