CH712718B1 - Pivot pin for clockwork movement. - Google Patents

Abstract

The invention relates to a pivot pin comprising a metal pivot (3) at at least one of its ends. The metal is a non-magnetic copper 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 hardened in depth with respect to the rest of the axis to a depth predetermined in order to harden the pivot (s) (3). The invention relates to the field of watch movements. A method of manufacturing such a pivot axis is also claimed.

Description

Field of the invention

The invention relates to a part for a timepiece movement and in particular to a non-magnetic pivot axis for a mechanical timepiece movement and more particularly to a balance axis, an anchor rod and a pinion. non-magnetic exhausts.

Background of the invention

The manufacture of a horological pivot axis consists, from a hardenable steel bar, in performing bar turning operations to define different active surfaces (bearing, shoulder, pivots etc.) then in subjecting the necked axis to heat treatment operations comprising at least one quenching to improve the hardness of the shaft and one or more tempers to improve its toughness. The heat treatment operations are followed by an operation of rolling the pivots of the axes, an operation consisting in polishing the pivots to bring them to the required dimensions. During the rolling operation the hardness as well as the roughness of the pivots are further improved. It will be noted that this rolling operation is very difficult or even impossible to carry out with most materials whose hardness is low, that is to say less than 600HV.

[0003] The pivot axes, for example the balance axles, conventionally used in mechanical clock movements are produced in grades of free-cutting steels which are generally martensitic carbon steels including lead and manganese sulphides 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 easily machinable, in particular of being suitable for bar turning and has, after quenching and tempering treatments, high mechanical properties which are very interesting for the production of horological pivot axes. These steels exhibit in particular high wear resistance and hardness after heat treatment. Typically, the hardness of the pivots of an axle made from 20 AP steel can reach a hardness exceeding 700 HV after heat treatment and rolling.

Although providing satisfactory mechanical properties for the horological applications described above, this type of material has the drawback of being magnetic and of being able to disturb the running of a watch after having been subjected to a magnetic field, and this in particular when this material is used for the production of a balance pin cooperating with a spring balance made of ferromagnetic material. This phenomenon is well known to those skilled in the art. It will also be noted that these martensitic steels are also sensitive to corrosion.

[0006] Tests to attempt to remedy 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 that does not allow them to be quenched and to achieve hardnesses and therefore wear resistances compatible with the requirements required 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 hardness greater than 500 HV to be obtained. Consequently, in the context of parts requiring great resistance to frictional wear and having to have pivots which present little or no risk of deformation, the use of this type of steels remains limited.

Another approach to try to remedy these drawbacks has consisted in depositing on the pivot axes of the hard layers of materials such as amorphous carbon known under the English name diamond like carbon (DLC). However, significant risks of delamination of the hard layer have been observed and therefore the formation of debris which can circulate inside the watch movement and disturb the operation of the latter, which is not satisfactory.

[0008] A similar approach, described in patent FR 2 015 873, provides for making a balance pin, at least the main part of which is made from certain non-magnetic materials. The pivots can be in the same material or in 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 axis made entirely of hardenable bronze. However, no information is given on the method of manufacturing the pivots. In addition, a part made of hardenable bronze has a hardness less than 450 HV. Such a hardness appears to a person skilled in the art as insufficient to carry out a treatment by rolling.

[0009] Also known from application EP 2 757 423 are pivot pins made of a cobalt or nickel alloy of the austenitic type and having 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

The object of the present invention is to overcome all or part of the drawbacks mentioned above by providing a pivot axis making it possible both to limit the sensitivity to magnetic fields and to obtain an improved hardness compatible with the requirements of resistance to wear and shock in the watchmaking field.

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

[0012] Another object of the invention is to provide a non-magnetic pivot axis which can be manufactured in a simple and economical manner.

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

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 outer surface of said at least one pivot is hardened in depth with respect to the heart of the axis according to a predetermined depth. The deeply cured outer surface has a hardness greater than 600 HV.

[0015] Consequently, a surface area or the entire surface of the axis is hardened, that is to say that the core of the axis can remain little or not modified. By this selective hardening of portions of the axis, the pivot axis makes it possible to 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 overall tenacity. Furthermore, the use of such a non-magnetic copper alloy is advantageous insofar as the latter have good machinability.

[0016] According to other advantageous characteristics of the invention: <tb> <SEP> - the predetermined depth represents between 5% and 40% of the total diameter d of the pivot, typically between 5 and 35 microns; <tb> <SEP> - the deeply hardened outer surface has scattered atoms of at least one chemical element;

[0017] In addition, the invention relates to a timepiece movement comprising a pivot axis according to one of the preceding variants, and in particular a balance axis, an anchor rod and / or a pinion. escapement comprising an axis as defined above.

