FASTENING PART FOR A HAIRSPRING

Abstract

An assembly (300) includes (i) a hairspring (2) made of a paramagnetic alloy including at least one of the following elements: Nb, V, Ta, Ti, Zr and Hf, notably an alloy including the elements Nb and Zr with between 5% and 25% by mass of Zr and an interstitial doping agent including oxygen, and (ii) at least one fastening part (1; 1′), notably two parts (1; 1′), in particular a stud (1) or a collet (1′), for an end (2a; 2b) of the hairspring (2), the at least one part (1; 1′) having a first portion (10; 10′) that is designed to come into contact with the hairspring (2) and that is made of titanium or titanium alloy or of tantalum or tantalum alloy, notably grade 2 titanium or grade 5 titanium.

Claims

1. An assembly comprising: a hairspring made of a paramagnetic alloy, and at least one fastening part having a first portion designed to come into contact with the hairspring and made of titanium, titanium alloy, tantalum, or tantalum alloy.

2. The assembly as claimed in claim 1, wherein the first portion has two bearing surfaces separated by a slot, each of the bearing surfaces being designed to come into contact with the hairspring.

3. The assembly as claimed in claim 2, wherein each of the bearing surfaces has, at at least one of ends of the bearing surfaces in the direction of the height of the hairspring, a positioning shape extending perpendicular or substantially perpendicular to the surface.

4. The assembly as claimed in claim 2, wherein each of the bearing surfaces has, at two of the ends of the bearing surfaces in the direction of a height of the hairspring, respectively a first positioning shape and a second positioning shape, the shapes extending perpendicular or substantially perpendicular to the respective bearing surface.

5. The assembly as claimed in claim 1, wherein the at least one fastening part includes a second portion designed to come into contact with a stud support or with a balance arbor.

6. The assembly as claimed in claim 1, wherein the surfaces are arranged substantially perpendicular to a plane of the hairspring and together form an angle.

7. The assembly as claimed in claim 1, wherein the surfaces are curved to form portions of a single cylinder of revolution or are made tangential to a single cylinder of revolution.

8. The assembly as claimed claim 7, wherein the at least one fastening part includes a collet and wherein the cylinder of revolution is centered on an axis of the collet.

9. The assembly as claimed claim 1, wherein the at least one fastening part includes a collet and wherein the collet includes at least one stop.

10. A manufacturing method for an assembly as claimed in claim 1, the method including: providing a stud, providing the hairspring, fastening the stud to the hairspring.

11. A manufacturing method for an assembly as claimed in claim 1, the method including: providing a collet, providing the hairspring, fastening the collet to the hairspring.

12. A manufacturing method for an assembly as claimed in claim 1, the method including: providing a stud, providing the hairspring, providing a collet, fastening the stud to the hairspring and fastening the collet to the hairspring.

13. The manufacturing method as claimed in claim 10, wherein the fastening step is performed by laser welding.

14. A clockwork oscillator or clockwork movement or timepiece including an assembly as claimed in claim 1.

15. The assembly as claimed in claim 1, wherein the at least one fastening part is selected from (i) a stud and (ii) a collet.

16. The assembly as claimed in claim 1, comprising two fastening parts.

17. The assembly as claimed in claim 1, wherein the paramagnetic alloy of the hairspring includes at least one of the following elements: Nb, V, Ta, Ti, Zr and Hf.

18. The assembly as claimed in claim 17, wherein the paramagnetic alloy of the hairspring includes the elements Nb and Zr with between 5% and 25% by mass of Zr and an interstitial doping agent including oxygen.

19. The assembly as claimed in claim 1, wherein the first portion is made of grade 2 titanium or grade 5 titanium.

20. The assembly as claimed in claim 2, wherein the slot extends in the direction of a height of the hairspring over a height greater than the height of the hairspring.

21. The assembly as claimed in claim 6, wherein the surfaces form an angle of between 150° and 179° considered from an axis of the hairspring.

22. The assembly as claimed in claim 7, wherein the surfaces are arranged substantially perpendicular to a plane of the helical spring.

23. The assembly as claimed in claim 1, wherein the surfaces are arranged substantially perpendicular to a plane of the helical spring.

24. The assembly as claimed claim 9, wherein the collet includes from two to five stops, distributed angularly about an outer periphery of the collet.

25. The assembly as claimed in claim 24, wherein the stops are distributed regularly about the outer periphery of the collet.

