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I think "backlash" is better understood as the distance one gear must be
turned in order to load the opposite side /face of the other gear. If
there is to be lash then both sides of the output gear tooth will be in
contact with both sides of the drive gear at all times. A small gap is
needed so the gears do not bind. A worm gear will make the slope look
exaggerated. In most gears the lubricant will occupy this space and
tighten up things. If you see a whole gear train you can get one gear to
rotate through 10's of degrees before the output drive moves, as all the
slope is taken up.
I still think it is best, as first suggested, no matter how good the
device is, to always approach the result by rotating the drum in the
same direction. Then the gear(s) are always loaded on the same side.
Just my 2 Cents worth

-----Original Message-----
From: Navigation Mailing List
[mailto:NAVIGATION-L@LISTSERV.WEBKAHUNA.COM] On Behalf Of Jared Sherman
Sent: Wednesday, May 25, 2005 2:14 PM
To: NAVIGATION-L@LISTSERV.WEBKAHUNA.COM
Subject: Re: Backlash


Alex-
 I would expect that any sextant will have some backlash, unless *every*
tooth in the sextant arm is cut at precisely the correct same angle on
both sides, referenced to the pivot point on the arm, and the helix on
the drum is similarly machined to exactly the same angle (bevel) on both
sides, again consistantly along the entire length.

 If there is any mismatch at any point, the drum would "rock" a bit at
that particular point, and even spring tension would not zero that out,
it would simply make the error more consistant from one time to the
next. At least, that's the way it appears to me. I know "engine turning"
was a big step in the industrial revolution  and in theory all of
those angles and faces could be perfectly uniform given good
manufacturing. Still, I suppose each manufacturer would have different
tolerances allowable in this.

 Presumably as a sextant wore a bit with age, it might "polish in" and
the angles become even more uniform. Or less--if the uneven use of
different parts of the arc, wear from salt grit, etc. made a difference.
Perhaps that is a topic you could explore with Plath when you get to
them.

 I have dealt with both Plath and Freiberger via email, and found both
firms to be outstanding examples of professionalism. If only every
business treated their customers the same way!

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