NavList:

In Navlist 791, George mentioned two possible causes of backlash. One
was a poor fit between the teeth of the arc and the tangent worm
screw. The second was some longitudinal movement of the shaft of the
tangent worm screw in its bearing, which is attached to the index arm.

Due to the spring holding the tangent worm screw against the arc,
(which George mentioned), I would discount this as a source of
backlash - unless the teeth were sufficiently worn that there was no
longer any clearance between the tops and bottoms of the threads.

Longitudinal movement in the shaft of the tangent worm screw is the
primary source of backlash and this can usually be adjusted out in
some way. Completely removing backlash by adjustment of an
anti-backlash nut is not good engineering however, as this increases
wear. An anti-backlash spring (which George also mentioned) usually
is sufficient to overcome backlash from longitudinal play.

However, radial play in of the tangent worm screw shaft in its
bearing is another source of backlash error. This occurs because the
hole in which the shaft rotates is too big, either through wear or
poor manufacture. I think the only real way to correct this problem
is to drill out and bush the bearing. Clive Sutherland also mentioned
the possibility of play in the tapered shaft holding the index arm
onto the frame. The symptoms of this would be exactly the same as
radial play in the tangent worm screw shaft.

Geoffrey Kolbe

At 09:56 22/07/2006, you wrote:


>Alex wrote in Navlist 858
>
>| 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?
>
>=====================
>
>I had also addressed the same matter, in Navlist 798, but Alex does
>not mention that message. Did he receive, and read it?
>
>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.
>
>
>
Border Barrels Ltd, Newcastleton, TD9 0SN, UK
Tel: +44 (0)13873 76253   Fax: +44 (0)13873 76214 


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