Abstract:
A rotary-torque adjuster that adjusts rotary torque of a rotational member includes a rotary shaft, the rotational member secured to the rotary shaft and rotating integrally with the rotary shaft, a plurality of clutch disks rotatably attached to the rotary shaft, a driver that drives the clutch disks, a plurality of presser components attached to the rotary shaft and rotating integrally with the rotary shaft, the presser components sandwiching opposite surfaces of the clutch disks by a predetermined pressure, and a drive transmission unit that makes a selection from among the plurality of clutch disks so as to transmit a driving force from the driver to the selected one or more clutch disks. The drive transmission unit changes the selection of the clutch disks to which the driving force from the driver is to be transmitted so as to change the rotary torque of the rotational member.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS  
       [0001]     The present invention contains subject matter related to Japanese Patent Application JP 2005-090484 filed in the Japanese Patent Office on Mar. 28, 2005, the entire contents of which are incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a rotary-torque adjuster that adjusts rotary torque in the process of transmission of a driving force from a driver, to an ink-ribbon conveying mechanism having such a rotary-torque adjuster used for conveying an ink ribbon, and to a printer equipped with such an ink-ribbon conveying mechanism.  
         [0004]     2. Description of the Related Art  
         [0005]     In printers that apply ink ribbons, if an ink ribbon is loose in the process of a printing operation, the ink ribbon may wrinkle, which may lead to a print error. Generally, an ink ribbon fed from a feed spool is given a back tension in order to avoid such looseness of the ink ribbon. Moreover, in a take-up spool, a take-up force is adjusted in view of the back tension. Accordingly, these measures are taken in order to maintain a constant ribbon tension between the feed spool and the take-up spool.  
         [0006]     There are various techniques for applying a ribbon tension to an ink ribbon. One example is providing each spool with a brake mechanism including, for example, a torque clutch or a torque limiter. In another example, a resilient member, such as a leaf spring, is used.  
         [0007]     However, as the diameter of the ink ribbon wound around the feed spool and the diameter of the ink ribbon wound around the take-up spool change gradually in the course of printing, the torque acting on each of the spools changes. Therefore, there are various disclosed techniques for applying a constant ribbon tension to the ink ribbon.  
         [0008]     For example, Japanese Unexamined Patent Application Publication No. 2002-234241 discloses a structure in which the rotary torque of a shaft that drives the take-up spool is determined by a sandwiching pressure of presser components. In this structure, a spring compression gear meshed with the shaft is rotated so as to change a biasing force applied to the presser components. Thus, the rotary torque is adjustable in accordance with the wound diameter of the ink ribbon around the spools.  
         [0009]     On the other hand, Japanese Unexamined Patent Application Publication No. 6-316139 discloses a technique in which a motor is controlled by counting the number of pulses generated in response to rotations of the take-up spool and the feed spool and then calculating a motor voltage related with motor torque and the rotating speed in accordance with the wound diameter of the ink ribbon around the spools.  
       SUMMARY OF THE INVENTION  
       [0010]     However, Japanese Unexamined Patent Application Publication No. 2002-234241 is problematic in that a long time is necessary for adjusting the rotary torque since the spring compression gear has to be rotated for every adjustment process.  
         [0011]     On the other hand, Japanese Unexamined Patent Application Publication No. 6-316139 is problematic in that the control operation is complicated, and that there may be variations in mechanisms depending on different devices, which may possibly impair the conveying stability of the ink ribbon.  
         [0012]     Accordingly, it is desirable to achieve stable torque with a low-cost structure.  
         [0013]     According to an embodiment of the present invention, there is provided a rotary-torque adjuster that adjusts rotary torque of a rotational member. The rotary-torque adjuster includes a rotary shaft, the rotational member secured to the rotary shaft and rotating integrally with the rotary shaft, a plurality of clutch disks rotatably attached to the rotary shaft, a driver that drives the clutch disks, a plurality of presser components attached to the rotary shaft and rotating integrally with the rotary shaft, the presser components sandwiching opposite surfaces of the clutch disks by a predetermined pressure, and a drive transmission unit that makes a selection from among the plurality of clutch disks so as to transmit a driving force from the driver to the selected one or more clutch disks. The drive transmission unit changes the selection of the clutch disks to which the driving force from the driver is to be transmitted so as to change the rotary torque of the rotational member.  
