Component for a timepiece movement

Abstract

The invention relates to a pivot arbor comprising a metal pivot (3) at each of its ends. The metal is a non-magnetic aluminium alloy in order to limit its sensitivity to magnetic fields, and at least the outer surface (5) of one of the two pivots (3) is deep-hardened to a predetermined depth with respect to the rest of the arbor to harden the pivot or pivots (3). The invention concerns the field of timepiece movements.

Claims

1. A pivot arbor for a timepiece movement comprising: a plurality of sections have different diameters, which define bearing surfaces and shoulders arranged between two end portions, and at least one metal pivot at at least one of said end portion, wherein said pivot has a conical shape, wherein the metal is a non-magnetic aluminium alloy so as to limit the sensitivity of the pivot to magnetic fields, wherein at least the outer surface of said pivot is deep-hardened to a predetermined depth relative to the core of the pivot arbor, and wherein the non-magnetic aluminium alloy is chosen from the group consisting of an aluminium-copper-lead alloy, an aluminium-silicon-magnesium-manganese alloy, and an aluminium-zinc-magnesium-copper alloy.

2. The pivot arbor according to claim 1, wherein the predetermined depth represents between 5% and 40% of the total diameter (d) of the pivot.

3. The pivot arbor according to claim 1, wherein the deep-hardened outer surface comprises diffused atoms of at least one chemical element.

4. The pivot arbor according to claim 1, wherein the deep-hardened outer surface has a hardness of more than 600 HV.

5. The pivot arbor according to claim 1, wherein said outer surface of said pivot has no hardening layer directly deposited on said outer surface.

6. The pivot arbor according to claim 1, wherein at least the outer surface of said pivot is rolled.

7. The pivot arbor according to claim 1, wherein the pivot arbor has two pivots.

8. A movement for a timepiece comprising a pivot arbor, wherein said pivot arbor comprises: a plurality of sections have different diameters, which define bearing surfaces and shoulders arranged between two end portions, and at least one metal pivot at at least one of said end portion, wherein said pivot has a conical shape, wherein the metal being a non-magnetic aluminium alloy so as to limit the sensitivity of the pivot to magnetic fields, wherein at least the outer surface of said pivot is deep-hardened to a predetermined depth relative to the core of the pivot arbor, and wherein the non-magnetic aluminium alloy is chosen from the group consisting of an aluminium-copper-lead alloy, an aluminium-silicon-magnesium-manganese alloy, and an aluminium-zinc-magnesium-copper alloy.

9. A movement for a timepiece wherein the movement comprises a balance staff, a pallet staff and/or an escape pinion comprising a pivot arbor comprising: a plurality of sections have different diameters, which define bearing surfaces and shoulders arranged between two end portions, and at least one metal pivot at at least one of said end portion, wherein said pivot has a conical shape, wherein the metal being a non-magnetic aluminium alloy so as to limit the sensitivity of the pivot to magnetic fields, wherein at least the outer surface of said pivot is deep-hardened to a predetermined depth relative to the core of the pivot arbor, and wherein the non-magnetic aluminium alloy is chosen from the group consisting of an aluminium-copper-lead alloy, an aluminium-silicon-magnesium-manganese alloy, and an aluminium-zinc-magnesium-copper alloy.

10. A method for fabricating a pivot arbor for a timepiece movement comprising the following steps: a) forming a plurality of sections have different diameters, which define bearing surfaces and shoulders arranged between two end portions, and at least one metal pivot at at least one of said end portion, wherein said pivot has a conical shape, wherein said metal being a non-magnetic aluminium alloy, to limit the sensitivity thereof to magnetic fields, wherein the non-magnetic aluminium alloy is chosen from the group consisting of an aluminium-copper-lead alloy, an aluminium-silicon-magnesium-manganese alloy, and an aluminium-zinc-magnesium-copper alloy; b) diffusing atoms by an ion implantation process to a predetermined depth in at least the outer surface of said pivot in order to deep-harden the pivot arbor in the main areas of stress while maintaining a high tenacity.

11. The method according to claim 10, wherein the predetermined depth represents between 5% and 40% of the total diameter (d) of the pivot.

12. The method according to claim 10, wherein the diffusion step comprises the diffusion of atoms of at least one chemical element.

