Non-magnetic pivot axis

Abstract

The present invention provides a pivot shaft made at least partially from pure tungsten or a tungsten alloy that has undergone surface hardening treatment to meet the shock resistance requirements in the watchmaking field. [Solution] The present invention relates to a pivot shaft for a watch movement, wherein at least a portion of it is made of pure tungsten or a tungsten alloy, and at least a portion of this portion is converted into a tungsten carbide layer (5).

Description

Technical Field

[0001] The present invention relates to a non-magnetic pivot shaft of a mechanical watch movement, and more particularly to a ten-jewel pivot.

Background Art

[0002] The production of a watch pivot shaft consists of performing bar turning operations using a hardenable steel bar to define various working surfaces (barrel, shoulders, arbors, etc.), and then performing a heat treatment operation on the turned shaft of the bar. The heat treatment operation includes at least one quenching operation to improve the hardness of the shaft and one or more tempering operations to improve toughness. After the heat treatment operation, a vanishing process is performed on the arbor of the shaft, in which the arbor is polished to the required dimensions. In the vanishing process, the hardness and roughness of the arbor are further improved.

[0003] Pivot shafts (such as ten-jewel pivots) conventionally used in mechanical watch movements are made of steel grades that can be turned on a bar, which are generally martensitic carbon steels containing lead and manganese sulfide to improve machinability. This type of steel called 20AP is usually used for such applications.

[0004] The advantage of this type of material is its ease of processing, especially its suitability for bar turning, and it has excellent mechanical properties that are extremely useful for the production of watch pivot shafts after quenching and tempering treatments. In particular, after heat treatment, these steels become hard and exhibit very excellent impact resistance. Usually, the hardness of the arbor of a shaft made of 20AP steel can reach a value exceeding 700 HV after heat treatment and vanishing.

[0005] Although this type of material has sufficient mechanical properties for the aforementioned watch applications, it has the disadvantage of being magnetic and may disrupt the rate of the watch when exposed to a magnetic field. In particular, this tendency is prominent when this material is used for a ten-jewel pivot that meshes with a hairspring made of a ferromagnetic material. This phenomenon is a well-known fact to those skilled in the art. It should also be noted that these martensitic steels are prone to corrosion.

[0006] To overcome these shortcomings, a tungsten alloy with non-magnetic properties would be ideal. However, the intrinsic hardness of this material is only around 400-550 HV, making it impossible to improve its properties using an effective quenching mechanism. This limited hardness can potentially cause problems with the impact resistance of the tenon. [Overview of the Initiative] [Problems that the invention aims to solve]

[0007] The present invention aims to improve upon the above-mentioned drawbacks by providing a pivot shaft that is at least partially made from pure tungsten or a tungsten alloy that has undergone surface hardening treatment to meet the shock resistance requirements in the watchmaking field. [Means for solving the problem]

[0008] Therefore, in order to achieve hardening, at least a portion of the shaft surface is heat-treated in a carbon-rich atmosphere, causing the tungsten in the shaft to react with the carbon in that atmosphere to form a tungsten carbide layer that has the property of reaching a hardness of over 2,000 HV.

[0009] More specifically, the present invention relates to a pivot shaft for a watch movement, characterized in that at least a portion of it is made of pure tungsten or a tungsten alloy, and at least a portion of the portion is converted into a tungsten carbide layer.

[0010] Tungsten and its alloys possess excellent corrosion resistance and extremely low magnetic susceptibility, while the tungsten carbide layer significantly improves the surface hardness of the pivot axis. This is an advantage for this timepiece, particularly in terms of the wear resistance and impact resistance of the tenons.

[0011] Other objects, advantages, and features of the present invention will become apparent from the following detailed description with reference to the accompanying drawings. [Brief explanation of the drawing]

[0012] [Figure 1] This is a diagram showing the pivot axis. [Figure 2] This is a partial cross-sectional view of a pivot axis containing a tungsten carbide layer according to the present invention. [Modes for carrying out the invention]

[0013] In this specification, the term “nonmagnetic” material means a paramagnetic, diamagnetic, or antiferromagnetic material with a magnetic permeability of 0.99 to 1.01.

