This application is claiming priority based on European Patent Application No. 22192771.8 filed on Aug. 30, 2022.
The present invention relates to a balance spring for a sprung balance assembly of a horological movement. The present invention further relates to a horological assembly comprising a balance spring and a stud.
In the horological field, a balance spring, associated with a balance, forms a regulating member commonly referred to as a sprung balance for mechanical timepieces. The balance spring is initially viewed as a very thin spring that is wound about itself in concentric coils when no stress is exerted thereon. In the mounted state, a first end of the balance spring, referred to as the inner first coil, is attached to a collet fitted on a staff of the balance, and a second end of the balance spring, referred to as the outer last coil, is attached to a stud which is a part typically attached by means of a stud holder in a balance bridge.
More specifically, the time base for mechanical timepieces, also referred to as the oscillating system, comprises a sprung balance assembly and an escapement. The balance consists of a balance staff pivoted between a first and a second bearing and connected to a balance rim by means of radial arms. The balance spring is attached, by its inner first coil, to the staff of the balance, for example by means of a collet, and is attached, by its outer last coil, to a fixed attachment point such as a stud carried by a stud holder.
The escapement, in a very widespread embodiment thereof, comprises a double roller system consisting of a table-roller carrying an impulse pin and a safety-roller in which a notch is made. The escapement further comprises a pallet-lever with a pallet-staff pivoted between a first and a second bearing. The pallet-lever consists of a lever that connects a fork to an entry arm and to an exit arm. The fork consists of an entry horn and of an exit horn, between which a dart extends. The travel of the fork is limited by an entry banking pin and an exit banking pin, which can be made in one piece with a pallet-bridge. The entry arm and the exit arm carry an entry pallet and an exit pallet respectively. Finally, the pallet-lever cooperates with an escape wheel comprising an escape wheel arbor pivoted between a first and a second bearing.
A balance spring is a spring which adopts the shape of a spiral when at rest. Wound in a horizontal plane, parallel to the plane of the horological movement, the balance spring serves only one purpose: to make the balance oscillate about its position of equilibrium, also referred to as the dead centre, at as constant a frequency as possible. When the balance leaves its position of equilibrium by pivoting in a given direction, it tensions the balance spring. This creates a restoring torque in the balance spring that causes the balance to return to its position of equilibrium. During this beat, the balance spring expands. However, as the balance has acquired a certain speed, and thus kinetic energy, it exceeds its position of equilibrium in the opposite direction to the previous, which again tensions the balance spring until the restoring torque it exerts on the balance stops it again and forces it to turn in the other direction.
The balance spring thus alternately expands and contracts: it is said to breathe. However, many factors can play a part in preventing a balance spring from developing isochronously during the expansion and contraction phases. In particular, the balance spring must withstand oxidation and magnetism, which cause the coils to stick together and work to stop the watch. The influence of atmospheric pressure, on the other hand, is low. For a long time, temperature has been the main problem, because heat expands the metal, while cold shrinks it. The balance spring must thus be resilient so that it can deform and yet always return to its original shape.
The material used to produce balance springs is usually steel. Such steels are ductile and must withstand corrosion. Recent developments also propose producing balance springs from silicon. Silicon balance springs are more precise than their steel predecessors, in particular because they are insensitive to magnetism. However, they have a higher cost price and, because they are fragile, they are more difficult to assemble.
A balance spring must be isochronous. Regardless of how far the balance spring turns, it must always take the same amount of time to oscillate. If the balance spring contracts by just a few degrees, it accumulates little energy and returns slowly to its position of equilibrium. If the balance spring has moved far from its position of equilibrium, it very quickly moves in the opposite direction. The important thing is that these two journeys take the same amount of time to complete. The underlying idea is that the energy available to the balance spring is not constant and that it must still function, whether the watch is fully wound or in the final hours of its power reserve.
