PLATEN GAP ADJUSTING MECHANISM FOR A PRINTER

Abstract: A platen gap adjusting mechanism includes a platen (1) for supporting a printing paper, a guide shaft (3) mounted in parallel with said platen (1), a carriage (4) composed of a carriage frame (5) and a base plate (7), a front end of the carriage (4) being rotatably mounted on said guide shaft (3), printing head (6) mounted on said carriage (4) for printing character data onto said printing paper, and a guide plate (20) fixedly mounted below a rear portion of said carriage (4) and substantially in parallel with said guide shaft (3) over the whole extent of the movement of said carriage (4). A parallel link mechanism is provided on the undersurface of the base plate and has a fixed part attached to the base plate, and a slider part (171) engaged with said guide plate (20) by means of a slit provided in the lower portion of said slider part, both parts being connected by means of two parallel flexible plate-shaped portions. An adjusting screw (19) is provided having a tip end thereof in contact with the slider (171) or the guide plate (20) and being screwed into a screw hole (172) from the upper portion of the carriage (4) in order to enable adjustment of the platen gap.

Description: The present invention relates to a printer and more particularly to a mechanism for adjusting the gap (hereafter referred to as the platen gap) between the platen and the printing head. A printer is known wherein a carriage having a printing head mounted thereon is movably mounted on a guide shaft and, with the printing head moved in parallel to a platen, any character may be printed on a sheet of printing paper held against the platen. In a printer of this type it must be ensured that the platen gap is correct in order to assure high quality printing. Accordingly a platen gap adjusting mechanism is provided. An example of such a platen gap adjusting mechanism is described in Japanese Laid-Open Patent Publication No. 58-90975. According to this platen gap adjusting mechanism, an eccentric bush is rotatably fitted in a carriage, which eccentric bush penetrates a guide shaft and, with the eccentric bush, a printing head is moved integrally with the carriage to adjust the platen gap. Another example of a platen gap adjusting mechanism is described in Japanese Laid-Open Utility Model Publication No. 58-175951. With this platen gap adjusting mechanism, one of a number of thrust bearing members corresponding to two guide shafts is shaped eccentrically and is rotatably engaged with the carrier, and the eccentric bearing is arbitrarily rotated to adjust the platen gap. However, such a platen gap adjusting mechanism suffers from several problems: (1) the mechanism is difficult in its adjustment with the eccentric bush; (2) the mechanism is complicated and thus expensive. Furthermore, a serial printer is known and described in Japanese Laid-Open Utility Model Publication No. 58-28851. This serial printer includes a guide rail adapted to have rack teeth formed over the surface of a rack and disposed in parallel with the platen for supporting the printing paper and a printing head integral with a motor, a pinion fixedly mounted on a rotary shaft of the motor, a guide piece formed on the bottom surface of the printing head. The pinion of the motor and the rack of the guide rail are inter-meshed with the pinion of the guide rail so as to hold the guide rail between the pinion and the guide piece. Rotating the motor, the motor and the printing head are integrally moved on the guide rail owing to thrust force caused by engagement between the pinion and the rack, and thereby printing is effected. However, such a printer also suffers from a problem. Namely, the guide rail is held between the pinion fixedly mounted on the rotary shaft of the motor and the guide piece formed on the bottom surface of the printing head, as described above. As a result, the printing head can not be moved perpendicularly to the axial direction of the platen. Therefore, adjustment of the platen gap is very difficult. Reference is also made to US-A-4268177 and EP-A-0038283, both of which describe platen gap adjusting mechanisms for printers in which a carriage rotatably mounted on a guide shaft mounts the printing head and means are provided for rotating the carriage and hence the printing head about the guide shaft to thereby adjust the gap between the printing head and the platen. The present invention seeks to provide a platen gap adjusting mechanism for a printer, which mechanism enables easy adjustment of the platen gap, and is simple and inexpensive. According to the present invention, there is provided a printer including a platen for supporting a printing paper and having a longitudinal axis, a printing head mounted in opposition to said platen so as to form a platen gap therebetween, and means for adjusting the size of said platen gap, said means for adjusting comprising: a guide shaft mounted to extend parallel to said axis of said platen; a carriage supporting said printing head and including a carriage frame mounted on said guide shaft for movement axially therealong and for movement rotatably thereabout and a base plate having a first end integral with said carriage frame