Balance for timepiece

Abstract

The balance wheel for a timepiece according to the invention comprises a structure (2) made of a first material and inertial masses (3) glued or brazed in respective housings (6) of the structure (2) and made of a second material, the second material being denser than the first material. Each inertial mass (3) is in point contact with the wall of the corresponding housing (6) at three points (B, C, D).

Description

[0001] The present invention relates to a balance wheel for a timepiece.

[0002] The balance wheel is the oscillating inertial element of a regulating organ serving as a time base in a timepiece. It is associated with a return spring - typically a balance spring or a flexible guide - arranged to return it to a position of equilibrium.

[0003] An important characteristic of balance wheels is their moment of inertia to mass ratio. The aim is to have, for a given moment of inertia, the balance mass as small as possible, in particular to limit energy losses and timekeeping defects.

[0004] It has thus become common to produce composite balances comprising a structure made of a low-density material (typically silicon) and denser metallic inertia masses located on or in the peripheral part of the structure. Such balances are described, for example, in documents WO 2008 / 135817, EP 2395402, EP 2485095, EP 2911012 and EP 3182214. The inertia masses are either formed on or in the structure by galvanic growth (WO 2008 / 135817, EP 2485095 and EP 3182214) or assembled to the structure by driving, bonding, brazing, welding or elastic mounting (EP 2395402 and EP 2911012). The galvanic growth technique is precise but has the disadvantage of being expensive and requiring a great deal of expertise. Joining techniques such as chasing, gluing, soldering, welding or elastic mounting are simpler to implement but are less precise.However, the positioning precision of the inertial masses influences the moment of inertia and the unbalance of the balance wheel, and therefore the chronometric precision.

[0005] The present invention aims to provide a composite balance wheel for a timepiece whose manufacture can be precise without requiring the inertia masses to be formed on or in the structure by galvanic growth.

[0006] To this end, the present invention relates to a balance wheel according to claim 1, particular embodiments being defined in the dependent claims.

[0007] The present invention also relates to a timepiece, in particular a watch, comprising such a balance wheel.

[0008] Other characteristics and advantages of the present invention will appear on reading the following detailed description made with reference to the appended drawings in which: there figure 1 is a perspective view of a balance wheel according to a particular embodiment of the invention; the figure 2 is a top plan view of a portion of the balance wheel shown in figure 1 .

[0009] In reference to the figure 1 , a balance 1 for a timepiece according to the invention comprises a structure 2 and inertial masses 3 rigidly fixed in the peripheral part of the structure 2. In the example shown, the balance 1 has a butterfly-type shape and comprises two inertial masses 3 symmetrical with respect to a geometric axis A of the structure 2 which coincides with the axis of rotation of the balance 1 when the latter is in use. Alternatively, however, the balance 1 could be circular in shape and have more than two inertial masses distributed around the geometric axis A.

[0010] The structure 2 is made of a low-density material, i.e. a material having a low density, typically less than or equal to 10 kg / dm 3< , or even 8 kg / dm 3< , or even 6 kg / dm 3< , or even 4 kg / dm 3< , and ideally less than or equal to 3 kg / dm 3< . The material of the structure 2 is for example based on silicon, diamond, quartz, glass, silicon carbide, titanium, aluminum, nickel, nickel-phosphorus, steel or copper, and is preferably silicon covered with a layer of silicon dioxide. Depending on its material, the structure 2 is produced by deep reactive ion etching DRIE, laser etching, electroerosion, LIGA or other. The structure 2 may be flat or, as shown, on two levels, with a first level comprising the base of the structure 2 and a second level comprising elements such as tenons 4 for mounting weights 5 for adjusting inertia and imbalance.

[0011] The inertial masses 3 are made of a denser material than that of the structure 2, typically made of a material with a density greater than or equal to 12 kg / dm 3< , or even 16 kg / dm 3< , and ideally greater than or equal to 19 kg / dm 3< . The material of the inertial masses 3 is for example a metal, an alloy or a loaded ceramic (for example loaded with tungsten), and is preferably gold or platinum. The inertial masses 3 are typically in the form of annular segments. Housings 6 are formed in the peripheral part of the structure 2 to respectively accommodate these inertial masses 3. Preferably, these housings 6 are through-shaped in the direction parallel to the geometric axis A, and thus open onto the upper and lower surfaces of the structure 2. Preferably, also, these housings 6 open radially onto the outer edge 7 of the structure 2. These housings 6 can be raised by parts 8 of the structure 2 located on the second level.