         [0014]     In the rotary-torque adjuster according to the above-referenced embodiment of the present invention, the rotational member is rotated in response to the transmission of the driving force from the driver, and the drive transmission unit makes a selection from among the plurality of clutch disks so as to transmit the driving force to the selected one or more clutch disks.  
         [0015]     Accordingly, on the basis of the selection of the clutch disks to which the driving force is transmitted, a plurality of rotary torques can be attained. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]      FIG. 1  is a perspective view of a printer according to an embodiment of the present invention;  
         [0017]      FIG. 2  is a perspective view showing a state in which a door of the printer is opened;  
         [0018]      FIG. 3  is a perspective view of an ink-ribbon conveying mechanism according to an embodiment of the present invention;  
         [0019]      FIG. 4  is a front view of the ink-ribbon conveying mechanism;  
         [0020]      FIG. 5  is a plan view of the ink-ribbon conveying mechanism;  
         [0021]      FIG. 6  is a vertical sectional view of a torque limiter included in the ink-ribbon conveying mechanism, as viewed from the front side thereof; and  
         [0022]      FIG. 7  is a graph showing a relationship between a wound diameter of an ink ribbon around a take-up spool and a ribbon tension. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0023]     In preferred embodiments of the present invention described below, there are provided a rotary shaft  41 , an engagement part  12 A engageable with a take-up spool and functioning as a rotational member, a motor  21  functioning as a driver, clutch disks  53  ( 53 A and  53 B), presser components  51  ( 51 A to  51 C), and a drive transmission unit defined by a worm gear  22 , a worm wheel  24 , a gear  25 , a sun gear  27 , a pendulum  29 , planet gears  30 A and  30 B, a planet gear  32 , an idle gear  34 , and gears  52 A and  52 B.  
         [0024]     The preferred embodiments of the present invention will now be described with reference to the drawings.  
         [0025]     A rotary-torque adjuster according to an embodiment of the present invention is applied in an ink-ribbon conveying mechanism, and the ink-ribbon conveying mechanism is installed in a printer.  
         [0026]      FIG. 1  is a perspective view of a printer  1  according to an embodiment of the present invention. The printer  1  includes a housing  2  and a door  3  attached to a front face of the housing  2 . The front face of the housing  2  is also provided with a power switch  4 . The door  3  has a door panel  5  attached thereto. The front surface of the door panel  5  is provided with an operating panel  6  having various switches, and a liquid-crystal panel  7  for displaying various messages. Furthermore, a catch tray  9  having an ejection opening  8  is attached to a lower end of the door  3 .  
         [0027]      FIG. 2  is a perspective view showing a state in which the door  3  of the printer  1  is opened. Moreover,  FIG. 2  shows a disassembled state of an ink ribbon  14  and a take-up spool  15  and feed spool  16  disposed at opposite ends of the ink ribbon  14 .  
         [0028]     The rear surface of the door  3  is provided with a thermal head  10 . Furthermore, a platen  11  is disposed inside the housing  2  at a position facing the thermal head  10  when the door  3  is closed. Although not shown, a grip roller and a pinch roller for conveying roll paper are disposed adjacent to the platen  11 .  
         [0029]     Furthermore, inside the housing  2 , an engagement unit  12  including a pair of engagement parts  12 A,  12 B engageable with the take-up spool  15  is disposed above the platen  11 . Moreover, an engagement unit  13  including a pair of engagement parts  13 A,  13 B engageable with the feed spool  16  is disposed below the platen  11 . In a state where the take-up spool  15  and the feed spool  16  are respectively engaged with the engagement unit  12  and the engagement unit  13 , the take-up spool  15  and the feed spool  16  are disposed parallel to the platen  11  in a rotatable fashion. The ink ribbon  14  is disposed so as to move above the platen  11 . When the door  3  is closed, the ink ribbon  14  is positioned between the platen  11  and the thermal head  10 .  
