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    Re: Backlash
    From: Jean-Philippe Planas
    Date: 2005 Nov 21, 14:07 -0800
    I do not notice on my SNO-T this lateral play you quote on yours. I don't know if it's related to backlash because backlash is essentially a longitudinal ("arcwise") gap between worm gears and arc teeth. Of course if you regulate backlash by backing a little the small drum screw, the worm and teeth will be in contact with more pressure from the spring, therefore inducing more difficult lateral displacement due to friction. But it's only a side effect that does not really correct a locally (at 65? as you quote) too wide adjustment.
    But as the measured angle is measured longitudinally, along the arc, I don't think this is likely to induce significantly degraded instrument precision.
    Anyway I would not recommend fussing too much with this small screw as long as your SNO-T is fresh from overhaul and adjustment by a renowned workshop.
    I will post some Lunars and Star-star distanc! es in a post to come.

    Alexandre E Eremenko <eremenko@MATH.PURDUE.EDU> wrote:
    Thank you for your interesting info.
    My sextant arm has a slight lateral movement
    (shaking) exactly near the "parking position"
    that is near 65 degrees.
    As I believe that this SNO was never used before
    (and I certainly use it VERY carefully),
    this can be the result of transportation.
    (I don't know how much transportation its it had in its
    lifetime, but it was delivered to me from Ukraine,
    and then I had a transatlantic roundrip flight,
    with sextant (packed as well as I could) in the
    checked in luggage.

    By the way, the Russian manual recommends disengaging
    the worm from the arc during transportation
    (just by putting tape on the levers that disengage)
    and securing ! the arm to the frame with another piece of tape.
    I did not do this, packing instead pieces of foam everywhere
    to prevent any motion or stress.


    On Mon, 21 Nov 2005, jcs wrote:

    > Some years ago when I was servicing a sextant for a friend I noticed an
    > excessive and variable (i.e. dependant on the part of the arc being used)
    > backlash problem.My surmise was that at sometime during its life the index
    > micrometer worm had been clumsily grated across the gear arc, and back lash
    > was occurring wherever the worm did not now properly engage. This only
    > occurred
    > only over certain parts of the arc, mostly near the parking position, as the
    > sextant was put away in its box.
    > This could occur on a sextant if the clamping lever had been insufficiently
    > gripped as the index arm was rapidly moved and the teeth on the gear (which
    > was made of a hard metal) come into contact with the! softer arc and damage
    > the tops of some the teeth. If you were to look at the shape of the damaged
    > teeth in profile their normal triangular shape would be turned into that of
    > a breaking wave.
    > My thoughts are that If you find backlash occurring on a old or second hand
    > sextant or anytime on
    > a plastic one, look carefully at the shape of the gear teeth. If any damage
    > is not very bad It may be carefully repaired. But! Beware, always look very
    > carefully if you are buying second hand. Even a new plastic sextant is not
    > immune, especially if "shop soiled!" This, in my opinion, is the greatest
    > risk with plastic sextants. They do need Extra Tender Loving Care!
    > CLIVE


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