Geoff,

At 10:35 AM 5/1/2011, you wrote:
>Why must that be ?
>
>Notebook paper is only 0.003"
>Why must a spring be so thin ?

When you bend a leaf spring into the sort of curve that you want for 
typical seismo designs, if the spring is too thick it will be 
stressed beyond its yield strength and it will get bent into a 
permanent curve--not good.

The relationship is: Radius = E x Thickness / (2 x Stress)
or Thickness = 2 x Stress x Radius / E

Where E is the Elastic Modulus - Assume 32E6 psi for steel
Yield Stress is well over 200,000 psi for good spring steel, so a 
safe design stress level might be 150,000 psi

For a minimum bend radius of 1 inch, which is about what we see in a 
9" long instrument, you have: Thickness = 2 x 150,000 x 1 / 32E6 = 
0.0094" maximum.  Thicker than that and the stress will exceed 
150,000 psi.  Smaller designs will require thinner springs.

>Also,
>
>This Zero Length spring stuff ?
>
>Do I understand right when I think
>a zero length spring does not change force as the spring length changes ?

No, it acts pretty much like any other coil spring, only that it is 
pre-stressed so that the force needed to make it begin to stretch is 
exactly the right value.  Then, *if and only if*, that spring is used 
with the LaCoste pendulum geometry, you get an 'infinite' period.

Similar results may be obtained with astatic leaf spring designs, 
however 'zero length' only applies to LaCoste coil springs.

Although having an infinite period pendulum sounds attractive, it has 
the great problem that tiny temperature changes will make the boom 
drift off to the stop.  You will want to give the boom at least a 
small tendency to center itself, which means that it will have a 
non-infinite period.

Feedback designs pretty much eliminate that issue.

Regards,
Brett 


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