Ink ribbon cassette
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to an ink ribbon cassette in which an ink ribbon drawn out from a supply spool is taken up onto a take-up spool. 
     2. Related Background Art 
     Generally, an ink ribbon cassette is such that as the diameter of a supply spool is decreased, the torque with which the ink ribbon on the supply spool is drawn out increases to make it difficult for the ribbon to be stably drawn out. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in view of the above-noted disadvantage and an object thereof is to provide an ink ribbon cassette which enables a ribbon to be stably drawn out and which has a reduced number of parts and is simplified in structure. 
     Other objects of the present invention will become apparent from the following detailed description thereof taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a printer in which is mounted an ink ribbon cassette according to an embodiment of the present invention. 
     FIG. 2 is an illustration of a partly broken-away plane of the ink ribbon cassette according to an embodiment of the present invention. 
     FIG. 3 is a cross-sectional view of a supply side ribbon spool. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A printer in which an ink ribbon cassette 7 is mounted according to an embodiment of the present invention is comprised of an operating unit 1 and a printing mechanism unit 2, as shown in FIG. 1. The printing mechanism unit 2 comprises a platen 3, a printing carriage 5 and a driving portion 6 therefor. A character wheel 8, a printing hammer 9 and the ink ribbon cassette 7 for supplying ink are mounted on the printing carriage 5. 
     FIG. 2 shows the details of the ink ribbon cassette. The cassette is divided into a take-up side A engaged with the printing carriage to take up the ink ribbon and a supply side B for supplying the ink ribbon so as not to interfere with the taking-up and printing operations. The cassette is comprised of a lower case 11 and an upper case 12, and various portions to be described are contained in the space therebetween. A central core 14 is rotatably fitted in a rotating boss 13 provided in the lower case 11, and the ink ribbon 15 is wound on the core 14 to thereby form a supply spool 16. The peripheral surface of that portion of the core 14 which is lower than the portion thereof on which the ink ribbon is wound is formed with a knurled uneven portion 14a for controlling the amount of unwinding of the supply spool 16. One arm 17a of a detent spring 17 fixed to the lower case 11 by the restraining portion 11a thereof is resiliently engaged with the knurled uneven portion 14a, and the other arm 17b of the detent spring 17 is resiliently urged against the outermost periphery of the ribbon spool 16. The ink ribbon 15 drawn out through such a detent mechanism is further biased by the arm 18a of a tension spring 18 for eliminating any slack of the ink ribbon, and is taken up by a take-up core 21 via guides 19 and 20. The take-up core 21 is rotatably provided on the end of a take-up lever 23 pivotable about a shaft 22. The take-up lever 23 is biased by a plate spring 24 toward ribbon feeding teeth 25 rotatably held between the upper case 12 and the lower case 11. Minute needles project around the ribbon feeding teeth 25 and penetrate into the take-up core 21 or the ink ribbon wound on the core 21 and thus, it becomes possible to take up the ink ribbon onto the take-up core 21 by rotating a manually operated knob 26 in the direction of arrow D, the knob 26 being fixed coaxially with the ribbon feeding teeth 25 and partly exposed from the upper case 12. As the ribbon is taken up onto the core 21, the winding diameter of the take up spool is increased. 
     FIG. 3 shows a cross-section of the supply spool. As can be seen in this Figure, the uneven portion 14a of the central core 14 at the lower end thereof is smaller in diameter than the portion of the central core on which the ribbon is wound and accordingly, the ribbon winding position becomes stable when the ribbon is wound onto this core 14. 
     In the above-described embodiment, the arm 17a of the detent spring 17 is engaged with the knurled uneven portion 14a of the core 14 to impart back tention to the ribbon, and further the arm 17b presses the peripheral surface of the ribbon spool 16 to impart back tension so as to ensure the ribbon spool 16 to be drawn out with the same torque. Also, back tension is further imparted to the ink ribbon by the tension spring 18. 
     The present invention is not restricted to the above-described embodiment, but for example, the tension spring 18 may be eliminated. Also, the ink ribbon drawn out toward the guide 19 may be passed over the arm 17b of the detent spring, and this arm 17b may be used to perform the function of the arm 18a of the tension spring 18. Thereby, the number of parts is reduced. 
     In the present invention, as described above, not only a detent force is imparted to the central core of the supply spool by the detent spring, but also a pressure force is imparted to the peripheral surface of the supply spool, whereby back tension can be effectively imparted to the supply spool by a small number of parts, thus stabilizing the draw-out of the ribbon.