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The Tourbillon Explained

written by A.Morgan - 7th Nov 2011

Gravity. We all use it every day and we barely even notice it, but shrink down to the size of the average watch movement and prepare to experience a whole new world of force…

Before we begin, let’s quickly catch up with those at the back that didn’t do last week’s homework and don’t understand the basic operation of a watch movement. First, power comes from a 20-30cm long metal strip that is coiled into a tight spring and wound either by the crown or by a weighted rotor. To stop this power exploding out all in one go, it is then transferred to the escapement, which is made up of an escape wheel, a pallet fork and a balance wheel. The escape wheel turns, and its jagged teeth rock the pallet fork left to right, which in turn oscillates the balance wheel back and forth, regulating the power and releasing it in small doses back through the escape wheel and into the gear train as the following video demonstrates:

As you can imagine, even the tiniest discrepancy in the escapement can cause deviations in accuracy, and so that leads on to one of the biggest problems faced by watchmakers; how to deal with the effects of gravity. Tolerances can be perfected, but the effects of gravity cannot be contained. The problem is this; when wearing a watch – particularly a pocket watch – it tends to hang in one position for long periods of time, and this affects the balance on the, er, balance wheel.

Imagine holding a metre length piece of rope with a weight attached to the other end. Then imagine spinning round, holding onto the rope so the weight lifts off the ground and begins orbiting around you, remaining equidistant from the ground for each rotation. All fine and dandy on a flat surface, but stand on a slope and then try to keep the weight spinning round level with the sloped ground and it becomes much more difficult. You fight gravity during one half of the rotation as you pull the weight away from the Earth’s core and work with it for the other half as it drops back down again. That imbalance is the same effect that gravity has on a balance wheel any time it isn’t perfectly flat.

So what can be done about it? After all, a standard movement is good for an accuracy of around three seconds per day, so surely chasing down those last three is just being pedantic? Well, let’s not forget that being a watchmaker requires a certain degree of pendency in the first place – particularly because the patience and concentration that goes into making a movement can become many shades of frustrating in the wrong hands. So it is the watchmaker’s urge to refine, perfect and improve everything they do.

The answer came in 1795 by a French-Swiss man called Abraham-Louis Breguet (inventor of other watchmaking revolutions such as the Breguet overcoil) who had the idea of rotating the complete escapement as it oscillates so gravity affects it equally from all angles. Thinking back to that imbalanced spinning weight on a rope – the theory behind the tourbillon would be similar in principle to constantly changing the angle of the slope – like a see-saw – to even out the effects of gravity.Try and spot the escape wheel and pallet fork rotating around the centre of the balance wheel as the tourbillon cage performs one rotation every minute:

Sounds easy when you say it, but of course in reality, it isn’t. There is a reason why the tourbillon is considered the ultimate complication, and why those that can make it are so highly revered; a tourbillon still has the same escape wheel, pallet fork and balance wheel to regulate the power of the mainspring, but it is all mounted in a rotating cage that – usually – rotates once every minute. Most manufacturers then strap a seconds hand to the top of it and it doubles up as a seconds sub-dial as well.

The mechanical complexity of a tourbillon is not actually that much more than a standard escapement, but the exacting compactness is what makes it such a challenge. Of course, the competitive nature of watchmakers means that now a bog-standard tourbillon is rather old hat, and since Breguet’s introduction of the complication, many other variations have been introduced. The standard tourbillon, for example, is known as single axis, because all the rotation is done through other one plane, but a double axis tourbillon adds a second plane, thus rotating the balance wheel both around its own edge and also back around on itself, just like a gyroscope. It’s quite a sight to behold. Then there are flying tourbillons (an off-centre version of the single axis), triple axis tourbillons, and even astro tourbillons, where the tourbillon cage revolves around the inside edge of the watch case.

So there you have it – a quick glimpse inside the brain-bending world of the tourbillon. Tiny, complex and beautifully engineered, they truly are the ultimate in micro-engineering, a feat of mad perfectionism no bigger than a sugar cube – and all just to save a few seconds a day.