Abstract:
The invention concerns a power reserve indicator mechanism for a timepiece of the type comprising:
       a mainspring ( 14 ),   a power reserve indicator moving between two end positions,   a differential gear ( 18 ), and   an intermediate wheel set ( 32 ) including first and second parts respectively kinematically coupled to the differential gear ( 18 ) and to the indicator and a coupling member coupling said parts, which rotate conjointly while the winding of the mainspring ( 14 ) is comprised between its end positions,       
 
     Said coupling member includes a friction mechanism arranged such that the first part can move with friction in relation to the second when the indicator has reached its first end position, and an elastic element arranged so as to be able to be cocked when the indicator has reached its second end position, by the movement of the first part with reference to the second part.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to the field of horology. It concerns, more specifically, power reserve indicator mechanisms for a timepiece movement of the type fitted with an energy source formed of a mainspring. In a conventional manner, the mechanism according to the invention includes a frame, a power reserve indicator and a differential gear with a first input coupled to a wheel set driven in rotation when the mainspring is being wound and a second input coupled to a wheel set driven in rotation when the mainspring is let down, and an output coupled to the power reserve indicator. In such a mechanism, the indicator is able to cover a given angle comprised between two end positions, the first of which is occupied when the mainspring is wound and the second, when the spring is let down. 
     DESCRIPTION OF THE RELATED ART 
     A mechanism of this type is described in “La montre Suisse à remontage automatique” (“The automatically wound Swiss watch”) by B. Humbert, Scriptar editions, Lausanne 1955, at page 85. It is fitted with an indicator friction mounted on a wheel set coupled to the output of the differential gear. The two end positions between which the indicator moves are defined by stop members. When the indicator reaches one of the stop members, at the upper winding or let down limit of the mainspring, it remains immobile whereas the mainspring continues to be wound or let down, the friction allowing the gear train to move while the indicator is still. 
     Such a solution thus enables the real winding of the mainspring to be estimated, but there may be significant differences from one cycle to another, due to the relative movement of the indicator in relation to the gear train. 
     In order to overcome this drawback, EP Patent Application No, 1 139 182 filed in the name of the Applicant, discloses a mechanism comprising an intermediate wheel set inserted between the output of the differential gear and the power reserve indicator. This wheel set includes an elastic element arranged such that the differential gear continues to rotate freely while the indicator remains abutting against the stop member or stopped, causing the elastic element to be cocked. 
     In practice, this arrangement can be used when the indicator is stopped in one or other of its end positions. But it is difficult to make the maximum winding position of the mainspring coincide perfectly with the end position of the indicator corresponding to the largest power reserve. This involves particular precautions and specific measures to be taken during assembly, particularly during after-sales service, for the indicator to display correct information. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to overcome the aforementioned drawback, while keeping the advantages provided by the mechanism of the aforecited EP Patent. 
     More specifically, the invention concerns a power reserve indicator for a timepiece movement of the type comprising a frame, an energy source, formed by a barrel and a mainspring housed in the barrel, a going train driven by the barrel and means for winding the mainspring. This mechanism includes:
         a power reserve indicator with a wheel set and a display device carried by the wheel set,   a differential gear kinematically coupled to the barrel via a first input, to the winding means via a second input and to the indicator via an output, and   an intermediate wheel set comprising first and second parts respectively kinematically coupled to the output and to the intermediate wheel set of the indicator and a coupling member coupling such parts, arranged such that the indicator occupies a position comprised between two end positions, the first reached when the winding of the spring is greater than an upper threshold value, and the second when the winding of the spring is less than a lower threshold value, the first and second parts rotating conjointly while the winding of the spring is comprised between its extreme values.       

     According to the invention, the coupling member includes a friction mechanism arranged such that the first part can move with friction in relation to the second when the indicator has reached its first end position, and an elastic element arranged so as to be able to be cocked when the indicator has reached its second end position, by the movement of the first part in relation to the second part. 
     Advantageously, the friction mechanism includes a stop member secured to one of the parts and a lever friction mounted on the other part, disposed and sized such that:
         when the indicator occupies its first end position and the mainspring continues to be wound, the lever is held against the stop member and rotates with friction on the part to which it is secured, such that the indicator remains immobile, and   as soon as the mainspring is let down, the indicator leaves its first end position, the first and second parts of the intermediate wheel set rotating together.       

     The lever is provided with a longitudinal slot enlarging into an aperture engaged on the pinion. 
