Re: [ATM] 6-pole

["Nils Olof Carlin" <nilsolof.carlin@telia.com>]

Arjan,

one definition of truss:
"A triangular arrangement of structural members that reduces nonaxial forces 
on the truss to a set of axial forces in the member."

Reminds me a bit of "Recursion, infinite: see Recursion, infinite" but I 
guess the triangles are essential.

In a practical telescope structure, my not-so-educated guess would be that 
tube truss poles just enough to withstand bending forces might be so thin as 
to bend easily or at least vibrate, or else be too thin-walled to take 
handling well. Thus, in practice, structural rigidity might not be the 
determinimg factor. BTW, in my only truss tube telescope, I use alu "T" 
profile, removing the "vertical" part at the ends, fastening the remaining 
flat piece with a screw with conical head. I like to do things differently 
;-)

In my smaller 'scopes, I make a tube of square cross-section, with walls of 
thin plywood right now some 3 mm of the cheapest stuff, but if I could find 
some really thin model airplane plywood (a mm or so), I'd use it - I don't 
expect it would bend much in its own plane, and anyway, it would form 
something like a true Serrurier truss, FWIW, with a square frame a little 
below midway supporting the trunnions.

Nils Olof


> This is precisely the point I try to make. You should really calculate how 
> well the definition of your structure is (call it rigidity) as a function 
> of the components and design. Tom Krajci had a link to a mechanical 
> calculator program for truss structures:
> http://www.grapesoftware.mb.ca/
>
> What you see very often is overdimensioning of the trusses, in diameter 
> and hence weight. Just take it a bit further and you can do with a single 
> pole (I think it is technically not right to call this a truss) like Russ 
> Porters' garden scope. Of course, your suggestion to use a hollow pole and 
> put the optical axis in the center has been used on several occasions.
>
> The original statement I responed to, was that 8 trusses would overspecify 
> the position. This is only true when the mounting points leave rotational 
> freedom and, like a real truss should do, only define distance between 
> mounting points.
>
> AtM
>
>> But the idea of a truss is that the members should not have significant 
>> change in lenght during load (even if they are not very resistant to 
>> bending) - a much less demanding requirement than stiffness against 
>> bending or rotation as required with less than 6 members. Thus, the 
>> primary requirement is sufficient cross-sectional area for the material 
>> and loads. But whether a 6-member truss or a 8-member one with 3/4 as 
>> large cross section and weight would be stiffer, I don't know. It should 
>> be calculable - there might even be ready-made solutions to be found.
>
> 

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