Deconstructing The Very Accurate Grand Seiko Spring Drive U.F.A.

There are 31,556,926 seconds in a year. That’s thirty-one million, five hundred fifty-six thousand, nine hundred twenty-six in words. As a reasonably logical non-stubborn person, I’d wager that’s basically identical to 31,556,906. Yup, I shaved off a measly 20 seconds from 31.5 million. To my eyes and arithmetical acumen (what little remains of it), 31,556,926 and 31,556,906 are what could be termed as approximately identical.

But what’s the deal with so many seconds.

All of this numerical nerdery makes mind-bending sense when you realize that a Grand Seiko Spring Drive U.F.A. Caliber 9RB2 deviates from a year’s 31,556,926 seconds by just 20. Yes, just 20!

That equates to a functional error of 0.000063% over a period of 365.25 days, a.k.a. the most accurate watch in the world powered by a mainspring.

The text below discusses how it’s made possible.

The Underlying Technology Of Spring Drive

Grand Seiko’s Ultra Fine Accuracy (U.F.A.) Caliber 9RB2 marks an evolutionary leap over the Spring Drive technology via a refinement of the latter’s mechanical-quartz synergy to achieve unprecedented levels of precision. To understand the U.F.A. better, a basic know-how of the Spring Drive is necessary.

Here’s how a typical Spring Drive movement functions:

A Spring Drive is a hybrid of electronic and mechanical watchmaking. Here, mechanical is employed for the driving force and energy storage, via a conventional mainspring and a barrel. There are no motors inside a Spring Drive caliber. Deconstructing it reveals the impressive volume of traditional watchmaking content that's packed in. Inside, you’ll find wheels, pinions, jewels, the keyless works, mainspring barrel, date jumping mechanism, the winding system - which in this case is the pawl based “magic lever” (similar to IWC’s Pellaton winding system), as well as the underlying mechanism for the power reserve. So, pretty standard mechanical features.

Also, connected to the mainspring via a geartrain is the governing wheel or the “glide wheel.”

Let’s now focus on how it all works (do refer to the figure above).

The mechanism is based on a Tri-Synchro Regulator system. Here, we have a mainspring in a barrel (that’s the big circle on the left) driving the glide wheel (which’s in the upper right-hand corner). Note that the glide wheel (rotor) is not toothed as it isn’t a conventional geartrain wheel. This wheel, which has a small permanent magnet at its center, turns in a magnetic field of two tiny electromagnets connected to wire-wrapped coils (stator).

The glide wheel, via the process of “electromagnetic induction,” generates an induced electrical current. The electricity produced activates a quartz crystal oscillator and an integrated circuit (small square at the top of the figure). The quartz oscillator vibrates at exactly 32,768 Hz, and this is transmitted as a precise reference signal to the IC. In a Spring Drive caliber, the integrated circuit compares the reference signal from the quartz oscillator with the revolution speed of the glide wheel, and intermittently exerts a magnetic braking force on the glide wheel, just enough to keep it rotating exactly at eight revolutions per second.

Here, the IC plays the role of the conventional balance and is the timing organ. Also, as opposed to a pure quartz movement, there are no power capacitors or batteries in a Spring Drive. It’s essential to emphasize that all of the energy comes from a mainspring.

Now, the governing wheel or glide wheel in turn is geared to the hands. So, in a brief: a barrel geared to a train turns the governing wheel which induces an electrical current that in turn creates an electromagnetic braking force from the integrated circuit to slow down and govern it which in turn is geared to the seconds, the minutes and the hours.

It’s notable to mention that as opposed to a conventional oscillating balance wheel, the governing wheel on a Spring Drive is unidirectional. So, there’s no tick-tock motion and no stop-start stagger. This is what allows the perfectly smooth Spring Drive glide.

By harmonizing three different energy types - mechanical, electrical, and electromagnetic, the Tri-Synchro Regulator represents an ingenious achievement that realizes quartz-level accuracy in mechanically powered movements.

Timekeeping Accuracy Via Spring Drive

Powered by a traditional mainspring and controlled by a quartz crystal oscillator and an integrated circuit (IC), Grand Seiko’s Spring Drive U.F.A. achieves a level of accuracy unimaginable in purely mechanical watches.

The inceptive iteration of a Spring Drive movement, the Seiko Caliber 7R68 from 1999 yielded ±15 seconds per month accuracy. With the Caliber 9R65 from 2004, Grand Seiko was able to attain an extended power reserve of 72 hours. In 2021 with the Caliber 9RA2, Grand Seiko achieved an even higher accuracy of ±10 seconds per month and a longer power reserve of 120-hours or approximately five days. This caliber featured a temperature correction function and a vacuum package IC for stabilizing accuracy.

Still, it wasn't enough to achieve a movement with annual accuracy.

Ingenuity Behind U.F.A.’s High Accuracy

Grand Seiko’s Caliber 9RB2 design team added a function to dynamically correct cumulative deviations in rate while also subjecting each component, including the crystal oscillator, to exhaustive technical scrutiny, addressing each of the factors that can impact long-term accuracy. The remarkable was realized via two key innovations.

Regulator Switch

A Spring Drive’s crystal oscillator, which vibrates in response to electrical stimulus, is a crucial accuracy-determining entity. Despite a quartz crystal being extremely stable, each one has its own characteristics and is prone to long-term frequency renormalization. For the Caliber 9RB2, Grand Seiko ages the quartz crystal for three months in order to eliminate any frequency instabilities. In addition, a regulator switch for micro-corrections has been added for allowing fine adjustments in case the quartz properties deviate from the optimum thus maintaining a stable rate. This regulator switch is a quartz crystal trimmer that adjusts for the drift of a quartz crystal over decades. So, this is built to last as a generational product.

Newly Designed IC

For the Caliber 9RB2, Grand Seiko has engineered a new hermetically vacuum-sealed package for the quartz crystal and the IC to prevent any impact from changes in temperature and humidity. The IC monitors ambient temperature 540 times per day deploying real-time compensation adjustments from any changes in temperature to further maintain long-term accuracy.

The Most Accurate Spring Powered Watch

In all regards, the Grand Seiko U.F.A. is an absolute spec-sheet behemoth. With a variation of no worse than 20 seconds per year, it’s simply one of the most accurate watches on the market. The Spring Drive U.F.A. Caliber 9RB2 powered SLGB001 And SLGB003 models were the standout novelties from Watches and Wonders 2025 and prove that Grand Seiko hasn’t run out of road in the pursuit of its core principles of precision, beauty, and ease of use. The brand can still deliver good value even while packing the highest degree of tech.