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


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{at}aha.org]
Sent:   Monday, January 06, 2003 1:49 PM
To:     Benjamin Robbins; atm{at}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{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
      The sender believes that this E-mail and any attachments were free of
any virus, worm, Trojan horse, and/or malicious code when sent.  This
message and its attachments could have been infected during transmission.
By reading the message and opening any attachments, the recipient accepts
full responsibility for taking protective and remedial action about viruses
and other defects. The sender's business entity is not liable for any loss
or damage arising in any way from this message or its attachments.

--- BBBS/NT v4.00 MP