Frequency: why is it (almost) always 4 Hz?

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Frequency:  why is it (almost) always 4 Hz?  - Frequency
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The most perceptive can hear it. Others can tell by looking at the movement. But most people don’t pay any attention at all to the frequency at which a watch beats – they leave that to the experts. But how do the experts decide?

When you’re reading through watch specs, there’s one line that tends to get no more than a cursory glance: “Frequency: 4 Hz (28,800 vph)”. We’re usually far more concerned about the materials, the diameter and the power reserve. It’s a bit like a car: you look at the top speed, not the engine capacity. Is that wrong? No, not at all! But watchmakers select the frequency of their movements with great care. Why is that?

It’s all about compromise

As with any engine, there’s no right answer, and there’s no universal solution. In watchmaking, as with most propulsion systems, the final specification is always a compromise between speed, precision, durability and torque, among other variables. A watch’s frequency is chosen according to the job it has to do. In some cases, the main criterion is precision. In others, power reserve will be more important, or durability. Ideally, a movement will fulfil all these criteria, within a price range that remains acceptable for mere mortals.

4 Hz all the way

Four Hertz seems to be the default frequency these days, although there was a time when the slower, gentler frequency of 3 Hz was more popular. Is this just a consequence of the increasingly frenetic pace of life? No, that has nothing to do with it. It’s about offering the greatest chronometric stability possible in a wristwatch. It’s no coincidence that the frequency changed around the middle of the 20th century. That was when watchmakers started to use increasingly precise measuring instruments – the famous “Witschi”, from the brand name of the most popular models used in watchmaking workshops.

Shock and acceleration

While pocket watches were well protected and remained relatively stable (in the vertical position), wristwatches were exposed to continually varying positions and angles, and were far more susceptible to shocks from the rough and tumble of everyday life, and from playing sport. “What determines the chronometric stability of a watch is the ability of its oscillator to return to its baseline state after a shock or acceleration,” explains Carole Kasapi, Director of Movements for TAG Heuer. “The faster an oscillator beats, the more quickly it returns to its initial state after a shock or a change of position.” So, why not push this principle to its logical conclusion? The higher the frequency, the more quickly the oscillator will revert to its optimum position. In a purely theoretical world, you could simply opt for frequencies of 5, 10 or even 50 Hz, and be assured of chronometric stability. Just look at quartz movements – they achieve a precision of just a few seconds’ deviation per year. Surely they offer a perfect example, with their frequency of 32 Hz?

Frequency:  why is it (almost) always 4 Hz?

Alas, theoretical projections all too often end up wrecked on the shores of reality. Choosing the highest possible frequency creates more problems than solutions. The most obvious is wear. Logically, a movement that beats at a frequency of 10 Hz will wear out approximately twice as fast as one with a frequency of 5 Hz. “And that’s not the only concern,” notes Carole Kasapi. “The dispersion of oils becomes a real problem. Modern chemistry has still not satisfactorily solved the lubrication problem of high-frequency movements.” Silicon, which is lighter and more resistant to wear, might offer a solution, but for the time being it is used only in escapements and balance springs, and they are often protected by patents. The final problem is that the faster an oscillator beats, the more energy it needs to work, which adversely affects the power reserve.   

The best possible compromise

At the end of the day, if 4 Hz appears to be de rigueur, it’s probably because it represents the best compromise. With the appropriate barrel and a relatively simple mechanism, it can provide a power reserve of three days. “One in three clients changes their watch, or doesn’t wear a watch at all, at the weekend. That means they need a power reserve that will last from Friday night to Monday morning,” Carole Kasapi explains.

Frequency:  why is it (almost) always 4 Hz?

In order to provide a 3-day power reserve with a frequency of 5 Hz or more, a watch would require a much bigger barrel. In that situation, the technical compromise would be replaced by an aesthetic compromise, which most brands refuse to accept. A frequency of 4 Hz allows for 70 or 80 hours of autonomy, which is sufficient for the majority of clients. Indeed, Rolex’s first action after bringing production of the El Primero calibre in-house (it was previously supplied by Zenith) was to reduce the rate from 5 Hz to 4 Hz. Several decades later, very few brands would quibble with this choice.

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