EP2652560B1 - Magnetic screening for timepiece hairspring - Google Patents

Description

The present invention relates to a device for protecting a spiral of a mechanical timepiece against disruptive magnetic fields coming from outside this timepiece.

In mechanical timepieces, the material constituting the hairspring is generally made of a metal alloy such as a steel capable of remanent magnetization if it is subjected to an external magnetic field. Although it is possible to envisage making this part in a non-magnetic material in order to neutralize the impact of an external magnetic field disturbing the proper functioning of such a mechanical timepiece, this disadvantage of susceptibility to magnetization for a spiral made with traditional materials is very largely offset by the excellent mechanical qualities that are conferred on it (ductility, elasticity, coefficient of thermal expansion, etc.). It has therefore been sought to protect this type of spirals against these disturbing fields. , so that if the motion is subjected to a field of the order of 4.8kA / m, the deviation does not exceed 30 seconds per day according to a clock normalization. Beyond this field of 4.8kA / m and unprotected this difference is very variable and reaches significant variations, up to several minutes per day. This difference is due mainly to the longitudinal magnetization of the turns of which the hairspring is made, this magnetization producing on the axis of the balance to which is connected the hairspring a torque that adds up or subtracts from the mechanical torque of normal operation. The gap is also influenced, but to a lesser extent, by the phenomenon of magnetostriction tending to lengthen or shorten the ribbon of which the spiral is made when subjected to a magnetic field.

To solve this problem of magnetic insulation of the regulating member, devices have already been proposed which ensure the protection of a timepiece against the disturbing influence of external magnetic fields of any kind, such as for example external fields. from permanent magnets or electric motors of any kind.

The simplest and most radical solution is to completely shield the movement of the timepiece to allow no disturbing line of fields to penetrate. This is the case proposed by the document CH 122391 where the movement of the watch is protected by a set of elements formed of a stainless alloy, high permeability and low hysteresis, forming a magnetic screen. The elements are a bowl disposed between the movement and the bottom of the watch, a dust cover ring forming a cap disposed between the movement and a casing ring, and an intermediate plate disposed between the watch plate and its dial. This way of doing things is extremely cumbersome and expensive. Indeed it requires three additional pieces that not only weigh down the watch but also increase its volume.

A lighter and less cumbersome solution than that proposed above is described in the document FR 1,408,872 . It is no longer a question here of completely surrounding the movement of the watch by a material with high permeability but only its bottom and its periphery. For this purpose, the device is constituted by a housing element having a sufficient permeance to magnetic fields, the housing element being completed by a casing ring of mild steel with which it forms a bowl enveloping the movement and forming a screen magnetic. The housing element is formed by the bottom of the housing, this bottom being made of stainless and polishable alloy having a homogeneous ferritic structure. Thus, in this embodiment, there is no addition of additional parts, the bottom and the casing ring being directly made of materials with high magnetic permeability. In addition no screen is disposed between the movement and the dial of the watch, the protection device being limited to a bowl without lid serving as housing for the movement of the watch.

A first disadvantage of this latter solution, however, is that the hairspring proper is not protected against a disturbing field whatever the orientation of this field prevailing in the plane of the hairspring. Indeed, as the hairspring is off-center with respect to the center of the movement and if omnidirectional protection is desired, it is a question of proposing a device centered with respect to said hairspring and not with respect to the movement as a whole as it is the case of the document cited above. Another disadvantage of this solution is to completely mask the movement, which is detrimental from an aesthetic point of view, especially for high-end watches.

Pendulums made of ferromagnetic materials are also known in the context of electronic watches, for example in the watches described in the patent documents. FR2063101 or CH361247 . The ferromagnetic material used for the balance does not constitute a magnetic shielding to improve the isochronism of the spiral, but is intended to cooperate with an electromagnetic circuit sustaining the oscillations. The document FR2000706 is an example of a similar solution of an electronic watch comprising a ferromagnetic balance-regulator in which a hairspring is arranged above the arms of the balance is thus very exposed to variations in external magnetic fields.

We finally know of the document CH289106 spirals made with particular alloys having advantageous elasticity and thermoelastic properties for attachment with a nickel balance. No particular property of magnetic shielding is however mentioned for the balance relative to the balance spring.

It is therefore an object of the present invention to provide a solution for improving the magnetic shielding of a hairspring and which is free from the above limitations.

These objects are achieved according to the invention by the main claim which, in addition to that it obeys what is explained in the first paragraph above, is original in that the protection device comprises a rocker made of amorphous ferromagnetic material .

An advantage of the proposed solution is to achieve a powerful magnetic shielding, due to the advantageous magnetic and anticorrosive properties of amorphous metals, while advantageously reusing certain elements of the movement as a shielding element, and thus not requiring the use of no additional parts or special surface treatments. Congestion is thus reduced to the maximum, as well as production costs. An additional advantage of the solution and to achieve a magnetic shield centered on the axis of rotation of the spiral to improve efficiency.

