EP1654597B1 - Thermally-compensated balance wheel - Google Patents

Abstract

The balance wheel (1) is produced by chemical etching of a sheet of material with a low thermal coefficient of expansion and a low density, such as, for example, diamond on which a number of balance weights (2, 20, 21, 22, 23, 24) are arranged. The form and position of the balance weights is determined such that the variation in the radius of gyration of the balance wheel caused by a variation in temperature is zero or negative, thus providing a balance wheel with a zero or negative thermal characteristic.

Description

The present invention relates to a pendulum of a watch movement made from a material having in particular a low coefficient of thermal expansion, such as diamond.

The dimensional variations of a balance, due to temperature variations, are one of the major problems of these objects; in fact, these dimensional variations, associated with those of the spiral spring associated with it, are a cause of variation of the oscillation frequency of the balance / hairspring system, respectively a cause of the lack of precision of a clockwork movement in function of the temperature. Means for compensating or correcting these variations have been proposed such as the bimetallic balance used in particular in marine chronometers. The disadvantage of such a rocker is its unfavorable aerodynamic shape causing poor penetration into the air during its oscillation movement and the difficulty of maintaining the center of gravity of the balance on its pivot axis. It can not be installed in a watch movement that must work in any position.

Furthermore, a watch movement balance must have a mass as low as possible for as much inertia as possible, while having excellent mechanical strength.

Advantageously, to make a rocker, one will choose a material having a low coefficient of thermal expansion, a low density and a high mechanical strength, these parameters being compared here to those materials usually used for the construction of rockers, as per for example, copper alloys or nickel.

Several classes of materials fulfill these conditions: for example the non-metallic materials of the carbon group comprising, for example, diamond or else metal oxides including for example corundum such as sapphire or ruby, the list of materials mentioned here being absolutely not exhaustive.

Processes for obtaining mechanical parts, especially parts used in watch movements, by chemical etching (Deep reactive ion etching) of a diamond plate have been described. The favorable physicochemical characteristics of diamond, in particular the low coefficient of friction, the high impact resistance, the mechanical strength, the low density, the high modulus of elasticity, the low coefficient of thermal expansion are among others parameters favoring its use in the aforementioned field.

According to machining methods similar to those mentioned above or other methods, other materials than diamond are also suitable for use in making mechanical parts such as clockwork balances.

It is therefore the low coefficient of thermal expansion, the low density and the high mechanical strength which are here favorable parameters to the use of these different materials for the manufacture of a balance.

An object of the invention is therefore to provide a balance whose characteristics are significantly improved relative to those existing balances, in particular its behavior, respectively its insensitivity to temperature variations and its moment of inertia / mass ratio.

There is also a watch movement provided with such a balance, as well as a timepiece equipped with such a movement.

A balance as desired is described in claim 1, while a watch movement and a timepiece as proposed are respectively described in claim 16 and claim 17.

• la figure 1 est une vue en plan d'un balancier selon l'invention, et
• la figure 2 est une vue en coupe d'un détail de la figure 1.

A particular embodiment of a rocker according to the invention comprising several variants is described below, this description being to be considered with reference to the appended drawing comprising the figures in which:

• the figure 1 is a plan view of a pendulum according to the invention, and
• the figure 2 is a sectional view of a detail of the figure 1 .

According to the form of execution of figure 1 , the balance 1 consists of a disc 10 obtained for example, by chemical etching using a plasma (Deep reactive ion etching), or by any other method, a diamond plate of generally constant thickness . A rocker pivot 11 is fixed in known manner to the center of the disk 10. A plurality of fastening means 12, eight in the example shown, are arranged on a circumference 120 close to the outer perimeter of the disk 10. Ferrules or weights 2 are fixed on the disc 10 by the known fastening means 12.

The rocker 1 is assembled in a known manner to a spiral spring, not shown in the figure so as not to overload, the latter being disposed under the plane of the disc 10 as shown in FIG.

The figure 1 shows some examples of execution of the weights 2, it being understood that generally a given balance has only weights 2 of a certain type.

The weights 2 are essentially of shape or asymmetrically fixed relative to the circumference 120 on the disk 10. In fact, considering that the disk 10 comprises eight weights in the form of elongate elements, as represented for example by the weight 20, and assuming that the balance 1 thus formed is subjected to an increase in temperature, the disc 10 made of very low coefficient of thermal expansion, for example diamond, will see its diameter increase only very slightly. On the other hand, the weights 20, that is to say made of a material with a thermal expansion coefficient that is much larger than that of the material constituting the disc 10, will have their dimensions increase by a ratio higher than that of the record 10.

