EP1258786A1 - Self-compensating spring for a mechanical oscillator of balance-spring type - Google Patents

Abstract

This self-compensating hairspring for mechanical oscillator balance-spring of clockwork movement or other precision instrument, made of Nb-Hf paramagnetic alloy having a Young's modulus thermal coefficient (CTE) such that it allows to substantially cancel the expression : ((1) / (E)) ((dE) / (dT)) + 3αs -2αb with: E: Young's modulus of the oscillating hairspring, (1 / E) (dE / dT) = CTE = thermal coefficient of Young's modulus of the oscillator balance spring, αs: coefficient of thermal expansion of the oscillator balance spring, αb: coefficient of expansion of the oscillator pendulum, contains between 2% and 30% at. from Hf. <IMAGE>

Description

The present invention relates to a self-compensating hairspring for mechanical balance-spring oscillator watch movement or other precision instrument, in Nb-Hf paramagnetic alloy with a thermal coefficient of the positive Young's module (CTE), able to compensate for thermal expansions of the balance spring and 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 torque exerted by the balance spring on the balance wheel and the moment of inertia of the latter, as indicated in the following relationship: F = 1 2π VS I

F = natural frequency of the oscillator with

C = constant of the restoring torque exerted by the oscillator hairspring

I = moment of inertia of the oscillator pendulum

avec:

E: module de Young du spiral de l'oscillateur 1 / E dE / dT= CTE = coefficient thermique du module de Young du spiral de l'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 with a positive Young's modulus thermal coefficient (hereinafter CTE), the thermal compensation of the mechanical oscillator is obtained by adjusting the CTE of the hairspring according to the coefficients of thermal expansion balance spring and balance wheel. Indeed, by expressing the torque and inertia from the characteristics of the balance spring and the pendulum, then by deriving equation (1) with respect to the temperature, we obtain the relative thermal variation of the natural frequency:

with:

E: Young's modulus of the oscillator hairspring 1 / E dE / dT = CTE = thermal coefficient of the Young's modulus of the oscillator hairspring

α _s : coefficient of thermal expansion of the balance spring of the oscillator

α _b : coefficient of thermal expansion of the pendulum of the oscillator

By adjusting the self-compensation term A = ½ ( CTE +3 α _s ) to the value of the thermal expansion coefficient of the pendulum, 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 widely used balance wheel materials, such as copper, silver, gold, platinum or steel alloys 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 its expansion, the spiral alloys must therefore have a corresponding self-compensation term. The precision desired for watches requires the ability to adjust the self-compensation term in manufacturing, in a controlled manner, with a tolerance of a few ppm / ° C around the value sought.

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

The CTE values sought for the balance springs produced with this family of alloys are adjusted by treatment thermal precipitation which also fixes the shape final hairspring by relaxation.

We have already proposed in CH-551 032 (D1), in CH-557 557 (D2) and DE-C3-15 58 816 (D3) alloys paramagnetic with high magnetic susceptibility and coefficient thermal of negative susceptibility, as an alternative to ferromagnetic alloys for the manufacture of self-compensating hairsprings 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 the drawing of the hairspring, but little of the work hardening rate, unlike ferromagnetic alloys. In addition, as mentioned in document D3, these alloys provide an area of thermal compensation for mechanical oscillators that extends over 100 ° C around of room temperature.

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

Document D3 cites in particular as being susceptible to be suitable for the production of balance springs for oscillators watch movements, alloys in which Nb or Ta are combined with Zr, Ti or Hf which found in these alloys in such proportions that they are able to precipitate in two phases.

We have also proposed in EP 0 886 195 (D4) a Nb-Zr alloy containing between 5% and 25% by weight of Zr and at minus 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 recrystallization which modifies texture and allows you to adjust the CTE. Oxygen influences precipitation and recrystallization and therefore the CTE.

