Dynamic Poising, 5: Finding the Balance Wheel’s Heavy Spot

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We need to find the balance wheel’s heavy spot to put it in poise. So far, we’ve discussed some of the principles involved in doing this and worked through some examples.

In this post, we’ll step back and revisit the procedure for isolating the heavy spot in the balance wheel in more detail. This role of amplitude in unmasking the cause of a positional error is one of the defining parts of dynamic poising, so we’ll go through an example step-by-step to make sure everyone gets it. Once you get this core basic idea, improving poise is fairly easy.

Poise Errors & Amplitude

A heavy spot will affect our rate in different positions, but its effect depends on amplitude. In the figure on the left, from Jendritzki’s book The Swiss Watch Repairer’s Manual, a balance wheel in a vertical position is shown. The wheel’s heavy spot (the solid black circle) is at the bottom in that position.

When the balance wheel’s amplitude is low, the rate is very fast, indicated by the larger black + signs. As the amplitude increases, the rate becomes less fast.

At 220 degrees, the effect of this heavy spot is compensated. 220 is a neutral zone in which small poise errors are cancelled.

Past 220 degrees, the heavy spot starts to slow the watch. It will lose increasingly large amounts of time as amplitude goes past 220.

An Illustration

In statistical lingo, we would say that the rate is an interaction between (1) poise and (2) amplitude. The effect of poise on rate depends on amplitude (whether it is low or high). Likewise, the effect of amplitude on rate depends on poise in that position.

To illustrate this interaction, we turn to a Hamilton 747 8/0 wristwatch, from a Clinton that I’ve been meaning to get around to. The wheel’s poise is pretty good, but we’re going to ruin it for the sake of our example.

Here’s the watch in pendant up (PU) position. I put a huge timing washer on the screw that’s directly below the balance staff when the watch is in this position. We thus know where the heaviest spot on the wheel is.

Now what we’ll do is run the watch in this position at different amplitudes, from low to 220 degrees to high. This will illustrate how poise errors affect rates as a function of amplitude.

Let’s start out low. When the watch is run at 124 degrees, the rate is really fast: +328 seconds a day.

At 182 degrees, a higher amplitude, the rate slows to +125 seconds.

As we approach 220 degrees, the watch’s rate slows some more: +41 seconds a day.

A bit more winding gets us to around 220 degrees of amplitude. Our rate is clearly better, around +10 seconds a day.

What happens when we get past 220 degrees, the “neutral zone” for small poise errors? As Jendritzki’s figure shows, the poise error should cause a losing rate once we get past 220. And indeed it does. At 246 degrees, for example, we’re now losing -38 seconds a day.

And at full wind, with a healthy amplitude of 285 degrees, the watch runs -79 seconds slow.

I encourage you to try this at home. Get a watch that runs fairly well and put a big timing washer under a screw. Set up the watch on the timing machine so that this screw is at the bottom, and then watch how the rate changes as you wind the watch.

Seeing how amplitude and poise errors interact teaches us a few things:

• We’re grateful for timing machines because dynamic poising is basically impossible without them.
• We can see why we want high amplitude: a heavy spot has a much smaller effect at high amplitudes. Even when the rate goes negative, the slowing effect is slighter than the earlier gaining effect.
• Most important, we can see a trick for finding a balance wheel’s heavy spot. Poise errors are huge at low amplitudes, so running the watch at a low amplitude is like holding a magnifying glass over the balance wheel—the heavy spot just pops out as the fastest rate.

Finding the Heavy Spot

Once again, with feeling: when its amplitude is low, a watch runs fastest when the balance wheel’s heavy spot is directly below the balance staff.

This makes it easy to find the heavy spot: wind the watch so amplitude is low (around 130 to 160 degrees), and then just run the watch in several positions.

We’ll illustrate this with the same Hamilton 747, which still has the heavy timing washer below the staff when the watch is pendant up. We’ll stick it on our trusty timing machine and see what we get.

We start with the 4 main positions, which as a reminder are defined from the dial side.

Here’s pendant up: +259 seconds a day.

And pendant left: +3 seconds a day.

Here’s pendant down: -353 seconds a day.

And here’s pendant right, the 6th position for a wristwatch: -122 seconds a day.

So far, the watch runs fastest when it is pendant up. The next step is to evaluate the 2 diagonal positions that flank our fastest position so far, which is pendant up. We needn’t check all 8 positions. We can check the main 4, find the fastest spot, and then check the 2 spots adjacent to it to see if one of them is faster.

Here’s one of the diagonals from PU: +170 seconds a day.

And the other diagonal: +72 seconds a day.

In this case, neither diagonal was faster than the pendant up position, which remains the fastest of them all. This isn’t shocking because we made it that way with our timing washer.

Finding the fastest position shows us where the heavy spot is, so we now have a target for adjustment. We can increase poise by removing weight at the heavy spot or by adding weight at the opposite spot.

• If the watch is running fast when dial up and down, or if the poise error is huge, we would add weight with timing washers or with heavier screws.
• If the watch is running slow when dial up or down, we would remove weight.

As before, I encourage you to do this at home with your practice watch. Use the watch you placed a heavy timing washer on and evaluate its rates in all 8 positions at low amplitude. You’ll find that the watch is fastest when the spot with the washer is directly below the staff, and slowest when the washer is directly above it.