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    Re: Visit to Freiberg
    From: Courtney Thomas
    Date: 2005 May 23, 17:02 -0500

    Bill,
    
    Thanks for your replies.
    
    I somehow missed the earlier post to which you refer concerning "power
    lines". Could you relay a copy please ?
    
    Cordially,
    Courtney
    
    
    On Mon, 2005-05-23 at 15:18, Bill wrote:
    > > What is backlash ?
    >
    > Courtney
    >
    > Any "slop" in the gears. If something is machined for a "press fit" you will
    > need and arbor press or some other method the fit one part inside the other.
    > Gears would wear rather quickly and be very hard to turn.  If the part
    > sliding in is machined smaller than the opening it goes into for a sliding
    > fit, there will be some slack/slop.  The easier they slide together, the
    > greater the slop.
    > >
    > > Without going to Germany, how do you know if it's a problem with your
    > > sextant ?
    >
    > I suggest power lines (see earlier post).
    > >
    > > Can you [yourself] approximately 'measure' it's significance with your
    > > sextant and correlate turning said 'screw', recheck and approximate a
    > > satisfactory adjustment ?
    >
    > Would have to see a SNO-T again to know how it is set up.  My table saw for
    > example, gives the operator the ability to move the blade-angle-adjustment
    > gears closer together to reduce backlash and compensate for wear. It will
    > stay where set. My Astra IIIB, according to Celestaire, uses a spring to
    > apply pressure on the worm gear to hold it against the arc teeth.
    >
    > The plus side of a spring is that the pressure of the worm gear against the
    > arc will remain almost constant along the arc, even if the arc is not
    > perfect.  IF the screw on the SNO-T regulates a spring pressure, the same
    > would apply.
    >
    > IF the screw on the SNO-T fixes the worm gear in a given position (like my
    > table saw) then the backlash will vary along the arc unless the arc is
    > perfectly equidistant from and tangent to the pivot point of the arm.
    >
    > My Astra appears to use a brass worm gear, so it will probably (slightly)
    > self lubricate against brass or a hard aluminum alloy arc.  It may also wear
    > faster than the body.  But easier to replace a worm gear than the sextant
    > body/arc.
    >
    > In either case the tradeoff for reducing backlash is increased gear wear and
    > increased effort to turn the drum.  Which points out the need for keeping
    > the gears clean and lightly lubricated.
    >
    > Bill
    
    
    

       
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