Abstract:
The invention relates to a method ( 31 ) for fabricating pallets ( 3 ) for an escapement system ( 1 ). According to the invention, the method ( 31 ) includes the following steps:
       a) forming ( 31 ), using a first material, the lever of the pallets ( 3 ) a first end of which includes the dart ( 15 ) and a second end includes two arms ( 11, 13 ) each for receiving a pallet stone ( 10, 12 );   b) over-moulding ( 35 ) a second material on said first end in order to form a pallets fork ( 17 );   c) adjusting ( 37, 39 ) said pallet stones and a staff ( 14 ) to enable the pallets ( 3 ) to be rotatably mounted.       
 
     The invention concerns the field of escapement systems for timepieces.

Description:
[0001]    This application claims priority from European Patent Application No. 11177438.6 filed Aug. 12, 2011, the entire disclosure of which is incorporated herein by reference. 
       FIELD OF THE INVENTION 
       [0002]    The invention relates to an escapement system for a timepiece and, more specifically, a system of this type comprising pallet fork wherein the function of the lever and that of the fork can be uncoupled. 
       BACKGROUND OF THE INVENTION 
       [0003]    Free escapement systems of the Swiss lever type are difficult to improve since they are a compromise between the lowest possible inertia and the best possible tribology of the pallet-stones and fork. Thus, it is difficult to prevent sticking between the impulse pin and the horns of the fork whilst maintaining minimum inertia and even insensitivity to magnetic fields. 
       SUMMARY OF THE INVENTION 
       [0004]    It is an object of the present invention to overcome all of part of the aforecited drawbacks by proposing a method for fabricating pallets having a low inertia lever and a fork with favourable tribological properties. 
         [0005]    The invention therefore relates to a method for fabricating pallets for an escapement system, characterized in that it includes the following steps:
       a) forming, using a first material, the lever of the pallets, a first end of which includes the dart and a second end includes two arms each for receiving a pallet stone;   b) over-moulding a second material on said first end so as to form a pallets fork;   c) adjusting said pallet stones and a staff to allow the pallets to be rotatably mounted.       
 
         [0009]    According to a first alternative embodiment, the fabricating method according to the invention differs in that it includes the following steps:
       a′) forming, using a first material, the lever of the pallets, a first end of which includes the dart and a second includes two arms each forming an integral pallet stone;   b) over-moulding a second material on said first end so as to form a pallets fork;   c′) mounting a staff to allow the pallets to be rotatably mounted.       
 
         [0013]    According to a second alternative embodiment, the fabricating method according to the invention differs in that it includes the following steps:
       a) forming, using a first material, the lever of the pallets, a first end of which includes the dart and a second end includes two arms each for receiving a pallet stone;   b′)over-moulding a second material on said first end so as to form a pallets fork and on said second end in order to form said pallet stones;   c′) mounting a staff to allow the pallets to be rotatably mounted.       
 
         [0017]    Thus, advantageously according to the invention, the mechanical features provided by the lever material are no longer dependent on those of the fork. Consequently, by way of example, the lever may advantageously offer low inertia and the fork optimised tribology. 
         [0018]    In accordance with other advantageous features of the invention:
       Step a) or a′) is achieved by a LIGA process, stamping or bar turning.   The first material is a metal or metal alloy.   The first material includes titanium, aluminium or an austenitic cobalt alloy.   Step b) or b) is achieved by injection into a mould formed using a LIGA process.   The second material is a polymer.   The second material includes polyoxymethylene.   The staff is formed of steel, brass or copper-nickel-zinc alloy.       
 
