Set of components of a movement

Abstract

Anchor, wheel, particularly escape wheel, hand, or bridge for a timepiece comprising a working surface (20) intended to come into contact by sliding and / or pivoting with another timepiece component and at least one first zone (10) distinct from and outside the working surface. The first zone (10) has a first surface texture with an epilame effect, making the first zone more oleophobic and / or hydrophobic than the working surface (20). The first surface texture is formed of microcavities, distributed regularly or irregularly, and arranged so that the surface of the first zone (10) between these microcavities is a continuous surface.The first texturing is chosen so that, for a given lubricant, the delta of wettability between the first zone (10) and the working surface (20), defined by the difference in absolute value between the spreading coefficient of the lubricant on the surface of the first zone (10) and the spreading coefficient of the lubricant on the working surface (20) is between 10 and 40 mN / m, preferably 30mN / m.

Description

[0001] The present invention relates to an anchor, a wheel, in particular an escape wheel, a hand or a bridge for a timepiece.

[0002] The use of lubricant or oil is widespread in watchmaking to reduce friction and facilitate many functions of the movement, such as the cooperation between the pallets of the anchor and the escape wheel or the pivoting of a wheel in a bridge. See, for example, EP 3 002 637 A1.

[0003] It is preferable that the lubricant be concentrated on the contact area, where friction is generated, and not spread elsewhere on the rest of the component and movement. For example, in the case of the anchor-escape wheel connection, the lubricant is only effective and necessary when concentrated on the inclined flanks of the pallets and on the impulse planes of the escape wheel.

[0004] To prevent oil or lubricant from spreading, watchmakers use a chemical coating called epilame. Epilame is a liquid product based on organic solvents intended to be applied in a very thin layer to the components of a timepiece and allowing the surface tension between the material forming the component and the lubricant to be reduced. By reducing the surface tension between the material and the lubricant, an imbalance is created with the surface tension of the lubricant with the ambient gaseous environment (air), and the lubricant will then concentrate in the form of drops, thus limiting its spreading.

[0005] The skilled person therefore seeks to cover with epilame only the surfaces of a component that do not need to be lubricated and wants to leave the lubricated contact surfaces bare. For an anchor, for example, the epilame is applied to the entire surface of the pallets by dipping the pallets, or even the entire anchor, in a chemical bath of epilame. The anchor is then placed in the escapement in operation for a brief period without lubricant (running in of the pallets). Thus, the friction of the pallets with the teeth of the escape wheel will remove the layer of epilame from the inclined sides of the pallets, where contact with the escape wheel is made. The lubricant can therefore spread over the inclined sides of the pallets, where contact with the escape wheel is made, but its spread is limited to all other surfaces of the pallets and / or the anchor covered with epilame.

[0006] The use of epilame, however, has many drawbacks: as a chemical product, it is subject to increasingly restrictive legislation, which has led to changes in composition and process in the past. There is therefore a risk that the composition will change again. A more sustainable solution would be preferable. Second, the epilame layer is not durable and has only limited resistance to washing (at best, the layer withstands three washes). This can be problematic in production but also during after-sales service, where washing the timepiece may require a new application of epilame. At the same time, it is also difficult to determine whether the epilame layer is intact or not on the surface of a component. It is also not possible to choose the precise area to be covered with epilame on components on a watchmaking scale.Therefore, as described above for the anchor, after applying the epilame, it is often necessary to manually wash the areas of the component in which epilame is not necessary or even counterproductive. Finally, the thermoplastic resin (shellac) used in particular to fix the pallets to the anchor adheres only with difficulty to the surfaces treated with epilame.

[0007] All of the above elements mean that the effective treatment of an anchor, a wheel, a needle or a bridge by epilame is a long and tedious process and its durability over time must be improved.

[0008] An objective of the invention is therefore to offer an alternative to the chemical treatment by epilame of the anchors, wheels, hands and bridges of a timepiece. The present invention therefore aims to produce an anchor, a wheel and in particular an escape wheel, a hand or a bridge arranged to contain a lubricant in a given area and / or prevent its spreading outside this area, but without the disadvantages of an epilame treatment.

[0009] The present invention relates to an assembly according to claim 1 and a timepiece comprising such an assembly according to claim 7.

