Chapter 4
A New Generation of Spring Drive Movements Born of Japanese Aesthetic Sensibilities
The quest for precision, durability and slimness
Caliber 9RA5 was released in 2020 to celebrate Grand Seiko’s 60th anniversary and was followed a year later by the Caliber 9RA2, in which the power-reserve indicator was moved to the case back. With an accuracy of ±10 seconds per month, the use of two differently sized barrels freed up limited space inside the movement for a power reserve that lasts around 120 hours. Born of Japanese craftsmanship and aesthetic sensibility, these movements combine superior robustness, durability, and shock resistance with a painstakingly finished exterior inspired by the rich natural environment of Japan’s Shinshu region, home of the Shinshu Watch Studio.
The vacuum-sealed package enables greater energy efficiency
The most taxing task for the developers was completing the high-precision Spring Drive package IC, which would subsequently help them achieve 10-second-per-month accuracy. The crystal oscillator is supposed to vibrate precisely 32,768 times per second, but temperature fluctuations can cause that frequency to vary slightly. To counteract this variation, a sensor measures the internal temperature 540 times a day to gauge the effect on performance. Whereas in a conventional configuration, the IC and crystal oscillator are mounted separately, the Caliber 9RA5 and Caliber 9RA2 achieve greater accuracy control by vacuum-sealing these in a single package. After sealing the package, the crystal oscillator is “aged” for three months. In addition, the internal design of the IC—an SOI-IC (silicon on insulator integrated circuit), which requires only minimal current to run—underwent improvements for better energy efficiency.
Meanwhile, installing all wiring connecting the crystal oscillator with the IC within the vacuum-sealed package prevents changes in humidity from influencing the movement in addition to shielding the movement from the influence of static electricity and light. However, adding the temperature compensation function meant the IC consumed more energy, so to offset this, the mainspring was improved, and the gear train was redesigned for more efficient transmission. The result was a power reserve of approximately 5 days.
Slimmer and more compact than ever before
The adoption of an Offset Magic Lever, in which the Magic Lever with crank wheel is positioned away from the center of the movement, helped reduce bulk, as did a reduction in the size of components like the barrel, which contains the spring, and the oscillating weight. The result was a movement 0.8 mm slimmer than the previous 9R6 series. The slight drop in winding efficiency caused by moving the Magic Lever away from the transmission wheel was resolved by creating a new a crank wheel that drives the Magic Lever. The center of gravity was lowered by reducing the distance from the bottom of the movement to the center of the crown and positioning the crown toward the case back as much as possible. To further increase the robustness, three discrete components—the barrel bridge, the center wheel bridge, and the winding stem cock—were combined into a single One-piece Center Bridge that forms the central supporting structure of the movement. This integration also resulted in improved transmission efficiency and the ability to set more parts simultaneously. Moreover, it reinforces the main plate at the 3 and 9 o’clock positions by containing most of the gear train within the structure.
Evolution 9 Collection SLGA021
Powered by the Caliber 9RA2 Spring Drive 5 Days movement, this timepiece is inspired by Lake Suwa stirred by the breeze before dawn. To the southeast of the Shinshu Watch Studio, where Spring Drive watches are made, one will find the gently lapping waters of Lake Suwa, a constant inspiration for Grand Seiko. The dial evokes the surface of this very body of water; to gaze at it is to see gentle waves dancing on the wind-swept lake in the pre-dawn stillness. And like a gentle wind on an otherwise tranquil body of water, the dark blue color manifests differently depending on the angle at which light catches it. Like flowing water, the seconds hand moves with the quiet, natural pace of time itself thanks to the Spring Drive movement just below the dial’s surface.
The strong, slender, and highly accurate Caliber 9RA5 are just one episode in the ongoing story: the evolution of Spring Drive as the ultimate fusion of Japan’s world-leading mechanical and quartz watch technologies.