Due to the small dimensions thereof, balance springs are difficult to assemble. However, the way in which the two ends of a balance spring are attached also has a major influence on the precision of the rate of the horological movement. In most mechanical horological movements, the two ends of the balance spring are inserted into a drilled part and rendered immobile by means of a pin which is forcibly assembled manually using pliers. This can cause the balance spring to rotate slightly, which is detrimental to the precision of the rate of the movement.
Another technique consists of attaching the ends of the balance springs using an adhesive. However, this technique also has its limitations. It has been observed that, due to its viscosity, the adhesive exerts a tensile force on the balance spring by capillary action and can press the ends of the balance spring against the walls of the stud in which these ends are engaged. The resulting deformation of the balance spring induces mechanical stresses therein, which mechanical stresses are detrimental to keeping a consistent rate.
The purpose of the present invention is to overcome the aforementioned problems as well as others by providing a balance spring whose outer last coil can be reliably attached to a stud without the use of adhesive or pins, or operations such as clamping or crimping, etc.
To this end, the present invention relates to a balance spring of a sprung balance assembly of a mechanical horological movement, the balance spring being formed by a succession of coils which extend between a first free end, referred to as the inner first coil, and a second free end, referred to as the outer last coil, the coils of the balance spring being arranged off-centre when the balance spring is in the free state, the outer last coil of the balance spring terminating in a stop means for the attachment thereof to a stud, the balance spring being attached, by its inner first coil, to a staff of the balance, and by its outer last coil, to the stud when the balance spring is in the mounted state inside the sprung balance assembly mounted in the mechanical horological movement, the coils being arranged concentrically when this balance spring is in the mounted state, the attachment of the balance spring to the stud inducing, in the coils of the balance spring, a resilient stress as a result whereof the stop means is attached in a captive manner to the stud.
The present invention further relates to a horological assembly formed by a balance spring and a stud for a sprung balance assembly of a mechanical horological movement, the balance spring being formed by a succession of coils which extend between a first free end, referred to as the inner first coil, and a second free end, referred to as the outer last coil, the coils of the balance spring being arranged off-centre when the balance spring is in the free state, the outer last coil of the balance spring terminating in a stop means, the stud comprising a base in which a recess is made, which recess receives the stop means, the coils being rearranged in a concentric manner when this balance spring is in the mounted state inside the sprung balance assembly mounted in the mechanical horological movement, the attachment of the balance spring to the stud inducing, in the coils of the balance spring, a resilient stress as a result whereof the stop means is engaged in a captive manner within the recess in the stud.
According to a special embodiment of the invention, the stop means is hook-shaped.
According to a special embodiment of the invention, the hook is T-, L-, U- or anchor-shaped.
According to another special embodiment of the invention, the balance spring is made of silicon, for example by plasma-arc cutting a silicon wafer.
Thanks to these features, the present invention provides a horological assembly formed by a stud and a balance spring, an outer last coil whereof can be reliably attached to the stud. More specifically, the passage of the balance spring from a position wherein its coils are arranged off-centre relative to one another when it is in the free state into a position wherein its coils are centred when the free end of its outer last coil is attached to the stud causes resilient tension to be applied to the coils of the balance spring, as a result whereof the stop means is engaged in a captive manner within the stud. Thanks to the invention, the balance spring can thus be attached to its stud without the need for adhesive or pins, or operations such as clamping or crimping, etc. This thus protects against issues related to the ageing of the adhesive, which can cause the balance spring to become detached from the stud, causing the watch to stop. Similarly, attaching the balance spring according to the invention requires a simple operation of engaging the free end of the outer last coil thereof inside the recess made in the stud. This avoids assembly operations as far as possible, reducing assembly and production times and thus reducing cost prices. Similarly, strictly limiting the assembly operations also ensures excellent reproducibility in terms of the functioning of sprung balance assemblies comprising a balance spring according to the invention. Thus, with the balance spring according to the invention, the coils S1, . . . , Sn−1 of the balance spring 1 are arranged off-centre when the balance spring 1 is in the concentric state, and rearrange themselves concentrically when the balance spring is in the mounted state inside the sprung balance assembly, this sprung balance assembly being at rest.
