FOR WANT OF A NAIL

April 12, 2014


“For want of a nail the shoe was lost; for want of a shoe the horse was lost; for want of a horse the rider was lost.”

I have no idea as to who first said this.  His or her name has been lost in time, but if you apply this anonomous saying to modern-day events, you would have to deduce that General Motors (GM) should have known better than substitute a less expensive ignition switch for one much more robust.   One that could take punishments known to exist over the life of an automobile.     Ignition switches used on cars today not only supply voltage to the starter but enable the air bag and other devices critical to smooth and continued operation.  These devices are certainly in the safety circuit and MUST fail safe when and if failures do occur.  THEY ARE CRITICAL TO QUALITY.   Critical to quality is a term used frequently in design, manufacturing and quality control to designate components that insure safe operation and performance of an electo-mechanical device or an assembly of devices.

During any component design phase, limits of acceptability are established.  These limits represent the maximum/minimum parameters defining the range of operation.  These limits “bracket” acceptable performance for all possible uses or misuses of the equipment or component.  Good quality programs monitor manufacturing and operational trends relative to the limits of acceptability so mid-course corrections during the manufacturing phase may be made.  This insures continued acceptable performance for the product.

An ideal component or assembly of components would exhibit six sigma (6 σ) levels or no more than 3.4 component defects per one million parts or assemblies produced.   This is the goal of every manufacturer—to achieve six sigma levels thereby improving manufacturing thru-put yield of component parts.  This can be very difficult in today’s world due to economic pressures and competition.  In the United States today, the average “sigma” value approaches four.

Component and system reliability is measured by Mean Time to Failure (MTTF) and Mean Time Between Failures (MTBF).  These times are established by reliability testing or cycle testing under varying conditions.  Cycle testing is usually performed by HALT (Highly Accelerated Life Testing) or HASS (Highly Accelerated Stress Testing).  Time to failure results from these performance tests.  MTBF determines maintenance times for the components and/or assemblies.  Understanding of this time frame is critical and determine preventative maintenance or replacement schedules.   Please note, evaluation laboratory testing and reliability testing  are two distinct methodologies to determine acceptability of a component or subassembly of components.  Both efforts are absolutely necessary and provide valuable information relative to manufactured parts.

One excellent predictive tool is FMEA (Failure Mode Effect Analysis).  This is a methodology used to discover probability of failure, severity of failure and frequency of failure for all possible failure modes.  Quantative estimates are given to each possibility thereby producing a conglomerate number that designates acceptable performance, marginable performance or unacceptable performance relative to failure type.  Generally, numerical values are derived by multiply the three estimates together.   An unacceptable value requires engineering to re-think the design thus eliminating the failure mode or providing for early detection of that failure mode.

I would hazard a guess that any ignition switch used by GM would have undergone similar investigations by virtue of testing.  These guys are not dummies and if that is the case, what happened?  I would surmise cost factors and margin played a significant role in the acceptance of these components.  At any rate, GM will have to account for the failures.  Thirteen (13) people were killed due to switch failure.  We are told the cost to rectify this problem would have been less than one dollar.  GM will not escape the repercussions—they know better.

I welcome your comments.

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2 Responses to “FOR WANT OF A NAIL”


  1. Great delivery. Great arguments. Keep up the good spirit.

    Like

    • cielotech Says:

      Hello m5000v5–Many thanks for the very kind comments. This, believe it or not, is one of the most-read posts I have written. People seem to enjoy the subject greatly. Really happy you enjoyed it. Take care and please do come back. Bob

      Like


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