RE: [ATM] Question About Dobsonian Design

["Alan Scott" <alanne1956@hotmail.com>]

Michael,
  I have seen roller bearings used, and they worked very well.  Actually, 
what was used was shower door wheels.  This was on a dob that did not have 
much weight change at the focuser - it used 1.25" eyepieces.  I basically 
made the same scope, but with a 2" focuser - and use heavy eye pieces.  I 
had nothing but trouble with bearings, and side a side friction system.  
Finally, I removed all four of the wheels, and replaced them with normal 
teflon bearings.  Works like a champ.

  The moral of the story from my viewpoint is that wheels work with a side 
friction system if you don't need too to create much friction with it.  You 
then will have the smoothest scope you have ever seen.  But, if you do 
require a bit more friction, just plain teflon bearings work MUCH better.

Alan Scott
http://www.xmission.com/~alanne/Astronomy.html


>From: "Michael Wolfson" <m_wolfson@rogers.com>
>To: <atm@atmlist.net>
>Subject: [ATM] Question About Dobsonian Design
>Date: Fri, 1 Apr 2005 23:21:34 -0500
>
>Hi.  I'm a relative newbie re ATM.
>
>I'm planning on building a lightweight dob for a 16 in mirror, mainly using 
>aluminum tubing of various sorts.  I already have a Dob Driver II which I'd 
>like to use.
>
>My main question is the necessity of using Teflon bearings.  I've read and 
>reread Kriege and Berry, and I think I understand the principals of 
>friction, stiction, moment arms and such in order to get the "buttery" feel 
>for manual use.  However, I understand that the Dob Driver would be 
>'happier" on frictionless (i.e. ball) bearings.
>
>Idea -- why not use ball bearings, and develop clutches for the Dob Driver, 
>and adjustable sources for additional friction as needed.  When the scope 
>is being driven by the motors, only the ball bearings would be in use.  
>However, when the Dob Driver was not at work (via some sort of mechanical 
>clutches that disengage the drive belt for alt, and drive wheel for az), 
>there would be some sort of mechanical clamp (e.g. with a thumb screw or 
>spring) that would press, say 2 square inches of Teflon, against an 
>aluminum surface with an 8 or 10 inch moment arm in alt and in az?  This 
>would then switch the scope into a mechanical mode that reproduces the kind 
>of friction advocated by Kriege and Berry.
>
>Any thoughts or suggestions would be most appreciated.  Thanks -- Michael
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