Another advantage of the proposed solution is to allow the visualization of the movement elements by the bottom of the watch, thus improving the overall aesthetics of the timepiece produced.

• les figures 1A et 1B montrent un spiral en perspective dans le plan ainsi qu'une portion de ce spiral;
• la figure 2 est une vue schématique et en perspective d'un mode de réalisation préférentiel de l'invention.
• la figure 3 est une vue schématique et en perspective d'un mode de réalisation alternatif de l'invention.

The invention will now be explained in detail below by several embodiments given as non-limiting examples, these embodiments being illustrated by the appended drawings in which:

• the Figures 1A and 1B show a perspective spiral in the plane as well as a portion of this spiral;
• the figure 2 is a schematic perspective view of a preferred embodiment of the invention.
• the figure 3 is a schematic perspective view of an alternative embodiment of the invention.

In the current context where the density of electromagnetic interference is increasing sharply, in particular because of the new generation of wireless cellular networks (3G) nomadic (Wi-Fi), but also the increase in the number of small permanent magnets used for the closure of handbags or mobile phone cases for example, it is important to find magnetic shielding solutions to ensure today the isochronism of the mechanical watch systems.

In doing so, the watchmaker, however, faces a space problem to house the shield on the plate and in the housing. Therefore, it has been sought to find optimal solutions that combine minimal footprint and effective attenuation of the magnetic field.

Rather than trying to reduce or completely eliminate the disturbing magnetic field at the hairspring by means of heavy and cumbersome solutions, it seems more advisable to orient or deflect this disturbing field, without necessarily reducing or removing it in directions where it is the least harmful from the point of view of its potential to polarize the magnetic material which is constituted by the spiral.

The regulating member of a mechanical watch is generally constituted by a spiral spring, as illustrated in FIG. Figure 1A . The hairspring is mounted about an axis of rotation Z and is wound in a plane perpendicular to this axis. The diameter of the spiral in this plane is noted d , while the height of the spiral along the Z axis is denoted h . The Figure 1B shows a portion of this spiral 1 which is a very long ribbon wound on itself, this ribbon preferably having a reduced height h and a very small thickness e . It follows from this that if it is polarized in the direction of the Z or orthogonally height, or in the direction of the thickness R or radially, little or no remanent magnetization will remain. On the other hand a polarization in the direction of the length L is to be avoided because it is the only one, especially on the outer turns of the hairspring, to cause a residual magnetization of the latter producing, as we saw above, an additional couple parasite causing a random variation of the return moment of the spiral affecting the isochronism of the regulating system. To avoid or greatly reduce this longitudinal polarization, it is therefore sufficient to orient the field lines in a more or less orthogonal and radial configuration to the plane of the hairspring 1.

In order to minimize the bulk, it is sought in the context of the invention to use advantageously elements of the movement so as not to require additional space for the magnetic shielding for a given caliber. The pendulum 2, of which a preferred embodiment with four branches is represented in figure 2 , appears therefore as the most suitable element, because of the location of its arms 3 in a plane parallel to the plane of the spiral 1, and their symmetrical configuration with respect to the axis of rotation Z of the spiral 1. This symmetrical arrangement of the arms 3 relative to the axis of rotation Z and the shielding provided by the casing ring 4, coaxial with the hairspring and of a height H chosen preferably much greater, for example at least 3 times greater than that of the height of the spiral h , not only greatly reduce the amplitude of the magnetic field applied inside the space in which the hairspring 1 rests, until the induced field is saturated in the casing ring. 4, but also to confer an omnidirectional protection with respect to the disturbing magnetic field, whatever the orientation of this field.

The casing ring 4 makes it possible to effectively protect the hairspring 1 against the disturbing magnetic fields, because the latter are deflected in greater numbers in the vertical direction of the Z axis of rotation, which is a direction of polarization according to which the hairspring is less sensitive. It will be noted however that the concentration of the field at the periphery of the arms 3 and at the level of the circle 4 always tends to locally increase this field, hence the need to provide a casing ring 4 of diameter D which is relatively large compared with the diameter. d spiral 1, preferably at least twice so that no part of the spiral, even at the outermost level, can suffer this undesirable effect of concentration. In order to improve the level of saturation of the induced field in the casing ring 4, it is possible to increase the section of the latter; however, a compromise must also be found with respect to the moment of inertia imparted to the pendulum, which must be maintained at a relatively low level to reduce the forces exerted by the hairspring 1. In order to increase the height of the casing ring 4 without increasing its mass, it will be possible to choose a section as tapered as possible, with for example a ratio between the height and width of this section greater than 10. Thus the polarization of the field lines will be more effective in the vertical direction Z.