[0012] The inertial masses 3 are glued into the housings 6 in a manner that positions them precisely so as to provide the balance with a well-determined moment of inertia relative to the geometric axis A without increasing the unbalance of the balance, which is ideally zero. Advantageously, the inertial masses 3 protrude from the outer edge 7 of the structure 2 to maximize the moment of inertia relative to the geometric axis A.

[0013] In reference to the figure 2 , each inertial mass 3 has an inner surface 9 facing the geometric axis A, an outer surface 10 opposite the inner surface 9 (and which therefore advantageously protrudes from the outer edge 7 of the structure 2) and first and second opposite lateral surfaces 11, 12 which connect the inner and outer surfaces 9, 10 by means of rounded sections 11a, 12a. For precise positioning of the inertial masses 3 in their housings 6, each inertial mass 3 is in point contact with the wall of the corresponding housing 6 at three points, these three points preferably consisting of two contact points B and C between the inner surface 9 and the bottom 13 of the housing 6 and a contact point D between the first lateral surface 11 and a lateral surface 14 of the housing 6. As can be seen in the figure 2 , the point contact between the inertial mass 3 and the lateral surface 14 of the housing 6 means that the inertial mass 3 can be slightly offset laterally relative to the housing 6. The glue used to hold the inertial mass 3 in the housing 6 is distributed between the contact points B, C and D and more generally in interstices 15 between the inertial mass 3 and the wall of the housing 6. A thin layer of glue may nevertheless remain between the inertial mass 3 and the wall of the housing 6 at the contact points B, C and D.

[0014] To obtain the three points of contact between each inertial mass 3 and the wall of its housing 6, several solutions are possible. A particularly advantageous solution in terms of positioning of the inertial mass 3 and effectiveness of the bonding is that illustrated in figure 2where the inner surface 9 of the inertial mass 3 is concave, more precisely in the shape of an arc of a circle centered on the geometric axis A of the structure 2, where the bottom 13 of the housing 6 is convex, more precisely in the shape of an arc of a circle, where the radius of curvature of the inner surface 9 is smaller than the radius of curvature of the bottom 13, where the two points of contact B and C between the inner surface 9 and the bottom 13 are therefore located at the ends of the inner surface 9, and where the lateral surface 14 of the wall of the housing 6 is convex and the two lateral surfaces 11, 12 of the inertial mass 3 are planar. Concerning the lateral surfaces 11 and 14, an inverse solution is of course possible where the lateral surface 11 of the inertial mass 3 would be convex and the lateral surface 14 of the housing 6 would be planar.In exemplary embodiments, the difference between the radii of curvature of the bottom 13 and the inner surface 9 is between 0.05 mm and 1 mm, preferably between 0.1 mm and 0.5 mm, preferably between 0.15 mm and 0.5 mm, more preferably between 0.2 mm and 0.3 mm. In exemplary embodiments, the radius of curvature of the convex side surface 11 or 14 is between 0.15 mm and 5 mm, preferably between 0.5 mm and 3 mm, preferably between 1 mm and 2 mm.

[0015] In order to facilitate positioning of the inertial masses 3 which does not generate unbalance, the housings 6 are symmetrical to each other according to an axial symmetry of order N with respect to the geometric axis A, where N is the number of housings / inertia masses, and the inertial masses 3 are symmetrical to each other according to this same axial symmetry of order N, which implies in particular that the possible lateral offset of the inertial masses 3 in their housings 6, seen from the geometric axis A, is the same (i.e. in particular is on the same side) for all the inertial masses 3.

[0016] The bonding of a given inertial mass 3 can be carried out by spreading glue over the entire wall of the housing 6, by introducing the inertial mass 3 into the housing 6 and by pressing the inertial mass 3 against the three contact points B, C and D until the glue polymerizes. To ensure the height positioning of the inertial mass 3, said pressure can be exerted on the inertial mass 3 by the force of gravity by placing the structure 2 and the inertial mass 3 on an inclined plane having a stop and by allowing the force of gravity to act while the inertial mass 3 is resting against the stop and the structure 2 is resting against the inertial mass 3, the inclined plane being made of a material to which the glue does not adhere. In a variant, the force of gravity can be replaced or supplemented by the force of a spring.

