Abstract:
An apparatus to drive a ribbon carrying a printing medium in an impact printer is implemented. The apparatus accommodates manual tightening of the ribbon by decoupling the drive motor from the ribbon cartridge without use of a clutch mechanism. A gear train effecting the required speed reductions between a drive motor and the ribbon cartridge spool also implements the decoupling. Reliability of the ribbon drive is improved thereby, and manufacturing and assembly costs are reduced over ribbon drives now used in impact printers.

Description:
This application is a division of Ser. No. 09/005,266, filed Jan. 9, 1998, and now U.S. Pat. No. 5,993,095. 
    
    
     TECHNICAL FIELD 
     The present invention relates in general to impact printers, and in particular, to a drive mechanism for the ink ribbon cartridge in such printers. 
     BACKGROUND INFORMATION 
     Impact printers using an ink ribbon to carry the printing medium from which characters on the printed page are formed must employ a ribbon drive mechanism to advance the ink ribbon so that fresh portions of the ribbon are available for printing. The mechanism driving the ribbon must accommodate manual tightening of the ribbon. 
     FIG. 1 illustrates an impact printer, printer  100 , in which the ink ribbon  104  is supplied in a ribbon cartridge, ribbon cartridge  101 . Printer  100  also includes a ribbon drive, ribbon drive  102 . Ribbon drive  102  may be a ribbon drive according to the prior art, or the ribbon drive of the present invention. A portion (see ribbon pinion  207  in FIG. 2) of ribbon drive  102  engages a spool (not shown) within ribbon cartridge  101  and advances the ink ribbon  104 . In order to manually tighten the ribbon  104 , ribbon cartridge  101  incorporates knob  103 , illustrated in the inset. By manually turning knob  103  in the same direction as the cartridge is driven, the ribbon  104  may be manually tightened. An enlarged view of such a ribbon-tightening knob is shown in FIG.  6 . Ribbon drive  102  must accommodate free rotation to permit manual tightening of the ribbon  104  while the ribbon cartridge is engaged with the ribbon drive  102 . This is complicated by the large speed reduction that is effected between the speed of the driving means for driving ribbon drive  102 , and the speed at which the ribbon  104  advances, which necessarily must be the rotation speed of an output of ribbon drive  102 . 
     Refer now to FIG. 2, in which is depicted a ribbon drive, ribbon drive  200 , in accordance with the prior art. Ribbon drive  200  includes ribbon motor  201  and gear mount bottom  202 , which supports gear  203  (shown in normal and inverted views), gear  204  (shown in normal and inverted views), and pinion gear  205 . These gears are enclosed by gear mount top  206 . Ribbon pinion  207  (which engages the spool (not shown) in ribbon cartridge  101 ) passes through an opening  210  in gear mount top  206  and is coupled to pinion gear  205 , through which it is driven. Gear  203 , gear  204 , and pinion gear  205  effect the speed reduction between the speed of ribbon motor  201  and the speed of ribbon pinion  207  which drives the ribbon  104 . Because the speed at which the ribbon  104  advances is substantially slower than the rotational speed of ribbon motor  201 , the speed reduction produced through gear  203 , gear  204 , and pinion gear  205  is also substantial. Consequently, the torque that would need to be applied to knob  103  in order to manually tighten the ribbon  104  would be considerable unless a means were incorporated to decouple ribbon pinion  207  from pinion gear  205 . This is achieved in the prior art by the use of spring clutch  208 . When ribbon pinion  207  is rotated manually through its engagement with knob  103 , spring clutch  208  decouples ribbon pinion  207  and pinion gear  205 , and thereby the rest of the ribbon drive  200 . In this way, knob  103  and ribbon pinion  207  may be freely rotated, and the ribbon  104  manually tightened, without rotating the remaining portions of ribbon drive  200 . 
     Achieving the required speed reductions through the use of a gear train employing multiple individual gears, and a spring clutch to decouple the gear train from the ribbon pinion increases manufacturing and assembly costs and decreases reliability. Thus, there is a need in the art for an improved ribbon drive that incorporates fewer parts, thereby reducing manufacturing and assembly costs, and improving reliability. 
     SUMMARY OF THE INVENTION 
     The present invention addresses the previously mentioned needs by providing an improved ribbon drive that requires only four parts to achieve the requisite speed reductions as well as accommodating manual tightening of the ribbon. A worm gear which is rotated by a drive means further rotates a compound worm gear. The compound worm gear includes a pair of gear structures on a common shaft. One is a pinion gear that engages the worm gear, and the second is a worm which is rotated as the common shaft is rotated by the action of the worm gear on the pinion gear. The worm on the compound worm gear further engages a pinion gear on a pinion. The pinion then drives the ribbon cartridge. The compound worm gear is supported within a worm gear frame, through which the worm gear passes via an opening in the worm gear frame, in order to engage the pinion gear on the compound worm gear. 
