Hairspring for clock resonator

Abstract

The spiral for a watch resonator according to the invention comprises a ferrule (2) having a geometric axis (A) and an integer number N, greater than or equal to 2, of coplanar elastic blades (3, 4) wound in spirals from and around the ferrule (2) and extending to external extremities (7, 8). The N elastic blades (3, 4) are arranged with rotational symmetry of order N about the geometric axis (A).The spiral is characterized in that, for each of the elastic blades (3, 4), on at least a portion of the blade extending from a first point located on the second turn of the blade counted from the ferrule (2) to a second point (11, 12) located on the outer turn of the blade, the stiffness of the blade is constant and the pitch is increasing, and on a portion (9, 10) of the outer turn of the blade located between the second point (11, 12) and the outer end (7, 8) of the blade, the stiffness of the blade is greater than on said portion of the blade, this portion (9, 10) of the outer turn being arranged to make the development of the blade more concentric during the oscillations of the spiral.

Description

[0001] The present invention relates to a spiral, or spiral spring, for a watch resonator, that is to say a spiral intended to serve as a return spring for a balance wheel of a watch part.

[0002] In a mechanical timepiece, the balance wheel and hairspring assembly is the timekeeping base. The balance wheel oscillates at a specific frequency, being periodically returned to an equilibrium position by the hairspring. Its oscillations are sustained by an escapement. The oscillation frequency of the balance wheel depends on its moment of inertia and the stiffness of the hairspring. Thus, to obtain the desired frequency, a balance wheel with a specific moment of inertia is paired with a hairspring of a specific stiffness during assembly, an operation called "matching."

[0003] It is known that the development of a classic balance spring (in the shape of an Archimedean spiral) during the oscillations of the balance wheel is eccentric. In other words, the center of gravity of the balance spring shifts during the oscillations of the balance wheel. This affects the accuracy of the resonator. Indeed, the eccentric development generates lateral forces between the pivots of the balance staff and the bearings in which they rotate, lateral forces that vary according to the amplitude of oscillation and modify the frequency. Furthermore, the eccentric development adds a perturbation in the vertical positions, related to the weight of the balance spring and the shifts in the center of gravity. This perturbation also varies with the amplitude of oscillation and differs depending on the vertical position of the resonator.

[0004] Several solutions have been proposed to make the development of a balance spring more concentric, including equipping the spring with a terminal curve that extends beyond the plane of the spring to bring the center of gravity back onto the axis of rotation (Breguet spring), stiffening a portion of the outer coil of the spring (patent EP 1473604), or varying the stiffness and pitch of the spring along its blade (patent EP 2299336). These solutions reduce, but do not eliminate, the eccentricity of the spring's development.

[0005] Other solutions do not aim to make the balance spring concentric but rather to compensate for the lateral forces exerted by the spring on the balance staff by superimposing another identical spring wound in the opposite direction or offset by 180°. French patent FR 2447571 describes such an arrangement in which both springs are Breguet springs offset by 180°. Mounting two superimposed springs on the balance staff produces forces or bending moments on the latter that do not cancel each other out. Furthermore, matching the balance wheel to obtain the desired frequency is difficult because it requires managing the combination of two stiffnesses. Finally, the superposition of two springs, like the terminal curve of a Breguet spring, increases the overall height of the resonator.

[0006] The drawbacks of the two-superimposed spiral arrangement have been resolved in the prior art by replacing the two spirals with a double-planar spiral made of two identical coplanar blades wound one inside the other and offset by 180°, or more generally with a multiple-planar spiral comprising N coplanar blades wound one inside the other and arranged with rotational symmetry of order N. Examples of such double or multiple spirals can be found in patents CH 700812 and EP 2154583, among others. The difficulty, however, with these spirals lies in ensuring that the individual blades do not touch during the spiral's development. To this end, patent EP 2154583 proposes two solutions.One approach, in the context of a double spiral, involves placing the points where the outer ends of the two blades are attached outside the plane of the spiral, so that the blades form two symmetrical cones on either side of the spiral's plane. This negates most of the advantages of the double spiral. The other proposed solution, for a double or multiple spiral, involves varying the winding pitch and the blade thickness along the entire length of each blade. Varying the blade thickness alters the spiral's stiffness, which necessitates a significant increase in the spiral's outer diameter, making it impossible to create a compact resonator.

