48TECHNIQUETEC
Constant force:
more than just torque
David Chokron
Constant force is attracting renewed interest as a
means of improving the rate of a watch, with no fewer
than five new developments in less than a year. They
take us back to the dawn of time measurement.
It’s all because of a curve, the diminishing power of the
unwinding mainspring. The force that powers a watch
is most irregular. A fully wound spring delivers a lot of
energy in the first few hours, levelling off for much of
the time that the spring unwinds. Then, with about twothirds of the power reserve used up, the force drops off
sharply and comes to an end. However for a mechanical movement to work properly, the balance must be
isochronous, which in theory means that the period of
each oscillation must be identical. To convert that into
practice – constant amplitude in equal periods – the
balance needs nothing to change, especially not the
energy it receives.
There are three ways to provide a constant force to the
balance. The first uses the fusee to compensate for the
declining power of the barrel. The second constitutes
a buffer store of energy in a remontoir, and the third
equalises the impulses at the level of the escapement.
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It’s an ancient conundrum. The sages of antiquity
noticed that in their clepsydras or water clocks the
water ran out faster when the tank was full and the
pressure high. They soon set up intermediate
tanks to equalise the pressure. In the Middle Ages,
long before the invention of the pendulum, the
amount of torque was the major issue in the precision of clocks. In those days of primitive metallurgy
it was essential to devise ingenious methods to
compensate for the poor efficiency and reliability of
the driving springs.
Gearshift. “The fusee and chain system is designed
to compensate for the loss of power by increasing
the torque,” is watchmaker Romain Gauthier’s definition. Torque is that rather abstract value denoting
the leverage of a force causing rotation as in a gear
train. It can be increased or decreased by gear
ratios. The first example of a fusee seems to go
back to 1430 in a clock owned by Philippe the
Good, Duke of Burgundy. The device was to endure
through the ages to accompany subsequent
CHNIQUETECHNIQU
(Left) Chain, fusee and barrel: A. Lange & Söhne makes
ample use of this "gearbox" in its Pour le Mérite models.
(Centre) The Breguet 7047BR places its tourbillon with
fusee and chain firmly in the foreground of its threedimensional architecture
(Right) Romain Gauthier replaces the chain by a snail
cam and fits ruby articulations in the fusee chains of his
Logical One.
horological developments. It was soon to be connected to the barrel with a metal chain, which
although stiff and badly lubricated was far superior
to the unpredictable elasticity of catgut. It works like
a variable-speed gearbox. When the barrel is fully
wound, it pulls the chain off the narrow top of the
fusee cone. When the barrel’s force has diminished, it takes the chain off the much larger diameter of the base of the cone, thus maintaining the
torque through increased leverage. This is the classic constant-force device, being easy to design but
difficult to make, for the links of the chain are half a
millimetre thick in a wristwatch. Breguet, DeWitt
and Cabestan have been using the fusee for years.
Even Zenith has taken it up, having it made by an
independent watchmaker, Vianney Halter.
It must nevertheless be recognised that in a good
mainspring the power curve remains flat for a long
period, which makes the fusee mostly redundant.
The system needs big barrels and a lot of force for
a limited running time. To tip the balance in his
favour, Romain Gauthier has fitted ruby articulations
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TECHNIQUETECHN
16
14
12
10
8
6
4
Couple du barillet sans force constante
Couple avec force constante
2
0
0
Christophe Claret presents a torque curve. Above and
shown in blue, the irregular disarming of the barrel. In red,
the stability provided by a remontoir.
in the fusee chains of his Logical One model to
reduce friction. Instead of a cone, he employs a flat
snail cam with an increasing diameter, similar to
that used in retrograde indications – with a marked
reduction in height.
Buffer dam. The most popular way of equalising
the torque works on the same principle as the
buffer reservoir of the ancient water clock. The
remontoir is a subsidiary power drive that fits in
near the escape-wheel. It comprises a blade spring
or coiled spring with a lock and release mechanism
– effectively an escapement. At regular intervals it
stocks up with enough energy to maintain the balance at a constant amplitude. The higher the frequency at which the remontoir spring is re-armed
the more constant the torque, as shown in the
curve supplied by Christophe Claret (fig.), who has
devised a new type of remontoir for his Kantharos
chronograph. It aims to reduce the inevitable friction in an escapement, which for him is the number
one problem. “It’s a question of the energy used to
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2
4
5.06
5.05
5.04
5.03
5.02
5.01
5.00
4.99
0.6
6
8
10
12
14
16
Variation couple
force constante
(rapporter au barillet)
0.62
0.64
0.66
0.68
0.7
operate the remontoir,” adds Stephen Forsey, cofounder of Greubel Forsey. “Every complex system
produces unwanted side-effects.” Thus the frequency at which the remontoir is recharged is a
matter of choice. Watchmakers usually settle for
once a second. The jumping seconds thus produced are an interesting by-product of this option,
as in François-Paul Journe’s Tourbillon à Remontoir
d’Egalité and Optimum models.
Flood control Saxon style. A. Lange & Söhne is
the company applying the most constant-force
devices. In 1994, the brand, newly revived from the
wasteland of the Eastern Bloc, was already offering
a tourbillon with fusee and chain, which has since
been implemented in four other models. However
some of its inventions leave it no choice but to use
a remontoir to make them work. The month-going
Lange 31 has two 1.85-metre mainsprings housed
in a massive barrel. “If that power were unleashed
directly it would break the escapement,” explains
Anthony DeHaas, head of product development.
