EP1258786B1 - Self-compensating spring for a mechanical oscillator of balance-spring type - Google Patents

Description

The present invention relates to a self-compensating hairspring for a mechanical pendulum oscillator-spiral watch movement or other precision instrument, paramagnetic alloy Nb-Hf having a positive Young's modulus thermal coefficient (CTE), able to compensate the thermal expansion of the balance spring and the balance.

• F = fréquence propre de l'oscillateur avec
• C = constante du couple de rappel exercé par le spiral de l'oscillateur
• I = moment d'inertie du balancier de l'oscillateur

All the methods proposed to compensate for these frequency variations are based on the consideration that this natural frequency depends exclusively on the ratio between the constant of the restoring moment exerted by the spiral on the balance and the moment of inertia of the latter, as indicated in the following relation: $$F=\frac{1}{2\mathit{\pi }}\sqrt{\frac{\mathrm{VS}}{I}}$$

• F = natural frequency of the oscillator with
• C = constant of the restoring torque exerted by the spiral of the oscillator
• I = moment of inertia of the pendulum of the oscillator

avec:

• E: module de Young du spiral de l'oscillateur $$\frac{1}{E}\frac{dE}{dT}=\mathrm{CTE}=\mathrm{coefficient\; thermique\; du}\mathrm{module\; de\; Young\; du\; spiral\; de}{\mathrm{l}}^{\prime }\mathrm{oscillateur}$$
  
• α_s : coefficient de dilatation thermique du spiral de l'oscillateur
• α_b : coefficient de dilatation thermique du balancier de l'oscillateur

Since the discovery of Fe-Ni-based alloys having a positive Young's modulus (hereinafter CTE) thermal coefficient, the mechanical oscillator thermal compensation is obtained by adjusting the spiral CTE as a function of thermal expansion coefficients. spiral and balance. Indeed, by expressing the torque and the inertia from the spiral and the pendulum characteristics, then by deriving the equation (1) with respect to the temperature, one obtains the relative thermal variation of the natural frequency: $$\frac{1}{F}{}^{\mathit{dF}}{/}_{\mathit{dT}}=\frac{1}{2}\left(\frac{1}{E}\frac{dE}{dT}+3{\alpha }_s-2{\alpha }_b\right)$$

with:

• E: Young modulus of the spiral of the oscillator $$\frac{1}{E}\frac{dE}{dT}=\mathrm{CTE}=\mathrm{thermal\; coefficient\; of}\mathrm{Young\text{'}s\; modulus\; of\; the\; spiral\; of}{\mathrm{l}}^{\text{'}}\mathrm{oscillator}$$
  
• α _s : coefficient of thermal expansion of the spiral of the oscillator
• α _b : coefficient of thermal expansion of the pendulum of the oscillator

By adjusting the autocompensation term A = ½ ( CTE +3 α _s ) to the value of the pendulum thermal expansion coefficient, it is possible to cancel equation (2). Thus, the thermal variation of the natural frequency of the mechanical oscillator can be eliminated.

The coefficients of thermal expansion α _b of the most used balance materials, such as alloys of copper, silver, gold, platinum or steel are in the range of 10 to 20 ppm / ° C. To compensate for the effects of temperature variations on the natural frequency of the oscillators due to their expansion, the spiral alloys must therefore have a corresponding self-compensation term. The desired precision for the watches requires to adjust in manufacture, in a controlled manner, the term of self-compensation with a tolerance of a few ppm / ° C around the desired value.

Ferromagnetic alloys based on iron, nickel or cobalt currently used for the production of spirals have an abnormally positive CTE in a range of about 30 ° C around the ambient temperature, due to the proximity of their Curie temperature. In the vicinity of this temperature, the magnetostrictive effects which decrease the Young's modulus of these alloys disappear, leading to an increase in the modulus. In addition to the fact that this temperature range is relatively narrow, these alloys are sensitive to the effects of magnetic fields. These modify the elastic properties of the spirals irreversibly and thus change the natural frequency of the mechanical oscillator. In addition, the elastic properties of the ferromagnetic alloys vary with the rate of cold work hardening, which requires to control exactly this parameter during the manufacture of the spiral.

The CTE values sought for the spirals made with this family of alloys are adjusted by a thermal precipitation treatment which also fixes the final shape of the spiral by relaxation.

We have already proposed in the CH-551 032 (D1), in the CH-557,557 (D2) and in the DE-C3-15 58 816 (D3) paramagnetic alloys with high magnetic susceptibility and thermal coefficient of negative susceptibility, as an alternative to ferromagnetic alloys for the manufacture of self-compensating spirals and precision springs. These alloys have an abnormally positive CTE and have the advantage of having elastic properties insensitive to magnetic fields. Their elastic properties depend on the texture created during drawing of the hairspring, but little on the rate of hardening, unlike ferromagnetic alloys. In addition, as mentioned in D3, these alloys provide a thermal compensation range of mechanical oscillators that extends over more than 100 ° C around the ambient temperature.

The physical causes that create the abnormally positive CTE of these paramagnetic alloys are explained in the aforementioned documents. According to them, these alloys have a high density of electronic states at the Fermi level, as well as a strong electron-phonon coupling, which generates this abnormal behavior of the CTE.

Document D3 cites in particular that it may be suitable for the manufacture of spirals for watch movement oscillators, alloys in which the Nb or Ta is alloyed with the Zr, Ti or Hf found in these alloys in such proportions that they are able to precipitate in two phases.