[0007] The present invention aims to remedy, at least in part, the aforementioned drawbacks of the prior art, and to this end provides a spiral according to claim 1, a watch resonator according to claim 12 and a watch part according to claim 13. Particular embodiments are defined in the dependent claims.

[0008] Other features and advantages of the present invention will become apparent from the following detailed description, made with reference to the accompanying drawings, in which: there figure 1 represents a spiral according to a first embodiment of the invention; the figure 2 represents a spiral according to a second embodiment of the invention; the figure 3 represents a spiral according to a third embodiment of the invention.

[0009] The spirals illustrated on the figures 1 à 3 The spirals, designated respectively by the markers 1, 1' and 1", are monolithic and typically produced by a process using the LIGA (Lithography, Galvanization and Forming) electroforming technique, the DRIE (Deep Reactive Ion etching) technique, or a molding technique. Depending on the process used, the spirals 1, 1' and 1" are made of metal or alloy, silicon, silicon coated with a layer of silicon dioxide, quartz, glass, ceramic (e.g., silicon carbide), or metallic glass, for example.

[0010] Each spiral 1, 1', 1" comprises in its central part a ferrule 2 intended to be fixed to the balance staff of a timepiece such as a watch. From and around the ferrule 2 are wound in spirals, one inside the other, two identical and coplanar elastic blades 3, 4. Each blade 3, 4 thus extends from an inner end 5, 6 joined to the ferrule 2 to an outer end 7, 8 intended to be fixed to the case of the timepiece by means of a stud. Each blade 3, 4 comprises several turns, including an inner turn (the first complete turn from the ferrule 2), an outer turn (the first complete turn from the outer end 7, 8), and intermediate turns. The ferrule 2 and the two blades 3, 4 have a common geometric axis A. The two blades 3, 4 are angularly offset by 180° around axis A, in other words they are symmetrical to each other in a rotational symmetry of order 2 around axis A.

[0011] Each blade 3, 4 has a constant cross-section, and therefore constant stiffness, along its entire length, except for a portion 9, 10 of its outer coil which is thicker than the rest of the blade. This stiffened portion 9, 10 is as described in patent EP 1473604. Its position on the outer coil, its length, and its thickness are chosen so that the development (contractions and expansions) of the blade during the oscillations of the balance staff are substantially concentric and, in any case, more concentric than if the portion 9, 10 had not been stiffened. As also described in said patent EP 1473604, a terminal part of the outer coil typically comprising all or part of the stiffened portion 9, 10 can be moved away from the penultimate coil relative to the spiral path to prevent the penultimate coil from touching the outer coil or the pin during large expansions of the blade.

[0012] The winding pitch of each blade 3, 4 increases at least over a portion of the blade extending from a first point located on the second turn counted from the ferrule 2 to a second point located on the outer turn, this second point preferably being located in the two-thirds of the outer turn closest to the outer end 7, 8, or even more preferably in the half of the outer turn closest to the outer end 7, 8, in angular length measured from the axis A. The pitch increase may be strict or occur in steps. In the example of the figure 1 The pitch is constant over several turns from ferrule 2, then increases strictly over approximately one turn, then remains constant over several turns up to a point 11, 12 on the outer turn located before the stiffened portion 9, 10. In the example of the figure 2 The pitch increases strictly from the ferrule 2 at least up to a point 13, 14 on the outer coil located before the stiffened portion 9, 10. In the example of the figure 3 The pitch is constant over several turns starting from ferrule 2, then increases strictly over several turns at least up to a point 15, 16 on the outer turn located before the stiffened portion 9, 10. In each embodiment, however, the pitch could be larger on the inner turn than on the subsequent inner turns. Furthermore, the inner turn of each blade 3, 4 could have a variable pitch (for example, to form a Grossmann curve) and / or a variable stiffness (for example, to form a stiffened portion), as proposed in patent EP 2917787. However, giving the inner turn the same stiffness and pitch as those of the subsequent inner turns (as in the examples of figures 1 And 3) or the same rigidity as the following inner turns and a pitch that evolves in the same way as that of the following inner turns (as in the example of the figure 2 ) facilitates the design of the spiral.

