Designing by Accident

Designing by Accident

Years ago, I was involved in a redesign project for some central office cabinets.  The equipment at that time was housed in refrigerator sized enclosures that lined up and would fill an area the size of a small home.  Today you could probably replace this equipment with a product the size of your home PC, but that’s another story.

The redesign effort was initiated due to increased concerns over EMI.  Processor speeds were increasing and the risk of radiated emissions causing interference with nearby radios (fire stations, police, etc) was becoming a real issue.  Today’s designer addresses EMI at the component and PCB level to minimize the macro effects.  But at the time of this effort our product was mature and total redesign was not in the plans, so we chose the Faraday cage approach.  The goal was to wrap the cabinet and contain the EMI.

My task was to address the cable entry and exit points so I’m going to pass the buck on the following design oversight (even though I probably would have overlooked it myself).  Another engineer was addressing the front door redesign.  The original cabinets had plexiglass doors, but the new design approach was to convert to an attractive molded plastic appearance and integrate a metal insert to provide structure and address the EMI issues.  Pretty straight forward, right?

Everything went smoothly thru the prototyping.  The hinges, gaskets, latches, injection molding, and sheetmetal all came together to produce a significantly better looking product.  The seal to the cabinet looked good as well.  Now all we had to do was perform some EMI tests.  This was going to be easy because we had an in-house outdoor test site.  We took the cabinets out, everything got cabled up, powered up, checked out, and ready to start the test the following morning.  Then came the surprise.

It was early April, so we were not totally finished with winter.  Overnight a cold front blew in and temperatures dipped into the 40F’s.  When we arrived to work to begin the tests, we were shocked to find that these beautiful new doors now looked like Pringles potato chips!  The latch and hinges constrained things a little, but the top and bottom of the 6-1/2 foot tall doors were pulled away from the cabinet by about 4″.

Then it became apparent what had happened.  The thermal contraction of the plastic was significantly higher than the contraction of the sheetmetal.  At 40F we measured a gap of about 4″.  As the day warmed up, the gap became smaller.  Add a scale and we just designed ourselves a very large thermometer!  (The solution was to add slots and shoulder washers to the attachment points between the sheetmetal and the plastic to allow them to expand and conctract independent of one another.)

Next week I’ll discuss how to design a clock using only a cell tower and the sun…

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