Sep 04, 2011,09:19 AM

Hello JMan,

Discussions of amplitude very often take place between two people with a less than completely thorough understanding of the subject.  

I once had a US watch industry executive (NOT my current employer) relay to me a story wherein he bragged to his Swiss counterparts that "his" watchmakers routinely got over 310o of amplitude where the Swiss watchmakers only got 300o .  I cringed when I heard this story because if his Swiss counterpart knew anything about watchmaking he surely thought to himself, "This guy is an idiot."  More amplitude is only necessarily better within a very limited, caliber specific context and only up to a point, beyond which it is undesirable.

Amplitude is not in any way a measure of accuracy, although it can sometimes represent a measure of the mechanical health of a movement.  If the amplitude (measured on a timing machine) is consistent across a variety of positions and within factory tolerances, a watchmaker knows a decent amount about the consistency of the power being delivered to the balance, which is critical to timekeeping.

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Amplitude is critically related to timekeeping for basically four inter-related (and sometimes contradictory) reasons: 1.) isochronism, 2.) poise errors, 3.) rate stability in response to disturbances and 4.) the potential for rebanking.  

1.) All watches strive for isochronism as much as possible which literally means that the rate will be the same regardless of the amplitude.  A very well designed oscillator will only change rate by 5 or 10 seconds per day across 70 or 80% of the power reserve of the watch, but all watches suffer from non-isochronism in some regard.  To the extent that undesirable rate deviations increase as the amplitude decreases, keeping the watch running with as much amplitude as possible is a good thing.

2.) Poise errors are rate deviations in the vertical positions associated with irregularities in the distribution of mass of combined balance and hairspring system (the balance complete in watchmaker parlance). This is one of the sources for rate variations between the 9H, 6H and 3H positions (the vertical positions commonly measured for chronometers). Ideally the poise should be adjusted as much as possible to reduce this error, but it will always exist to some extent, and since poise error is closely tied to amplitude, it's best to have the amplitude of the balance optimally suited to minimize poise errors when worn. 

This is a complicated discussion no matter how you slice, but 220o  is the amplitude at which poise errors are negated completed and they become dramatically more pronounced below 180o  and above 300o .  If the watch loses 25o  of amplitude from the horizontal to vertical positions and approximately 50o  over the course of 24 hours (the usual run time for a hand wound watch with a traditional power reserve) and we want the amplitude curve to be centered around 220o  (in the vertical positions), we'll want our maximum amplitude at full wind to be 270o  if minimizing poise errors is our only goal.  Allowing for some loss in amplitude as the oils deteriorate, maybe we'd shoot for 275o  or 280o .  Please see Daniels Watchmaking for more info and alter calculations for automatics or extended power reserves as desired.

3.) A significant, real-world problem for mechanical watches, and especially those worn on the wrist, is the frequent shocks, seemingly random twisting and turning motions and other g-force inducing perturbations they are subjected to.  Good amplitude is a good defense against these perturbations because significant amplitude drops when disturbed represent a smaller percentage of the total amplitude and thus less deleterious to timekeeping. A high beat rate is an even better defense in this context and high beat escapements generally result in more consistent amplitude between the horizontal and vertical positions, so it's a win-win.

4.) The problem with too much amplitude (in addition to the magnification of poise errors mentioned previously, which is of minor consequence when compared to our current topic) is the risk of rebanking.  If the amplitude gets so high that the roller jewel on the balance strikes the back side of the pallet fork, it's VERY bad for timekeeping.  A watch that is rebanking can gain minutes per hour or can even break off the roller jewel and stop completely.  

As the lubricants deteriorate inside the barrel of an automatic watch (accelerated by the slipping of the mainspring bridle when fully wound), amplitude can sometimes increase and cause the watch to rebank even if it was running with a perfectly safe amplitude previously.  Also, periodic rebanking can be triggered by arm movements even if the amplitude is not explicitly excessive when the watch is stationary, something that is more problematic for watches with large, heavy balances.  For this reason, most manufacturers have pretty well defined amplitude guidelines for the various calibers they produce, with some watches running optimally with 310o  and others becoming dangerously high above 260o .

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As you can see, it's a complicated subject and it should not be generalized about casually in my humble opinion (and I have left escapement and pinning-point errors out of this discussion entirely for the sake of simplicity).  This is one of the reason it bothers me when publications like WatchTime include timing machine readings in their reviews and then associate some kind of score with the readings as if such a practice is anything other than nonsensical.  

Don't misunderstand, I'm sure some of the watchmakers advising on these reviews are very knowledgeable, but it gives the reader an erroneous sense of their own understanding when the reviewer says, "The amplitude was only 290o  at maximum, so we could not give the full 10 points in this category."  This kind of casual reductionism of complicated topics is NOT helpful. For you car guys, imagine if a car mag reviewer said, "This car only only red-lines at 5500 rpm, so we deducted a point from its overall score." Huh?


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