Wednesday, July 20, 2011

Time keeping

What a week! After receiving 127 requests to disclose the cherry strudel recipe and 16 requests to disclose Yelena's phone number (neither possible, sorry) finally a watch-related email from a fellow enthusiast:

"Can you explain in simple terms how the watch balance wheel is regulated to keep 'spot on time' ?"

Dear J.N.,

I am glad you've asked for a simple explanation because this will save us time :-) You've indirectly asked two questions and I'll try to respond accordingly.

But before we get into it, let me just say that I cannot stress enough the importance of this subject. Unfortunately I know of many watch retailers, dealers, collectors - and even watch repairers who handle high grade watches on a daily basis, yet most of them have only vague knowledge of time keeping adjustment.

Therefore if you care to immerse yourself in my brief reply, you will be miles ahead from them all!

Time keeping and regulation are fundamental to watchmaking and our ability to tell the time accurately. Let me just say that mechanical watches never keep 'spot on time' but neither do 'quartz' watches. Even atomic clocks are not perfect but they are getting better: In the 1950s they were able to keep time within 1 second per 2000 years, while today we got them 'adjusted' to 1 sec per 20 million years. Which is not really all that bad.

Back to mechanical watches. Instead of 'spot on time' may I propose we settle for the more practical term: 'reasonably accurate'. Let's say 2 seconds per day.

While 2 sec/day is way above any practical requirement (especially if you travel on Sydney buses) your watch will still be 1 minute off at the end of the month. But then again, this is at least twice better than official Swiss chronometer certifications.

Which makes the Swiss standard look pretty lame. Or does it?

Before we jump into any conclusions, let's put things into perspective.

The balance wheel assembly is the heart of the watch. It is directly responsible for time keeping.

In essence, the balance wheel is a mechanical oscillator - it is the heart of the watch. Or to put it simply, this is the device which makes ticks and tocks.

And those ticks and tocks are what the time is made of!

Take a look at the photo below.

One second consists of EXACTLY 4 ticks and 4 tocks (*for 28.800 bph movement).
No more, no less. Consequently, one hour is made of 28,800 ticks. Or 691,200 heart beats per day! That is almost seven hundred thousand beats!

Let's say that instead of 691,200 beats your watch heart ticks just 16 beats more (or less) per day That would be an error of 0.00002 % which is the equivalent of those 2 magic seconds we are chasing!

You don't have to be a mathematician to figure out that such a tight tolerance is nothing but a miracle - especially for a mechanical device.

Actually there is no other mechanical device - including precision laboratory instruments - which would or could keep such a level of accuracy 24 hours per day over a period of 5 years without any maintenance while submerged under water, or up in the air, exposed to a temperature range of 50 degrees Celsius or more. Not to mention game or two of golf, sledge hammer and occasional drop to tile floor!

Indeed, the mechanical watch heart is a MIRACLE of human engineering and it's ability to tick with such an amazing accuracy is a very unique property.

Now that we've learned how precious that little heart is, we can get into more detail about time keeping adjustment or regulation.

The heart (oscillator) consists of a number of components, but two are directly responsible for ticks and tocks are:

- spring (often called a hair spring) and
- balance wheel

If you like formulas, you'll love this one: it is a real beauty! Actually the
greatest thing about Newtonian physics is that everything makes perfect sense!
(Unlike with confusing quantum stuff which came later :-)

The above formula describes the relation between time keeping, the hair spring
and the balance wheel.

Why is it a beauty? Because this is exactly what we need to know! It tells us that period of oscillation T [ticks and tocks] is determined by the position of weight on a balance wheel [I, inertia] and the stiffness of the hair spring [k].

In other words, in order to make our watch tick faster or slower, we could make adjustments to either the hair spring, the balance wheel - or both!

Rolex (and some other makers like Patek or the new Omega Coaxial) have designed their tickers in such a fashion that adjustment to time keeping is made by the variation of inertia of the balance wheel.

With most calibres of Rolex watches, regulation is achieved by adjusting the position of
regulating weights. The tiny little weight (marked A) is set closer or further away from the balance wheel rim (B). The closer to the rim: more inertia, slower period of oscillation, and the watch goes slower. The weight is effectively a nut; one quarter of a turn of one weight (4 shown on photo) equals approximately 1 sec per day.

Other makers opted for variation of stiffness of hair spring as a means of time keeping adjustment. Here, the inertia is constant (no weight on wheel) but the hair spring is
either shortened (stiffness increased) or extended by sliding the regulator lever [B].

As one would expect, both designs have advantages and disadvantages, and yes, there is more than one way to skin a cat.

Since by now I have most likely lost most of my subscribers, let me just wrap it up by clarifying a couple of Internet misconceptions.

The myth: watches with higher oscillator rates are better time keepers. Well this one has been busted big time. If you are to remember just one thing today, then remember this: never buy a watch just because of high beat rate!

A 36,000 bph watch is not necessarily a better time keeper than a watch ticking at 19,800 bph. The higher beat also comes with a few side effects: it requires a very special kind of lubricant for the pallets / escape wheel; more frequent overhaul and if movement is not serviced properly there would be more wear and tear.

This is the main reason why Rolex Daytonas produced with Zenith El Primero movements
(Pre - 2000. production) have 'modified' movements: the original Zenith bph was reduced to 28,000 and true to tradition, Rolex preferred free sprung balance wheel with variable inertia adjustment.

But the 36,000 bph has one fantastic property: it allow us not only to mechanically split the second to 10 beats (5 ticks+5 tocks) but to display the 1/10 of a second on the dial as well. And yes, the high beat oscillators are less sensitive to outside forces which are trying to throw the oscillator out of balance.

The common bph for mechanical wrist watches are 18,000 - 19,800 - 21,600 - 25,200 - 28,800 - 36,000 corresponding to 5 / 5.5/ 6 / 7 / 8 and 10 beat per second.

For those who like to think "outside the circle": you can actually chose ANY rate for your mechanical oscillator, as long as you have a wheel cutting machine to cut a wheel with 64.9754 tooth and don't mind "hour" hand displaying 3.483 hours per day :-)

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