The second task in dynamic poising is to align the 4 vertical rates. For vintage watches, the vertical rates usually differ by a lot. My trusty 12s Illinois, for example, was initially adjusted to only 3 positions, and it has been through many hands in the past 90 years—some skilled, some clumsy.
To adjust the 4 vertical rates, we need to understand why watches keep different time in different vertical positions.
The poise of the balance has a huge effect on vertical rates. Simply put, the balance wheel has a heavy spot somewhere. Just as bicycle tires and lawnmower blades will always have a spot that’s slightly heavier, a balance wheel invariably has one spot that’s a bit heavier.
If we improve the wheel’s poise—either by removing weight from the heaviest spot or adding weight at the lightest spot—we will align the vertical rates.
(There is much more to vertical timing than poise, but we’ll work our way toward some of the more obscure, technical, or hard-to-adjust reasons [e.g., pinning point of the hairspring at the collet, overcoil vs flat hairsprings, and the like] as this blog meanders on.)
Dynamic vs Static Poise
In the old-school books, you’ve read about poise in the context of using poising tools after replacing a balance staff. The balance with the roller (but not with the hairspring) is placed on a poising tool, spun around to discern heavy spots, tweaked a few times, and then declared ready to go.
This approach is called static poise. It has some limitations, which is why modern adjustment emphasizes dynamic poise—the poise of the oscillating system when it is actually operating, also called “poise in use.”
Poise should be evaluated dynamically—when the balance is actually swinging in the watch—because many factors affect poise in use beyond the balance wheel, staff, and roller. For example, the hairspring itself has issues of poise, such as the cut-out slot in the collet.
Dynamic poise—poise in use—is not simply the sum of all the individual poise errors. When you bring together all the factors large and small, from the poise of the staff, roller, wheel, and hairspring to tiny deviations and flaws in regulator pins, banking pins, pallet forks, and the like, you have some complex interactions at work. As a result, you can’t really judge how the complete balance system will perform in-use by spinning the wheel in an old-school poising tool.
If you work on cars, essentially identical issues are involved in balancing a tire. Because tires aren’t perfectly round and even, their weight varies slightly. When a tire is put on a metal rim that itself is slightly out of poise, the interactions between the tire, rim, and road create issues of poise-in-use that are not simply the sum of individual poise-at-rest issues.
Poise and Amplitude
Balance poise is the biggest reason why a watch has different vertical rates, but the effect of poise varies as a function of amplitude. Stated differently, the biasing effect of a heavy spot is larger at small amplitudes and weaker at large amplitudes.
Thus, we want high amplitude—at least 270 degrees, measured dial up (DU) and dial down (DD)—for two reasons.
- a high amplitude indicates that the watch is globally clean and healthy
- a high amplitude diminishes the biasing effect of poise errors
Jendritzki, in Watch Adjustment, has a helpful figure that shows how poise errors interact with amplitude to affect rates:
The X-axis shows the position of the heavy spot; the left Y-axis shows amplitude; and the right Y-axis shows rate. This one figure captures much wisdom for watch adjusting:
- poise errors are largest when amplitude is low
- poise errors are bigger when the heavy spot is above or below the balance staff and smaller when they are to the side
- at low amplitudes, a heavy spot below the balance staff causes a gain; a heavy spot above the balance staff causes a loss
- poise errors are equalized when the amplitude is 220 degrees
- when amplitude exceeds 220, we find small poise errors in the other direction
In general, a watch with an amplitude of 270 in the horizontal positions will have an amplitude of at least 220 in the vertical positions. The amplitude drops off due to the higher friction. When horizontal, the balance rests only on a single point—the small balance pivot. When vertical, however, the balance staff rests on two points—the sides of the jewels.
The watch adjuster should stick to venerable, time-honored amplitude guidelines. A watch isn’t ready for adjusting unless:
- its amplitude is at least 270 degrees in both the DU and DD positions
- its rate is reasonably similar in the DU and DD positions
- its amplitude is at least 220 degrees in all 4 vertical positions
Now that we understand why dial up and dial rates can differ and why the four vertical rates can differ, our next post in the series brings it all together by demonstrating how to adjusting a vintage Hamilton wrist watch to 5 positions.