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    Re: A-10 Sextant Manual
    From: Bill Morris
    Date: 2009 Jun 11, 03:43 -0700

    Douglas
    
    You have been very brave to dissassemble the worm shaft assembly. Presumably 
    the cross-pins came out without too much trouble.
    
    You write "The spring on the worm 
    shaft, plus the spring of the index prism sector  are the only devices 
    pushing downwards to keep the worm shaft in contact with the lower washer. 
    This position must be maintained in all rotations and movements of the worm 
    shaft. For an absolute positive position determining method this is poor 
    design,...", 
    
    but this is the principle used, for example, in precision grinder bearings, 
    where a spring box is used to take up clearance in the opposed tapered 
    bearing (see attachment). Lesser machines simply apply a fixed pre-load to a 
    pair of opposed tapered bearings, but this method is not proof against 
    increase in shaft length as it warms up. 
    
    and you write :"...or other 
    possibilities such as the washer not being exactly flat."
    
    Here I think you are considering axial float. It is not sufficient for one 
    thrust face to be wedge-shaped or out-of-flat. Axial float occurs when the 
    thrust faces are not at right angles to the axis of the shaft, so that there 
    is a periodic shift axially with each revolution of the shaft.
    
    I don't think there is anything wrong in an engineering sense with the worm 
    shaft, providing lubrication is maintained. It has not been a problem in the 
    instruments I have restored, at least, once lubrication was renewed. Other 
    methods of removing end float would take up more room and not take into 
    account the differential expansion of the aluminium alloy frame of the 
    assembly and the steel of the shaft. There does however seem to be a major 
    problem with the index prism bearing which was apparently notorious for 
    getting out of kilter in service. It did not need sixty five years for this 
    to happen. 
    
    Do I see a ball bearing in your instrument and is this a Denny special? If so, 
    it is only the second time I have seen a ball bearing used in a sextant, the 
    other being the ball-recording sextant which used a pair of opposed angular 
    contact bearings for the index arm. I assume there is another bearing on the 
    other side of the frame.
    
    But all this is getting rather off the topic of navigation and it might be 
    best if we had any further engineering discussion off list.
    
    Bill Morris
    engineer{at}clear.net.nz
    
    
     
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