It was further proposed in the EP 0 886 195 (D4) an Nb-Zr alloy containing between 5% and 25% by weight of Zr and at least 500 ppm by weight of a doping agent formed at least in part of oxygen. With this alloy, the CTE is controlled by the texture. The precipitation that occurs during the fixing process induces a recrystallization that modifies the texture and makes it possible to adjust the CTE. Oxygen influences precipitation and recrystallization and therefore CTE.

The adjustment of the CTE during the fixing operation is difficult to control. Indeed, the texture that controls the CTE is changed during fixing by recrystallization. However, in the Nb-Zr-O alloys, the initiation of the recrystallization and its progress depend on the oxygen concentration, the rate of work hardening and the temperature. It was found that with these alloys, the temperature range on which the recrystallization takes place is very narrow (about 50 ° C). In addition, the variation of CTE induced is large, of the order of 150 ppm / ° C between the beginning and the end of recrystallization. The narrow temperature range in which recrystallization occurs and this large variation in CTE make the CTE adjustment of alloys Nb-Zr-O difficult to reproduce. The narrowness of this temperature range is due to the fact that this reaction is triggered by the precipitation of the Zr-rich phases from the solid solution.

While the D3 document is based on the ability of the components of the alloy to precipitate in two phases, the spring with abnormally positive CTE is made from the annealed alloy at high temperature and then rapidly cooled to obtain a solid solution supersaturated. In this state, the alloy is then cold deformed to more than 85%. This strong deformation induces a texture favorable to a positive CTE. To adjust the CTE to the desired value, the alloy is finally heat-treated in a temperature range that allows precipitation of the supersaturated solid solution. The phases precipitating from the solid solution have lower CTEs, which results in a decrease in the overall CTE and allows its adjustment to the desired value. Recrystallization after two-phase precipitation is relatively difficult to control. Moreover, in the case of Hf, the proportion of Hf must be greater than 30 at%, since up to this concentration, this element is in solid solution in Nb. The capacity of deformation is thus reduced.

The object of the present invention is an alloy which makes it possible to remedy, at least in part, the disadvantages of the abovementioned alloys.

It has surprisingly been found that Nb-Hf alloys with very small proportions of Hf, i.e., proportions that are well below the limit from which Hf precipitates, allowed to obtain a positive CTE, this limit falling to 2% at.

The subject of the invention is therefore a self-compensating hairspring for a mechanical balance-spring oscillator. watch movement or other precision instrument, of paramagnetic alloy Nb-Hf having a positive Young's modulus (CTE) thermal coefficient, able to compensate for the thermal expansion of the balance spring and the balance, according to claim 1.

The alloy from which the spiral object of the invention is made has several advantages.

Hf is in solid solution in Nb over a very wide concentration range (up to 30% at.).

The contribution of Hf to the positive CTE is very strong, so that small proportions of Hf are needed. That's how about 2% at. of Hf suffice to make the CTE positive. It turned out, after tests, that a Nb-Hf alloy 4% at. has a CTE of 13 ppm / ° C after partial recrystallization, which corresponds quite to the values required in the case of a balance spring system.

1. 1. La variation de CTE au cours de la recristallisation n'est que de 50 ppm/°C, soit trois fois moins que pour un alliage Nb-Zr.
2. 2. La recristallisation n'étant pas déclenchée par une précipitation, elle est plus lente et a lieu sur une très large plage de température (env. 400°C) comme le montre la figure annexée.

With this Nb-Hf 4% at. Alloy, CTE adjustment is easier to control because:

1. 1. The variation of CTE during recrystallization is only 50 ppm / ° C, which is three times less than for an Nb-Zr alloy.
2. 2. Since recrystallization is not triggered by precipitation, it is slower and takes place over a very wide temperature range (about 400 ° C) as shown in the attached figure.

Finally, the low concentration of Hf required to have the required CTE of 13 ppm / ° C improves the deformability of the hairspring and facilitates the drawing operations.

The spiral alloy Nb-Hf may further contain one or more additional elements such as Ti, Ta, Zr, V, Mo, W, Cr in concentrations such that no precipitation occurs during the form fixing operation of the spiral.

The effect of oxygen on the spiral Nb-Hf was found to be weak or even nil.

Claims (2)

1. A self-compensating spiral spring for a mechanical balance-spiral spring oscillator for a watch or clock movement or other precision instrument, made of an Nb-Hf paramagnetic alloy possessing a thermal coefficient of Young's modulus (TCE), such that it enables the following expression to be substantially equal to zero: $$\frac{\mathrm{1}}{\mathrm{E}}\frac{\mathrm{dE}}{\mathrm{dT}}\mathrm{+}\mathrm{3}{\mathrm{\alpha }}_{\mathrm{s}}\mathrm{-}\mathrm{2}{\mathrm{\alpha }}_{\mathrm{b}}$$

   

   
   where:

   E:Young's modulus of the spiral spring of the oscillator;

   $\frac{\mathrm{1}}{\mathrm{E}}\frac{\mathrm{dE}}{\mathrm{dT}}\mathrm{=}\mathrm{CTE}\mathrm{=}\mathrm{thermique\; coefficient\; of\; Youngʹs\; modulus\; of\; the\; spriral\; spring\; the\; oscillator}\mathrm{;}$

   

   α_s : thermal expansion coefficient of the spiral spring of the oscillator;

   α_b : thermal expansion coefficient of the balance the oscillator,

   characterized in that it contains between 2 at% and 30 at% Hf.
2. The spiral spring as claimed in claim 1, wherein the alloy contains less than 10 at% Hf.

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