Abstract:
A thermal printer having a reciprocable carriage on which a thermal head is carried. The printer further includes a toothed rack and a driving plate both of which are rotatably held to a shaft. The rack acts to rotate the thermal head toward and away from a recording sheet. The driving plate has a protrusion for rotating the rack in one direction. A pin protruding from the driving plate on opposite side to the rack is fitted in a grooved cam, which serves to rotate the driving plate. A tension spring is mounted between the rack and the driving plate to cause the protrusion of the driving plate to angularly bias the rack in one direction so that the force of the tension spring urges the head toward the recording sheet.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a thermal printer and, more particularly, to a thermal printer having a carriage that is equipped with a thermal head and that is capable of reciprocating along a line to be printed on a recording sheet and is capable of automatically advancing a print tape mounted on the carriage when printing. 
     BACKGROUND OF THE INVENTION 
     A thermal printer has been developed in which print tape having a heat-fusible material is disposed between paper and a thermal head that is equipped with heater elements. As the thermal head is moved, the heater elements are selectively heated to melt the heat-fusible material in the tape. The molten material is then transferred to the paper. This printer has the advantage that during printing operation it generates less noise than other kinds of printers. 
     The conventional thermal printer is shown in FIGS. 9-14, of which FIG. 9 is a plan view of the printer, showing the whole structure of the printer. In this figure, a sheet of paper (not shown) is placed on a platen 1. A rubber member 2 is mounted in front of the platen 1, i.e., at the print position. A paper guide 3 acts to guide the paper wound on the platen 1. A thermal head 4 is disposed opposite to the rubber member 2, and has a plurality of heater elements. The head 4 is mounted on a carriage 5. Print tape 6 has a heat-fusible material that is to be transferred to the paper. The tape 6 is provided in a tape cassette 7, which is detachably mounted to the carriage 5. The tape 6 is guided between the thermal head 4 and the paper and it is automatically advanced during printing. 
     The carriage 5 is movably mounted on a carriage guide plate 8. Referring also to FIG. 10(a), the plate 8 is rotatably supported at locations 9. As shown in FIGS. 10(a) and (b), a carriage guide shaft 10 (two shown) is firmly secured to the carriage 5 and is guided by a groove 11 in the plate 8. The plate 8, the guide shaft 10, and the groove 11 constitute a carriage guide mechanism that guides the carriage 5 along the front surface of the platen 1. A compression spring 12 urges the carriage 5 on the carriage guide plate 8, and hence the thermal head 4, toward the rubber member 2 so that the thermal head 4 presses against the print tape and paper during printing. 
     Referring to FIG. 9, a wire 13 has its ends connected to opposite ends of the carriage 5. The wire 13 is wound on pulleys 14 and 15 that are disposed on one side of the carriage guide plate 8. The wire 13 is also wound on a driving pulley 16 having gears, for example, at its sides. The wire 13, the pulleys 14, 15, and the driving pulley 16 constitute a carriage-moving means that moves the carriage 5 along the platen 1. The paper is pressed by a paper feed roller 17 which is secured to a paper feed shaft 18. The roller 17 and the shaft 18 constitute a paper feed means that transports the paper in the direction indicated by the arrow A in FIG. 9. 
     Referring still to FIG. 9, a stepper motor 19 has a motor gear 20 mounted on its output shaft. An idle gear 21 which is in mesh with the gear 20 is in mesh with the gear on one side of the driving pulley 16. A first intermittent gear 22 is in mesh with the gear on the other side of the pulley 16. A second intermittent gear 23 is in mesh with the first intermittent gear 22. A paper feed gear 24 engages with the second intermittent gear 23. A movable contact is mounted to a mount 25. A ratchet 26 is in mesh with the paper feed gear 24. Another ratchet 27 can come into and out of engagement with the ratchet 26. A ratchet spring 28 urges the ratchet 27 into engagement with the ratchet 26. One end of the spring 28 is retained by a washer 29. A knob 30 that is manually operated is provided to move the ratchet 27 away from the ratchet 26. The knob 30 has a gear on its periphery, the gear being capable of engaging with a gear formed on the ratchet 27. The knob 30 is rotatably held to a lever 31. 