         [0030]      FIG. 3  is a perspective view of an ink-ribbon conveying mechanism  20  according to an embodiment of the present invention.  FIG. 4  is a front view of the ink-ribbon conveying mechanism  20 .  FIG. 5  is a plan view of the ink-ribbon conveying mechanism  20 .  FIG. 6  is a vertical sectional view of a torque limiter  50  included in the ink-ribbon conveying mechanism  20 , as viewed from a front side thereof.  
         [0031]     Referring to FIGS.  3  to  5 , the motor  21  functioning as a driver for the ink-ribbon conveying mechanism  20  has a rotary shaft  21   a.  The rotary shaft  21   a  has the worm gear  22  fixed thereto. A rotatable supporting shaft  23  is disposed adjacent to the worm gear  22 . The worm wheel (spiral gear)  24  is secured to an upper portion of the supporting shaft  23  and is meshed with the worm gear  22 . On the other hand, the gear  25  is secured to a lower portion of the supporting shaft  23  and is disposed concentric to the worm wheel  24 .  
         [0032]     Furthermore, another rotatable supporting shaft  26  is disposed adjacent to the supporting shaft  23 . The sun gear  27  is attached to the supporting shaft  26  and is meshed with the gear  25 . Moreover, the supporting shaft  26  also has the pendulum  29  attached rotatably thereto.  
         [0033]     Two ends of the pendulum  29  are respectively provided with supporting shafts  28  and  31 . The supporting shaft  28  has the planet gears  30 A and  30 B rotatably attached thereto, which are arranged in two levels in the vertical direction. The planet gears  30 A and  30 B are meshed with the sun gear  27 .  
         [0034]     On the other hand, the supporting shaft  31  has the planet gear  32  attached thereto in a rotatable fashion. The planet gear  32  is meshed with the sun gear  27 .  
         [0035]     To the right of the pendulum  29  is disposed the torque limiter  50 . The torque limiter  50  is provided with the gears  52 A and  52 B that are arranged in two levels in the vertical direction.  
         [0036]     The upper gear  52 A meshes with the idle gear  34  in a rotatable fashion, but the lower gear  52 B does not mesh with the idle gear  34 . The idle gear  34  is rotatably supported by a supporting shaft  33 .  
         [0037]     The idle gear  34  is provided for maintaining the rotational direction of the gears  52 A and  52 B in one direction regardless of whether the motor  21  rotates in the forward or reverse direction.  
         [0038]     Referring to  FIG. 5 , when the pendulum  29  rotates clockwise around the supporting shaft  26 , the planet gear  32  meshes with the idle gear  34 . In contrast, when the pendulum  29  rotates counterclockwise in  FIG. 5  around the supporting shaft  26 , the two planet gears  30 A and  30 B respectively mesh with the gears  52 A and  52 B.  
         [0039]     The torque limiter  50  will now be described in detail.  
         [0040]     Referring to  FIG. 6 , the engagement part  12 A for the take-up spool  15  is secured to a lower end of the rotary shaft  41 . Thus, the rotary shaft  41  and the engagement part  12 A rotate integrally. Furthermore, the rotary shaft  41  supports a presser component  51 A, a clutch disk  53 A, a presser component  51 B, a clutch disk  53 B, and a presser component  51 C disposed one on top of another in that order from top to bottom in  FIG. 6 .  
         [0041]     The rotary shaft  41  is D-shaped in cross section. Each of the presser components  51 A to  51 C has a D-shaped shaft hole so as to be engageable with the rotary shaft  41  having the D-shaped cross section. Accordingly, when the presser components  51 A to  51 C are fitted to the rotary shaft  41 , the presser components  51 A to  51 C are rotatable integrally with the rotary shaft  41  and are supported in a shiftable fashion only in the axial direction of the rotary shaft  41 .  
         [0042]     On the other hand, each of the clutch disks  53 A and  53 B has a circular rotational hole. The clutch disks  53 A and  53 B are thus rotatable around the rotary shaft  41  and are supported in a shiftable fashion in the axial direction of the rotary shaft  41 .  