[0010] The attached figures schematically illustrate an embodiment of a pallet of an anchor, a bridge and a needle according to the invention. There figure 1 illustrates the contact angle of a drop of liquid on a rigid solid surface. figures 2a et 2b illustrate a palette of an anchor seen in perspective and profile. The figures 3a et 3b illustrate a first embodiment of a pallet of an anchor according to the invention seen in perspective and in profile. The figures 4a et 4b illustrate a second embodiment of a pallet of an anchor according to the invention seen in perspective and in profile. The figure 5a et 5b illustrate an example of a pattern for a surface texturing of an anchor, wheel, needle or bridge according to the invention: the figure 5a illustrates the basic pattern then repeated to obtain the figure 5b . There figure 6 illustrates an embodiment of a bridge according to the invention. The figure 7 illustrates an embodiment of a needle according to the invention.

[0011] A drop of liquid placed on a solid, such as a drop of lubricant on the surface of a watch pallet, can be more or less round (or more or less flat). Adhesive forces between the solid and the liquid cause the drop to spread out, while cohesive forces internal to the liquid cause the drop to round off and avoid contact with the solid. The contact angle θ of a liquid on a solid as illustrated in figure 1 is the angle formed by the surface of the solid with the tangent to a drop of liquid deposited on this solid passing through the edge of the drop. Wettability describes the behavior of the drop on the surface of the solid and qualifies its degree of spreading. Wettability is total when the liquid spreads completely on the surface: the contact angle θ is then 0°. Wettability is partial when the liquid forms a drop on the solid. Wettability is high for a contact angle θ strictly less than 90°: the liquid will spread over a large part of the surface of the solid. Wettability is low for a contact angle θ between 90° and 180° inclusive: the liquid will concentrate in drops and minimize contact with the surface of the solid. The contact angle θ, which decreases the more the drop tends to spread on the surface of a solid, is therefore inversely proportional to wettability.

[0012] Wettability can also be qualified by means of the spreading coefficient S defined by: S = γ SG − γ SL + γ LG where γ SG denotes the solid-air interfacial energy or surface tension, γ SL denotes the solid-liquid interfacial energy or surface tension, and γ LG denotes the surface tension of the liquid in air. The spreading coefficient is expressed in N / m. The sign of the spreading coefficient S determines the wettability: S ≥ 0 indicates total wettability, while S < 0 indicates partial wettability.

[0013] In theory and for a perfect solid, the three interface energies γ SG , γ SL , and γ LG must balance each other, which is summarized by Young's equation: 0 = γ SG − γ SL − γ LG cosθ .

[0014] Combining Young's equation with the definition of the spreading coefficient, we obtain the Young-Dupré equation: S = γ LG cosθ − 1 .

[0015] To compare the wettability of two surfaces for the same liquid, we consider for example a first surface A on which a drop of a liquid L is deposited, all in the ambient air (gaseous environment G) and a second surface B on which a drop of the same liquid L is also deposited in the same gaseous environment G. We also determine the contact angle θ A of the drop L on the surface A and the contact angle θ B of the drop L on the surface B. We obtain the following cases: θ A = θ B : the contact angles are equal, the wettability is the same for both surfaces A and B, and the spreading coefficients SA , SB are also equal. θ A < θ B : the drop L spreads more on the first surface A than on the second surface B, the wettability is therefore higher for the first surface A than for the second surface B. This is also confirmed by calculating the spreading coefficients: SA > SB . θ A > θ B : the drop L spreads more on the second surface B than on the first surface A, the wettability is therefore higher for the second surface B than for the first surface A. This is also confirmed by calculating the spreading coefficients: SB > SA .

[0016] To compare the wettability of two surfaces for the same liquid, we can therefore define the notion of wettability delta: D = S A − S B .

[0017] A surface is an "oleophobic" site if the contact angle θ of a drop of greasy liquid on said surface is strictly between 90° and 150°. For a contact angle greater than 150° (and a hysteresis of said contact angle less than 10°) the surface is said to be "superoleophobic". In the following, oleophobic is used as a general term to describe surfaces characterized by a contact angle θ of a drop of greasy liquid on said surface strictly greater than 90° (i.e. oleophobic is used to designate a surface that is simply oleophobic or superoleophobic).