     The aforementioned motor gear 20, idle gear 21, driving pulley 16, first intermittent gear 22, second intermittent gear 23, and paper feed gear 24 constitute a gearing which operates the carriage-moving means and the paper feed means in an interlocked relation. That is, this gearing reciprocates the carriage 5, and moves the paper a certain amount in the directions indicated by the arrow B in FIG. 9 whenever the carriage 5 makes one reciprocation. The aforementioned ratchets 26, 27, and the knob 30 constitute a manual paper feed mechanism that permits one to manually move the paper backward, i.e., in the direction indicated by the arrow C in FIG. 9. 
     As shown in FIGS. 9 and 12, a driving gear 32 is mounted on the shaft extending from the gear 20 on the motor 19. This gear 32 is coupled to a contact gear 34 via an idler 33. The contact gear 34 is composed of a fixed gear 34a and an abutment gear 34b. The fixed gear 34a is in mesh with the driving gear 32. The abutment gear 34b is urged into abutment with the fixed gear 34a by a spring 35. The contact gear 34 is in mesh with a rack member 36 disposed opposite to the gear 34. The rack member 36 consists of two rows of teeth, one of which has an incomplete tooth portion 36a. This tooth portion 36a is missing teeth at its periphery, and is in mesh with the fixed gear 34a. The other row of teeth is a complete tooth portion 36b that is in mesh with the abutment gear 34b. The driving gear 32, the contact gear 34, the rack member 36, and other components constitute a cam operation means. A T-shaped protrusion 37 formed on the rack member 36 is reciprocable in a space 38d formed in a cam 38, which is composed of a lower portion 38a, a higher portion 38b, and an inclined portion 38c formed between them as shown in FIG. 11. 
     The cam 38 abuts on the shaft portion 40 of a receiving portion 39 extending from the support portion 9 of the carriage guide plate 8, as shown in FIGS. 10 and 11. Accordingly, when the pin 40 protruding from the receiving portion 39 rides on the lower portion 38a of the cam 38 as shown in FIG. 11(a), the thermal head 4 presses down toward contact with the platen 1. When the pin 40 rides on the higher portion 38b of the cam 38 as shown in FIG. 11(b), the thermal head 4 is urged away from the platen 1 against the action of the compression spring 12. Under this condition, the carriage 5 is moved, i.e., returned, by the aforementioned wire 13. 
     The driving gear 32 of the cam operation means is always driven by the motor 19. The stroke that the cam 38 or the rack member 36 travels is made constant by a stopper 41. Therefore, the rack member 36 is designed to consist of the two rows, i.e., the incomplete tooth portion 36a and the complete tooth portion 36b. The fixed gear 34a of the contact gear 34 is in mesh with the incomplete tooth portion 36a. It is designed so that when the pin 40 is placed at any arbitrary position on the cam 38, i.e., when the contact gear 34 is placed at either end of the rack member 36, the driving force of the motor 19 is not directly transferred to either the rack member 36 or the cam 38, the fixed gear 34a is not in mesh with the rack member 36, and the abutment gear 34b is caused to run idle. 
     Further, in order to prevent the components from being adversely affected by the rapid change in the speed of the motor 19 as it is reversed, the protrusion 37 on the rack member 36 is situated in the space 38d in the cam 38, and a clearance is formed between the rack member 36 and the cam 38. As shown in FIG. 13, the wire 13 engages the carriage 5 in the manner described below. A clearance D is formed between an enlarged portion 13a formed on the wire 13 and a frame 5a that is formed on the carriage 5. Thus, the carriage 5 is not allowed to move until the platen 1 and the thermal head 4 completely assume their other arbitary states. 
     The mechanism for winding the print tape 6 is now described by referring to FIGS. 14 and 15. As shown in FIGS. 10(a) and 14, a winding rack 42 is mounted below the rubber member 2 and extends along the whole length of the region in which the carriage 5 can move. A winding gear 43 which can come into mesh with the winding rack 42 is mounted in the carriage 5. The gear 43 is connected to a winding bobbin unit 47 via a first intermediate gear 44, a second intermediate gear 45, and a third intermediate gear 46. The winding gear 43 can move slightly from the center of rotation of the first intermediate gear 44 toward the rack 42. A spring member 48 resiliently urges the winding gear 43 toward the rack 42. The gear 43 is made movable as described above to prevent the addendums of the rack 42 and of the gear 43 from becoming damaged when the gear 43 engages the rack 42. That is, the addendums of the gear 43 cease to be in contact with the rack 42 immediately after the gear 43 comes into mesh with the rack 42. 
