EP1562087B1 - Balance for a watch mechanism - Google Patents

Abstract

The balance (1) has an axle (2) made up of steel for supporting a balance wheel comprising arms (4) connected with a felloe (3). The arms and felloe are made up of a single piece with titanium or titanium-base alloy. The arms are radially projected from a drilled central part (5) that moves on a bearing of the axle by supporting against a shoulder.

Description

Background of the invention

The present invention relates to a pendulum for a watch movement, comprising an axis, a serge and arms connecting the serge to the axis, and intended to be associated with a spiral spring to form conventionally the mechanical oscillator which determines the basic frequency of the movement of a timepiece, in particular a watch. A known construction of a balance of this kind is illustrated for example in the patent CH 494 992 or the patent CH 621 669 .

Currently, in a pendulum for a watch movement, the wheel-shaped part comprising the serge (or rim) and the arms is made of a copper-based alloy, especially cupro-beryllium or nickel silver, or nickel. Such an alloy offers an advantageous combination of qualities which include, in particular, its non-magnetic nature, good chemical stability and sufficient mechanical characteristics. The density of these alloys is greater than 8 kg / dm ³ . Their coefficient of thermal expansion, which is about 17 · 10 ^-6 / ° C for CuBe, about 15 · 10 ^-6 / ° C for nickel and about 21 · 10 ^-6 / ° C for the nickel silver is not particularly favorable.

où I est le moment d'inertie du balancier autour de son axe de rotation et M est le couple élastique du spiral, exprimé en Nm/rad. Les fréquences usuelles des oscillateurs de montre s'échelonnent de 2,5 Hz à 5 Hz, par pas de 0,5 Hz afin qu'une durée d'une seconde corresponde à un nombre entier d'alternances de l'oscillateur. Un mouvement est donc conçu pour une fréquence donnée et l'ensemble balancier-spiral doit avoir cette fréquence-là. Dans la formule ci-dessus, on voit que le paramètre pertinent du balancier est le moment d'inertie. Comme la part des bras du balancier est très faible dans le moment d'inertie, celui-ci dépend avant tout des dimensions (diamètre et section transversale) et de la densité de la serge.The oscillation frequency f of a balance-spring oscillator is given by: $$\mathrm{1}\mathrm{/}\mathrm{f}\mathrm{=}\mathrm{2}\mathrm{\pi }\mathrm{\cdot }{\left(\mathrm{I}\mathrm{/}\mathrm{<figure-callout\; id="5"\; label="M"\; filenames="imgf0001.png"\; state="\{\{state\}\}">M</figure-callout>}\right)}^{\mathrm{0}\mathrm{,}\mathrm{5}}$$

where I is the moment of inertia of the balance around its axis of rotation and M is the elastic torque of the spiral, expressed in Nm / rad. The usual frequencies of the watch oscillators range from 2.5 Hz to 5 Hz, in steps of 0.5 Hz so that a duration of one second corresponds to an integer number of alternations of the oscillator. A movement is therefore designed for a given frequency and the sprung balance assembly must have this frequency. In the formula above, we see that the relevant parameter of the pendulum is the moment of inertia. As the arm of the pendulum is very small in the moment of inertia, it depends primarily on the dimensions (diameter and cross-section) and the density of the serge.

In some cases, the designer of a watchmaking movement may wish to use a comparatively large diameter pendulum, for example for aesthetic reasons. Increasing the diameter without changing the moment of inertia can be done either by reducing the section of the serge, or by using a lower density material. In both cases, the balance will have a smaller mass, which reduces the friction in the bearings, therefore the disturbances of the isochronism of the balance according to the positions (vertical and horizontal) of the movement. However, a Reduced cross section becomes too weak, especially if it has to be fitted with adjusting screws. We can then consider the use of a lighter material.

The FR Patent 1,275,357 plans to make a lightened watch pendulum, by combining a serge made of a light metal such as aluminum with a wheel-shaped elastic support element, constituted by a circle and spokes, the circle having legs outside for its attachment to the serge. This allows the wheel to be made of a material with high mechanical properties, for example a spring steel. A similar solution, but without brackets, is provided in the FR 1 301 938 .

However, such a two-piece pendulum construction made of different materials does not offer the same guarantees of durability and stability of form as a one-piece construction, in particular because of the large differences in thermal expansion between the steel. , aluminum-based alloys and copper-based alloys. These dilations and the deformations that they can induce significantly modify the moment of inertia and therefore the frequency of oscillation, especially in the case of an aluminum serge. On the other hand, with this two-piece construction, it is difficult to center the serge with respect to the axis of rotation.

