Adjustment doesn’t correct for flaws; it builds upon an already excellent watch movement. As a result, not all watches are ready for adjustment. As all of our textbooks emphasize, a watch must be running well before it is a candidate for the various mechanical machinations needed for positional timing.
One requirement we’ve already described is amplitude: we need at least 270 degrees in the horizontal positions and 220 degrees in the vertical positions.
Stable rates are another requirement. By this, we don’t mean long-term isochronism, such as when a watch has the same rate at full wind and after 24 hours. Instead, we’re focusing on volatility in the rates across shorter time spans, such as a few minutes.
Examples of Rate Stability
Our friendly neighborhood timing machine will reveal whether a watch’s rates are stable or variable. When a watch’s rate is stable, the estimated daily rates stay essentially the same over a period of several minutes. The timing traces will be as straight as Kansas railroad tracks. A new ETA 2824, for example, will have essentially zero deviation within a short period. For vintage watches, with their looser tolerances and decades of wear, we will need to be more lenient.
Nevertheless, plenty of old watches will have stable rates. Here are the timing traces from an Illinois 12s 405 pocket watch, one of our example watches. It is running a bit fast, but the lines are straight. This watch will run at +6 for several minutes, so it’s ready for adjusting.
Here’s an example of a watch with unstable rates. This is a Hamilton 987E in the pendant down position. It is clean and has good amplitude, but notice that the traces are wavy. (The beat error is bad, too.) Within a 60 second span, this watch ranged from -17 to +25 seconds per day—that’s pretty variable.
A watch with this level of variability isn’t ready for adjustment. The reasons should be obvious. First, adjusting seeks to bring the rates close to zero in all positions. But if the rates can’t be pinned down, they are essentially undefined. Second, the wavy traces are a symptom of some flaw in the watch that ought to be corrected.
Causes of Unstable Rates
What causes unstable rates? The answer, as usual, is many things. The root cause is unstable power transmission to the balance wheel. Anything that causes inconsistent power delivery will cause inconsistent rates. Here are some of the more common culprits:
- the watch is way out of beat
- a wheel is out of round or out of flat
- a pivot is out of round or bent
- a jewel hole is chipped, cracked, or out of round
- a gear tooth is worn, misshaped, or dirty
- a pinion leaf is worn, misshaped, or dirty
Because we’re focusing on short-term (high frequency) variability, our culprits are usually the escape wheel or 4th wheel. As Jendritzki points out in Watch Adjustment,
the peaks of the wavy diagram are separated more or less from one another, according to the location of the effect which has caused them. If the escape wheel is out of round, the distance between two consecutive peaks corresponds to the time of rotation of the escape wheel, usually six seconds; this distance will be ten times as great if the fourth wheel is out of round, as this goes around once in sixty seconds.
In short, if the peaks in the waves appear roughly every 6 seconds, the escape wheel is the culprit. Commonly, a pivot is slightly bent or a tooth is grimy or chipped.
If the peaks appear roughly every minute, the 4th wheel is the culprit. Bent lower pivots are very common. The subsecond hand attaches to this pivot, so it gets bent when people remove the hand by lifting up the dial (a bad practice) or mash the hand back on. A slight bend is enough to affect how the teeth engage with the escape wheel, causing inconsistent power delivery.
As with many things, a good cleaning often fixes things. Polishing the escape wheel’s pivots, teeth, and pinion leaves will often straighten things out, so to speak.
In some cases, such as some of our example watches, you can’t pin down the cause of the wavy traces. Many small errors, taken together, can cause eccentric waviness. When a watch gets a jolts, perhaps from being dropped or from an old mainspring snapping, many small changes to gear teeth and pinion leaves will happen. Collectively, these small flaws will interact to create wavy traces, so no single change can cure the problem.