RIBBON FEED MECHANISM

Abstract: The present invention relates to a ribbon feed mechanism for use in a heat transfer printer. The ribbon feed mechanism includes first, second, and third ribbon feed components (53, 55, 54) fitted on a shaft (60), a friction plate (56) mounted between the first and second ribbon feed components (53, 55), a spring (57) mounted between the second and third ribbon feed components (55, 54), a cylinder which is in contact with the third ribbon feed component (54) at one end thereof, and a ribbon feed presser (65) for pressing the other end of the cylinder (66) toward the third ribbon feed component (54). The rotational torque is always kept constant, and therefore no unmatching of colors occurs when the mechanism is applied to a printer.

Description: The present invention relates to a ribbon feed mechanism for use in a printer. In recent years, heat transfer colour printers have been used as computer aided design terminals or in computer graphics or video tap recording. Fig. 1 is a perspective view of a known beat transfer colour printer. The heat transfer colour printer includes: a lower cabinet 1; a circuit section 2; a power source panel 3; a pinch roller 4; a capstan roller 5 with which the pinch roller 4 makes contact to drive it; and a platen roller 6. The platen roller 6, the pinch roller 4, and the capstan roller 5 together form a paper feed mechanism. The pinch roller 4 and the platen roller 6 are driven by a main motor 7. The printer also includes: a pinch roller lever 8 for contacting the pinch roller 4 with the capstan roller 5 and separating the pinch roller 4 from the capstan roller 5 in synchronization with the movement of a cam; a peel-off roller 9 for separating an ink film from a sheet of paper; a paper feed roller 10 for contacting the sheet of paper with the platen roller 6; a paper guide 11 for guiding the sheet of paper; a paper support 12 for housing a roll of paper; a cover frame 13 which is opened and closed by the disengagement and engagement respectively of a locking lever 14 with a locking lever shaft 15, the cover frame 13 having a ribbon feed gear (not shown) for positioning one end of a ribbon feed which comprise a roll of ink film and a ribbon feed presser 17 for pressing the opposing end of the ribbon feed; a printing head bolder 18; a printing head 19 fixed to the printing head holder 18; a head arm 20 which is moved in synchronization with the cam and to which the printing head holder is fixed; a cooling fan 21 for radiating heat from the main motor 7 and the circuit section 2; an operation panel 22; a top cover 23; and an upper cabinet 24. In such a heat transfer colour printer, if a ribbon-shaped ink film wound in a roll is to be fed, it is necessary that a feed mechanism for the ink film includes a slipping mechanism whose operation is in some way related to the diameter of the roll or the feed speed thereof. A known ribbon feed structure having a slipping function will be described below with reference to Fig. 2. Fig. 2 is a schematic cross-sectional view of a known ribbon feed mechanism of a thermal type printer. This thermal printer is of a heat transfer type which employs an ink film 32. A sheet of recording paper 30 from a recording paper roll 29 is passed between a capstan roller 25 and a pinch roller 26. The sheet of recording paper 30 is held between these two rollers and is conveyed towards a platen roller 27, where it is brought into contact with the ink film 32. At this point, the sheet of recording paper 30 and the ink film 32 are compressed between a thermal head 28 and the platen roller 27. The thermal head 28 serves to convert electrical signals to thermal signals, and these thermal signals are used to effect recording of characters or images on a sheet of heat-sensitive paper (not shown). Alternatively the thermal signals are used to heat transfer an ink of the ink film 32 onto the sheet of paper 30, so that the characters or images are sequentially recorded on the sheet of recording paper 30 in accordance with the electrical signals. The sheet of recording paper 30 is then cut into predetermined lengths by a cutter 31. During this process, the ink film 32 is rolled out from an ink film roll 33 and is conveyed by a supply feed mechanism 34 and winding feed mechanism 35. The supply feed mechanism 34 and the winding feed mechanism 35 of the ink film in general are provided with a slipping mechanism because the sheet of recording paper 30 and the ink film 32 are often conveyed by the same motor (not shown) and the diameter of the ink film roll 33 varies in accordance with the quantity of ink film which has been fed. Fig. 3 shows an sample of a known slipping mechanism. A friction plate 43 is fixed to a first ribbon feed component 40. A second ribbon feed component 41 is mounted adjacent the friction plate 43 in such a manner that it makes contact with the friction plate 43. A third ribbon feed component 42 is placed on the second ribbon feed component 41 through the intermediary of a spring 44 in such a manner that the third ribbon feed component 42 is rotationally fixed to the second ribbon feed component 41 and is secured on a shaft 46 by a C ring 45. More specifically, the first ribbon feed component 40 and the second and third ribbon feed components 41 and 42 are fixed on the shaft 46 in that order with the friction plate 43 being interposed between the first ribbon feed component 40 and the second ribbon feed component 41. A sliding friction generated between the second ribbon feed component 41 and the friction plate 43 is determined by the axial force imparted by the spring 44. In this case, a paper reel 47 mounted on the third ribbon feed component 42 is indirectly driven by the first ribbon feed component 40 which serves as a driving source. An ink film is