How Spring Drive Works

The most discussed, debated and mysterious movement in the world today is the Seiko Spring Drive.  Most associated with the high-end brand Grand Seiko, spring drive movements are now available in many high end Seikos, from Ananta to Galante to Credor.  But despite the success and fame of this radical design, very few watch collectors really understand how it works.  Until now.

The Introduction

Let’s start with the basics:  what is a spring drive?  It’s a radical new design  for a watch movement.  In fact, it’s the most radical change in watchmaking since the quartz movement debuted in the first wristwatch in 1969, and not coincidentally, that revolution was also created by Seiko.

Oversimplified, the spring drive combines the accuracy of quartz with the mechanical beauty of a mechanical movement to create something altogether new.  Virtually the entire movement is mechanical, not fundamentally dissimilar to the mechanical Grand Seiko 9S55 movement.  But a tiny, yet critically important, area of the mechanical movement has been replaced:  the escapement.  In its place now resides the tri-synchro regulator, the key to the entire design.  The result is that the movements are beautifully made, automatic or hand wound, almost completely mechanical and yet supremely accurate.  Spring drives have other unique advantages too:  the seconds hand is perfectly smooth and the watch is silent.

How Does It Do That?

Let’s start with the easy part, which is only really complicated.  That part is the mechanical aspect, which makes up the vast majority of a spring drive movement.

Here we can see the rotor, a heavy oscillating weight used to wind the watch with the movement of the wearer.  You may be wondering how this component differs in a spring drive from another high-end automatic watch, and the answer would be it doesn’t.  Like virtually all Seikos, except for the 9S65 and 9S85, spring drives use the incredibly elegant “magic lever” winding system.

This system allows the rotor to wind the watch regardless of whether the rotor moves clockwise or counterclockwise with a minimum number of parts and complexity.  Virtually all spring drive movements are automatic, but there are a small number of hand wound spring drives.

The mainspring is also pretty much identical to that of an ordinary high-end mechanical watch.  It does use Seiko’s proprietary Spron alloy technology which allows it, like the mechanical 9S65, to achieve an impressive 72 hour power reserve, but it’s functionally identical to any other high-end mechanical watch.

So the spring drive, so far, is just an ordinary high-end automatic movement.  This is where that all changes–the tri-synchro regulator (TSR).  The TSR regulates three different kinds of energy–kinetic, electrical, and electromagnetic.  But before we understand why that’s so important, we need to take a look at the conventional design it replaces.

What we’re looking at is the balance wheel from a Grand Seiko 9S65 movement.  This is basically the same design you would find in many great watches, like Zenith, A Lange & Sohne, ETA and Vacheron Constantin to name only a few.  The balance wheel oscillates back and forth very quickly, and each swing and return allows the escapement to move one step, momentarily supplying power to the rest of the watch and letting the hands move.  The escapement, in short, is the part of the mechanical watch that keeps time–all components before it are subservient to it, and all components after it are controlled by it.  It’s the brain.

For all its beauty and precision, its mechanical design is very susceptible to all sorts of environmental influences that cause it to be unstable.  Changes in position mean that the movement is affected by gravity in different ways, ultimately causing the watch to behave quite differently when it is in different positions relative to the earth.  Vibration can cause the movement to speed up.  The amount of energy in the mainspring, which is different from full to nearly empty, also imparts more energy to the escapement and affects timekeeping.  But there is the imprecision in the parts to take into account as well–they must be incredibly finely made in order to achieve chronometer-level results.

In Hegelian philosophical terms, the escapement represents the thesis.  Inevitably, then, if Hegel is to be believed, it must be met with an antithesis.  And it was.

That antithesis took the form of the quartz movement.  Seiko developed the first portable quartz clock, first quartz wall clock and first quartz wristwatch, innovations that would permanently alter the landscape of horology.  Quartz movements were, functionally speaking, vastly superior to mechanical ones.  They are supremely accurate–this 9F quartz, for instance, is rated for just 10 seconds a year.  Furthermore, they are inherently immune to positional instability and to isochronism and are also many times more shock resistant.  They quickly became the primary design used in watches, at that time both entry level and high-end.  But for all their benefits, they lacked the complex beauty of a well made mechanical movement.  They required battery changes.  They had precision, but they lacked passion.  The stage was set for thesis and antithesis to meet, and 30 years later, they did.

