Mechanical Watch – Bartosz Ciechanowski

In the world of modern portable devices, it may be hard to believe that merely a few decades ago the most convenient way to keep track of time was a mechanical watch. Unlike their quartz and smart siblings, mechanical watches can run without using any batteries or other electronic components. Over the course of this article I’ll explain the workings of the mechanism seen in the demonstration below. You can drag the device around to change your viewing angle, and you can use the slider to peek at what’s going on inside: What you see here is known as the movement – the inner part of a mechanical watch that’s usually enclosed in a metal case. In this article I’m focusing on a watch movement itself, since beautiful watch cases merely hide the intricate mechanisms which are the real stars of the show. The entire watch movement has a lot of parts, and in this blog post I’ll explain the purpose of each one. The world of watchmaking is jargon-heavy, so many of the components may have unfamiliar names, but you shouldn’t feel pressured to remember them – the names and parts will be color-coded for easy reference. In a functioning watch many parts are in constant motion. By default all animations in this article are enabled, but if you find them distracting, or if you want to save power, you can globally pause all the following demonstrations.disabled, but if you prefer to have things moving as you read you can globally unpause them and have animations running. While the entire watch movement has many parts, the timekeeping system, which forms the core function of any watch, consists of just seven major elements which we can lay out in a straight line: It may not look like much, but these parts still have a lot of interesting details about them that contribute to the second hand rotating at a correct pace. We’ll start exploring these details by focusing on the source of power for this entire contraption. Power Purely mechanical devices have a few different ways to power themselves, but one of the simplest methods to store energy is to use a spring. Most springs we see in daily life are coil springs. In the demonstration below, you can move the mass attached to this type of spring to see it bounce: When a spring like this is compressed, it stores some energy that is then released when the compressing tension is removed. Mechanical watches typically use a different kind of spring – a spiral torsion spring. This type of spring is loaded when it’s twisted.

When let go, the spring unwinds in the opposite direction to eventually settle in its natural state: In a mechanical watch, we ultimately want to show rotating hands, so a spinning motion that a torsion spring provides is particularly useful. A spring in a typical mechanical watch has a slightly more complicated shape – you can see it below in its relaxed state. By dragging the slider you can try to wind it midair, but as soon as you let go, it will snap back to its original shape: As you can see, this spring is quite strong and it wants to expand very rapidly. To contain the spring we have to put it in a casing known as a barrel: Once in the barrel, the spring still wants to expand to its original state, but the barrel’s wall keep it in place. This spring is the storage of energy for the watch and its name, the mainspring, reflects its importance. Unfortunately, we can’t really get any useful work from the mainspring in this state – it has already expanded to the largest possible size. To store more energy in it we need to wind it tightly using the arbor that we’ll first attach on the inner side of the mainspring: If you look closely, the mainspring has a little hole near its end – you can see it in the center of the demonstration. The arbor has a little hook that grabs onto that hole: When the arbor is turned, it pulls the mainspring with it, causing it to wind. In the demonstration below, we’re holding the barrel tight, and you can turn the arbor by dragging the slider: Notice that as soon as you let go of the arbor by releasing the slider, the mainspring will turn the arbor right back. This is less than desired – we want the barrel to turn instead, so that it can power the other parts of the watch. To get some useful work from the mainspring, we’ll have to keep holding on to the arbor and instead let the barrel go when we want to use the stored energy: We’ll soon see how this is accomplished in practice, but for now we’ll assume that the arbor is held tight and the mainspring ends up rotating the barrel, just like in the demonstration above. Before we finish up with the mainspring and the barrel, let’s discuss two other details that make this mechanism more reliable. Let me bring up the relaxed spring one more time: The metal strip attached to the mainspring provides additional tension to its outer part.

