Part for clock movement

Abstract

The invention relates to a pivot axis for a watch movement comprising at least one pivot (3) made of a non-magnetic metallic material (4) at at least one end to limit its sensitivity to magnetic fields. Said non-magnetic metallic material (4) is a non-magnetic light metal or a non-magnetic alloy of said light metal, and at least the external surface of said pivot (3) is coated with an anodic layer of oxide of said material (5), obtained by anodizing. The invention relates to the field of watch movements.

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.

[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, in particular, have a high hardness after heat treatment, allowing very good shock resistance to be obtained. 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, i.e. 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 shock resistances compatible with the requirements for the production of watch pivot axes. The axes obtained then show marks or severe damage in the event of shocks which will then have a negative influence on the chronometry of the movement. 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 pivots with high shock resistance, the use of this type of steel remains limited.

[0007] Another approach to try to overcome 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, resulting in the formation of debris that can circulate inside the watch movement and disrupt its operation, which is not satisfactory.

[0008] Also known from application EP 2 757 423 are pivot pins made of austenitic cobalt or nickel alloy and having an external surface hardened to a certain depth. However, such alloys can be difficult to machine for the manufacture of pivot pins. In addition, they are relatively expensive due to the high price of nickel and cobalt.

[0009] We also know the document JP2010209473A which discloses a pivot axis for watch movements, this axis being made of anodized aluminum.

[0010] Also known from document CH 707986 is a timepiece, characterized in that it is made of a ceramicized aluminum alloy comprising between 88 and 94% aluminum, between 5 and 10% zinc, between 1 and 2% magnesium, between 0 and 0.1% iron and between 0 and 0.1% silicon, possibly oxidized on the surface. It can be a bridge, a plate, a balance spring, a balance wheel, a wheel, an escapement, a pinion, a mobile.

[0011] Also known from DE 10 2009 046 647 is a watch mechanism comprising a component having friction surfaces. The component is made of aluminum or an aluminum alloy and is coated with a hard anodized layer on the friction surfaces. The coating has open pores on its free surface in which a lubricant is deposited. The component may be a movable drive part such as an anchor or an escape wheel or a fixed drive part. Summary of the invention

[0012] The aim of the present invention is to overcome 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.

[0013] The invention also aims to provide a non-magnetic pivot axis which can be manufactured simply and economically.

[0014] For this purpose, the invention relates to a pivot axis for a watch movement as defined in claim 1. Preferred embodiments of this axis are defined in dependent claims 2 to 7.

[0015] Furthermore, as defined by dependent claims 8 and 9, the invention relates to a timepiece movement comprising a pivot axis as defined above, and in particular a balance staff, an anchor stem and / or an escape pinion comprising an axis as defined above.

[0016] Finally, the invention relates to a method of manufacturing a pivot axis as defined in claim 10. Preferred variants are defined in claims 11 to 13. Summary description of the drawings

[0017] Other features and advantages will become clear from the description given below, for informational 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. Detailed Description of Preferred Embodiments

[0018] 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.

[0019] An alloy of an element is an alloy containing at least 50% by weight of said element.

[0020] The invention relates to a part for a watch movement and in particular to a non-magnetic pivot axis for a mechanical watch movement.

[0021] 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.

[0022] 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.

[0023] 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.

[0024] Thus, the pivot 3 is made of a non-magnetic metallic material 4 in order to advantageously limit its sensitivity to magnetic fields.

[0025] According to the invention, said non-magnetic metallic material 4 is a non-magnetic light metal or a non-magnetic alloy of said light metal.

[0026] Preferably, said non-magnetic metallic material 4 used in the invention is chosen from the group comprising aluminum, titanium, magnesium and their non-magnetic alloys.

[0027] In a particularly advantageous manner, said non-magnetic metallic material 4 is chosen from the group comprising a 6000 series aluminum alloy (AI Mg Si), a 7000 series aluminum alloy with copper (AI Zn Cu), a Grade 5 titanium alloy (comprising from 5.5 to 6.75% Al and 3.5 to 4.5% V), an Mg-Zr alloy, the proportions of the different elements of the alloys being chosen to give them non-magnetic properties as well as good machinability. These alloys have in particular the property of being able to be machined and of being suitable for anodizing.

