NavList:

I should have written first that another cause of backlash is a loosening of the screw that holds the prism sector shaft in place. It is on the end of the prism sector shaft and can be accessed after removing the averager plate on the right side of the instrument. This screw (item 2, figure 17, attached)  presses against washer (item 3) to hold the prism sector shaft (item 7) and sector teeth fully engaged with the worm assembly (item 14). If the dowel pin (item 1) is missing,  the washer can rotate in use and thereby cause loosening of the screw which then allows the sector teeth to move out of full engagement with the worm causing backlash.

So, instead of taking the sextant apart to check the sector spring (item 15) first tighten up the screw (item 2) and see if that cures the problem.

I am also attaching pages that describe the difficult procedure  for reinstalling the sector spring (pages 18 and 19.)

gl



Gary LaPook wrote:

Sounds like you may have a backlash problem with your A-10 since that is 
the range that backlash causes. Are you sure the "sector spring" is 
properly installed since its purpose is to eliminate backlash?

To check for backlash take a series of 20 sun shots alternating by 
bringing the sun up into the bubble and down into the bubble. If you 
overshoot, start that sight over again. Then plot the two separate 
series on graph paper and see if they differ, if they do, you have backlash.

To deal with it, if you do not want to disassemble the sextant, always 
approach the center of the bubble the same way, either moving the object 
up or moving the object down and work out an index correction for this.

For use in flight, in which the bubble is always moving around, you will 
be approaching the bubble approximately the same number of times from up 
and down so the differences will average out over a two minute shooting 
period of 120 recorded sights but you will need to work out an index 
correction for the average.

gl

douglas.denny@btopenworld.com wrote:
  

Mr. Morris.  With reference to the Fairchild A10; you say:-

"A grub screw attaches the 
one-toothed clutch to the shaft of the counter and the latter is provided 
with a screw-driver slot to make the adjustment easier." 
------------

Many thanks for this.  I didn't look hard enough to realise (having just filled the bubble chamber and wanting to get on and play with comparisons of the A10 with my definitive Hughes MkIX BM; ...though I suspected there had to be some means of disconnecting the main drive shaft and worm unit from the counter to set the counter to zero (or whatever) then disconnect the counter and adjust the index prism.

I have to say I am not impressed with the accuracy achived in my comparisons with sun shots between the two sextants getting very variable results from the A10 up to plus or minus 15 or even 20 minutes of arc compared with the MkIX.  Though to most differences were around 8 to 10 minutes of arc.  That is still too much.

This of course is partly due to no telescope with the A10, with a relatively small image of the sun and bubble compared to the MkIX; but even so, the results are not good at all. I even wonder if the worm is distorted, though I have not done definitive checks yet to assesss the linearity of the worm screw thread and tangent sector gear.
I would not have been happy in a B29 Superfortress bomber mid- Pacific with that sextant to get me home,  as the Americans were doing in WW2.

In my opinion the Hughes MkIX is the definitive sextant of its type; approaching if not actually being the very best that can be achieved with a bubble sextant.

Douglas Denny.

Chichester. England.

P.S. Pic enclosed of the two sextants.
    

  

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