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The calibre 240’s hour of glory

The calibre 240’s hour of glory

By: Jean-Philippe Arm

Born in 1977, it remains after 40 years the delight of Patek Philippe and of the happy owners of a variety of watch models, some unique and all elegant. And there’s no fuss about it because when it comes to celebrating anniversaries the stars are usually watch collections. This time, at the start of 2017 it’s the turn of a watch movement to be in the limelight, and deservedly so.

This thin selfwinding calibre, designed in 1970 at the height of the quartz crisis as a subtle counter to the electronic revolution, contributed to the resurgence of high-level mechanical watchmaking. In constant use since its birth, it has evolved over the decades, benefitting from the most advanced technological breakthroughs. It can thus be seen to exemplify the development of watchmaking since then.

The calibre 240 might mean little to the world at large but it remains significant for horological specialists and professionals. Its 40th anniversary is therefore worth celebrating here.

Henri and Philippe Stern’s commission in the mid-1970s took their employees in research and development by surprise, since they were expecting to work on quartz movements. But no, there was no question of getting into quartz movements. Nevertheless, their advantages had to be retained, namely convenience and thinness. The idea was therefore to develop the thinnest possible selfwinding movement. The solution was to employ a micro rotor.

The calibre 240 was constructed by a Gérard Berret, who was then in charge of the technical department and had already developed two or three other movements for Patek Philippe. He had joined the company in 1968 from Universal Genève, which had had a micro rotor movement since 1958. He had himself constructed this particular mechanism so was familiar with its advantages, its drawbacks and its challenges. His experience enabled him to design and develop the calibre 240 for Patek in an exceptionally short time.

Was there ever a desire or even a need to go so fast ? “Not really,” says Daniel Jaquet, who joined Patek in 1964. “It was more a matter of opportunity because hurry was never part of the corporate culture.” Jaquet is now in active retirement as a mentor, having long been in charge of production. He was well placed in the technical department and then in the workshops to oversee the epic development of the calibre 240. Another witness to the movement’s iterations and its latest achievements is Philip Barat who has been at Patek Philippe since 1992 and is now its Head of Development.

Winding worries
Micro rotor selfwinding systems were not widespread in the 1960s and 70s, since a small off-centre rotor would necessarily be less effective or even ineffective at winding the mainspring than the conventional central rotor. “This was our greatest worry,” recalls Daniel Jaquet. “Furthermore, it would require a larger movement of 27 to 27.5 mm in diameter. That was big for the time, but laughable today.”

But the real advantage was to achieve the aim of a very thin movement. Without claiming to match the accuracy of quartz, its purpose was to power elegant mechanical watches that didn’t have to be wound. This was how those at the Jonction factory and the Rue du Rhône headquarters intended to take a stand against the electronic tsunami.

To ensure winding efficiency while remaining thin required a number of choices to be made, one of them being the balance frequency. The standard was then 2.5 Hz, but sights were set higher. A frequency of 4 Hz was unthinkable for reasons of size and power reserve, so 3 Hz was the reasonable choice, with the Gyromax free-sprung balance that had been patented in 1953.

Choosing direction
There was also the choice to be made between unidirectional and bidirectional winding – a matter of endless debate. Both systems were tested on prototypes and unidirectional winding turned out to give the better results. “We always take this approach without prior assumptions,” remarks Philip Barat. “In this case it was a micro rotor that didn’t have the same dynamic behaviour as a central rotor and its winding efficiency in a single direction turned out to be better.”

For the record, this prototyping phase took place in 1976 in the Jonction factory. It was then cased up in the Rue du Rhône workshops and presented as a new model at the Basel show in the spring of 1977, the official date of its launch. Forty years have gone by since then. Seen in different models from the bridge side, it looks as if it hasn’t changed, but in fact it has evolved considerably.

The order was satisfactorily fulfilled. Until then, Patek Philippe had a selfwinding movement that was 4.6 mm thick (the 27-460) which powered a perpetual calendar. With the calibre 240, the gauge went down to 2.4 mm up to the bridges, but 2.5 mm up to the gold rotor. Initially it was 2.4 mm overall, hence the calibre number. “However it failed to wind properly and it needed a bigger rotor, which is why we added a tenth of a millimetre to the rotor that was sunk as far as possible into the movement.”

The thin selfwinding movement had to use the minimum amount of energy. As we shall see, a series of improvements has today made it more efficient. To keep it thin, the centre wheel was offset. “We had to use a few tricks to avoid superimposed wheels,” Philip Barat disclosed. “Instead of indenting the cannon pinion in the usual way we used an ordinary split pin.” Watchmakers will appreciate this. Ordinary people will know that the jargon hides real technical issues and that the unceasing quest of the masters of time involves contrivances touching on concrete details. If the devil is in the details, so is salvation. It has always been so, from the water clock to the atomic clock.