     The elastic element is secured to the part including the stop member and cooperates with the lever such that, when the indicator occupies its second end position, it is immobilised, whereas the mainspring drives the going train and the differential gear, and that the elastic element is cocked, then let down when the mainspring is let down. 
     The mechanism further includes the following features:
         the elastic element is formed of an elastic strip fixed by a first of its ends to the wheel, whereas its second end, which is free, is disposed such that it presses the lever against the first end,   the intermediate wheel set includes a wheel formed of a plate whose periphery is provided with a toothing, a pinion formed of a sleeve provided at one of its ends with a toothing, a spacer disposed on the sleeve, the lever being snap fitted onto the sleeve to hold said wheel which is mounted to rotate freely on the spacer, and   the mainspring is of the slipping spring type.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other advantages of the description will appear more clearly upon reading the following description, made with reference to the annexed drawing, in which: 
         FIG. 1  is a cross-sectional view of a date indicator mechanism according to the invention, 
         FIGS. 2 and 3  are top and cross-sectional views of the intermediate wheel set, and 
         FIG. 4  is a diagram showing the variation in couple as a function of the mainspring winding angle. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The mechanism according to the invention is intended to be placed in a watch movement. It is mounted on a plate  8  that is partially visible in the drawing. The movement includes an energy source formed of a barrel  10 , mobile in rotation about an arbour  12  pivoting on plate  8  and in which there is housed a mainspring  14  provided with a slipping spring, not explicitly shown in the drawing. Arbour  12  carries a ratchet wheel  16  which, driven in rotation by means of a winding crown that is not shown, winds mainspring  14 . 
     Barrel  10  drives the going train which is also not shown in the drawing. 
     The mechanism further includes a differential gear  18 , described in detail in the aforementioned EP Patent Application. In summary, it includes an arbour  20  placed at its output, a planet carrier  22  secured in rotation to arbour  20  and carrying a planet gear  24  with a conical toothing. A first and a second input are respectively formed by a first  26  and a second  28  wheel mounted freely on planet carrier  22 . 
     Wheels  26  and  28  are each formed by a plate identified by the letter a and the periphery of which carries a toothing b, and a pinion c provided with a conical toothing d. Wheel  26  meshes, via its toothing  26   b , with barrel  10 . Wheel  28  meshes, via its toothing  28   b , with an intermediate wheel  30  that itself meshes with ratchet wheel  16 . Finally, toothings  26   d  of pinion  26   c  and  28   d  of pinion  28   c  are meshed with planet gear  24 . 
     The gear ratios between barrel  10  and first input wheel  26 , on the one hand, and ratchet wheel  16  and second input wheel  28 , on the other hand, are equal, such that, for the same given angle traveled by barrel  10  and ratchet wheel  16 , wheels  26  and  28  travel equal angles. 
     Moreover, the mechanism includes an intermediate wheel set  32 , particularly illustrated in  FIGS. 2 and 3 , mounted so as to pivot on plate  8 . It includes a pinion  34  formed of a sleeve  34   a  provided, at one of its ends, with a toothing  34   b . Sleeve  34   a  is formed of a cylindrical portion  34   c  and a conical portion  34   d  that are enlarged from portion  34   c  outwards. 
     Wheel set  32  further includes a wheel  36  formed of a plate  36   a  the periphery of which is provided with a toothing  36   b  which meshes with arbour  20  of differential gear  18 . A spacer  37  is driven onto portion  34   c  of sleeve  34   a . A wheel  36  is mounted freely in rotation on spacer  37  and can thus rotate on pinion  34 . 
     According to the invention, a lever  38  of rectangular shape and whose length is around 0.75 times that of the diameter of wheel  36 , is provided with a longitudinal slot  41  that enlarges into an opening  42  located approximately at third thirds of its length. Lever  38  thus has, on either side of opening  42 , a large arm  38   a  and a small arm  38   b . It is snap fitted onto the conical part  34   d  of pinion  34  so as to hold wheel  36  axially. Pinion  34  and lever  38  thus form a friction mechanism, whose threshold couple is defined by the elasticity of arms  38   a  and  38   b.    
     Moreover, a spring  40  formed of an elastic strip in the shape of an arc of a circle embracing an angle of around 270° is fixed, via one of its ends  44  and in a known manner, to the plate of wheel  36 . Its other end  46 , which is free, forms an elbow  46   a  bent towards the exterior of the wheel and disposed such that, at rest, it abuts against the large arm  38   a , in proximity to opening  42  and thus holds the lever pressed against end  44  of the strip. 
