To: atm{at}shore.net
From: tkrajci{at}san.osd.mil
Reply-To: tkrajci{at}san.osd.mil


> From: Mark Holm 

> ...I used the formula in Kriege & Berry to calculate that if I
> increased the tube diameter from 1/2 inch to 3/4 inch, I should get
equivalent
> stiffness despite the longer length.

Which formula exactly did you use?  Are you talking here about the tube's
second moment of inertia...which scales as the fourth power of the radius? 
Your 'truss' in this design is cantilevered tubes, yes?  If so, then
scaling by radius ^ 4 should be correct in your application.

>...I have been contemplating a slightly more radical
design.
> The three tubes would be arranged in an off center tetrahedron.

>...This is somewhat like Mel Bartels' little
4.25"
> except that his has two parallel tubes plus the two angling tubes.
>
> A possible problem has entered my mind and I would like to have others
thoughts
> on it.  It seems to me that the truncated tetrahedon truss is not
inherently
> stiff in torsion about an axis parallel to (but offset from) the
mirror's
> optical axis.  Am I likely to get vibration troubles because of this?

I don't know.  I don't know if GRAPE can help with this, but if you give me
more specifics...dimensions, wall thicknes, materials, estimate of weight
at the upper end, etc....I may be able to work up a model in GRAPE.  If
nothing else, it can then tell us the stiffness in rotation of this
truss...and depending on angular inertia (I don't think GRAPE calculates
that...you'd have to figure that out by hand)...I suppose we could figure
out the fundamental oscillation frequency.  But I can't model damping
effects, so this may not be an accurate model that I cook up.

You probably need some testimony from Mel or others that have built such
truss arrangements on their experience in the field.

Tom Krajci
Tashkent, Uzbekistan

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