The standards for what makes a "good watch" vary from person to person. However, some easy-to-understand points include the finish, construction, and performance. This article explains how to "distinguish a good watch" from these points, and is about how to appraise a high-quality watch. This time, we will focus on the balance spring, one of the heart of a watch.
http://www.webchronos.net/features/119515/
Interview and text by Masayuki Hirota (Chronos-Japan)
[Article published in the July 2020 issue of Kronos Japan]
A good watch as seen from the balance spring
Because it is produced by etching, theoretically it is not possible to wind the outer end of a silicon balance spring. However, Breguet has developed a silicon winding balance spring by connecting cut silicon parts. However, according to Breguet's vice president, "we placed more importance on appearance than theoretical performance." Due to the manufacturing costs, it is only used in a few models.
The balance wheel is the heart of a watch. It consists of a balance wheel and a hairspring. What is important about the hairspring is that it is resistant to magnetism, resistant to shock, and that it does not shift from its center even when it expands and contracts. Let's take a look at hairsprings from these three points.
"Curly beard" and "flat beard"
Considering performance alone, the conditions for a good balance spring are set: it must be resistant to magnetism, not easily deformed when subjected to shock, and not easily misaligned when expanding or contracting.
Magnetism is largely dependent on the material, but the latter two can be improved. To make it less likely to deform, simply use a stiffer balance spring. High-frequency movements generally have stiffer balance springs, which improve shock resistance and reduce positional error. To make it less likely to shift center when expanding or contracting, a practical solution is to wind up the outer end of the balance spring (winding hair). This is an attempt to keep the balance centered by lifting the outer end of the balance spring inward even when the balance spring expands or contracts. However, a regular Breguet hairspring does not significantly improve isochronism. To achieve more desirable results, the "Phillips-type" winding hairspring developed by Edouard Phillips is considered ideal.
Surprisingly, all current Rolex models use a highly isochronous hairspring. Rolex is the only manufacturer capable of mass-producing a winding balance spring that, in theory, has a variable winding time. It's called the Parachrom hairspring. Made from an 85% niobium and 15% zirconium alloy, it's oxidized to provide far superior anti-magnetic and shock resistance compared to cobalt- or nickel-based hairsprings.
Grand Seiko's new movement also features a free-sprung balance, as well as a highly isochronous winding hairspring. While details are unknown, it was apparently created through 8 simulations. Furthermore, by twisting the rotating hairspring, the outer end of the winding hairspring can be adjusted to adjust accuracy. This is likely the only mechanism in the world. The material is the same sprung balance as the existing movement.
However, this is currently only used by a very small number of manufacturers, including H. Moser & Cie., Renaud & Papi tourbillons, and some independent watchmakers. As far as I know, the only watch that is superior is Voutilainen's "Vintuit," which uses a hairspring with a Phillips-type outer end curve and a modified inner end curve (the part that unwinds from the collet), resulting in better isochronism.
However, in recent years, we have seen the appearance of theoretically superior balance springs. A. Lange & Söhne has completely redesigned its balance spring, evolving its winding hairspring to achieve even greater isochronism. Furthermore, the new winding hairspring used in Seiko's 9SA5 also has a shape derived from 8 simulations. While details are unknown, its theoretical performance is said to be superior to existing winding hairsprings. However, winding hairsprings are not suitable for mass production, and because they are wound by hand, there is considerable individual variation. François-Paul Journe is skeptical of the use of winding hairsprings, saying, "Rather than using a poorly winding hairspring, it is better to use a stable flat hairspring."
Compared to a hairspring wound around the periphery, a flat hairspring makes it easier for the balance wheel's center of gravity to shift. However, it is extremely difficult to mass-produce a hairspring with consistent quality. For this reason, many manufacturers have adopted the approach of using a flat hairspring to improve accuracy. One of the most successful examples is Breitling, which uses a hard hairspring that is less likely to deform and improves accuracy through careful adjustments.
Thin movements cannot have a winding balance spring either. For this reason, all thin movements use a standard flat balance spring. A typical example is Nomos. Nomos invested 12 million euros to establish a system for producing their own balance springs. As a result, most of their movements use in-house balance springs, but only the Alpha uses Nivarox. Normally, high-end mechanical watches adjust their accuracy in five positions, but Nomos does it in six.
However, there are also improvements to the standard flat balance spring. Nivarox flat balance springs are annealed on the outside to soften them and improve the balance spring's shock resistance. The company calls them Nivacourbe.
The Choice of a Silicon Balance Spring
The silicon balance spring is an attempt to simultaneously improve both magnetic resistance and shock resistance. In particular, its magnetic resistance is incomparable to existing balance springs. In addition, the latest silicon balance springs have been designed with an innovative shape to create a winding hairspring effect. Good examples include the silicon winding hairsprings of Patek Philippe and Breguet (!), and Rolex's Syloxi hairspring. Also, in recent years, there have been attempts to make the collet smaller, taking advantage of the advantage of being able to mold the collet as a single piece.
Patek Philippe uses silicon balance springs. Although they are flat, they theoretically have better isochronism than winding balance springs. This is because, rather than shaping the inner and outer end curves, the thickness (i.e. elasticity) is changed in parts. This has the same effect as deforming and reinforcing the balance spring, making it less likely for the center of gravity to shift. This is a balance spring that takes advantage of the benefits of silicon, which is processed by etching.
Rolex has announced a new silicon hairspring called the Syloxi hairspring. According to the company's materials, it boasts excellent magnetic resistance, maintains high stability despite temperature changes, and is 10 times more shock-resistant than conventional hairsprings. Its patented shape also ensures isochronism in any position. This is one of the reasons why Rolex women's movements achieve an accuracy of ±2 seconds per day even after casing.
Furthermore, considering the ease of maintenance, it is better for the balance spring to be made of metal rather than silicon, and it is preferable for the outer end of the balance spring to be fixed to the balance stud with a traditional rivet or screw fastener rather than laser welding. As shown in the photo, Voutilainen's Cal. 28 has the outer end of the balance spring attached with a screw.
As befits a high-end watch, the Cal. 28 uses a winding balance spring. The material (ébauche) is manufactured by Precision Engineering. The outer end is wound according to a typical Phillips curve, but unusually for current models, the inner end also has a Grossmann-type curve. As far as I know, there are only a few watch movements in existence that have been modified down to the inner end. In theory, this makes it difficult for the balance spring's center of gravity to shift, resulting in excellent isochronism.
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