One of the advantages of the mainspring’s coiled shape is a more nearly equal distribution of tension along the entire length of the spring, which goes hand in hand with relatively constant torque. A fully wound mainspring provides very strong driving torque, and a mainspring in its middle range between fully wound and totally exhausted delivers a relatively constant level of torque. The force declines significantly as the mainspring’s tension lessens. The barrel and the mainspring inside it play a fundamental role in the design of every movement. The center of a classically constructed caliber is occupied by the hour wheel and the cannon pinion, so the maximum diameter of the barrel is limited by the radius of the plate. The barrel’s rotational speed defines the loss of torque during the first 24 hours after the mainspring has been fully wound. Afterward the mainspring in a hand-wound watch is usually given a complete tightening.
A small translation ratio to the center wheel’s pinion and fast rotations of the barrel combine to minimize loss of torque. A typical mainspring should unwind after eight to 10 turns. From these parameters, engineers calculate the ideal thickness of the mainspring, which should fill half to 55 percent of the barrel. Torque arriving at the escape wheel should decline by no more than 15 percent the first day. The calculation of the entire gear train is oriented using these criteria so the watch’s movement runs at a regular rate.
The frequency of the balance plays an essential role in all calculations. As the balance’s frequency increases, the movement’s running autonomy decreases. Skillful construction can achieve up to a week’s running autonomy with only one barrel: the Swiss company Hebdomas proved this in 1913, when it introduced a watch with a big barrel that covered the entire movement. Ulysse Nardin used a similar construction in its Freak in 2001. The 1930s saw the advent of eight-day movements with standard barrels, modified gear trains and tiny balances paced at a frequency of 2.5 hertz. Today’s four-hertz rapid oscillators can usually run for 72 hours without a fresh dose of energy. IWC’s Calibers 51011, 51111 and 59210 demonstrate that a single barrel and a balance paced at 28,800 vph can achieve a full week of running autonomy.
Dividing the driving torque between two mainsprings is by no means a new idea. Henri Louis Jaquet-Droz employed this technique as early as 1785. Abraham-Louis Breguet also devoted considerable attention to optimizing the energy supply for his chronometers: two barrels acting simultaneously on one center pinion not only replaced the conventional chain-and-fusée system, they also enabled Breguet to reduce the mainspring’s thickness by half. The Favre-Leuba company caused a small furor in the watch world when it debuted its three-mm-slim caliber family 25x and 27x in 1962: both movements were hand wound and, similar to Breguet’s invention, each had two barrels acting on the center pinion. The thickness – or rather the thinness – of the springs was quite impressive: each was a mere 0.05 mm thick, yet produced 9 1/4 barrel rotations and approximately 40 hours of running autonomy. The advantages lie in reducing one-sided bearing pressure on the minutes wheel and in the fact that thinner mainsprings develop their force more uniformly. On the other hand, the height increases with parallel switching of two barrels and their collaborative action on the center pinion.
Glashütte master watchmaker Alfred Helwig took a different approach. He relied on a pair of serially switched barrels that act one after the other. After the first mainspring has been fully wound, it then begins to tighten its counterpart. This arrangement doubles the overall length of the springs. Longines first used this principle in its caliber family 89x with two piggyback barrels in 1975. The subsequent Caliber L990 was much slimmer, partly because its two barrels were positioned side by side. The advantages of two quickly rotating energy reservoirs are apparent: lower torque reduces the forces acting on the gear trains while simultaneously enhancing performance. The winding mechanism can work more efficiently in self-winding movements. Partly because of these advantages, this principle can be found increasingly in new constructions. Running time also increases when the length of one mainspring is added to that of its companion. Depending on the details of their construction, most of these calibers run for two to eight days. The pinnacle was reached by A. Lange & Söhne and its Caliber L.034.1 in the Lange 31, which can run for 31 days without a fresh dose of energy.
Panerai installed three serially arranged barrels in Calibers P.2002, P.2003, and P.2004, which resulted in a minimum of eight days of running autonomy. Blancpain followed suit in automatic Calibers 5235DF and 6938, each of which achieves 192 hours of running autonomy. Chopard packed a quartet of barrels into the L.U.C Quattro: four mainsprings, each 470 mm long, give Caliber L.U.C 1.98 a running time of at least 216 hours, or nine days. Vacheron Constantin’s tourbillon Caliber 2253 runs for 14 days with one winding. TAG Heuer’s linearly self-winding Caliber V4 has 52 hours of running autonomy.
Enrico Barbasini, Michel Navas and Mathias Buttet, founders of the now defunct Swiss movement maker BNB Concept SA, joined with Jacob & Co. to create its hand-wound Quenttin tourbillon with Caliber 5 that has a frequency of three hertz, seven barrels arranged side by side, and 31 days of running autonomy.
Far more important than increasing the number of barrels is the technical evolution of the energy reservoir per se. With an overall concept designed to maximize energy efficiency, Cartier’s concept watch, the ID Two, achieves more than a month of running autonomy with four normal-sized, polymer-coated barrels made of fiberglass-reinforced material.
This article first appeared in the April 2013 issue of WatchTime Magazine.
Dear Mr. Brunner,
A very interesting article. Thank you for supplying so many physical characteristics for mainspring material. However I wish you could clarify one point. You sad: “… mainspring … have an extremely high tensile strength of up to 3,000 megapascals (equivalent to 300,000 meters)”. As I know the tensile strength we are measure in pascals or PSI or Kg/mm2 or … but not in meters. A meter is a unit of length. So, 3,000 megapascals = 3 MPa = 435 PSI = 0.3 KG/mm2 = 305914.9 KG/m2
Note: (m2 = sq m).