The synthesis takes the form of the spring drive, the design that combined the passion of a mechanical with the precision of a quartz, and this group of components (the ones closest to us in the photo), collectively known as the Tri-Synchro Regulator, made it all possible.  In a spring drive, like a conventional mechanical, it’s important to understand how the escapement or TSR functions on a fundamental level.  Most people perceive them as a “gas pedal” in a car, which applies power exactly when needed to advance the hands at the right rate.  In reality, mechanicals and spring drives work the other way around–the gas is always fully depressed, but the escapement and TSR operate the brake pedal, letting off the brake just the right amount of times in a second (or the right amount in a spring drive) to advance the rest of the movement accurately.  So escapements and TSRs are basically two different kinds of braking system–if you removed them, but kept the rest of the movement intact, the hands would spin quickly until the mainspring was depleted of energy.

This is a Grand Seiko mechanical escapement, which is functionally similar to the mechanical watch you’re probably wearing right now.  Each turn of the balance wheel moves the pallet fork, on our right, which unlocks and locks the escape wheel, on the left.  This back and forth motion causes the characteristic “tick-tock” sound and the movement of the seconds hand.  That movement is not truly smooth, it just advances so many steps in a single second it can look that way–a seconds hand advances exactly 8 times in a second in most mechanical watches.  This smooth movement of the seconds hand is the signature of virtually all mechanical watches and is often held out as one of the properties that make mechanicals aesthetically superior to quartz watches, which almost always tick just one time per second in order to conserve battery life.

This function is accomplished in a spring drive by means of an electromagnetic brake.  Simplifying it a bit, an electromagnet is finely controlled to provide precisely the correct amount of resistance to apply to the glide wheel to keep it running perfectly accurately.

The technical details of how exactly that works are a bit more complex.  Basically, a powerful permanent magnet is affixed to the glide wheel, called the stator.  This is really the true brilliance of the design, because it elegantly combines different functional mechanisms into a very small number of parts.  That simple little permanent magnet is basically the hairspring of a spring drive, if such comparisons can even be made.  But it’s also so much more–it’s an important part of the electric generator, the brake, and the sensor that detects speed.

Let’s look at those functions in depth and see precisely how they work.  When you take a dead spring drive watch and wind it up, the glide wheel (and everything else) is allowed to freewheel for a moment.  You can actually see this as the seconds hand moves far too quickly briefly.  This rapid motion generates the initial electrical energy needed to power the rest of the TSR.  How does that work?  Well, basically like any conventional electric generator–essentially, wire is moved inside of a magnetic field, or the reverse in this case, which generates electricity.  But where does that electricity go?

That electricity is used to power the integrated circuit (IC) and the electromagnetic brake.  Let’s start with the IC first.  If the tri-synchro regulator is the brains of the spring drive, then the integrated circuit is the brains of the tri-synchro regulator.  That electricity provides the resource that circuit needs to operate itself and the quartz crystal oscillator, which is the “reference” of the system–the speed of the glide wheel is ultimately governed by the information from the quartz crystal.  This is the only element of the entire spring drive that is associated with conventional quartz movements.  In this regard, it is basically the same as any quartz movement, but it’s what the spring drive does with that information that really sets it apart.  The stator provides the necessary information, basically sending a pulse once per rotation.  Now we have two sources of information:  that collected from the glide wheel and that collected from the quartz crystal.  A phase comparison circuit “compares” the waveforms from each.  The goal is for them to be perfectly in sync.  In reality, a properly functioning spring drive’s glide wheel will always be running faster than it needs to, and thus, the goal here is to see exactly how much the glide wheel needs to be slowed down to match the quartz oscillator.  That data is passed along to the brake control circuit which decides how much to slow the glide wheel down.  Then the electromagnetic brake is applied, slowing the glide wheel, and the rest of the watch, exactly the right amount to keep it in line with the quartz oscillator.  This process occurs 8 times per second and is extraordinarily precise–the only inaccuracy that results in a spring drive is a consequence of the fact that even quartz oscillators are not perfectly accurate–but at least 8 times per second, the glide wheel will be perfectly in sync with it.