Description

[0084] The attached figures show, by way of example, one embodiment of a timepiece incorporating an embodiment of a stud according to the invention and an embodiment of a collet according to the invention.

[0085] FIG. 1 is a front view of an embodiment of a stud according to the invention.

[0086] FIG. 2 is a perspective view of the embodiment of a stud according to the invention.

[0087] FIG. 3 is a partial perspective view of an oscillator incorporating the embodiment of a stud according to the invention.

[0088] FIGS. 4 to 6 are detail views of the embodiment of a stud according to the invention.

[0089] FIGS. 7 to 11 show an embodiment of a collet according to the invention.

[0090] FIG. 12 is a diagram showing an embodiment of a timepiece according to the invention including a stud according to the invention and a collet according to the invention.

[0091] FIG. 13 is a graph showing the improvements in adherence strength of the hairspring on a stud according to the invention.

[0092] FIG. 14 is a graph showing the mean rate (M) of a timepiece, averaged for the different position of the timepiece, as a function of the amplitude (A) of the balance spring in free isochronism.

[0093] One embodiment of a timepiece 600 is described below with reference to FIG. 12. The timepiece is for example a watch, in particular a wristwatch. The timepiece includes a clockwork movement 500, in particular a mechanical movement, which in turn includes an oscillator 400, such as a balance spring oscillator having a balance pivoted on an axis A1 and a hairspring arranged mainly in a plane P1 perpendicular to the axis A1. The axis A1 is also the axis of the hairspring.

[0094] The oscillator 400 has a helical spring assembly 300 including a hairspring 2, a first part 1′ for fastening the inner end 2b of the hairspring to a balance arbor, i.e. a collet 1′, and a second part 1 for fastening the outer end 2a of the hairspring to a frame of the movement, notably a balance bridge 4, possibly via a stud support 3, as shown in FIG. 3. The second fastening part is a stud.

[0095] Advantageously, the hairspring is made of a paramagnetic alloy including at least one of the following elements: Nb, V, Ta, Ti, Zr and Hf. In particular, the hairspring includes at least 2%, or at least 5%, by mass of one of the following elements: Nb, V, Ta, Ti, Zr and Hf. Preferably, the hairspring is made of an alloy including the elements Nb and Zr with between 5% and 25% by mass of Zr and an interstitial doping agent including oxygen. Preferably, the hairspring is made of an alloy including 85% by mass of Nb, 14.95% by mass of Zr and 0.05% by mass of oxygen. The alloy may also include other impurities, for example within the following limits: Hf<7000 ppm, Ta<1000 ppm, W<300 ppm, Mo<100 ppm, others <60 ppm.

[0096] Preferably, the stud 1 includes a portion 10 designed to come into contact with the hairspring 2. Advantageously, the stud is made of: [0097] titanium, or [0098] titanium alloy, notably grade 2 titanium or grade 5 titanium, or [0099] tantalum, or [0100] tantalum alloy.

[0101] Equally and preferably, the collet 1′ includes a portion 10′ designed to come into contact with the hairspring 2. Advantageously, the collet is made of: [0102] titanium, or [0103] titanium alloy, notably grade 2 titanium or grade 5 titanium, or [0104] tantalum, or [0105] tantalum alloy.

[0106] “Titanium” preferably means any material with a mass percent of titanium greater than 99%, or greater than 99.5%.

[0107] “Titanium alloy” preferably means any other material whose main or dominant element by mass is titanium, such as grade 5 titanium (Ti6Al4V).

[0108] “Tantalum” preferably means any material with a mass percent of tantalum greater than 99%, or greater than 99.5%.

[0109] “Tantalum alloy” preferably means any other material whose main or dominant element by mass is tantalum, such as tantalum TaW containing between 2.5% and 10% of W by mass or tantalum TaNb containing approximately 40% of Nb by mass.

[0110] Making the collet and/or the stud from titanium or titanium alloy is particularly suited to welding a hairspring made of a niobium-based alloy that has between 5% and 25% by mass of Zr, in particular an alloy including the elements Nb and Zr with between 5% and 25% by mass of Zr and an interstitial doping agent including oxygen. Indeed, Nb and Zr are entirely soluble in Ti.

[0111] Making the collet and/or the stud from tantalum or tantalum alloy is particularly suited to welding a hairspring made of a titanium base that has between 17% and 62% by mass of one of the elements Nb or Ta, for example at least 17% by mass of Nb and for example a maximum of 62% by mass of Ta. Making the collet and/or stud from tantalum or tantalum alloy is advantageous for welding an Nb—Hf hairspring comprising between 2% and 30% by mass of Hf.

