Mechanical watches are powered by a spiral spring, which accumulates elastic energy, and releases it gradually and constantly, operating the hands and giving us back the measure of time.
To keep the watch working, the mainspring must always be wound. It can be wound manually, in which case it is a manually wound movement, or through a system that employs moving the watch while it is being worn, in which case we are referring to a self-winding movement, or automatic.
Since 1700, many watchmakers have struggled to find a mechanical system which would replace manually winding the watch, to keep it always wound. The invention of this system is yet another stroke of genius by Abraham-Louis Breguet. The Neuchâtelsi watchmaker set himself the challenge of maintaining his watches wound, which at the time was carried out only manually, using a key carried separately from the watch.
In 1780, Breguet presented his invention, consisting of an oscillating weight capable of transforming the jolts of the wearer into energy, thanks to an arm connected to the mainspring.
This extraordinary innovation, however, did not immediately achieve great success: the reason is to be found in the nature of the pocket watch, widespread at that time, to which Breguet applied his idea; in fact, a watch carried in the pocket will hardly receive sufficient oscillations along the vertical axis, needed for fully winding the spring.
We will have to wait for the introduction of the wristwatch, between the late 1800s and early 1900s, to see a more practical application of automatic winding.
… But what makes an automatic watch work?
The functioning of the self-winding mechanical movement is due to the kinetic energy released by the movement of the arm; such a “prodigy” is caused by the oscillating weight, a small rotor that oscillates with every movement of the wrist, effectively taking the place of the crown and spool group, a winding element present in manual winding mechanical movements.
The oscillating weight or rotor, initially in the shape of an anchor, is a circular sector suitably weighted in the outermost band, to improve its moment of inertia; with each movement of the wrist it rotates around its axis and, through a series of additional gears, transmits its energy to the winding spring, wound on itself inside the barrel, a small flat metal cylinder.
The mainspring is connected from one end to the shaft located in the center of the barrel, and from the other end (the outer one) to the wall of the barrel itself.
By its nature, once wound up, the spring tends to unwind, releasing energy: this is the “fuel” that allows the watch movement to work.
The first automatic movements used a rotating mass applied and superimposed on the mechanism of a manual mechanism, pivoted in a decentralized manner with respect to its axis, in such a way as to overcome the moment of inertia and oscillate with each movement of the wrist; the rotating system was connected by means of small gears to the winding barrel and thus ensured the continuity of running of the watch.
Initially, the oscillating masses had movement limitations also due to the presence of springs that had to reverse the direction of rotation or cushion the end of stroke, with consequent poor reserve of the charge.
It was Rolex who modified the rotor, in the shape of a half moon, by making it rotate continuously, after overcoming another obstacle: charging in both directions of rotation and no longer in a unidirectional way, as it originally happened.
In 1948, Eterna advanced the technology of self-winding watches, with the development of the Eterna-matic automatic movement. The use of five strategically placed ball bearings made the movement very efficient and greatly reduced friction and drag on the oscillating weight that wound the mainspring. This innovation has reduced wear on internal parts, increasing the accuracy and life of the movements.
How to prevent excessive movement of the oscillating weight from overloading the spring, causing it to break?
The first solution that was found was direct and simple: when the winding spring is completely wound, it operates a pin that physically blocks the movement of the oscillating mass. However, this method proved to be inefficient as the pin, which blocked the rotor, was subject to excessive wear due to impacts with the mass itself.
The definitive solution was found with the adoption of a winding spring with breda; when the winding spring reaches maximum tension, it slides against the grooved inner edge of the barrel instead of squeezing until it breaks.
By combining this device with automatic winding, the current method of winding automatic mechanical watches is achieved: the oscillating weight always wounds and the winding spring “slips” when it is at maximum tension, avoiding breaking.
Which automatic to choose? A non-trivial decision that affects technique and durability.
In this historical moment it is evident that the self-winding mechanism is the most popular among watchmaking enthusiasts, it satisfies the convenience of not having to remember to wind up the watch every day, and satisfies the suggestion of precision mechanics.
When choosing an automatic watch, after identifying additional functions, brand, price and aesthetics, it is still necessary to make a technical consideration that does not depend on quality.
The design choices of the various Maisons produce more or less efficient automatic recharging systems, that is, more or less rapid in reloading the winding spring.
There are two different categories of automatic, in a classification across all brands, which refer to the type of winding that the oscillating weight supplies to the winding spring, that is unidirectional and bidirectional.
In the former, the rotor operates the spring only when it rotates in one direction, as in Patek Philippe watches.
In the latter, thanks to reversing gears, the winding action takes place in both directions. An example are Rolex movements, whose red reversing gears are easily identifiable.
This system makes it possible to exploit both directions of rotation of the rotor, both clockwise and anticlockwise. The driving wheel, which is fixed integrally to the rotor, meshes with the mobile transmission pinion thanks to the inversion lever, which has two limiters to avoid excessive penetration of the teeth, which could jam and block the train of the automatic.
According to the direction of the rotor, the return pinion moves to mesh with one of the two wheels, in contact with each other, according to the same direction.
One meshes with the other, which always turns in the same direction regardless of that of the rotor; through the continuation of the automatic gearbox train, the motion of this wheel reaches the spool of the barrel, loading the spring.
There are also reloading systems such as IWC’s Pellaton, in which the oscillating mass does not act on the axis of the barrel, but the rotor drives two cams that hook a toothed wheel integral with the barrel, making it rotate and reload it in a very functional way.
Therefore, despite the 200 years of evolution, studies on automatic winding have not yet stopped. Improvements have been introduced in recent years on the components, to increasingly reduce the friction and wear of the mechanism. The rotor no longer turns around the mechanism but rotates on it and has the same diameter, consequently the movements used are larger, increasing its accuracy.
Being a movement that strongly depends on the user and his conscious and unconscious movements, to maximize its performance, watchmakers have become good observers of human habits.
Author: Andrea Muratore
Translated by: Enrico Dalla Guerra
