Re: [ATM] 6-pole

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

Provided the attachment points of the 6 truss rods form equilateral 
triangles of similar size at both ends, the truss rods will define a hexagon 
around the light path. The circumscribed to inscribed diameters are c:a 1.15 
(sqrt(4/3)). In the case of 8 rods in the usual arrangement, it is a square 
around the light path (similar ratio sqrt(2) - hardly better). In either 
case, you need some way to keep a shroud from "cutting corners" - I don't 
see much of a difference, or problem, there.

I once used a 6 truss arrangement joining a "square" mirror box and a 
hexagonal upper cage - no particular problem.

But I wonder, Arjan, what you mean by  "Even a single pole could completely 
define position, when mounted rigidly to top and bottom structures." If 
highly resistant to all kinds of deformation like bending, of course, and 
the best way of realizing this in practice would be IMHO the (not uncommon) 
way to use a single, hollow pole, with inner diameter a little larger than 
the actual aperture and centered around the optical axis ;-)

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.

Nils Olof

----- Original Message ----- 
From: "Arjan te Marvelde (hetnet)" <arjan.te.marvelde@hetnet.nl>
To: <atm@atmlist.net>
Sent: Wednesday, April 09, 2008 9:39 AM
Subject: Re: [ATM] 6-pole


> Doesn't this depend on how the poles are attached?
> Even a single pole could completely define position, when mounted rigidly 
> to
> top and bottom structures.
>
> You could go for lighter poles when you use more than 6. There must be an
> optimum in weight and rigidity somewhere between a single and many poles, 
> or
> ultimately even a closed tube.
>
> AtM
>
>> LDL> 6 poles completely define the position.
>> LDL> 8 poles over constrain the position.
>>
>> Completely correct, hence the Hexapod. One disadvantage of 6 pole
>
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> 

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