Moreover, it is also of interest to note that, contrary to the prior art where it is always ensured that the free end of the outer last coil of the balance spring can be attached to the stud while generating as little stress as possible in order to preserve the isochronal qualities of the resulting regulating assembly, in the case of the invention, the stopping means is attached to the stud under stress, this stress inducing a mechanical tension in the balance spring which will guarantee that the outer last coil of the balance spring is locked onto the stud while simultaneously guaranteeing the timing performance of the regulating assembly by the concentric rearrangement of the coils of the balance spring.
Other features and advantages of the present invention will be better understood upon reading the following detailed description of one embodiment of the balance spring according to the invention, said example being provided for the purposes of illustration only and not intended to limit the scope of the invention, given with reference to the accompanying drawing, in which:
The present invention was drawn from the general inventive idea consisting of providing a balance spring which, in the non-mounted state, when no stress other than the force of gravity is exerted thereon, has coils that are off-centre, such that the space separating two consecutive coils from the following two coils is not the same as the distance of the coils from the centre of the balance spring materialised by its inner first coil increases. On the other hand, the balance spring according to the invention is arranged such that, when it is attached to the stud by the free end of its outer last coil, its coils are centred, such that its coils extend concentrically. According to one advantage of the invention, the passage of the balance spring from its free state, wherein its coils are off-centre, to the state wherein it is attached to the stud, and wherein its coils are centred, causes resilient tension to be applied to its coils, as a result whereof the stop means provided at the free end of its outer last coil is engaged in a captive manner inside the recess made in the stud. The balance spring according to the invention is thus attached without the use of adhesive or specific tooling. This attachment is thus simpler and quicker to produce, and is more reliable compared to balance springs of the prior art. Moreover, since the operation of attaching the balance spring according to the invention to its stud requires virtually no assembly other than sliding the stop means into the recess in the stud, the functioning of the resulting sprung balance assemblies is less dependent on the skill of the operators or on the correct setting of the machines for attaching the balance springs and is thus far more reproducible.
One example embodiment of a balance spring according to the invention is shown in
The balance spring 1 according to the invention can, for example, be formed from a silicon bar having a width w and a thickness t (see
The balance spring 1 according to the invention can also be obtained by means of the manufacturing method described in the international application WO 2019/180177 A1. Briefly described, this method for manufacturing a silicon balance spring consists of:
The balance spring 1 according to the invention can also be made of metal or a metal alloy, for example by means of the LIGA process (Lithographische Galvano Abformung in German): after a layer of photosensitive polymer has been deposited on a substrate by centrifugation, this layer of photosensitive polymer is used to form a recessed structure corresponding to the desired contour of the balance spring 1 by photolithography. To this end, the layer of photosensitive polymer, the thickness whereof corresponds to the desired height of the coils of the balance spring 1, is exposed to light through a photolithography mask and then chemically etched to obtain the recessed structure corresponding to the desired contour of the balance spring 1. The recessed structure is then filled with a metal or metal alloy, for example by electroplating or by compression and sintering (U.S. Pat. No. 4,661,212), and finally the recessed structure is chemically dissolved and the balance spring 1 is released.
It goes without saying that the present invention is not limited to the embodiment described hereinabove and that various simple alternatives and modifications can be considered by a person skilled in the art without leaving the scope of the invention as defined by the accompanying claims. In particular, it should be noted that during a period of operation of a sprung balance assembly equipped with a balance spring 1 according to the invention, when the balance spring 1 alternately contracts and expands, a pulling/pushing force F4 directed along the outer last coil 6 is exerted on the stop means without there being any risk of the stop means separating from the stud 14, this being prevented by the engagement of the foot 10 of the hook 8 in the groove 18. Moreover, other shapes of the hook 8 can of course be envisaged, for example an “L” shape, oriented inwards (see
Number | Date | Country | Kind |
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22192771.8 | Aug 2022 | EP | regional |