Abstract:
A slip mechanism for a timepiece comprises a rotational shaft having a pinion, a gear and a spring wherein the rotational shaft has a flange for supporting the gear, a mounting portion formed adjacent the surface of the flange for rotatably mounting the gear and slots formed above the mounting portion for engagement with the spring. The slots coact with recessed portions of the spring for preventing the spring from rotating with respect to the shaft.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a gear train structure for a timepiece and, in particular, to improvements in a slip mechanism for use in the gear train. 
     In conventional slip mechanisms, it has heretofore been practiced generally, for example, to combine a pinion with a gear placed on it and a spring mounted further thereon by way of calking thereby intensely urging the gear to the pinion by the spring to connect them frictionally. 
     Such method is, however, disadvantageous from an economical point of view since connecting steps such as calking are required and a great loss result for each faulty or defective assembly produced. 
     Another conventional method has been known in which a gear is fabricated from metal and integrally formed together with a pinion upon plastic molding of the latter. 
     This method is also defective in that the slip torque is greatly changed with even a slight variation in forming conditions because the slip torque is determined by the force resulted between the metal and the plastic upon forming of the assembly. 
     A further method of providing a flexible arm to either one of a gear and a pinion made of plastic to obtain a slip torque by frictionally connecting them with the flexible arm is defective in that a reliability of the slip mechanism is very low since the slip torque tends to change with even slight changes in temperature and humidity. 
     It is, accordingly, an object of this invention to eliminate the foregoing drawbacks and provide a structure which is inexpensive and quickly and easily highly reliable and capable of connection with a gear train assembly. 
     The foregoing defects can be overcome by a slip mechanism comprised of a gear, pinion and a spring, in which the pinion is formed with a guide portion for rotationally guiding the gear, a supporting portion for bearing the height of the spring and an anti-rotation portion for the spring, and designed such that the pinion and the spring are rotated together upon slipping. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     This invention will now be described by way of a preferred embodiment thereof referring to the accompanying drawings, wherein 
     FIG. 1 is a sectional view of a gear train showing a preferred embodiment of this invention; 
     FIG. 2 is a plan view of the slip mechanism; 
     FIG. 3 is a sectional view taken along a line A--O--A in FIG. 2 and 
     FIG. 4 and FIG. 5 are perspective views of the slip mechanism parts shown in FIG. 1. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENT 
     Referring to FIG. 1, an upper plate 1, an intermediate plate 2 and a lower plate 3 for rotationally supporting a gear train are secured to each other way of posts 4 and 5 and screws 6 and 7 which extend through the posts and are screwed into respective one of the plates. 
     A second shaft 8 is rotationally supported by the above upper and the lower plates 1 and 2 and is securely connected to a second wheel 9 driven by a motor M and a first pinion 10. A shaft 11 is also rotationally supported by the upper and the lower plates 1 and 2 and fixedly provided with a second pinion 11a for engagement with a minute wheel 15. The shaft 11 frictionally and rotatably holds a first intermediate gear wheel 12 that meshes with the second wheel 9. The minute wheel 15 is fixed with a third pinion 14 and a pipe 17 having a minute indicator attached at its top. A second intermediate gear wheel 18 for engagement with the third pinion 14 is fixed together with a fourth pinion 20 to a shaft 19 which is rotatably supported by the intermediate and the lower plates 2 and 3. An hour wheel 21 meshes with the fourth pinion 20 and is secured to an hour indicator pipe 22 rotationally supported by the lower plate 3. The second intermediate gear wheel 18 meshes with a time amend wheel 23. In the gear train of the above constitution, if the slip torque is set lower than the static coupling force of the motor, since the first intermediate wheel 12 slips against the shaft 11 upon rotation of the time amend wheel 23 when a watch user wants to amend the present time, only the portion of the gear train behind the minute wheel 15 is rotated. 
     Reference will now be made in detail to the frictional coupling mechanism between the first intermediate gear wheel 12 and the shaft 11 shown in FIG. 2 and in the succeeding figures. As described above, the shaft 11 is formed at its lower portion with a second pinion 11a and a rotational sliding portion 11b which is rotationally supported by the aperture in the intermediate plate 2, as well as at its top with a second rotational sliding portion 11c which is rotationally supported by the upper plate 1. Further, in the middle of the shaft 11, are formed a flange portion 11d for rotatably supporting thereon the first intermediate gear wheel 12 and a mounting portion 11h formed above the flange portion 11d for mounting the intermediate gear wheel 12. Also formed are spring bearing portions 11f and 11f&#39; for defining the height of the spring 24 and interengaging portions 11g and 11g&#39; comprising slats in the shaft 11 for the planar positioning of the spring and coacting with complementary interengaging portions of the spring 24 to prevent relative rotation 24. 
     Referring now to the constitution of the spring 24, it has a generally U-shaped, bent configuration having interengaging portions 24a and 24b for engagement with the complementory interengaging portions 11g and 11g&#39; of the shaft 11, flexible portions 24c and 24d, guide portions 24i and 24j and, with anti-slip off portions 24e . . . 24h for preventing the spring 24 per se from slipping off. The U-shaped spring 24 has two spring arms connected at one end by a base portion and when viewed in side elevation, the spring arms have a generally V-shape with recessed portions 24i and 24j being formed at the vertex of the V for engaging the inner walls of the slots 11g and 11g&#39;. 
     A description will now be given of the manner of coupling of the shaft 11, the first intermediate wheel 12 and the spring 24. The first intermediate gear wheel 24 12 is first inserted to the shaft 11 from above while guided along the rotational guiding portion 11h of the shaft 11 and then the spring 24 is inserted to the shaft 11 laterally along the anti-spring rotation portions 11g and 11g&#39; whereby the rotating direction of the spring 24 is restricted by the embracing portions 24a and 24b of the spring 24. Upon further insertion of the spring 24, the spring arms expand their outwardly by resiliency and the vertical movement of the spring 24 is regulated by the upper face of the first intermediate gear wheel 12 and the spring bearing portions 11f and 11f&#39; of the shaft 11, whereby the spring 24 is vertically flexed and its flexible portions 24c and 24d advance along the anti-spring rotation portions 11g and 11g&#39;. When the flexible portions 24c and 24d pass over its anti-spring rotation portions 11g and 11g&#39;, the spring resumes the initial state by its own returning force. In the assembly state, the spring 24 is retained by the anti-slip off portions 24e . . . 24h of the shaft 11 and can not be readily disengaged. In the above coupled state, since the spring 24 is resiliently held between the upper face of the first intermediate wheel 12 and the spring bearing portions 11f and 11f&#39; and the shaft 11 is urged upwardly with respect to the first intermediate gear wheel 12, a constant torque is produced between them. The first intermediate gear wheel 12 and the shaft 11 can thus be coupled by simple manual operation. 
     While the slip mechanism is provided in the first intermediate wheel in the above embodiment, similar effects can be obtained as well by providing it between the minute wheel and its pinion. In addition, the intermediate wheel and the shaft can be made either with metal or plastic to attain the similar effects. 
     According to this invention, a slip mechanism of an excellent economical advantage and a high relliability can be obtained and, hence, this invention has much practical importance.