and a second end directed away therefrom; a guide plate fixed at a position below said second end of said base plate and extending parallel to said guide shaft; said printer being characterised in that said means for adjusting further comprises: a parallel link mechanism including a fixing member attached to a lower surface of said second end of said base plate and a slider member connected to said fixing member, said slider member having therein a slot within which fits said guide plate, whereby upon movement of said carriage axially of said guide shaft, said parallel link mechanism and said second end of said base plate are guided by said guide plate; adjusting screw means, threaded through a hole in said second end of said base plate and having a tip end in engagement with said slider member, for, upon rotation within said hole, moving said slider member and said second end of said base plate relatively toward or away from each other and thereby moving said carriage and said printing head rotatably about said guide shaft to increase or decrease said platen gap; and said parallel link mechanism including means for, upon said moving said slider member and said second end of said base plate relative to each other, preventing said slider member from becoming inclined relative to said guide plate and thereby maintaining a parallel relation therebetween, said means for preventing comprising flexible means connecting said slider member to said fixing member. In order that the invention may be better understood, several embodiments thereof will now be described by way of example only and with reference to the accompanying drawings in which:- Figures 1, 4, 7 and 10 are respectively perspective views, partly cut out, illustrating an embodiment of a platen gap adjusting mechanism according to the invention; Figures 2, 5, 6, 8, 9 and 11 are side elevational views each illustrating the platen gap adjusting mechanism; Figure 3 is a perspective view illustrating a parallel link member; Figure 12 is an exploded perspective view illustrating an inked ribbon cartridge, pedestal and base plate for mounting the inked ribbon cartridge and the pedestal; and Figure 13 is an exploded perspective view of the inked ribbon cartridge pedestal. As shown in Figure 1, designated at 1 is a platen supporting a printing paper (not shown), both ends of which are rotatably supported on a respective side plate (not shown) of a base plate 2. Designated at 3 is a guide shaft disposed in parallel with the platen 1, both ends of which are fixed on a respective side plate. Designated at 4 is a carriage with a base plate 7 described later and a carriage frame 5 having a cylindrical part rotatably mounted on the guide shaft 3 and movable along the shaft. Designated at 6 is a printing head, which is mounted on the carriage 4 and is adapted to move with respect to the platen 1 with the movement of the carriage 4. Designated at 7 is the base plate which is fixed by means of a carriage frame 5 and a spring 8. The base plate 7 has a connector 9 carried thereon, which connector is connected with a further base plate (not shown) provided on the bottom of the printing head 6 for delivering and receiving a printing head control signal. A motor 10 is fixed on the under surface of the base plate 7. Designated at 11 is the output shaft of the motor 10, 12 is a pinion mounted on the shaft 11, 13 is a slit disk for controlling the rotation of the motor 10, 14 is a plurality of slits formed in the slit disk 13, 15 is a sensor for detecting the slits 14, 16 is a rack fixed at its ends to respective side plates of the base plate 2 and which is engaged with the pinion 12. By means of these members, the carriage 4 moves at a prescribed speed along the guide shaft 3. Namely, energisation of the motor 10 causes the rotary shaft 11 to rotate, and thereby the pinion 12 mounted on the rotary shaft 11 also rotates. Thereupon, since the pinion 12 and the rack 16 are in engagement with each other, the carriage 4 moves at a prescribed speed along the guide shaft 3. A parallel link member 17 is further fixed on the lower surface of the base plate 7 by means of a screw 18. An adjusting screw 19 is provided on the upper surfa ce of the base plate 7, whose tip is brought into contact with the surface 177 of a slider part 171 described later (refer to Figure 3) of the parallel link member 17. The parallel link member 17 is constructed by connecting a slider part 171 having a slit 170 therein engaging with a guide plate 20 described later with a fixed part 173 having a screw hole 172 for the screw 18 by means of two parallel flexible plate-shaped parts 174, 175, and integrally formed by resin moulding. Designated at 176 is a stopper integrally formed with the slider part 171 for restricting the rotation of the carriage 4, and 177 is a surface in contact with the adjusting screw 19. As shown in Figure 1, the guide plate 20 is integrally formed with the base plate 2 or mounted on the base plate 2. The engaging slit 170 of the slider part 17 is slidably engaged with the guide plate 20. In Figure 2 illustrating a portion of the printer, designated at 21 is a cover for covering therewith the motor 10, slit disk 13, and the sensor 15 of Figure 1, 22 is