[0013] The combination of the stiffened sections 9 and 10 and the increasing pitch of the blades 3 and 4 prevents the two blades 3 and 4 from touching during the development of the balance spring. It also ensures sufficient spacing between the coils to prevent the two blades 3 and 4 from attracting and sticking to each other. These advantages are achieved without compromising the compactness of the balance spring, which maintains a constant blade thickness except at the stiffened sections 9 and 10, allowing the desired order of magnitude of stiffness to be reached without increasing the external diameter of the balance spring. Furthermore, the symmetry of the balance spring makes the balance-spring resonator isochronous (its oscillation frequency is virtually independent of the oscillation amplitude) and reduces the resonator's rate deviations between different vertical positions (the rate deviations between vertical positions are now only the result of an imbalance on the balance rim).By virtue of its flat structure, which remains flat during operation, and its symmetry, the balance spring according to the invention does not exert uncompensated forces or moments on the balance staff. Finally, the balance spring can be paired with a balance wheel in the same way as for a conventional balance spring resonator.

[0014] In variants, the arrangement of the rigidified portions 9, 10 could be modified to compensate for a lack of isochronism due to the escapement, in accordance with the teaching of the aforementioned patent EP 2917787.

[0015] An alternative to increasing the thickness of the blades to obtain the stiffened portions 9 and 10 would be to increase the height of the blades. Portions 9 and 10 could also be stiffened by means other than increasing the cross-section of the blades, for example, by heat treatment or by doping the material.

[0016] The number of coplanar blades of the spiral according to the invention is not limited to two. Generally, the spiral according to the invention comprises an integer number N of blades, greater than or equal to 2, these blades being arranged around the geometric axis A according to a rotational symmetry of order N.

Claims

1. Spiral (1; 1'; 1") for clock resonator, comprising a ferrule (2) having a geometric axis (A) and an integer number N, greater than or equal to 2, of coplanar elastic blades (3, 4) wound in spirals from and around the ferrule (2) and extending to external extremities (7, 8), the N elastic blades (3, 4) being arranged according to a rotational symmetry of order N around the geometric axis (A), characterized in that, for each of the elastic blades (3, 4): - on at least a portion of the blade extending from a first point located on the second turn of the blade counted from the ferrule (2) to a second point (11, 12; 13, 14; 15, 16) located on the outer turn of the blade, the stiffness of the blade is constant and the pitch is increasing, - on a portion (9, 10) of the outer turn of the blade located between the second point (11, 12; 13, 14; 15, 16) and the outer end (7, 8) of the blade, the stiffness of the blade is greater than on said portion of the blade, this portion (9, 10) of the outer turn being arranged to make the development of the blade more concentric during the oscillations of the spiral.

2. Spiral (1; 1") according to claim 1, characterized in that the pitch has a first constant value over at least several complete turns counted from the first point.

3. Spiral (1; 1") according to claim 2, characterized in thatthe pitch presents the first constant value also of the ferrule (2) at the first point.

4. Spiral (1) according to claim 2 or 3, characterized in that the pitch has a second constant value, greater than the first constant value, over at least one complete turn after the said several complete turns counted from the first point.

5. Spiral (1) according to claim 4, characterized in that the step has the second constant value at least up to the second point (11, 12).

6. Spiral (1") according to claim 2 or 3, characterized in that the pitch is strictly increasing at least over several complete turns after said several complete turns counted from the first point and at least up to the second point (15, 16).

7. Spiral (1') according to claim 1, characterized in that the pitch is strictly increasing at least on said portion of the blade.

8. Spiral (1') according to claim 7, characterized in thatthe pitch is strictly increasing at least from the ferrule (2) to the second point (13, 14).

9. Spiral (1; 1'; 1") according to any one of claims 1 to 8, characterized in that the pitch is increasing at least from the ferrule (2) to the second point (11, 12; 13, 14; 15, 16).

10. Spiral (1; 1'; 1") according to any one of claims 1 to 9, characterized in that the rigidity of the blade is constant at least from the ferrule (2) to the second point (11, 12; 13, 14; 15, 16).

11. Spiral (1; 1'; 1") according to any one of claims 1 to 10, characterized in that the second point (11, 12; 13, 14; 15, 16) is located in the two-thirds of the outer loop closest to the outer end (7, 8), preferably in the half of the outer loop closest to the outer end (7, 8), in angular length measured from the geometric axis (A).

12. Clock resonator comprising a balance wheel and a balance spring (1; 1'; 1") according to any one of claims 1 to 11.

13. Timepiece comprising a clockwork resonator according to claim 12.

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