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From: George Huxtable (no email)
Date: Tue May 24 2005 - 11:29:32 EDT
Robert Eno wrote-
>A late acquaintance of mine who repaired sextants and other nautical
>instruments for decades, told me that even the finest sextants can have a
>bit of backlash. His suggestion to me was to always rotate the tangent screw
>one way when making observations so as to preclude the possibility of
>backlash errors. I have followed this practice for years even though I own a
>C.Plath which, theoretically, should have no backlash whatsoever.
That is indeed sensible practice.
>But back to removing backlash: my understanding is that this is not a
>repairable problem; that backlash is a result of poor machining of the arc,
>hence my earlier question about how the instrument shop removed this error.
As I see it, not so much from poor machining of the arc. I am unfamiliar
with the fine details of "proper" metal sextants (and better acquainted
with the plastic variety), but presumably the teeth of a rack are made with
a tapered profile. The worm which engages them, having a similar tapered
profile, and being spring-loaded against the rack, is kept wedged into firm
contact against both faces. This simple arrangement should minimise
backlash without requiring any great precision in the machining of the
rack. (The great precision called for is in the accurate PITCHING of the
teeth of the rack, to avoid calibration error)
But then there's another stage in the motion, where backlash can arise.
That's in transferring the lengthwise motion of the worm, along its axis,
to shift the index arm to and fro. Backlash here can be minimised using
some sort of spring, applying constant pressure in one direction or
another, against an end-stop on the bearing of the worm-wheel. This will
work as long as the overall friction of the arm is always dependably less
than the pressure provided by that spring.
The snag about applying such spring-pressure against any sort of rubbing
contact is that it can give rise to wear, after long-term use. So an
alternative to spring-loading is to have more rigid, and adjustable,
positoning of the two moving parts, so that they touch, but only just
touch. This would require some skill in adjusting out any lost-motion, but
ensuring that it isn't overtightened, and doesn't bind at extreme
temperature or after ageing. Differences betwwen different makes and models
are no doubt to be found is such design details; I have no knowledge about
which model does what.
George.
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contact George Huxtable by email at , by phone at
01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
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