[0028] For example, a particularly preferred alloy is aluminum alloy EN AW 6082, aluminum alloy EN AW 7075, and aluminum alloy EN AW 7068.

[0029] Composition values ​​are given as a percentage by weight. Elements without a composition value are either the remainder (majority) or elements for which the percentage in the composition is less than 1% by weight.

[0030] Obviously, other non-magnetic alloys based on light metal are possible provided that the proportion of their constituents gives them non-magnetic properties as well as good machinability.

[0031] The non-magnetic light metal-based metallic material used in the invention generally has a hardness of less than 250 HV, or even 100 HV. In addition to the advantage of being non-magnetic, this material has low inertia due to its low weight.

[0032] According to the invention, at least the external surface of said pivot 3 is covered with an anodic layer of oxide of said material 5, obtained by anodization growth. This layer of oxide obtained by growth has excellent adhesion to the base material of the pivot making it possible to avoid any subsequent delamination during use. Said layer of oxide obtained by growth also has an improved hardness compatible with the requirements of resistance to wear and shocks in the watchmaking field.

[0033] Thus, the formed anodic layer 5 has a hardness greater than 300 HV, preferably greater than 400 HV, and more preferably greater than 500 HV.

[0034] In addition, the anodic layer 5 formed has a thickness of between 2 µm and 50 µm, preferably between 10 µm and 30 µm.

[0035] It is obvious that other layers not having a hardening function can then be deposited on the anodic layer 5. Thus, it is possible to deposit a lubricating layer on the anodic layer 5, for example.

[0036] Therefore, at least the external surface of the pivot 3 is hardened, i.e. 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 zones while having good resistance to corrosion and fatigue.

[0037] 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 at least one pivot 3 made of a non-magnetic metallic material at each of its ends, to limit its sensitivity to magnetic fields, said non-magnetic metallic material being a non-magnetic light metal or a non-magnetic alloy of said light metal; b) subjecting at least the external surface of said pivot 3 to an anodizing treatment in order to grow on said surface an anodic layer of oxide of said material by anodizing to form on the surface of said pivot 3 a hard layer at least at the level of the main stress zones.

[0038] The anodic oxide layer 5 is formed by anodization according to step b) and has a thickness of between 2 µm and 50 µm, preferably between 10 µm and 30 µm.

[0039] Advantageously, the anodizing treatment of step b) is a conventional anodizing process or a micro-arc oxidation process.

[0040] Conventional anodizing is carried out by imposing a direct current on the system. Conventional anodizing can be of different types and can be carried out for example in an oxalic or sulfuric medium. An oxalic medium is preferred. The various parameters of conventional anodizing to be taken into account, such as the composition of the electrolytic bath, and in particular the choice and concentration of the acid, the operating conditions such as the temperature of the electrolyte, the pH, the intensity of the anodizing current, to obtain an anodic oxide layer of thickness and hardness appropriate for the invention, are known to those skilled in the art.

[0041] Classic anodizing treatments for aluminum can be as follows: Treatment 1: Bath 1 H 2 SO 4: 150 g / l ± 5 Al 3+<: 20g / l ± 5 Temperature: 8°C ± 3 Current intensity: 1.5 A / dm 2< ± 0.5 Treatment 2: Bath 2 H 2 SO 4: 150 g / l ± 10 Oxalic acid: 50g / l ± 10 Temperature: 16°C ± 2 Current intensity: 1 A / dm 2< ± 0.5

[0042] Conventional anodizing treatments for titanium are carried out in accordance with AMS 2488 using alkaline anodizing. The thickness of the anodic layer is approximately 3 µm.

[0043] Micro-arc oxidation (MAO) is a surface electrolytic process based on the principle of conventional anodization while differing from the latter both by the electrical power supply and by the nature of the electrolyte. It causes plasma micro-discharges to appear on the surface of the material during the treatment. Electrolytic plasma oxidation processes are generally carried out in weakly concentrated alkaline baths with current densities generally less than 1 A / cm 2< and voltages greater than 200 V. A person skilled in the art knows how to choose the parameters relating in particular to the electrical power source, namely the current mode (density, frequency and waveform of the current), the charge density and the current density, and to the electrolytes, in particular aqueous-based (composition and concentration) to obtain an anodic oxide layer of thickness and hardness appropriate for the invention.