[0018] Similarly, a surface is a "hydrophobic" site if the contact angle θ of a drop of aqueous liquid on said surface is strictly between 90° and 150°. For a contact angle greater than 150° (and a hysteresis of said contact angle less than 10°) the surface is said to be "superhydrophobic". In the following, hydrophobic is used as a general term to describe surfaces characterized by a contact angle θ of a drop of aqueous liquid on said surface strictly greater than 90° (i.e. hydrophobic is used to designate a surface that is simply hydrophobic or superhydrophobic).

[0019] It is possible for a surface to be both oleophobic and hydrophobic (see for example the surface coating used on mobile phone screens).

[0020] The first aspect of the invention relates to an anchor for an escapement mechanism intended to equip a timepiece. In a well-known manner, the anchor carries two pallets intended to cooperate with the teeth of the escape wheel during the impulse and release phases. One of these pallets is illustrated in figures 2a à 4b .

[0021] The pallet 1 is a small parallelepiped-shaped element intended to be embedded in one of the arms of the anchor. The pallet 1 is terminated by an inclined flank 2, a portion of which, hereinafter called the working surface 20, receives the impulse from the teeth of the escape wheel and cooperates directly with the impulse plane of said teeth. Generally, a lubricant is used to improve the contact between the working surface 20 of the pallet 1 and the teeth of the escape wheel. Ideally, the lubricant should remain on the working surface 20 and on the impulse plane of the teeth of the escape wheel and not spread to the rest of the pallet 1 or the movement.

[0022] Usually, the watchmaker dips the entire pallet 1 in an epilame bath and then washes the inclined flank 2 and the working surface 20 to remove the chemical product (lapping in the anchor). The surface of pallet 1 covered with epilame is then characterized by a lower spreading coefficient than that of the washed working surface (i.e. without epilame) of the inclined flank 2 of pallet 1: that is to say that the lubricant will spread more on the washed working surface 2 than on the rest of pallet 1.

[0023] To replace the use of a chemical treatment such as epilame, the disadvantages of which have been mentioned above, the present invention uses a mechanical surface treatment.

[0024] According to the invention, at least a first zone of the paddle 1 chosen outside the working surface 20 of the inclined flank 2 has a first surface texturing so that the first zone constitutes an epilame effect zone, more oleo- or hydrophobic than the working surface on which a lubricant is applied. That is to say that said lubricant will spread less on the first zone than on the working zone or that the contact angle of a drop of lubricant on the working zone will be smaller than the contact angle of a drop of the same lubricant on the first zone. For a given lubricant, the wettability delta between the first zone and the working surface of the paddle 1 will preferably be between 10 and 40 mN / m and preferably equal to 30 mN / m.

[0025] By texturing and texturing, we mean creating a relief structure formed from the regular repetition of a geometric pattern and extending over an entire determined area of ​​the surface of an object.

[0026] The texturing can be formed by circular or rectilinear grooves, holes, or any other suitable geometric shape.

[0027] Texturing is carried out on the first area of ​​palette 1 by any suitable engraving means, manual or automated (chemical, physical, laser, etc.). Preferably, texturing is obtained by laser or by deep reactive ion etching (known by its English acronym DRIE). Other processes may be considered depending on the materials used, in particular 3D printing techniques.

[0028] Any other known and suitable surface treatment can be carried out before texturing the surface of the first zone: for example, it is possible to carry out plasma cleaning or watch polishing of the entire palette or the first zone beforehand.

[0029] According to the invention, the surface texturing of the first zone is chosen to form microcavities in this first zone, distributed regularly or irregularly, and arranged so that the surface of the first zone between these microcavities is a continuous surface (see the figure 5b ).

[0030] The working surface 20 may also have a second surface texturing, or be the direct result of the manufacturing of the part or even result from another surface treatment (watch polish, plasma cleaning, etc.).

[0031] In a first embodiment illustrated in figures 3a et 3b , a first zone 10 of the pallet 1 directly adjacent to the inclined flank 2 has a first surface texturing so that this first zone 10 constitutes an epilame effect zone, more oleo- or hydrophobic than the inclined flank 2. According to the invention, for a given lubricant, the contact angle of a drop of said lubricant on the inclined flank 2 will be smaller than the contact angle of a drop of the same lubricant on the first zone 10. According to the invention, the wettability delta D between the inclined flank 2 and the first zone 10 is between 10 and 40 mN / m.