an arm part provided on the carriage frame 5, and 23 is a roller rotatably fixed on the arm part 22, the roller 23 being held between the pinion 12 and the rack 16. The platen gap adjusting mechanism will now be described with reference to Figure 2. The procedure for reducing the distance between the surface of the platen 1 and the tip end surface of the printing head 6 facing the platen 1 surface, i.e. the platen gap will first be described. In this case, the adjusting screw 19 is so rotated that the tip end thereof goes forward toward the surface 177 of the slider part 171. Hereby, the slider part 171 is pressed by the screw 19, and engaged with the guide plate 20 through the engaging slit 170. Accordingly, the flexible plate-shaped parts 174, 175 between the slider part 171 and the fixed part 173 are deflected whereby the fixed part 173 is lifted above the slider part 171. The flexible plate-shaped parts 174, 175 can be deflected in such a way as to prevent the slider part 171 from being inclined with respect to the guide plate 20, thus keeping a parallel relation therebetween. Hereby, the rear end of the base plate 7 is pushed up and thereby the carriage frame 5 on which the base plate 7 is fixed is rotated around the guide shaft 3 in the direction of arrow a. Since the printing head 6 is fixed on the carriage frame 5, the printing head 6 is also rotated in the direction of arrow a and the tip end surface thereof is brought into contact with the surface of the platen 1. Thus, the platen gap can be reduced. Next, the procedure for increasing the platen gap will be described. In this case, the adjusting screw 19 is rotated in the opposite direction to the case described herebefore, and thereby the tip end thereof is forced to go back toward the base plate 7. Hereupon, the flexible plate-shaped parts 174, 175 return to the original flat state owing to their inherent restoring force. Thus, the rear end of the base plate 7 is lowered, whereby the carriage frame 5 is rotated around the guide shaft 3 in the direction of arrow b with the printing head 6 rotated integrally with the carriage frame 5 and thus the tip end surface is separated from the surface of the platen 1. In such a manner, the platen gap is increased. Since the platen gap can be increased or decreased as described above, the platen gap can be adjusted to the optimum for printing by rotating the adjusting screw 19 in the desired direction. Operation of the platen gap adjustment in the present embodiment is as described above, and thereafter the platen 1 keeps a printing paper (not shown). An inked ribbon cassette is mounted on the base plate 7 and printing is effected by means of the printing head 6. The carriage 4 is, in the above printing operation, moved along the guide shaft 3 owing to thrust force caused by rotation of the pinion 12, in engagement with the rack 16, mounted on the rotary shaft 10 rotated by driving the motor 10, and thereby the slider part 171 of the parallel link member 17 fixed on the base plate is forced to slide on the upper surface of the guide plate 20. Thus, even with the flexible plate-shaped parts 174, 175 of the parallel link member 17 deflected due to adjustment of the platen gap, the slider part 171 is not inclined with respect to the guide plate 20 as described before without caused any biassed abutment, and thus the carriage 4 is able to move smoothly. Now another platen gap adjusting mechanism will be described. Illustration of Figure 4 is the same as that of Figure 1 exclusive of the rear end of the base plate 7. The platen gap adjusting mechanism will be described with reference to Figures 5 and 6. Designated at 200 is an adjusting screw, the tip end of which is in contact with the guide plate 20 and is slidable thereon. First, with the platen gap as shown in Figure 5, the adjusting screw 200 is so rotated that the tip end thereof is moved toward the guide plate 20. Hereupon, since the tip end of the adjusting screw 200 is in contact with the guide plate 20, the rear part of the base plate 7 is lowered with rotation of the adjusting screw 200. Hereby, the carriage frame 5 on which the base plate 7 is fixed is rotated around the guide shaft 3 in the direction shown by the arrow in Figure 6. The printing head 6 is, since fixed on the carriage frame 5, also rotated in the direction of the arrow, and thereby the tip end surface thereof is moved away from the surface of the platen 1. Accordingly, the platen gap is increased as shown in Figure 6. Hereupon, t₁>t is assumed. Rotated the adjusting screw 200 in the opposite direction to the above description, the tip end surface of the printing head 6 is brought closer to the platen 1 surface to reduce the platen gap. Next, another platen gap adjusting mechanism will be described. Illustration of Figure 7 is the same as that of Figure 1 excepting the rear end of the base plate 7. The platen gap adjusting mechanism in the present case will be described with reference to Figures 8 and 9. Designated at 300 is an adjusting screw, with the upper surface of which a flat mounting shaft 301 is integrally formed, to which shaft 301 a gear 302 is fitted movably up and down. Designated at 303 is a support shaft provided on the base plate 7, 304 is