Interesting points.
The Bayer guys say this about the gravitational 'sag'
Apparently since the two hairsprings oscillate in two different directions simultaneously and asymmetrically the center of gravity is kept in the the center on average
i assume something like this
Moser's Double Hairspring
escapement with two opposing Straumann Hairsprings prevent the gravitational
error from occurring in the first place.
In a watch, the gravitational error disrupts the stability of
its accuracy. This means that when the watch is in a horizontal position – i.e.
when the dial is at the top – the watch runs differently than when the dial is
positioned vertically, i.e. laterally. The watchmaker counteracts this
phenomenon by adjusting the watch so that the slower vibrations of the
balance in vertical position are compensated for as precisely
as possible by the more rapid vibrations in the horizontal position. This
produces an average level of accuracy, although this is dependent on the way in
which the owner wears his watch. This is because the average level of accuracy
is only achieved if both positions occur with approximately the same frequency.
In practice, the vast majority of watches are therefore adjusted in fi ve
standardized positions in order to minimize the discrepancies. At Moser we
pursue this practice to the limit, as we adjust our watches in all six possible
positions. So where, then, does this gravitational error come from? And how can
we reduce its impact even further?
These questions have preoccupied watchmakers for many decades.
In this respect, it is important to know the type of attachment at the outer
end of the balance spring. This is realized either by a flat curve or a Breguet
curve. The socalled flat curve, which is used in most mechanical watches, can
be implemented very easily. It does, however, present the disadvantage that the
balance spring assumes an asymmetrical shape when it vibrates. In doing so, the
centre of gravity in the spring moves away from the middle. If we now imagine
that the spring adopts a vertical position, it then becomes clear that due to
the shift in the centre of gravity, the vibrations in the “downward” direction
are accelerated by the earth’s gravitational pull. At the same time, vibrations
in the “upward” direction are impeded, and consequently slowed down. On the
other hand, if the spring is in a horizontal position, this effect does not
play a role. This is not a good starting position for the stability of the
accuracy.
In contrast, the Breguet curve was developed in order to avoid
precisely this asymmetrical vibration of the fl at curve. It does this by
ensuring that the outer end is curved upwards over the high edge, and is then
further curved inwards. This task demands all the skills and ability of the
adjusters, since the procedure is predominantly carried out by hand. This
complicated and from the point of view of craftsmanship highly demanding form
of manufacture is the reason why the Breguet curve is only found in watches of
very high quality. As a result, the vibrations of a spring with a Breguet curve
are almost completely symmetrical. But only almost. There is still a small
residual error.
The
approach to the double hairspring escapement with a pair of Straumann
hairsprings is now very simple: the springs are arranged one above the other,
with one vibrating to the left and the other vibrating to the right. If both
springs have the same mechanical characteristics, then when they vibrate, the
centre of gravity moves outwards from the centre – just as it does with a
single spring and a flat curve. However, as both springs are vibrating
asymmetrically in opposite directions as a result of the different direction of
rotation, the centre of gravity, on average, remains exactly in the centre. A
gravitational error due to the asymmetrical vibrations of the springs can
therefore not occur in the first place. So why was a tourbillon invented
to compensate for the error caused by gravity?
At first, the tourbillon was developed to compensate for the
gravitational error in a cut bimetallic balance with a steel spring. With this
type of balance, the effect of temperature on the accuracy of the watch was
prevented by the individual arms of the bimetallic balance bending outwards or
inwards with changes in temperature. It is easy to see how the two arms of the
balance never moved in a uniform fashion, which invariably caused a much
greater gravitational error than the asymmetrical vibrations of a spring with a
flat curve. This technique was used in pocket watches, which under normal
conditions would be in a vertical position in the waistcoat or gilet
customarily worn at the time. A tourbillon therefore made perfect sense,
because the entire escapement could, for example, complete one revolution
around itself per minute. The changing centre of gravity thus had an
accelerating effect before slowing down half a minute later in the opposite
direction. On average, the existing error was therefore compensated for within
one complete revolution. This only works, however, if the watch remains in the
same position for at least as long as the tourbillon requires to carry out one
complete revolution. It is easy to see that this would rarely be the case with
a wristwatch.
Nowadays, with the use of self-compensating alloys – as in
Straumann hairsprings – combined with a Glucydur balance, an imperfectly
working bimetallic balance is no longer used. The tourbillon can now be used to
compensate for the much smaller gravitational error relating to the vibrations
of a fl at curve spring. However, this too only works properly if the watch
remains in the same position for as long as the tourbillon requires to complete
one revolution around itself.
As a general rule, we can therefore certainly say that it is
better for an error not to occur in the fi rst place than to try and compensate
for it afterwards. It is for this reason that we at Moser have developed the
Straumann double hairspring escapement system.