13. The method according to claim 10, wherein the method does not comprise any step of depositing a hardening layer directly on the outer surface of the pivot.

14. The method according to claim 10, wherein the pivot is subjected to a rolling/polishing step after step b).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Other features and advantages will appear clearly from the following description, given by way of non-limiting illustration, with reference to the annexed drawings, in which:

(2) FIG. 1 is a representation of a pivot arbor according to the invention; and

(3) FIG. 2 is a partial cross-section of a balance staff pivot according to the invention, after the diffusion treatment operation via ion implantation and after the rolling or polishing operation.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

(4) In the present description, the term “non-magnetic” means a paramagnetic or diamagnetic or antiferromagnetic material, whose magnetic permeability is less than or equal to 1.01.

(5) An aluminium alloy is an alloy containing at least 50% by weight of aluminium.

(6) The invention relates to a component for a timepiece movement and particularly to a non-magnetic pivot arbor for a mechanical timepiece movement.

(7) The invention will be described below with reference to an application to a non-magnetic balance staff 1. Of course, other types of timepiece pivot arbors may be envisaged such as, for example, timepiece wheel set arbors, typically escape pinions or pallet staffs. Components of this type have a body with a diameter preferably less than 2 mm, and pivots with a diameter preferably less than 0.2 mm, with a precision of several microns.

(8) Referring to FIG. 1, there is shown a balance staff 1 according to the invention, which comprises a plurality of sections 2 of different diameters, preferably formed by bar turning or any other chip removal machining technique, and defining, in a conventional manner, bearing surfaces 2a and shoulders 2b arranged between two end portions defining two pivots 3. These pivots are each intended to pivot in a bearing typically in an orifice in a jewel or ruby.

(9) With the magnetism induced by objects that are encountered on a daily basis, it is important to limit the sensitivity of balance staff 1 to avoid affecting the working of the timepiece in which it is incorporated.

(10) Surprisingly, the invention overcomes both problems at the same time with no comprise and while providing additional advantages. Thus, the metal 4 of pivot 3 is a non-magnetic aluminium alloy so as to advantageously limit the sensitivity of the staff to magnetic fields. Further, at least the outer surface 5 of pivots 3 (FIG. 2) is deep-hardened to a predetermined depth with respect to the rest of pivot 3 advantageously by means of an ion implantation process, so as to offer, advantageously according to the invention, a superior hardness on said outer surface while maintaining high tenacity.

(11) Indeed, according to the invention, the deep-hardened outer surface of pivots 3 has a hardness of more than 600 HV.

(12) Preferably, the non-magnetic aluminium alloy is chosen from the group comprising an aluminium-copper-lead alloy, an aluminium-silicon-magnesium-manganese alloy, an aluminium-zinc-magnesium-copper alloy, wherein the proportions of the alloys is chosen to give them both non-magnetic properties and good machinability.

(13) For example, the non-magnetic aluminium alloys used in the present invention, designated according to the standard DIN EN-673-3, are:

(14) EN AW-2007 having the formula AlCu4PbMgMn (named Avional Pb118)

(15) EN AW-2011 having the formula AlCu6BiPb (named Decoltal 500)

(16) EN AW-6082 having the formula AlSi1MgMn (named Anticorodal 110/112)

(17) EN AW-7075 having the formula AlZn5.5MgCu (named Perunal 215)

(18) The aluminium alloy 7449 having the formula AlZn8Mg2Cu may also be used.

(19) The composition values are given in mass percent. The elements with no indication of the composition value are either the remainder (aluminium) or elements whose percentage in the composition is less than 1% by weight.

(20) Of course, other non-magnetic aluminium-based alloys may be envisaged provided the proportion of their constituents confers non-magnetic properties and good machinability.

(21) It has been empirically demonstrated that a hardening depth of between 5% and 40% of the total diameter d of pivots 3 is sufficient for application to a balance staff. By way of example, if the radius d/2 is 50 μm, the hardening depth is preferably approximately 15 μm all around pivots 3. Evidently, depending upon the application, it is possible to provide a different hardening depth of between 5% and 80% of the total diameter d.

(22) Preferably according to the invention, the deep-hardened outer surface 5 of pivots 3 comprises diffused atoms of at least one chemical element. For example, this chemical element may be a non-metal such as nitrogen, argon and/or helium. Indeed, as explained below, through the interstitial supersaturation of atoms in non-magnetic aluminium alloy 4, a surface area 5 is deep-hardened with no need to deposit a second material over pivots 3. Indeed, the hardening occurs within the material 4 of pivots 3 which, advantageously according to the invention, prevents any subsequent delamination during use. Consequently, outer surface 5 of pivot 3 comprises a hard surface layer, but has no additional hardening layer deposited directly on said outer surface 5. It is evident that other layers not having a hardening function may be deposited. Thus, it is possible, for example, to deposit a lubrication layer on the outer surface of the pivot.

(23) Consequently, at least one surface area of the pivot is hardened, i.e. the core of pivots 3 and/or the rest of the arbor may remain little modified or unmodified without any significant change to the mechanical properties of balance staff 1. This selective hardening of pivots 3 of balance staff 1 makes it is possible to combine advantages, such as low sensitivity to magnetic fields, hardness and high tenacity, in the main areas of stress, while offering good corrosion and fatigue resistance.

(24) The invention also relates to the method of manufacturing a balance staff as explained above. The method of the invention advantageously comprises the following steps:

(25) a) forming, preferably by bar turning or any other chip removal machining technique, a balance staff 1 comprising at least one metal pivot 3 at each of its ends, said metal being a non-magnetic aluminium alloy, to limit its sensitivity to magnetic fields; and

(26) b) diffusing atoms to a predetermined depth by an ion implantation process at least in the outer surface 5 of pivots 3 so as to deep-harden the pivots in the main areas of stress.

(27) Diffusion step b) comprises the diffusion of atoms of at least one chemical element, for example, a non-metal, such as nitrogen, argon and/or helium. This method has the advantage of not limiting the type of diffused atoms and of allowing both interstitial and substitutional diffusion.

(28) The depth of hardening of outer surface 5 may advantageously be increased with the aid of a heat treatment performed during or after the ion implantation treatment step b).

(29) According to a preferred embodiment, pivots 3 are rolled or polished after step b) in order to achieve the final dimensions and surface finish required for pivots 3. This rolling operation after treatment makes it possible to obtain arbors presenting improved resistance to wear and shocks compared to arbors whose pivots have simply been subjected to a hardening operation. Consequently, at least outer surface 5 of pivots 3 of the invention is rolled.

(30) Advantageously according to the invention, regardless of the embodiment, the method can be applied in bulk. Finally, advantageously, it was discovered that the compressive stresses of the method improve fatigue and shock resistance.

(31) The method according to the invention does not comprise any step of depositing an additional hardening layer directly onto outer surface 5 of pivot 3.

(32) The pivot arbor according to the invention may comprise pivots treated according to the invention or be entirely made of non-magnetic aluminium alloy. Further, the diffusion treatment of step b) may be performed on the surface of the pivots or over the entire surfaces of the pivot arbor.

(33) The pivot arbor according to the invention may advantageously be made by bar turning or any other chip removal machining technique using non-magnetic aluminium alloy bars with a diameter preferably less than 3 mm, and preferentially less than 2 mm. Aluminium alloys are known to those skilled in the art for being too soft to be able to be rolled and for wear resistance during use. However, in a surprising and unexpected manner, the use of such materials according to the invention makes it possible to make pivot arbors presenting a hardness of more than 600 HV which allows rolling to be performed and satisfactory longevity to be achieved during motion. To achieve the present invention, those skilled in the art had to overcome bias to use a non-magnetic aluminium based alloy to make a component of very small dimensions by means of a method comprising a step of bar turning (or any other chip removal machining method) and of rolling.

(34) Against all expectations, the method of the invention makes it possible to obtain a timepiece pivot arbor wherein at least the pivots are formed by bar turning (or any other chip removal machining method) and rolling using a non-magnetic aluminium alloy.

(35) Of course, this invention is not limited to the illustrated example but is capable of various variants and alterations which will be clear to those skilled in the art. In particular, it is possible to envisage entirely or virtually entirely treating pivots 3, i.e. treating more than 80% of the diameter d of pivots 3, although this is not necessary for the application to pivot pins such as timepiece balance staffs.