[0032] In this first embodiment, the first surface texturing of the first zone 10 is chosen so that the spreading coefficient of the lubricant on the first zone 10 after texturing is lower than the spreading coefficient of the same lubricant on the surface of the inclined flank 2: that is to say so that a drop of lubricant on the surface of the inclined flank 2 spreads more than a drop of the same lubricant on the surface of the first zone 10. Thus, in this example, the first zone 10 acts as a barrier (epilame effect zone) to prevent the lubricant from spreading in this zone, but rather remains contained on the inclined flank 2 and in particular on the working surface 20.

[0033] Preferably, the first zone 10 is chosen to extend on all sides of the pallet so as to surround the inclined flank 2 and to ensure that the lubricant is contained on the inclined flank 2 and does not spread to the rest of the pallet 1.

[0034] Preferably, the first zone 10 does not occupy the entire surface of the pallet 1 excluding the inclined side 2, but a second zone 3 of the pallet 1 is left free, that is to say without surface texturing with an epilame effect, and in particular with a spreading coefficient sufficiently high to allow good attachment of the shellac used to fix the pallet to the anchor.

[0035] In a second embodiment illustrated in figures 4a et 4b , the first zone 11 comprises a portion of the inclined flank 2 directly adjacent to the working surface 20 of the pallet 1. In this case, the texturing of the surface of the first zone 11 is chosen so that the spreading coefficient of the lubricant on the first zone 11 after texturing is lower than the spreading coefficient of the same lubricant on the working surface 20 or so that this first zone 11 constitutes an epilame effect zone, more oleo- or hydrophobic than the working surface. According to the invention, for a given lubricant, the contact angle of a drop of said lubricant on the working surface will be smaller than the contact angle of a drop of the same lubricant on the first zone 11.

[0036] According to the invention, the wettability delta D between the working surface 20 and the first zone 11 is between 10 and 40 mN / m.

[0037] This second embodiment also has the advantage of allowing a wettability delta between the working surface 20 of the inclined flank 2 and the first zone 11, while leaving the rest of the pallet 1 “neutral”, that is to say without treatment modifying the wettability and in particular with a spreading coefficient sufficiently high to allow good attachment of the shellac used to fix the pallet to the anchor.

[0038] Alternatively, it is possible to combine the first zone 11 of the second embodiment and the first zone 10 of the first embodiment, the two zones having the same texturing or a different texturing but always chosen so that the spreading coefficient of the lubricant after texturing is lower than the spreading coefficient of the same lubricant on the working surface 20, so as to make the first zones 10, 11 more oleo- or hydrophobic than the working surface.

[0039] As these examples illustrate, the first area of ​​the palette that has a first surface texturing is preferably adjacent to the working surface, and the goal is to make this area an epilame barrier (more oleo- or hydrophobic area) preventing the spread of the lubricant.

[0040] With regard to the materials, the present invention applies in particular to an anchor, a wheel, a needle or a bridge made from materials traditionally used in watchmaking (ruby, brass, metal alloy) as well as to more recently used materials from the family of glasses, ceramics, oxides or polymers.

[0041] Examples of glasses include silica, soda-lime glasses, borosilicates, non-alkali glasses, vitreous silica, aluminosilicates, sapphire, and fluoride glasses.

[0042] Three families of ceramics can also be identified: oxides, non-oxides and composites, for example zirconium oxide, silicon oxide, ruby, alumina, carbides and nitrides, and silinvar ®< (composite formed of a silicon core and a silicon oxide shell).

[0043] Polymers are, for example, high mechanical performance polymers, such as those among polyaryletherketones (PAEK) like polyetheretherketone (PEEK) or polyamides.

[0044] The anchor, wheel, needle or bridge according to the invention can also be made of silicon.

[0045] As for the lubricant used, it can be a natural or synthetic oil, a mineral lubricant, a soft or hard grease, natural or synthetic, preferably thixotropic, or a fluid gel (between a grease and a liquid) or water-based. Such lubricants are marketed for example under the product names 9010, D5 or 9415 by the Moebius company and are well known to watchmakers.

[0046] As for the size of the first area with a first surface texture, this is preferably at least as large as the average diameter of a drop of the lubricant in question. It is obvious that the larger the first area, the more effective the barrier will be (the less the lubricant will spread into unwanted places).