         [0026]    Finally, the invention relates to a timepiece, characterized in that it includes an escapement system with pallets obtained from the method according to any of the preceding embodiments. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0027]    Other features and advantages will appear clearly from the following description, given by way of non-limiting illustration, with reference to the annexed drawings, in which: 
           [0028]      FIG. 1  is a diagram of pallets according to the invention. 
           [0029]      FIG. 2  is a partial cross-section of a timepiece including pallets according to the invention. 
           [0030]      FIG. 3  is a diagram of unfinished pallets according to the invention. 
           [0031]      FIG. 4  is a diagram of an over-moulding step according to the invention. 
           [0032]      FIG. 5  is a block diagram of the method according to the invention. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0033]    The examples illustrated in  FIGS. 1 and 2  show an escapement system  1  for a timepiece. As seen in cross-section in  FIG. 2 , the escapement system  1  includes, in particular, pallets  3  for cooperating with a roller  5 . Roller  5  is preferably driven onto the balance staff  2  and includes a small roller  4  provided with a notch  6  and a large roller  8  provided with an impulse pin  7 . 
         [0034]    Pallets  3  preferably include a lever  9  formed of a first metallic material which is preferably selected for its low density, low sensitivity to magnetic fields and possibly its capacity to change, for example, ruby pallet stones  10 ,  12 . 
         [0035]    Thus, preferably, the material of lever  9  may be, for example, titanium, aluminium or an austenitic cobalt alloy. 
         [0036]    Lever  9  includes arms  11  and  13  for carrying pallet stones  10  and  12  in order to cooperate with at least one escape wheel (not shown), of escapement system  1 . The pallet staff  14  is mounted between these two arms  11  and  13  for pivotably mounting pallets  3 . Staff  14  may be made, for example, of a material like steel, brass or copper-nickel-zinc alloy. Finally, at the opposite end to arms  11  and  13 , lever  9  has a dart  15  and a pallets fork  17 . 
         [0037]    Advantageously according to the invention, dart  15  is integral with lever  9  and preferably in the extension thereof. Dart  15  is for cooperating with the small roller  4  to prevent accidental movements of pallets fork  17 . 
         [0038]    pallet fork  17  preferably includes an over-moulding  19  formed by a second material which is preferably different from the first material of lever  9  and intended to improve the tribological properties of fork  17  on impulse pin  7  while avoiding adversely affecting the overall inertia of pallets  3 . Preferably, the material of fork  17  is also paramagnetic in order to reduce the sensitivity of pallets  3  to magnetic fields. 
         [0039]    The material of pallets fork  17  is ideally a material exhibiting low wear and having a low friction coefficient in association with impulse pin  7  of large roller  8 . This material may be a polymer. 
         [0040]    Preferably, the polymer used is polyoxymethylene (POM) for its particular shock damping properties, low sensitivity to magnetic fields and very good tribological properties. During development it was thus discovered that the hertz pressure exerted by the ruby impulse pin on the polyoxymethylene horns was limited to 54 Mpa, i.e. an 85% reduction in mechanical stresses compared to standard steel horns. 
         [0041]    Fork  17  is preferably over-moulded onto lever  9  at dart  15  and includes two horns  16  and  18  intended to enter into contact with impulse pin  7  of large roller  8  in order, after the rotation of pin  7 , to pivot pallets  3  about staff  14  in a to-and-fro motion. 
         [0042]    Advantageously according to the invention, separating the materials of pallets fork  17  from that of lever  9 , optimises the properties of pallets  3  between the desired sensitivity to magnetic fields and/or the desired inertia of lever  9  on the one hand, and on the other hand the low wear and low friction coefficient of pallets fork  17  in association with impulse pin  7 . 
         [0043]    Preferably, the over-mould of pallets fork  17  traps lever  9  so as to ensure that it does not become detached. Thus, as seen in  FIGS. 2 and 4 , lever  9  includes notches  25 ,  26  enabling the over-moulding to form bridges  20  and  21  of polymer material locking fork  17  against lever  9 . 
