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Calibre SH21 Masterclass

In these series of Masterclasses, we explore the complex art of mechanical watchmaking through the study of our in-house movement.




Part 1 – The escapement

In part 1, we look into the fundamentals of regulating time, with the escapement.

This is an area of watchmaking where experience, knowledge and feeling come to the fore as results of simulations or computer analyses often fail. Watchmakers work with an orchestra made up of imperfect systems, arranged so that one ‘problem’ will be compensated by another. The precision and the reliability of the whole watch is a result of good or bad orchestration – or compensation – for these individual faults.

How the escapement works

The illustration below shows us the familiar tick-tock of the anchor wheel which indicates the beats of time. To make sure that the time between the ticks is identical, it’s necessary to move the anchor very precisely, with constant frequency. This is the work of the main part of the assortment; providing balance with the hairspring to regulate time.



In wristwatches, the hairspring provides a ‘reset force’ similar to that of a pendulum in a clock. The anchor, then, has a double function. It should stop and free the wheel train in short sequences, delivering power (which comes out of the turning of the anchor wheel to the balance) to ‘feed’ the swinging of the anchor against friction. Achieving the balance required between hairspring and anchor is one of the more subtle arts of the watchmaker – the challenge is to stabilise the swinging of the system to have exactly same spells between ticks. To achieve this, power must be identical for each push of the anchor, with friction constant enough to achieve stable angles of deflection. It’s not so easy to do!

Different frictions affect the bearings between horizontal and vertical positions of the balance wheel, so to increase the power in the system and mitigate and increase reliability, watchmakers either opt for a heavier balance wheel or increase frequency – both of which reduce power reserve. The trade-off is difficult to prove through tests other than daily use; this is because all precision tests, such as the ones undertaken by COSC, are static, even as the power of the main spring reduces over time. There are no common tests for the impact of daily use and really, only the wearer will learn how his different watches compare. It could be possible that one watch loses 15 seconds per day, and another gains 10 seconds, despite both watches being correctly certified by COSC.

Theoretically, all influences on these balance systems are known; you can read hundreds of pages in dozens of books about them. These include, for example: the location compared to the main position; the number of winds of the hairspring; its thickness and height; its end curves; forms and materials and balances together with these combinations; different escapement systems; anchor materials and angles; constant force systems; double balances and tourbillion. But there’s no ‘best’ way to resolve one influence in isolation because there is no ‘best’ resolution altogether.

Many individual solutions conflict, and every solution is a compromise. Even today, it’s not possible to foresee these characteristics before the first prototype of a movement is made. But help is at hand! The individual results are regulated at the end of the assembly by a regleur or regleuse – a vital role in an industry where the costs of industrialisation can equal a nice pre-war Bentley.


Part 2 – The Double Barrel
In a second horology masterclass, we debate the pros and cons of parallel and series connections in double-barreled configurations.

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Part 3 – The Wheel Train
Previous sections of the masterclass covered about the power storage in Calibre SH21’s barrels, as well as its escapement. Between the barrel and the escapement is the wheel train – and that’s what we focus upon here.

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