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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 3 – The Wheel Train

The wheel train is a system of wheels that transmit the force of the timepiece’s power source, the mainspring, to the escapement to drive the balance wheel. Movements such as ETA’s 2824 and 6497 calibres have the following wheels:

•  A minute wheel – often in the centre, mostly one revolution per hour like 6497
•  A first wheel which is faster than the minute wheel
•  A second wheel which usually has one revolution per minute
•  Escapement wheel (which is the interface to the escapement)

The minute wheel often carries the minute hand (e.g. our own Calibre SH21) and the second wheel often directly carries the second hand. As we have only one second wheel it is vital that its location is decided during construction. The face of the watch will change if the second wheel is at the 6H Position (ETA 6498), at the 9H Position (ETA 6497) or in the centre (ETA 2824) as the second hand will be displayed on the wheel position. During the development of SH21 we wanted to be able to determine the position of the second wheel during assembly. From 1940 to 1960, most watch designs incorporated a central second hand rather than the traditional positions of 6H and 9H. To overcome this issue, some watch constructions have two second wheels, both turning at the same speed, which creates a new problem. The additional second wheel is like a dead-end – it isn’t located within the gear train where the power goes through. This means the second hand is powered by the other wheels which can lead to slackness and, in some circumstances, a temporary stopping of the second hand.

All wheels have play between their teeth, but in the general gear train where the power is going through, the wheels are all pressed by the barrel to one flange of the teeth. One principle of SH21 from the outset was that any hands should move without slack or play, thus precluding us from using the additional ‘dead-end’ second wheel approach.

To bring the power to the central and small second position we have installed a more complex gear train which goes through both positions on each watch. Due to power being delivered from the first wheel to the centre second pinion, via a second first wheel to the small second pinion located on the opposite direction, only one extra pinion and wheel is needed.

As a result, this takes the play out of the central second without any additional springs or pieces. You can actually check this function on any SH21 watch when it is nearly unwound. Firstly, give the crown five revolutions to supply a minimum amount of power to the barrel. If you then pull the crown to the second position and turn the hands backwards and forward, the second hand will also jump around three to four seconds backwards and forward; this is the normal play of the gear train. In our construction, the power of the barrel takes out any play and pushes the central second hand in a forward motion. With this power on the central hand it must move in a continuous motion.

This ‘jump back’ function of the seconds hand requires the power in the barrels to be lower than the friction of the hand which is why the test described above is on a partially wound watch.

Reliability, precision, robustness and flexibility were the guiding principles of Calibre SH21. The design of the wheel train is an example of how these qualities were designed and incorporated into our movement from the very beginning.

Part 1 – The Escapement
In part 1, we look into the fundamentals of regulating time, with the escapement.

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.