         [0044]    Of course, lever  9 , pallet stones  10 ,  12  or staff  14  may be different. Thus, by way of example, according to a first alternative, pallet stones  10 ,  12  may be integral with lever  9  if one does not wish to be able to change pallet stones  10 ,  12 . For the same purpose, according to a second alternative, pallet stones  10 ,  12  may be over-moulded in an identical or similar manner with respect to pallets fork  17 . i.e. with a polymer such as, for example, polyoxymethylene. 
         [0045]    The method  31  of fabricating pallets  3  will now be explained with reference to  FIG. 5 . Method  31  includes a first step  33  for forming, using a first material, the body of pallets  3 , a first end of which includes dart  15  and a second end includes the two arms  11 ,  13  each for carrying one pallet stone  10 ,  12 . 
         [0046]    Step  33  may be achieved by numerous techniques, such as for example a LIGA process, stamping or bar turning. This step  33  allows lever  9 , arms  11 ,  13  and dart  15  to be fabricated in a single piece. As seen in  FIG. 3 , lever  9  has a hole  22  for fitting staff  14 . Moreover, each arm  11 ,  13  has a hole  23 ,  24  for fitting a pallet stone  10 ,  12 . 
         [0047]    Of course, in the case of the first alternative cited above, step  33  is also for forming an integral pallet stone  10 ,  12  on each arm  11 ,  13 . 
         [0048]    Finally, as explained above, step  33  also preferably forms notches  25 ,  26  which will be filled with the future over-mould(s). 
         [0049]    Method  31  continues with the second step  35  for over-moulding a second material on the first end, i.e. near dart  15 , to form pallets fork  17  and, possibly, on said second end in order to form pallet stones  10 ,  12 . Preferably, to guarantee optimum structural quality, the mould  27  used for over-moulding is achieved by a LIGA process, i.e. photolithography followed by galvanoplasty. 
         [0050]    Mould  27  includes stop members  28  and  29  for locking arms  13 ,  11  and lever  9  respectively. Only cavity  30  of mould  27  is intended to be filled. Of course, several over-moulds could be made in the same mould  27 , i.e. mould  27  could include several cavities  30  and several pallets levers could be placed in the mould and/or other parts of lever  9  could be over-moulded, for example to make the second alternative embodiment. 
         [0051]    As explained above, the second material is preferably a polymer. Consequently, step  35  preferably consists in over-moulding the lever of the pallets by injecting polymer into cavity  30  of mould  27  in order to form the body of pallets fork  17  with, in particular, horns  16  and  18  and, possibly, according to the second alternative embodiment, to form pallet stones  10 ,  12  on arms  11 ,  13 . 
         [0052]    The third step  37  of method  31  is for mounting pallet stones  10 ,  12  in holes  23 ,  24  and staff  14  in hole  22  so that pallets  3  can be rotatably mounted. At the end of these last two steps, pallets  3  are finished and can be mounted in a timepiece as an element in a Swiss lever escapement system  1 . 
         [0053]    Of course, in the case of the first and second alternative embodiment, step  37  must be considered to be limited to mounting staff  14  in hole  22  so that pallets  3  can be rotatably mounted. 
         [0054]    Consequently, pallets  3  are of the composite type, i.e. formed from two distinct materials. Thus, as shown in  FIG. 2 , the height of pallets  3 , preferably on axis A, is entirely formed by the second material owing to notch  25  made in step  31  forming, in particular, an aperture in lever  9 . 
         [0055]    Of course, this invention is not limited to the illustrated example but is capable of various variants and alterations that will appear to those skilled in the art. 
         [0056]    In particular the shapes of pallets  3  and/or roller  5  may be different. Pallets  3  may also include, for example, at least a third arm for carrying at least a third pallet stone for the application of the invention to a coaxial escapement system. 
         [0057]    It is also possible to imagine adapting the shape of pallets fork  17  and more specifically that of horns  16  and  18  to fit another type of roller or another type of material of impulse pin  7  to prevent wear and sticking during successive contacts. 
         [0058]    Finally, the different first and second materials may also be a difference in external coating, although the core of the materials is the same. By way of example, lever  9  could also be formed of polymer coated silicon without departing from the fact that the first material of lever  9  is different from the second material of fork  17 .