Mechanical watch accuracy

[Alcan]

I've read several times that manual wind watches tend to run faster when the mainspring is wound down. To me, this seems counter-intuitive and that the watch would run slower. Is it because there is less pressure on the escapement when the mainspring is unwound?

The phenomenon that a watch tends to run faster as it winds down is not exclusive to manually wound watches, as automatics do the same. The reason this happens is that the balance amplitude will decrease as the mainspring winds down, and the frequency of the balance will go up (because it isn't swinging as far on each beat). As the main spring winds down, there is less energy transferred to the balance - think of the energy output as a curve that stays mostly flat until the end where the spring unwinds, where it drops off.

The tendancy of a watch to maintain the frequency regardless of balance amplitude is called "isochronism." There are specific things that can be done in the design of the watch to help the balance maintain it's frequency as the main spring winds down (breguet overcoil hairspring for instance).

Back to automatics and manuals - my understanding is that most manual watches are more fined tuned to compensate for the loss of energy as the spring unwinds, because it is known that the spring will wind down over time (how long depends on the movement design, number of barrels, etc.). For automatrics, this is considered (generally) less important since the watch is assumed to be near full wind most of the time since it is being wound all the time it is worn.

Hope this helps.

[dman]

Al has you covered VERY well.

[Alcan]

Originally Posted by dman

Al has you covered VERY well.

He sure did. Thanks, Al. There's some of the value of a website like this one, lots of knowlegable people to help out.

So, was the old fusee design with the tapered drum and chain drive to compensate for the reduced mainspring tension as it unwound?

[padi56]

Excellent answer Al, frequently worn automatics are usually at or near a full state of wind most of the time,You normally get erratic motions of an escapement when the mainspring is too tightly wound hence the slipping clutch not to overwind. You might think they can be adjusted to be more accurate. This is not necessarily so,as a manual-wind watch that is wound consistently once per day.They can be regulated so that the day to day variation is again very small. In short,IMO there is no definite performance advantage to an automatic/over a manual wind it is just mostly down too a pure simple convenience.

Originally Posted by Alcan

He sure did. Thanks, Al. There's some of the value of a website like this one, lots of knowlegable people to help out.

So, was the old fusee design with the tapered drum and chain drive to compensate for the reduced mainspring tension as it unwound?

Yes, that is the purpose of the fusee design.