That metal strip really wants to snap back to its straight shape, so it pushes against the wall of the barrel, creating a lot of friction that keeps the mainspring in place: This locks the outer end of the mainspring when the arbor moves the inner. If we were to keep winding the spring past its maximum capacity, we’d overpower that friction letting the mainspring slip inside – this acts as a safety mechanism to prevent the parts from breaking. As we’ve seen, in its relaxed state, the mainspring forms an S-shape with varied curvature throughout. This helps to balance the tension in mainspring’s different sections when it is inside the barrel. Notice that the inner sections of the wound spring have a much smaller radius than the outer parts. If the relaxed spring was just a straight piece of metal, then after winding, the inner parts would be bent much more than the outer parts. With the S-shaped spring the outer sections of the spring are also under a similar tension because they want to get back to their curve that is bent in the opposite direction. To secure the mainspring and prevent dust from getting in we close the barrel with a lid that snaps into its place: We’ve managed to make some parts rotate and one could naively think that we could just attach a watch hand to the barrel to make it track time. Unfortunately, that won’t really work – you can witness this in the demonstration below. You can see how this “watch” behaves after you wind the mainspring with the slider and let it go: We clearly have some work to do – the hand spins way too fast and it only does a few rotations before the mainspring inside the barrel runs out of the stored energy. Clearly, this contraption won’t let us track time in any reliable way. If we wanted our watch to run continuously for around 40 hours on a single wind, we’d need the minute hand to complete 40 rotations in that time. Moreover, the second hand should cover around 40 × 60 = 2400 complete rotations in that time. We need to find a way to convert a small number of revolutions of the barrel into a large number of revolutions of the hands. This is where gears come in. Gears I’ve talked about gears on this blog before, so let me just recap things very briefly. Gears can be used to change the speed of rotation between two different axes.

In the demonstration below, you can witness that by observing little dots I put on each gear – the yellow gear, which is powered by the bigger red gear, takes much less time to finish a single revolution: An important aspect of two matching gears is their number of teeth. Each tooth in one gear meets with a space between teeth in the other gear, so within a unit of time both gears rotate by the same number of teeth. If the number of teeth in two gears is different, those gears can take a different amount of time to complete a single rotation. In the demonstration below, you can change the ratio of the number of teeth between the driving red gear and the driven yellow gear to see how it affects the speed of rotation of that yellow gear: These gears are intended to work with each other so the ratio of teeth is equivalent to the ratio of the gear radii. When the driving gear has more teeth than the driven gear, the driven gear makes more rotations than the driving gear. We can use this behavior to make the second hand of a watch rotate many times on a single rotation of the barrel. Let’s consider how much of a speed increase we have to do here. The barrel can rotate close to 7 times on a single wind, but we want the second hand to complete around 2400 revolutions in the same time. We need the ratio of teeth, or the ratio of radii, to be around 343:1. Let’s see how that would look in practice. In the demonstration below, you can use the slider to look at the two gears from further away: As you can see, these proportions are ridiculous – to make the red gear fit in any reasonably sized watch, the yellow gear would have to be absolutely tiny and both gears would have to have very fragile, microscopic teeth. Instead, mechanical watches use a train of gears with multiple gears working in pairs – each pair increases the speed to some extent. In the demonstration below, you can see the four wheels participating in this reduction. Notice that there are two gears on most axes of rotation. You can control the speed of rotation of this gear train using the slider: The barrel acts as the first wheel, it drives the second wheel, which drives the third wheel, which finally drives the fourth wheel. Notice that each big gear drives a smaller gear called a pinion.

A pinion is mounted on the same shaft as the next big gear so we’re able to keep increasing the speed on each axis. This approach has significant advantages – we’re able to make the overall mechanism much smaller and we’ll soon use one of the intermediate wheels that rotates at a slower rate to drive minute and hour hands. Before we finish up with gears, let me quickly mention the shape of their teeth. While many bigger machines use an involute shape for the profile of their gear teeth, mechanical watches commonly use cycloidal profiles which are obtained by rolling a circle on the surface of another circle. Let’s see how the so-called going train that we’ve assembled works when we wind the mainspring through the arbor and let the watch run: We’ve certainly achieved the goal of the second hand rotating many times on a single rotation of the barrel, but the speed of revolution of that hand is still completely untamed. We need to find a way to control the rate of release of the energy stored in the mainspring – we’ll do this with the escapement. Escapement Let’s start by looking at the two components that create the escapement – the escape wheel and the pallet fork: Notice the unusual shape of the teeth of the escape wheel – it’s very different than the gears we’ve seen before. Its top part hosts a regularly shaped gear that can be used to turn that wheel. The pallet fork itself is made of metal, but notice the two pinkish transparent parts at its end. These are jewels made from synthetic ruby. That compound is not only very hard, which prevents its wear, but it also has a low coefficient of friction with steel. Let’s see why these properties are important by observing how these two components interact with each other: The escape wheel wants to rotate as indicated by the red arrow. The pallet fork prevents that motion, but as we pivot that pallet fork back and forth we let the escape wheel briefly escape from that jail only to be stopped again. We’ll see the details of that interaction in a few paragraphs, but right now this mechanism lets us control the rotation of the escape wheel by simply moving the pallet fork from one side to another. Let’s see how these pieces fit into the rest of the assembly. In the demonstration below, I’ve wound the spring for you so the barrel, through the gear train, ends up trying to rotate the escape wheel.