[0014] An alloy of an element is an alloy containing at least 50% by weight of that element.

[0015] This invention relates to a non-magnetic pivot shaft for a mechanical watch movement. The invention is described below in the context of its application to the balance shaft 1. Naturally, other types of watch pivot shafts are also possible, such as movable shafts for watches, typically pinions for escapements, and even anchor shafts. These types of parts preferably have a body diameter of less than 2 mm and a pivot diameter of less than 0.2 mm, and the diameter of the end of the pivot can be reduced to about 50 μm.

[0016] Referring to Figure 1, a tenon joint 1 is shown having multiple sections 2 of different diameters. These sections are preferably formed by turning bar stock or other machining techniques involving chip removal, and conventionally defined tenons 2a and shoulders 2b are positioned between two ends that define two tenons 3. Each of these tenons is intended to rotate on a bearing, usually located in a hole in stone or ruby.

[0017] According to the present invention, at least a portion of the pivot shaft, in particular the portion of the shaft intended to rotate in contact with at least other parts, is made of pure tungsten or a tungsten alloy. Furthermore, according to the present invention, this portion or part of this portion that is made of pure tungsten or a tungsten alloy is made of WC, W2C, or non-stoichiometric WC carbide. (1-x)It is covered with a tungsten carbide layer, which may be present.

[0018] In the illustrated example, at least a portion of the tenon joint 1, namely the tenon 3, is made of pure tungsten or a tungsten alloy, and at least a portion of this is converted into a tungsten carbide layer 5 (Figure 2). The core 4, made of pure tungsten or a tungsten alloy with the tungsten carbide layer 5, is visible.

[0019] "Pure tungsten" means a material containing 99.5% or more, preferably 99.95% or more, of tungsten by weight. Optional impurities such as Mo, C, Fe, and O can be included as additives to achieve 100% by weight. For example, it may contain a minimum of 99.95% by weight of W, a maximum of 0.020% by weight of Mo, and a maximum of 0.030% by weight of other impurities.

[0020] For example, tungsten alloys may contain lanthanum oxide, rhenium, or copper and nickel.

[0021] More specifically, the alloy of tungsten and lanthanum oxide, i.e., La2O3, contains lanthanum oxide in a weight percentage of 0.2 to 2%, preferably 0.3 to 1.5%, and more preferably 0.5 to 1%. Advantageously, this alloy consists of tungsten, lanthanum oxide in the aforementioned proportions, and impurities, with the total impurity content being 1.5% by weight or less.

[0022] More specifically, the tungsten-rhenium alloy contains 2-30%, preferably 5-20%, of rhenium by weight. Advantageously, this alloy consists of tungsten, the aforementioned proportions of rhenium, and impurities, with the total impurity content being 1.5% by weight or less.

[0023] More specifically, the alloy of tungsten with copper and nickel contains copper and nickel, and the total content of these two elements is 2 to 20 wt%, preferably 5 to 10 wt%. Advantageously, this alloy consists of tungsten, copper and nickel in the aforementioned proportions, and impurities, and the total content of impurities is 1.5 wt% or less. It is advantageous that nickel is more than copper. That is, when the total of copper and nickel is 100%, the proportion of nickel is 55% or more and the proportion of copper is 45% or less.

[0024] The hardness of tungsten and tungsten alloys according to the present invention is 500 HV0.05 or more in the case of pure tungsten and 400 HV0.05 or more in the case of tungsten alloys. The Vickers hardness (HV) is the hardness measured according to the ISO 6507-1:2018 standard. Tungsten and tungsten alloys have a low magnetic susceptibility, and usually the value is less than 8×10 -5 less, preferably less than 7×10 -5 less.

[0025] According to the present invention, at least a part of the pivot shaft is made of tungsten or an alloy thereof, and then heat-treated on the surface to form a tungsten carbide layer. It is possible to perform heat treatment on the whole of this at least part of the pivot shaft or only on a part of this part. In the latter case, the part that does not cause the reaction to form the tungsten carbide layer is protected by, for example, a ceramic or refractory metal joint that exposes only the part to be treated.

