More mainspring tension slows watch?

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  1. #1
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    More mainspring tension slows watch?

    My 2824-2 based watch has been running pretty consistently at +6 per day. It had been running about +9 until a couple of forum members suggested I store the watch crown up at night. I was pleasantly surprised that such a small change resulted in a substantial improvement.

    Up to about a week ago, I had relied completely on auto winding. But recently there were a few days in a row when I knew I would only be wearing the watch for a few hours, so I supplemented by manual winding. Then out of a developing habit, I wound it a few turns each night even though I had worn the watch all day. Unexpectedly, I have noted an increase in accuracy to about +4.

    I find this counter-intuitive. I would not think that maintaining greater mainspring tension would lead to the watch running slower. Does this make sense?

  2. #2
    Member lysanderxiii's Avatar
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    Re: More mainspring tension slows watch?

    More precisely, reduced torque from the mainspring causes the movement to speed up.

    As the mainspring runs down, it provides less torque to the gears. This means the impulse from the pallet fork to the balance drops as spring tension drops. Since the impulse to the balance drops the distance the balance swings before reversing drops. This means the time it tales to complete one cycle of the balance drops, or the movement speeds up.

    Most likely your winding of the watch is keeping it nearer full wind state during the 24 hour period you are timing the watch.. If you measure the rate with a timing machine and compare a fully wound rate to the rate after 24 hours, you would see a very noticable difference.
    Last edited by lysanderxiii; April 12th, 2010 at 19:32.
    familiaritas parit contemptum; raritate admiratione wins.- Lucius Apuleius
    est necessry, accurate ad secundo? - Lysander magna
    iustum est horologium - Obscurus Genius

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