wound around the paper reel 47. In this known structure, the torque that rotates the paper reel 47 is determined by the condition of surface contact between contacting surfaces of the friction plate, 43 and the second ribbon feed component 41 and the axial force of the spring 44. However, there is a noticeable difference between static friction generated before the sliding starts is started and dynamic friction generated after the sliding has started. In consequence, the rotational torque is not constant and is therefore unstable. Thus, the known structure does not ensure a constant rotational torque, and it is therefore difficult to feed the ink film 32 at the same speed at which the sheet of recording paper 30 is fed. In particular, when this structure is applied to a colour printer which requires matching of three primary colours of yellow, magenta and cyan with a high degree of accuracy, unmatching of these colours occurs. In view of these problems an object of the present invention is to provide a ribbon feed mechanism having a constant torque mechanism which has a simple structure and which ensures stable rotational torque. It is known to provide a ribbon feed mechanism comprising first, second and third ribbon feed components fitted on a shaft and capable of being rotated in a rotating direction, a friction plate disposed between said first and second ribbon feed components, a mechanical biasing means disposed between said second and third ribbon feed components, a cylinder having a first end in contact with said third ribbon feed component, and a ribbon feed presser contacting another end of the cylinder for pressing the cylinder towards the third ribbon feed component. The present invention is characterised in that it provides a ribbon feed mechanism wherein the second and third ribbon feed components are mechanically coupled by the mechanical biasing means which acts between said components in the rotating direction, said second and third ribbon feed components each having an inclined toothed engaging surface which under the action of said mechanical biasing means are pressed into partial contact with each other such that a force exerted by the mechanical biasing means between said components in the rotating direction acts through inclined surfaces of contact between the inclined toothed engaging surfaces to create an axial force, that produces a friction force between said second ribbon feed component and said friction plate, and when a driving torque applied to the ribbon feed mechanism exceeds said friction force it causes the second ribbon feed component to move slightly in the rotating direction in advance of the third ribbon feed component thereby creating a gap between the inclined surface of contact between the inclined toothed engaging surfaces resulting in a reduction of the magnitude of the friction force between the second ribbon feed component and the friction plate. The foregoing and further features of the present invention will be more readily understood from the following description of preferred embodiments, by way of example, by referance to the accompanying drawings, of which:- Fig. 1 is a perspective view of a known heat transfer colour printer; Fig. 2 is a schematic view of a thermal type printer; Fig. 3 is a cross-sectional view of part of a known slipping mechanism; Fig. 4a illustrates the principle on which the present invention is based; Figs. 4b and 4c illustrate the relationship between the friction generated and the distance moved; Fig. 5 is a cross-sectional view of one embodiment of the present invention; Fig. 6a is an exploded perspective view of the essential part of Fig. 5; Fig. 6b is an enlarged view of a ribbon feed component; Fig. 7 is a side elevational view of a ribbon feed mechanism, showing another embodiment of the present invention. Fig. 4a illustrates the operational principle on which the present invention is based, and Figs. 4b and 4c show the relationship between friction and distance moved. The principle of the operation will be first described with reference to Fig 4a. A driving member 49 is fitted in a U-shaped base 48. A sliding member 50 is provided within the base 48 in contact with a friction plate 51 fixed to the base 48. The sliding member 50 has a toothed surface which engages with a toothed surface of the driving member 49. A spring 52 is provided between the driving member 49 and the sliding member 50 so as to urge the sliding member 50 by a given force in the sliding direction of the driving member 49. The toothed contact surfaces therefore receive the vertical force which acts there upon in the manner indicated by the arrows, so that a friction force is generated between the friction plate 51 and the sliding member 50. When the driving member 49 is to be moved in the direction indicated by the arrow shown at the left of Fig. 4a from its stationary state relative to the base 48, a friction force shown by the solid line in Fig 4b is generated between the sliding member 50 and the friction plate 51 when a force indicated by the broken line in fig. 4b acting in the intended direction of movement is applied to the driving member 49. The force applied to the driving member 49 is gradually increased. As it exceeds the compression force of the spring 52, the spring 52 contracts instantaneously. At this point, since there is a gap A between the toothed engaging surfaces, the driving portion 49 moves slightly leftward, i.e., in the direction which ensures that a gap is formed between the toothed contact surface B, as shown in Fig. 4a. As a result, the contact force between the toothed contact surfaces in the manner indicated by the arrows in Fig. 4a decreases. Thereafter, the friction force between the sliding member and friction plate decreases and the spring 52 restores its force. As it restores a certain force, a friction force is restored. This process is repeated in a short period of time, so that the friction force remains constant to all intents and purposes. A point C in Fig. 4c at which the force applied to the driving member 49 is identical to the friction force corresponds to a point D in Fig. 4c at which the sliding member 50 starts to move. Alter this point, if a constant force is applied to the driving member 49, the sliding member 50 will continue to move by the same distance as that of the driving member 49. This is because, if the driving member 49 moves in the direction in which a gap is formed between the toothed contact surfaces B of Fig 4a, the sliding member 50 immediately follows the movement of the driving member and moves in the direction in which that gap is reduced. The difference between the static friction and the dynamic friction is reduced by the force acting in such a manner that the friction force is reduced when the sliding member 50 starts to move, so as to ensure smooth starting of the movement. When this principle is used in a rotating device, a constant torque can be set by the spring 52 and the angle of the inclination of the toothed contact surfaces B, thereby providing stable slipping torque. The relationship between the above-described principle and one embodiment of this invention will be described below with reference to Fig. 5 which is a cross-sectional view of a ribbon feed mechanism and Figs. 6a and 6b which are exploded perspective views of a fixed torque device. The base 48, the driving member 49, the sliding member 50, the friction plate 51, and the spring 52 of Fig. 4a correspond to a first ribbon feed component 52, a third ribbon feed component 54, a second ribbon feed component 55, a friction plate 56 and a spring 57 of Figs. 5 and 6, respectively. The toothed contact surfaces of Fig. 4a corresponds to toothed surfaces 58 and 59 of Fig. 6a. Fig. 6b is an enlarged view of the third ribbon feed component 54. As is seen in Fig. 5, the first ribbon feed component 53 is fitted on the shaft 60, to which the friction plate (having a doughnut-like shape) is fixed. Next, the second ribbon feed component 55 is fitted on the shaft 60 in a state where its toothed surface 59 is directed upwards so that the surface thereof which is not toothed makes contact with the friction plate 56. This surface which is in contact with the friction plate determines the friction. Next, the third ribbon feed component 54 is fitted on the shaft 60, its toothed surface 58 directed downwards so that it makes contact with the toothed surface 59. The spring 57 is a coil spring whose two ends are inserted into spring fixing holes 61 and 62 formed in the third and second ribbon feed components 54, 55 respectively as clearly shown in Fig. 6a. These two ends are arranged such that the shaft 60 passes through the centre of the coiled spring 57. Finally, these components are fixed on the shaft 60 by a washer 63 and a C ring 64. A ribbon feed presser 65 is pressed against a spool 66 which is set on the third ribbon feed component 54. The thus-arranged ribbon feed mechanism is operated in the manner described below. When the first ribbon feed component 53 is rotated by a motor, the pressure between opposing contacted inclined surfaces of the toothed contact surfaces (59, 58) of the second and third ribbon feed components 55 and 54 decreases in accordance with the principle described with reference to Figs. 4a, 4b and 4c so that the force which drives the ink film (not shown) provided on the paper reel 66 through the third ribbon feed component 54 acts in such a manner that the friction force between the second ribbon feed component 55 and the friction plate 56 is reduced. When this driving force exceeds the friction force, slipping occurs, and the ink film is fed at a fixed torque. More specifically, when a torque that feeds the ink film becomes larger than the torque set by the ribbon feed mechanism, slipping occurs, and the ink film is not fed. When the first mentioned torque is smaller than the second mentioned torque, the ink film is conveyed at a fixed torque at which the ribbon feed mechanism has been set. This embodiment differs from the device shown in Fig. 4a since the coil spring is arranged in a different manner: the circumferential direction of the spring 57 employed in this embodiment coincides with the rotational direction of the ribbon feed mechanism with both ends of the spring 57 being fitted in the holes 61, 62 formed in the components 54, 55. It therefore acts as an element which transmits the drive force in the rotational direction which corresponds to the sliding direction shown in Fig. 4a, and the spring force thereof can be set by the twisting angle formed between the ribbon feed components 55 and 54. The friction plate 56 of this embodiment may be made of a cork or a cork containing rubber. A friction plate made of cork containing rubber has advantages in that it does not cause problems involving stick-slip and locking that might occur under high humidity. More specifically, since the cork containing rubber has a hardness of 80 to 90 Hs, the coefficient of friction obtained when the pressure is applied is small and its fluctuation is also small, preventing occurrence of stick-slip. The cork containing rubber contains smaller amount of air than the conventional cork, and therefore has a low water absorption. This property of the material enables prevention of locking that might occur when the friction plate 56 absorbs water under high humidity and swells. Thus, it is possible to stabilize the rotational torque of a ribbon feed mechanism with the use of arrangement according to the present embodiment. In particular, it is possible to make the rotational torque constant which might otherwise