[0017] In another embodiment of the invention, the inertia masses 3 are brazed in the housings 6 instead of being glued. For this, the same method as described above can be implemented by replacing the glue with solder, with the difference that the wall of the housings 6 is previously metallized, for example by physical or chemical vapor deposition, in the case of a structure 2 made of a non-metallic material, that likewise the surfaces of the inertia masses 3 are previously metallized if the inertia masses 3 are made of a non-metallic material and that a heating operation is implemented, for example in a furnace, to make the solder act on the structure 2 and the inertia masses 3.

Claims

1. Balance wheel (1) for a timepiece comprising a structure (2) made of a first material and inertia masses (3) glued or brazed in respective housings (6) of the structure (2) and made of a second material, the second material being denser than the first material, the structure (2) having a geometric axis (A) which is an axis of rotation of the balance wheel (1) when the latter is in service, characterized in that each inertial mass (3) is in point contact with the wall of the corresponding housing (6) at three points (B, C, D).

2. Balance wheel according to claim 1, characterized in that each inertial mass (3) has an inner surface (9) facing the geometric axis (A), an outer surface (10) opposite the inner surface (9) and first and second opposite lateral surfaces (11, 12) connecting the inner and outer surfaces (9, 10), and in thatthe three points (B, C, D) consist of two contact points (B, C) between the inner surface (9) and a bottom (13) of the corresponding housing (6) and one contact point (D) between one (11) of the first and second opposite side surfaces (11, 12) and a side surface (14) of the corresponding housing (6).

3. Balance wheel according to claim 2, characterized in that the inner surface (9) of each inertial mass (3) is concave, in that the bottom (13) of each housing (6) is convex and in that the radius of curvature of the inner surface (9) of each inertial mass (3) is smaller than the radius of curvature of the bottom (13) of the corresponding housing (6).

4. Balance wheel according to claim 2 or 3, characterized in that, for each inertial mass (3), one of said lateral surface (14) of the housing (6) and of the corresponding lateral surface (11) of the inertial mass (3) is convex and the other of said lateral surface (14) of the housing (6) and of the corresponding lateral surface (11) of the inertial mass (3) is flat.

5. Balance wheel according to claim 2 or 3, characterized in that , for each inertial mass (3), said lateral surface (14) of the housing (6) is convex and the corresponding lateral surface (11) of the inertial mass (3) is flat.

6. Balance wheel according to one of claims 1 to 5, characterized in that the housings (6) open onto the outer edge (7) of the structure (2).

7. Balance wheel according to claim 6, characterized in that the inertial masses (3) protrude from the outer edge (7) of the structure (2).

8. Balance wheel according to one of claims 1 to 7, characterized in thatthe housings (6) are through in the direction parallel to the geometric axis (A).

9. Balance wheel according to one of claims 1 to 8, characterized in that the housings (6) are symmetrical to each other according to an axial symmetry of order N with respect to the geometric axis (A), where N is the number of housings (6), and in that the inertial masses (3) are symmetrical to each other according to this same axial symmetry.

10. Balance wheel according to one of claims 1 to 9, characterized in that the structure (2) is made of a material having a density less than or equal to 10 kg / dm 3 , preferably less than or equal to 8 kg / dm 3 , preferably less than or equal to 6 kg / dm 3 , preferably less than or equal to 4 kg / dm 3 , preferably less than or equal to 3 kg / dm 3 .

11. Balance wheel according to one of claims 1 to 10, characterized in thatthe structure (2) is made of a material based on silicon, diamond, quartz, glass, silicon carbide, titanium, aluminum, nickel, nickel-phosphorus, steel or copper.

12. Balance wheel according to one of claims 1 to 11, characterized in that structure (2) is made of silicon covered with a layer of silicon dioxide.

13. Balance wheel according to one of claims 1 to 12, characterized in that the inertial masses (3) are made of a material having a density greater than or equal to 12 kg / dm 3 , preferably greater than or equal to 16 kg / dm 3 , preferably greater than or equal to 19 kg / dm 3 .

14. Balance wheel according to one of claims 1 to 13, characterized in that the inertial masses (3) are made of metal, alloy or ceramic.

15. Balance wheel according to one of claims 1 to 14, characterized in that the inertial masses (3) are made of gold or platinum.

16. Timepiece comprising a balance wheel (1) according to one of claims 1 to 15.

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