Non-magnetic pivot pin

Abstract

The invention relates to a pivot pin (1) for a timepiece movement, which is at least partially made of tungsten or a tungsten alloy.

Description

LABELED P IVOTEM ENT AXIS Technical field of the invention

[0001] The present invention relates to a part for a watch movement and in particular to an amagnetic pivot axis for a mechanical watch movement and more particularly to an amagnetic balance shaft, an anchor rod and an escape pinion. Technological background

[0002] The manufacture of a watch pivot axis involves machining a bar of hardenable steel to define various active surfaces (bearing, shoulder, pivots, etc.). The machined axis then undergoes heat treatment, including at least one quenching to improve its hardness and one or more tempering processes to enhance its toughness. These heat treatments are followed by pivot rolling, a process that polishes the pivots to the required dimensions. During this rolling process, the hardness and surface finish of the pivots are further improved.

[0003] Pivoting axes, such as balance staffs, commonly used in mechanical watch movements, are made from free-cutting steel grades that are generally carbon martensitic 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 easily machinable, particularly suitable for turning, and, after quenching and tempering treatments, exhibits high mechanical properties that are very interesting for the production of watch pivots. These steels, in particular, exhibit high hardness after heat treatment, making it possible to obtain very good shock resistance. Typically, the hardness of pivots on an axle made of 20 AP steel can reach a value exceeding 700 HV after heat treatment and rolling.

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

[0006] Attempts to address these drawbacks have been made with austenitic stainless steels, which are characterized by their non-magnetic properties, meaning they are paramagnetic, diamagnetic, or antiferromagnetic. However, these austenitic steels have a crystallographic structure that prevents them from being hardened to achieve the hardness and shock resistance required for watch pivots. The resulting pivots exhibit marks or severe damage upon impact, which negatively affects the movement's timekeeping. One way to increase the hardness of these steels is through work hardening, but this process does not achieve hardnesses exceeding 500 HV. Consequently, the use of this type of steel remains limited for components requiring pivots with high shock resistance. Summary of the invention

[0007] The invention aims to overcome the aforementioned drawbacks by proposing a pivot axis that simultaneously limits sensitivity to magnetic fields and to obtain mechanical properties that meet the shock resistance requirements in the watchmaking industry.

[0008] To this end, the invention relates to a pivot axis for a watch movement made at least partially of tungsten or a tungsten alloy. Preferably, the tungsten alloys are alloys comprising either lanthanum oxide, copper, or nickel with the addition of one or more elements selected from Fe, Cu, Co, and Mo. In particular, the lanthanum oxide acts as a chip breaker, which improves the machinability of the tungsten alloy.

[0009] Tungsten and its alloys combine high hardness with values ​​greater than or equal to 500 HV0.5, very good corrosion resistance and very low magnetic susceptibility.

[0010] This material is known to be very difficult to machine, which has hindered its use in watchmaking until now.

[0011] Other features and advantages of the invention will become apparent from the detailed description that follows, with reference to the attached drawings. Brief description of the figs

[0012] Figure 1 represents a pivot axis according to the invention.

[0013] Figure 2 shows a partial cross-section of a pivot of a pivot axis according to the invention having a coating on its external surface.

[0014] Figure 3 illustrates the paramagnetic behavior of tungsten exhibiting magnetic susceptibility of 6.8x10-5 . detailed description of the invention

[0015] In this description, the term "non-magnetic" material means a paramagnetic, diamagnetic, or antiferromagnetic material with a magnetic permeability between 0.99 and 1.01.

[0016] An alloy of an element is an alloy containing at least 50% by weight of that element.

[0017] The invention relates to a component for a watch movement, and in particular to a non-magnetic pivot axis for a mechanical watch movement. 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 axes are conceivable, such as axes for moving parts, typically escapement pinions, or anchor rods. Components of this type preferably have body diameters less than 2 mm and pivots with diameters less than 0.2 mm, with a precision of a few microns.

[0018] Referring to Figure 1, we 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.

[0019] With the magnetism induced by everyday objects, it is important to limit the sensitivity of the balance staff 1 otherwise it may influence the operation of the timepiece in which it is incorporated.

[0020] Thus, at least part of the balance shaft axis 1, namely the pivot 3, is made of a non-magnetic metallic material in order to limit advantageously its sensitivity to magnetic fields. According to the invention, said material is pure tungsten or a tungsten alloy.

[0021] Pure tungsten is defined as a material comprising tungsten in a percentage by weight greater than or equal to 99.5%, preferably greater than or equal to 99.7%. It may contain possible impurities such as Mo, C, Fe and O, cited by way of example.

[0022] Tungsten alloys are alloys comprising either lanthanum oxide, copper, or nickel with the addition of one or more elements chosen from Fe, Cu, Co, and Mo.

[0023] More specifically, the tungsten alloy with lanthanum oxide, namely La20s, comprises the latter in a weight percentage of between 0.2 and 5%, preferably between 0.3 and 3%, more preferably between 0.5 and 2%. Advantageously, the alloy consists of tungsten, lanthanum oxide in a weight percentage of between 0.2 and 5%, preferably between 0.3 and 3%, more preferably between 0.5 and 2%, and possible impurities with a total content for the latter less than or equal to 1.5% by weight.

