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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 2 – The Double Barrel

The barrels are the powerhouse of the movement, so it’s unsurprising the double barrel calibre (as opposed to one with a single barrel) delivers an increased power reserve thanks to its two mainsprings, each of which are housed within tooth-edged barrels.

In developing Calibre SH21, our aim was to focus upon power, precision and stability, as opposed to making something that was extra-slim. Creating a movement with two barrels, we had a choice of establishing them in either a parallel or series configuration. As stability was our main focus, a series connection was best suited to Calibre SH21; however, below provides a better explanation of the positive and negatives of setups.

The Barrels (above) – For power storage, usually with an integrated main spring, these deliver energy into the Wheel Train.

Parallel Construction

Pros for parallel construction
Two barrels push one centred minute pinion. As the minute is relatively heavily loaded, it makes sense to install the parallel barrels opposite to this pinion, thereby enormously reducing the bearing pressure. The turning direction of both barrels is the same, helping to charge them. With just half the power stored in each barrel it’s easier to achieve extremely flat barrels, making for an ultra-slim movement.

Cons for parallel construction
This exerts double the force on the minute pinion, making it necessary to halve the power of the mainspring, thereby leaving it open to the risk of sticking.

If a spring sticks, its power delivery to the minute pinion halves again, reducing precision and stability. It’s often difficult to install the barrels opposite to the centre wheel and still have both barrels touching the central wheel in the main layout. Some watchmakers have tried to compensate for this by using a side pinion which delivers power from one side to the centre wheel; however this doesn’t give them the bearing pressure reduction achieved by having both barrels driving the central wheel.

Series Construction

Pros for series connection (as found in Calibre SH21)
Power in the mainspring is equal to a single-barrel solution, with equal power in each barrel. If one spring sticks, the other can catch up and will reduce the impact to the minute pinion.

The power-to-revolutions ratio remains much more even over time, meaning the power delivered is much more stable within the first day or two after full winding. Calibre SH21 loses much less energy on the first day compared to those with one barrel or parallel constructions. This means its COSC certification is not only valid on its first day; in fact, it’s only after its third day that power losses start to affect accuracy. This makes for a far more stable movement.

Cons for series connection
Delivering power into the barrels isn’t as straightforward as they can’t be charged via the two barrel-axes. In Calibre SH21’s case, power is delivered from the rotor to the outer teeth of the first barrel. The spring of the first barrel delivers power in to the axis. Power is then delivered to the second barrel via a gear train system of three wheels (which equal the turning direction of the two barrels). The second barrel then operates like a one barrel construction. This means the addition of two to three extra wheels into the movement, all with their attendant pinions and additional jewels. It’s a far more powerful, stable and accurate movement – but it’s not small!

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

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.