[0112] One embodiment of a stud according to the invention is described below in detail and with reference to FIGS. 1 to 6.

[0113] The stud is for example made from a single piece, as in the embodiment illustrated. The stud notably has an overall square shape formed by two branches of substantially the same size. The two branches may be connected to one another by a radius fillet.

[0114] The stud 1 includes a first portion 10 designed to be welded to the hairspring 2, in particular by laser welding, at the outer end 2a of the hairspring, as shown in FIG. 2. The stud also has a second portion 100 designed to be fastened, notably inserted, conventionally into a groove of the stud support 3, which is mounted on the balance bridge 4, as shown in FIG. 3. The first and second portions may be made of different materials and assembled on top of one another.

[0115] The first portion 10 has a first bearing surface 10b and a second bearing surface 10c that are separated by a slot 10a. Each bearing surface is designed to come into contact with the hairspring. In the embodiment shown, the slot extends in the direction of the height h of the hairspring, preferably over a height H10 greater than the height of the hairspring. The slot 10a enables the first and second bearing surfaces 10b, 10c to be separated or distinguished from one another. The slot 10a is advantageously oriented substantially in the direction of the height H10 of the portion 10 of the stud 1. Such an arrangement enables heat conduction to be completely interrupted when welding the blade of the helical spring to each of the first and second bearing surfaces 10b, 10c and to prevent the occurrence of interference between two zones of the hairspring affected thermally during welding. This arrangement enables the necessary energy to be applied to the weld, optimizing preservation of the mechanical properties of the alloy of the hairspring.

[0116] The slot can be formed through part of the thickness of the stud, i.e. without passing through the stud. Alternatively, the slot can pass through the entire thickness of the stud.

[0117] As an alternative to the foregoing, the slot may be oriented perpendicular to the height h of the hairspring. The slot may also be oriented in another direction.

[0118] The first surface and the second surface are designed to receive two zones of a single helical spring face. Preferably, the two zones are separated from one another in the direction of the helical spring, i.e. in the direction in which the helical spring mainly extends at the zones. There is therefore a space between the zones (space measured in the direction in which the helical spring mainly extends at the zones). It is therefore not possible to move from one point of a zone to a point of the other zone without travelling a distance in the direction in which the helical spring mainly extends at the zones.

[0119] The first bearing surface 10b has a first relief 103b or 104b at one of the ends 101b or 102b of same. This first relief provides a positioning stop for the hairspring, notably an axial positioning stop for the hairspring. Indeed, the blade of the hairspring, in bearing contact with the first surface, can be moved to come into contact against the first relief such as to accurately position the hairspring in relation to the stud in the direction of the height H10 of the stud. The first relief extends for example perpendicular or substantially perpendicular to the first surface 10b, such as to form a stop. Advantageously, the first bearing surface 10b has a second relief 103b or 104b at the other of the ends 101b or 102b of same. This second relief provides a positioning stop for the hairspring. The second relief extends for example perpendicular or substantially perpendicular to the first surface 10b, such as to form a stop.

[0120] Similarly, the second bearing surface 10c may have a third relief 103c or 104c at one of the ends 101c or 102c of same. This third relief provides a positioning stop for the hairspring. Indeed, the blade of the hairspring, in contact against the second surface, can be moved to come into contact against the third relief such as to accurately position the hairspring in relation to the stud in the direction of the height H10 of the stud. The third relief extends for example perpendicular or substantially perpendicular to the second surface 10c, such as to form a stop. Advantageously, the second bearing surface 10c has a fourth relief 103c or 104c at the other of the ends 101c or 102c of same. This fourth relief provides a positioning stop for the hairspring. The fourth relief extends for example perpendicular or substantially perpendicular to the second surface 10c, such as to form a stop.

[0121] The positioning reliefs described above enable the blade of the hairspring to be positioned accurately in relation to the stud, thereby enabling the hairspring to be accurately fitted after the hairspring has been welded onto the stud. Welding may comprise two spot welds s1, s2 made respectively at each of the bearing surfaces 10b, 10c or on the edge of each of the bearing surfaces 10b, 10c. Preferably, the third and fourth spot welds s3, s4 are made respectively at each of the bearing surfaces 10b, 10c or on the edge of each of the bearing surfaces 10b, 10c, in addition to the spot welds s1, s2, as shown in FIG. 2. To ensure this precise positioning, where one or two of the bearing surfaces each have two positioning reliefs, said reliefs are spaced apart by a distance greater than the height h of the spring blade. Advantageously, this height play is less than 0.04 mm, or less than 0.03 mm. The positioning reliefs described above form a second slot 10d oriented substantially perpendicular to the first slot 10a in order to support and/or guide the blade of the hairspring, as shown in FIG. 1.