     FIG. 15 is a front elevation of the aforementioned winding bobbin unit 47. As can be seen from this figure, a compression spring 51 is mounted between a winding bobbin 50 and a unit gear 49 that comes into mesh with the third intermediate gear 46. The gear 49 is pressed against a friction member 52 on a bobbin pulley 53 by the resilience of the spring 51, the friction member 52 being made of felt. The frictional resistance produced in this way permits the rotating force of the gear 49 to be transmitted to the bobbin pulley 53 to thereby rotate the bobbin 50. When the load applied to the winding bobbin unit 47 exceeds a certain value, the pulley 53 slips on the unit gear 49, so that the winding of the print tape is terminated. 
     In the conventional thermal printer constructed as described above, the winding rack 42 is fixed on the side of the platen 1 as shown in FIG. 10(a), and the carriage 5 that supports the winding gear 43 is rotated. Thus, the gear 43 can come into and out of mesh with the teeth of the rack 42. Since the angular range through which the thermal head can rotate relative to the platen must be large enough to afford a sufficient clearance, the angular range through which the carriage 5 must move is also large. Because of the required clearance, the module for the winding gear 43 or other gear cannot be made very large. Thus, the gear 43 may fail to come into mesh with the winding rack 42 if the rack 42 is slightly bent. Under this condition, winding of the print tape is not assured. 
     FIG. 16 is a diagram showing the characteristic of the load that is applied to press the thermal head against the platen in the conventional thermal printer. In this diagram, point X indicates the load when the thermal head 4 is away from the platen 1, i.e., the head is up. Distance Y indicates the load when the head 4 just comes into contact with the platen 1. Point Z indicates the load when the head 4 is pressed on the platen 1, i.e., the head is down. Point F indicates the force applied to the platen 1 by the head 4. 
     The mechanism for rotating the carriage 5 in the conventional thermal printer is designed as shown in FIG. 11, and therefore the tensile force of the compression spring 12 presses the head on the platen when the head is down. When the head is up, the spring 12 is stretched further and so the load needed for the stretch is considerably larger than the force applied to press the head on the platen. Consequently, the electric power consumed by the driving motor for stretching the spring is large. 
     SUMMARY OF THE INVENTION 
     It is the object of the present invention to provide a thermal printer which is free of the foregoing difficulties with the prior art and which consumes a smaller amount of electric power. 
     This object is achieved in accordance with the teachings of the invention by a thermal printer comprising: a platen; a thermal head means consisting of either the combination of a thermal head and a carriage or only a thermal head, for example, and capable of reciprocating along the line to be printed, the head means having an engaging portion, the head means being angularly moved to the platen when the line is to be printed, the head means being angularly moved away from the platen such that the engaging portion comes into engagement with a portion of a first driver means when no line is printed; a shaft for rotatably supporting the first driver means and a second driver means, a portion of the first driver means coming into engagement with the engaging portion of the thermal head means to angularly move same, the second driver means having a protrusion for angularly moving the first driver means in one direction, the second driver means further having a cam engagement portion; a cam body connected to the second driver means via the cam engagement means for angularly moving the second driver means; and a tension spring mounted between the first and second driver means in a stretched manner to impart a tensile force to the second driver means in such a way that the protrusion of the second driver means angularly moves the first driver means in one direction. 
