COMPONENT FOR CLOCK MOVEMENT

Description

Field of invention

[0001] The invention relates to a part for a watch movement and in particular to a non-magnetic pivot axis for a mechanical watch movement and more particularly to a non-magnetic balance shaft, anchor stem and escapement pinion. Background of the invention

[0002] The manufacture of a watch pivot shaft consists, starting from a hardenable steel bar, of carrying out bar turning operations to define different active surfaces (bearing, shoulder, pivots, etc.) and then subjecting the bar turned shaft to heat treatment operations including at least one quenching to improve the hardness of the shaft and one or more tempering operations to improve its toughness. The heat treatment operations are followed by a rolling operation of the pivots of the shafts, an operation consisting of polishing the pivots to bring them to the required dimensions. During the rolling operation, the hardness and roughness of the pivots are further improved. It should be noted that this rolling operation is very difficult or even impossible to carry out with most materials whose hardness is low, i.e. less than 600HV.

[0003] Pivot shafts, such as balance shafts, typically used in mechanical watch movements are made from free-cutting steel grades, which are typically martensitic carbon steels containing lead and manganese sulfides to improve their machinability. A steel of this type, designated 20AP, is typically used for these applications.

[0004] This type of material has the advantage of being easy to machine, in particular of being suitable for bar turning and, after quenching and tempering treatments, has high mechanical properties that are very useful for the production of watch pivot axes. These steels have, in particular, high wear resistance and hardness after heat treatment. Typically, the hardness of the pivots of an axis made of 20 AP steel can reach a hardness exceeding 700 HV after heat treatment and rolling.

[0005] Although providing satisfactory mechanical properties for the watchmaking applications described above, this type of material has the disadvantage of being magnetic and of being able to disturb the running of a watch after being subjected to a magnetic field, particularly when this material is used for the production of a balance staff cooperating with a balance spring made of ferromagnetic material. This phenomenon is well known to those skilled in the art. It should also be noted that these martensitic steels are also sensitive to corrosion.

[0006] Tests to try to overcome these drawbacks have been carried out with austenitic stainless steels which have the particularity of being non-magnetic, that is to say of the paramagnetic or diamagnetic or antiferromagnetic type. However, these austenitic steels have a crystallographic structure which does not allow them to be hardened and to achieve hardnesses and therefore wear resistances compatible with the requirements for the production of watch pivot axes. One way to increase the hardness of these steels is work hardening, however this hardening operation does not allow hardnesses greater than 500 HV to be obtained. Consequently, in the context of parts requiring high resistance to wear by friction and which must have pivots with little or no risk of deformation, the use of this type of steel remains limited.

[0007] Another approach to address these drawbacks has been to deposit hard layers of materials such as amorphous carbon known as diamond-like carbon (DLC) on the pivot axes. However, significant risks of delamination of the hard layer have been observed, leading to the formation of debris that can circulate inside the watch movement and disrupt its operation, which is not satisfactory.

[0008] A similar approach, described in patent FR 2 015 873, provides for the production of a balance staff of which at least the main part is made of certain non-magnetic materials. The pivots can be made of this same material or of steel. It is also possible to provide for the deposition of an additional layer applied galvanically, chemically, or from the gas phase (for example in Cr, Rh, etc.). This additional layer presents a significant risk of delamination. This document also describes a balance staff made entirely of hardenable bronze. However, no information is given on the manufacturing process of the pivots. In addition, a part made of hardenable bronze has a hardness of less than 450 HV. Such hardness appears to the skilled person to be insufficient for rolling treatment.

[0009] Also known from application EP 2 757 423 are pivot pins made of austenitic cobalt or nickel alloys with an external surface hardened to a certain depth. However, such alloys can be difficult to machine by chip removal. In addition, they are relatively expensive due to the high price of nickel and cobalt. Summary of the invention

[0010] The aim of the present invention is to overcome all or part of the drawbacks mentioned above by proposing a pivot axis which makes it possible both to limit sensitivity to magnetic fields and to obtain improved hardness compatible with the requirements of resistance to wear and shocks in the watchmaking field.

[0011] The invention also aims to provide a non-magnetic pivot axis having improved corrosion resistance.

[0012] The invention also aims to provide a non-magnetic pivot axis which can be manufactured simply and economically.

[0013] To this end, the invention relates to a pivot axis for a watch movement comprising at least one metal pivot at at least one of its ends according to the characteristics of claim 1.

[0014] According to the invention, the metal is a non-magnetic copper alloy in order to limit its sensitivity to magnetic fields, and at least the external surface of said at least one pivot is hardened in depth relative to the core of the axis according to a predetermined depth.

