Marine: Unexplained Holes in Hamilton's Model 21 Lower Plate

Discussion in 'Chronometers' started by Ralph, Apr 13, 2016.

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  1. Ralph

    Ralph Registered User
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    Jan 22, 2002
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    I noticed these holes, circled in red, while examining a Model 21. While looking at other examples, I find the holes in about half of the ones I examined. Does anyone know their purpose??

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    Ralph
     
  2. Tom McIntyre

    Tom McIntyre Technical Admin
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    Do they line up with the break circuit mounting?

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  3. doug sinclair

    doug sinclair Registered User
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    With the chronometer face down, the plate referred to becomes the top plate. When the chronometer is face up, the plate referred to becomes the bottom plate. :)

    My model 21 is serial number 6125, and it has the same two holes. On mine, the inboard hole is threaded, and it is blind. The outboard one is also threaded, slightly larger diameter, and goes right through the plate.

    The picture of mine shows the same two holes. I have also shown an image from page 9 of the model 21 overhaul manual which shows a component I have never seen on a model 21. On page 82 of the manual, this part is listed arm-balance wheel locking, part number 42299. It is also shown in the exploded view of the chronometer, page 83, and is listed as item 31. The only thing that would seem to fit anywhere near those two holes would appear to be the balance locking arm. The image from page 9 would certainly seem to me to be for the balance locking arm screw. But why two holes? The balance locking arm screw is part # 37204, and the washer is part # 42251. There is no screw shown in the parts list for a second hole. So Why?

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  4. Ralph

    Ralph Registered User
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    Doug, I think you solved it.... and #34 is the stop pin for the locking arm. That accounts for the second hole.

    Thanks, Ralph
     
  5. DeweyC

    DeweyC Registered User

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    Doug is correct.
     
  6. artbissell

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    Apparently the balance lock arrangement using the two holes was not often used. I have perhaps a more commonly used method. artbissell

    IMG_3485ax.jpg
     
  7. doug sinclair

    doug sinclair Registered User
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    That one looks decidedly un-Hamilton!
     
  8. artbissell

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    So the careful use of corks obviated the need for any balance mechanical lock. ? Although using a balance rim hole with the homemade one seems simpler and better to me. Art
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  9. Tom McIntyre

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    Art, I think the balance interference solution does not protect from shock like both of the other solutions do. The motion does not need to be very great to have shock damage.
     
  10. artbissell

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    So it seems to me many 21's have survived without adequate shock protection? Art
     
  11. doug sinclair

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    image.jpeg

    The most likely scenario for a balance staff to suffer might be considered to be when the instrument was being transported? Certainly not the only risk, for sure. The carry case on these instruments would offer some buffering against damage. But immobilizing the balance wheel by any means was more to protect the three escapement jewels than to protect the balance staff, in my view. I'm not certain any of the means of immobilizing the balance could totally eliminate damage to a pivot. However, a pair of cork wedges strategically placed might offer more shock protection than other methods, including the brace that Hamilton later installed when the instruments were sent for servicing. That's how I see it, anyway.
     
  12. DeweyC

    DeweyC Registered User

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    Doug is correct, the point was to prevent the wipeout to the escapement when the instrument is wound down and the impulse jewel jams onto an escape wheel tooth. AFAIK, all locking modifications were done by the USN (as reported by Whitney), most notably at the Norfolk yard instrument shop. I know of no evidence the Hamilton was ever involved in these modifications. The main problem with the design that locks onto a MTp screw is that most machinist mates on ship were pretty ham handed and they broke the balance staff. Hence, the removals. Better to cork or the fork brake design.

    These instruments withstood salvos from 14 inch guns, explosions and typhoons. Shock is not their weak point. It was and still is mishandling.
     
  13. artbissell

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    Very much appreciate the detailed balance shock protection info. My 21 was bought from antique store 30+ years ago and its corks never removed by friend who sold it to me and another friend who traded me an Austrian sporting rifle for it. I recently obtained again from his estate sale. Is it best left unused and not serviced? No records with it. Is there a way to know its users? Looks unused to me. Art

    IMG_3444a.jpg IMG_3483a.jpg
     
  14. doug sinclair

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    Good looking instrument. The Hamilton records show it as being finished and sold on Nov. 2, 1944. Sold to 64-1618. Does anyone know who that might be? As to running it without servicing it? I would say an emphatic NO to that! It may well run, but suppose for a minute the detent jewel has been replaced with a metal pin (as many have), you'll destroy the escape wheel! Likewise the impulse roller jewel. And the effect of that large, heavy balance wheel running in dry bearings? Shudder! It's up to you to whether you should run it, or not. But if it were mine, I'd service it, first. And check out all the other possible areas for concern.
     
  15. Tom McIntyre

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    Doug, I am pretty sure the numbers are the military purchase contract numbers. It would be nice to have the other side documents that would tell the disposition of the material.
     
  16. artbissell

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    Thanks for the emphatic NO. At least for the last 30 years we have been afraid to run it. Art.
     

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