NavList:

Greg asked-

"Sort of an off-the-wall question that occurred to me this past
weekend while doing practice sights... I've read (and always
practiced) that because of possible mechanical backlash in the
micrometer screw mechanism you should do your final altitude
adjustment in the same direction (i.e. either bringing the body up or
down, not back and forth).

But wouldn't any backlash (i.e. any lag in the body moving up or down
when turning the micrometer dial in the opposite direction) be
immediately apparent to the person taking the sight? Both of mine seem
to start moving immediately when I do that, but maybe there really is
some subtle lost motion happening there that isn't obvious to the
observer?

I'll try to remember next time I'm out at the beach to take a series
of sights using both methods (racking in one direction vs. up and down
to find the horizon "kiss") and see what the results look like."

==================

GregR makes a valid point.

Backlash, or lost-motion (same thing) can arise at two places.

One is in the engagement between the teeth of the worm and the teeth
of the rack. Usually, both will be a vee shape, and have a somewhat
different profile, held into contact by a light spring pressure, so
that tangential contact occurs along both flanks of each tooth. It's
done that way to minimise backlash, and does so very successfully.

The other is in the lengthwise location of the screw with respect to
the index arm. Backlash here is minimised, either by maintaining a
slight lengthwise spring pressure against a thrust bearing, or by a
careful adjustment that takes up all such axial slack, while leaving
the screw still free to turn; a trick requiring a compromise that is
hard to get just right..

Even in a cheapo instrument such as my plastic Ebbco sextant, with an
aluminium screw working against a rack that's machined in the plastic,
backlash is controlled sufficently well that I can't detect its effect
within the usable reading-accuracy of the instrument. That is, within
about an arc-minute or so. Nor can I detect any backlash by "feel".
And in general, I think that's true of most sextants. Nav-l members
were surveyed at one time and most reported negligible backlash. If my
Ebbco had been subjected to years of hard use (it hasn't) then I might
expect the first signs of wear would be a developing backlash.

Those that are trying to measure to a small fraction of a minute,
perhaps for lunars, will be most conscious of the need to defend
against minute amounts of backlash. The recommendation, as GregR says,
is always to measure with the final movement of the drum always in the
same direction (and, importantly, always the same way as used for the
index error). Is that vital? Maybe not, for ordinary altitude
navigation, to within a few minutes, unless an old instrument has got
particularly sloppy. But it's not a bad rule; it's just as easy to
always turn it the same way, as not to. So why not keep things
consistent, and remove one unnecessary variable?

George.

contact George Huxtable at george@huxtable.u-net.com
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.





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