Does engaging the chrono's really impact power reserve? Not as straight forward as one might think.
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  1. #1
    Member Wish2K's Avatar
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    Does engaging the chrono's really impact power reserve? Not as straight forward as one might think.

    One might think it does, but it doesn't make sense. If the spring is wound, it is still essentially fixed. So you are guaranteed a certain number of balance wheel oscillations & escapement steps for each full wind irrespective of how much load is on the entire mechanism. The watch will just run slower or faster delivering to completion those oscillations. As such, I'm gaining about 6 - 10 seconds per day with the chrono engaged (which is to be expected with the reduced balance wheel amplitude) which confirms my suspicions.

    If I'm wrong, I'd be interested otherwise.

  2. #2
    VRT
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    Re: Does engaging the chrono's really impact power reserve? Not as straight forward as one might thi

    I've asked similar question some time ago - https://forums.watchuseek.com/f2/when...ed-714083.html . lysanderxiii explained it. I'll rephrase his reply: when chrono is running the watch will stop sooner (what might look like RP is shorter) but at this point the spring has the same amount of power that it would have if it would run without chrono; it happens because the chrono requires bigger torque to run than the spring can provide at this point - if you turn the chrono off the watch might start running again.

  3. #3
    Member Wish2K's Avatar
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    Re: Does engaging the chrono's really impact power reserve? Not as straight forward as one might thi

    Quote Originally Posted by VRT View Post
    I've asked similar question some time ago - https://forums.watchuseek.com/f2/when...ed-714083.html . lysanderxiii explained it. I'll rephrase his reply: when chrono is running the watch will stop sooner (what might look like RP is shorter) but at this point the spring has the same amount of power that it would have if it would run without chrono; it happens because the chrono requires bigger torque to run than the spring can provide at this point - if you turn the chrono off the watch might start running again.

    Hmm. I'm still not getting it. If the chrono is engaged and increases torque stress by about (lets say for the sake of arguments I know this is subjective) 30%, this doesn't mean the spring is unwinding faster to keep up. The watch should run nearly on time as long as the balance wheel is oscillating. If the stress was say +%100, then things would simply halt.

    There's enough mainspring tension to make the escapement move according to its fixed amount - or not...

    Power reserve should mean the amount of spring tension needed to keep the balance wheel and escapement moving. If there's too much friction for the spring tension to overcome (engaged chronos), then it comes to a halt sooner.

    I'm trying to figure out how this correlates with a watch that seems to stop 7 - 8 hours sooner than it's 40 hour power reserve rating. It doesn't, and looks more like a faulty mainspring.

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  5. #4
    Member lysanderxiii's Avatar
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    Re: Does engaging the chrono's really impact power reserve? Not as straight forward as one might thi

    Quote Originally Posted by Wish2K View Post
    Hmm. I'm still not getting it. If the chrono is engaged and increases torque stress by about (lets say for the sake of arguments I know this is subjective) 30%, this doesn't mean the spring is unwinding faster to keep up. The watch should run nearly on time as long as the balance wheel is oscillating. If the stress was say +%100, then things would simply halt.

    There's enough mainspring tension to make the escapement move according to its fixed amount - or not...

    Power reserve should mean the amount of spring tension needed to keep the balance wheel and escapement moving. If there's too much friction for the spring tension to overcome (engaged chronos), then it comes to a halt sooner.

    I'm trying to figure out how this correlates with a watch that seems to stop 7 - 8 hours sooner than it's 40 hour power reserve rating. It doesn't, and looks more like a faulty mainspring.
    What I stated was based on empirical evidence. Here's why it works like it does....

    Basically, engaging the chronograph places a higher torque requirement on the mainspring. Since the torque output of the spring drops as the spring unwinds, the point where the mainspring can no longer provide enough torque is reached sooner. (See graph below, the Y axis is the torque output of the spring, the X axis is time, the red line in the power curve of the spring. The upper blue line is the torque requirement with chronograph on, the green is with it off). Sometimes, there is not enough torque in the mainspring to overcome the static friction* on the pivots so the movement will not resume working when the chronograph is turned off, unless the balance is jump started.



    ________________________
    * Static friction is always higher that sliding friction, so if a watch is already running, it takes less power to keep it running than the start it from a dead stop.
    Last edited by lysanderxiii; January 1st, 2013 at 01:02.
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  6. #5
    Member Wish2K's Avatar
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    Re: Does engaging the chrono's really impact power reserve? Not as straight forward as one might thi

    Quote Originally Posted by lysanderxiii View Post
    What I stated was based on empirical evidence. Here's why it works like it does....

