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Chronometry: A question about Earnshaw detent escapements in chronometers

waynet7

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May 29, 2007
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I have been trying to understand some aspects of the evolution of the Earnshaw spring detent escapement. I have seen references to 'old' and 'new' styles. I found a thread here that suggested that the main difference is the diameter of the roller table. Are there other possible differences? I am wondering if there were differences in the impulse jewel as well? Perhaps some examples not employing an actual jewel? I wish there were more pictures available on the net to study, but most depict only the modern Hamilton, Swiss, and Russian examples. Thanks!
 

doug sinclair

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In the modern reprint of Rupert Gould's book on the marine chronometer edited by Johnathan Betts, fig. 34 facing page 118 shows renderings of both the Earnshaw escapement as designed by Earnshaw, and also the modern version of the escapement. Owing to restrictions imposed by the publishers of the book, I am reluctant to upload the image. However, on page 271 of the book Time and Timekeepers by Willis Millham copyright 1923, an image of an earlier version of the detent escapement is shown. While both versions function the same way, there are differences in proportion as you suggest.

image.jpg
 

waynet7

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Sorry! I Did not mean to be ungrateful. I do thank you for your response. I've been preoccupied with other matters these last few days. Please accept my apologies for my lack of hasty response. As to my OP, I've spoken to an area watchmaker, AWCI trained, who has said that not all 19th century chronometers actually employed an impulse jewel, and I am curious as to how this was accomplished, as I personally have not handled many such instruments.
 

Tom McIntyre

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I have never seen one, but there is no reason in principle why one could not be made with no jewel on the impulse roller notch. It would just be an unwise thing to do. Earnshaw, early on, had problems with the jewel cutting the escape tooth due to an awkward impulse angle. It is a fairly violent encounter. :)
 

gmorse

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This arrangement of the impulse roller is much more commonly found in duplex escapements, where the impulse action is very similar.
 

waynet7

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Thank you for the replies! I did actually lay eyes on a chronometer balance assembly that had an angled as opposed to curved cut-out in the roller table with no jewel present. The point of the cut-out had a radius, and I wonder if this is where the jewel would have been seated. I wish I could post a picture, but I don't have access to the part or the instrument. I tend to think that the jewel would need more support than that shallow radius would provide. This is what prompted my curiosity.
 

Tom McIntyre

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I presume you know that the term "chronometer balance" has nothing to do with the intended escapement the balance will be attached to. Chronometer balance is used to describe any balance with temperature compensation built in with compensation screws and mean time screws.

In the chronometer escapement, the escape wheel is locked by the detent jewel on the spring or pivoted detent. When the releasing jewel on the roller moves the detent to release the escape wheel, the escape wheel accelerates to catch up with the balance and gives impulse to the impulse jewel on the balance. This jewel is normally set in the wall of the angle cutout of the roller. There is no shallow cutout on the roller of a spring detent escapement. The location of the impulse jewel on the roller could have a variety of shapes so long as the jewel is in position to receive the impulse from the escape wheel. The reverse swing of the balance flips the gold passing spring away from the detent but does not move the detent itself.

In the similar acting duplex escapement, the long tooth of the escape wheel rests on the locking jewel that surrounds the balance arbor. That jewel has a notch in it that releases the escape wheel when it is turning in the same direction the escape wheel wants to move (in the opposite rotation of the balance the tooth just bounces on the notch). When the escape wheel is released, the short upright arms catch up to and strike the impulse piece on the balance. (This can be a jewel but usually is not.) The impulse from the escape wheel give momentum to the balance.

There are lots of variations on these escapements, but the above is the basic operation of the two most common types.
 
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