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06/10/2024
 6 minutes

Development, History, and Technology of the Microrotor

By Tim Breining
Microrotor-2-1

Development, History, and Technology of the Microrotor

Of the 586,000 watches listed on Chrono24, more than 380,000 are automatic timepieces, making them the most popular category. You’ll also find 58,000 hand-winding watches here, much less frequent than the 114,000 quartz timepieces on Chrono24. And among the 380,000 automatic watches, you’ll find a remarkably small number of watches whose rotors occupy only a peripheral space among the bridges and wheels of their movements, rather than being enthroned on top of them. We’re talking about microrotor calibers, of course. Why were they invented? What are their advantages and disadvantages? Which brands are using them in their watches and furthering the technology? Read on to find out.

Microrotor watches: exclusive, complex, and expensive?

Microrotor watches are now most commonly found in haute horlogerie brands like Bulgari, Patek Philippe (in its 240 caliber series), the Parmigiani Fleurier Tonda, the Laurent Ferrier Classic Micro-Rotor, or the Czapek Antarctique. Our colleague Sebastian took a look at a few of these in his article What Is a Microrotor Watch? Three Current Models in Comparison.

Patek Philippe Kaliber 240 HU
The Patek Philippe 240 HU Movement

Brands sometimes like to use a microrotor movement even in their most complicated timepieces. One reason for this is that this caliber design allows a clear view of the fine surface engraving and finishing without a large rotor getting in the way. This may lead some aficionados to believe that microrotor watches are difficult and expensive to manufacture, and are therefore reserved for the higher end of expensive watchmaking. But that’s actually not how it is.

The Buren Watch Company’s Brilliant Innovation

At the end of the 1950s, two Swiss watch brands released their microrotor innovation around the same time, with Buren presenting the Super Slender featuring a microrotor caliber, and Universal Genève releasing its Polerouter. Apparently unaware of what the other was doing, the companies had worked on this movement simultaneously. Universal Genève coined the name “microrotor” and proudly engraved it on one of the bridges. Hans Kocher, who was in charge of developing the microrotor at Buren, preferred the term “planetary rotor” instead, which never caught on and is now largely forgotten in the watch world.

Today, many attribute the creation of the microrotor to the cult brand Universal (which is currently enjoying a revival of sorts). Something that probably helps fuel this belief is the fact that design legend Gérald Genta was involved in the creation of the Polerouter.

But who was the actual pioneer? Well, depending on your sources, you’ll find conflicting information from both brands when it comes to the exact date of invention. What we can be sure of, however, is that the existing patents make it legally clear that the Buren Watch Company was the first to patent microrotor technology. Buren’s chief design engineer, Hans Kocher, filed patent CH329804A in Switzerland in 1954, while Universal Genève filed their patent CH329805 in 1955.

Zeichnung aus einem späteren Patent von Hans Kocher für die Buren Watch Company, Quelle: Patent US3306025A
Later patent designs by Hans Kocher for the Buren Watch Company (Source: Patent US3306025A)

As a result, Universal Genève had to engrave “PATENTED RIGHTS PENDING” on its movements for a time, which they did on the plate on which the microrotor sits. At the same time, Buren, which also licensed its design to other companies, agreed with Universal on a reduced royalty of four Swiss francs per movement for the use of Buren’s technology. Universal’s patent was granted in 1958, although it remains unclear how this was possible given that Buren’s was clearly the first to be filed.

Why re-invent the automatic rotor?

Now that we know the backstory, why exactly was the microrotor invented in the 1950s, when mechanical watches were the only practical way to tell time on the go?

That was back when timepieces had to be reliable, elegant, and compact, and not necessarily beautifully crafted luxury watches. In a conventional self-winding caliber, the rotor contributes significantly to the overall thickness of the watch. In his book Automatische Uhren, Kocher mentions patents from the 1940s that already suggested the idea of moving/integrating the automatic winding mechanism into the caliber, rather than on top of it. It was Kocher who would go on to achieve this and patent it.

Since the development of the microrotor was predicated on aesthetic ideas, some compromises had to be made in terms of the watch’s functionality. To understand this, let’s take a look at the basic design aspects of a watch’s winding rotor.

