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Feature: How An Automatic Watch Works

When you wear an automatic watch, it never stops, never runs out of power. Is this magic? Some breakthrough in perpetual motion? No, of course it isn't, but it is still pretty clever, and it's taken the best part of a millennium to perfect. Let's see what all the fuss is about.

Watch our video review of the Patek Philippe Calatrava 5180/1R-001 to find out how an automatic watch works.

Watches and clocks have been ticking under mechanical power in some form or another since the third century BC, but it was the ground-breaking invention of the escapement in the 13th century that moved timekeeping into a new era. Regulating the release of power from its source, the escapement allowed clockmakers to explore new methods of providing drive to their creations, such as weights and, later, springs.

The advent of spring power allowed the mechanisms to be shrunk small enough to fit inside a portable case, and so, in the 15th century, the pocket watch emerged. All it needed was a quick wind every day or so and it would tick merrily away with no problems—and that's how things stayed until 1777, when Abraham-Louis Perrelet, presumably sick of the exertion needed to wind his watch every morning, developed a mechanism that converted movement to power.

This was a great idea, in theory ... except that it was for a pocket watch, which doesn't experience much in the way of movement. A few other companies mimicked the idea, including Breguet, but they all came to the same conclusion. The idea was shelved for another century and a half.

With the turn of the 20th century came a change in fashion. Driven by wartime convenience, the pocket watch had emigrated to the wrist, and now the automatic movement started to make sense. British watchmaker John Harwood made the connection first, in 1923, fashioning a pivoting weight to his watches that wound the mainspring with the wearer's movement.

The idea had legs this time, but it wasn't perfect. The spinning weight could only travel so far, bouncing between two stops as it moved. Rolex jumped on the chance to polish the idea, launching the Perpetual movement eight years later. This time, the weight could spin freely in either direction, unrestricted and capable of harvesting more power from less motion. This is the automatic watch as we know it today.

The Patek Philippe Calatrava 5180/1R-001

The Patek Philippe Calatrava 5180/1R-001

To aid this journey through the innards of an automatic watch, Patek Philippe has very kindly stripped away much of the movement that would typically hide the mechanism in this 5180 Calatrava. Let's take a closer look at the calibre 240 SQU.

It all starts at the rotor weight, here embedded into the movement as a micro rotor. As the wearer moves through the day, the mass of the rotor swings its centre of gravity as low as possible, pivoting on its free axis. Teeth engage its motion to the keyless works—so-called because of the absence of a winding key, previously required on older pocket watches—to transfer the power into the reduction wheels, which are geared to increase the torque needed to tighten the mainspring from the gentle wind of the rotor weight. By overlapping the wheels and connecting the drive with pinions—smaller wheels—the gearing can be more compact.

But what if the rotor weight spins the wrong way, does it unwind the movement? Thankfully not, and that's achieved in this example through the disengagement of the transfer wheel, which rocks into place when turned in the correct direction, and out when it is not. Some movements even use click wheels or switching rockers to provide power whichever way the rotor spins.

The power travels through to the mainspring via the ratchet wheel—also known as the first wheel—but that's not the only way it can get there; the movement can be hand wound as well. Turn the crown and the winding stem drives the crown wheel, which also feeds the ratchet wheel. Either way, the turning of the ratchet wheel tightens the coil of the mainspring, which is connected via a hooked post called an arbour. The click spring, a sprung pawl, prevents the ratchet wheel from spinning back on itself and unwinding the mainspring.

The skeletonised calibre 240 SQU hides nothing

The skeletonised calibre 240 SQU hides nothing

At the other end of the coiled mainspring is the mainspring barrel, which it sits snugly inside. For automatic watches like this one, notches and a special grease are used to allow the mainspring to slip to prevent over-winding. As it is wound tight, it naturally wants to uncoil, but the click spring prevents the ratchet wheel from unwinding, and forces the mainspring barrel to turn instead. Teeth on the edge of the mainspring barrel engage with the centre wheel—also known as the second wheel—advancing the power to the motion works that drive the hands. But what stops the mainspring simply unwinding all in one go and spinning the hands into a frenzy?

This is where the escapement comes in. Also driven by the centre wheel, the power comes down through the third wheel, then the fourth wheel, gearing the hourly rotation of the centre wheel into one rotation per minute. The fourth wheel is also used to drive the second hand when it is located in a sub-dial.

The fourth wheel turns the escape wheel, whose teeth are very different to all the other wheels in the motion works, which enable it to facilitate a unique function. As it turns, one carefully angled tooth gives the long entry jewel in the pallet lever a nudge. This nudge pivots the pallet lever about its staff, which engages the exit jewel with the escape wheel, locking it at the next tooth.

At this point, the watch should jam and come to a halt, but the pivoting of the pallet does something else as well: it knocks the impulse jewel on the balance wheel, which spins on its staff, coiling the balance spring. As the spring coils, it slows the balance down, then reverses its direction. The balance spins back again, knocking the pallet lever back to centre, moving the exit pallet to unlock the escape wheel.

As the exit pallet clears the escape wheel, the escape wheel gives the exit pallet a nudge. This swings the pallet lever back to its original position, locking the entry pallet against the escape wheel and giving the impulse jewel on the balance wheel a push. The balance wheel swings, slows, returns and knocks the pallet lever back to centre, releasing the entry pallet from the escape wheel—back where we started.

A micro rotor provides power when the watch is worn

A micro rotor provides power when the watch is worn

This intricate dance, where the escape wheel is momentarily locked and unlocked over and over again—here, six times per second—is what controls the unwinding of the mainspring, regulating its speed. Now, the motion works can be used to drive the hands.

Whilst on most movements the centre wheel usually sits in the centre, and—as it turns once per hour—directly drives the minute hand, here on this calibre 240 SQU the centre wheel is offset. Instead, the centre wheel pinion turns the minute wheel, which drives the hidden cannon pinion—the cannon pinion is usually connected directly to the centre wheel—onto which the minute hand is mounted.

A thin pinion on the minute wheel meshes with the hour wheel, which slips over the cannon wheel and spins freely. The hourly rotation of the minute hand is geared by a factor of twelve to the hour wheel—here's where the hour hand sits. For watches with central seconds, the drive from the fourth wheel is fed through the centre of the cannon pinion to the second hand.

And there you have it. That's how an automatic watch is able to take the motion of your wrist and convert it into hours, minutes and seconds.

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