     Finally,  FIG. 1  also shows that the mechanism comprises an indicator wheel set  48  mounted so as to pivot on a pivot-shank  50  driven into plate  8 . It includes a plate  48   a  provided, at its periphery, with a toothing  48   b  meshed with pinion  34 , and a pipe  48   c  extending beyond plate  8  and intended to carry a power reserve indicator hand, which is not shown in the drawing. 
     Plate  48   a  includes a cut out portion  48   d  in the form of an annular section embracing an angle of around 150° corresponding to the angle of displacement of the hand. A pin  52 , driven into plate  8  is engaged in cut out portion  48   d  and acts as a stop for wheel set  48 , for which first and second end positions are thus defined. 
     In a watch movement fitted with the mechanism that has just been described, mainspring  14 , while letting down, drives in rotation and in a conventional manner, barrel  10  which meshes with the going train. 
     The aforecited EP Patent Application describes in detail the operation of differential gear  18  during the winding and letting down of mainspring  14 . It will not, therefore, be described again here. 
     In normal operation, i.e. when the mainspring is sufficiently wound to drive the going train of the movement properly and the indicator wheel set is comprised between its two end positions, spring  40  then drives, via its end  44 , lever  38 , which is gripped between ends  44  and  46   a  of spring  40 . 
     Mainspring  14  is slowly let down driving the going train. As can be seen in  FIG. 4 , the couple of mainspring  14 , represented by curve  58   a , decreases until it reaches a threshold value  60   a.    
     The power reserve indicator then indicates that the latter is exhausted and the indicator wheel set is immobilised, the end of cut out portion  48   d  abutting against pin  52 . 
     Like the mechanism described in the aforecited EP document, if the user of the watch still does not wind mainspring  14 , the latter continues to be let down while causing the watch movement to work. Differential gear  18  thus continues to rotate and, with it, wheel  36 . Since pinion  34  meshes with indicator wheel set  48 , it is blocked. Wheel  36  thus has a relative movement in relation to pinion  34 . This movement is made possible because wheel  36  is coupled to pinion  34  by spring  40  which is being wound. As curve  62   a  shows, the couple generated by spring  40  increases from point  64  to point  65  where the couple of mainspring  14  and spring  40  compensate for each other, such that the watch stops. 
     Then, the user winds mainspring  14  illustrated by curve  58   b . Spring  40  is let down (curve  62   b ), then, when lever  38  returns to its rest position, the mainspring couple reaches a certain value at point  60   b , the indicator wheel set is again driven and the indicator moves with respect to the scale of the dial. During this operation of winding the mainspring, end  44  of spring  40  drives lever  38  with a lower couple than the friction couple of the lever on the pinion and thus drives, with it, pinion  34 . Then, when the power reserve indicator displays the maximum load, the indicator wheel set is immobilised. The corresponding couple of mainspring  14  is shown at point  66  of curve  58   b . Pinion  34  is then blocked, since it is meshed with the indicator wheel set. 
     If the user of the watch continues to wind mainspring  14 , he must then overcome the friction of lever  38  on sleeve  34   a , which is illustrated by portion  68  of curve  58   b . Lever  38 , abutting against end  44  of spring  40 , thus pivots on pinion  34  until the maximum winding of mainspring  14 . Owing to the slipping spring system, the couple of the latter is stabilised at plateau  70 , despite the continuing winding. The various elements of the indicator mechanism are thus perfectly adjusted or oriented and correspondence is obtained between the end positions of the lever and the indicator and the maximum winding of the mainspring, without any particular measures having to be taken when the constituent parts of the mechanism are set in place. 
     The cycle following letting down occurs as described hereinbefore. Then, during the next rewinding, owing to the adjustment achieved during the first cycle, the user knows that when the indicator reaches its end position, the mainspring is completely wound. In the device described, no particular measures are required to set the various components in place, both as regards production and after sales service, since the indicator is automatically adjusted in its correct position, owing to the fact that, during winding, lever  38  rotates with friction on pinion  34 , whereas during letting down, the adjustment is maintained, because spring  40  is being wound. It is clear that, in order to allow these arrangements, the winding couple of spring  40  must always remain less than the friction couple of lever  38  on conical portion  34   d.