From: martin.cibulski{at}t-online.de (Martin Cibulski)
To: 
Reply-To: martin.cibulski{at}t-online.de (Martin Cibulski)


Ross,

I will try to explain my thoughts why both constructions give equal stiffness:

The stiffness comes from the resulting spring constant at the upper points
of the truss triangles. That means how much force generates the
construction against a small lateral movement. Lateral movements occurr
when the scope is pointing nearly horizontally and this is the worst case
to be handled by the contruction.

In a conventional truss assembly one pipe is compressed while the other one
is lengthened a little bit. This occurs until the resulting lateral force
holds the weight of the upper ring.

Adding the string brings both pipes always under pressure. Pointing
horizontally results in more compression of the lower pipe while the upper
one is released a bit but still under pressure.

In both cases the compressibilities of both pipes add to the resulting
lateral stiffness at the top point of the triangle. The angle of the
triangle is the same in both cases. The stiffness of the string doesn't
matter because its purpose if only to put a constant force on top of the
triangle. So its smaller angle is not problem. Even a rubber which is
strong enough (but not stiff like metal) would serve the purpose.

Martin

>I seems to me that without clamps on the poles the
>strings will have to handle all the tensile loads in
>the truss, which they are poorly positioned to do in
>this case.  For that reason I strongly suspect that
>the arrangement you propose will be less stiff than a
>conventionally-clamped 8-pole truss.

>Ross

>>Currently I'm building the mechanical parts for my 14" scope.
>>The tube will have 8 trusses. Instead of fixing the 16 ends
>>of the trusses with clamps or screws I added four strings.
>>Their tension will put the trusses under pressure and so fix them.
>>
>>A drawing of one truss triangle between the upper ring and the
>>mirror box can be seen at:
>>
>>http://martin.cibulski.bei.t-online.de/truss_triangle_with_string.jpg
>>
>>The construction is easy to build because I don't need any truss clamps.
>>Setting up the truss tube after transport requires fixing of only four
>>screws.
>>
>>The stiffness should be the same compared to the usual 8 truss
construction
>>without the string. The string is actually not responsible for the
>>stiffness. It
>>only must have enough tension to put always both trusses under pressure.

>>Martin

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