Chapter 5
Caliber 9RB2 with an astounding annual accuracy of ±20 seconds
The world's most accurate wristwatch movement powered by a mainspring
Grand Seiko’s Spring Drive Caliber 9RB2 far surpasses conventional daily and monthly standards with its annual rate of ±20 seconds, making it the pinnacle of precision among spring-driven movements. In recognition of its superb accuracy, Caliber 9RB2 receives the new U.F.A. designation (Ultra Fine Accuracy). This movement achieves its remarkable performance eight years after it was first conceived. But how was this daunting level of annual accuracy realized?
Leveraging years of development to achieve superb accuracy
Caliber 9RB2 incorporates a quartz oscillator that has been aged for three months in a process developed for the Caliber 9RA series. It also features an ultra-low-power IC with a compensation function to handle accuracy fluctuations caused by changing temperatures, with the IC and wiring sealed in a vacuum package to safeguard stability and precision. Minimizing residual stress (the tendency for oscillators to return to their original shape) during processing enhances the annual rate by reducing the tiny frequency deviations that can develop over time. In addition, all components, including the oscillator, were thoroughly reviewed, and by redesigning the IC, even the smallest variations in accuracy were further minimized to achieve ultra-high precision. The torque output of the mainspring was boosted for enhanced stability, and drive train gear shapes were optimized to enhance transmission efficiency. Thanks to the reduced drive load, the power reserve and energy generation performance have been enhanced.
The first-ever regulation switch for Spring Drive and comprehensive magnetic resistance
Caliber 9RB2 is the first movement in Spring Drive history to feature a regulation switch that corrects for time gain and loss. The switch offsets tiny deviations in accuracy caused by changes in oscillator frequency over long-term use. Precision adjustments carried out during after-sales service, tailored to caliber characteristics, guarantee a superb annual rate over many years. Also notable is the use of magnetic resistance, which reduces the influence of magnetic effects and facilitates miniaturization. While the conventional approach is to place a soft-iron magnetic ring around the movement’s perimeter, Caliber 9RB2 divides this shielding into four sections placed strategically in the movement's clearance spaces on both sides. Such continuous refinements in conception and technology deliver a smaller movement that maintains exceptional performance.
A single-barrel design with higher transfer efficiency
The layout of the Offset Magic Lever was redesigned to accommodate a single barrel. To counteract the loss in winding efficiency that results from the offset, the shape, rotation axis location, and thickness of gears were revised. Mainspring length and width, as well as thickness and barrel volume ratio, were also reconceived to allow 72 hour-power reserve and a lower center of gravity for even better on-wrist comfort. The One-piece Center Bridge structure provides increased durability, and the movement was moved closer to the case back to deliver a slimmer profile and a pleasing view through the sapphire crystal. Bridge edge, side faces, and holes on the bridge are adorned with diamond-cut edges, and the oscillating weight is engraved with “SPRING DRIVE ULTRA FINE ACCURACY.”
Evolution 9 Collection SLGB003
The 9RB2 movement allows Grand Seiko to use a case with a diameter of just 37 mm, the smallest among 9R-equipped models. The dial ring is thinner than that used with the 9RA series, and a sapphire box crystal opens up a wide view of the dial. The case and bracelet are formed from High-Intensity Titanium, while the dial features an exquisite pressed pattern evoking the beauty of ice forests in the Shinshu region, the home of Spring Drive watchmaking.
Caliber 9R Spring Drive debuted in 2004 with functionality engineered to meet Grand Seiko's high standards. Since then, this movement platform has continued to evolve, culminating most recently in Caliber 9RB2, the world's most accurate wristwatch movement powered by a mainspring. Research and development will continue on Spring Drive, a fusion of Japan's classic mechanical watchmaking and pioneering quartz technology.