The oscillation frequency of the balance 1 depends in part on its moment of inertia, the latter parameter being equal to: $$\mathrm{l}\mathrm{=}\mathrm{m}\mathrm{.}{\mathrm{<figure-callout\; id="2"\; label="r"\; filenames="imgf0001.png"\; state="\{\{state\}\}">r</figure-callout>}}^{\mathrm{2}}$$

• l : moment d'inertie
• m. masse du balancier
• r : rayon de giration du balancier

With:

• l: moment of inertia
• m. mass of the pendulum
• r: radius of gyration of the pendulum

The radius of gyration is the radius of the circumference over which the whole mass m of a pendulum having the same moment of inertia as that considered would be concentrated.

The oscillation frequency of the balance 1 also depends on the torque c supplied by the balance spring to said balance, this torque also varies with the temperature.

• l / c = constante en fonction de la température.

For a balance / hairspring assembly, we therefore seek to obtain a report:

• l / c = constant as a function of temperature.

The mass m does not vary with the temperature, the shape, the dimensions and the positioning of the weights 20 will be determined so that for any temperature, the variation of the radius of gyration caused by the variation in diameter of the disk 10 is compensated by a corresponding variation of the radius of gyration caused by the dimensional variations of the weights.

For example, in case of temperature increase, the disc 10 expands very slightly, thereby increasing its own radius of gyration. Due to the increased diameter of the disk 10, the attachment points 12 of the weights 20 also move on a circumference 120 of larger diameter. The weights 20 also increase their length, by a factor greater than the increase in diameter of the disc 10 since the coefficient of thermal expansion of the weights 20 is significantly higher than that of the material of the disc 10, and given the asymmetrical arrangement of weights on their attachment point, the portion of the mass of the weight placed on the side of the center of the disk 10 being generally larger than the portion of the mass of the weight placed in the direction of the outer perimeter of the disk, the center of gravity of the each weight moves towards the center of the disc 10, respectively the radius of gyration due solely to the flyweights decreases, partially compensating, completely or overcompensating the variation of the radius of gyration due to the disc. The phenomenon is exactly reversed in the event of a drop in temperature.

Thus, by judiciously choosing the shape, the volume, the degree of asymmetry around the fastening as well as the material, respectively the coefficient of thermal expansion of the weights 2, one can obtain an overcompensation of the thermal effect so that the radius of gyration of the balance decreases as a function of temperature, thus giving the balance a negative temperature coefficient. By associating this balance with a negative temperature coefficient to a spiral with a positive temperature coefficient, it is thus possible to obtain a balance / hairspring system absolutely insensitive to temperature variations.

The positioning of the weights 2 on the disc 10 makes it possible to adjust the thermal coefficient of the balance, respectively of the balance / balance system to a positive, zero or negative value as required.

The figure 1 also shows other possible forms of weights 2, it being understood that they are represented only by way of example and in no way limit the possible forms for obtaining the desired effect described above.

The flyweight 21 is essentially triangular in shape with rounded corners, since its mass is distributed asymmetrically around its center of gravity, the adjustment of the degree of thermal compensation can be done by rotating the flyweight around its attachment point. The flyweight 22 is common at two attachment points. The weight 23 is in the form of an elongate bar, it can adjust its degree of compensation by varying the angle α that makes the axis of the weight to the radius of the disc. Another means of adjusting the degree of thermal compensation is shown on the weight 24 which has, for example an oblong hole 240 for adjusting the radial position of the weight.

In order to obtain balance of the balance, the weights of each pair of two radially opposed weights are adjusted symmetrically, so as not to create unbalance. To achieve a fine static and dynamic equilibrium, at least one pair of orientable ferrules 25, generally known in the art, can be added to the device.

In order to improve the aerodynamic behavior of the balance during its oscillation movement, the flyweights can be cut and formed in order to reduce their friction in the air, as represented by way of example in the figure 2 .

It is therefore understood that by mounting weights of one of the types shown as examples of possible embodiments at 20, 21, 22, 23 or 24, and optionally a pair of orientable ferrules 25, it is possible to 'finely adjust during assembly, both the temperature coefficient of the balance and its static equilibrium and dynamic. Once these settings have been made during editing, there is usually no need to make further adjustments.

The example described above and represented on the figure 1 shows a balance wheel provided with eight attachment points 12 of flyweights. It is understood that the balance wheel may include any number of attachment points, different from eight.