The adjustment of the CTE during the fixing operation is hard to handle. Indeed, the texture that controls the CTE is modified during fixing by recrystallization. However, in the alloys of Nb-Zr-O, the triggering of the recrystallization and its course depend on the concentration oxygen, work hardening rate and temperature. It has been found that with these alloys, the range of temperature on which the recrystallization takes place is very narrow (around 50 ° C). In addition, the variation of CTE induced is large, around 150 ppm / ° C between start and the end of recrystallization. The narrow temperature range in which the recrystallization takes place and this strong variation in the CTE makes the adjustment of the CTE of Nb-Zr-O alloys difficult to reproduce. The narrowness of this temperature range is due to the fact that this reaction is triggered by the precipitation of the phases rich in Zr from the solid solution.

Whereas document D3 is based on the capacity of components of the alloy to precipitate in two phases. The spring with abnormally positive CTE is made from alloy annealed at high temperature and then rapidly cooled so as to obtain a supersaturated solid solution. In this state, the alloy is then cold deformed to more 85%. This strong deformation induces a favorable texture to a positive CTE. To adjust the CTE to the desired value, the alloy is finally heat treated in an interval of temperature which allows the precipitation of the solution supersaturated solid. The phases that precipitate from solid solution have lower CTEs, which results in a decrease in the overall CTE and allows its adjustment to the desired value. Recrystallization after precipitation in two phases is relatively difficult to master. Furthermore, 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 deformation capacity is therefore reduced.

The object of the present invention is an alloy which allows the disadvantages to be remedied, at least in part of the above-mentioned alloys.

It has surprisingly been discovered that alloys Nb-Hf with very small proportions of Hf, that is to say, proportions that are well below the limit from which Hf precipitates, allowed to obtain a positive CTE, this limit lowering up to 2% at.

The invention therefore relates to a self-compensating hairspring for mechanical balance-spring oscillator watch movement or other precision instrument, in Nb-Hf paramagnetic alloy with a thermal coefficient of the positive Young's module (CTE), able to compensate for thermal expansion of the balance spring and balance wheel, depending on the claim 1.

The alloy from which the hairspring object of the invention has several advantages.

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

The contribution of Hf to positive CTE is very strong, from so that small proportions of Hf are required. This is how about 2% at. of Hf are enough to make the CTE positive. It turned out, after testing, that an Nb-Hf alloy 4% at. has a CTE of 13 ppm / ° C after recrystallization partial, which corresponds perfectly to the values required in the case of a balance-spring system.

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. 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, the adjustment of the CTE is easier to control because:

1. The variation in CTE during recrystallization is only 50 ppm / ° C, ie three times less than for an Nb-Zr alloy.

2. Since recrystallization is not triggered by precipitation, it is slower and takes place over a very wide temperature range (approx. 400 ° C) as shown in the appended figure.

Finally, the low concentration of Hf necessary for having the required CTE of 13 ppm / ° C improves the deformation capacity hairspring and facilitates wire drawing operations.

The Nb-Hf alloy hairspring may still contain a or several additional elements like Ti, Ta, Zr, V, Mo, W, Cr in concentrations such that no precipitation has place during the fixing operation of the hairspring shape.

The effect of oxygen on the Nb-Hf hairspring was revealed weak or even zero.

Claims (3)

Self-compensating hairspring for mechanical oscillator balance-spring of clockwork movement or other precision instrument, made of Nb-Hf paramagnetic alloy having a Young Modulus thermal coefficient (CTE) such that it allows to substantially cancel the expression: 1 E of dT + 3α s - 2α b with: E: Young's modulus of the oscillator hairspring
1 / E dE / dT = CTE = thermal coefficient of the Young's modulus of the oscillator hairspring α _s : coefficient of thermal expansion of the balance spring of the oscillator α _b : coefficient of expansion of the pendulum of the oscillator, characterized in that it contains between 2% and 30% at. from Hf. Spiral according to claim 1, wherein the alloy contains at least one of the following additional elements : Ti, Ta, Zr, V, Mo, W, Cr in concentrations such no precipitation takes place during the operation fixing its shape. Spiral according to one of the preceding claims, wherein the alloy contains less than 10% at. from Hf.

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