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Experiences

Balance of power

the escapement mechanism

Text — Boris Schneider Date2015-03-04T10:08:59

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Unruhreif_Spirale.jpg
—In principle, the balance fulfils the same function as a suspended pendulum but continues to function regardless of its position

A mechanical watch continues to show the correct time even as the tension in the mainspring diminishes. Making this possible is a mechanism that has been gradually improved for over 300 years: the escapement. Essentially, it consists of a pendulum—here in the shape of a ring—and a special lever. The lever rocks back and forth, alternately locking and releasing the wheel train as it does so and allowing the hands to advance in regular, even steps. Ultimate precision is only achieved, however, when the system has undergone painstaking precision adjustment by hand.

Anyone who looks into a mechanical movement is inevitably captivated by the rapid interplay between the balance and pallets. However, what the untrained eye might see as the watch’s driving force is, in fact, more of a brake. “The escapement allows the mainspring to distribute energy in small, equally divided portions, pretty much the same way that a turnstile allows only one person in a crowd to pass through at a time,” is how Raphael Frauenfelder, watchmaker and project manager for industrialization at IWC in Schaffhausen, neatly describes it.

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A ring-shaped pendulum sets the pace

When wound, the mainspring drives a chain of toothed wheels upon which the hands are also mounted. The escapement is the final stage in the wheel train and acts as a brake. Without it, the hands would simply whiz around the dial at high speed and the watch’s store of energy would be exhausted in a matter of seconds. But the escapement not only keeps the mainspring in check: it also distributes its energy in precisely portioned pulses. The pendulum has proved itself the most efficient physical means of handling the necessary information. Its greatest advantage is that it continues to oscillate at virtually even intervals regardless of the amount of energy in the drive.

In a grandfather clock, the pace is set by a pendulum that swings leisurely back and forth. For portable watches, however, whose position constantly changes and which are subject to rapid acceleration, that system is unsuitable. In 1670, the Dutch mathematician Christian Huygens hit upon the idea of allowing a ring to oscillate on a balance spring. The rim replaces the slowly moving body while the spring emulates gravity. In principle, the so-called balance fulfils the same function as a suspended pendulum but continues to function regardless of its position. After the escapement, it is the last sub-assembly in the movement.

Spirale_Breguet-Endkurve
—The Breguet balance spring improves precision

The escapement is the final stage in the wheel train and acts as a brake

Nothing is allowed to influence the balance

A watch’s regulatory system should be designed in such a way that it is affected as little as possible by external influences. Temperature fluctuations represent a special challenge. They cause certain metals to expand or contract. In the balance, this can have a negative influence on precision. For many years, for instance, balance springs were made of carbon steel. But with this material, a temperature change of just one degree Celsius can result in inaccuracies of around ten seconds per day. It was for this reason that someone had the idea of using bimetallic balances made of brass and steel. “If the temperature goes down, the diameter of the rim expands to compensate for the increased elasticity in the balance spring,” explains Frauenfelder.

Since the beginning of the 20th century, progress in metallurgy has pushed forward the development of new materials. An alloy developed in the 1930s is still the basic material found in the balance springs produced today. It comprises mainly cobalt, molybdenum, tungsten and beryllium. A very hard combination of copper and beryllium has established itself¬—among other things—as the material of choice for the balance. “Both materials are virtually immune to temperature fluctuations, rust-free and less affected by magnetism than steel,” says Frauenfelder, explaining the benefits of working with them. More recently, there has been experimentation with balance springs made of silicon. However, it remains to be seen whether this semimetal, well known from computer technology, will establish its place in watchmaking.

Incabloc-Stosssicherung
—A special shock protection system – the INCABLOC (™) ensures that impacts have no influence on the delicate parts

A special coil increases precision

The sensitive mechanism is affected not only by temperature fluctuations. In around 1800, Swiss watchmaker Abraham-Louis Breguet observed that a flat balance spring expanded eccentrically when oscillating. This effect, particularly when combined with changes in position, has a negative effect on the watch’s accuracy. Breguet hit upon the idea of securing one end of the balance spring at a higher level, which enabled the spring to “breathe” concentrically and more evenly. A French mathematician, Edouard Phillips, later calculated the shape of this special coil at the end of the spring and perfected the principle. At IWC, the overcoil on the Breguet balance spring is still bent into position by hand. The job demands an enormous amount of skill and experience from the watchmaker.

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calibre_98295
—The IWC-manufactured 98295 calibre's balance can be adjusted using the index

Shock absorbers to protect the sensitive mechanism

Protection of the oscillating system against impacts merits special attention. The balance is constantly in motion and is one of the parts in the movement most subject to wear and tear. At a frequency of 4 Hertz, a balance completes 28,800 beats per hour. The balance rim, which is about a centimetre in diameter, is mounted on an arbor with bearing pivots that are less than one-tenth of a millimetre in diameter. “In order to ensure that impacts can have no influence on the rate or damage the delicate parts, the balance staff bearings are fitted with special shock absorbers,” explains Frauenfelder.