Disk 10 is described above and shown as consisting of a solid disk. This embodiment is particularly advantageous in view of the friction in the air caused by the oscillation movements of the balance around its pivot, a balance in the form of a solid disc having a better aerodynamic behavior than a compound beam a center and a serge connected by arms. This form of execution as a solid disc is achievable due to the low density of the material; the realization of a solid disc does not pejorant excessively the mass of the disc 10. However, if a reduction in the mass of the disc is absolutely necessary, given the strong mechanical strength of the material, in particular diamond, openings, as represented in FIG. 13 or 130 can be arranged. Similarly, the disc 10 may not be absolutely flat and of constant thickness, it may also for example have a serge in the form of a flange on the outer perimeter of the disc.

A rocker as described above, according to one or the other of its variants as well as made with one or the other of the materials adapted for this purpose, can be used advantageously with a made spiral it is also made of a material with a low coefficient of thermal expansion, such as diamond, as well as with a metal balance spring. The heat coefficient of the balance is then chosen so as to compensate for the thermal expansion of the spiral, low in the case of a diamond spiral and strong in the case of a metal balance spring.

Claims (17)

1. A balance wheel for a timepiece movement, comprising:

   a disc (10) obtained from a plate in a material having a heat expansion coefficient as well as a density less than that of nickel, characterized in that said disc includes a plurality of attachment means (12) distributed over a circumference (120) close to the outer perimeter of the disc, and

   a plurality of balance weights (2, 20, 21, 22, 23, 24) each being attached to one of said attachment means, said balance weights (2, 20, 21, 22, 23, 24) consisting in a material having a heat expansion coefficient larger than that of the material making up the disc,

   the disc (10) being made by chemical etching by means of a plasma, of a plate of said material making up said disc.
2. The balance wheel according to claim 1, characterized in that said material making up said disc (10) is from the group of carbon.
3. The balance wheel according to claim 1, characterized in that said material making up said disc (10) is from the group of metal oxides.
4. The balance wheel according to claim 2, characterized in that said material is diamond.
5. The balance wheel according to claim 3, characterized in that said material is corundum such as sapphire or ruby.
6. The balance wheel according to claim 1, characterized in that for each balance weight, the mass portion of the balance weight located outside the circumference (120) on which the attachment means (12) are set up is different from the mass portion of the balance weight located inside said circumference.
7. The balance wheel according to claim 6, characterized in that the ratio between the mass portion of the balance weight located outside the circumference (120) on which the attachment means (12) are set up and the mass portion of the balance weight located inside said circumference is selected so that the variation depending on the temperature of the radius of gyration of the balance wheel is zero, providing said balance wheel with zero temperature coefficient.
8. The balance wheel according to claim 6, characterized in that the ratio between the mass portion of the balance weight located outside the circumference (120) on which the attachment means (12) are set up and the mass portion of the balance weight located inside said circumference is selected so that the variation depending on the temperature of the radius of duration of the balance wheel is negative, respectively that the radius of duration decreases for an increase in temperature, providing said balance wheel with negative temperature coefficient.
9. The balance wheel according to any of claims 7 or 8, characterized in that the adjustment of the ratio between the mass portion of the balance weight located outside the circumference (120) on which the attachment means (12) are set up and the mass portion of the balance weight located inside said circumference is obtained by pivoting the balance weight (21, 23) around its attachment means.
10. The balance wheel according to any of claims 7 or 8, characterized in that the adjustment of the ratio between the mass portion of the balance weight located outside the circumference (120) on which the attachment means (12) are set up and the mass portion of the balance weight located inside said circumference is obtained by radial displacement of the balance weight (24) on its attachment means.
11. The balance wheel according to any of the preceding claims, characterized in that it further comprises at least one pair of orientable collets (25) with which the balance wheel may be balanced statically and dynamically.
12. The balance wheel according to any of the preceding claims, characterized in that the balance weights are cut so as to have low aerodynamic resistance.
13. The balance wheel according to any of the preceding claims, characterized in that the disc (10) is solid.
14. The balance wheel according to any claims 1 to 12, characterized in that the disc (10) has at least one aperture (13, 130).
15. The balance wheel according to any of the preceding claims, associated with a spiral spring, characterized in that the ratio of the moment of inertia of the assembly divided by the torque provided by the spiral spring of the balance wheel is constant versus temperature.
16. A timepiece movement comprising a balance wheel according to any of the preceding claims.
17. A timepiece comprising a timepiece movement according to claim 16.

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