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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@clear.net.nz


 
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