Isochronism: how is it adjusted in practice?

[ifibrin]

How is isochronism adjusted in practice? Is this something best left to the designer of the movement (I.e. hairspring design, choice of mainspring), or is it something that can be done by a watchmaker? I assume the lowering in amplitude over 24 hours is the cause of the increase in rate, but what can be done to compensate for this? Does the inherent positional variation further complicate adjustments for isochronism?

[jdm]

Sorry, I don't know much about the fine art adjusting except for the famous sentence: "Adjustment starts at the barrel". Which taken literally would mean that the mainspring is not part of it?!?

However allow me share some considerations which are somehow preliminary to the question.
Premise, I find worth to remind that in watchmaking when we say isochronism, that is is synonymous with "mainspring isochronism". That was the first problem that clock and watchmakers had to solve after a more-or-less isochronous escapement was introduced. I like to be faithful to the meaning of the Greek-derived word and call "problem of isochronism" anything that has to do with maintaining good timekeeping in different conditions. By the way, maybe not everyone knows that a similar word - plesiochronous (almost synchronous, which describes well many mechanical watches) - is used in telecommunications.

We know that the due to the materials available, and the technology of the time, the first  mainsprings had very variable torque output depending on the winding state. For that the fusee device was invented and became common, which counteracted that the problem with more introduced alteration of the torque. At some point, the mainspring manufacturing (and I suspect, the introduction of the Swiss lever which is more tolerant of power torque variations) allowed a big step forward.

Then keyless winding was introduced, which made the operation much easier and likely to break the spring, or introduce dirt into the mov.t. People could do it at any time with one hand, even without thinking about it. 

At that point, smaller (pocket) watches were introduced, which people really liked, as well ladie's watches and then the wristwatch. Not having much room on these, power reserve was reduced, which minimized the difference between full winding and "any  other state". All one needed to do was to wind approximatively at the same time every day, keep the watch well serviced and regulated, and good timekeeping was almost assured. 

Finally, the automatic winding was introduced. With that, the watch is always fully (or almost) wound. Only 8 - 10 hours of mainspring isochronous behavior are needed, not a big deal really, as a bonus with even better materials and technology accessible to the masses, one could to even skip one day of wearing the watch.

 In conclusion, leaving aside the obvious case of fitting a new mainspring when proven necessary, on modern and contemporary watches I think there is nothing to worry about mainspring isochronism. Because all the best practices which are beneficial to that (absolute cleanliness, meticulous inspection, and perfect lubrication of the train) are to be strictly followed anyway.

[Klassiker]

You can change the mainspring to give yourself a longer power reserve, which should deliver more constant power over a longer time. This is the easiest improvement you can make, if the existing spring is weak. If the rate is higher when the amplitude is lower (all positions?) check the hairspring is centred in the regulator pins. If it is centred, then opening up the gap between the pins (or boot and pin) very slightly will slow the rate, more so for low amplitudes. 

Edited November 16, 2021 by Klassiker

[nickelsilver]

In a watch with an overcoil hairspring, the overcoil is calculated to be isochronous. If there is an error (and all other things are as well adjusted as possible, escapement, train is free, barrel properly adjusted with a good spring etc.) then the curve can be modified to compensate the error*. In a watch with a flat spring, you can adjust the gap between the regulator pins, and that's about it. In some cases a small adjustment to the hairspring at the collet can improve things, but it's a try-and-see technique with no real method behind it. There are watches where the balance and hairspring are very well matched, proportionally, that simply perform very well. In others, in particular smaller watches, it can be very hard to get a great timing result let alone a good isochronal performance.

 

A 30mm Omega movement from the 50s will almost always perform pretty much the same, and excellently, at full wind and after 24h, regardless of the fact that there is perhaps a 40 degree drop in amplitude. A 5x7 ligne A. Schild will probably be difficult to get a delta in 6 positions under 40 seconds at full wind, with a notable drop in performance at 24h.

 

In addition, the quality of the hairspring has a real effect. Many commercial grade watches in the past used Nivarox 3 or 5 springs, which simply don't perform as well as Nivarox 1. Watches with steel springs are a different animal; at high amplitude there is a centrifugal effect on the balance arms which tends to slow the rate, while the hairspring is spending less time between the regulator pins, raising the rate- and the opposite at lower amplitudes, so it can have a somewhat compensating effect (but not necessarily). A springer who worked at Gruen told me they had to modify the overcoil forms when they switched from steel hairspring/split balance to modern alloy springs with solid balances.

 

*To adjust an overcoil to raise the rate at lower amplitudes, the overcoil body is reshaped to be closer to the balance center. To reduce the rate at lower amplitudes, it's adjusted to be further from the balance center. Obviously this is tricky work, and more harm can easily be done than good. The beat will invariably be thrown out and require adjusting the collet position. In school we did these exercises on American railroad grade pocket watches, it's really watches of that quality that merit and respond well to such adjustments. James L. Hamilton's small (green) book Timing Manipulations explains the process (and many other things) very well; it's long out of print but does come up on Ebay if you keep your eyes open.

Edited November 16, 2021 by nickelsilver

[Klassiker]

18 hours ago, ifibrin said:

Does the inherent positional variation further complicate adjustments for isochronism?

Yes, because of the different and changing Interactions of the hairspring with the regulator. For the rest of your questions, just read @nickelsilver's post.

[Klassiker]

@nickelsilver When are you going to write a book?

[nickelsilver]

39 minutes ago, Klassiker said:

@nickelsilver When are you going to write a book?

Ha, when I have something to add- I just regurgitate stuff I read in the books that already exist!

[ifibrin]

6 hours ago, Klassiker said:

If it is centred, then opening up the gap between the pins (or boot and pin) very slightly will slow the rate, more so for low amplitudes. 

 

5 hours ago, nickelsilver said:

In a watch with a flat spring, you can adjust the gap between the regulator pins, and that's about it.

Thanks for all the replies! They were really helpful!

How do you adjust the gap between the regulator boot and pin for a flat hairspring, since only the pin can be slightly bent? Isn’t the regulator boot pretty much fixed in shape?

Only the Etachron system allows you to adjust the gap on both sides of the spring pretty much easily by rotating the regulator stud.