At first glance, the mainspring may not be one of the most exciting watch components. As opposed to dials and cases, it’s rarely the subject of heated discussion among enthusiasts. It just chugs along slowly, enclosed in the barrel, without making a sound. Nevertheless, this component – in addition to the escapement and the gear train – is one of the central functional elements of mechanical watches both manual and automatic. It may look unspectacular, but the mainspring has two important jobs: providing energy and sharing responsibility for the watch’s precision. Unsurprisingly, the materials and elaborate processes that go into mainspring production are well-guarded secrets among manufacturers and suppliers. Various design tricks and the use of multiple barrels allow for space-saving slim watches that still can still offer power reserves of over a week. So, it’s high time we took a closer look at this underrated component.
Springs make it possible for a watch to store energy, regardless of its position. That’s why they are the perfect choice for wearable timepieces, such as pocket watches and wristwatches, that can’t use the time-tested weight-driven methods of freestanding or wall clocks. The mainspring’s characteristic coiled spiral form was the result of the spacial limitations presented by a watch movement, as well as the need for rotation. In theory, other designs could work, but they would take up too much space and require the conversion from linear motion to rotation.
Essentially, the spiral-shaped spring behaves no differently from a simple flexible or leaf spring. Leaf springs are comparable to a ruler attached to the end of a table, where you press down on the free end. However, in the coiled spiral version, the entire length of the spring occupies as little space as possible. In this way, springs that are several inches or even well over a yard long, can be housed in a barrel which itself only takes up only a portion of the movement.
Layout and Special Features of the Mainspring
A mainspring’s stiffness depends on the material, as well as its length, width, and thickness. When you double the thickness, the stiffness increases eightfold. Length affects this property inversely: Double the length, and it will be eight times less stiff. The relationship between width and stiffness is linear, so if you double the width of the spring, you also double its stiffness. Because the width directly affects the thickness of the movement (and thus of the watch itself), wide mainsprings are best avoided. In summary, for any given material, the main variables are spring thickness and spring length. Since their adjustment theoretically affects the mechanical properties to the same degree, other factors must be taken into account, including the spring’s installation in the barrel.
Typically, a watch is wound using the arbor in the center of the barrel. This arbor hooks into a hole on the mainspring. The outer end of the spring meets the inner wall of the barrel, either with a fixed connection, or (in automatic watches) with a slipping mechanism that allows the end of the spring to slide around the inside of the barrel when fully wound. The mainspring thus occupies the space between the arbor and the inner wall of the barrel, and the goal is to use that space as efficiently as possible. Minimizing the arbor’s diameter has some natural limitations. The mechanical strength of the arbor must be preserved. In addition, a small arbor diameter results in increased bending at the inner end of the mainspring, which must not exceed the physical limits of the spring material. In order to withstand this bending, the spring cannot be too thick. Ultimately, the amount of space available dictates the limit for the barrel’s diameter.
It becomes clear that there is no one “perfect” mainspring for every watch. Several factors must be considered, and the adjustment of one of the spring’s geometrical parameters inevitably influences the others. That’s why calculating the dimensions of a mainspring and its barrel is an iterative process, made simpler by empirical values and approximation formulas. Just because the barrels are the same size doesn’t mean that different movements can use the same mainspring. Ideally, the mainspring should be custom tailored to the given movement, and it should be designed to achieve the maximum power reserve (a certain number of rotations) while suppling sufficient power for the watch to run.
It should also be noted that the spiral-shaped mainspring’s compact form does come with some inherent disadvantages. One general property of springs presents a problem for implementation in watches: The spring force increases with deflection (i.e., additional winding), and this fluctuating driving force causes a movement’s frequency to vary over time. We’ve already discussed the remedy to this issue in a previous article. However, this solution generally only appears in the most expensive timepieces, so the overwhelming majority of watches must simply make do with this problem. You can never get rid of it completely, but there are some tricks that reduce it to a minimum. One such trick is a reverse curve in the mainspring, which can only be seen when the spring is removed from the barrel. In their relaxed state, modern mainsprings don’t look like a pure spiral, but rather their ends are bent backwards, forming an S-shape. When installed in the barrel, this shape results in a more even power output as the spring unwinds.