NIQUETECHNIQUE
The remontoir serves to provide a force that is not
only constant but also bearable – “with an amplitude
variation of more or less two degrees,” Mr DeHaas
points out. Lange’s Zeitwerk model needs constant
force to govern the three discs that indicate the
hours and minutes. With between one and three
jumps every 60 seconds it needs to accumulate a
lot of energy, which it releases all at once. The combination of a remontoir and a centrifugal governor –
as used in minute-repeaters – mitigates the torque
and protects the escapement from its strength. In
both cases the supply of constant energy to the balance is a collateral benefit.
(Left) The Tourbillon Souverain by François-Paul Journe
uses a blade-type remontoir rather than a spiral spring.
(Right). To avoid excessive tension, A. Lange & Söhne
equips its chains with a stopwork. It should be pointed out
that they are just 0.4 mm thick!
(Below) Heritage Watch Manufacture has patented a
constant force escapement featuring three pallet levers
and two balance-springs.
Constant impulse at the heart. The third solution is
perhaps the most aristocratic, but like open-heart surgery, it is also fraught with risk. A precise amount of
energy is directly supplied to each impulse of the balance within the escapement itself. Taking this device
as a reference, no other solution in current production
can truly claim to supply constant force. The device
by Heritage Manufacture Watch, although ingenious
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TECHNIQUETECHN
The new silicon Echappement Constant by Girard Perregaux, which achieves concomitant constancy of energy and of rate.
with its three pallet levers and a second coiled spring
rearmed at each impulse, is not in series production.
Nor does IWC’s effort meet the strict constant-force
requirement. Although the remontoir is fitted within
the tourbillon carriage of its calibres 94800 and
94900, it is not rearmed at each impulse but at every
second, which means that it jumps. Even worse, it
needs so much energy that it is only effective for the
first 48 hours of the 96-hour power reserve.
Thereafter the tourbillon carriage no longer jumps but
returns to its conventional pace. “But during those 48
hours the torque curve is perfectly smooth,” remarks
Stefan Ihnen, co-manager of R&D at IWC. He
nonetheless admits that the running time would be 25
to 30% longer without the constant-force device.
“The problem with this sort of mechanism is the accuracy to friction ratio. If you solve one problem, you
create another,” he concludes.
Finally Girard-Perregaux’s Echappement Constant
was on show at Baselworld 2013 after several
years in gestation, representing the most inventive
solution to date. Two torsion blades within a large
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butterfly-shaped component in silicon provide
resistance to the force delivered through the going
train. When the force reaches a peak the two
blades bend the other way in a fraction of a second
to allow the escape-wheel to advance. The constancy of the energy and of the rate together is thus
maintained at each impulse.
Overloading the camel. Controlling energy is a
complex
and
counter-intuitive
process.
Watchmaking is supposed to progress by minimising the friction and mass of escapements, and here
they are burdened with a complexity of secondary
mechanisms. The barrels put into service are huge
and none gives more than 100 hours of running
time. Yet energy can be managed more economically by straightening the high and the low ends of
the torque curve. IWC uses the well-tried stopping
mechanism. Its 51000 movement series will run for
eight days, but after seven days indicated on the
dial a planetary gear stops the barrel before its
power drops to an insufficient level. Journe has a
NIQUETECHNIQUE
Richard Mille carefully controls the state of wind of its
RM030. Its rotor disengages for 10 hours when the power
reserve reaches 50 hours.
similar idea. The power-reserve indicator on his
Octa watches shows 120 hours, although the
movement will keep going for 160. Greubel Forsey
is working on the rate and force of the unwinding
barrels. “Instead of having a gear ratio of around
4000 as in a conventional movement, our ratio in
the Double Tourbillon 30° is between 1700 and
1800,” Stephen Forsey explains. “The reduction is
lower, there’s less loss thanks to weaker springs
and our curves are much lower at the start
(0 hours) and flatter overall.”
The simple solution is too difficult. But there’s an
even simpler way to achieve constant force with a
device that is both efficient and yet so commonplace
as to escape notice: automatic winding. When worn
by an active person, the selfwinding movement
stays fully wound, thus providing constant torque. To
capitalise on the optimum state of wind, Richard
Mille uses winding rotors fitted with variable-inertia
vanes to govern the energy generated by the
wearer. His RM030 model is further equipped with
an automatic clutch that disengages the rotor for 10
hours when the power reserve reaches 50 hours. It
thus avoids the exponential peak in torque when the
barrel is fully wound, as well as the variations
caused by the mainspring slipping in the barrel.
Although the constancy of force is not as clear-cut
as in the other options, the minimal cost and complexity of this solution makes it unbeatable. But as
tradition and marketing would have it, the most precise chronometers are always manually wound, and
have to be kept at an optimum state of wind. This
level of attention can be tiresome, even for the
chronometrical purists, especially if they have several watches to look after. Constant torque also
comes at the cost of squandering energy. Yet ensuring constant force demonstrates watchmaking mastery. Its timekeeping value, is not, as is often the
case, the main consideration, however real it may
be. And because of this, a constant force-watch is
not subject to market criteria any stricter than for any
other watch. The most difficult thing in watchmaking
is evidently simplicity. •
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