[0015] Therefore, a surface area or the entire surface of the shaft is hardened, meaning that the core of the shaft may remain little or not modified. By this selective hardening of portions of the shaft, the pivot shaft can combine advantages such as low sensitivity to magnetic fields, and in the main stress areas, hardness, in addition to good corrosion resistance while maintaining good general toughness. Furthermore, the use of such a non-magnetic copper alloy is advantageous insofar as they have good machinability.

[0016] For example, the predetermined depth represents between 5% and 40% of the total diameter of the pivot, typically between 5 and 35 microns.

[0017] According to the invention, the deep-hardened external surface comprises diffused atoms of at least one chemical element.

[0018] According to the invention, the deep-hardened external surface has a hardness preferably greater than 600 HV.

[0019] Furthermore, the invention relates to a watch movement comprising a pivot axis according to one of the preceding variants, and in particular a balance shaft, an anchor stem and / or an escape pinion comprising an axis as defined above.

[0020] Finally, the invention relates to a method of manufacturing a pivot axis according to the characteristics of claim 8.

[0021] Therefore, by diffusion of atoms in the copper alloy, a surface area or the entire surface of the pivots is hardened without having to deposit a second material over the pivots. Indeed, the hardening is carried out directly in the material of the pivot axis which advantageously makes it possible according to the invention to avoid any subsequent delamination as can occur in the case of the deposition of a hard layer on the axis.

[0022] For example, the predetermined depth represents between 5% and 40% of the diameter d total pivot.

[0023] According to the invention, the atoms comprise at least one chemical element.

[0024] According to the invention, step b) consists of an ion implantation process followed or not by a diffusion treatment.

[0025] According to the invention, the pivots can be rolled or polished after step b). Summary description of the drawings

[0026] Other features and advantages will become clear from the description given below, for information purposes only and in no way limiting, with reference to the attached drawings, in which: there figure 1 is a representation of a pivot axis according to the invention; and the figure 2 is a partial section of a balance shaft pivot according to the invention after the diffusion treatment operation and after the rolling or polishing operation. Detailed Description of Preferred Embodiments

[0027] In this description, the term “non-magnetic” material means a paramagnetic or diamagnetic or antiferromagnetic material, the magnetic permeability of which is less than or equal to 1.01.

[0028] A copper alloy is an alloy containing at least 50% copper by weight.

[0029] The invention relates to a part for a watch movement and in particular to a non-magnetic pivot axis for a mechanical watch movement.

[0030] The invention will be described below in the context of an application to a non-magnetic balance staff 1. Obviously, other types of watch pivot staffs are conceivable, such as for example watch mobile staffs, typically escapement pinions, or even anchor stems. Parts of this type have body diameters preferably less than 2 mm, and pivots with a diameter preferably less than 0.2 mm, with a precision of a few microns.

[0031] Referring to the figure 1we can see a balance shaft 1 according to the invention which comprises a plurality of sections 2 of different diameters, preferably formed by turning or any other machining technique by chip removal, and conventionally defining bearing surfaces 2a and shoulders 2b arranged between two end portions defining two pivots 3. These pivots are intended to each pivot in a bearing, typically in an orifice of a stone or ruby.

[0032] With the magnetism induced by objects encountered on a daily basis, it is important to limit the sensitivity of balance staff 1, otherwise it will influence the running of the timepiece in which it is incorporated.

[0033] Surprisingly, the invention makes it possible to solve both problems at the same time without compromise and by providing other advantages. Thus, the metal 4 of the pivot 3 is a non-magnetic copper alloy in order to advantageously limit its sensitivity to magnetic fields. In addition, at least the external surface 5 of the pivots 3 ( Figure 2 ) is hardened in depth relative to the rest of the pivot 3 to a predetermined depth in order to offer, advantageously according to the invention, high hardness at the level of said external surface while maintaining high toughness.

[0034] Indeed, according to the invention, the deep hardened external surface of the pivots 3 has a hardness greater than 600 HV.

[0035] Preferably, the non-magnetic copper alloy is chosen from the group comprising a brass (Cu-Zn) or a special brass (Cu-Zn with Al and / or Si and / or Mn), a copper-beryllium, a bronze (Cu-Sn), an aluminum bronze, a copper-aluminum (optionally comprising Ni and / or Fe), a copper-nickel, a nickel silver (Cu-Ni-Zn), a copper-nickel-tin, a copper-nickel-silicon, a copper-nickel-phosphorus, a copper-titanium, the proportions of the different elements of the alloys being chosen to give them non-magnetic properties as well as good machinability.

[0036] For example, brasses can include the alloys CuZn39Pb3, CuZn37Pb2, or CuZn37.

[0037] Special brasses may include the alloys CuZn37Mn3Al2PbSi, CuZn23Al3Co or CuZn23Al6Mn4Fe3Pb.

[0038] Nickel silver can include the alloys CuNi25Zn11Pb1Mn, CuNi7Zn39Pb3Mn2 or CuNi18Zn19Pb1.