The approach of producing movement parts made of ferromagnetic material, that is to say having a magnetic susceptibility (generally noted by the Greek letter X) very high, had until now never been considered by the man of the watchmaking business because of the strong tendency to oxidation of the usual ferromagnetic materials, in particular by the presence of iron and the deficiency of chromium in these alloys. However, it is now possible to surface treat this type of materials with anticorrosion agents to avoid this inconvenience, while not changing the magnetic properties. The high magnetic saturation material used to make the casing ring 4 and the arms 3 is an amorphous iron-based alloy, for example an iron-nickel, iron-cobalt or iron-chromium alloy, or alloys of the same type. iron-nickel-molybdenum, iron-nickel-copper. This type of alloy is recognized for its low coercivity properties and high magnetic permeability, that is to say with very narrow hysteresis cycles, and with a very high slope, are also very resistant to corrosion and thus particularly suitable for the implementation of the invention. The chemical nature of the alloy is chosen so that the magnetic behavior of the material has a high magnetic permeability and saturation level, such as a Permenorm iron-nickel alloy with a nickel content of 45 to 50%.

According to the preferred embodiment illustrated on the figure 2 , the rocker 2 comprises at least four flattened arms which extend in the winding plane of the spiral. When using the watch, the balance is constantly rotated and a substantially flat surface is emulated to form a magnetic shield in this plane. According to this variant illustrated, where the attenuation is of the order of half between an external field and the field where the hairspring 1 is located, whose diameter d and height h preferably respect the ratios stated above with respect to those D, H of the casing circle 4.

In order to further improve the effectiveness of the shielding, it is possible to increase the number of arms and / or their thickness in order to increase the protection surface. When the plurality of arms 3 covers an area equal to more than a quarter of the virtual disk delimited by the casing ring 4 in the arm rotation plane 3, an attenuation of disturbances relative to the walking gaps which can reach ratios higher than 3, in particular for induction values greater than 10 millitesla (mT), ie approximately 8kA / m for a three-armed balance with the ratio of area evoked with respect to the virtual disk delimited by the casing circle . It is possible to further improve these ratios to values of 6-7 with a solid disc instead of arms 3; however, this solution has the disadvantage of increasing the mass of the system and consequently the moment of inertia and the energy consumed. Therefore, in order not to increase the total mass of the system, maximum flattened arms will be favored for a given mass, that is to say whose dimensions extend as much as possible in their plane of rotation, so that that the polarization of the field is optimal in the vertical direction Z. Whatever the number of arms used in the context of the invention, it can be considered that the arms are described as flattened when the ratio between the width and the length of the their section is greater than about 10, so that they cover the largest possible area in the plane of the virtual disk delimited by the casing ring 4.

To achieve such flattened arms for the balance, the amorphous metal alloy used in the context of the invention is here again particularly advantageous because of the elastic deformation properties and mechanical strength conferred, which make it easy to obtain a very flattened shape for a given mass. This flattened shape makes it possible to orient the lines of an external magnetic field more effectively without requiring increasing the weight of the balance, and consequently its moment of inertia, which would be detrimental to the performance of the regulating system for a given spiral spring. .

In order to further improve the effectiveness of the magnetic shielding, the device according to the invention may comprise a second series of arms 3 surmounted on said casing ring 4, as illustrated in FIG. figure 3 . The series of arms 3 'may preferably be angularly offset or of a different or complementary but symmetrical geometrical shape. We can also imagine that the two sets of arms are identical to that of the lower arms 3, so that the two series of arms 3 and 3 'are superimposed. The advantage of covering the magnetic shield on the top by rotating arms makes it possible on the one hand to constitute a symmetrical and totally closed space inside which the spiral 1 is arranged, which makes the shielding effective both in terms of attenuation and isotropy; on the other hand, similar to the attenuation values measured with the only arms 3 the mass efficiency of the shielding is greatly improved compared to a solid surface such as a disk. The piece forming the balance with two sets of arms 3,3 'may be formed integrally for example by a LIGA type process, or by interlocking a rib in a groove of male-female parts each having a series of arms and each forming part of the casing ring 4.

Those skilled in the art will also be able to observe that one advantage of all the proposed embodiments is not to hinder the visualization of the movement, in particular by the bottom of the case, as is the case with conventional shielding. This possibility can therefore be used for the manufacture of skeleton or tourbillon watches where at least part of the movement is intended to be visible to the user.

Claims (5)

1. Device for protecting a balance spring (1) of a mechanical timepiece against interfering magnetic fields origination outside said timepiece, characterized in that the device includes a balance (2) made of an amorphous ferromagnetic alloy.
2. Device according to the preceding claim, characterized in that said balance (2) has at least four flattened arms (3).
3. Device according to any of the preceding claims, characterized in that the balance (2) is formed of a plurality of arms (3) covering a surface area equal to more than a quarter of the disc delimited by a casing ring (4).
4. Device according to any of the preceding claims, characterized in that the balance (2) includes a second series of arms (3') mounted on said casing ring (4).
5. Device according to any of the preceding claims, characterized in that said balance (2) has a diameter (D) at least two times greater than the diameter (d) of said balance spring (1).

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