         [0043]     Furthermore, the gear  52 A mentioned above is provided around the outer periphery of the clutch disk  53 A. Similarly, the gear  52 B is provided around the outer periphery of the clutch disk  53 B.  
         [0044]     Furthermore, felt sheets  54 A functioning as friction members are adhered to opposite surfaces of the clutch disk  53 A. The felt sheets  54 A are provided for generating a predetermined frictional force between the presser component  51 A and the clutch disk  53 A and between the clutch disk  53 A and the presser component  51 B. In this embodiment, the felt sheets  54 A are annular-shaped (donut-shaped). The felt sheets  54 A are given a dimension such that the two felt sheets  54 A are respectively in contact with outer periphery areas of the presser component  51 A and the clutch disk  53 A and outer periphery areas of the clutch disk  53 A and the presser component  51 B.  
         [0045]     Similarly, felt sheets  54 B composed of the same material as the felt sheets  54 A (having the same coefficient of friction) are adhered to opposite surfaces of the clutch disk  53 B. The felt sheets  54 B are annular-shaped like the felt sheets  54 A, but are given a dimension different from that of the felt sheets  54 A. Specifically, the felt sheets  54 B are given a dimension such that the two felt sheets  54 B are respectively in contact with inner periphery areas of the presser component  51 B and the clutch disk  53 B and inner periphery areas of the clutch disk  53 B and the presser component  51 C.  
         [0046]     Furthermore, as shown in  FIG. 6 , a (compression coil) spring  42  functioning as a bias member is engaged with an upper portion of the rotary shaft  41 . A lower end of the spring  42  in  FIG. 6  is in contact with the presser component  51 A, whereas an upper end is in contact with a stationary member  43  secured to the rotary shaft  41 . On the other hand, the presser component  51 C is retained in position by a step portion  41   a  of the rotary shaft  41 . Consequently, the spring  42  presses the presser components  51 A to  51 C and the clutch disks  53 A and  53 B downward in  FIG. 6  in the axial direction of the rotary shaft  41 .  
         [0047]     Accordingly, if the clutch disks  53 A and  53 B are rotated in a state where a load applied to the rotary shaft  41  is low, a slip (relative movement) is not generated between the presser component  51 A and the clutch disk  53 A nor between the clutch disk  53 A and the presser component  51 B due to the felt sheets  54 A. Therefore, the clutch disk  53 A and the presser components  51 A and  51 B rotate integrally. Likewise, a slip is not generated between the presser component  51 B and the clutch disk  53 B nor between the clutch disk  53 B and the presser component  51 C due to the felt sheets  54 B, and therefore, the clutch disk  53 B and the presser components  51 B and  51 C rotate integrally. Since the rotary shaft  41  is rotated integrally with these components and disks, the engagement part  12 A (and the take-up spool  15 ) is rotated accordingly.  
         [0048]     As the load applied to the rotary shaft  41  becomes higher, a slip (relative movement) is generated between the clutch disk  53 A and each of the presser components  51 A and  51 B and between the clutch disk  53 B and each of the presser components  51 B and  51 C. As a result, only the clutch disks  53 A and  53 B rotate.  
         [0049]     When the motor  21  is rotated in, for example, the forward direction (CW) as viewed from an electric-contact side of the motor  21 , the worm wheel  24  and the gear  25  rotate counterclockwise in  FIG. 5 . Thus, the sun gear  27  rotates clockwise in  FIG. 5 , allowing the pendulum  29  to rotate clockwise. This moves the planet gear  32  to a first position where it meshes with the idle gear  34 . Subsequently, the planet gear  32  rotates counterclockwise and the idle gear  34  rotates clockwise, thus allowing the gear  52 A (the clutch disk  53 A) to rotate counterclockwise. Accordingly, the rotary shaft  41  and the engagement part  12 A are rotated counterclockwise (CCW).  
