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THE PALLETS (LEVER)

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Figure 1

Diagram of pallets (lever) (steel and synthetic ruby)

Click on the thumbnail to enlarge it

1. General description

With the escapement wheel and the double roller, the Pallets (lever) is a component of the Distribution mechanism (here, the Swiss lever escapement).

The shape of the Pallets (lever) and its interaction with the escapement wheel and the double roller (itself attached to the Balance Axis) allow it to transform the rotational motion of the finishing Train (wheel train) into an oscillatory movement. Moreover, when the Balance freely performs its oscillation, the pallet stones of the Pallets (lever) block the escapement wheel and thus the entire finishing Train (wheel train) and the Barrel.

The Pallets (lever) owes its name to its shape and appears in different types of escapements. The Pallets (lever) of the Swiss lever escapement features two ruby Pallet stones that successively lock the teeth of the escape wheel. When the Balance completes its free Oscillation, the Impulse pin (roller jewel) releases the Pallets (lever), which pivots slightly. The tooth of the escape wheel, previously locked, transmits, in a brief rotational movement, its Impulse to the Pallets (lever) via the impulse plane of the Pallets (lever), which, through a rocking motion, is directly transmitted to the Balance by the Fork of the Pallets (lever) located opposite the Pallet stones.

2. Composants et matériaux

Selon sa fabrication, l’ancre se compose d’un nombre variable de composants. Dans une fabrication traditionnelle les composants sont les suivants:

La tige d’ancre

Fabriquée en acier, il s’agit d’un axe pivoté sur laquelle la planche de l’ancre est chassée ou vissée. De récents développements, visent à produire une tige d’ancre en céramique par usinage femto-laser. Une telle pratique vise à supprimer l’influence des champs magnétiques dans l’échappement et l’organe régulateur mais demeure, à ce jour, anecdotique. Il est à noter que pour garantir la parfaite liberté de l’ancre, les pivots de la tige d’ancre ne doivent jamais être lubrifiés.

La planche de l’ancre

C’est le composant principal de l’ancre. Traditionnellement fabriquée en acier, elle vient chassée ou vissée sur la tige de l’ancre. De part et d’autre de son point de pivotement se déploient les deux bras qui portent les palettes. L’asymétrie des deux bras permet de positionner les plans de repos des palettes de manière équidistante au point de pivotement de l’ancre. À l’opposé des deux bras, se déploie la baguette, à l’extrémité de laquelle est découpée, sur le même plan, la fourchette. Enfin, à la base de la fourchette un perçage permet de recevoir, chassé, le plot de dard.

Les palettes (ou levées)

Celles-ci sont traditionnellement fabriquées en rubis synthétiques afin de minimiser leur usure ainsi que les frottements dans leur interaction avec les dents de la roue d’échappement. De leur géométrie et de leur positionnement dans les bras de la planche dépend, notamment, le bon fonctionnement de l’échappement (cf. : l’achevage et l’organe de distribution).  

Le dard

Il est traditionnellement fabriqué en laiton dont la malléabilité permet facilement de le limer ou de le forger afin de garantir le bon fonctionnement de l’échappement. Il constitue une pièce monobloc avec le plot de dard qui permet de le chasser dans la planche de l’ancre et de le positionner en dessous de cette dernière. Le dard est un organe de sécurité qui permet d’éviter le renversement (cf.: l’organe de distribution et l’achevage).

Notes importantes

• Dans le cas des ancres fabriquées en nickel-phosphore (UV-LIGA), le dard et son plot sont  également fabriqués en nickel-phosphore et constituent un élément monobloc avec la planche, lequel est amagnétique.
• Dans le cas des ancres fabriquées en silicium (gravure DRIE), l’ensemble de l’ancre (palettes incluses) est fabriqué d’un seul bloc amagnétique. Malgré leurs nombreux avantages, cette technologie et ce matériau ne permettent aucune opération d’achevage.