This is why I explained how it was designed to run fast earlier–if you think about it, because 100% of the motion of the gears and hands is mechanical, the spring drive cannot do anything about a slow running movement.  It doesn’t have the ability to speed it up, only slow it down.  Thus, like a mechanical watch, the “gas pedal” is permanently pressed to the floor, and the brains of the operation are just deciding when to stop it (in a conventional mechanical) or to slow it down (in a spring drive).

But if the accuracy of a spring drive is ultimately governed by a quartz crystal, why does it so easily exceed ordinary quartz performance?  Seiko grows all of their quartz crystals in house and Grand Seiko gets first pick–that is to say Seiko tests all of their crystals and Grand Seiko gets the best of the best.  Then the crystals go through a 3 month aging process.  The result is increased performance relative to an ordinary quartz.  The absolute best of even that batch is chosen for the Grand Seiko 9R15 movement, a higher performance version of the 9R65 that is rated for 33% better accuracy.  The quality of the quartz used in the spring drive is likely identical to that used in the amazingly accurate 9F quartz movement, which outperforms the 9R/5R spring drive largely due to thermocompensation.

Suffice to say, even the quartz oscillator of the tri-synchro regulator is anything but ordinary.

Ok, we’re out of the weeds.  Let’s walk through the whole process one step at a time and we can move onto more abstract considerations for the spring drive.


First, the mainspring is wound, either by means of winding the crown (all spring drives can be handwound) or by the movement of the rotor due to the motion of your wrist.  In automatic spring drives, which are virtually all of them, the rotor is connected to the magic lever winding system.  That system winds the mainspring regardless of whether the rotor turns clockwise or counterclockwise.  One way or the other, the mainspring becomes wound and turns the gears inside the movement.  Those gears lead to a glide wheel, the first component of the tri-synchro regulator.  The glide wheel turns rapidly, which moves the permanent magnet (stator) that is attached to it.  That rotation operates as part of an electric generator, supplying electrical power to the IC, quartz oscillator, and electromagnetic brake.  The IC “detects” the speed of the glide wheel and compares it to the information from the quartz oscillator.  That information is used to control an electromagnetic brake which precisely, but gently, applies braking force to the glide wheel 8 times per second.  Therefore, the speed the glide wheel rotates is precisely controlled and, ultimately, the movement of the hands is nearly perfectly accurate.

So Is It A Mechanical Or A Quartz?

Ultimately, it’s a question of semantics, but I’d argue that the answer is much simpler than most suggest.  The answer to the question “is spring drive mechanical or quartz” is “no.”  It’s neither.  A spring drive is a spring drive.


Let me explain.

A spring drive cannot be a mechanical movement, because the implication is that it’s totally mechanical, which it isn’t.  It’s almost entirely mechanical, but the most crucial part of the whole movement is the tri-synchro regulator, so named because it regulates kinetic (mechanical), electric and electromagnetic energy.  It is partly electronic by its own admission, therefore, it cannot purely be mechanical.

But is it a quartz?  Well, certainly not under the conventional understanding, where a battery powers a motor to move the hands.  Sure, a crucial component is a quartz oscillator but to put it in the category of quartz watches is to dismiss the fact that there is much more to the function of a watch than simply a time base.  Watch movements combine lots of different independent functional units to make the whole.  Who can deny that the mainspring is a crucial component of a mechanical watch even if it’s not the mainspring that ultimately keeps time?  Or the automatic winding system?  The going train? The complications connected to it?  Or, in a quartz watch, the motor and battery play crucial roles even if they aren’t the quartz oscillator.


In other words, there’s much more to being a quartz or mechanical watch than just an escapement or quartz oscillator–you can’t reasonably reduce movement taxonomy to that fundamental level without losing crucial detail.  We can’t talk about spring drives like quartz watches without specifying that they’re spring drives–they’re too different.  And there’s a reason for that–they are different.


Thus, for reasons of both accuracy and to make language less confusing, it warrants its own distinct classification:  spring drive.

So We Know How It Works And What It Is:  Why Should We Want It?