[0047] Alternatively, it is possible to choose a second texturing for the chosen working surface to reduce the spreading coefficient of the lubricant on this working surface, i.e. to make it more oleo- or hydrophilic. In addition, this second texturing could in particular have the shape of a serpentine or a zigzag, covering a part of the working surface and thus forming a reservoir for the lubricant: it is thus possible to use the surface texturing to create a physical reservoir in which the lubricant spreads well and is also physically retained there.

[0048] The preferred pattern for the first surface texturing is a pattern made of holes. Preferably, the holes have a constant depth. Also preferably, the depth of the holes is greater than or equal to the radius of the holes. Even more preferably, the depth of the holes is greater than or equal to the diameter of the holes. It is also possible to have holes of different radii and different depths.

[0049] Preferably, the ratio between the actual area (area of ​​the continuous surface between the microcavities) and the apparent area (total area of ​​the first surface before texturing) of the first zone is between 5 and 50%, preferably between 10 and 40% and even more preferably between 15 and 30%.

[0050] Knowing the properties of the chosen lubricant, the person skilled in the art is able to choose the appropriate first texturing pattern as well as the size of the first zone.

[0051] According to the invention, the surface texturing of the first zone is chosen to form microcavities in this first zone, distributed regularly or irregularly, and arranged so that the surface of the first zone between these microcavities is a continuous surface (see the figure 5b ).

[0052] The following table illustrates for example different possible textures obtained from the pattern of figures 5a et 5b by varying the distance a between two adjacent holes (distance between centers) and the diameter d of the holes: a d Surface totale St = a 2< sin(60) / 2 Surface vide Sv = π (d / 2) 2< Surface pleine St-Sv Rapport surface pleine / totale (µm) (µm) (µm 2< ) (µm 2< ) (µm 2< ) % 28 21 339 173 166 49 28 22 339 190 150 44 28 23 339 208 132 39 28 24 339 226 113 33 28 25 339 245 94 28 28 26 339 265 74 22 28 27 339 286 53 16 28 28 339 308 32 9 20 15 173 88 85 49 20 16 173 100 73 42 20 17 173 113 60 35 20 18 173 127 46 27 20 19 173 142 32 18 20 20 173 157 16 9 20 21 173 173 0 0 25 18 271 127 143 53 25 19 271 142 129 48 25 20 271 157 114 42 25 21 271 173 98 36 25 22 271 190 81 30 25 23 271 208 63 23 25 24 271 226 45 16 25 25 271 245 25 9 25 26 271 265 5 2

[0053] By performing surface texturing according to the pattern of the figures 5a et 5b and the parameters of the table above on the first zone 10 of palette 1 of the execution mode illustrated in figure 3a , an oleophobic zone is created on the pallet which prevents the lubricant from spreading (reduced spreading coefficient) and a wettability delta of approximately 30 mN / m is obtained between this first zone 10 and the inclined side 2 of the pallet 1 without texturing for the same lubricant.

[0054] The concepts developed above for an anchor according to a first aspect of the invention remain valid for the other aspects of the invention; wheel, in particular escape wheel, hand or bridge. The working surface designates a surface of these elements intended to come into sliding and / or pivoting contact with another watch component (pallet and impulse plane of the teeth of the escape wheel, hand and cannon-pinion, bridge and mobile pivoted on said bridge, wheel and jumper / pawl / other wheel, etc.). Said contact on the working surface may be recurrent as in the case of an anchor or a wheel working with another mobile in motion or unique as in the case of a hand driven onto the cannon-pinion for which contact with the cannon-pinion occurs only during driving.With respect to a wheel and in particular the escape wheel, the surface texturing may be applied to an area directly adjacent to the working surface of each tooth (for example the working surface of the impulse plane of each tooth) or to the flank of the tooth.

[0055] As with the palette above, we choose to texture an area of ​​the wheel, needle or bridge to reduce the spreading coefficient of the lubricant and make this area an epilame effect barrier preventing the lubricant from spreading (area more oleo- or hydrophobic than the working area).

[0056] With respect to a bridge, the working surface may refer to a bearing carried by the bridge or a support surface intended to support a moving part and / or its pivoting, while the first area may be an area adjacent to the bearing or the support surface or to the entire bridge. figure 6 illustrates a bridge 5 in which a slider 6 is pivoted. The working surface 50 of the bridge 5 is the surface of the inner walls of the hole in which the slider 6 is pivoted. The first zone 51 having a first texture is a surface of the bridge immediately adjacent to the hole and preferably extends over the entire surface above which the slider 6 moves. The present invention can also be applied to a plate, a plate, a spring, a jumper, a cam, a barrel, an escape pinion, a barrel drum (inner) or a pin for a timepiece.