[0044] The method according to the invention may comprise a preliminary surface treatment step necessary to clean the axes before subjecting them to anodization.

[0045] The pivot axis according to the invention may comprise pivots treated according to the invention by applying step b) to the pivots only or be made entirely of a non-magnetic metallic material based on light metal, its external surface being able to be entirely covered with an anodic layer of oxide of said material by applying step b) to all the surfaces of the pivot axis.

[0046] The method according to the invention may further comprise, after step b), a finishing treatment step c). The finishing treatment may be a rolling or polishing operation in order to achieve the desired final surface condition for the pivots 3. In the particular case of micro-arc oxidation, the finishing treatment may be a lapping operation in order to eliminate the porous surface layer.

[0047] The following example illustrates the present invention without, however, limiting its scope.

[0048] Balance shafts made of 6082 aluminum are produced in a known manner and treated according to the method of the invention according to the classic anodizing treatment 1: Bath 1: H 2 SO 4 : 150 g / l ± 5 Al 3< + : 20g / l ± 5 Temperature: 8°C ± 3 Current intensity: 1.5 A / dm 2< ± 0.5

[0049] After conventional anodizing treatment, the 6082 aluminum balance staff is covered with a 5.8 µm thick anodic layer of aluminum oxide. The core hardness is measured at 119 HV0.01. The hardness in the anodic oxide layer is measured at 695 HV0.01. The result is a light metal (aluminum) balance staff combining the advantages of low sensitivity to magnetic fields, high hardness and toughness in the main stress areas while having good resistance to corrosion and fatigue.

Claims

1. Pivot arbor (1) for a timepiece movement comprising at least one pivot (3) made of a non-magnetic metal material (4), at at least one of the ends thereof, to limit the sensitivity thereof to magnetic fields, said non-magnetic metal material (4) is a non-magnetic light metal or a non-magnetic alloy of said light metal, characterized in that at least the external surface of said pivot (3) is coated with an anodic oxide layer of said material (5) having a thickness comprised between 2 µm and 50 µm and a hardness greater than 300 HV.

2. Pivot arbor (1) according to claim 1, characterized in that the arbor is made of a non-magnetic metal material, said non-magnetic metal material being a non-magnetic light metal or a non-magnetic alloy of said light metal in order to limit the sensitivity thereof to magnetic fields, and in that the external surface thereof is coated with an anodic oxide layer of said material.

3. Pivot arbor (1) according to any of the preceding claims, characterized in that the non-magnetic metal material (4) is chosen from the group comprising aluminium, titanium, magnesium and their non-magnetic alloys.

4. Pivot arbor (1) according to any of the preceding claims, characterized in that the non-magnetic metal material (4) has a hardness of less than 250 HV.

5. Pivot arbor (1) according to any of the preceding claims, characterized in that the anodic oxide layer (5) has a thickness comprised between 10 µm and 30 µm.

6. Pivot arbor (1) according to any of the preceding claims, characterized in that said anodic oxide layer (5) has a hardness greater than 400 HV, and more preferentially greater than 500 HV.

7. Pivot axis (1) according to any of the preceding claims, characterized in that the non-magnetic metal material (4) is chosen from the group comprising a 6000-series aluminium alloy, a 7000-series aluminium alloy with copper, a Grade 5 titanium alloy, and an Mg-Zr alloy.

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

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

10. Method for manufacturing a pivot arbor (1) for a timepiece movement comprising the following steps: a) forming a pivot arbor (1) comprising at least one pivot (3) made of a non-magnetic metal material (4), at at least one of the ends thereof, to limit the sensitivity thereof to magnetic fields, said non-magnetic metal material (4) being a non-magnetic light metal or a non-magnetic alloy of said light metal; b) subjecting at least the external surface of said pivot (3) to an anodizing treatment to grow on said surface an anodic oxide layer of said material (5) by anodization in order to obtain an anodic oxide layer with a thickness comprised between 2 µm and 50 µm and a hardness greater than 300 HV.

11. Method according to claim 10, characterized in that the anodic oxide layer (5) has a thickness comprised between 10 µm and 30 µm.

12. Method according to any of claims 10 and 11, characterized in that the anodizing treatment of step b) is a conventional anodizing process or a micro arc oxidation process.

13. Method according to any of claims 10 to 12, characterized in that after step b), the method also comprises a finishing treatment step c).

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