[0122] Advantageously, the first and second bearing surfaces 10b and 10c are designed to perfectly fit the curve of the end blade of the hairspring. To do so, the first and second surfaces 10b, 10c are inclined in relation to the surface defined by the bottom of the slot 10a or to the face of the stud that is visible in the view in FIG. 1. Preferably, the first and second surfaces 10b, 10c are inclined at two different angles, which may for example be between 5° and 15°. Consequently and as shown in FIGS. 5 and 6, the first and second surfaces 10b, 10c may together form an angle α (i.e. a non-zero angle), notably an angle α of between 150° and 179° considered from the axis A1 of the balance or of the hairspring. In other words, the axis A1 is within the obtuse dihedral formed by two half-planes passing through the first and second surfaces respectively. The first and second surfaces may also be arranged perpendicular or substantially perpendicular to the plane P1 of the helical spring. The first and second surfaces may be flat faces. The faces may be tangential to a single surface, notably a single cylinder of revolution or a cylindrical surface of revolution or a more complex surface formed by a portion of the end curve of the hairspring. At least one of the first and second surfaces 10b, 10c can form a non-zero angle with a plane that is parallel and orthoradial to the axis A1.

[0123] Alternatively, the first and second surfaces may be curved surfaces designed to best fit the blade of the hairspring seated therein. For example, the first and second surfaces may each be a portion of a single cylinder of revolution or a cylindrical surface of revolution or a more complex surface formed by a portion of the end curve of the hairspring.

[0124] In the embodiment of the stud shown, the first and second surfaces are discontinuous. However, alternatively, the first and second surfaces may be uninterrupted, i.e. forming a single surface. This single surface may be a “continuous tangent”, i.e. have no edges.

[0125] Ideally, these first and second surfaces are identical to the surface, which is not necessarily cylindrical, of the outer end 2a of the hairspring.

[0126] Such a stud design advantageously provides at least two contact points between the stud and the end blade of the hairspring. The assembly precision, notably the welding precision, of a hairspring on such a stud is thus optimized, and is no longer only guaranteed by the assembly means. In the techniques known in the prior art, the assembly means are formed such as to minimize, before fastening of the blade of the hairspring to the stud, the movements of the blade of the hairspring about the theoretical contact point of same defined exclusively by the curve of the spring and a single and unique receiving plane of the stud. This degree of freedom enabling the blade to oscillate through an angular range of approximately 4°, or 8° about the theoretical contact point of same enables torque to exist in the blade at the outer fitting of the hairspring, once the blade has been fastened to the stud. This phenomenon may cause the non-concentric arrangement of the hairspring, thereby resulting in chronometry problems, in particular in the isochronism curve and “flat-hanging difference”.

[0127] FIG. 14 is a graph showing the mean rate M in seconds per day of a timepiece, averaged for the different position of the timepiece, as a function of the amplitude A in degrees of the balance spring in free isochronism. The dashed lines, showing the isochronism curves for a balance spring assembly representative of the prior art, in which the end of the end curve of the hairspring has been moved through an angle of ±4° about the theoretical contact point of same with the stud, define an envelope in which the mean rate of the timepiece varies as a function of the nominal position of the blade of the hairspring in relation to the stud.

[0128] The unbroken line N shows a function with an optimized isochronism curve representing operation of a balance spring assembly provided with a stud according to the invention, with a hairspring in which the end of the end curve is accurately positioned by the first and second bearing surfaces of the stud. Notably, such an arrangement in practice reduces the isochronism curve and “flat-hanging difference” in the timepiece containing the balance spring.

[0129] One embodiment of a collet according to the invention is described below in detail and with reference to FIGS. 7 to 11.

[0130] The collet includes a first portion 10′ designed to be welded to a hairspring 2, in particular by laser welding, at the inner end 2b of the hairspring, as shown in FIG. 8. The collet also has a second portion 100′, in the form of a central opening 100′, that is for example designed to be pressed against the balance arbor 5, as shown in FIGS. 8 to 11. The first and second portions may be made from a single part. Alternatively, the first and second portions may be made of different materials and assembled on top of one another.