     Other objects and features of the invention will appear in the course of description that follows. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of main portions of a thermal printer according to the present invention; 
     FIG. 2 is a side elevation of main portions of the printer shown in FIG. 1, for showing the relation between the winding rack and the winding gear when the head is up; 
     FIG. 3 is a view similar to FIG. 2, but showing the relation when the head is down; 
     FIG. 4 is a side elevation of main portions of the printer shown in FIG. 1, for showing the relation between the winding rack and the carriage when the head is up; 
     FIG. 5 is a view similar to FIG. 4, but showing the relation when the head is down; 
     FIG. 6 is a side elevation of main portions of another thermal printer according to the invention, for showing the relation between the winding rack and the carriage; 
     FIGS. 7 and 8 are diagrams showing the characteristics of loads applied to press the thermal heads of the embodiments of the invention against their platens; 
     FIG. 9 is a plan view showing the whole structure of a conventional thermal printer; 
     FIG. 10(a) is a perspective view of the carriage and its surroundings of the printer shown in FIG. 9; 
     FIG. 10(b) is a cross-sectional view of the carriage and its surroundings of the printer shown in FIG. 9; 
     FIG. 11(a) is a fragmentary side elevation of the printer shown in FIG. 9, for showing the operation for lowering the thermal head; 
     FIG. 11(b) is a fragmentary side elevation of the printer shown in FIG. 9, for showing the operation for elevating the thermal head; 
     FIG. 12 is a plan view partially in cross section of the cam-driving mechanism of the printer shown in FIG. 9; 
     FIG. 13 is a fragmentary bottom view of the printer shown in FIG. 9, for showing portions by means of which the carriage is connected to a wire; 
     FIG. 14 is a schematic representation for illustrating the print tape winding mechanism of the printer shown in FIG. 9; 
     FIG. 15 is a front elevation of the winding bobbin unit of the printer shown in FIG. 9; and 
     FIG. 16 is a diagram showing the characteristic of the load applied by the conventional thermal head. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A thermal printer according to the present invention is shown in FIGS. 1-5. This printer includes a platen 1, a carriage 5, and a shaft 60 extending parallel to the platen 1. The carriage 5 is rotatably held to the shaft 60 so as to be slidable axially on the shaft. As shown in FIGS. 2 and 3, a winding gear 61 is mounted on the bottom of the carriage 5. This gear 61 is splined to a bobbin shaft 62, and is urged downwards at all times by a coiled spring 63. A winding bobbin unit including the bobbin shaft 62 has a slip mechanism that is substantially similar to that of the conventional printer. When a load exceeding a certain value is applied, the bobbin 64 is not allowed to rotate. A hook 65 facing downwards protrudes from the bottom of the carriage 5. (FIGS. 4-6) 
     As shown in FIG. 1, a winding rack 66 having an L-shaped cross section is disposed parallel to the shaft 60. Protruding from both ends of the rack 66 are connector portions 67 (one shown), through which the shaft 60 extends at their both ends. Thus, the rack 66 can rotate about the shaft 60. A number of teeth 66a are formed on the front side of the rack 66 to come into mesh with the winding gear 61 (see FIG. 3). Also, the teeth 66a are capable of engaging the hook 65. A driving plate 68 is rotatably held to the shaft 60 in such a way that it overlaps one connector portion 67 of the winding rack 66. As shown in FIG. 1, a protrusion 69 which engages with the upper end of the connector portion 67 is formed on the end portion of the driving plate 68 which extends toward the winding rack 66. A tension spring 70 is mounted between the upper end of this end of the driving plate 68 and the lower end of the rack 66 in a stretched manner. A pin 71 protrudes from the end portion of the plate 68 which extends on the opposite side to the rack 66. The front end of the pin 71 is fitted in a groove 73 formed in a cam 72. 
     Also shown in FIGS. 2 and 3 are a tape cassette 74 having print tape therein, paper 75 to be printed, a protrusion 76 that performs a pushing operation, and a carriage stopper 77. 
     FIGS. 2 and 4 show the condition in which the head is up, away from the platen. In this state, the pin 71 is at the end of the groove 73 that is closest to the center of rotation of the cam. Accordingly, the other end of the driving plate 68 which is on the side of the winding rack 66 is at a first position which is lower than a second position assumed in the head-down condition shown in FIGS. 3 and 5. Under this head-up condition of FIGS. 2 and 4, the connector portions 67 for the rack 66 are pushed down by the protrusion 69, so that the teeth 66a of the rack 66 face downwards away from the winding gear 61. The front ends of the teeth 66a push the hook 65 on the carriage downwardly, keeping the carriage 5 angled back so that the thermal head 4 is away from the platen 1. The rack 66 disengages from the winding gear 61 in this head-up state. Therefore, when the carriage 5 reciprocates, the bobbin 64 will not turn, and the print tape will not advance. 
