Sudden change in time keeping question

[jb335]

I have a 16750 8.4 serial (1984) that I acquired a couple months ago with unknown service history. I only have a two watch collection so I split wearing them pretty much every other day so they stay wound. The 16750 has been keeping AMAZING time in the two months I've had it with consistently only gaining 1 second a day. Unbelievably good.

Well, I just got back from a ten day vacation out of the country where I did not bring my 16750. I just left it home so it sat motionless and unworn for nearly two weeks.

When I got back I gave it a few wounds and started wearing it. Within the first 3 hours or so it gained 7 seconds. I was surprised and a bit concerned. However, I continued to wear it daily for the next several days and it went back to only gaining about a second a day. Maybe closer to 2 seconds a day now. But pretty much back to its very consistent self.

My questions is, is this normal for a watch to sit unwound for a couple weeks to have a relatively large swing in time keeping within the first day of wearing it again? I'm tempted to just shrug it off and not think twice about it as long as it continues to be under 2 seconds a day. Just wondering if anyone thinks this is a known indicator of an issue or at least needing a service?

Maybe my GMT is just punishing me for not being taken along on the vacation?

[SeaDweller50]

"Few wounds" is your answer. Watch will run faster with a slack mainspring. Always fully wind your watch 30 full turns.

[diverdoug]

Originally Posted by Airborne

"Few wounds" is your answer. Watch will run faster with a slack mainspring. Always fully wind your watch 30 full turns.

Wow, that is counter-intuitive. I would have guessed that it runs faster when fully wound. Thank you for teaching me something today.

[Tools]

Originally Posted by diverdoug

Wow, that is counter-intuitive. I would have guessed that it runs faster when fully wound. Thank you for teaching me something today.

Nope. A full mainspring puts maximum thrust to the movements pallet-lever and pushes the balance to full swing... A full swing takes longer, hence, more time (amplitude) between ticks.

A slack or unwound mainspring only imparts a modest push to the balance and it swings a lesser amount (less amplitude), which equals faster ticking.

[Drewkeffler]

Originally Posted by Airborne

"Few wounds" is your answer. Watch will run faster with a slack mainspring. Always fully wind your watch 30 full turns.

Very informative. Thank you

[kilyung]

Think of a pendulum. Pushing on the pendulum is equivalent to the torque from the mainspring. Proper timing is based on the period of that pendulum (in this case the balance wheel). You need a flat torque curve from the mainspring for good isochronism. Too little and you're not likely to get the period you need.

[drm20]

Originally Posted by kilyung

Think of a pendulum. Pushing on the pendulum is equivalent to the torque from the mainspring. Proper timing is based on the period of that pendulum (in this case the balance wheel). You need a flat torque curve from the mainspring for good isochronism. Too little and you're not likely to get the period you need.

Great info, thanks!!

[jb335]

Wow, very informative. I had no idea about the benefits of fully winding. Thanks all.

[Bob the brush]

It hated being left. Getting it's own back!

[jb335]

Quick update for anyone that cares....it's now been a week since I got back from vacation and started wearing my 16750 again, and it is back to it's amazing reliability of barely gaining ONE second a day. After that initial quick 7 second leap because it was not fully wound, it's been barely a second a day of gain. Such an amazing watch.

[DPE]

I find that some mechanical watches if not used for a short period need to settle in again when you use them again