Well, we’ll start with the obvious:  it’s really, really accurate.  How accurate are we talking about?  Officially, +/- 1 second a day or about 15 seconds a month.  The spring drive has been out for years now, however, and I’m comfortable saying that virtually all owners report much better accuracy than that–some reporting as little as a couple seconds per month.  Only the best mechanical watches can match that in a day, much less a month.  Even given that it has a quartz oscillator, how does it achieve better than average quartz accuracy?  The quartz crystals are grown in house, cherry picked for Grand Seiko (both the 9F quartz and 9R spring drive) and then aged–so even though it has a quartz oscillator, it’s one of the finest ever made.


But it’s not just more accurate than a mechanical watch, it’s inherently more stable as well.  Mechanical watches are greatly affected by their position relative to the earth–that is to say by gravity.  Spring drives aren’t, however.  It will achieve exactly the same performance whether it’s dial up or crown up, a comparison where mechanical watches are very lucky to have a difference of just 5 seconds.  It’s also much less susceptible to shock than a mechanical watch.  Where vibration tends to cause mechanicals to run fast, it will have no effect on the timekeeping of a spring drive.


An oft-overlooked, but very significant, source of instability is due to the difference in the amount of energy the escapement receives from the mainspring.  The mainspring does not contribute the same amount of energy at all states of wind–that is to say, the amount varies from fully wound to nearly empty–and that affects time keeping quite a bit.  But spring drives are totally immune to this issue.

Perhaps the most unique advantage of the spring drive, however, is the beauty of the seconds hand.  It is literally the only seconds hand ever made that is truly smooth.  Even high frequency mechanical movements, like Zenith’s El Primero or Grand Seiko’s own 9S85, or high frequency electronic watches, like Bulova’s Precisionist or Accutron, only provide the illusion of smoothness.  In fact, as you can hear when you listen closely, they are simply ticking very fast.


The spring drive never ticks.  There is no balance wheel to rotate back and forth like a mechanical.  The glide wheel always rotates in the same direction and is always advancing.  As a result, the only time the seconds hand stops is when the watch is dead.  While the glide wheel is slowed 8 times per second, the amount it is slowed, the fact that is never stopped, and the high frequency with which it occurs means that it not observable to a human, even in super slow motion.  The slow downs are simply too small and too frequent to be experienced consciously.  Consequently, the spring drive is usually considered to have the most beautiful sweep seconds of any watch ever made.  As an interesting side effect, it is also perfectly silent, without the characteristic ticking of all other analog designs.  This is why Seiko calls it “the silent revolution.”

Of course, like any movement design, it’s not perfect.  Like both mechanicals and quartz, it is susceptible to thermal instability–that is to say, its accuracy is slightly affected by changes in temperature.  The difference is very small, however, despite not employing a thermocompensated quartz system, and will not be noticeable on a day to day basis.  Even bad cases will not result in a difference of more than a few seconds a month.


The other concern of the spring drive is that it’s basically a mechanical watch and therefore, needs service much more frequently than a “pure” quartz watch.  The TSR may be extremely robust, far more wear and shock resistant than an equivalent escapement, but the rest of the movement is still “just” a high-end mechanical watch and those parts need oil and cleaning, just like your Omega or Zenith does.


But ultimately, unless you’re wearing a thermocompensated quartz like the 9F already (and to a small degree even then), those are issues you’re already dealing with.  The mechanical watch you have on needs occasional service and changes accuracy due to temperature.  The quartz watch you have on probably needs battery replacements and is also susceptible to loss of accuracy due to temperature.  Put another way, the spring drive has no new disadvantages, it simply shares some of the disadvantages that almost all high-end watches do.

The Conclusions

I’ve attempted to at least take a superficial look at what a spring drive is, how it works, and how we define it.  Now I’m sure Junya Kamijo would like to object to my vast oversimplification of this brilliant design, but this is the best that mere mortals like myself can do.

The spring drive is Seiko’s attempt to combine the strengths of quartz, that mainly being accuracy and stability, with the beauty of a mechanical, to create something altogether new.  They succeeded and the result has a beautiful advantage that neither of its constituent elements have:  a perfectly smooth and silent sweep hand.

The design has been out for more than a decade now, so the results are in:  it works very well.  The spring drive isn’t the end of the story, of course–Grand Seiko still loves their quartz and mechanical watches too.  But it’s the completion of a Hegelian triad–thesis, antithesis and synthesis.  There’s a place in our collections for all of these.


The Video

See the perfectly smooth seconds hand and beautiful movement of the Seiko spring drive in action here.

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