NavList:

Consider a clockwise rotation of the drum. If there was radial play
in the tangent worm screw shaft, the screw would tend to climb the
arc slightly before starting to slide. Looking towards the bottom of
the index arm, the tangent worm screw would be seen to rotate
slightly in the clockwise direction.

Now consider a clockwise rotation of the drum where there is radial
play in the shaft of the index arm. The frictional force acting on
the tangent worm screw will create a clockwise turning moment on the
index arm, as seen from the bottom of the index arm. Due to the
looseness of the index arm shaft, the index arm will rotate slightly
in the clockwise direction. Thus the tangent worm screw (fixed as it
is to the index arm) will be seen to rotate slightly in the clockwise
direction, as above.

The same scenario will be true if "anti" is put before clockwise
above. Thus the lost motion error _will_ depend on whether the drum
is being turned clockwise or anti-clockwise.

Of course, the index mirror is attached to the index arm and if there
is any _significant_ radial play in the index arm shaft, rotating the
drum one way then the other will be seen as a sideways movement in
the image reflected off the index mirror when testing for index
error. If no sideways movement is seen, I would think any radial play
in the index arm shaft would not lead to any significant lost motion error.

Geoffrey Kolbe

At 17:27 22/07/2006, George Huxtable wrote:


>I agree with all Geoffrey Kolbe's posting about backlash except for
>his final sentence, when he wrote-
>
>|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.
>
>=================
>
>No, I think the symptoms of such looseness would be rather different,
>because it occurs at the other end of the index arm. It would not
>depend on whether the drum was being turned clockwise or
>anti-clockwise, but it could be affected by any tilt of the
>instrument, or by whether it had just been tilted, in the fore-and-aft
>direction, or perhaps just according to the way the index adjustment
>was being gripped. It could differ depending on whether it was the
>upper part of the arc range, or the lower, that was being read. I have
>never heard of such an error being apparent in practice in a sextant,
>but that doesn't imply that it can't occur.
>
>My cheap Ebbco plastic sextant has a non-tapered bearing as its index
>arm pivot (plastic in plastic, polycarbonate) and I can detect no
>shake there by feel. I don't know what a dial gauge would show, and
>wonder whether prolonged daily use might result in wear and subsequent
>looseness. At present it will be contributing little to the overall
>inaccuracy.
>
>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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