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


> From: "Roy Diffrient" 

> Tom, I think you're on the right track in using the FEA software to
analyze
> your truss ideas.  I  have only these few points.

> - - Unless I missed something, there seems to be no allowance for the
weight
> of the truss itself in your examples....

Thank you.  I need to learn more about this software, but yes, I may have
to add in the mass of the truss members if the software does not take it
into account.  I'll keep this in mind as I get better with FEA.

> - - I didn't see the standard truss listed in the results table, but the
> deflection for that design can be easily calculated, and that could be a
> good check on the FEA software, setup, etc.  The deflection is
calculated by
> the formula W L^3 / (c^2 A E), where W = load (plus half the truss
weight,
> as above), L = truss member length, c = base spacing between members, A =
> cross sectional area of the material in a member, and E = elasticity
modulus
> (for steel, 30 million psi).

Thank you again.  This gives me a way to cross-check results, which I need.

> - - Those apparently useless members of the typical 8-element truss form
a
> parallelogram linkage which nicely keeps the upper end parallel to the
> primary for all deflections.  The purpose of the truss, after all, is to
> hold the optics in collimation.
> - - For trusses where all sides and elements are equal, like all those
shown

> on your site, the deflection will not vary as the truss is rotated around
> the optical axis.  The deflection of each side due to tension/compression
> does change depending on the rotation angle, but the vector sum of the
> deflections remains the same, just as the total load remains the same
> regardless of rotational changes in tension/compression force vectors in
the

> truss elements.
> - - Don't forget Mark Serrurier's original reason for using his truss
design
> on the Hale telescope: It was not because it was stiff enough (it
wasn't).
> It was (in part) because the two ends of the tube assembly could be
designed
> to have equal deflection.  Deflection of the bottom end could thus cancel
> out the deflection of the top end.  There's no reason that principle
can't
> also be applied to your design, especially if you plan to use a truss on
> both ends.  So there's more to it than just truss stiffness.  Don't get
me
> wrong - improvement of truss design and stiffness is a noble goal, and
our
> trusses might well be improved or optimized using FEA.  But so far, it
looks
> like you're only modeling half of the tube assembly.

Yes, the Serrurier truss is "if you can't beat 'em...join em" in
terms of truss stiffness and collimation.  However, for a smaller scope (16
inch versus 200), and a slower f/ratio (f/4.5 versus f/3.33)...I may be
able to get away with a non-Serrurier truss.  I'll do some more analysis to
see how large my error budget is...and wheter or not my truss will meet
those needs.

However, I may be using a box frame to attach my tube assembly to my fork
arms.  In that case, the door opens to a Serrurier truss arrangement very
nicely.  I may kill two birds with one stone this way if I design the truss
properly.

Tom Krajci
Tashkent, Uzbekistan

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