How a watch work
In a new series, Atelier Manager Damian Ahcin explains what goes on inside a watch in a simple and understandable way. Part 1: Energy storage.
Have you ever wondered where the energy comes from that powers all the gears, complications and hands of a wristwatch? Where and how is the energy stored, and what is needed to prevent it from dissipating wastefully?
With a wall clock, the principle is simple: a chain hangs from a drum with a weight dangling from the end. You turn the drum, winding the chain around it and pulling the weight upwards. As it is pulled down by the force of gravity (but is prevented from doing so by a blocking mechanism), this creates tension. The controlled unrolling of the chain causes the drum to rotate, and it is this movement that energises the gear train.
So far, so good. But obviously you can’t use dangling weights in a wristwatch. So how do we store energy? And how do we get the rotary movement to drive the gear train?
The solution is a small, fine but extremely strong strip of steel: the mainspring. It slumbers curled up in the mainspring barrel – sometimes tightly coiled (when the watch is freshly wound), sometimes a little more relaxed (when the watch urgently needs to be wound again). In between is pure energy, because the spring wants nothing more than to expand into its original state, but is only allowed to in the tiniest increments, tick by tick.
So that the spring can maintain its tension and develop the necessary force, it has to be made of a special steel alloy. Try asking a watchmaker – his eyes will light up: for him, the structure of this alloy is as thrilling as the secret formula of Coca-Cola. Nobody knows the exact proportions of the composition except the very few specialised manufacturers.
But back to our steel spring: the longer it is, the longer the watch can run without having to be wound. This is known as the power reserve. Eight days is quite possible with one mainspring i.e. with a single barrel. Two would be ideal to distribute the energy better. But it can also be taken to extremes: Hublot has developed a model in which eleven barrels connected in series guarantee a power reserve of 50 days. The length, thickness and diameter of the mainspring vary from calibre to calibre; there is no standardisation. The mainspring of a Unitas pocket watch movement from the Beyer watchmaking course measures 44 centimetres in length (in a barrel with a diameter of 14.5 millimetres!) and is just 0.175 millimetres “thick”. The spring in the movement of a small ladies’ watch, on the other hand, can be 18 centimetres short and 0.065 millimetres thin. By comparison, the diameter of a normal hair is about 0.07 millimetres.
So we have the mainspring, and we have an axle around which the spring is coiled, called the barrel arbor, to which the mainspring is fixed. If you have a hand-wound watch, the other end of the mainspring is also fixed to the inner wall of the barrel. By turning the winding crown, you cause the barrel arbor to rotate (via a transmission), while the barrel remains fixed. The mainspring tightens more and more around the core – until you can feel it has reached a stop. With a hand-wound watch, you should stop winding immediately; otherwise the end hook with which the mainspring is attached to the barrel arbor may break.
TINY STORAGE SPACE
Simply through the winding of the spring, the movement is able to store all the energy it needs to power its components.
- The mainspring barrel is a cylindrical case. The barrel arbor rotates in the centre. An indentation in the barrel wall allows the mainspring to be fixed in place.
- The mainspring is a very thin strip of metal that wants nothing more than to return to its original state.
- The barrel arbor is a rotating axle that pivots between the bridge and the mainplate. Mounted on it is the core, a cylindrical component with a hook to secure the inner end of the mainspring.
- The barrel cover closes the case and acts as a bearing for the barrel arbor. On the bridge above the barrel there sits what is known as the click, which stops the barrel arbor from turning back by blocking the gear wheel.
THE BUZZ OF THE CLICK
In an automatic watch, which winds itself thanks to its oscillating weight, what is known as a slipping spring is attached to the barrel arbor only. At the other end, it divides into two lips that are not fixed. Why? Because the oscillating weight works continuously and does not stop producing energy even when the mainspring is fully tensioned. If the end were fixed, at some point it would break. Although the two lips of the slipping spring provide resistance by spreading with all their might, they are simply dragged along as the fully wound spring continues to rotate. That’s why you can wind your automatic watch with the crown for as long as you like – you won’t do it any harm.
So our mainspring is now tightly wound around the arbor. To prevent it from unwinding, there’s an attachment called a click. Using a tiny pawl, it engages the toothed gear that is mounted on the arbor and holds it in place. The work of the click can be heard when a watch is wound by hand. It’s that subtle buzzing sound: with each winding movement, the pawl briefly pops out of the gear wheel and immediately locks in the next gap.
In other words: when we actuate the winding crown or mobilise the oscillating weight, we rotate the gears that turn the barrel arbor, coiling the spring around itself. In this way, we store the energy generated by the winding motion. In the next issue of beyond, we’ll be explaining how this energy is released in a controlled manner and how the gear train, as a transmission device, ensures that nothing goes too fast and nothing goes too slow.
There’s another nice detail in antique clocks that we often come across in our workshop: in the old days, it was customary for watchmakers to immortalise themselves on the mainspring after a service or an intervention, using a scriber’s needle to inscribe the year or their name, the latter sometimes in elaborate lettering. This means that for watchmakers, old mainsprings are a secret that we look forward to discovering when opening a movement: “Another detail that keeps such a watch, its creators and a bygone era alive.”
