NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: "Lost Motion" Question
From: Clive Sutherland
Date: 2006 Jul 22, 05:14 -0500
Alex;
In a good modern sextant the bearing of the Index arm is usually a cone
shaped shaft working in a ground taper socket with a fixing screw that can
be adjusted to take up the slackness in the fit.
I dont know at what point in time this method was first applied or if it is
used in your instrument, but it should be examined if you are worred by the
instruments performance. There could be backlash from this or if the bearing
itself was loose in its attachment to the frame. (this bearing is sometimes
separate and attached to the frame with fixing screws)
If the taper is loose in its socket it would be simple to check and
readjust. if the bearing is a simple parallel bearing and it has worn or
damaged then it would require a good modern workshop to put it right.
For example, If a sextant with a 150mm arc radius has about 0.015 mm play
in its pivot , it would have a backlash of about 0.3 minutes or plus or
minus 0.15 min uncertainty in its reading.
Note, this error would be the same as if the backlash were in the
micrometer.
Clive.
----- Original Message -----
From: "Alexandre E Eremenko" <eremenko@math.purdue.edu>e
To: <NavList@fer3.com>
Sent: Saturday, July 22, 2006 8:59 AM
Subject: [NavList 858] Re: "Lost Motion" Question
>
>
> Dear Frank,
> This statement is very interesting (and misterious):
>
>> I'll have to dig up the reference on this,
>> but it's interesting to note that
>> the earliest references to backlash come from the 18th century
>> --long before
>> the micrometer.
>
> Please post the reference when you dig it out.
>
> When I think of the construction of the usual vernier sextant
> I cannot imagine any backlash in its operation,
> and I don't see how friction of the arm
> against the arc can be relevant.
> Maybe they (and you) mean that the arm of the sextant
> gets slightly bent in a direction parallal to the arc,
> because of the friction in the pivot?
>
> Alex.
>
>
> >
>
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To , send email to NavList-@fer3.com
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From: Clive Sutherland
Date: 2006 Jul 22, 05:14 -0500
Alex;
In a good modern sextant the bearing of the Index arm is usually a cone
shaped shaft working in a ground taper socket with a fixing screw that can
be adjusted to take up the slackness in the fit.
I dont know at what point in time this method was first applied or if it is
used in your instrument, but it should be examined if you are worred by the
instruments performance. There could be backlash from this or if the bearing
itself was loose in its attachment to the frame. (this bearing is sometimes
separate and attached to the frame with fixing screws)
If the taper is loose in its socket it would be simple to check and
readjust. if the bearing is a simple parallel bearing and it has worn or
damaged then it would require a good modern workshop to put it right.
For example, If a sextant with a 150mm arc radius has about 0.015 mm play
in its pivot , it would have a backlash of about 0.3 minutes or plus or
minus 0.15 min uncertainty in its reading.
Note, this error would be the same as if the backlash were in the
micrometer.
Clive.
----- Original Message -----
From: "Alexandre E Eremenko" <eremenko@math.purdue.edu>e
To: <NavList@fer3.com>
Sent: Saturday, July 22, 2006 8:59 AM
Subject: [NavList 858] Re: "Lost Motion" Question
>
>
> Dear Frank,
> This statement is very interesting (and misterious):
>
>> I'll have to dig up the reference on this,
>> but it's interesting to note that
>> the earliest references to backlash come from the 18th century
>> --long before
>> the micrometer.
>
> Please post the reference when you dig it out.
>
> When I think of the construction of the usual vernier sextant
> I cannot imagine any backlash in its operation,
> and I don't see how friction of the arm
> against the arc can be relevant.
> Maybe they (and you) mean that the arm of the sextant
> gets slightly bent in a direction parallal to the arc,
> because of the friction in the pivot?
>
> Alex.
>
>
> >
>
--~--~---------~--~----~------------~-------~--~----~
To post to this group, send email to NavList@fer3.com
To , send email to NavList-@fer3.com
-~----------~----~----~----~------~----~------~--~---
