Randy and Chris,

In the FBV designs we use a pair of short single flexures, which work 
well for them.  We found that when making certain tests we wanted to 
put the flexures in compression from gravity and I worried that they 
might buckle.  So I did a quick analysis to see what it looked like.

http://bnordgren.org/seismo/Buckling.pdf

This should be taken as only a rough idea of what will happen, but 
the results are interesting, nevertheless.  Note that the flexure 
ends are actually constrained slightly differently from the model I 
used.  In the actual design one end is clamped while the other is 
only clamped to remain vertical while being allowed to move 
horizontally.  The model is assuming one clamped and one free 
end--similar, but not quite the same.  In reality, the buckling load 
should be greater than the analysis is showing.

I was analyzing two 1/2" wide 0.002" thick steel flexures working 
over a 0.012" gap.  For that and similar cases, the results were 
interesting.  The formulas should be useful in evaluating flexures 
having other dimensions.

On another subject, we have had good results using non-metallic 
(Kapton) flexures in situations where reducing flexure stiffness was 
particularly important.  They are much more flexible, though weaker, 
than steel and seem to be more resistant to being damaged, which was 
sometimes occurring with the steel flexures.  Also, we have never 
observed any of the creep issues we were expecting, although in such 
feedback designs, those effects would likely be well masked.

Brett


>----- Original Message ----- From: "Randall Pratt" 
>To: 
>Sent: Sunday, June 12, 2011 7:08 AM
>Subject: RE: Seismograph Photo
>
>>Hi Chris,
>>
>>I am using strips of steel from a steel measuring tape for the pivot.  I cut
>>strips about 1/8" wide across the tape and I use 2 pieces spaced about an
>>inch to give side to side stiffness in boom motion.  A regular scissor will
>>cut the tape and may also work to trim a double edge blade.  You are correct
>>the boom is a bit high in the picture.  Before mounting the boom it can be
>>hung from a thread in the pivot clamp and the line of the thread will point
>>to the cg.  I think in most AS-1 types the cg is actually below the boom and
>>the boom should be up and not level.  On my boom the coil below the arm and
>>the extended damping plate below the arm tend to put the cg well below a
>>horizontal arm at about the edge of the mass and the hinge side of the mass.
>>The boom can also be balanced horizontally before mounting to get a cg for
>>the lift test type of calibration or for a calibration coil computation.
>>
>>With my adjustable spring mounts I played with angles down to 30 degrees
>>while I had the optical sensor.  Without temperature control it is to much
>>of a struggle to maintain stability at lower angles and longer periods.
>>The spring I have is negative length -0.191 inch.  I measured 6 weights
>>between 7000 and 14500 grains and used linear regression on the
>>measurements.  An easy way to do this is to hang a small plastic or paper
>>cup from the spring and add lead bullets to the cup.   In the US many of us
>>have reloading scales which can measure to 1/10 grain up to 500 at 7000
>>grains per pound.  A vernier caliper can then measure extension from a fixed
>>point to the cup rim allowing k to be determined fairly accurately.  After
>>assembly, period measurement can then confirm the total boom mass since it
>>is harder to weigh beyond the scale capacity.
>>
>>There is a hidden advantage to brazing the frame.  When the garage is 10
>degrees F the torch is warmer than a drill press!!!
>
>Randy


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