3. Profile and terminology of the pallet stones (lifts)

Figure 2

Profile of the Pallets (lever) pallet stones

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4. History

The use of Pallets (lever) in different types of clock and later watch escapements dates back to the early 18^th century. And Thomas Mudge invented the Swiss lever escapement in 1754. This type of escapement has been by far the most widely used since the dawn of the 20^th century. In itself, the Pallets (lever) has not undergone major changes since its invention. Until the end of the 19^th century, watchmakers frequently compensated for the balance defects of the Pallets (lever) caused by its asymmetrical shape by attaching a counterweight to it. History would reveal that it is preferable to favor the lightness of the Pallets (lever) over its balance, and counterweights were subsequently abandoned. Other historical milestones are linked to the materials used. Thus, the pallet stones of the Pallets (lever), once made of natural Rubis / Jewel, have since the middle of the 20^thcentury replaced by synthetic corundum Pallet stones, just like all the jewels in a watch. Finally, the emergence of the manufacturing process UV LIGA (for nickel-phosphorus Pallets (lever)) and DRIE (Deep Reactive Ion Etching) Engraving (for Silicon Pallets (lever)) at the beginning of the 21^st century makes it possible to produce Pallets (lever) that are lighter, more robust, and antimagnetic.

5. Artisanal manufacturing of a Pallets (lever)

The most artisanal method for manufacturing a Pallets (lever) consists of tracing then cutting with a Piercing saw (jeweler’s saw)the profile of the Pallets (lever) in a Steel plate. The final dimensions will be achieved by filing and smoothing the surfaces. The stud holding the Dart (safety pin) will be transferred and fixed to the Pallets (lever) plank by two feet for precise and stable positioning. The cylindrical Dart (safety pin) at the stud is flattened by forging and filing to ensure its function. The Pallets (lever) shaft will be turned then pivoted. Old, manual machines, or more recent motorized ones, are used to grind the Ruby / Jewel Pallet stones in order to give them their exact dimensions and to control the angle of the Impulse planes. Once the components of the Pallets (lever) are made and decorated, the craftsman proceeds to its Assembly. The Dart (safety pin) is thus driven into its stud, the plank is generally screwed onto the Pallets (lever) shaft and the Pallet stones are held in their clamps with shellac, which, when slightly heated, allows the exact position of each Pallet stone to be adjusted and firmly secured once cold.

6. Industrial production of a Pallets (lever)

On an industrial scale, the Pallets (lever) shaft will be turned on an Automatic lathe before its pivots are rolled. Stamping is particularly well suited to the manufacture of the pallet bridge, although it can also be machined. Pallet stones are produced on automatic machines equipped with diamond grinding wheels. Depending on the level of finishing required, the pallets (lever) bridges are polished either by hand or in bulk in polishing drums. Finally, the use of glue to fix the pallet stones has now largely replaced shellac in industrial production.

7. High-tech methods for producing pallets (lever)

Microfabrication technologies have profoundly transformed the production of certain watch components, particularly the components of the Escapement. Among these processes are manufacturing by UV-LIGA and by Silicon etching (DRIE (Deep Reactive Ion Etching)) allow obtaining pallets with extremely high geometric precision, difficult to achieve with traditional machining methods.

These processes rely on techniques derived from microelectronics and enable the production of parts with complex profiles, controlled surface finishes, and near-perfect industrial repeatability in materials known to be non-magnetic.

Unlike conventional methods (milling, stamping), UV-LIGA and silicon processes:

• Eliminate mechanical stresses during manufacturing
• Involve non-magnetic materials
• Eliminate defects related to the tool (milling cutter radius, burrs)
• Allow for superior precision and repeatability

They are thus part of an evolution toward high-precision watchmaking, combining traditional craftsmanship with advanced technologies.

The manufacturing of Pallets (lever) using UV-LIGA and in Silicon using the DRIE (Deep Reactive Ion Etching) method illustrates the integration of microfabrication technologies into contemporary watchmaking. These processes make it possible to improve the precision, regularity, and performance of Escapements, while opening the way to new architectures.

They now constitute a complement to traditional methods, particularly in the field of high-end technical watchmaking and the industrial production of high-value-added components.