Precision adjustment by hand

For a watch to be precise, the balance’s oscillations must be exactly equal. In order to achieve this, the ring-shaped pendulum must be painstakingly set by hand. The watchmaker can adjust the balance, for instance, using the index. This small lever modifies the active length of the balance spring. It can be clearly seen on the reverse side of the IWC-manufactured 98295 calibre, which has an elongated index known as the Jones arrow. The same principle, incidentally, applies to a guitar: the frets on the neck alter the strings’ length and, with it, the tone they emit.

Depending on the design, the oscillating mechanism can also be set using eccentric cams or weight screws. If these are turned outwards, the oscillations become slower; if they are turned inwards, the movement speeds up. The result is similar to what happens if we rotate on a swivel chair and try holding our arms close to our body or sticking them out. To ensure that the one-millimetre-long screws have the greatest possible mass despite their volume, they are made of pure gold.

For a watch to be precise, the balance's oscillations must be exactly equal.

The escapement measures out the beats

The escapement is responsible for transferring the pace dictated by the balance to the wheel train. Over the centuries, countless systems have been devised to do this. Of all these, the one that has established itself is the Swiss club-tooth lever escapement. It consists of an escape wheel, a pallet lever (anchor) and an impulse pin. The escape wheel is the last wheel in the wheel train and is driven by the mainspring. The pallet lever is made of steel or brass and shaped like a ship’s anchor. It provides the connection to the balance. Attached to each of its two arms are tiny jewels, the pallet stones, which mesh with the teeth on the escape wheel. At the top end, the impulse pin is mounted on a disc on the balance rim and moves back and forth.

When the balance oscillates in one direction, one of the pallet stones on the anchor locks the wheel train. When it returns to its starting point, the impulse pin drops into the pallet fork. The pallet lever is pushed to the other side, and the pallet stone released. The mechanism is unblocked and the hands can move forward. For this brief moment, the balance is the driving force in the entire process.

balance_wheel
—The swiss tooth-lever escapement has proven to be an efficient solution
Ankerpaletten.jpg
—Thanks to the regular impulses provided by the pallet lever, the balance remains in motion

Engine and brake in one

But now things are reversed. Driven by the wheel train, the unblocked escape wheel tooth slips onto the impulse face of the pallet stone and gives the pallet lever a push. The impulse pin—or roller jewel—transfers this impulse to the balance, which now oscillates in the opposite direction. But now the other pallet stone comes within the radius of the escape wheel and blocks the wheel train once again. This procedure is repeated with every vibration in the balance rim, alternating in both directions. The sound of the pallet stones striking the metal teeth of the escape wheel produces the watch’s audible tick-tock sound. A film of viscous lubricant minimizes friction and ensures that the process runs smoothly.

“Thanks only to the regular impulses provided by the pallet lever does the balance remain in motion. Otherwise, the frictional loss of momentum would cause it come to a standstill after a short time, and the watch would stop running,” explains Frauenfelder. As the tension in the mainspring decreases, the impulses transmitted to the balance become weaker, and the amplitude is reduced. Nevertheless, the intervals at which the impulse pin passes its starting point and the pallet lever releases the wheels remain almost exactly the same.

Complex theories, tips and tricks

However, in order to achieve an extremely high level of precision over the watch’s entire running time, we need to understand many complex theories. Watchmakers at IWC are able to dig deep into a repository of special tricks that they can use to increase a watch’s precision even further. For example, the play between the two pins that secure the index to the balance spring can be altered by a few thousandths of a millimetre to optimize the timing of the oscillating system.

The setting of the pallet lever and escape wheel calls for enormous dexterity because the angles need to be precise. The pallet stones, which are just one millimetre in length, are secured in their settings on the pallet lever with shellac, a resinous adhesive made from the secretions of the lac insect. Only if this job is carried out to an accuracy of just a few thousandths of a millimetre will the parts interact with the necessary precision and the heart of the tiny machine beat at the same regular intervals.

“There are many processes in the watch industry that are being increasingly industrialized, but the escapement has remained the exclusive domain of manual craftsmanship,” sums up Frauenfelder. And it is, perhaps, precisely this feature that has preserved the fascination of centuries-old mechanical watchmaking to this day.

Explore More Articles
IWC Da Vinci Perpetual Calendar Sketch
Putting eternity on your wrist

During his time as head watchmaker at IWC, Kurt Klaus translated the Gregorian calendar with all its many irregularities into a mechanical program that will continue running perfectly until 2499 with virtually no corrections from outside.

Ingenieur Constant-Force Tourbillon
The constant is the force

IWC's constant-force mechanism ensures that the escapement delivers an absolutely even supply of power and delivers unprecedented precision.

breguet_spring
Increased precision
down to the overcoil

In some IWC calibres, the balance rim oscillates to and fro on a Breguet spring. The terminal coil is painstaking shaped by hand and plays a significant role in ensuring that the balance oscillates with perfect regularity, thus increasing the watch's precision.

IWC Portugieser Annual Calendar
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The New Small Eternity

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Pellaton's ingenious automatic meets state-of-the-art engineering

A matter of adjustment

For an IWC watch to run accurately, the oscillations of the balance require careful adjustment.

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Proprietary 52000-Calibre
Family

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