Small seconds
The calibre 240 does not have a centre seconds hand nor did it have a small seconds hand originally – an anomaly that is worth a second look. In its particular going train the seconds wheel does not go around once a minute despite its name. This is not a flaw and furthermore it is between 4 and 5 o’clock. There was no question of displaying the seconds in the initial model, but they did appear in this unusual position 20 years later. The recollection brings a smile to Philip Barat. “In 1976 when the movement was designed, putting the seconds there was unheard of. We would have been the laughing stock of our colleagues. The proper place for small seconds was at 6 o’clock, at 3 o’clock or at 9 o’clock, but never between 4 and 5. It was out of the question.” Daniel Jaquet adds : “If we wanted to put it at 6 or 9 o’clock we would have had to increase the diameter and the watch would have been slightly thicker, and that was ruled out. That is why the original calibre 240 didn’t have a small seconds.”

It appeared in 1992 in the unusual position between 4 and 5 o’clock, and again in 1993 with a small complication between 7 and 8 o’clock for visual balance, along with along with moonphases and a power-reserve indicator.

Later in 2005, the small seconds again appeared between 4 and 5 o’clock. It subsequently moved to 6 o’clock in 2011 with the 31-260 REG QA movement, which was inspired by the calibre 240. But then the diameter increased to 31 mm instead of 27.5 mm. Let us forget the small seconds and rewind.

Serving the perpetual calendar
The first model fitted with the calibre 240 in 1977 was the Golden Ellipse reference 3738.

Four years later it came out in a skeleton version. Then in 1985 it powered a perpetual calendar constructed and made available a few years earlier by Dubois-Dépraz, a partner in the Joux Valley and a valuable supplier of components and modules. “They could offer springs and jumper springs that were extremely thin, which are essential for perpetual calendars,” Daniel Jaquet recalls. Let’s not forget that vertical integration by the brands is a recent phenomenon. It was then normal for such suppliers to contribute ideas, projects, designs and prototypes. “We were really excited, but we hesitated to bring it out as it was. It was not a typical Patek Philippe construction. To keep it thin we had to integrate the perpetual calendar in the barrel bridge and the baseplate. That was how it finally came out.”

This second version of the calibre 240 was officially launched in 1985 as the reference 3940. “At 3.95 mm is still one of the thinnest perpetual calendars,” says Philip Barat. “It was a real achievement, thanks to the calibre 240 and indications by hands.”

Small complications
It took eight years for the second incarnation to appear and another 12 before we saw the calibre 240 fitted in the 1997 reference 5055 with small seconds, dates, moonphases and power-reserve indicator. After a first attempt in 1993 and a year after the first annual calendar, it was the real start of the small useful complications with products that were more affordable than perpetual calendars.

The saga of the thin calibre 240 continued, punctuated by the complications with which it was associated. Were these bolt-on modules ? Philip Barat shakes his head. “Not necessarily. Part of the 1985 perpetual calendar was a module and therefore independent, but another part was integrated into the baseplate. Moreover the norm here is integration or semi integration. These are not additional plates that we fit on top. The world time model of 2000, for example is not a module ; the calibre 240 HU has its own baseplate for the world time.”

A family
The integration issue is of course driven by the constant preoccupation of achieving the thinnest possible movement and therefore the thinnest possible watch, whatever functions it might contain. “If that requires an additional module rather than integration, we will do it. That might also happen because of production costs.”

The brands often describe a modified calibre as new. “In this case we instead call them a family of movements. The determining factor is the centring, or the relative positions of the barrel arbor, the balance shaft and the escapement pivot.” It nevertheless remains the calibre 240 with inscriptions, letters and numbers referring to the associated functions and indications.

While on the bridge side it appears to be the same calibre 240, the dial side is evolving constantly. In 2000 it was the world time watch ; in 2002, the Celestial with its revolving starry sky – “the Sky Moon’s little sister.” The latest development was the limited-series universal time watch of 2014 with a large moonphase in its centre. What’s next ? “I can’t say any more,” says Philip Barat with a smile, “but there are quite a few things under development that are based on the 240.”

Apart from indications, it has also evolved within, taking advantage of basic technical developments – “all of them to do with reliability.”

Its Achilles’ heel, as we have seen, is its winding ability. “In 1992, when we recalculated the going train to bring out the small seconds, new software was available. We were able to design new tooth profiles, changing the shape of their points, reduce energy loss to friction and make the gearing more efficient. Furthermore, the ball bearings for the rotor and its clutch have evolved favourably from steel to zirconium, making lubrication unnecessary.”

In 2004, the Gyromax balance changed from two arms with eight weights to two arms with four weights. Then came the silicon balance spring, or rather its Patek Philippe derivative called Silinvar ®. “An outside bulge was added to a flat spring to obtain the same rate advantages as provided by the Breguet or Phillips terminal curve, but without an increase in height.” Since then the calibre 240 has been fitted with such a spring.

The launch of the reference 5550 in 2011 was the calibre 240’s high-tech consecration, since it powers the Patek Philippe Advanced Research perpetual calendar that introduced the Oscillomax ®. This brings together the three regulatory organs made of Silinvar ® : the Spiromax ® balance spring, the GyromaxSi ® balance wheel and the Pulsomax ® escapement.

“This was a great leap forward for the calibre 240,” exclaims Philip Barat. “Its power reserve jumped from 48 to 75 hours, or in practical terms, a long weekend.”

It came in a limited series of 300 copies according to Patek Philippe’s policy for its advanced-research products. Only time will tell whether this is the right technological approach or if others will turn out to be more fruitful. In any event the calibre 240 will surely have a role to play.