Examining the Escape Wheel and Pallet Fork
The story of the beating heart of the mechanical watch comes full circle with a closer inspection of the escape wheel and the pallet fork
The three-part journey detailing the workings of the mechanical escapement finally comes to a close with this story. Having dealt with the hairspring (issue #54) and balance wheel (issue #55), the time is ripe to meet the escape wheel and the pallet fork. Basically, these components are the ones you encounter most directly in your daily interactions with your watch. The sounds of the movement beating are the sounds of the pallet fork and escape wheel making repeated and regular contact. For such a pivotal part, it receives perhaps the least amount of attention, when compared with the hairspring and the balance wheel.
Part of the reason for this is that the conversation about the mechanical escapement revolves around the production of hairsprings and balance wheels. The escape wheel and its pallet fork partner are almost after-thoughts. Indeed, they can be considered part of the wheel train, but are properly part of the escapement.
In our previous stories, we noted that the balance spring and balance wheel must work as a unit to create an effective regulating unit. The analogy was to the pendulum in the pendulum clock. Well, the escape wheel and the pallet fork already existed in this type of clock, and indeed pre-date the invention of this kind of timepiece. Of all the components of the escapement, this is the oldest. Our journey to the past will take us well past the European Renaissance, all the way to antiquity.
Before moving forward, it should be noted that the Rolex side story here was originally published in issue #42, and was written by Jamie Tan.
It seems that elements of the wheel train were already in play in the era of the clepsydra, otherwise known as the water clock. Then as now, there was an escapement in place to regulate the flow of energy. There is plenty to be amazed about here but sadly, none of these ancient devices survived to the current age. What we know about them comes from historians, philosophers, writers, and scientists of the ancient era. To sum it up, to the ancients, the escapement was already familiar – just not in exactly the way it looks today.
The most famous example of a historical escapement associated with a water clock comes not from Europe but Tang Dynasty China. In 723 (or 725), the world’s first clockwork escapement debuted in China, in a water clock, according to Wikipedia. Some sources disagree here but that is because this escapement is not a mechanical one. This feels like a moot point because the escapement is simply a mechanism. Of course, this becomes clearer when one understands the function of the pallet fork and the escape wheel.
Let us fast-forward directly to the anchor escapement as used in pendulum clocks in the 17th century. This mechanism is very similar to the pallet fork and escape wheel that we know today because there is an anchor, which is what a pallet fork somewhat resembles, and a wheel with angled outward facing teeth, which is exactly what the escape wheel in a contemporary watch looks like.
The shaft of the anchor connects to a swinging pendulum, which moves it to-and-fro. The arms of the anchor have one protrusion each, typically called pallets, and these make contact with the escape wheel, as dictated by the pendulum. This creates a start-stop motion effect on the escape wheel, as one pallet engages one tooth. Each such impulse received by the wheel moves it forward, only for the momentum generated to be arrested by the opposite pallet making contact with a tooth of the wheel. The escape wheel is connected to the rest of the gear train, to which it delivers the controlled kinetic bursts generated by its interaction with the anchor.
Once again, that is how mechanical timekeeping works. The above description differs slightly from what is on Wikipedia or even Berner’s Illustrated Professional Dictionary of Horology, but it all amounts to the same thing. Indeed, whether it is a clock or a portable timekeeper, the function of the escapement is the same. Of course, we do not have to go into how the balance wheel and hairspring combine to simulate the effect of the pendulum because we did that in issue #55. If you are reading this story online, that is the benefit of splitting the escapement story into three parts.
For those jumping back and for the between the three issues, you will have noted that the description of how the Swiss lever escapement works in a wristwatch (#55) is almost identical to how the anchor escapement works in the pendulum clock. One key difference to note is that the Swiss lever system is self-starting while the anchor escapement is not. The swinging motion of the pendulum needs to be initiated for the anchor escapement to work.
Before commencing on details about the escape wheel, we will list the components that make up the escape wheel and pallet fork in full. This article does not purport to be a textbook or manual, and a complete list of components here is the clearest indication of our intentions. This list is reproduced in full from the Dictionary of Horology, which is a kind of textbook:
- The escape wheel
- The escape pinion
- The pallet-arms with two pallets
- The lever, otherwise known as the anchor
- The banking pins, which limit the movement of the lever
- The fork or notch
- The guard pin, which resides with the notch
- The safety roller
- The table roller
- The impulse pin, which resides on the table roller
- The balance staff
- The pallet staff
The escape wheel interacts with the balance via the pallet fork, but it also connects with the gear train. The escape wheel receives unregulated energy from the mainspring via the gear train, and transmits it to the balance wheel and balance spring. In this way, the balance wheel and balance spring get a push, and then that assembly returns regulated energy to the escape wheel via the pallet fork.