[0131] As with the stud 1, the portion 10′ has a first slot 10a′ defining two bearing surfaces 10b′, 10c′ of a blade portion of the inner end of the hairspring 2. Thus, the first portion 10′ has a first bearing surface 10b′ and a second bearing surface 10c′ that are separated by a slot 10a′. Each bearing surface is designed to come into contact with the hairspring. In the embodiment shown, the slot extends in the direction of the height h of the hairspring, preferably over a height H10′ greater than the height of the hairspring. The slot 10a′ enables the first and second bearing surfaces 10b′, 10c′ to be separated or distinguished from one another. The slot 10a′ is advantageously oriented substantially in the direction of the height H10′ of the portion 10 of the stud 1. Such an arrangement enables heat conduction to be completely interrupted when welding the blade of the helical spring to each of the first and second bearing surfaces 10b′, 10c′ and to prevent the occurrence of interference between two zones of the hairspring affected thermally during welding. This arrangement enables the necessary energy to be applied to the weld, optimizing preservation of the mechanical properties of the alloy of the hairspring. The slot can also be used as a visual marker for accurately positioning the spot welds around the periphery of the collet.

[0132] As an alternative to the foregoing, the slot may be oriented perpendicular to the height h of the hairspring. Alternatively, the slot may be oriented in another direction.

[0133] In an embodiment not shown, the first bearing surface may have a first relief at one of the ends of same. This first relief provides a positioning stop for the hairspring. Indeed, the blade of the hairspring, in contact against the first surface, can be moved to come into contact against the first relief such as to accurately position the hairspring in relation to the collet in the direction of the height of the collet. The first relief extends for example perpendicular or substantially perpendicular to the first surface 10b′, such as to form a stop. Advantageously, the first bearing surface 10b′ may have a second relief at the other of the ends of same. This second relief provides a positioning stop for the hairspring. The second relief extends for example perpendicular or substantially perpendicular to the first surface 10b′, such as to form a stop.

[0134] Similarly, the second bearing surface 10c′ may have a third relief at one of the ends of same. This third relief provides a positioning stop for the hairspring. Indeed, the blade of the hairspring, in contact against the second surface, can be moved to come into contact against the third relief such as to accurately position the hairspring in relation to the collet in the direction of the height of the collet. The third relief extends for example perpendicular or substantially perpendicular to the second surface 10c′, such as to form a stop. Advantageously, the second bearing surface 10c′ may have a fourth relief at the other of the ends of same. This fourth relief provides a positioning stop for the hairspring. The fourth relief extends for example perpendicular or substantially perpendicular to the second surface 10c′, such as to form a stop.

[0135] The positioning reliefs described above enable the blade of the hairspring to be positioned accurately in relation to the stud, thereby enabling the hairspring to be accurately fitted after the hairspring has been welded onto the collet. Welding may comprise two spot welds s1′, s2′ made respectively at each of the bearing surfaces 10b′, 10c′ or on the edge of each of the bearing surfaces 10b′, 10c′. Preferably, the third and fourth spot welds s3′, s4′ are made respectively at each of the bearing surfaces 10b′, 10c′ or on the edge of each of the bearing surfaces 10b′, 10c′, in addition to the spot welds s1′, s2′, as shown in FIG. 9. To ensure this precise positioning, where one or two of the bearing surfaces each have two positioning reliefs, said reliefs are spaced apart by a distance greater than the height h of the spring blade. Advantageously, this height play is less than 0.04 mm, or less than 0.03 mm. The positioning reliefs described above can then form a second slot oriented substantially perpendicular to the first slot in order to support and/or guide the blade of the hairspring.

[0136] Advantageously, the first and second bearing surfaces 10b′ and 10c′ are designed to perfectly fit the curve of the blade of the hairspring. To do so, the first and second surfaces 10b′, 10c′ may together form an angle α′, notably an angle α′ of between 150° and 179° considered from the axis A1 of the balance or of the hairspring. In other words, the axis A1 is within the obtuse dihedral formed by two half-planes passing through the first and second surfaces respectively. The first and second surfaces may also be arranged perpendicular or substantially perpendicular to the plane P1 of the helical spring. The first and second surfaces may be flat faces. The flat faces may be tangential to a single surface, notably a single cylinder of revolution. The precise positioning of the hairspring in relation to the collet also helps to achieve chronometric improvements of the same type as those obtained by the precise positioning of the hairspring in relation to the stud.

