PSN-L Email List Message
Subject: Re: Reinforced concrete base for Lehman instrument
From: beezaur beezaur@..........
Date: Sat, 08 Mar 2003 20:48:40 -0800
> Carbon could be OK, but how do you plan to make the fittings?
> Metal is easier to fabricate and fit. You can easily get nickel plated
> piano wire down to 8 thou. from a music shop, it does not have any
> 'bending' memory and it works just fine. See http://www.daddario.com/
> Suggest that you forget kevlar or other plastics. A ball bearing in the
> end of the arm gives an excellent bottom suspension up against a
> stainless steel razor blade glued flat onto the crossbar.
> Chris Chapman
You are right that steel is more rigid than concrete, by about 8x, but a
cement/sand base has easily 8x the cross sectional area of steel, so
becomes more rigid because of its greater size.
Arrow shafts can be ordered hollow. You would normally glue in a
threaded insert (female) for the tip and push in a nock. One could
easily glue in two tip inserts. Glue a ball bearing on one end and
thread the other to the mass. You can drill a hole in a lead weight and
glue an arrow tip in that. This way the boom (by this I mean just the
compression member between the mast and weight) has a natural period in
the tens of Hz and has next to no mass, so should not bother the pickup
hardware, even if it does vibrate some. Also, making the boom so much
lighter than the weight at the end should help sensitivity.
As for tension member, My thought is that using bowstring material
(650,000 psi breaking strength vs. maybe 100,000 psi for steel) simply
reduces the mass and therefore increases the natural frequency. I have
all this stuff anyway for my bow.
My working hypothesis is that if I make the structure as stiff as
possible and keep all mass on the boom concentrated, I will get a
A day without math is like a day without sunshine.
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