Patent Abstract:
A spiral spring ( 10 ) for the movement of a watch, includes a plurality of coplanar leaves ( 10   a,    10   b ) wound into one another. Furthermore, the inner ends of each leaf are rigidly connected to a single collet ( 12 ). The leaves ( 10   a,    10   b ) and the collar ( 12 ) are produced as a single part.

Full Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to the field of mechanical horology. It more particularly relates to a spiral spring intended to equip a regulator organ of a mechanical watch. 
       BACKGROUND OF THE INVENTION 
       [0002]    In clocks, pendulums, and mechanical or electronic watches, there is always a regulator organ making it possible, as its name indicates, to regulate the running of the timepiece. In the case of a mechanical watch, the regulator organ is formed by a balance and a spiral spring. 
         [0003]    Traditionally, the spiral is a leaf, in general metal, with a rectangular section wound on itself in the shape of a spiral of Archimedes. It is fixed in its center on the balance-staff, by a piece called a collet. The outside of the spiral is fixed to a balance, called balance-cock, by a piece called a balance-spring stud. The balance-spring stud is fixed either directly to the balance-cock, or via a mobile balance-spring stud support. 
         [0004]    Such an assembly of a spiral is not optimal for the isochronism of the watch. In fact, the center of the spiral moves during its development, which causes reaction forces at the pivots of the balance-staff. The intensity of the forces exerted on the pivots participate greatly in the isochronism that is generally observed. 
         [0005]    Timepiece are known provided with two spirals mounted on the balance-staff, in opposite directions, arranged in different planes. The house H. Moser &amp; Cie offers an escapement provided with two spirals arranged on either side of the balance, in opposite directions. Document EP2063325 proposes a mechanism with two traditional spirals arranged concentrically and coplanarly. It is known that it is difficult to obtain precise characteristics for spirals made traditionally and that therefore, two spirals will most often be different, even just slightly. Thus, the adjustment of such a balance provided with two spirals having different characteristics, and the alignment of the forces exerted by the two spirals, present difficulties. Furthermore, the likely difference between the two spirals makes the result of the forces exerted at the balance-staff, in most cases, non-null and difficult to control. 
         [0006]    The present invention therefore aims to propose a spiral making it possible to improve the isochronism of a watch, while remaining simple to implement. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0007]    More specifically, the invention relates to a spiral spring of a watch movement as defined in claim  1 . 
         [0008]    Other features of the invention are provided in the dependent claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    Other features of the present invention will emerge more clearly upon reading the following description, done in reference to the appended drawing, in which 
           [0010]      FIGS. 1 to 6  show, in top view, diagrammatic views of different embodiments according to the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0011]      FIG. 1  shows a flat spiral spring  10 . It comprises a first inner end associated with a collet  12 . 
         [0012]    Particularly to the invention, from the collet  12 , several leaves are deployed, the drawing showing two of them. There is therefore a first  10   a  and a second  10   b  leaf, wound in the same plane and in the same direction. The leaves  10   a  and  10   b  are wound into one another, the coils of one being inserted between the coils of the other. The first  10   a  and second  10   b  leaves are arranged at 180° relative to one another. The leaves are identical, so that the ends of the leaves are situated on a circle and are situated at 180° from one another. 
         [0013]    Advantageously, the outer ends of the leaves are connected to each other by a rigid frame  14 , i.e. the frame does not (or practically does not) contribute to the elastic torque exerted by the spring. The shape of the frame  14  is circularly symmetrical relative to the center of the spiral. Preferably, the frame follows a circular path, concentric to the spiral. 
         [0014]    Typically, as allowed by shaping techniques with silicon-type materials, the collet  12  is made in one piece with the rest of the spiral. Advantageously, for the leaves to have identical elastic characteristics, the two leaves  10   a  and  10   b , the collet  12  and the frame  14  are made in a single piece, monolithically. To that end, the spiral according to the invention can be made from materials that can be shaped using deep etching techniques, particularly silicon-based spirals, in particular made from monocrystalline silicon, potentially covered by a layer of silica, but also spirals made from diamond, obtained by growth then deep etching, or spirals made from DCS (Diamond Coated Silicon), i.e. silicon spirals covered with diamond. 
         [0015]    Owing to the symmetrical arrangement of two identical leaves  10   a  and  10   b,  each of the leaves exerts a force on the balance-staff offsetting the force exerted by the other leaf. Thus, the reactions on the staff are minimized, or practically null, which makes it possible to improve the isochronism of the oscillator. 