[0039] Bronzes may include CuSn9 or CuSn6 alloys.

[0040] Aluminum bronzes can include CuAl9 or CuAl9Fe5Ni5 alloys.

[0041] Copper-nickel alloys may include the CuNi30 alloy.

[0042] Copper-nickel-tin alloys may include CuNi15Sn8, CuNi9Sn6 or CuNi7.5Sn5 alloys.

[0043] Copper-titanium alloys may include the CuTi3Fe alloy.

[0044] Copper-nickel-silicon alloys may include the CuNi3Si alloy.

[0045] Copper-nickel-phosphorus alloys may include the CuNi1P alloy.

[0046] Copper-beryllium alloys can include CuBe2Pb or CuBe2 alloys.

[0047] Composition values ​​are given as a percentage by weight. Elements without a composition value are either the remainder (Copper) or elements for which the percentage in the composition is less than 1% by weight.

[0048] The non-magnetic copper alloy can also be an alloy with a mass composition of between 14.5% and 15.5% Ni, between 7.5% and 8.5% Sn, a maximum of 0.02% Pb and the remainder Cu. Such an alloy is marketed under the brand name Toughmet ®< by the company Materion.

[0049] Obviously, other non-magnetic copper-based alloys are possible provided that the proportion of their constituents gives them non-magnetic properties as well as good machinability.

[0050] It has been empirically shown that a hardening depth of between 5% and 40% of the total diameter d of the pivots 3 is sufficient for application to a balance shaft. For example, if the radius d / 2 is 50 µ m, the hardening depth is preferably around 15 µ m all around the pivots 3. Obviously, depending on the applications, a different hardening depth of between 5% and 80% of the total diameter d can be provided.

[0051] Preferably according to the invention, the deep-hardened external surface 5 of the 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 boron. Indeed, as explained below, by interstitial supersaturation of atoms in the non-magnetic copper alloy 4, a surface zone 5 is deep-hardened without having to deposit a second material over the pivots 3. Indeed, the hardening is carried out directly in the material 4 of the pivots 3 which advantageously makes it possible according to the invention to avoid any subsequent delamination during use. As a result, the external surface 5 of the pivot 3 comprises a hard surface layer but does not have any additional hardening layer deposited directly on said external surface 5. It is obvious that other layers not having a hardening function can be deposited.It is therefore possible to deposit a layer of lubrication on the external surface of the pivot, for example.

[0052] Consequently, at least one surface area of ​​the pivot is hardened, i.e. the core of the pivots 3 and / or the rest of the shaft can remain little or not modified without significant modification of the mechanical properties of the balance shaft 1. This selective hardening of the pivots 3 of the balance shaft 1 makes it possible to combine advantages such as low sensitivity to magnetic fields, high hardness and toughness, in the main stress areas while having good resistance to corrosion and fatigue.

[0053] The invention also relates to the method of manufacturing a balance shaft as explained above. The method advantageously comprises, according to the invention, the following steps: a) forming, preferably by turning or any other machining technique by chip removal, a balance shaft 1 comprising a pivot 3 made of metal at each of its ends, said metal being a non-magnetic copper alloy to limit its sensitivity to magnetic fields and; b) diffusing atoms to a predetermined depth at least in the external surface 5 of the pivots 3 in order to harden the pivots in depth at the level of the main stress zones.

[0054] According to a first preferred embodiment, the pivots 3 are rolled or polished after step b) in order to achieve the desired final dimensions and surface condition for the pivots 3. This rolling operation after treatment makes it possible to obtain axles having improved resistance to wear and impact compared to axles whose pivots have only undergone a hardening operation. As a result, at least the external surface 5 of the pivots 3 of the invention is rolled.

[0055] Advantageously according to the invention, whatever the embodiment, the method can be applied in bulk. Thus, step b) can consist of a thermochemical diffusion treatment such as a boriding of several balance shafts and / or several balance shaft blanks. It is understood that step b) can consist of interstitially diffusing atoms of a chemical element, for example a non-metal, into the non-magnetic copper alloy 4. Finally, advantageously, it has been found that the compressive stresses of the method improve fatigue resistance and shock resistance.

[0056] Step b) could also consist of an ion implantation process and / or a thermal diffusion treatment. This variant has the advantage of not limiting the type of atoms diffused and allows both interstitial and substitutional diffusion.

[0057] When the treatment carried out during step b) is an ion implantation process, the hardening depth of the external surface 5 can advantageously be increased using a heat treatment carried out during or after step b) of ion implantation treatment.

[0058] The method according to the invention does not include any step of depositing, directly on the external surface 5 of the pivot 3, an additional hardening layer.