     FIGS. 3-5 show the other condition in which the head is down. To move from the head-up state to the head-down state, the cam 72 is rotated in a clockwise direction (as viewed in FIGS. 1-4) the pin 71 moves along the groove 73 in the cam and slowly moves away from the center of rotation of the cam 72. This rotates the driving plate 68 about the shaft 60 in a clockwise direction. The winding rack 66 is urged to rotate in a clockwise direction by being pulled by the tension spring 70. Then, the rack teeth 66a move away from the hook 65 and come into mesh with the teeth of the winding gear 61. Simultaneously, the upper surface of the rack 66 bears against the protrusion 76 formed on the bottom of the carriage 5. 
     As the cam 72 turns further, the driving plate 68 and the winding rack 66 further rotate clockwise, rotating the carriage 5 toward the platen 1. As a result, the thermal head 4 comes into abutting engagement with the rubber member 2 via the paper 75. If on rotation of the rack 66, the rack teeth 66a and the teeth of the winding gear 61 abut with each other without properly coming into mesh with each other, then the rotation of the rack 66 pushes the gear 61 upwardly by compressing the coiled spring 63 until the teeth of the gear 61 can mesh with the rack teeth 66a during the movement of the carriage relative to the rack 66. Then, the spring 63 pushes the gear 61 downwardly, so that the teeth of the gear 61 come into mesh with the teeth 66a. As a result, the gear 61 will assuredly rotate on further movement of the carriage 5. 
     The cam 72 turns further, and the driving plate 68 continues to rotate clockwise. However, the winding rack 66 cannot rotate further because the thermal head 4 bears on the rubber member 2. Accordingly, after the rotation of the rack 66 is stoped by the thermal head 4 bearing on the platen&#39;s rubber members the driving plate 68 continues to rotate and stretches the tension spring 70. This stretching force accumulates and presses the head 4 against the platen, after sufficient rotation of the cam 72. When the pin 71 arrives at the end of the cam groove 73 that lies farthest from the center of rotation, as shown in FIGS. 3-5, a desired force F is applied to the head 4, by the tension spring 70 pressing it against the platen. 
     FIG. 7 is a diagram showing the characteristic load applied to the thermal head of the thermal printer of this example to press the head against the platen when changing from the head-up condition to the head-down condition. As can be seen from this diagram, the load is zero in the head-up condition (point X) until the thermal head 4 just comes into contact with the platen 1 (point Y). Then, as the head is lowered, a load a is applied to stretch the tension spring 70 until the desired force F is obtained (point Z). 
     If the carriage 5 is pulled while pressing the head 4 against the platen in this way, the winding gear 61 rotates on the teeth 66a of the winding rack 66. Then, the print tape in the tape cassette 74 is advanced via the bobbin 64, corresponding to the distance traveled by the carriage 5. 
     In order to restore the printer to the head-up condition shown in FIGS. 2 and 4 from the head-down condition shown in FIGS. 3 and 5, the cam 72 is rotated in the opposite direction, i.e., in a counterclockwise direction. Then, operations opposite to the foregoing are performed. These operations need not be described herein. 
     Referring next to FIG. 6, there is shown another thermal printer according to the invention which is similar to the printer described above except that it further includes an auxiliary spring 78 to rotate the winding rack 66 clockwise about the shaft 60, i.e., in the direction to press the thermal head 4 against the platen. 
     The load characteristic of this modified example of thermal printer is shown in FIG. 8. In this diagram, bent line p indicates the characteristic of the load associated with the tension spring 70, and bent line q indicates the characteristic of load associated with the auxiliary spring 78. Where the auxiliary spring 78 is used as in this modified example, its force adds to the force of the tension spring 70 and the desired force F can be obtained by applying a load a that is substantially half the desired load F, provided that the tensile spring 70 and the auxiliary spring 78 are equal in tensile strength, because both springs 70 and 78 act to press the head against the platen. In this diagram, a indicates the maximum value of the load applied to the cam. 
     Since the novel thermal printer is constructed as described above, almost no load is applied during the head-up condition, unlike the aforementioned conventional printer. Consequently, it consumes less electric power and requires less operating expenses.