Congratulations to Moritz Grossmann for having its Tefnut Twist nominated for this year's Grand Prix d’Horlogerie de Genève (GPHG). The Tefnut Twist with the Grossmann strap winder makes it possible to wind the watch in no time.
Grossmann's watchmakers fulfilled a long-standing wish when they created the new winding mechanism for the Tefnut. The quest for a robust alternative to the conventional crown winder culminated in an ingeniously simple solution.
The Grossmann strap winder allows the watch to be wound with turns of the strap attachment located at 6 o'clock in the case. It's fast and works even with gloves or freshly applied nail polish. It takes some dexterity to wind a watch by its crown, but the strap offers a big enough surface for a firm grip. With just a few back-and-forth movements, the watch can be fully wound. The process is very comfortable and a real advantage especially for small watches.
The Grossmann strap winder necessitated extensive modifications of the caliber 102.2. It is based on the caliber 102.1 and has a compact diameter of 26mm. To create the strap winder, a rotatable strap attachment was integrated in the case at 6 o'clock. It converts turns of the strap into winding torque via a toothed stem.
The movement is again designed as a pillar caliber. A separately removable winder module incorporates all of the strap winder functions. The escape wheel and the pallet lever are mounted on a separate cock which simplifies the removal of the individual escapement parts. The size of the escape wheel cock was minimized to keep the view of the escapement as uncluttered as possible.
A very ingenious mechanism transfers the force needed to tension the mainspring from the winding stem in the case at 6 o'clock via a wheel train – which is visibly arranged on the movement – to the mainspring barrel. A special bevel gear pair establishes the connection between the inclined winding stem and the horizontally aligned movement. While winding, the robust strap transmits ample power which is transformed into speed. A backstop ratchet allows bidirectional winding. Only very few twists of the strap are needed to fully wind the watch.
Since the strap winder gear ratio is not suitable for hand setting, the winding and setting functions were separated in the Tefnut Twist. As usual, the hands are set with the crown, which in this case is in the less exposed 4 o'clock position.
Since the winding stem could turn with the strap while the watch is being worn, the backstop tolerates up to 20° of angular deflection without causing the stop click to instantly engage the stem. The ratchet wheel has only eight teeth, so the stopwork cannot be overloaded when very fast winding movements are performed.
In the caliber 102.2, a mainspring with a bridle – commonplace in self-winding watches – prevents damage that could be caused by excessive force. The bridle contacts the inside of the mainspring barrel and resists an amount of frictional torque that as a rule is 10% higher than the maximum torque of the spring. So, when the mainspring is fully wound, turning the crown is met with resistance but does not lurch.
The Grossmann balance for the Tefnut calibers is clearly smaller than in the larger Atum and Benu movements. It is designed for a high moment of inertia with minimal mass and, beyond the mass and poising screws, offers convenient precision adjustment options.
The stop-seconds mechanism is activated when the setting crown is pulled. A spring firmly attached to the main plate presses against the safety roller and thus prevents inadvertent contact with the polished outer surface of the balance-wheel rim or maladjustment of the delicate poising screws. The spring is controlled by a linkage-type slide that for the strap winder is connected directly with the arresting spring and is thus uncoupled from the winding motion.
The form factor of the Tefnut Twist was modified to accommodate the strap winder. Its bezel and the striking strap attachments please the eye with balanced proportions. With the seconds subdial at 7 o'clock and the setting crown at 4 o'clock, the visual focus is subtly shifted in the direction of the strap winder. Depending on the version, the strap bars of the prominent attachments are adorned with precious stone cabochons or gold screws.
The Tefnut Twist with the strap winder comes in three versions: "Fancy," "Classic" and "Gent" with rose-gold and white-gold cases. All versions are fitted with nodal hands that are of course manually crafted and annealed to a brown-violet hue to match the numerals. Their slenderness is a unique expression of pure watchmaking artistry.
The Tefnut Twist "Classic" projects a balanced combination of rare beauty and superb functionality. A centred guilloche pattern with the numerals of the raised hour circle and the crisply detailed minute scale project suave symmetry. Positioned at 7 o'clock, the seconds subdial adds an endearing accent. Full scales assure precise readings of minutes and seconds. Elegant Arabic numerals round out the personality of the classic wristwatch.
The Tefnut Twist "Fancy" is a high-end jewelry watch with a multi-faceted face. The main dial and the seconds subdial are framed by white brilliant-cut diamonds. Mother-of-pearl facing and the radially guilloched passepartout visually highlights the off-centre seconds display. The scythe-shaped hour display attracts attention with five vibrantly styled numerals on a mother-of-pearl background. Precious-stone cabochons grace the crown and the strap bars of the precious ladies' watch.
The Tefnut Twist "Gent" exemplifies reduction to the essence. As in the Tefnut 36, the dial already reveals the perfect face of a classic precision timekeeping
instrument. Its bright silvery surface highlights the succinctly drawn indications for the hours, minutes, and seconds. The case alone, with the crown
at 4 o'clock and the strap attachments discreetly terminated with four gold screws, quickly reveal that the watch is endowed with the Grossmann strap