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How A Chronometer Works Part III |
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Articles -
Watchmasters Corner
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Written by Ziggy Zumba
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Page 3 of 4
Note what happened to the tilting pinion, it was forced away from the seconds wheel.
Now this poses a problem for us. Why is this a problem?? If we stop the chronograph, there is nothing holding the seconds wheel in place, and we need a way to hold the wheel in position, otherwise it will flop around the dial (remember my flea experiment).
The seconds wheel is held stationary with a brake lever. Here is the brake lever, and here’s how it’s controlled by the cam.
Re-Starting the chrono seconds
To re-start the chrono from the stopped position, all you do is press on the top pusher once more, the brake is released, and the tilting pinion tips over and turns the seconds wheel again.
Reset
You can only reset from the OFF position. So pressing the top pusher again stops the chrono. The tilting pinion disengages, and the brake lever holds the seconds wheel in place.
When you reset the chrono, nothing is going to happen to the tilting pinion, it is already out of engagement, and that’s where we want it to be and it stays there.
As for the seconds wheel, resetting has to bring it back to the “12” position on the dial. How can we do that if the brake lever is holding the wheel in place? We can’t, the brake has to be released before the seconds wheel can move back to “12”.
As the cam rotates, the first thing to take place is the brake lever is released:
Now that the brake is released, the wheel is free to rotate to the “12” location on the dial.
How are we going to do this? Remember I said earlier that all chrono wheels have heart pieces on them for centering. This one is no different. Here is the heart piece on the seconds wheel, easier to see now, and it does look like a heart.
So the tilting pinion is disengaged, the brake lever released, the seconds wheel is free to turn, and now the Hammer is going to snap down on the heart piece and force it to go to “12”.
This is the hammer, this is how it’s controlled (by the cam of course):
This is the spring that activates it, it slams down on the heart pieces with a good snap. Hard enough to spin the hand on the shaft and get it out of alignment with the off position on the dial.
Depending where the seconds hand is stopped on the dial, will depend on how fast and hard the hand goes back to the “12” position on the dial. The “6” o’clock position of the seconds wheel is the most stressful one to reset from, you should avoid resetting the chrono when the hand is here. My recommendation: always reset the chronograph with the seconds hand at + - 10 seconds from 12, i.e. between 10 am and 2 pm on the dial. This is less stressful on the hand, and should avoid problems with the hand getting out of alignment (unless you have it glued in place, like I did).
The problem with the hand not resetting to the correct location can’t be caused by the heart piece or hammers, just look at it, it will always go to reset position. It can’t slip. It can break, but then it would not reset at all, there is a difference between the hand resetting to the incorrect position, or it not resetting at all.
The reason the chronograph seconds does not reset to “12”; is always caused by the hand slipping on the shaft or the tube assembly. The solution is to remove the hand, put epoxy glue on it and on the shaft, and reinstall it. This is what I did to my 188, and 4 months later it is still working great, and I always reset from “6” to try and break it, no luck so far, always resets between the dots.
Here is the hand, and where you should put the epoxy to secure it:
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Last Updated ( Tuesday, 14 August 2007 )
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