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The Escapement for Dummies

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
Recoil: Verge

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.

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What’s the Frequency Kenneth?

When you come to servicing a modern watch, chances are you will have access to the technical information for that specific calibre. In the information pack you will find the watch’s reference FREQUENCY. This is the frequency at which the hairspring is expected to operate in order to output the correct time. A variation from the reference frequency will manifest in either a gain or a loss. Given the fact that a hairspring is a physical object, we can expect certain imperfections to exist in its structure, meaning we should expect a fluctuation in frequency, which for a 21,600 train count should be 3Hz (21,600 vph/60 minutes in an hour/60 seconds in a minute = 6 vibrations per second. 1 hertz is equal to one oscillation (twice a vibration) and therefore you divide 6 by 2 to reach 3 hertz).

So if, for example, you expect your watch to operate at 3Hz but, upon placing it on the timing machine you discover the frequency VARIES between 2.995Hz and 3.005Hz, how can you expect this to manifest in timekeeping errors?

This example is unrealistically straightforward, as the +/- variations of the frequency are exactly the same. Normally, you would have to do the following calculation twice, but in this case once is enough. The result of this calcutlation will show you the potential error in the daily rate.

Gain/loss = 0.005 either way.

0.005 x 3,600 (seconds per hour) x 24 (hours in a day) / 3 (ref frequency)

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A Short Glossary of Parts Found in an Automatic Watch

When reading about watches, you will hear a lot of part names thrown around. Below is a list of some of the most common parts of an automatic watch and a brief description of the function of the more common (or interesting) bits.

1. Crown Wheel: the crown wheel engages with the winding pinion, which is sits on the stem, which is attached to the crown, which is the little knob on the side of the watch that you pull out to wind the watch or set the time. When you turn the crown, the stem rotates the winding pinion which in turn rotates the crown wheel, which engages with the…

2. Intermediate Wheel: not all automatic watches have an intermediate wheel, but they sometimes used in conjunction with the…

3. Auxiliary Wheel (Wig Wag): the auxiliary is not an essential component, but it does offer a useful benefit in automatic watches. The auxiliary wheel, or ‘wig wag’ wheel, has a floating axis enabling the rotor or oscillating weight to be disengaged in the act of winding.

4. Ratchet Wheel: the ratchet wheel sits on top of the barrel, which contains the mainspring and when it is turned, the mainspring is wound around the post in the barrel, which we call the barrel arbour.

The following parts are rare and their functions might be confusing at this stage, so I will skip straight on to describing the more run-of-the-mill components.

5. Ratchet Driving Wheel Pinion
6. 3 Arm Spring
7. Ratchet Driving Wheel
8. Reduction Wheel & pinion
9. Winding Wheel & Pinion
10. Stop Pawl
11. Stop Pawl Spring

12 & 13 Reversers: reversers are really what make a bi-directional weight work so well. The reversers, thanks to a series of internal brakes and springs, take the rotation of the weight, no matter which direction it is turning, and convert it into ONE direction, so the mainspring can be wound constantly. This is essential. Without the reversers, bi-directional winding would be impossible as the mainspring can only be wound in one direction, around the barrel arbour (rotating it in the opposite direction would UNWIND it).

Another couple of rare ones:
14. Mobile Platform
15. Intermediate Pinion

16. Pinion attached to Oscillating Weight: this pinion engages with the reverses, or the intermediate pinion if that separates them, and transfers the movement of the weight to them.

17. Oscillating Weight: the usually semi-circular weight that winds the watch by way of the wearer’s movement. It is also called a rotor weight, or just a rotor.

18. Winding Stem: the bit attached to the crown that is pulled into different positions when you wind or set the watch. The winding and sliding pinions both sit on the stem and are moved into different locations in order to perform different functions with every position of the crown.

19. Winding Pinion: here we are, back to the beginning again! The winding pinion sits on the stem and engages with the crown wheel so that the watch can be WOUND.

If you have heard of any other parts that you would like an explanation of, or would like further information on any of the above parts, please comment below and I will respond as quickly as I can.

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The Mainspring and its Barrel

The mainspring in its barrel is the energy accumulator (motor organ) of a mechanical watch (in an electronic watch, the battery is the energy accumulator).

The approximate thickness of a wristwatch mainspring is 1/100th of the internal diameter of the barrel.

There are three parts to the Barrel: the Barrel lid (obvious), barrel drum (the main body of the barrel that bears the barrel teeth and receives the spring) and the barrel arbour (the central post around which the spring winds and on which the barrel pivots).

Have you ever wondered what the main difference between the lubrication of the barrel wall in an automatic watch and a mechanical is? I’ll tell you: you do NOT lubricate the barrel wall in a manual wind watch; you only grease the internal bottom of the barrel drum and the inside of the lid and the spring itself. In an automatic watch, you would put a small amount of graphite grease (such as MR1) in the recesses on the walls to allow the slipping bridle to slip without excessive friction.

The barrel arbour normally takes up one third of the internal diameter of the barrel, as does the fully wound mainspring, which leaves one third free at all times.

With that in mind, let’s try and calculate the thickness of a mainspring given an arbour core RADIUS of 1.65mm.

Assuming that the Barrel Arbour Diameter (3.3) is one third of the Interior Barrel diameter (9.9), then the theoretical thickness of the mainspring (1/100th of the interior Barrel diameter) should be 0.099, or 0.1 when rounding up.


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Mainspring in Focus

The term ‘Creep’ refers to the slipping of the welded bridle around the outside of a lubricated barrel Creep.

To compensate, some brands have grooved the barrel wall to stop the bridle from slipping. The problem is, though, that the torque generated from the winding with a grooved wall sends too much power to the escapement.

The creep is something of a compromise.

Shortening the bridle, which is spot welded to the spring, and straightening it, reduces creep but leaving the watch with elevated amplitude.

The magnet test with determine whether a mainspring is good quality and made of nickel/cobalt/chromium as these materials are anti-magnetic.

The term Strength, when used in regards to the mainspring is the thickness and width measured in Newton Metres – torque.

The mainspring should be the height of the barrel wall less 0.5mm clearance.

It is important to have the correct spring for the barrel is to ensure the watch gains the maximum power reserve possible i.e. number of turns of barrel. To do this, you need to figure out how much space you have inside your barrel. First things first, you need to work out the area of the barrel. To find the area of the circle, you have to square the radius and multiply by pi. But don’t forget you have to compensate for the barrel arbour, which takes up roughly one third of the barrel’s diameter.

Here’s a working example for you:

If a barrel of 10mm has a radius of 5mm then the area would be 78.57 (25×3.1428), but that’s before you’ve subtracted the area occupied by the barrel arbour. Since the barrel arbour tends to occupy one third of the would-be functional space of the barrel and the mainspring in either a relaxed of wound state occupies another third (the final third remains for the tensioning of the spring), you need to perform the following calculation:

The equation for working out the length of the mainspring is thus:

Pi (R2 – r2) / 2e

R= barrel radius
r = arbour radius
e = spring thickness

You have to double the thickness of the spring to account for the necessary space.

In similar equations, N will stand for the number of turns between the wound and unwound extremes of the spring and L the length of the spring.

To find the number of turns to unwind a barrel (N’), do the following equation:

N’ = 1/e (R – the square root of R2 – (Le/Pi)

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