Summary of the invention

The present invention aims at making it possible to produce a balance-spring oscillator having a larger diameter than usual for the same frequency, or having a higher frequency with the same dimensions as a usual oscillator, avoiding the aforementioned drawbacks. The invention aims in particular to lighten the balance by maintaining a sufficient mechanical strength and good dimensional stability vis-à-vis temperature variations.

For this purpose, a rocker according to the invention is characterized in that the serge and the arms are made of titanium. Preferably, the serge and the arms are made in one piece, which is a great advantage over the two French patents mentioned above, but it is not excluded to manufacture these parts separately, then to assemble them by welding or a other way.

If the choice of titanium among other light metals, to make a watch wheel, had never been considered so far, since aluminum has been for decades, it is probably because of expected machining difficulties. In the utility model DE 1 987 070 published in 1968, it had been mentioned the possibility of using a light metal such as aluminum or titanium instead of cupro-beryllium to make a watch exhaust wheel, which is a flat wheel and rotating quite slowly. However, no industrial use of titanium in a wheel of this kind has been made to our knowledge. On the other hand, the physical properties required of the materials of a balance wheel are very different or higher than for another wheel of a clockwork movement. It is surprising that the selection of titanium for this particular application has a set of technical advantages that make it possible to achieve a balance wheel both lightweight and high quality: non-magnetic nature, low density, high mechanical strength, low coefficient of thermal expansion, resistance to corrosion. Compared to cupro-beryllium, titanium is almost two times lighter and expands thermally by half, while also offering good mechanical properties. Compared with aluminum, titanium is a little heavier, but has much better mechanical characteristics and thermal expansion three times less. The invention therefore makes it possible to produce a one-piece balance wheel whose serge is relatively light despite relatively large dimensions, while the arms are thin and elastic while being sufficiently strong.

On the other hand, in comparison with a balance of conventional material having a relatively thin serge, a balance of the same diameter titanium may have a higher serge (in the direction parallel to the axis of rotation), which allows to spare in the serge of tapped holes for balancing screws in cases where this would not be possible in the serge made of conventional material.

Brief description of the drawings

• la figure 1 est une vue en perspective du balancier, et
• la figure 2 est une vue en coupe suivant la ligne II-II de la figure 1.

Other features of the invention will appear in the following description of an embodiment of a pendulum for a watch movement having a titanium balance wheel, presented by way of non-limiting example of the invention with reference to the attached drawings, in which:

• the figure 1 is a perspective view of the pendulum, and
• the figure 2 is a sectional view along line II-II of the figure 1 .

Detailed description of an embodiment of the invention

The pendulum 1 represented in the Figures 1 and 2 comprises a conventional steel shaft 2 which supports a balance wheel comprising a serge 3 and for example three arms 4 made in one piece with the serge. This piece is made of titanium, for the reasons of lightness exposed above. The arms 4 radiate from a pierced central portion 5 which is driven on a bearing 6 of the axis 2 abutting against a shoulder 7. In a conventional manner, the shaft 2 supports in addition to a ferrule 8, intended for fixing a spiral not shown, and a double exhaust plate 9 for cooperating with an anchor.

The following alloys, for example, can be used: Titanium grade 2: AFNOR T40 (Fe 0.25%, O 0.048%, C 0.06%, N 0.05%, H 0.013%, Ti for the rest) Titanium grade 5: AFNOR TA6V6E2 (Al 5.5%, V 5.5%, Fe 0.6%, N 0.04%, Sn 2%, Cu 0.6%, Ti for the rest).

Due to the low density of the titanium, the serge 3, having in this case a trapezoidal cross section, is sufficiently high, thick and strong to include threaded holes for receiving adjustment screws if necessary. In this case, balancing balancer is not achieved by means of adjusting screws, but by milling depressions 11 in the outer face of the serge.

Starting from the idea that the pendulum 1 is made of titanium (density 4.5 kg / dm ³ ) in order to have a larger diameter than a conventional cupro-beryllium balance (density 8.25 kg / dm ³ ) having the same moment of inertia, keeping in addition the same cross-section of the serge, it can be calculated that the average diameter of the serge of the titanium balance will be enlarged by 22%. The effect obtained from the point of view of aesthetics is significant, without causing loss of mechanical characteristics of the serge.

Claims (3)

1. Balance (1) for a watch or clockwork, comprising a staff (2), a rim (3) and arms (4) connecting the rim to the staff, characterized in that the rim and the arms are made of titanium.
2. Balance according to claim 1, characterized in that the rim (3) and the arms (4) are made in a single piece.
3. Balance according to claim 1, characterized in that the staff (2) is made of steel.

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