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Horology 101 - positional error

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Published on 08-13-2009 10:57 AM

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Positional Error

What’s the deal with some of these mechanical watches? Some days they run a little fast - sometimes slow. Some guys say they can lay them in a certain position at night to gain or loose a little time. I will attempt to explain positional error.

Before we start, let me say that I will be discussing a "perfectly" running watch with no other problems that would cause positional error. I am talking about a balance wheel that is not perfectly balanced. And after all, nothing is perfect. I will talk about some other causes of errors at the end.

A balance wheel should be just that - balanced. That is how it got it's name. The reason that such great effort is made to make sure it is balanced is to avoid positional error. If there is a heavier place on the balance, it will have error.

Watchmakers have worked since the invention of the balance wheel to minimize the effects of this problem. Along with the efforts to compensate for temperature, this is the reason for most of the material and design changes for the hairspring and balance wheel. They use specialized tools to analyze and minimize these problems.

There is no positional error when the balance is flat in the dial up and dial down positions. Positional error only shows itself in the sideways positions. In the flat positions, the pull of gravity has no special effect on the rate of the oscillations.

In the sideways positions, any heavier spot on the balance causes it to move faster and slower as it moves up and down. Picture a balance scale with more weight on one side. The same physics that causes the heavy side of the scale to go lower is what causes the balance wheel to run at a different rate.

Now it gets even more complicated. Most balance wheels move back and forth about 285 degrees from the center point. As the mainspring unwinds, the degrees of oscillation become less and the error starts to diminish until it gets to the 180 degree mark. At this point, there is no positional error. Since the balance wheel is turning exactly half way around, it cancels out. As the motion continues to lessen, the error once again starts to increase, but this time the opposite way. A watch that was running slightly fast will run slow and by the same amount.

So why don’t they just balance that wheel?

It’s easier said than done. Every effort is made, but once the hairspring is attached, other problems arise. The outside end of the hairspring is fixed (having no effect on the balance), but the rest of the hairspring winds and unwinds as the balance oscillates. This introduces a constantly changing distribution of the weight. So even if the balance wheel itself was perfect, the hairspring throws it off. Through the years, many different designs of overcoil hairsprings have been used to help minimize this problem.

What’s the answer?

Dynamic poising helps. A watchmaker can check the rate of the movement in several different positions using his timing equipment. If he understands the theory, he can then tell where the heavy spot is. He removes a little weight or adjusts screws on the balance wheel perimeter and then tries it again (yes, trial and error). It sounds difficult, but with practice it is a very efficient way to reduce positional error.

In practice, dynamic poising is not done during the manufacturing process on any but the highest grade watches. Most watches, direct from the factory and properly adjusted, will meet the standards for which they are designed without dynamic poising.

Incidentally, this problem with positional error is why the tourbillon was invented. In a tourbillon movement, the entire balance wheel assembly rotates around about once a minute. If there is a positional error, it will cancel out when the tourbillon gets 180 degrees around. So rather than try to solve a problem which can’t be completely solved, they figured out a way to cancel it out!

So what’s the deal with my watch?

Most watches are going to have some degree of positional error. But there is no set standard for which positions might be fast or slow. This is determined when the watch is made and there is no guarantee that the same models will have the same error in the same positions. So just because Joe’s watch gains a couple of seconds when left overnight in a certain position, it doesn’t mean yours will too. Each watch is different and you will just have to experiment on your own.

Is that all there is to it?

There are other reasons for positional error that don’t have to do with balancing. Most have to do with use (or abuse), wear and lubrication.

There are pivots on the end of the axel that the balance wheel turns on. They are less than a tenth of a millimeter and rest in spring-loaded jewels to absorb any shocks. The condition of these pivots is extremely important to good timekeeping. They must be perfectly straight, properly shaped, polished, and lubricated. Any changes in this condition will effect the timekeeping. If the oil runs dry or if they get dirty, they will start to wear. If the watch receives a hard enough shock, they can be bent and this will throw the balance out of poise. Shock damage to the hairspring can bend it slightly and cause similar problems. Most of these issues are not seen until a watch is a few years old, but you never know when they can appear.

Magnetism is another problem that should be mentioned. I recently had a watch that showed more that a minute fast in all positions. I demagnetized it and it immediately returned to COSC specifications - an instant cure. This was a dramatic example. Usually there is just a slight amount of magnetism that causes odd and unpredictable readings. Watches have been antimagnetic for years, but every watchmaker has a demagnetizer near his bench and uses it before he even tries to make any adjustments.

Remember I said that positional error is not seen in the dial up or down positions? Theoretically it is impossible on a “perfectly” running watch. This type of error is caused by a difference in the top and bottom pivots - depending on which is carrying the weight. If one is dirty, worn or improperly oiled, there will be a difference. How much of a difference depends on the condition of the pivots.

My advice

First, don’t go crazy. There is no perfect watch. It is completely normal for a watch to vary in different positions and from day to day. The better the movement, the less the variation. As you wear it, the rate on a watch changes throughout the day. Second, remember that there are many factors influence the rate. This is why the COSC standard is a range and an average. Get to know your watch’s “personality”. It is the changes in it's behavior that will let you know if something is going wrong. I am continually amazed that these little pieces of machinery are as accurate and durable as they are.

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Horology 101 - positional error
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