NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: "Lost Motion" Question
From: George Huxtable
Date: 2006 Jul 18, 03:49 -0500
Frank wrote-
it's interesting to note that
| the earliest references to backlash come from the 18th
century --long before
| the micrometer. Supposedly because of possible differences in
friction
| between the index arm and the arc, early quadrant users were told
to make small
| adjustments always in the same direction.
That seems plausible, in the early days of the Vernier, when index
arms were long (18 inches, even!) and wooden, so therefore much less
rigid than later, when they were always brass. Also, when the bearing
of the arm pivot was a simple pin working in wood, it's easy to
imagine stiction occurring there, particularly as the wood swelled or
shrank with humidity, even with the best ebony. Error could then
result, because the index mirror might then fail to follow small
movements at the far end of the index arm. It could be minimised, to
some extent, by always making the final adjustment in the same sense,
but not eliminated.
It's lost motion, just as we have been discussing with micrometers,
but with a completely different origin. In later times, with chunkier
all-metal instruments, I can't see such backlash affecting Vernier
instruments significantly; but it would be interesting to find out
what the recommendations were (see below) . The backlash we have been
considering up to now has been confined to the micrometer mechanism,
so I wonder whether 20th century Vernier sextants will be affected by
backlash at all, if the pivot is kept in reasonable order.
Lecky's "wrinkles", at a quick glance, doesn't seem to mention the
matter, but Raper, "Practice of Navigation", 1864, dealing with
finding index error, addresses it, saying-
"In consquence of the spring or elasticity of the index-bar, the error
will be different for the onward and the backward motion of the index.
It has been recommended, therefore, to turn the tangent-screw right
and left alternately, in making successive contacts, by which a
partial compensation is obtained. This source of discrepancy is,
however, effectively removed by taking all observations, including
that for index-error, with the same motion of the index-bar. The
onward motion being adapted as the most natural, the tangent-screw is
always employed to close the object and the reflected image, and is
thus always turned in the same direction." It should be kept in mind
that the tangent-screw, in those days, was no more than a mechanism
which could be clamped and unclamped to the index arm to carefully
control its motion, and played no part in the readout, which was done
by the Vernier.
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.
--~--~---------~--~----~------------~-------~--~----~
To post to this group, send email to NavList@fer3.com
To , send email to NavList-@fer3.com
-~----------~----~----~----~------~----~------~--~---
From: George Huxtable
Date: 2006 Jul 18, 03:49 -0500
Frank wrote-
it's interesting to note that
| the earliest references to backlash come from the 18th
century --long before
| the micrometer. Supposedly because of possible differences in
friction
| between the index arm and the arc, early quadrant users were told
to make small
| adjustments always in the same direction.
That seems plausible, in the early days of the Vernier, when index
arms were long (18 inches, even!) and wooden, so therefore much less
rigid than later, when they were always brass. Also, when the bearing
of the arm pivot was a simple pin working in wood, it's easy to
imagine stiction occurring there, particularly as the wood swelled or
shrank with humidity, even with the best ebony. Error could then
result, because the index mirror might then fail to follow small
movements at the far end of the index arm. It could be minimised, to
some extent, by always making the final adjustment in the same sense,
but not eliminated.
It's lost motion, just as we have been discussing with micrometers,
but with a completely different origin. In later times, with chunkier
all-metal instruments, I can't see such backlash affecting Vernier
instruments significantly; but it would be interesting to find out
what the recommendations were (see below) . The backlash we have been
considering up to now has been confined to the micrometer mechanism,
so I wonder whether 20th century Vernier sextants will be affected by
backlash at all, if the pivot is kept in reasonable order.
Lecky's "wrinkles", at a quick glance, doesn't seem to mention the
matter, but Raper, "Practice of Navigation", 1864, dealing with
finding index error, addresses it, saying-
"In consquence of the spring or elasticity of the index-bar, the error
will be different for the onward and the backward motion of the index.
It has been recommended, therefore, to turn the tangent-screw right
and left alternately, in making successive contacts, by which a
partial compensation is obtained. This source of discrepancy is,
however, effectively removed by taking all observations, including
that for index-error, with the same motion of the index-bar. The
onward motion being adapted as the most natural, the tangent-screw is
always employed to close the object and the reflected image, and is
thus always turned in the same direction." It should be kept in mind
that the tangent-screw, in those days, was no more than a mechanism
which could be clamped and unclamped to the index arm to carefully
control its motion, and played no part in the readout, which was done
by the Vernier.
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.
--~--~---------~--~----~------------~-------~--~----~
To post to this group, send email to NavList@fer3.com
To , send email to NavList-@fer3.com
-~----------~----~----~----~------~----~------~--~---