Finally, the invention relates to a method of manufacturing a pivot pin comprising the following steps: <tb> <SEP> a) forming, preferably by bar turning or any other machining technique by chip removal, a pivot axis comprising at least one metal pivot at at least one of its ends, said metal being an alloy non-magnetic copper, to limit its sensitivity to magnetic fields; <tb> <SEP> b) diffusing atoms to a predetermined depth at least in the external surface of said pivot in order to harden the pivot axis in depth at the level of the main stress zones while maintaining high toughness.

[0019] Consequently, by diffusion of atoms in the copper alloy, a surface area or the whole of the surface of the pivots is hardened without having to deposit a second material on top of the pivots. Indeed, the hardening is carried out directly in the material of the pivot axis which advantageously allows 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. .

[0020] According to other advantageous characteristics of the invention: the predetermined depth represents between 5% and 40% of the total diameter d of the pivot; the atoms have at least one chemical element; step b) consists of a thermochemical diffusion treatment; step b) consists of an ion implantation process followed or not by a diffusion treatment; the pivots are rolled or polished after step b).

Brief description of the drawings

Other features and advantages will emerge clearly from the description which is given below, by way of indication and in no way limiting, with reference to the accompanying drawings, in which: FIG. 1 is a representation of a pivot axis according to the invention; and FIG. 2 is a partial section through a pivot axis of the balance according to the invention after the diffusion treatment operation and after the rolling or polishing operation.

Detailed description of the preferred embodiments

In the present 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.

[0023] A copper alloy is an alloy containing at least 50% by weight of copper.

[0024] The invention relates to a part for a timepiece movement and in particular to a non-magnetic pivot axis for a mechanical timepiece movement.

The invention will be described below in the context of an application to a non-magnetic balance axis 1. Obviously, other types of horological pivot axes can be envisaged such as for example axes of watch mobiles, typically exhaust gears, or even anchor rods. Parts of this type have, at the level of the body, diameters preferably less than 2 mm, and pivots with a diameter preferably less than 0.2 mm, with an accuracy of a few microns.

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

With the magnetism induced by objects encountered on a daily basis, it is important to limit the sensitivity of the balance axis 1 under penalty of influencing the rate of the timepiece in which it is incorporated.

Surprisingly, the invention makes it possible to solve the two problems at the same time without compromise and while 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 outer 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, a high hardness at said surface. external while maintaining high tenacity.

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

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 various elements of the alloys being chosen to give them non-magnetic properties as well as good machinability.

For example, the brasses can comprise the alloys CuZn39Pb3, CuZn37Pb2, or CuZn37.

[0032] The special brasses may include the CuZn37Mn3AI2PbSi, CuZn23Al3Co or CuZn23AI6Mn4Fe3Pb alloys.

The Maillechort can comprise the alloys CuNi25Zn11Pb1Mn, CuNi7Zn39Pb3Mn2 or CuNi18Zn19Pb1.

Bronzes can include CuSn9 or CuSn6 alloys.

Aluminum bronzes can include CuAl9 or CuAl9Fe5Ni5 alloys.

The copper-nickel alloys can include the CuNi30 alloy.

Copper-nickel-tin alloys can include CuNi15Sn8, CuNi9Sn6 or CuNi7.5Sn5 alloys.

[0038] The copper-titanium alloys can comprise the CuTi3Fe alloy.

The copper-nickel-silicon alloys can include the CuNi3Si alloy.

The copper-nickel-phosphorus alloys can comprise the CuNi1P alloy.

The copper-beryllium alloys can include CuBe2Pb or CuBe2 alloys.

The composition values are indicated in percentage by mass. The elements without indication of a composition value are either the remainder (copper) or elements for which the percentage in the composition is less than 1% by weight.

The non-magnetic copper alloy can also be an alloy having a mass composition of between 14.5% and 15.5% of Ni, between 7.5% and 8.5% of Sn, at most 0.02% of Pb and the remainder of Cu. Such an alloy is marketed under the trademark Toughmet® by the company Materion.

Obviously, other non-magnetic copper-based alloys can be envisaged as long as the proportion of their constituents gives them non-magnetic properties as well as good machinability.

It has been shown empirically that a hardening depth of between 5% and 40% of the total diameter of the pivots 3 is sufficient for the application to a balance axis. By way of example, if the radius d / 2 is 50 μm, the hardening depth is preferably around 15 μm all around the pivots 3. Obviously, depending on the applications, a different hardening depth of between 5% and 50% of the total diameter can be expected.

Preferably according to the invention, the deeply hardened outer surface 5 of the pivots 3 comprises diffused atoms of at least one chemical element. For example, this chemical element can 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 hardened in depth without having to deposit a second material over the pivots 3. In fact, 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. Therefore, the outer surface 5 of the pivot 3 comprises a hard surface layer but does not have any additional hardening layer deposited directly on said outer surface 5. It is obvious that other layers having no hardening function can. be filed. Thus, it is possible to deposit on the outer surface of the pivot a lubricating layer for example.