         [0050]     On the other hand, when the motor  21  is rotated in the reverse direction (CCW) as viewed from the electric-contact side, the worm wheel  24  and the gear  25  rotate clockwise in  FIG. 5 . Thus, the sun gear  27  rotates counterclockwise in  FIG. 5 , allowing the pendulum  29  to rotate counterclockwise. This moves the planet gears  30 A and  30 B to a second position where they respectively mesh with the gears  52 A and  52 B. Subsequently, the planet gears  30 A and  30 B rotate clockwise, thus allowing the gear  52 A (the clutch disk  53 A) and the gear  52 B (the clutch disk  53 B) to rotate counterclockwise. Accordingly, the rotary shaft  41  and the engagement part  12 A are rotated counterclockwise (CCW).  
         [0051]     In the case where the motor  21  is rotated in the forward direction as viewed from the electric-contact side, since the idle gear  34  and the gear  52 A (the clutch disk  53 A) are meshed with each other, the presser components  51 A and  51 B are accordingly rotated due to the frictional force of the felt sheets  54 A provided on the opposite surfaces of the clutch disk  53 A. Accordingly, this allows the clutch disk  53 B, the presser component  51 C, the rotary shaft  41 , and the engagement part  12 A to rotate integrally.  
         [0052]     In contrast, in the case where the motor  21  is rotated in the reverse direction as viewed from the electric-contact side, since the planet gears  30 A and  30 B respectively mesh with the gears  52 A and  52 B, the presser components  51 A to  51 C are accordingly rotated by the clutch disks  53 A and  53 B and the frictional force of the felt sheets  54 A and  54 B. Accordingly, this allows the rotary shaft  41  and the engagement part  12 A to rotate integrally.  
         [0053]     Consequently, the frictional force acting on the torque limiter  50  is greater when the motor  21  is rotated in the reverse rotation than in the forward rotation by an amount of friction generated by the felt sheets  54 B between the presser component  51 B and the clutch disk  53 B and between the clutch disk  53 B and the presser component  51 C. In other words, the rotary torque of the take-up spool  15  is increased by that amount of friction.  
         [0054]     The rotational direction of the motor  21  is switched in accordance with a wound diameter of the ink ribbon  14  wound around the take-up spool  15 .  
         [0055]      FIG. 7  is a graph showing a relationship between the wound diameter of the ink ribbon  14  around the take-up spool  15  and a ribbon tension. In the graph, the horizontal axis represents a progression rate from an initial value point (i.e. initial take-up point) S to a final take-up point (i.e. maximum wound diameter) E, and the vertical axis represents a ribbon tension (g).  
         [0056]     Referring to  FIG. 7 , a line  61  indicates a mode in which the gear  52 A and the idle gear  34  are meshed with each other such that a driving force is transmitted only to the clutch disk  53 A. A line  62  indicates a mode in which the gears  52 A and  52 B are respectively meshed with the planet gears  30 A and  30 B such that a driving force is transmitted to both clutch disks  53 A and  53 B.  
         [0057]     In the mode for transmitting the driving force only to the clutch disk  53 A, the ribbon tension at the initial value point S is 600 g. As the wound diameter of the ink ribbon  14  increases, the ribbon tension decreases. In this embodiment, the wound diameter increases by about two times at a point where the ink ribbon  14  is completely used up, and the ribbon tension is reduced to 300 g at the final take-up point E.  
         [0058]     On the other hand, in the mode for transmitting the driving force to both clutch disks  53 A and  53 B, the ribbon tension at the initial value point S is 900 g. As the wound diameter of the ink ribbon  14  increases, the ribbon tension decreases. The ribbon tension is reduced to about 450 g at the final take-up point E.  
         [0059]     As mentioned above, in this embodiment, the felt sheets  54 A adhered to the clutch disk  53 A and the felt sheets  54 B adhered to the clutch disk  53 B have different inner diameters and outer diameters. In  FIG. 7 , the line  62  shows characteristics in which the ribbon tension is greater than that of the line  61  by about 1.5 times. There are various parameters for determining the degree of frictional force of the clutch disks  53 A and  53 B. The parameters include, for example, shapes, materials, and adhering locations of the felt sheets  54 A and  54 B. Accordingly, these parameters may be selected in accordance with design conditions.  