[0024] More specifically, the tungsten-copper alloy comprises the latter in a weight percentage of between 2 and 49%, preferably between 3 and 45%, more preferably between 5 and 40%. Advantageously, the alloy consists of tungsten, copper in a weight percentage of between 2 and 49%, preferably between 3 and 45%, more preferably between 5 and 40%, and any impurities with a total content of the latter less than or equal to 1.5% by weight.

[0025] More specifically, the tungsten-nickel alloy comprises nickel in a weight content of between 0.5 and 20%, preferably between 1 and 15%, more preferably between 1.5 and 10%, and comprises one or more elements selected from Fe, Cu, Co, and Mo in a total content of said element(s) between 0.2 and 10%, preferably between 0.5 and 5%. Advantageously, the alloy consists of tungsten, nickel in a percentage by weight between 0.5 and 20%, preferably between 1 and 15%, more preferably between 1.5 and 10% and any impurities with a total content for the latter less than or equal to 1.5% by weight.

[0026] Tungsten and tungsten alloys according to the invention have a hardness greater than or equal to 500 HV0.5. They have low magnetic susceptibility, typically with values ​​below 8 x 10 -5 , preferably at 7x10' 5 .

[0027] According to one variant, at least part of the balance staff 1, namely the pivot 3, is made of the aforementioned non-magnetic material 4, which is coated, at least on part of its external surface, with a layer 5 ensuring the pivot's wear resistance. This variant is shown in Figure 2. Preferably, the layer is made of Ni or NiP. The phosphorus content can preferably be between 0% (resulting in pure Ni) and 15% by weight. Preferably, the phosphorus content in the NiP can be a medium level between 6% and 9%, or a high level between 9% and 12%. It is clear, however, that the NiP can contain a low phosphorus content.

[0028] Furthermore, when the coating is NiP with a medium or high phosphorus content, it can be hardened by heat treatment. Typically, the heat treatment is carried out between 200°C and 400°C for a time ranging from 20 minutes to 24 hours.

[0029] The Ni or NiP layer preferably has a hardness greater than 400 HV0.5, and more preferably greater than 500 HV0.5. Particularly advantageously, the unhardened Ni or NiP layer preferably has a hardness greater than 500 HV0.5 but less than 600 HV0.5, i.e., preferably between 500 HV0.5 and 550 HV0.5. When heat-treated, the NiP layer can have a hardness between 900 HV0.5 and 1000 HV0.5.

[0030] Advantageously, the Ni or NiP layer can have a thickness of between 0.5 pm and 10 pm, preferably between 1 pm and 5 pm, and more preferably between 1 pm and 2 pm.

[0031] Preferably, the layer is a NiP layer, and more particularly a chemical NiP layer, that is to say, deposited by chemical means.

[0032] To improve the adhesion of the Ni or NiP layer, the pivot may include at least one bonding sublayer deposited between the non-magnetic material and the Ni or NiP layer. For example, a gold sublayer and / or a galvanic nickel sublayer may be provided under the Ni or NiP layer.

[0033] According to the invention, the Ni or NiP layer is deposited by a process selected from the group including PVD, CVD, ALD, galvanic and chemical deposition, and preferably chemical.

[0034] It should be noted that only the external surface of the pivots, or even just a portion of the external surface of the pivots, can be coated with the Ni or NiP layer. Alternatively, the entire external surface of the balance staff, including the pivots, can be coated with the Ni or NiP layer.

[0035] Hardness and magnetic susceptibility measurements were performed on several uncoated samples.

[0036] For magnetic properties, the hysteresis curve M(H) was characterized at room temperature by varying the field applied to the samples with a MicroSense EZ9 vibrating sample magnetometer (VSM). Figure 3 shows the curve for pure tungsten. The magnetic susceptibility (μm) is equal to 6.8 × 10⁻¹⁰ -5 Similar values ​​were obtained for tungsten alloys according to the invention.

[0037] The measured hardness values ​​are HV0.5. A value of 540 HV0.5 is obtained for the tungsten alloy with 1 wt% oxide of lanthanum. Values ​​in the range of 500-600 HV0.5 were obtained for other tungsten alloys and pure tungsten.

Claims

DEMANDS 1. Pivot axis (1 ) for watch movement made at least partly of tungsten or of a tungsten alloy, said pivot axis being characterized in that it is made of a tungsten alloy comprising lantana oxide with a weight content of between 0.2 and 5%, preferably between 0.3 and 3%, more preferably between 0.5 and 2%.

2. Pivot axis (1) according to claim 1, characterized in that it is made of tungsten, lantane oxide with a lantane oxide weight content of between 0.2 and 5%, preferably between 0.3 and 3%, more preferably between 0.5 and 2% and possible impurities with a total content for said possible impurities less than or equal to 1.5% by weight.

3. Pivot axis (1) according to any one of the preceding claims, characterized in that said at least part is coated with a layer (5) of Ni or NiP.

4. Movement for a timepiece, characterized in that it comprises a pivot axis (1) according to one of the preceding claims.

5. Movement for a timepiece, characterized in that it comprises a balance staff (1), an anchor rod and / or an escapement pinion comprising a staff according to any one of claims 1 to 3.

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