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RE: [ATM] Friction and sticking in multipoint mounts
I have a 54 point cell made for a 38" mirror. It is simply constructed and
virtually no friction in the system what so ever. Works perfectly. The
problem with most floatation systems are that thay are too complicated.
There is no need for bearings and low friction devices. You can design a
simple system with virtually no friction using off the shelf components.
Balancing the trianges is completely unecessary in a properly designed
cell.
A flotation cell is designed to support ALL parts of the mirror even if the
mirror is not flat. there is no need to worry about all points faling into
a plane, there is no need to worry about friction, there should be none.
Remember that once the glass is placed in the mirror cell, NOTHING moves!!
It it does move, then you lose collimation or focus.
Worry less, observe more.
Kreig
Original Message:
-----------------
From: artbianconi@blast.net
Date: Mon, 06 Sep 2004 12:15:12 -0400
To: nilsolof.carlin@telia.com, atm@atmlist.net
Subject: [ATM] Friction and sticking in multipoint mounts
A 27 point mount is enormously heavy, regardless of aperture and
simply not reliable given the technology, the tools used to make
it and the level of skill needed to fabricate it.
A famous writer-philosopher once volunteered that if you bite
into an egg and taste something wrong, it's pointless to keep
eating it, expecting things to be different.
To foist such a bad egg on amateurs and create an expectation
based on book-theory is unfair to them. As proof of this I offer
you the simple fact that this entire string started as a
consequence of one person trying to overcome an enormous friction
problem. How many others are out there whose friction levels are
beneath the level of human detection but still affecting the
image?
Other ATM's have volunteered opinions similar to mine and I have
not even begun to explore the problems that occur when oxidation
sets it or the binding that will occur when torsional loads are
imposed and flexing occurs in this complex, interactive system.
I didn't pursue the matter any further with the guys from Bell
Labs as I was not looking to them for solutions but for
confirmation of my own estimates. I took a bite, didn't like it
so why continue?
My cell will be a super rigid "wheel". one with four layers of
cored and laminated carbon fiber separated by spokes with an "I"
beam cross section made the same way.
The 15" mirror will "float" on a thin "liquid like" material and
be secured with a new breed of polymer developed by NASA. There
will be no measurable deflection under normal loads and the
mirror will maintain it's shape. For all practical purposes, this
will be a super rigid system.
My 6 inch will employ the same cell technology only I won't
bother with the thin "liquid like" material interface.
I am not going to defend the design against arm-chair nay sayer's
and theorists many of which think nothing of making a 1/12 wave
mirror and then mounting it in plywood or a cardboard tube from
Home Depot! It's been tested to death by people at two different
companies who specialize in far more sophisticated finite element
analysis than PLOP. They have verified my design and know the
environment in which it must work. One FEA analyst said it best:
"Art, you did it again. . . . . you over-designed the damn
thing!"
It's stronger and has greater deflection resistance than the half
inch boiler plate steel GEM my 12.5" sits in and that thing
weighs in at close to half a ton when assembled!
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