The release of energy from the balance is thought of as an “escape,” with the balance capturing the raw power of the mainspring, with small amounts “escaping” to return to the gear train. This is why the wheel is called the escape wheel and the entire system, the escapement. It is notable that the escape wheel is made in steel, traditionally. The gear train, by way of contrast traditionally uses brass wheels.
The teeth of the escape wheel, and the pallets that make contact with them, are also important. For example, the number of teeth the escape wheel sports will shape the frequency of the movement, otherwise known as the vibrations per hour. The shape of the teeth also have implications, because the Swiss lever escapement is characterised by squared-off teeth, while the English lever escapement has sharp teeth. A key difference here is in how the impulse is received by the escape wheel. In the Swiss version, the impulse is shared between the pallet stones and the impulse faces of the teeth. In the English version, the impulse is taken entirely by the pallet stones.
The pallet fork is so-named for its resemblance to a fork, although it more closely resembles an inverted anchor. In watches, it is typically made of nickel-plated brass, unlike the escape wheel, and features two jewels on its ends, which are the pallet stones. These synthetic jewels mitigate friction as the fork and escape wheel connect. At the other end are the banking pins, which limit the movement of the pallet fork, and the notch. The roller pin, attached to the balance sits in this notch.
Using contemporary approaches, it is possible to reduce the overall number of components here, as demonstrated by multiple Swatch Group brands using silicon for both pallet forks and escape wheels. The most significant change is in the pallet stones, which can be done away with entirely. Indeed, the pallet fork is subject to a lot of research and development as brands try to find a way to make power transmission possible with as little contact as possible between the escapement and the gear train.
This brings us to the function of the pallet fork once again. To sum it up, this component facilitates the transfer of energy between the balance and the escape wheel. It acts as a brake of sorts to the gear train, keeping the mainspring from unwinding all at once. As to the movement of the pallet fork, this is enabled initially by the escape wheel advancing. This sends an impulse via the pallet fork to the balance wheel; the pallet fork then stops the escape wheel turning with its opposing arm, while it waits for a return impulse from the balance wheel. This return impulse comes from the oscillating balance wheel, which moves the pallet fork in the opposite direction, freeing the escape wheel to advance a notch. This process repeats until the mainspring stops sending energy to the escape wheel.
Thus, the power supply sent by the mainspring is regulated by the balance assembly, via the start-stop action of the pallet fork. It is important to note that the pallet fork does not move the escape wheel; it merely regulates its motion.
With this, we know how the heart of the mechanical watch works. As usual, we are indebted to the work of various authors for their research. Our sources include Berner’s Illustrated Professional Dictionary of Horology, Worn and Wound, the Journal of Haute Horlogerie, and the press departments of various brands.
From here, we will go into various Swiss lever executions in contemporary watches, as well as alternatives to this system, including quartz.
If the unlocking of the escape wheel requires the arm of the pallet fork that is in contact with it to slide off, then it stands to reason that this interaction can be optimised simply by minimising this sliding component. Rolex has sought to do so in its most recent escapement development.
Rolex’s efforts to improve the lever escapement culminated in its proprietary Chronergy escapement, which the manufacture debuted at BaselWorld 2015. Chronergy addresses the lever escapement’s sliding friction in a few ways, beginning with a different lever and tooth profile.
Compared to a typical lever escapement’s pallet fork, which engages the escape wheel squarely, the Chronergy’s is offset to the side, which according to Rolex, increases the lever effect of the pallet fork to improve its efficiency. This is complemented by the reworked profiles of the contact surfaces – the pallet stones are thinner in the Chronergy, while the escape wheel’s teeth are correspondingly larger to compensate. Coupled with the escape wheel’s skeletonised construction, which reduces its mass and inertia, the reversal in the relative sizes of the components contributes to higher efficiency as well.
Of course, the efficacy of a design only goes as far as it can be manufactured in real life. To that end, the Chronergy escapement’s escape wheel is manufactured via UV LiGA for greater precision, and every tooth is measured at two different points to ensure that its tolerances – to the tune of just a few microns each way – are met.
Taken in isolation, the Chronergy escapement has an improved efficiency of 50 per cent, significantly higher than a traditional lever escapement that transmits only 35 per cent of the mainspring’s energy. This alone accounts for an increased power reserve of 10 hours. Together with other technical features, the Calibre 3255 movement that the Chronergy debuted in achieved a total going time of 70 hours.
Teeth profile and lever angle aside, Rolex also flexed its muscles in material research when developing the Chronergy escapement. Although efficiency is not directly improved by these measures, reliability has been. The first concerns the usage of new lubricants that Rolex has developed in-house, which promise greater long-term stability to allow the watch to have longer service intervals. Both the escape wheel and pallet fork are also rendered in nickel phosphorus, a paramagnetic alloy that makes the watch less susceptible to magnetic fields.