Dynamic vs Static Poising: Or, “Why Would a Smooth Balance Wheel Ever Be Out of Poise?”

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The shift from balance wheels with timing screws to smooth balances was a major event in watchmaking.

Screwed balances, of the sort we see in all the pocket watches we have profiled, use timing screws to adjust the wheel’s poise. For example, here’s a Waltham balance wheel from the Waltham 645 we worked on a while back that keeps miraculously good time.

This 110-year-old wheel is more high-tech than it looks.  The screws can be modified to improve the wheel’s poise—you can add or remove weight, or even replace entire screws.

The wheel is also designed to compensate for temperature changes. The wheel itself is made of two different metals that expand at different rates as the temperature changes. The cut-out slots in the rim allow the arms to move toward or away from the center as the temperature changes, thus changing the moment of inertia and hence the rate.

But for the purposes of positional timing, the screws were the big thing about these wheels. They allowed adjusters to vary the distribution of weight around the wheel fairly easily.

Keeping It Smooth

A lot of engineering and innovation went into those old wheels, but innovation continued apace. Eventually, alloy wheels came on stage. These wheels were made of alloys that were self-compensating for temperature, so no cut-outs or bimetallic forms were needed.

But their biggest difference was that they were smooth: no balance screws, no nothing.

For example, here’s an image from Sinn, which has a glossary of watch terms on its webpage.

The wheel is smooth, with no screws. But notice the thin line on the rim. Some weight has been carved out to poise the wheel.

Here’s the alloy wheel from the Wostok 2209 we recently repaired. Adjusting it to keep better time across positions involved dimpling some weight from under the rim with a pivot drill.

Removing weight is cleaner and more precise in a factory environment, without a doubt. Here’s what it looks like in modern times at the Vostok watch factory (see the 5:13 point).

 

Dynamic vs Static Poising

Someone recently asked me why smooth alloy balance wheels need to be poised at all, and it’s a good question.

There are two reasons, really.

The first comes from manufacturing. It is hard enough to make a bicycle wheel or automotive rim fully poised, let alone a miniscule wheel. The challenges of precision manufacturing, when paired with a watch company’s aspirations for high accuracy, guarantee that most wheels will need some adjusting to be fully poised.

The second reason, though, is the big one: the difference between static poising (called simply poising in the old books) and dynamic poising.

In static poising, the balance wheel and roller are spun on a poising tool to identify the heavy spot. Here’s a great overview of static poising for an alloy balance wheel.

There’s nothing wrong or shameful about static poising. It is fun to do and is how adjusting was done for a long, long time. But it has some limitations that explain why dynamic poising eventually took over.

  • It isn’t sensitive to tiny variations in poise. A modern timing machine can magnify small errors more easily than a static poising tool.
  • It doesn’t take into account whether the watch is overall fast or slow, so it isn’t clear if weight should be added or removed. In the old watch factories, balance wheels were poised before hairsprings were fitted, so it didn’t matter if a wheel was a bit heavy or light—the hairspring was vibrated to make it work. But in repair work, you can make a wheel too heavy or too light, causing the DU and DD positions will diverge from the horizontal ones.
  • It doesn’t take the hairspring assembly into account.

This last issue is what distinguishes dynamic poising from static poising. The hairspring assembly itself has complex poise issues. It consists of a stud (often asymmetrical), a coiled spring, and a collet. The collet is asymmetrical, with a cut-out slot and a hole for a pin.

The poise of the hairspring assembly and the balance assembly will interact in complex ways. A perfectly poised hairspring attached to a perfectly poised wheel won’t necessarily yield a perfectly poised result. Likewise, poise errors in one assembly might cancel out poise errors in the other. It’s simply too hard to say a priori.

So that’s the limitation of static poising: we can get a wheel to be perfect, but then we stick an out-of-poise hairspring assembly on it and place it in a complex, dynamic escapement system. What works on the poising tool won’t necessarily work in the moving system.

And this is why a smooth alloy wheel will still be out of poise. Even if manufactured to perfection, the roller, jewel, and hairspring will introduce poise errors.

We thus do for watches what tire guys do for tires: evaluate poise dynamically, while the part is in operation.

When we assess the wheel’s poise while it is operating within the complete system of the watch, we can modify its poise so that it works well within that system.