[0057] With respect to a needle 7 as illustrated in the figure 7, the working surface 70 is the surface of the needle in contact with the axis 8 onto which the needle 7 is driven. The first zone 71 of the needle 7 having a first texture is chosen to prevent the lubricant from rising by capillarity along the axis and spreading to the rest of the surface of the needle 7.

[0058] Thus, thanks to the present invention, it is possible to produce an anchor, a wheel, and in particular an escape wheel, a hand or a bridge shaped to avoid the spreading of said lubricant outside the working surface of the watch component without having to resort to traditional epilame, which is difficult to implement and not very durable. The surface texturing proposed by the present invention is resistant to washing, allows the area to be treated to be chosen very precisely as well as the final characteristics (spreading coefficient in particular) of said area once textured. The present invention thus offers a more precise and more versatile result, which can be adapted to the precise situation of the component (material, lubricant used, etc.) for greater efficiency. The use of potentially dangerous chemical products as well as time-consuming steps such as running-in of the anchor are also avoided.The use of surface texturing is also applicable to existing traditional components: it is possible to avoid a new epilame bath during after-sales service by applying surface texturing according to the invention to at least one area of ​​the anchor, wheel or bridge. Since the surface texturing step can be carried out after the manufacture of the anchor, wheel or bridge, it generally does not require any modification to the manufacturing process of these parts. Finally, the surface texturing operation can be carried out in large series on production tools, unlike the quenching of a component in a chemical bath.

Claims

1. Assembly formed from a first timepiece component selected from a pallet (1, 2), a wheel, in particular an escapement wheel, a hand (7) or a bridge (5) for a timepiece, from a second timepiece component and from a lubricant, the first timepiece component comprising a working surface (20; 50; 70) intended to come into sliding and / or pivoting contact with the second timepiece component and at least one first zone (10, 11; 51; 71) distinct from and outside of the working surface (20; 50; 70), the lubricant being intended to improve said contact at the working surface of the first component, characterised in that the first zone (10, 11; 51; 71) of the first timepiece component has first surface texturing with an epilame effect rendering the first zone (10, 11; 51; 71) more oleophobic and / or hydrophobic than the working surface (20; 50; 70); in that the first surface texturing is formed of microcavities, distributed in a regular or irregular manner, and arranged so that the surface of the first zone (10, 11; 51; 71) between these microcavities is a continuous surface; and in that the first texturing is selected so that, for said lubricant, the delta of wettability between the first zone (10, 11; 51; 71) and the working surface (20; 50; 70), defined by the difference in absolute value between the spreading factor of the lubricant on the surface of the first zone (10, 11; 51; 71) and the spreading factor of the lubricant on the working surface (20; 50; 70) is between 10 and 40 mN / m, preferably 30 mN / m.

2. Assembly as claimed in the preceding claim, characterised in that the first zone (10, 11; 51; 71) of the first timepiece component is adjacent to the working surface of the first timepiece component.

3. Assembly as claimed in one of the preceding claims, characterised in that the first surface texturing is selected so that the ratio between the area of the continuous surface of the first zone (10, 11; 51; 71) of the first timepiece component between the microcavities and the total area of the first zone (10, 11; 51; 71) before texturing is between 5 and 50%, preferably from 10 to 40% and more preferably between 15 and 30%.

4. Assembly as claimed in one of the preceding claims, characterised in that the first texturing is formed by the repetition of holes, the depth of which is constant and greater than or equal to their radius, preferably greater than or equal to their diameter.

5. Assembly as claimed in one of the preceding claims, characterised in that the working surface (20; 50; 70) of the first timepiece component further comprises second texturing selected to reduce the spreading factor of the lubricant on this working surface (20; 50; 70) and to render it more oleophilic or hydrophilic when said first timepiece component is a pallet, a wheel or a bridge.

6. Assembly as claimed in the preceding claim, characterised in that this second texturing forms a reservoir for a lubricant in the form of a zigzag or serpentine shape and extending over the majority of said working surface.

7. Timepiece comprising an assembly as claimed in one of claims 1 to 6.

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