         [0016]    The frame  14  is arranged to be able to support an attachment organ, preferably a balance-spring stud, to connect the spiral to a balance-spring stud fixed on the movement. In the embodiment of  FIG. 1 , the frame has a portion with a width adapted to receive a slotted balance-spring stud, known by those skilled in the art. More specifically, the frame has a wide zone  14   a  and a slimmer zone  14   b  to receive the balance-spring stud. The latter is fixed to the frame using an adapted technique, such as adhesion or welding, chosen by one skilled in the art. 
         [0017]      FIGS. 2 to 5  propose different embodiments to produce the frame  14 . In  FIGS. 2 to 4 , the frame  14  is semi-circular and connects the two ends of the leaves arranged at 180°. In  FIG. 2 , the wide zones  14   a  of the frame are hollowed out, which makes it possible to lighten the spiral in the outer zone thereof, which is always interesting to limit the threes on the balance-spring stud in case of impact. 
         [0018]    In  FIG. 3 , the frame  14  has several slim zones  14   b,  making it possible to position the balance-spring stud at several locations around the frame, which can provide flexibility for the construction of the movement. 
         [0019]      FIG. 4  proposes a frame provided with a hole  16  to receive a non-slotted balance-spring stud, which can be housed and fixed in the hole. The frame  14  of  FIG. 5  is arranged in the same way, but makes a full circle by connecting the outer ends of the leaves by their two sides. The distribution of the masses is thus completely symmetrical. It will be noted that, even in the configuration where the frame is provided with a hole, the frame could be hollowed out. Several holes can also be arranged on the frame. 
         [0020]    It will also be noted that the function of the frame can be fulfilled by a balance-cock including several balance-spring studs (one balance-spring stud per leaf), making it possible to fix the ends of the leaves. In that case, the frame, as such, is eliminated and only the collet and the leaves are then monolithic. 
         [0021]    Although the figures only show examples in which the spiral spring  10  includes two leaves, more can be provided. It is thus possible to have n identical leaves, distributed at 360°/n around the collet, the outer ends also being distributed at 360°/n. Such an arrangement makes it possible to improve the distribution of the leaves and forces around the balance-staff, and therefore allows better compensation. 
         [0022]    In such a case, with n leaves, a frame  14  in the shape of an arc of circle defines an angle of 360°/n, or a multiple of that value, as chosen by the person skilled the art. A complete circular frame is also possible. 
         [0023]    Each leaf being wound between the coils of other leaves, it is understood that, for a spiral with a given dimension, increasing the number of leaves causes, on the one hand, a reduction in the active length of each leaf. Thus, relative to a conventional spiral occupying a surface area S, provided with a single leaf whereof the active length is L and thickness e (the thickness being the dimension of the leaf in the plane of the spiral) and with pitch p between the coils, a spiral according to the invention with n leaves of thickness e, pitch p for each coil, occupying the same surface S, each leaf will have an active length of length L/n. This consequently increases the stiffness of the leaf, but that can be offset by decreasing the thickness of each leaf, which makes it possible to increase the active length and decrease the stiffness. It is thus easy to obtain a total desired torque in compliance with the torques obtained with conventional spiral springs. It is also possible to consider making spirals occupying a larger surface area in order to obtain leaves with the desired length. From a practical perspective, spirals with 2, 3 or 4 leaves will be preferred. 
         [0024]      FIG. 6  proposes a spiral spring  10  whereof the outer coil of each leaf is provided with a reinforcement  18  making it possible to correct the centering of the spiral and to bring the center of gravity of the active part to the action center of the elastic torque, i.e. to the center of the spiral. Such a reinforcement  18  makes it possible to improve the concentricity of the development of the spiral and to further reduce the reactions at the pivot. It will be noted that the reinforcement  18  in itself does not participate in defining the elastic torque of the spiral. As shown in  FIG. 6 , the pitch separating the last spire is at a constant distance from the next-to-last coil, including at the reinforcement. It would also be possible to consider the distance between the last coil and the next-to-last coil being smaller than the pitch separating the other coils, particularly at the reinforcement. 
         [0025]    This description was provided solely as a non-limiting illustration of the invention and one skilled in the art can also provide various alternatives following directly from the description provided above, without going beyond the scope defined by the claims. In particular, the pitch of each leaf can be constant, as shown in the drawing, but it can also vary. Furthermore, the balance-spring stud can he replaced by another fixing mode, in particular a screw to secure the frame directly with the balance-cock.

Technology Classification (CPC): 6