Balance motion, like many aspects of watch adjustment, has been studied in great detail from a theoretical point of view. Still, in practice, even the people who made high-grade watches designed them empirically. I was told by a really fine watch adjuster that the older Hamilton railroaders like the 992 were set up for a total balance motion, in the horizontal positions, of 580 degrees while the 992bs were given mainsprings that produced a 540 degree motion to cut down on the timing problems and breakage of roller jewel that results from that comes from excessive motion. He found the newer watches were harder to adjust to his standards. In the process of carrying a watch it is always possible to add or subtract energy from the balance. This changes temporarily that motion of the watch. With single impulse watches like chronometers if you accidentally arrest the motion they wont automatically restart and the watch sets. This isnt a problem with lever watches but the opposite problem of increasing balance motion over the maximum of 720 degrees (less the angular width of the fork and jewel pin) will cause the watch to run very fast until the motion drops below 720 degrees. For this reason the balance motion should be kept low enough that this is not likely to happen. On the other hand if the motion is to low the watch will be hard to adjust because the normal motion of carrying the watch will add and subtract a large enough percentage of energy to the balance to exceed the isochronal characteristics of the balance spring.
Different watch companies set their watches up differently, Walthams, especially the high-grade 1888s have very light balances. Even a slightly too strong mainspring can lead to excessive motion. Accordingly Waltham listed 3 different thicknesses of mainspring for the 1888s (the strength a mainspring varies as the cube of the thickness so a .05mm difference in thickness make a substantial difference in strength). On the other hand Elgins 18 3/4 plate railroad watches ha very heavy balances and very large mainsprings. As a result they are great time keepers in the pocket, at least until the balance pivots get damaged.
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Bill Tapp
Different watch companies set their watches up differently, Walthams, especially the high-grade 1888s have very light balances. Even a slightly too strong mainspring can lead to excessive motion. Accordingly Waltham listed 3 different thicknesses of mainspring for the 1888s (the strength a mainspring varies as the cube of the thickness so a .05mm difference in thickness make a substantial difference in strength). On the other hand Elgins 18 3/4 plate railroad watches ha very heavy balances and very large mainsprings. As a result they are great time keepers in the pocket, at least until the balance pivots get damaged.
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Bill Tapp