     Manual ribbon tightening is accommodated through pivotal motion of the worm gear frame. The worm gear frame is free to pivot about the worm gear. When the worm gear is being rotated by the drive means, the engagement of the worm gear with the pinion gear on the compound worm gear tends to cause the compound worm gear also to rotate. Moreover, the force of the worm gear on the teeth of the pinion gear produces a moment which tends to cause the compound worm gear and the worm gear frame in which it is supported to pivot about the worm gear. This tendency to pivot on the part of the compound worm gear and the worm gear frame is restrained by the pinion. The worm on the compound worm gear engages the pinion gear on the pinion and the tendency of the compound worm gear to pivot acts to force the worm against the teeth of the pinion gear on the pinion. 
     The rotation of the compound worm gear, and hence the worm on the compound worm gear, then rotates the pinion. Rotation of the pinion then advances the printer ribbon. When the drive means is off, rotation of the pinion during manual tightening of the ribbon leads to a decoupling of the gear train in an analogous fashion, as will now be described. 
     With the drive means off, rotation of the pinion, and thereby the pinion gear attached thereto, forces the teeth of the pinion gear into the worm of the compound worm gear. This produces a moment about the worm gear. This moment in turn causes the compound worm gear to tend to pivot about that gear. The compound worm gear is supported in the worm gear frame, and thus, the worm gear frame also tends to pivot under the action of this moment. Because there is nothing to restrain this pivoting, the compound worm gear and the worm gear frame pivot away from the pinion. They will continue to pivot, at least, until the teeth on the pinion gear of the pinion no longer engage the worm on the compound worm gear. This effects the decoupling of the gear train that is necessary to accommodate the manual tightening of the tape. When the drive means subsequently is energized, the pivoting action previously described causes the worm on the compound worm gear to reengage the pinion gear on the pinion thereby rotating the ribbon. 
     The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 illustrates a printer incorporating a ribbon drive in accordance with an embodiment of the present invention. 
     FIG. 2 illustrates a ribbon drive in accordance with the prior art. 
     FIG. 3 illustrates an exploded view of a mechanical transmission in accordance with an embodiment of the present invention. 
     FIG. 4 illustrates a partial assembly of a manual transmission in accordance with an embodiment of the present invention. 
     FIG. 5 illustrates a bottom view of a pinion in accordance with an embodiment of the present invention. 
     FIG. 6 illustrates a view of a ribbon-tightening knob of FIG. 1 in accordance with an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     In the following description, numerous specific details are set forth to provide a thorough understanding of the present invention. However, it will be obvious to those skilled in the art that the present invention may be practiced without such specific details. Refer now to the drawings wherein depicted elements are not necessarily shown to scale and wherein like or similar elements are designated by the same reference numeral for the several views. 
     Refer now to FIG. 3 in which is depicted an exploded view of an improved mechanical transmission  300  in accordance with an embodiment of the present invention. Mechanical transmission  300  may be a ribbon drive, such as ribbon drive  102 . Drive means  301  rotates worm gear  302 . Worm gear  302  passes through an opening  303  in worm gear frame  304  to engage pinion gear  305  on compound worm gear  306 . Journal  307  on worm gear  302  is supported in bearing  308  on worm gear frame  304 . Worm gear frame  304  is pivotally suspended about worm gear  302  and is free to pivot through angle “A” in the “a-b” direction. Compound worm gear  306  also includes worm  309  at an end opposite that of pinion gear  305 . Journals  310  of compound worm gear  306  are supported in frame bearings  311  in worm gear frame  304 . 
     The relationship between worm gear  302 , worm gear frame  304  and compound worm gear  306  can be appreciated by now referring to FIG.  4 . Depicted therein is a partial assembly  400  of an improved ribbon drive  300 . Worm gear  302  engages pinion gear  305  on compound worm gear  306  which is held in worm gear frame  304 . Compound worm gear  306  is rotatably supported by journals  310  in frame bearings  311 . 
