Amplitude?

Canuck

I have the Joseph Bulova School of Watchmaking, 9th edition, from 1968. If the edition you are waiting for hasn’t changed from my 9th edition, you will find there is no index to assist you in researching the topic of amplitude. You’ll likely have to read the whole book to glean snippets of information that discuss amplitude. You will learn lots about the design and structure of watches from 50 years ago, and much before that, from the Bulova book. But after you peruse it, you‘ll likely find you have hardly scratched the surface of what comprises modern watches.

When you plod through the a Bulova book and come up short on the topic of amplitude, I suspect you will find there is a lot more information in this MB archives regarding amplitude, than you’ll likely find almost anywhere else. Amplitude stats on the performance of a given watch are not the be all and end all pertaining to the condition of a given watch.

Archer

Back in the times before there were timing machines that would actually tell you the amplitude, watchmakers would look at the balance and judge by eye if it had "good action" or not...and if it looked okay, they didn't worry about it too much. Some still don't consider it important.

In modern watchmaking things are a bit different. Companies have standards for minimum, and often maximum, amplitude. It will vary by the movement, so there really is no "one size fits all" answer to what good amplitude is. Where specs from the maker aren't available, some judgement is involved. Also knowing how the amplitude numbers in each position relate to each other is important.

Most companies do not have a minimum amplitude requirement at full wind. Instead they give you a minimum amplitude number at 24 hours after full wind. Even within one maker of movements, the standard for the movements will vary, so what might be good minimum amplitude for one movement won't be sufficient for another.

What some companies will do is give you a maximum amplitude number at full wind, and this is to ensure that the amplitude doesn't get so high that you encounter rebanking. This is where the roller jewel travels far enough around to strike the outside of the pallet fork horn, and then it will bounce back quicker than the balance spring would normally return it. It will then bounce back off the other fork horn, and the cycle repeats - this leads to a very fast rate.

There are some companies and movement that are known not to have high balance amplitude. Seikos for example (in particular the cheaper kind) will often not have balance amplitudes over 250 even when brand new at full wind. There are some Omega movements that also don't often have high balance amplitude. So there is some experience required to understand what the movement should have.

There's no simple answer to this unfortunately, but the numbers you have given above in your post, those ranges are a good starting point. However for vintage watches there are many that will have the amplitudes you are saying for modern watches. One common issue I run into on Cal. 321's for example is amplitude that is too high, causing rebanking. These watches will begin to rebank around 305 to 310, so often after a full service the movement is rebanking on these.

Hope this helps.

Cheers, Al