Optomechanics – Forensic Observation on Shock Test

Colleagues:

I was peering into the AEH archives the other day and got a shock.  I needed the source code that I use to analyze the effects of a variety of shock pulse shapes.  What I found was a complete set of mechanical engineering drawings for the medium-weight hammer blow shock test machine at the Hughes-Fullerton facility!  John Martin, who ran the test lab, gave them to me after AEH succeeded in getting a client’s Optical-Nav system qualified for Navy shipboard minesweeper service.

AEH’s job was to specify the shock isolators.  To understand the problem I had prepared a massive Nastran model of the system (6 feet high, two feet square and 550 pounds) mounted on an array of Aeroflex wire-rope isolators.  The numbers had said it should pass the shock test, but it failed.  AEH was called to the lab.  The bolts holding the isolators had stripped the threads in the isolator flanges!  I spent lunch-time with the Nastran output file reviewing the loads and forces and couldn’t make sense of the results.  The client thought that larger/stronger isolators were required.

I went back to the test lab and re-inspected the failed isolator mounts.  I found that the threaded ends of all the failed bolts were flush with the inside surface of their isolator flanges and that all of the un-failed bolt-ends protruded into that sway-space by about 3/16ths of an inch. 

I returned to the Nastran file, looking at displacements in stead of forces.  It predicted that the isolators would use virtually all of their sway-space.  The failed bolts had actually been pushed out when the isolators prematurely bottomed on the bolt-ends, not pulled out when the isolators stretched to their limits.  The bolts were too long.  Larger isolators were not necessary.

One fix was to put extra flat washers under the heads of the bolts so their ends would not protrude into the sway-spaces.  With that accomplished we returned to test the next day and PASSED!

That was AEH’s second successful adventure in John’s shock-test lab.  He graciously offered AEH the drawings for his test machine and invited me to present a paper at the annual conference of the Institute of Environmental Sciences, of which he was President.  I accepted both.

Engineers need to keep their forensic skills sharp, too!

Thank you John Martin!

Al H.
6-1-16

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