     When worm gear  302  is driven by drive means  301 , motions of compound worm gear  306  and worm gear frame  304  are produced. Driving worm gear  302  in the direction illustrated, clockwise when viewed from above in FIG. 4, causes a rotation of compound worm gear  306  in frame bearings  311 . The force acting on pinion gear  305  giving rise to this rotation acts on a thread of worm gear  302 . This force has both a vertical and horizontal component. The vertical component tends to rotate pinion gear  305  and is resisted by journals  310  against frame bearings  311 . The horizontal component is resisted by worm gear frame  304 , and since the thread of worm gear  302  is displaced a distance from a center line through worm gear  302 , it produces a moment about that center line. This moment tends to cause compound worm gear  306 , and thereby worm gear frame  304 , to pivot about worm gear  302  in direction “a” as shown. This motion is restrained by pinion  312  in FIG.  3 . 
     As shown in FIG. 3, pinion  312  extends through guide  313  on housing  314  and engages a spool (not shown) in ribbon cartridge  101 . Partial assembly  400  is inserted in housing  314  whereby worm  309  on compound worm gear  306  engages pinion gear  315  on pinion  312 . A bottom view of pinion  312  is shown in FIG. 5, whereby pinion gear  315  may be better appreciated. As worm gear frame  304  tends to pivot in the direction “a” in FIG. 3 under the action of the moment about the center line of worm gear  302 , as described hereinabove, it is forced into engagement with pinion gear  315  on pinion  312 . Compound worm gear  306  simultaneously is rotating in frame bearings  311 . 
     The rotation of compound worm gear  306  and thence worm  309  causes a rotation of pinion gear  315  and pinion  312  of which it is a part. The rotation is a consequence of the force acting on the teeth  501  of pinion gear  315  by the thread forming worm  309 . The teeth  501  react back on worm  309 . There is a component of the reaction force along a center line of compound worm gear  306 . This force is in the direction “c” shown in FIG. 3 when pinion  312  is driven in the direction of rotation, as shown in FIG.  3 . This force acts on the thread forming worm  309 , and because this thread is displaced laterally from the center line of worm gear  302 , the reaction force produces a moment about the center line. This moment tends to cause compound worm gear  306 , and thereby worm gear frame  304 , to pivot in the direction “a” about worm gear  302 . Thus, the reaction of teeth  501  on worm  309  produces a moment about worm gear  302  that compliments the moment due to the force of worm gear  302  on pinion gear  305 , as previously described. 
     When the drive means  301  is not energized, the printer ribbon  104  can be manually tightened by rotation of knob  103 . Pinion  312  is rotated by knob  103 , with which it is engaged through a spool (not shown) in ribbon cartridge  101 , in the driven direction as shown on FIG.  3 . Now teeth  501  act on the thread forming worm  309 , tending to cause compound worm gear  306  to rotate. More importantly, this force has a component along the center line of compound worm gear  306  in the direction “d” shown in FIG.  3 . This force produces a moment about worm gear  302  as previously described, but now the moment causes compound worm gear  306  and thereby worm gear frame  304  to pivot in direction “b” in FIG.  3 . Furthermore, unlike the driven case, there is nothing corresponding to pinion  312  to oppose this tendency, so worm gear frame  304  is free to pivot until worm  309  disengages from pinion gear  315 . In an embodiment of the present invention in which frame  317  of housing  314  encloses partial assembly  400 , openings  316  in frame  317  are such that worm gear frame  306  is free to pivot sufficiently to allow this disengagement to occur. 
     An embodiment of the present invention may have an electric motor as a drive means. In an alternative embodiment, an electric motor drive means may be a direct current (D. C.) electric motor. 
     The rate of rotation of pinion  312  and thereby the printer ribbon  104 , is reduced from the rate of rotation of the drive means by the action of worm  309  and pinion gear  315  along with worm gear  302  and pinion gear  305 . The reduction ratio is determined by the number of teeth on pinion gear  305  and pinion gear  315 , as well as the number of threads forming each of worm  309  and worm gear  302 . Embodiments of the present invention may include multithreaded worms in either worm  309 , worm gear  302 , or both. In such embodiments, the reduction ratio value is smaller than the reduction ratio value in an embodiment in which worm  309  and worm gear  302  are singly threaded, provided the numbers of teeth on pinion gear  305  and pinion gear  315  are otherwise unchanged. It would be understood by an artisan of ordinary skill that embodiments employing singly threaded worms, multithreaded worms or a combination thereof are within the scope of the claimed invention, as are pinion gears having any predetermined numbers of teeth. 
     It would be further understood by one of ordinary skill in the art that an alternative embodiment of the mechanical transmission of the present invention may be used when it is necessary to drive a load from two motive devices. 
     Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims.