NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: A-10 Sextant Manual
From: Douglas Denny
Date: 2009 Jun 9, 14:37 -0700
From: Douglas Denny
Date: 2009 Jun 9, 14:37 -0700
Having just come onto this interesting forum I am amazed by the quality and willing support from so many people. Thank you all. Mr. Morris and Mr. Le Pook, thank you for your help and excellent quality pictures/diagrams ------------- I have not had time to take the sextant apart again to look into the problem more carefully (in fact may do so after this e-mail exchange), but can make some comments already. The sextant is in fine condition with no corrosion or any mechanical deterioration at all. The inside looks as if made yesterday. Except for: the bubble chamber was in need of reconditioning and filling (done as described with silicone fluid); the horizon view 45 degree glass was broken; and the main index prism has a piece of glass shard broken away on its 'base' - which is not affecting the optical properties at all. The latter was stuck back in position with cyanoacrylate glue and is of no consequence. All the ball bearings are in absolutely perfect condition and must have been lubricated and still were covered in oil film. The index mirror holder assembly is a perfect push fit onto the slotted 'drive' shaft. The index segment and worm gear are in perfect condition with not the slightest pitting or corrosion anywhere. Even the black crackle finish is nearly perfect. This particular sextant was probably used and handled by Sir Francis Chichester in the late wartime, as it was obtained from a gentleman who worked at Henry Hughes (later Kelvin Hughes) as part of a whole collection that had been put to one side in a storeroom, were to be thrown out, and he 'saved' them. The collection was part of a number of sextants which were assessed for performance by Francis Chichester when he was a consultant for H.Hughes, the Air Ministry and the RAF on aircraft sextants, for presumably checking the relative efficiency of them to assist in their designing their own. There are some interesting ones including a Japanese model which is, I believe, literally a Japanese copy of an American design. -------- back to the A10: The returning action of the spring on the index segment arc onto the worm is, I am quite sure, acting properly as it should now I have ensured full tension possible and mover the leverage further up the segment. I am quite sure now it was working perfectly alright before my modifying it slightly. The backlash here is only in the order of a few minutes of arc anyway when the prism is rocked gently to test for backlash, and returns eactly and very positively, whilst the variable element with the adjusting gearing backwards and forwards is up to 25 or even 30 minutes of arc. The variable movement simply must be inherent in the worm shaft end-float. There is no other explanation. I can see from your excellent photograph of the A10 the left hand side upper bearing near the counter is the area of interest. I rememember the helical spsring on the worm shaft but did not test its action as I was concentrating on the index segment spring. Having not taken it apart yet again, I cannot say for sure, but the diagrams indicate a cone shape on the worm shaft, and the photo a flat end piece, so I assume the bearing is a tapered (cone) plain bearing directly into the upper bearing boss. If so, this is surprisingly poor qulity engineering for an instument of this calibre, and it really needs a taper-roller bearing here for efficient engineering and positive action. A basic design flaw? - and no wonder they have "backlash". This would have been done for cost-effectiveness I guess as taper-roller bearings would have been expensive in wartime and more costly to fit into the design. Surprising because the Americans did not usually stint on costs for their military equipment, but I suppose costs eventually have to be considered somehwere eventually in manufacture. Even the shaft resting on a single ball-bearing at the end, with the spring pushing the shaft onto the ball-bearing would have been a more positive method in fixing the shaft position but allowing rotation - with low friction too. A simple plain or ball-race at the upper bearing would then suffice. This is becoming more interesting. Douglas Denny. Chichester. England. --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---