    Basically, engaging the chronograph places a higher torque requirement on the mainspring. Since the torque output of the spring drops as the spring unwinds, the point where the mainspring can no longer provide enough torque is reached sooner. (see graph below, the Y axis is the torque output of the spring, the X axis is time, the upper blue line is the torque requirement with chronograph, the green is without). Sometimes, there is not enough torque in the mainspring to overcome the static friction* on the pivots so the movement will not resume working when the chronograph is turned off, unless the balance is jump started.



    ________________________
    * Static friction is always higher that sliding friction, so if a watch is already running, it takes less power to keep it running than the start it from a dead stop.
    No argument there. I'm in concurrence. The power depletion and it's effect on power reserve seems to really matter only at the very tail end. Not through the life of the wind. The dilemma I'm having is why my movement is missing about 7 hours of reserve power when it's rated at 40. Some will say it's because chronos deplete the power reserve faster. But it doesn't make sense under closer inspection.

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    Member mleok's Avatar
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    Re: Does engaging the chrono's really impact power reserve? Not as straight forward as one might thi

    Quote Originally Posted by Wish2K View Post
    No argument there. I'm in concurrence. The power depletion and it's effect on power reserve seems to really matter only at the very tail end. Not through the life of the wind. The dilemma I'm having is why my movement is missing about 7 hours of reserve power when it's rated at 40. Some will say it's because chronos deplete the power reserve faster. But it doesn't make sense under closer inspection.
    I'm not sure why you think you're in concurrence, when you seem to be missing the point. Without knowing the torque curve as the mainspring nears the end of its normal power reserve, and the additional drag that the chronograph adds, you can't tell how much the power reserve will be affected by running your chronograph. On the one extreme, if the additional friction added exceeds the maximum torque supplied by the mainspring, your power reserve would go from 40 hours to 0 hours.


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  8. #7
    Member lysanderxiii's Avatar
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    Re: Does engaging the chrono's really impact power reserve? Not as straight forward as one might thi

    Quote Originally Posted by Wish2K View Post
    No argument there. I'm in concurrence. The power depletion and it's effect on power reserve seems to really matter only at the very tail end. Not through the life of the wind. The dilemma I'm having is why my movement is missing about 7 hours of reserve power when it's rated at 40. Some will say it's because chronos deplete the power reserve faster. But it doesn't make sense under closer inspection.
    Running the chronograph does not deplete the spring faster, the spring just reaches a point where it can no longer supply enough power to run the movement sooner:

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  9. #8
    Member Wish2K's Avatar
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    Re: Does engaging the chrono's really impact power reserve? Not as straight forward as one might thi

    Quote Originally Posted by mleok View Post
    I'm not sure why you think you're in concurrence, when you seem to be missing the point. Without knowing the torque curve as the mainspring nears the end of its normal power reserve, and the additional drag that the chronograph adds, you can't tell how much the power reserve will be affected by running your chronograph. On the one extreme, if the additional friction added exceeds the maximum torque supplied by the mainspring, your power reserve would go from 40 hours to 0 hours.
    Fair enough. I'm currently testing this by timing my watch with chronos on until the movement stops. Will turn chronos off and start it up again to see how much longer it runs without winding. Maybe my 7 or so hours are still in there?
    Last edited by Wish2K; January 1st, 2013 at 01:21.

  10. #9
    Member lysanderxiii's Avatar
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    Re: Does engaging the chrono's really impact power reserve? Not as straight forward as one might thi

    Quote Originally Posted by Wish2K View Post
    Fair enough. I'm currently testing this by timing my watch with chronos on until the movement stops. Will turn chronos off and start it up again to see how much longer it runs without winding. Maybe my 7 or so hours are still in there?
    It might, it might not.

    As I stated, the static friction will be higher that the sliding friction. Once the movement stops the torque must be high enough to over come the static friction, this is not true for all cases, as temperature, oil contamination and other factors will affect the torque required the re-start a movement at (or near) the end of the power reserve.

    If you bump the watch to jump start it, you might get your 'missing' seven hours.....

    In my original tests, the average power reserve after restarting (sometimes by bumping, sometimes without) got very close to the nominal PR.

    I found you lose about 10% of the nominal PR with the chronograph engaged.
    Last edited by lysanderxiii; January 1st, 2013 at 01:31.
    familiaritas parit contemptum; raritate admiratione wins.- Lucius Apuleius
    est necessry, accurate ad secundo? - Lysander magna
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  11. #10
    Member Wish2K's Avatar
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    Re: Does engaging the chrono's really impact power reserve? Not as straight forward as one might thi

    Understood. Actually static friction with chronos off can be likened to sliding friction with chronos on? Name:  hmmm.gif
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Size:  376 Bytes Yes hardly scientific. Anyway will find out the truth once it finally winds down. So far it's 25 hours.

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