From: "Good, Donald" 
To: 'Benjamin Robbins' , atm{at}shore.net
Reply-To: "Good, Donald" 


It is true only if the tension on the chains is high enough to compensate
for the cage moment in the horizontal position.  The horizontal is the
highest stress case for the tube and chain structure.  The chain from the
highest point on base in this position to the cage forms an angle with the
horizontal.  The tension in the chain can then be "separated"
into a vertical and horizontal component.  The vertical component (V) (the
one we are interested in) is the chain tension (T) times the rise from cage
to base (H) (in the horizontal) divided by the chain length (L): V=T*H/L 
If this force from the contributing chains is greater than the weight of
the cage plus 1/2 the weight of the tubes and chains, then droop is
minimized.  You have a chain on each side so they add up.
2*V>W[cage] + 1/2*(W[tubes] + W[chains])
If V is too small, then the cage droops.  This increases H because the
angle increases.  The cage stops drooping when the increase in H causes
enough of an increase in V to balance the cage, etc.

The problem with PVC is that tightening the turnbuckles on the chain
increases the tension only a little.  It mostly just causes the PVC to bend
more.  At some point, it will just crack.

In looking at the chain arrangement on your web page, I just noticed
something.  The side chains appear to be mounted about 3 inches for the
"top" side (the side at the top when the scope is horizontal). 
If you move that mount point as close to the "top" edge as
possible, increasing H, it might be enough without changing to aluminum. 
Put it right in the corner, if possible.

Also, when rigging for use, the other chains should have less tension than
the side chains since they do not support the weight of the cage.  Only
enough for stiffness in wind and when moving around the sky.



-----Original Message-----
From: Benjamin Robbins [mailto:benjaminr{at}highwire.com]
Sent: Monday, January 06, 2003 1:39 PM To: Good, Donald; atm{at}shore.net
Subject: RE: ATM - Travel Scope


But we were under the impression that the bending of the tubes was of no
consequence, that it is the tension of the chains that causes the secondary
structure not to move, that the poles could bend all they want as long as
the chains stay under tension. Is this incorrect theory?

Thanks,
Benjamin

--- BBBS/NT v4.00 MP