From: "Tom Krajci" 
To: , ,
        , 
Reply-To: "Tom Krajci" 


>From: "Bill Byrd" 

>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


-+-
Outgoing mail is certified Virus Free. Checked by AVG anti-virus system
(http://www.grisoft.com). Version: 6.0.483 / Virus Database: 279 - Release
Date: 5/19/2003

--- BBBS/NT v4.01 Flag-5