be large until the mechanism starts to move due to large static friction and become suddenly small owing to sudden reduction in the friction that takes place at the time when the mechanism starts to move. Further, the present embodiment employs a coil spring for pressing the component against the friction plate whose circumferential direction coincides with the rotational direction of the ribbon feed mechanism and whose axis coincides with that of the ribbon feed mechanism. It is therefore possible to adjust the spring force easily by changing the twisting angle of the spring. A second embodiment of the present invention will be now described. If the ribbon feed pressing is not made uniform, an ink film may be wrinkled while it is being wound, even if the constant torque mechanism according to the present invention which constitutes the first embodiment is employed. The present embodiment is directed to obviating this problem. Referring to Fig. 7 which shows the ribbon feed pressing portion of a heat transfer printer, a spool 66 is supported by a ribbon feed gear 67 and a conical or tapered ribbon feed presser 65. Therefore, even if the spool 66 swells by absorbing moisture or by being thermally expanded, backlash is absorbed by the tapered portion of the ribbon feed presser 65. Even if the paper reel 66 is not positioned coaxially with respect to the ribbon feed gear 67 and the tapered ribbon feed presser 65, the end of the spool 66 which is closer to the tapered ribbon feed presser is corrected in position at the most stable portion on the tapered portion of the ribbon feed presser 65 during the feeding of the ink film, thereby enabling the spool 66 to be positioned coaxially with respect to the ribbon feed gear 67 and the ribbon feed presser 65. In this embodiment, the ribbon feed presser 65 is tapered. However, the ribbon feed gear 67 may be tapered in place of the ribbon feed presser 65. Alternatively, both of the ribbon feed presser 65 and the ribbon feed gear 67 may be tapered. Whereas the spool 66 is made of paper in this embodiment, it may be of a plastic or a metal. Thus, it is possible, according to the present embodiment, to prevent occurrence of backlash of ribbon feed and of wrinkling of the ink film which results from the occurrence of backlash so as to provide a heat transfer printer which ensures a high quality of printing. As will be understood from the foregoing description, it is possible to stabilize the rotational torque of the ribbon feed mechanism according to the present invention. In particular, it is possible to make the rotational torque constant which might be otherwise large before the movement of the mechanism is started due to large static friction and which becomes small suddenly after it has been moved due to sudden reduction in the friction.

Claims: A ribbon feed mechanism comprising first, second and third ribbon feed components (53, 55, 54) fitted on a shaft (60) and capable of being rotated in a rotating direction, a friction plate (56) disposed between said first and second ribbon feed components (53, 55), a mechanical biasing means (57) disposed between said second and third ribbon feed components (55, 54), a cylinder (56) having a first end in contact with said third ribbon feed component (54), and a ribbon feed presser (65) contacting another end of the cylinder (66) for pressing the cylinder (66) towards the third ribbon feed component (54), characterised in that the second and third ribbon feed components (55, 54) are mechanically coupled by the mechanical biasing means (57) which acts between said components (55, 54) in the rotating direction, said second and third ribbon feed components (55, 54) each having an inclined toothed engaging surface (59, 58) which under the action of said mechanical biasing means (57) are pressed into partial contact with each other such that a force exerted by the mechanical biasing means (57) between said components (55, 54) in the rotating direction acts through inclined surfaces of contact between the inclined toothed engaging surfaces (59, 58) to create an axial force that produces a friction force between said second ribbon feed (55) and said friction plate (56), and when a driving torque applied to the ribbon feed mechanism exceeds said friction force it causes the second ribbon feed component (55) to move slightly in the rotating direction in advance of the third ribbon feed component (54) thereby creating a gap between the inclined surfaces of contact between the inclined toothed engaging surfaces (59, 58) resulting in a reduction of the magnitude of the friction force between the second ribbon feed component (55) and the friction plate (56). A ribbon feed mechanism as claimed in Claim 1, characterised in that it includes a driving means for applying a driving torque to said ribbon feed mechanism for rotating said first ribbon feed component (53) around said shaft (60). A ribbon feed mechanism according to Claim 1 or 2 characterised in that a part of the ribbon feed presser (65) which contacts an end of the cylinder (66) is tapered. A ribbon feed mechanism according to any preceding claim, characterised in that said friction plate (56) is made of cork containing rubber. A ribbon feed mechanism according to any preceding claim, characterised in that the mechanical biasing means (57) comprises a coil spring having coils which lie in the plane of the rotation of said first, second and third ribbon feed components (53, 55, 54), and the two ends of said coil spring (57) are respectively coupled to said second and third ribbon feed components (55, 54).

Inventors: ITOH, TAICHI, TAKITA, MASAAKI, TANAKA, MASUMI, TSURU, YOSHIKAZU, USHIHARA, MASAHARU
Applicants: MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1988
Main classification: B41J33
IPC codes: B41J17, B41J33