Feature: How Does A Tourbillon Work?

In a cake made of watch parts, the tourbillon is surely the cherry. It's revered, it's worshipped, it's—wait a second, what actually is a tourbillon, and why should I want one? Here to answer that question are a couple of watches from two of the oldest and best watchmakers in the world: the Breguet Classique Complications 3657PT/12/9V6 and the Blancpain Villeret Tourbillon 6025-3642-55B.

Before we talk about what a tourbillon is, let's address why it was needed in the first place. The balance wheel, the beating heart of a mechanical movement, is subjected to the constant force of gravity as it swings backwards and forwards. Imagine a weight on the end of a rope. You're holding the other end of that rope. You spin, the rope pulls taught, the weight follows, leaving the ground and orbiting the vertical axis of your body as you rotate. As the weight goes around and around, it remains equidistant from the ground; all forces are even.

The tourbillon was invented by Abraham-Louis Breguet in 1795

Now imagine the same thing, but with the orbit of the weight spinning around a horizontal axis, like a plane's propeller. The weight dips down, then swings back up again as it comes around. It's easy to let it drop with gravity, but hard to get it to rise up again as it goes against gravity. It speeds up as it falls, and slows as it rises. The same principles apply to a balance wheel, which is why you'll see phrases like 'adjusted in 5 positions' engraved on some movements. A watch must be able to perform regardless of its orientation.

The Blancpain has a flying tourbillon, which cantilevers the balance wheel in the tourbillon cage

In 1795, watchmaker Abraham-Louis Breguet wondered, 'what if the balance wheel can be in all positions at once?' If it were possible, it would average out all those gravitational variations and provide a more uniform beat. After all, the pocket watch was commonplace at the time, which, for packaging reasons, hung the balance in a way that was constantly working against gravity.

In Breguet's words, 'I have succeeded in cancelling, through compensation, the anomalies caused by the different positions of the centres of gravity of the regulator movements ...'

The tourbillon was invented to average poise errors by rotating the escapement

What he came up with was a device that encapsulated the entire escapement, rotating it so that each part of the balance wheel took its turn to work hardest, therefore evening out the effects of gravity. He named it 'whirlwind'—which, in French, is 'tourbillon'.

How did Breguet do it? It's actually easier to see on the Blancpain, which has what's known as a 'flying tourbillon'. Where the balance is centralised on the Breguet's traditional tourbillon, the Blancpain's is cantilevered, allowing it to 'fly' around the central axis of the tourbillon cage. For us, this opens up the tourbillon and lets us see what's going on.

The tourbillon increases accuracy for watches held in the upright position

While a traditional movement sends its power to the locking and unlocking mechanism of the escapement to prevent it winding down all in one go, a tourbillon watch sends the power first to the tourbillon cage, which houses the escapement. The cage sits directly in between the gear train and the escape wheel, and as the cage turns, the escape wheel orbits it. Were it not for the fixed wheel below the tourbillon cage, the escape wheel would do nothing. A pinion connecting the escape wheel to that fixed wheel makes the escape wheel turn.

And from there, everything is business as usual. The escape wheel pushes the pallet fork, which locks the escape wheel and spins the balance wheel—the balance wheel bounces back again on the balance spring, knocking the pallet fork, unlocking the escape wheel and allowing it to advance one more tooth. Just like a normal movement. If you want to see how an escapement works in more detail, watch our previous video, 'How Does A Mechanical Watch Work?'.

Tourbillons are especially useful in pocket watches because how they are worn

As a whole, the tourbillon is an intricate solution, simple in its idea yet complex in its execution. And yet, somehow, it gets even more so: the tourbillon has actually been developed further to add a second and even a third axis of rotation, distributing gravity over an even greater area.

The thing is, Breguet's invention was specifically for a pocket watch, which hung in that one same position all day long. Wristwatches rarely sit in one position for any extended period of time, and if they do, it's usually dial side up, where the least gravitational imbalance is experienced.

In a wristwatch, a tourbillon is a beautiful mechanism that showcases the highest skill

What this means, really, is that the tourbillon is completely pointless, engineering for engineering's sake. But it's pointless in the same way the Mona Lisa is pointless. It's engineering for engineering's sake much in the same way as the International Space Station. Tourbillons still exist ... well, just because. If they didn't, we'd probably be no worse off, but having them around certainly seems better.

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