Then came 1973 and with it a new chronograph caliber that survived the turmoil of the quartz revolution – during which the mechanical wristwatch almost died out – without complaint: the Valjoux 7750 . With him, designer Edmond Capt placed particular emphasis on ease of service and energetic efficiency. The amazing thing: the rotor of the automatic movement only worked in one direction. Criticism that he does his job inadequately can hardly be heard from experts either. On the contrary: the good elevator performance and reliability are repeatedly praised. Not least because of this, this automatic caliber, despite its comparatively simple technology, is now one of the undisputed bestsellers among movements that are available for mechanical wristwatches.
In 1994 Girard Perregaux returned to the ranks of manufacturers that produce automatic watches with the 3xxx caliber family. The base caliber 3000, diameter 23.9 and height 2.98 mm is one of the particularly flat representatives of the species with a central rotor. The energy for the automatic is provided by a ball bearing rotor in one direction of rotation. The decision to dispense with a gearbox was made after careful consideration, because in the opinion of the designers, rotors that only wind up on one side react much more sensitively to the movements of the wrist and wristwatch.
Jaeger-LeCoultre , for example, shared this view with the 975 “Autotractor” launched in 2004, and Patek Philippe with the development of the automatic caliber 324. Thus, the question of whether movements with unidirectional or bidirectional winding work more efficiently belongs to the scholarly dispute. The valid answer is still a long time coming. Both systems have their advantages and of course also disadvantages, not least in practice and for those who wear automatic watches.
The reduction gear: Indispensable for the automatic watch
In contrast to the change gear, the reduction gear is indispensable in the automatic movement. Its task is to convert the relatively fast rotor movements into slower ones with higher torque. This is what it takes to tension the mainspring.
Because too much of a good thing would be harmful, the outer spring end is not attached directly to the barrel. Rather, it hangs on the hook of a sliding chamber, which slides along the inner wall of the barrel after reaching the predefined maximum tension.
Rotor yes, but what reason for discussion is the shape, size and arrangement of the flywheel. Regardless of whether it is a central or microrotor: every system has its advantages, but also its disadvantages. Supporters of the classic central rotor see no technical reason to use a rotor with a smaller diameter and integrate it somewhere in the basic movement i.e starting version of fossil brand. For example, if you take an “El-Primero” automatic movement from Zenith, the torque of the central rotor can be calculated as follows:
Movement diameter: 30 mm
Weight of the flywheel: 5.95 grams
Turning radius: 7.34 mm
Maximum static torque of the flywheel: 43.67 gr x mm
Sliding moment of the mainspring: 2,000 gr x mm
With the same shape of the flywheel and the bearing moved two millimeters from the center, its diameter must logically be reduced accordingly. This leads to the following result:
Weight of the flywheel: 4.73 grams
Turning radius: 6.34 mm
Maximum static torque of the flywheel: 30 gr x mm
Loss of static torque: 31 percent
A different reduction gear is therefore required to compensate. Depending on the characteristics, the elevator process can slow down here.
When designing very flat or even ultra-flat automatic movements, designers have resorted to the option of fully integrating the rotor into the factory level since the 1950s. In this case one speaks of a microrotor. A calculation example for this:
Weight of the flywheel: 2.88 grams
Turning radius: 3.57 mm
Maximum static torque of the flywheel: 10.28 gr x mm
Loss of static torque: 76 percent
By calculating the reduction gear accordingly, the loss of static torque can also be counteracted here. Compared to centrically or slightly eccentrically arranged centrifugal masses, microrotors allow an unobstructed view of the entire clockwork with its decorations as well as the oscillation and escapement system in addition to a particularly flat design. And this is a particularly serious decision criterion for many lovers of automatic watches and of course hand-wound watches.
Movement is (almost) all s of an automatic watch
Regardless of the size and positioning of the flywheel, regardless of whether the winding is acting on one or both sides, all movements have one thing in common for automatic watches: If they are to function properly without the occasional manual tuition, a sufficient amount of movement is required, which the watch can do through wearing or is fed by a watch winder. Because the movement alone generates kinetic energy for tensioning the mainspring. This is accompanied by a more uniform drive torque, which in turn can be reflected in better performance of the wristwatch.
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