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THE TOURBILLON

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1. Definition and principle

The tourbillon is a mechanism intended to improve the precision of mechanical watches. Its principle consists in grouping the Escapement and the Regulating organ within a rotating cage. By making the regulating organ rotate on itself, a continuous mixing of vertical positions is achieved. Thus, errors in running caused by gravity are compensated on average. This idea was patented by Abraham-Louis Breguet in 1801, a founding date for complications in the history of watchmaking. The tourbillon is classified among precision complications. Its benefit is particularly significant for pocket watches, worn permanently in a vertical position. For wristwatches, however, the chronometric advantages are more debatable.

2. The effects of gravity on the regulating organ

Since the invention of the Balance-Hairspring, horologists observed that gravity disturbs regulation in vertical positions. Indeed, the Balance and the Hairspring (balance spring) inevitably present slight Mass asymmetries. These imbalances result in a rate error specific to each vertical position. Thus, with the Crown up, a Movement that gains five seconds per Day would lose the same amount when the Crown is oriented downward. In horizontal position, these effects disappear. For a pocket watch worn standing in a pocket or a wristwatch worn in vertical positions, gravity acts continuously in the same direction. The problem is therefore persistent and significant for portable precision instruments.

3. The Tourbillon cage: structure and operation

The Tourbillon cage is the rotating structure at the heart of the mechanism. It brings together in a single assembly the Escapement wheel, the Pallets (lever), the Balance and the Hairspring (balance spring). This assembly rotates slowly around a vertical axis, driven by the finishing Train (wheel train). Inside the cage, the escape wheel meshes with a fixed toothed ring gear attached to the Main plate. This epicyclic device allows the escape wheel to turn while the cage is rotating. As it rotates, the cage moves the regulating organ successively through all vertical positions. The errors specific to each position then mutually compensate for one another. This principle, simple in its conception, nevertheless requires execution of very great precision.

4. The pivoted-cage Tourbillon

The pivoted-cage Tourbillon, or between-pivots Tourbillon, is the original construction devised by Breguet. The cage rotates around its central axis like an ordinary wheel. Its axis has a lower pivot connected to the Main plate and an upper pivot held by the Bridge of the tourbillon. This dual fixation gives the mechanism great mechanical stability. The tourbillon bridge is one of the most visible elements of the movement. It is often given special care in terms of finishing and Decoration. This construction offers the best compromise between robustness, ease of Regulation / Adjustment and precision.

5. The flying tourbillon

The flying tourbillon was developed by the German Alfred Helwig in 1920. Its particularity is to eliminate the upper pivot of the cage. It is then held only by a single lower Bearing (today and generally: a ball bearing). This design has the advantage of reducing the total thickness of the tourbillon cage. It also offers increased visibility of the mechanism, without Bridge concealing the cage. However, the absence of an upper pivot increases friction and creates an unfavorable lever-arm effect. This type of construction saw notable growth starting in 1985. The appearance of precise miniature ball bearings made it possible to offset its inherent drawbacks. Since then, these bearings have been systematically used as the sole bearing for modern flying cages.

6. Rotation speed and Power reserve

The rotation speed of the Tourbillon cage directly influences the effectiveness of the compensation. Logically, a faster rotation blends the positions with greater frequency. However, this acceleration has a direct energy cost on the Movement’s Power reserve. The vast majority of cages complete a full rotation in sixty seconds. This one-minute cycle allows the cage to coincide with a seconds indication. Thus, a Hand carried by the upper axis of the cage or an index (hour marker) on the periphery, directly indicates the current second. Rarer constructions adopt a rotation in thirty seconds or four minutes. These variants illustrate the permanent tension between chronometric precision and energy autonomy.

7. Lightness of the cage and inertia of the balance

The chronometric performance of a tourbillon results from a delicate balance between several contradictory parameters. On one hand, the cage must be as light as possible to minimize its impact on energy consumption. This is why titanium is frequently used for its components, due to its low density. On the other hand, a balance with high inertia guarantees better stability of the frequency. However, a heavy Balance mechanically weighs down the cage and increases the Barrel’s consumption. Designers must therefore resolve four interrelated parameters: Balance Inertia, cage Mass, Frequency, and Power reserve. This balance constitutes one of the central challenges of any Tourbillon design.

8. Assembly

The Tourbillon cage is one of the most demanding Assemblies in mechanical horology. It brings together between fifty and eighty very small components, united within an extremely reduced volume. While the manufacture of many components can now be industrialized, their Assembly remains the preserve of experienced Horologists (watchmakers). Each pivot, each Train (wheel train), and each Bearing must be positioned with extreme precision. An error in the alignment of the Axes compromises the dynamic balance of the cage and impairs chronometry. Furthermore, friction between components must be minimized through suitable Lubrication and careful Polishing. The Assembly of a Tourbillon can thus represent several dozen hours of work for a qualified Horologist (watchmaker).