no clue what you mean with hairspring amplitude, but the minimum amplitude of the balance can easily be estimated: The balance only continues oscllating if the whole oscillation periode exeeds the lift angle. For a typical escapement with 52° lift angle, and without beat error the balance must swing 26° to both sides. This is of course an ideal value which is in practice a bit higher due to friction loss, and because the lift angle is not perfectly centered to the rest position of the balance.
I don’t think I’ve seen anything below about 120 degrees on my timegrapher...may be the lower limit of what it can calculate. I’ve used slow motion video to look at some running watches that the timegrapher couldn’t measure and have seen some running at about 50 degrees (total swing only a little more than 90 degrees).
Thanks. I asked because I am a newbie and I was practicing on an old beat up Elgin Sportsman watch. I took it completely apart, and after putting it back together, it was working again, but it was beating at about 90 degrees (45 to each side). I almost couldn't believe it continued beating at all! It was a pretty small movement for someone as experienced as me. I had a tough time placing the wheel train bridge as well as getting the lower balance pinion back in its jewel pivot. This was just for practice and I didn't clean it and I don't have a timegrapher. I am guessing I bent a pivot and that is causing excess friction somewhere.
I am getting more confident taking movements apart and getting them back together working. Besides more practice with the tweezers and pegwood, the next steps for me will be cleaning and oiling.
6 degrees...and arguably this is the "amplitude" of the hairspring for the ZO340 movement.
See here: ZENITH - DEFY LAB
As for conventional balance: with vintage pockets watches, there was a time when 225 degrees was considered to be the "perfect" amplitude (bearing in mind that this is in the stem up pendant position)...with 180 considered acceptable and 90 being the minimum (at full mainspring power).
Most timegraphers have a difficult time calculating low amplitudes (which may be due to the correspondingly "quieter" sounds used in the calculations). Cheap Chinese machines without manual gain adjustment do a miserable job of this.
If a movement is designed for du/dd 270 operation at full power...and you are seeing 60 degrees at full power, the PR will likely be very short and the movement will stop with quite a bit of power remaining on the ms...and accuracy will be miserable.
A test of the quality of your work: the balance should sustain oscillation (after a nudge to get it started) with 6-8 clicks of power on the ms (ie from no power wind for 6-8 teeth of the ratchet wheel). If it requires more than 1/4 turn of the ratchet wheel, there is likely an issue that should be addressed.
At the end of the day, amplitude is one parameter used to assess whether the movement is operating "in specification".
Something that's often misunderstood is what we are measuring when specifying an amplitude. Those familiar with AC circuits are also familiar with Peak-to-Peak measurements and may look at a rotating balance while applying the same concept. This is not the case. Amplitude is measured Zero-to-Peak.
The OP's numbers are really an amplitude of 45 degrees, not 90.
If there's any question in your mind, a watch rebanks at about 330-340? Take a balance wheel and manually turn it all the way to rebank. Most of a full turn, right? Do it in the other direction. Same. If amplitude were Peak-to-Peak, it would rebank closer to 720 degrees.
Amplitude is always a fun discovery for us novices. You've done some watches, and they started right up when you lowered the balance into place. They run more or less accurately (as long as you're not finicky and don't time over more than a day), and you feel like a genius!
Then you either get a Timegrapher or you learn how to use your smartphone to take slow-mo video, and you realize your watch has maybe 180 degrees of amplitude.
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