Hello,

Regarding springs of a Lehman design the best one you can makeup
is represented in the design (that i did not sketched but i have only 
pictures) here: http://mariottim.interfree.it/doc02hl_e.htm

If you look carefully at the pictures describing also the LVDT sensor
you can see how a couple of hinges made by a thick steel of 1/10mm
works in traction and not in pressure.

This is the best design available for an amateur.
The element represented in the picture is a solid aluminum plate
milled to have the shape required of this design.
With some work at bench you do the work assembling some aluminum plates.

Regarding a vertical seismometer:

A seismometer using the "C" spring as is it in the picture of Roger 
Sparks cannot be considered a vertical polarized.
The couple of springs in C shape keep the boom rather steady but
not enough to assure that no horizontal motion is present.
This effect is know as "cross-sensitivity" and represent how much
of a cross signal is picked by the sensor.

An alternative solution can be to keep the C spring very short
reducing a lot the cross sensitivity. THis would not reduce the
sensor period if the boom is kept long.
Anyway a good vertical design remains the "Lacoste" with a spring
that can resemble the infinte period if accurately dimensioned.

Even if in compression it works much better than you can imagine.

A new idea that I did not tested is to go to your dentist and
purchase from him a couple of tool they use to remove nerves from teeth 
roots, these tools are very thin but impressively robust since they are 
made with titanium. You can use a couple of the making them to work
in torsion. Of course you need to lock them in an proper axial 
configurations.

best regards
Mauro

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