The word ‘escapement’, is one you’ll no doubt have encountered multiple times when reading about watches. But what exactly is an Escapement, and why is it important that you should know?
Simply put, an escapement delivers energy from the barrel, via the train wheels, to the regulating unit (balance or pendulum) to maintain its oscillation. It is the ‘distributing organ’ of the watch and also counts the oscillations of the regulating unit.
That’s what it does in a nutshell, but it has not always been that way. There have been many different types of escapement throughout the history of watchmaking, each one exploiting the available technology of the day and attempting to redefine the tried and tested methods to increase accuracy and efficiency. The most common in use today is the Swiss Lever escapement (the Co-Axial escapement, invented by the late George Daniels, is a modified version of this and currently taking the world by storm). The Swiss Lever Escapement is a DETACHED escapement (which means when the escapement is at rest (at the dead point), its constituent parts are not touching (in theory).
So what are the other types of escapement? Well, there’s the Frictional Rest and the Recoil, but they aren’t very often used these days. An awareness of variations between escapements is imperative when fixing old watches (Pin Lever, or Pin Pallet Escapements were, thanks to their cheapness, incredibly common for a while).
Check out the below categorisations:
Detached: Swiss lever, Detent, Pin lever
Frictional Rest: Cylinder, Duplex
The most common of which is the Swiss Lever Escapement, which you can assume to be present in all modern watches unless otherwise stated. The presence of another escapement is almost certainly an attempt to improve the escapement’s efficiency or to sell you something on the back of a stylistic, material or operational quirk. Either way, any deviation from the norm will not be allowed to pass unnoticed.
So let’s take a closer look at the most common escapement.
We’ve talked about the escapement before and defined it loosely as the distributing organ of the watch. It controls the release of the mainspring’s energy, which powers the train. Exactly how it does that is what we are going to focus on over the next couple of weeks.
It isn’t always simple, and some of the explanations may not be easily digestible at first, so if you need any further clarification on anything at all, please ask. Often diagrams and physical examples work well with getting to grips with the escapement, but the latter is particularly difficult to arrange (although I might be able to do some videos in the future).
So let’s start off wit covering something very basic: the unlocking of the escapement. You may find this laughably obvious, but it occurred to me that the term itself must not be taken for granted. So much happens during the unlocking phase of the escapement it is important to have a thorough understanding of the process.
Right, follow this example:
The watch is fully wound so that the mainspring is as tightly coiled around the barrel arbour as it can be. The watch has as much power as it can ever have.
Imagine that there is a brake pressing on the balance wheel so everything is static. There is no movement at all in the watch. The mainspring is coiled and all that energy is being held in place by the brake on the balance wheel.
Now, what this energy, which lives in the barrel, is trying to do, is to drive the wheels in the watch. The barrel wants to turn, but cannot because the brake is on. If it was able to spin freely (if the pallets and balance wheel were removed) the centre wheel, third, fourth and escape wheels would zip round suddenly and would not stop spinning until all the tension of the spring in the barrel had been released. But this would never do. The release of the energy needs to be constant and controlled. This is the job of the escapement.
So now imagine we remove the brake and the watch starts ticking. The balance wheel is swinging at a regular pace.
What is the balance wheel doing? How does its swinging affect the wheels in the train? Well, underneath the balance wheel the is a small jewel known as the impulse jewel protruding downwards. As the balance wheel swings to and fro, so does the impulse jewel. Aligned with the impulse jewel is a T-shaped piece of metal with a U-shaped recess at the bottom of the T-shaped stem, and Jewels (pallet stones) at the end of either arm of the T.
Now, those two pallet stones that project from the arms of the T-shaped ‘pallet fork’ at an angle of somewhere between 13 and 16 degrees, engage alternately with the angled teeth of the escape wheel, acting as mini brakes.
Every time the balance swings, the impulse jewel enters the U-shaped notch of the pallet fork, disengaging one pallet stone and allowing the escape wheel to progress by one tooth, before the other pallet stone engages and stops the escape wheel in its tracks.
Then, on the return journey of the balance wheel, the impulse pin performs the same action but in reverse, allowing the escape wheel to progress another tooth (remember, the force from the barrel is driving the wheels in ONE DIRECTION. As soon as the pallet stones allow the escape wheel freedom to move, it will do so in ONE DIRECTION, regardless of which pallet stone is locking and unlocking.
That is, in a nutshell, how the escapement works. When I say unlocking, I am referring to the pallet stones disengaging from the escape wheel tooth, allowing the escape wheel to move under the influence of the mainspring in its barrel.
There are five shocks (impacts) produced in the Swiss lever escapement, during the process of Unlocking and Locking.
Chronological order of shocks and their descriptions:
1. Unlocking: the impulse jewel striking the notch
2. Beginning of Impulse for the Escape Wheel: the escape wheel catches up with the impulse face of the pallet
3. Beginning of the Balance Wheel Impulse: the notch catching up with the impulse jewel
4. Drop: The escape wheel tooth strikes the locking face of the exit pallet and…
5. Safety Action: …simultaneously, the lever hits the banking pin
Order of importance (loudness): 4-5-1-2-3
The Swiss Lever escapement has an inherent loss. Impulse before the centre line (0 degrees) causes a gain. Impulse after the line of centres causes a loss. A disturbance before the line of centres causes a loss. A disturbance after the line of centres causes a gain.
Since the unlocking (a disturbance) occurs before the centre line, causing a loss, and the majority of the impulse occurs after the line of centres, a further loss is caused.
Therefore the effect of the escapement interference causes a loss. As the lift angle is 52 degrees (in a standard watch), and becomes proportionally more influential as the amplitude decreases, the loss too becomes greater.
The lift angle is the angle of travel through which there is contact between the impulse jewel and the notch. It is classified as a disturbance and thus as the amplitude drops and the lift angle remains the same, it becomes a relatively longer and more influential disturbance.