Die Universal Genève Polerouter unterlag im Patentstreit mit Buren
Patent battle runner-up: the Universal Genève Polerouter

Automatic Watch Winding Rotors: The Basics

Automatic watch rotors have several aspects that affect their winding performance. One of these, of course, is their weight.

Watchmakers make rotors from materials that are as dense as possible and don’t cost a fortune. A great option here is pressed tungsten alloys with a density second only to that of platinum, which is very expensive (fun fact: even gold is lighter than tungsten).

Furthermore, static torque acts on the winding mechanism as the wearer moves his or her forearm gently throughout the day, and the rotor’s moment of inertia comes into play when the wearer moves his or her arm quickly and suddenly, such as during exercise. Both of these forces are particularly strong with a rotor that has as much of its mass as possible on its outer edges; this is especially true of the moment of inertia.

Ein konventioneller Rotor, hier aus einem Rolex-Werk, mit einem Kreisbogen kleiner als 180 °
A conventional rotor from a Rolex movement with an arc length of less than 180°

A rotor optimized for fast winding is not fully semicircular, but has an arc length of between 120° and 160°. This means that it will wind the watch less effectively if the wearer makes gentle movements with the forearm; if the wearer wants the watch to wind faster with this type of rotor, he or she will have to move the forearm faster and more abruptly, e.g., when playing sports. In fact, most rotors have an arc length of 180°, making them ideal for winding by slow, gentle forearm movements.

Disadvantages of a Microrotor

These principles make it easy to understand why the winding performance of a microrotor is often not up to par with that of its larger counterpart. As we saw above, it’s not just sheer mass, but also the mass of the rotor being as far away from the axis of rotation as possible that makes the difference in an automatic movement’s ability to wind its mainspring; a microrotor simply can’t match the performance of a conventional one. Although a microrotor embedded in the movement itself can be positioned for effective winding performance, it can never deliver the same winding as a centrally positioned, larger rotor that is centered and directly over the watch’s axis of rotation.
And while a microrotor allows for a thin watch, it takes up space in the movement itself, which means less room for the gear train and a large mainspring. Average winding performance and reduced power reserve are far from an ideal combination, and are very noticeable in some microrotor watches from years past. However, modern microrotor calibers like the Yema CMM.20, with its 70-hour power reserve and four-hertz pulse, show that this theoretical disadvantage doesn’t necessarily have to be the case, thanks to very efficient, optimized components. On the other hand, Patek Philippe’s time-tested, albeit old-fashioned, 240 series calibers offer a power reserve of 38 hours in some of the watches they power – not exactly what we’ve come to expect from contemporary timepieces.

Parmigiani Fleurier Tonda Microrotor
Parmigiani Fleurier Tonda Microrotor

The Future of the Microrotor

Not every decision made about mechanical watches falls into the “perfectly rational” category. So it’s not surprising that the microrotor is generally enjoyed by the watch world, especially when considering how thin some of the watches are that have one. Enthusiasts have the notion of exclusive watchmaking in their heads when they think of a microrotor, which is reflected in the marketing for these watches any time new models are released. Along with Yema’s new CMM.20 caliber mentioned above, Vaucher Fleurier makes a standard microrotor movement for its in-house partner Parmigiani. The Chinese Hangzhou 5000a microrotor caliber, ticking away inside the Baltic MR01, made a big splash for the French watchmaker, with this timepiece going for sums well over its list price – a Patek homage for every budget. The BVL 138 powering the raved-about, much-loved Bulgari Octo Finissimo is a microrotor movement whose ultra-thin design puts the brand’s watchmaking prowess on full display.

As we can see, there’s no shortage of new or popular watches powered by microrotor movements. Is it any wonder that enthusiasts continue to be fascinated by this charming technology? It’s great to see newer brands making microrotor watches in the affordable segment as well. I can’t wait to see what timepieces with this wonderful construction are yet to come.


About the Author

Tim Breining

My interest in watches first emerged in 2014 while I was studying engineering in Karlsruhe, Germany. My initial curiosity quickly evolved into a full-blown passion. Since …

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