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Re: [ATM] ATM Digest, Vol 30, Issue 4

>Message: 2
>Date: Thu, 1 Jun 2006 16:38:08 -0500
>From: RP Sutton 
>Subject: Re: [ATM] Help with Strut Flexure in my 3 Truss DOB
>To: 
>Message-ID: <20066116388.087416@softbuye-qh14rl>
>Content-Type: text/plain; charset="us-ascii"
>
>Hi John and Mike -
>
>the scope was built on designs but Albert Highe - please see here:
>
>http://pw2.netcom.com/~ahighe/home.html
>
>I have emailed him asking for help as well but I have not heard back from him 
>yet. the three single pole design has worked succesfully in may scopes just not 
>mine and I cannot figure out why.
>
>Thanks again -
>
>Patrick
   
  Patrick, I too have a 3 tube Dob (not a truss). It's a 12.5" F5.3 and I use 1.5" 0.058" wall
6061T6 aircraft tubing. I have no problem with flexing at all as the structure is quite stiff, but
I don't use a single ring upper cage assembly.
   
  I'm curious as to the design of your upper support ring and hardware as 6.5 lbs (IIRC) seems
rather heavy for a single ring. My upper cage uses a full plywood shell with clamps top and bottom
but only weights about 8 lbs complete. The upper rings in the designs from the website you listed are
using composite materials and would yield very light upper structures.  This is essential for
avoiding flexure with this type design (single ring). 
   
  Just increasing the tube diameter to 1.5" using the same wall thickness (0.058") will gain you a
stiffness increase of about 75%, if I've done my math correctly comparing second moments of area
  for the two tube sizes. This may be enough to reduce the flexure to an acceptable level.
   
  Pics of the scope here:
   
  http://tinyurl.com/hbmtv
   
  Curt Diggs


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