[0137] Advantageously, the surfaces 10b′, 10c′ are portions of a single cylinder of revolution in which the directrix is the circle A of center CA, which may or may not be centered on the axis A1 of the balance. In the embodiment shown in FIG. 10, the center CA is not on the axis A1, such as to minimize or eliminate the movement of the surfaces 10b′, 10c′ onto which the hairspring is welded when pressing the collet 1′ onto the arbor 5.

[0138] The collet 1′ may include arms 1A′, 1B′, 1C′, 1D′, which may be deformable or otherwise and have variable sections or otherwise, such as to optimize the force required to press the collet onto the balance arbor and/or the holding torque of the collet on the balance arbor. Preferably, the contact between the collet and the arbor is cylinder-cylinder. The central opening 100′ may be a circular borehole 100′ designed to fit the cylindrical periphery of the balance arbor 5, such as to minimize the stress inside the collet when pressing the collet onto the balance arbor. Preferably, the collet has at least one peripheral portion or stop 1E′, 1F′, 1G′ against which the inner turn of the hairspring can bear in the event of impact, before the elastic limit of the material used to make the hairspring is exceeded. These stops are distributed angularly, regularly or otherwise, around the outer periphery of the collet, as shown in FIG. 11. Preferably, these stops are semicircle portions tangential respectively to the circles E, F, G with centers CE, CF, CG. In the embodiment shown, the circles E, F, G have different diameters to best fit the shape of the inner turn of the hairspring. The centers CE, CF, CG are in this case the same and coincide with the axis Al or the center CB of the balance arbor 5, and are different from the center CA. The stops 1E′, 1F′, 1G′ are located at respective distances RE, RF, RG from the axis A1 that increase in the direction of the helical spring moving outwards from the point where the hairspring is joined to the collet.

[0139] An embodiment of the manufacturing method for an assembly 300 including: [0140] a hairspring, and [0141] a stud 1, and/or [0142] a collet 1′,
is described below.

[0143] The method includes the following steps: [0144] Provision of the hairspring as described above, [0145] Provision of the stud as described above and/or of the collet as described above, [0146] Fastening of the stud to the hairspring and/or fastening of the collet to the hairspring.

[0147] Advantageously, the fastening step or steps include the following sub-steps: [0148] Positioning of the stud in relation to the hairspring and/or positioning of the collet in relation to the hairspring, [0149] Welding, notably laser welding, of the stud to the hairspring and/or welding, notably laser welding, of the collet to the hairspring,

[0150] Advantageously, the welding sub-step includes making at least one spot weld, in particular two spot welds, on each of the first and second surfaces of the stud designed to receive the hairspring and/or making at least one spot weld, in particular two spot welds, on each of the first and second surfaces of the collet designed to receive the hairspring.

[0151] FIG. 13 is a comparative graph highlighting the gains of an assembly carried out in accordance with the manufacturing method described above. The graph shows different situations on the X-axis and the intensity of the pull forces on the Y-axis. Considering a reference force FA required to pull an Nb—Zr hairspring comprising approximately 15% by mass of Zr from a stud made of steel, the studies carried out by the applicant demonstrate that the force FB required to pull a given Nb—Zr hairspring from a given stud made of grade 5 titanium is around 3 times the reference force FA, with the forces FA and FB applied directly to the blade of the helical spring near to the stud and arranged in the plane of the helical spring and oriented substantially towards the center of the helical spring.

[0152] Considering a reference force FC required to pull an Nb—Zr hairspring comprising approximately 15% by mass of Zr from a collet made of steel, the studies carried out by the applicant demonstrate that the force FD required to pull a given Nb—Zr hairspring from a given collet made of grade 5 titanium is around 1.1 times the reference force FC, with the forces FC and FD applied directly to the end portion of the blade of the helical spring at the collet and arranged in the plane of the helical spring in a direction substantially tangential to the semicircle portion of the collet receiving the hairspring.

[0153] The invention makes it possible to optimize the strength of the weld of a hairspring made of a paramagnetic alloy, notably in the event of impact, by choosing fastening parts in which the portion designed to come in contact with the hairspring is made of titanium or titanium alloy or tantalum or tantalum alloy. Such a pairing of materials helps to achieve a quality weld due to the total solubility of the solid phases, thereby preventing the appearance of fragile intermetallic compounds, as well as a low solidification range that limits the risk of solidification cracks.