ATM 2 Piece Poles - and how trusses really work

["Tom Krajci" <tkrajci@san.osd.mil>]

>From: "Bill Byrd" <web01@flash.net>

>The 5" test pole flexes 
>very little, if any, when tightened securely. 

I just had a thought about truss telescopes and 'sectioned' poles (often
done for traveling/compactness purposes.)

First, a truss, by definition, has all members under tension and
compression...and *only* tension and compression.  No bending/moments
are acting on any members.  (At least in the ideal model/concept.)

See:
http://astro.umsystem.edu/atm/ARCHIVES/APR00/msg00921.html (to introduce
the principle of a truss under tension/compression...and not
bending/moments)
An important quote from this post:
"It can be done using just Hooke's Law, which defines
change in length for elastic deformation under pure tension or
compression.
There is no bending involved in this."

See:
http://astro.umsystem.edu/atm/ARCHIVES/APR00/msg00974.html
for a minor correction in the math.  (But the principles of forces used
are still valid.  Tension and compression, but no bending.)

See:
http://overton.tamu.edu/aset/krajci/truss-analysis/truss-analysis.htm
for some examples of possible truss setups for telescopes.  Note that
the Berry/Kriege book has some errors in terms of describing truss
stiffness versus 'flexure resistance'...that can lead you to some faulty
analysis and conclusions.
(and this page is part of 
http://overton.tamu.edu/aset/krajci/scope-design.htm
which is part of
http://overton.tamu.edu/aset/krajci/  )


OK...so what point am I making?

I submit that there is a fairly widespread misunderstanding among many
in the ATM community when it concerns truss design (and testing)
concepts.

Truss members are under tension and compression, not bending.

However...how do many ATM's 'test' their multi-part truss tubes?

By bending them.

This does not measure the stiffness of the truss made from these
multi-part tubes.  This only measures the ultimate failure strength of
the truss...because that’s how a truss fails...but a member bending
beyond a certain limit.

Do ATM's design and build trusses for telescopes to operate near their
ultimate yield strength?

No.  (Unless you did a bad job with your design calclulations.)
Frequently the goal is a stiff structure to maintain collimation, not a
'high load capacity' truss to hold a 200 pound secondary mirror assembly
at the end of the truss.

Bending your truss tube to 'test' it for stiffness in your final truss
is a flawed approach.

What you should do is hang your truss tube, and measure its extension
when put under a tension load.  (If you have a one piece truss
tube...then you only need the material, the length, and the cross
section area of the tube...and can run the numbers to determine its
'spring constant'...but for a multi-piece truss tube...your connectors
are tougher to model...therefore you may be forced to real-world test
your setup.)

Test your multi-part truss tubes under tension or compression...to make
sure that your joints do not significantly reduce the 'spring constant'
of your tube...compared to a one-piece tube of similar
material/length/cross section area.

I am not saying that multi-part truss tubes are a bad idea.  I'm saying
that if you use a connector technique with a rather skinny bolt (small
cross section area), then you may have significantly reduced the
stiffness of your final truss...and you may not hold decent collimation
in your portable scope.  

Make sure the connectors are stiff in terms of tension/compression.  The
'bending test' is not appropriate here.  A tension/compression test is
far better...in my humble opinion.

Thanks to Roy Diffrient for keeping this stuff straight in my head!


But is it straight in my head?  Where have I made errors in thinking in
this post?  I don't want to lead myself or anyone else down a wrong
road.



Tom Krajci
Tashkent, Uzbekistan


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