You can often encounter terms like “escapement” from watch magazines, or “broken staff” and “balance wheel not spinning” from your watchmaker when you try to get your watches fixed. Unless you are familiar with inner workings of watches, you probably have no idea what exactly those terms mean. Fortunately, the basic principle of clockwork is actually very straight forward. I will try to explain it using a pullback toy car.

A pull back toy car has a spring coil. When you push the car backward, the spring coil winds and stores the kinetic energy. Once you release the finger, the spring coil releases the kinetic energy, so the wheels start spinning to drive the car forward. The watch minding mechanism works exactly the same way.

Now, let’s add a pendulum. A swinging pendulum is actually one of the earliest timekeeping mechanisms, as the time that pendulum takes to complete one cycle of the whole movement is close to a constant (but diminishes slowly). The problem is that the pendulum will eventually stop swinging due to the gravity and friction. However, if you put a moving toy car upside down and let it touch the pendulum, you will see the spinning wheel pushes the pendulum away and cause the pendulum to start swinging. Now, simple clockwork is born just by putting a pendulum and a toy car together. Of course, you can immediately point out many drawbacks of this design. One is: the spring coil in the toy car cannot maintain its power for too long. The toy car wheels probably will stop spinning after a few seconds.

Now, consider the setup in this figure. Instead of a pendulum, we replace it with a wheel that has teeth (red B in the figure). When the wheel B touch the toy car wheel at point A (in blue), the teeth will prevent the toy car wheel from spinning. However, the spinning force of the toy car wheel will eventually push the teeth away and start spinning. When this occurs, the wheel B rotates in clockwise direction and compresses spring C. The spring C will push the wheel B back immediately and cause wheel B to reverse its rotation until the teeth of wheel B touches the toy car wheel again. The car wheel will be stopped and tries to push the teeth away again. As you can see, wheel B will be swinging back and forth. Due to the periodic contacts of the wheel B teeth, the toy car wheel will also be stopped regularly, thus conserve energy stored in the spring coil. If the setup is done correctly, it will form an equilibrium swinging state for timekeeping until the spring coil in the toy car is exhausted

It is not too hard, right? Now, you can also understand if you increase the spring force C, the watch will run faster (because the wheel B will bounce back faster) and vice versa. This is usually how watchmakers adjust the timekeeping of your watches.

In fact, this is exactly how a mechanical watch works. In a typical watch, wheel B is called balance wheel; the pivot of wheel B is called balance staff, and the spring C is called hairspring. Of course, this setup oversimplifies many details. For example, the ticking sound you hear from a watch is actually the sound of escapement, which would be the contact point A where the wheel B teeth touches the toy car wheel. A tick sound is generated every time when wheel B hit the toy car wheel. An escapement has to be durable while maintaining constant accurate motion of catching and releasing of the toy car wheel. The design of a working escapement takes years of fine tuning. Now, almost all mechanical watches uses lever escapement, but if you collect antique watches, you will encounter watches that uses different types of escapement. Of course, there are new types of escapements being developed in the recent years like coaxial or constant escapement. You do not need to understand details of different escapement types if you don’t want to. Just remember that escapement is actually a complex mechanism of catching and releasing the driving gear.

You might also wonder what jewels or rubies do in a watch. In fact, due to the constant motions, watch parts wear out very quickly. Parts made out of durable materials, such as ruby, can extend lifespan of a watch dramatically. If you were a watch designer, where would you put the rubies in our simple setup? A good place would be at the pivot of wheel B because of its constant movement and direction changes. A ruby placed at the pivot can ensure that wheel B can turn smoothly for a very long time. Other places would be axes of the toy car wheels and inner gears. Under the same design, a watch that utilizes more rubies (i.e. more number of jewels) means the watch is more durable. 15~17 jewels are usually considered as typical. Of course, luxury watches and watches that has more complications/functions will use more jewels in their parts. Please keep in mind that those rubies are just low cost synthetic corundum for industrial use, so no one, not even your watchmaker, would want to steal them from your watch.

Let’s apply what we have learned to a real manual winding mechanical watch that uses level escapement. In the watch diagram, you can see the power from the spring coil D is being transferred through a series of gears (along with dotted brown line) until point A, the contact point where toy car wheel touches balance wheel B. It is not too difficult to understand now, isn’t it? Please note that I purposely used the same color on A, B and C to mark the gears of the same functionality on both diagrams to illustrate their similarity.

Next time, if your watchmaker tells you that your watch has a broken staff, you will immediately know that the pivot of wheel B is broken. If the wheel B cannot rotate freely to complete its swinging motion, the equilibrium cannot be formed; that means the watch is dead (and needs to be fixed).
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