         [0060]     In the printer  1  according to this embodiment, a tolerance range of the ribbon tension for maintaining a high print quality is within 400 to 600 g, as shown in  FIG. 7 . Therefore, a control operation is implemented by driving the clutch disk  53 A singularly from the initial value point S to a two-thirds point of the progression rate at which the lower tolerance limit is 400 g. Subsequently, the operation switches to another mode in which both clutch disks  53 A and  53 B are driven until reaching the final take-up point E. In other words, the motor  21  is rotated in the forward direction up to the two-thirds point of the progression rate, and is then rotated in the reverse direction from the two-thirds point onward. This allows the ribbon tension to be maintained within the tolerance range.  
         [0061]     Although not shown, there are various known types of detection mechanisms for detecting the wound diameter of the ink ribbon  14 . In one example, a detector may be disposed in direct contact with the outermost periphery of the ink ribbon  14  wound around the take-up spool  15  so as to detect the wound diameter in a mechanical fashion. In another example, the wound diameter may be calculated on the basis of FG pulses output from rotary detectors provided for the feed spool  16  and the take-up spool  15 .  
         [0062]     Accordingly, the ribbon tension that varies in accordance with the wound diameter of the ink ribbon  14  can be maintained constantly within the tolerance range by simply switching the rotational direction of the motor  21  on the basis of a detection result of the wound diameter. This ensures a stable print quality.  
         [0063]     The ribbon tension can be adjusted to a preferred value on the basis of, for example, the specifications of the clutch disks  53 A and  53 B, the dimensions and frictional forces of the felt sheets  54 A and  54 B, and the biasing force of the spring  42 .  
         [0064]     Furthermore, the rotary torque can be changed instantaneously by simply rotating the motor  21  functioning as a driver in the reverse direction. Moreover, the motor  21  serves as a common driver for the switching of the clutch disks  53 A and  53 B and for the driving of the take-up spool  15 . Accordingly, this contributes to a simplified structure, lower manufacturing costs, higher reliability, and an overall size reduction.  
         [0065]     Furthermore, since the mechanism of the rotary-torque adjuster is not installed in every ink-ribbon cassette, the above embodiments achieve lower costs from manufacture to disposal and are advantageous from an environmental standpoint.  
         [0066]     The technical scope of the present invention is not limited to the above embodiments, and modifications are permissible within the scope and spirit of the present invention.  
         [0067]     In the embodiments described above, a desired rotary torque is attained by moving the pendulum  29  on the basis of a detection result of the wound diameter of the ink ribbon  14  and changing the number of clutch disks  53  to which a driving force is to be transmitted. Alternatively, the rotary torque may be adjusted by switching between the clutch disks  53 A and  53 B of the plurality of clutch disks  53 .  
         [0068]     Although gears are used in the drive transmission unit for transmission between the clutch disks  53  and the rotary driving system in the above embodiments, the gears may be replaced with, for example, rubber rollers. In other words, the drive transmission unit may be any type of a unit as long as it has the capability to transmit a driving force by means of a predetermined frictional force.  
         [0069]     Furthermore, the number of clutch disks  53  is adjustable. For example, although the ribbon tension is switched once in the course of a take-up process in the above embodiments, the number of clutch disks  53  may be increased so that the ribbon tension can be switched twice, three times, and so on.  
         [0070]     In the above embodiments, the felt sheets  54 A and  54 B are adhered at different locations (with different surface areas). Alternatively, the felt sheets  54 A and  54 B may be adhered at corresponding locations (with the same surface area). Furthermore, the felt sheets  54 A may have a coefficient of friction different from that of the felt sheets  54 B.  
         [0071]     According to the above embodiments, the rotary torque of the engagement part  12 A functioning as a rotational member can be adjusted by selecting the clutch disks to which a driving force is to be transmitted. Consequently, even if a load on the engagement part  12 A changes, the rotary torque is maintained within a predetermined range.  
         [0072]     It should be understood by those skilled in the art that various modifications, combinations, sub-combinations and alterations may occur depending on design requirements and other factors insofar as they are within the scope of the appended claims or the equivalents thereof.