[0059] The pivot axis according to the invention may comprise pivots treated according to the invention or be made entirely of non-magnetic copper alloy. In addition, the diffusion treatment of step b) may be carried out on the surface of the pivots or on all of the surfaces of the pivot axis.

[0060] The pivot axis according to the invention can advantageously be produced by bar turning or any other machining technique by chip removal from non-magnetic copper alloy bars with a diameter preferably less than 3 mm, and preferably less than 2 mm. Copper alloys are known to those skilled in the art to be too soft to be able to be rolled and to withstand wear in motion. However, the use of such materials according to the invention makes it possible, in a surprising and unexpected manner, to produce pivot axes having a hardness greater than 600 HV allowing rolling and achieving satisfactory longevity in motion.To achieve the present invention, the person skilled in the art had to overcome the prejudice against using a non-magnetic copper-based alloy to produce a part of very small dimensions by means of a process comprising a step of bar turning (or any other machining technique by chip removal) and rolling.

[0061] Against all expectations, the method of the invention makes it possible to obtain a watch pivot axis of which at least the pivots are formed by turning (or any other machining technique by chip removal) and rolling from a non-magnetic copper alloy.

[0062] Of course, the present invention is not limited to the illustrated example but is susceptible to various variants and modifications which will appear to those skilled in the art. In particular, it may be envisaged to totally or almost totally treat the pivots 3, that is to say to treat a percentage greater than 80% of the diameter d of the pivots 3 even if this is not necessary for the application to pivot axes such as watch balance axes.

[0063] Of course, the present invention is not limited to the illustrated example but is susceptible to various variants and modifications which will appear to those skilled in the art without departing from the scope of the invention as defined by the appended claims.

[0064] In particular, it may be envisaged to totally or almost totally treat the pivots 3, that is to say to treat a percentage greater than 80% of the diameter d of the pivots 3 even if this is not necessary for the application to pivot axes such as watch balance axes.

Claims

1. Pivot arbor (1) for a timepiece movement comprising at least one metal pivot (3) at least one of the ends thereof, characterized in that the metal is a non-magnetic copper alloy so as to limit the sensitivity of the pivot to magnetic fields, and in that at least the outer surface (5) of said pivot (3) is deep-hardened to a predetermined depth relative to the core of the pivot arbor, the deep-hardened outer surface (5) has a hardness of more than 600 HV, the deep-hardened outer surface (5) comprising diffused atoms of at least one chemical element chosen from nitrogen, argon and / or boron, so that said outer surface (5) of said pivot (3) has no hardening layer directly deposited on said outer surface.

2. Pivot arbor (1) according to claim 1, characterized in that the predetermined depth represents between 5% and 40% of the total diameter (d) of the pivot (3).

3. Pivot arbor (1) according to any of the preceding claims, characterized in that the non-magnetic copper alloy is chosen from the group consisting of a copper and zinc based brass, a copper-beryllium, a nickel silver, a bronze, an aluminium bronze, a copper-aluminium, a copper-nickel, a copper-nickel-tin, a copper-nickel-silicon, a copper-nickel-phosphorus, a copper-titanium, an alloy having a mass percent composition of between 14.5% and 15.5% Ni, between 7.5% and 8.5% Sn, at most 0.02% Pb and the remainder copper.

4. Pivot arbor (1) according to any of the preceding claims, characterized in that at least the outer surface (5) of said pivot (3) is rolled.

5. Pivot arbor (1) according to any of the preceding claims, characterized in that the pivot arbor has two pivots.

6. Movement for a timepiece characterized in that the movement comprises a pivot arbor (1) according to any of the preceding claims.

7. Movement for a timepiece characterized in that the movement comprises a balance staff, a pallet staff and / or an escape pinion comprising a pivot arbor (1) according to any of claims 1 to 5.

8. Method for fabricating a pivot arbor (1) for a timepiece movement comprising the following steps: a) forming a pivot arbor (1) comprising at least one metal pivot (3) at one of the ends thereof, said metal being a non-magnetic copper alloy, to limit the sensitivity thereof to magnetic fields; b) diffusing atoms at least one chemical element chosen from nitrogen, argon and / or boron to a predetermined depth in at least the outer surface (5) of said pivot (3) in order to deep-harden the pivot arbor (1) in the main areas of stress while maintaining a high tenacity, until a hardness of more than 600 HV is obtained.

9. Method according to claim 8, characterized in that the predetermined depth represents between 5% and 40% of the total diameter (d) of the pivot (3).

10. Method according to any of claims 8 to 9, characterized in that step b) consists of an ionic implantation process which may or may not be followed by a diffusion treatment.

11. Method according to any of claims 8 to 10, characterized in that the method does not comprise any step of depositing a hardening layer directly on the outer surface (5) of the pivot (3).

12. Method according to any of claims 8 to 11, characterized in that the pivot (3) is subjected to a rolling / polishing step after step b).