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    Re: A-10 Sextant Manual
    From: Douglas Denny
    Date: 2009 Jun 12, 00:52 -0700

    Bill Morris:
    
    I was in error referring to a washer at the lower bearing - as I thought at 
    first on disassembly, stuck to the bearing boss - but is the bearing itself 
    which is a press fit into the boss. And on reflection admit the bearing 
    should be adequate for location and positional accuracy providing the shaft 
    does indeed press fully into the bearing due to the springs. Any friction in 
    the shaft in its bearings mitigates against this. A proper thrust bearing 
    would not go amiss as a proper job to reduce friction here, but the loading 
    is so slight it is hardly necessary on second thoughts providing the whole 
    shaft is clean, oiled and free.  It was not perfectly so, with some 
    'stickage' - not surprising after 60 years, but even when cleaned and 
    re-assembled the problem persists.
    
    I am now baffled, as having cleaned and re-assembled with a slightly stronger 
    spring to ensure good location of the whole assembly on rotation of the worm 
    the "backlash" problem is still there with a vengeance.
    
    I can only conclude that the very slight leeway in the shaft end-float is the 
    problem still, being completely convinced (though not proven) the sector to 
    worm gear is adequate; and that friction of the gearing from the adjustment 
    knob through the helical gear to the worm shaft keeps the shaft slighty one 
    way, then the other depending on the direction of rotation.
    I conclude it is an inherent fault consequent of the design of the gearing 
    mechanism itself, probebly due to the high friction of the helical gear 
    (transfering motion to the marking disc) - and is very frustrating.
    
    Attention finally moves to the counter mechanism, but that appears to be 
    direct gearing to the first dial (seconds) and does not give the impression 
    backlash would be present here. It is annoying I have not been able to cure 
    the problem.
    Any further ideas are welcome.
    
    Douglas Denny.
    
    Chichester. England.
    
    P.S.  If you wish to converse in more detail which as you say is not strictly 
    of navigational interest here, my e-mail address is:
    
    douglas.denny{at}btopenworld.com
    
    
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