As mentioned above, mainsprings are made from highly specialized materials. As the “heart” of the watch, the balance spring is usually the main focus of reports and advertising campaigns from manufacturers. With proprietary alloys or the use of silicon and carbon, considerable progress has been made in this area over the past decades. However, the development of alloys for mainsprings has reached some milestones of its own.
Collectors of vintage watches manufactured before the second half of the 20th century are familiar with the most common downside of historical mainsprings: The carbon steels used back then were subject to rust, wore out over time, and tended to break due to their brittleness. Relief came from engineer Reinhard Straumann, whose invention “Nivaflex” made the “unbreakable” mainspring suitable for mass production. If you notice a similarity to the name Nivarox, you’re on the right track because this well-known alloy found in balance springs is also attributable to Straumann, as is the establishment of Nivarox SA in 1951. Nivaflex consists primarily of cobalt, nickel, and chrome, with small quantities of iron and tungsten and a fraction of one percent beryllium. Both Nivarox and Nivaflex are produced today by the highly specialized company Vacuumschmelze in Hanau, Germany. Nivaflex has anti-magnetic propertied, is corrosion-resistant, and boasts a high-performance spring material that is resistant to breakage. On modern manualwatches, you really have to make an effort to break a mainspring by hand.
Manufacturing a Mainspring
Spring manufacturers receive Nivaflex as a wire, which is turned into a strip by a rolling process. Sections of the long strip are cut off, and an eyelet is punched out on one end for the hook of the arbor. After bending the ends into the S-shaped reverse curve, there is a heat treatment, which sets the spring’s final mechanical properties. The end hook or slipping spring (depending on whether the spring is for a hand-wound or automatic timepiece) is then welded onto the outer end. Because friction is undesired between the individual turns, the springs get a Teflon coating. The spring is then delivered, either pre-lubricated and installed in the barrel, or wound up in a transport ring.
The most famous manufacturers of mainsprings include companies like Générale Ressorts and Schwab-Feller in Switzerland. The fact that Patek Philippe, Rolex, and the Richemont company took shares in Schwab-Feller in 2015 attests to the importance of high-quality mainsprings. Carl Haas of Schramberg, Germany is a spring manufacturer that makes not only mainsprings but also balance springs for NOMOS Glashütte, among others.
When One Spring Isn’t Enough: Watches with Two or More Barrels
Since the debut of the Lange & Söhne Lange 1 with double barrels, fitting more than one barrel into a movement has become an indicator of exclusivity. Chopard‘s L.U.C-Quattro takes it a step further with four barrels. The high power reserves that go hand-in-hand with multiple barrels are particularly practical for collectors and owners of many watches. They enable you to leave such watches alone over the weekend or even a whole week without having to reset the time and date afterward. This extended power reserve is typically achieved through a series of interconnected barrels, which in effect increases the movement’s mainspring length. The impressive record holder in this type of construction is the Hublot MP-05 LaFerrari, in which ten interconnected barrels allow for a power reserve of 50 days. This watch comes with an electrical winding tool. Since the ratchet wheels of the barrels, which are connected to the arbor, have to mesh with one another, the movement requires additional gears.
An alternative is to use two barrels in parallel. This version does not require additional gears because both barrels act on the center wheel directly. If there is enough space to arrange the barrels symmetrically around the center wheel, then the otherwise one-sided bearing forces on the center wheel can be virtually balanced out. The spring forces are added together in the parallel construction, so when the spring lengths are equal, a second barrel results in twice the force as opposed to the single-barrel design. In practice, this is used to decrease the height of the springs by half, allowing for particularly flat movements without having to make sacrifices in the driving force or power reserve. However, this type of construction is rather rare compared to serial barrel arrangements. There are also models, such as those in the Jaeger-LeCoultre Duomètre collection that have separate barrels driving different functions in the watch (for example, one for the movement and one for the chronograph).
Whether your watch has one, two, or even more barrels, these unimposing cylinders contain more power than you might initially assume. Particularly with manual watches, the wearer interacts directly with the mainspring. In automatic watches, this interaction is less direct, but the mainspring component is no less relevant. Perhaps you’ll think about this the next time you look at your favorite timepiece.