[0026] The heat treatment is carried out in a gas atmosphere rich in carbon so that tungsten and carbon can react to form tungsten carbide. For example, the atmosphere can contain neutral gases such as methane (CH4) and argon (Ar). The proportion of methane may be 5 to 20 vol%, usually 10%, and the rest is argon. The treatment is carried out at a temperature of 1,000°C to 1,300°C, ideally 1,150°C, and the plateau time at the target temperature is 5 minutes to 2 hours, ideally 15 minutes.

[0027] The maximum hardness of the tungsten carbide layer obtained in this way is 2,000–2,500 HV 0.05. Typically, the hardness of the tungsten carbide layer may vary slightly along the thickness direction, and this value is in the range of 2,000–2,500 HV 0.05.

[0028] The thickness of the resulting tungsten carbide layer depends on the processing time and temperature settings. A layer 20-30 microns thick can be obtained at 1,150°C and a holding time of 15 minutes. Typically, the thickness is 5-50 microns, preferably 20-35 microns.

[0029] After heat treatment, the shaft may be subjected to one or more friction finishing processes to remove the carbon-rich surface layer and / or to improve the surface roughness of the components.

[0030] Prior to heat treatment, the shaft is manufactured from a rod-shaped ébauche. For example, the manufacturing process (not shown) for an entire shaft made of tungsten or a tungsten alloy is as follows: - Prepare a rod made of pure tungsten or tungsten alloy. - Preferably, the shaft is formed by turning the bar material using laser ablation. - Burnish the tenon. - Deburring and polishing are performed.

[0031] Please note that burnishing can be performed after heat treatment. [Explanation of Symbols]

[0032] 1 Tenshin 2a with body 2b Shoulder 3 mortise 4 cores 5. Tungsten carbide layer

Claims

1. A pivot shaft for a watch movement, wherein at least a portion of it is made of pure tungsten or a tungsten alloy, and at least a portion of the said portion is converted into a tungsten carbide layer (5).

2. The pivot shaft according to claim 1, characterized in that the thickness of the tungsten carbide layer (5) is 5 to 50 microns, preferably 20 to 35 microns.

3. The pivot shaft according to claim 1, characterized in that the maximum hardness of the tungsten carbide layer (5) is 2,000 to 2,500 HV 0.

05.

4. The pivot shaft according to claim 1, characterized in that it is made of pure tungsten having a tungsten content of 99.5% by weight or more, preferably 99.95% by weight or more, with the remaining impurities totaling 100%.

5. The pivot shaft according to claim 1, characterized in that the tungsten alloy is an alloy with lanthanum oxide, an alloy with rhenium, or an alloy with copper and nickel.

6. The pivot shaft according to claim 5, characterized in that the alloy of tungsten and lanthanum oxide contains 0.2 to 2%, preferably 0.3 to 1.5%, more preferably 0.5 to 1%, of lanthanum oxide and unavoidable impurities by weight percentage, wherein the total content of the unavoidable impurities is 1.5% by weight or less.

7. The pivot shaft according to claim 5, characterized in that the alloy of tungsten and rhenium contains 2 to 30%, preferably 5 to 20%, of rhenium by weight percentage, and unavoidable impurities, wherein the total content of the unavoidable impurities is 1.5% by weight or less.

8. The pivot shaft according to claim 5, characterized in that the alloy of tungsten, copper, and nickel contains copper and nickel in a total weight percentage of 2 to 20%, preferably 5 to 10%, and unavoidable impurities, wherein the total content of the unavoidable impurities is 1.5% by weight or less.

9. The pivot shaft according to claim 1, wherein the pivot rod (1) includes the portion formed by the tenon (3), and the tenon is located at the end of the pivot rod (1).

10. A method for heat treatment of a pivot shaft, - A step of preparing a pivot shaft in which at least a portion is made of pure tungsten or a tungsten alloy, - A step of heat-treating at least a portion of the aforementioned portion in a gas atmosphere containing a large amount of carbon, at a holding temperature of 1,000°C to 1,300°C for 5 minutes to 2 hours. A heat treatment method for a pivot axis, including the method described above.

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