Steel is an incredibly complicated section of metallurgy, one of the most important subsections for watchmakers is spring steel. Without spring steel, it would not be possible to have any mechanical watches, and we would be stuck with gravity-driven pendulum clocks. Spring steel was key to the development of the pocket watch, not only for the mainspring but also for the hairspring that maintains the timing of the balance wheel. It is hard to imagine that the same basic materials used to make a watch case can be reconfigured to provide very different physical properties. This is the wonder of the iron-carbon complex that is the basis of all steel.
How is Spring Steel Different
I wrote a previous blog about steel and how it is an alloy of carbon and iron. This led us down the path of low carbon austenitic stainless steel to manufacture watch cases. For watch cases, the key mechanical properties are corrosion resistance, anti-magnetic, and good physical properties for accurate machining and finishing.
Spring steel is very different. This is a purely practical component in the movement and needs to be manufactured to deliver the best functional performance. So what is the key functional performance we are looking for?
Ultimately the spring is there for one purpose and one purpose only, to store and release energy. Thus, it can be thought of as a mechanical battery. As the spring is wound up, energy is stored in the atomic structure of the steel, and then as it unwinds, the energy is released.
Spring Steel For Energy Storage
The key aspect here is that the spring needs to go through many cycles of being wound up and then unwinding without reducing its performance. This is known as the elastic region of the material. How is this achieved?
To understand this, we need to look at what happens to steel when stress is applied to the material. This is commonly represented in a graphical form called a stress-strain graph.
As you can see above, the graph starts as an angled straight line known as the elastic region. As the material stretches within the elastic region, the material will return to its original shape and size if the stress is removed.
There is a kink at the top of the elastic region where the material will fail through plastic deformation. This is characterized by the material becoming thinner; this is a reversible process. The point at which plastic deformation starts to occur is called plastic deformation.
The key part to understanding is that springs need to work within the elastic region of the stress-strain graph of the material in question. Therefore, if the forces put into the spring increase past the yield point, permanent deformation will occur, and the spring will not return to its original size.
Protecting the Elastic Region
There are 2 ways to ensure that the mainspring will always remain in the elastic working range. The first is to engineer the steel to have a high yield strength. The second is to provide a mechanical release so that the spring cannot be wound past its yield strength.
Engineering the Spring Steel
First, let’s look at the key properties required of spring steel. Strength is the key property, and this can be described as having a high yield strength. The strength of the spring steel can be manipulated by varying the alloying elements in the steel and principally by increasing carbon content. Other alloying elements, principally manganese, boron, and molybdenum, can be added to adjust the exact characteristics of the spring steel. To keep things simple, we will focus on carbon as this is the key alloying element to increase the strength of the steel.
Spring steels have between 0.5% and 1% carbon by weight, so as you can see from the phase diagram below, when heated above 900oC, the steel will be in its austenitic phase (indicated by γ in the diagram below). In this phase, the steel is in a face-centered cubic structure. If this steel is left to cool slowly, over several hours back to room temperature, it will change into two distinct phases within the steel, cementite (Fe3C) and ferrite (Fe), and will revert to a body-centered cubic structure.
The trick to creating strong steel comes by speeding up the cooling process. Rapid cooling frustrates the transformation from face-centered to the body-centered cubic structure and creates a new martensite phase. Martensite has a different crystal structure again, but we shall ignore that to keep things as simple as possible.
Martensite, at a microscopic level, is a series of small interlocking plates. These interlocking plates are very hard, strong, and brittle. The magic appears by manipulating the alloying elements so that the amount of martensite is controlled by the chosen cooling process. For knives, where the hardness is key, they are typically 75% martensite, but for spring steel, where flexibility is also necessary, martensite is typically 50% of the microstructure.
Spring Steel In Practice
In practice, a watch mainspring is approximately a 30cm long thin ribbon of spring steel. The ribbon can be manufactured in several different ways, but it is the finishing processes that will dictate its final physical properties. In this case, we can assume the ribbon is hot-rolled at a temperature above 900oC. The ribbon is then placed into a cooling solution, usually an oil bath or an acid bath.
The advantage of an acid bath is it will create a blue color on the surface of the spring; this surface coating provides a barrier to corrosion. The other aspect to understand is that if the spring is heated above 900oC and left to cool slowly, it will lose its physical spring properties as the martensite will have dissolved back to austenite and then, on slow cooling, reverted to cementite and ferrite.
Mechanical Protection of the Mainspring
The spring has been made, but it is still important to ensure that the spring is not subject to greater stress than its yield strength while it is in its working environment. If the mainspring were to exceed its yield strength, then the mainspring would be rendered useless. In the world of watches, the mainspring exceeding its yield strength is referred to as overwinding the mainspring. What is happening in the spring has been wound so tightly that it bends and is then permanently deformed or bent.
A straightforward but effective mechanism has been developed to protect the mainspring from being damaged in this manner. First, the mainspring is attached to the arbor in the center of the barrel but is not attached to the barrel directly. Instead, the mainspring is attached to a bridle held by another spring to the inside of the mainspring barrel. The bridle has teeth on it, and the mainspring barrel also has teeth on its inside surface. These teeth on the inside of the barrel and the bridle are tuned by selecting the bridle spring carefully so that when the mainspring reaches its maximum working stress, the bridle will slip around the inside of the mainspring barrel. This friction protection mechanism will ensure that your mainspring can never be overwound and protects the mainspring from damage.
Atomic Structure and Macro Engineering
I always find it amazing to look into the details of the materials and implementation of the mechanical systems. Watches are an incredible illustration. How the materials are selected, and then the implementation of each piece is remarkable. The materials are engineered to accentuate the required physical properties, and then a mechanical system is put in place to protect against any weaknesses that the material has.
We have managed to review many of the technical considerations of the mainspring in 3 minutes, but what needs to be remembered is that the technology to create this elegant solution has unfolded over centuries. And developments continue with some springs made from newly configured and how they are deployed in any mechanical system are key to a successful outcome, and this is just a spring!
Even more fascinating is how the spring is utilized in the mainspring barrel to ensure consistent energy into the timekeeping movement. Constant torque in the wheel train is imperative to ensure the balance wheel keeps regular time. If you would be interested in knowing more about how the mainspring maintains constant torque regardless of its winding level, please comment below.