Watch hairspring and method for making same

• US 7,077,562 B2
• Filed: 10/30/2003
• Issued: 07/18/2006
• Est. Priority Date: 11/25/2002
• Status: Active Grant

• Current Assignee: CSEM Centre Suisse D'Electronique et de Microtechnique SA
• Original Assignee: CSEM Centre Suisse D'Electronique et de Microtechnique SA
• Inventors: Bourgeois, Claude, Perret, André, Hoogerwerf, Arnold Christiaan
• Primary Examiner(s): MISKA, VIT W
• Application Number:
  US10/533,442
• Publication Number:
  US 20050281137A1
• Time in Patent Office: 992 Days
• Field of Search: 368/127, 368/140, 368/175, 265/166
• US Class Current: 368/175
• CPC Class Codes: F16F 1/021 characterised by their composition, e.g. comprising materials providing for particular spring properties composition and manufacture of clock or watch springs G04B1/145
• CPC Class Codes: F16F 1/024 Covers or coatings therefor F16F1/24 takes precedence
• CPC Class Codes: F16F 1/10 Spiral springs with turns lying substantially in plane surfaces F16F1/326 takes precedence
• CPC Class Codes: F16F 2222/02 temperature-related F16F2228/002 takes precedence
• CPC Class Codes: G04B 1/145 Composition and manufacture of the springs compositions and manufacture of components, wheels, spindles, pivots, or the like G04B13/02; compositions of component escapements G04B15/14; composition and manufacture or hairsprings G04B17/066; compensation for the effects of variations of temperature of springs using alloys, especially for hairsprings G04B17/227; materials for bearings of clockworks G04B31/00; heat treatment and chemical or mechanical treatment for control of the structure C21D8/00; iron and steel alloys C22C; non-ferrous alloys C22C and B22F
• CPC Class Codes: G04B 17/066 Manufacture of the spiral spring locking of the spiral spring by the regulating lever G04B18/026; spiral spring with temperature compensation G04B17/227; fixation of the spiral spring on the collet G04B17/32; mainspring G04B1/14

Abstract

The invention relates mainly to a hairspring intended to equip the balance wheel of a mechanical timepiece. It is in the form of a spiraled rod (10) cut from an {001} single-crystal silicon plate. Its structure and its dimensions are calculated so as to minimize the thermal drift of the hairspring/balance-wheel assembly by means of the first and second thermal coefficients of its spring constant.

Claims

1. A new hairspring intended to equip the balance wheel of a mechanical timepiece and in the form of a spiraled rod (10) cut from an {001} single-crystal silicon plate having a first thermal coefficient (C₁) and a second thermal coefficient (C₂) of its spring constant C, the turns of said hairspring having a width w and a thickness t, characterized in that said rod comprises a silicon core (12) and an external layer (14) of thickness ξ

formed around the silicon core and made of a material having a first thermal coefficient of the Young'"'"'s modulus of opposite sign to that of the silicon.

2. The hairspring as claimed in claim 1, characterized in that said external layer (14) is made of amorphous silicon oxide (SiO₂).

3. The hairspring as claimed in claim 1, characterized in that the dimensional ratio ξ

/w is defined so as to obtain a first thermal coefficient (C₁) of its spring constant C of predetermined value.

4. The hairspring as claimed in claim 3, characterized in that the dimensional ratio ξ

/w is defined so as to minimize the first thermal coefficient (C₁) of its spring constant C.

5. The hairspring as claimed in claim 4, characterized in that the dimensional ratio ξ

/w is about 0.06.

6. The hairspring as claimed in claim 1, characterized in that the width of said rod is modulated, periodically, as a function of the angle θ

that defines the orientation of each of its points in polar coordinates in order to minimize the second thermal coefficient (C₂).

7. The hairspring as claimed in claim 5, characterized in that the width of said rod is modulated so that its local flexural stiffness is constant in order to minimize the second thermal coefficient (C₂).

8. The hairspring as claimed in claim 7, characterized in that the modulation is effected according to the formula:

$w=w_0<br><br>\sqrt[3]{1-\frac{1-\frac{{\stackrel{\_}{s}}_{12.0}}{{\stackrel{\_}{s}}_{11.0}}-\frac{1}{2}<br><br>\frac{{\stackrel{\_}{s}}_{44.0}}{{\stackrel{\_}{s}}_{11.0}}}{2}<br><br>{\sin }^2<br><br>\left(2<br><br>\mathrm{\theta <br><br>}\right)}$

in which {overscore (s)}₁₁, {overscore (s)}₄₄ and {overscore (s)}₁₂are the three independent elastic coefficients of silicon along the crystallographic axes.

9. The hairspring as claimed in claim 6, characterized in that, in order to minimize the first thermal coefficient (C₁) and the second thermal coefficient (C₂), the thickness t of the rod, its width w in the {100} plane and the thickness ξ

of the silicon oxide layer have values for which the thermal drift of the spring constant C of the hairspring is a minimum within a given temperature range.

10. A method for determining the optimum dimensions of the hairspring as claimed in claim 9, characterized in that it consists, in succession, in:

mathematically expressing the stiffness of the hairspring as a function of its thickness t, its width w modulated in the plane of the hairspring, the thickness ξ

of the silicon oxide layer, the elastic anisotropy of the silicon and the temperature;

calculating the thermal behavior, in particular the first two coefficients (C₁, C₂) of the spring constant of the hairspring for all combinations of possible values of the parameters t, w and ξ

within a given temperature range; and

adopting the t, w, ξ

combinations for which the thermal drifts of said coefficients (C₁and C₂) are minimal.

11. The method as claimed in claim 10, characterized in that it consists, finally, in calculating the width w of the spiral at any point from the formula:

$w=w_0<br><br>\sqrt[3]{1-\frac{1-\frac{{\stackrel{\_}{s}}_{12.0}}{{\stackrel{\_}{s}}_{11.0}}-\frac{1}{2}<br><br>\frac{{\stackrel{\_}{s}}_{44.0}}{{\stackrel{\_}{s}}_{11.0}}}{2}<br><br>{\sin }^2<br><br>\left(2<br><br>\mathrm{\theta <br><br>}\right)}$

in which {overscore (s)}₁₁, {overscore (s)}₄₄ and {overscore (s)}₁₂ are the three independent elastic coefficients of silicon along the crystallographic axes.