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> 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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