When checking a watch on a timing machine, we want to see stable rates: a set of parallel lines, not wavy curves. A watch might be too slow or too fast, but it should be consistently so. Here are some stable rates, for example, from our “Hulk green” Russian Poljot watch.
And here are some wavy ones from a 1960s Poljot 2409 in the pendant down position. This watch has unstable rates over a short time frame. The lines are parallel but wavy, drifting up and down.
Watches with unstable rates can’t be adjusted because you can’t pin down the rate in any given position. Needless to say, if the movement is +20 one minute but -20 a minute later, it isn’t ready for fine-tuning.
Plate Jewels & Unstable Rates
In our post on the fundamentals of rate instability and wavy traces, we noted that many things can cause unstable rates. Flaws in the jewels are a common cause—not the most common, but common enough if you work on older watches that have been dropped and bumped a few times over the decades.
A good vintage watch has many kinds of jewels, from the roller to the pallets to the plates. Here we’ll focus on the plate jewels, which serve as bearings for the gear train. In a standard 17-jewel watch, these are the jewels in the plate and bridges that keep the wheels—center, third, fourth, and escape wheels—and pallet fork upright and rotating with minimal friction.
Plate jewels are often cracked and chipped. The cracks can be subtle, and the chips can be small “flea bites” on the edge of a hole. Here’s an example of a Hamilton 747 8/0 watch movement with both problems. The center hole jewel is cracked, and the third wheel jewel has a small chip. Here’s the center hole jewel, with its two big cracks:
And here’s the lower third wheel jewel, with a small chip in the edge of the hole.
These cracks and chips probably came from a jolt to the watch, such as a drop, bump, or snapped mainspring.
You see minor jewel flaws like these a lot in vintage watches. The watch will usually run, often fairly well, with minor chips and cracks in plate jewels, so someone who is simply trying to get the watch to tick won’t bother to repair them. Nevertheless, minor flaws in the plate jewels can cause the kinds of unstable rates and wavy traces that make a watch unadjustable. (As an aside, the balance wheel’s jewels—the hole jewels and cap jewels for the upper and lower pivots—are a different beast. They are commonly cracked, as our many example watches show, usually resulting in severe, unignorable problems.)
This Hamilton 747, for example, ran with good amplitude, but the rates were wavy, and the positional variances were high. Let’s think about why even minor chips and cracks in plate jewels can cause wavy traces and unstable rates.
Wheels running out of upright. First, for some jewel flaws, the wheels won’t run truly upright. A slight wobble in a wheel as it turns will cause uneven power transmission to the next wheel in the gear train, causing a classic example of a consistently wavy trace.
Greater friction in some positions. Second, for some minor chips and cracks, the unstable rates will appear in some positions more than others. A watch with a chipped jewel might run with great amplitude when dial-up and dial-down but show unstable rates in only some vertical positions. You can visualize how it would happen. Because of the slight tolerance of the jewel hole, the pinion will have more contact with the chipped side of the jewel in some positions than in others. The differences in friction and the uneven surface will cause timing positional problems. This sort of pattern—clean, even timing traces in most positions but not in one of them—can be vexing to troubleshoot, so be sure to inspect those jewels closely when cleaning your watches.