Consequently, at least one superficial zone of the pivot is hardened, that is to say that the heart of the pivots 3 and / or the rest of the axis, can remain little or not modified without significant modification of the mechanical properties. of the balance axis 1. This selective hardening of the pivots 3 of the balance axis 1 makes it possible to combine advantages such as low sensitivity to magnetic fields, high hardness and tenacity, in the main stress areas while having good resistance to corrosion and fatigue.

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: <tb> <SEP> a) forming, preferably by bar turning or any other machining technique by chip removal, a balance shaft 1 comprising a metal pivot 3 at each of its ends, said metal being a copper alloy non-magnetic to limit its sensitivity to magnetic fields and; <tb> <SEP> 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 areas.

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 allows to obtain pins with improved wear and impact resistance compared to pins whose pivots have only undergone a hardening operation. Therefore, at least the outer surface 5 of the pivots 3 of the invention is rolled.

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 axes and / or several blanks of balance axes. It will be understood that step b) can consist in diffusing interstitially in the non-magnetic copper alloy 4, atoms of a chemical element, for example a non-metal. Finally, advantageously, it has been found that the compressive stresses of the process improve fatigue resistance and impact resistance.

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

When the treatment implemented 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 ) treatment by ion implantation.

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

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

The pivot axis according to the invention can advantageously be produced by bar turning or any other machining technique by removing chips from non-magnetic copper alloy bars of diameter preferably less than 3 mm, and preferably less to 2 mm. Copper alloys are known to those skilled in the art for being too soft to be able to be rolled and for resistance to wear in motion. However, the use of such materials according to the invention surprisingly and unexpectedly makes it possible to produce pivot pins having a hardness greater than 600 HV making it possible to carry out rolling and to achieve satisfactory longevity in motion. To achieve the present invention, those skilled in the art had to overcome the prejudice of using an alloy based on non-magnetic copper to produce a part of very small dimensions by means of a process comprising a turning step (or any other technique of 'machining by chip removal) and rolling.

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

Of course, the present invention is not limited to the example illustrated but is susceptible to various variants and modifications which will appear to those skilled in the art. In particular, it can be envisaged to completely or almost completely treat the pivots 3, that is to say 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 clockwork balance axes.

Claims (14)

1. Pivot pin (1) for a watch movement comprising at least one pivot (3) made of metal at at least one of its ends, characterized in that the metal is a non-magnetic copper alloy in order to limit its sensitivity to magnetic fields, and in that at least the outer surface (5) of said pivot (3) is hardened in depth relative to the core of the axis to a predetermined depth, the outer surface (5) hardened in depth having a hardness greater than 600 HV. 2. Pivot pin (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 pin (1) according to one of claims 1 or 2, characterized in that the outer surface (5) hardened in depth comprises diffused atoms of at least one chemical element. 4. Pivot pin (1) according to one of the preceding claims, characterized in that the non-magnetic copper alloy is chosen from the group comprising a brass based on copper and zinc, a copper-beryllium, a nickel silver, a bronze , an aluminum bronze, a copper-aluminum, a copper-nickel, a copper-nickel-tin, a copper-nickel-silicon, a copper-nickel-phosphorus, a copper-titanium, an alloy having for mass composition between 14.5 % and 15.5% of Ni, between 7.5% and 8.5% of Sn, maximum 0.02% of Pb and the rest of Cu. 5. Pivot pin (1) according to one of the preceding claims, characterized in that said outer surface (5) of said pivot (3) has no hardening layer deposited directly on said outer surface. 6. Pivot pin (1) according to one of the preceding claims, characterized in that at least the outer surface (5) of said pivot (3) is rolled. 7. Pivot pin (1) according to one of the preceding claims, characterized in that it comprises two pivots. 8. Movement for a timepiece, characterized in that qull comprises a pivot pin (1) according to one of the preceding claims. 9. Movement for a timepiece according to the preceding claim, characterized in that the pivot axis (1) is a balance axis, an anchor rod or the axis of an escape pinion. 10. A method of manufacturing a pivot pin (1) for a watch movement comprising the following steps: a) forming a pivot axis (1) comprising at least one metal pivot (3) at at least one of its ends, said metal being a non-magnetic copper alloy to limit its sensitivity to magnetic fields; b) carrying out a thermochemical diffusion treatment or an ion implantation process followed or not by a heat treatment, according to a predetermined depth at least in the external surface (5) of said pivot (3) in order to harden the pivot axis in depth (1) at the level of the main stress areas while maintaining high toughness, the deeply hardened outer surface (5) exhibiting a hardness greater than 600 HV. 11. The method of claim 10, characterized in that the predetermined depth represents between 5% and 40% of the total diameter d of the pivot (3). 12. Method according to one of claims 10 or 11, characterized in that the thermochemical diffusion treatment step comprises the diffusion of atoms of at least one chemical element. 13. Method according to one of claims 10 to 12, characterized in that it does not include any step of depositing, directly on the outer surface (5) of the pivot (3), a hardening layer. 14. Method according to one of claims 10 to 13, characterized in that the pivot (3) undergoes a rolling or polishing operation after step b).