RE: ATM - Travel Scope

[Benjamin Robbins <benjaminr@highwire.com>]

Donald,

How is it that we can arbitrarily move our chains from their current
location? Here is a quote from Jane's 16 web page
(http://www.tms-usa.com/grayarea/janes16/jane16.htm):

"Probably the most unique part of the design of Janes telescope is that
instead of trusses between the mirror box and the secondary cage, there are
3 triangles of Spectra fiberglass string, and two fiberglass spring poles.
The concept behind this is the fact that the fiberglass string has near zero
stretch characteristics. The two fiberglass poles are spring loaded, and
keep the fiberglass strings taught. 
To further explain the concept, imagine two strings anchored in your garage
floor about 2 feet apart, and tied together about 5 feet in the air. Grab
the knot with your finger, pull the strings taught, and now you can only
move the string in an arc, forward and back, not left to right. Now make
another pair of strings, and anchor them to the floor, 120 deg's apart from
the first pair. Grab the knot with your other hand, and it also is only able
to move in it's own arc, 120 deg's from the first arc. Add a third pair
another 120 deg's apart, and have your imaginary friend hold it at its knot,
and it too has its own arc. Now tie the three knots together with a
secondary cage, and now the only way to move the secondary cage is to slack
a string. Keep enough tension on it, and it will stay in the same place
relative to the garage floor. Now it should be easy to see how it would work
on a telescope."

Our design is based on the chains being rotated 120 degrees from each
other...if we move them to the corner than that is all out of whack isn't
it?

Thanks for all the feedback!
Benjamin



 -----Original Message-----
From: 	Good, Donald [mailto:dgood@aha.org] 
Sent:	Monday, January 06, 2003 1:49 PM
To:	Benjamin Robbins; atm@shore.net
Subject:	RE: ATM - Travel Scope

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@highwire.com]
Sent: Monday, January 06, 2003 1:39 PM
To: Good, Donald; atm@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
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