a gap changeover lever rotatably mounted on the support shaft 303. The changeover lever 304 has a lug on one end thereof, the other end is fan-shaped. Gear teeth engaging with the gear 302 are provided in the fan-shaped position of the lever 304. Designated at 305 is a slider having a channel-shaped cross section, a lower part of which slider 305 is located on the lower surface side of the guide plate 20, and on an upper part of which slider 305 a post 306 is formed toward the lower part of the slider and fitted in a hole provided in the base plate 7. Designated at 307 is a leaf spring, and the gear 302 is energised upward so as to be engaged with the gap changeover lever 304 by an arm part formed on the central part of the leaf spring 304. One end of the leaf spring 307 engages the underside of the upper part of the slider 305 and pushes the slider 305 upward. Hereby, the lower part of the slider 305 is energised with the lower surface of the guide plate 20 to prevent the rear part of the carriage 4 from being lifted up during movement of the carriage 4. Next, operation of the arrangement of the platen gap adjusting mechanism described above will be described with reference to Figures 8 and 9. Here, designated at 308 is a press member for preventing the gear 302 from coming out from the mounting shaft 301 while fixing the leaf spring 307 on the base plate 7. First, as shown in Figure 8, the gear 302 fitted movably up and down in the mounting shaft 301 integral with the adjusting screw 300 is energised upward by the leaf spring 307 and engaged with the gap changeover lever 304. Hereupon, pushing down the gear 302 from this state against the force of the leaf spring 307, the gear 302 is released from engagement with the gap changeover lever 304 (refer to Figure 9). Thereafter, by rotating the gear 302 clockwise or counterclockwise by means of fingers, etc., the adjusting screw 300 may be rotated integrally with the gear 302, and thereby the rear part of the base plate 7 is moved upward or downward. Accordingly, the carriage 4 is rotated clockwise or counterclockwise together with the printing head 6 around the guide shaft 3, and thereby the platen gap is changed and thus adjusted to a desired value. In such a manner, an initial setting of the interval of the platen gap is effected, which interval has been predetermined for use as a reference. Then, letting go operator's hold of the gear 302, the gear 302 is pushed up by the restoring force of the leaf spring 307 and again engaged with the gap changeover lever 304. Thereafter, in adjusting the platen gap, holding the lug on one end of the gap changeover lever 304 between the operator's fingers and turning it in the desired direction, the adjusting screw 300 is rotated via the gear 302, and thereby the carriage 4 is rotated around the guide shaft 3 together with the printing head 6. Thus, a proper adjustment of the platen gap can be assured in accordance with the thickness of the printing paper to be used. Moreover, it is likely in adjusting the platen gap, that the pinion 12 strikes the rack 16. It may be considered in that case as a countermeasure to previously provide proper backlash between the rack 16 and the pinion 12, or to allow the rack 16 to have a certain measure of flexibility. With this countermeasure, the amount of rotation of the carriage 4 in adjusting the platen gap can be absorbed without any trouble in practical use. Successively, another platen gap adjusting mechanism will be described. Illustration of Figure 10 is the same as that of Figure 1 excepting the rear end of the base plate 7. Figure 11 is also the same as that of Figure 2 excepting the rear end of the base plate 7. Moreover, the parallel link member 17 of Figure 10 is the same as that of Figure 1. In the following, the rear end of the base plate 7 shown in Figures 10 and 11 will be described. In Figure 10, an inked ribbon cartridge pedestal 410 is mounted on the upper part of the base plate 7 as shown in Figure 12, and an inked ribbon cartridge 400 is further mounted on the pedestal 410. Designated at 413, 414 and 415 are pawls for fixing the cover 21 on the base plate 7, 415 is a hole for mounting a screw q, 417 and 418 are holes for mounting a connector 9, 419 is a hole for mounting a screw 18, and 420 and 421 are holes for mounting the inked ribbon cartridge pedestal 410, a projection 422 of the inked ribbon cartridge pedestal 410 being inserted into the hole 420. In addition, the projection (not shown) is inserted also into the hole 421. Designated at 423 is a gear mounted on a shaft 11 of the motor 10, 424 is an adjusting screw, 425 is a gear, and 426 is a coil spring. Here, the adjusting screw 424 is screwed into a screw hole provided in the base plate 7, and the tip end of the adjusting screw 424 makes contact with the surface 177 of the slider part 171 of the parallel link member 17 in the same way as the adjusting screw 19 of Figure 1. Accordingly, the adjusting screw 424 serves in the same way as the adjusting screw 19 with respect to adjustment of the platen gap. A gear 425 is mounted on the adjusting screw 424 and is movable up and down. A coil spring 426 provided between the base plate 7 and the gear 425 acts to push the gear 425 which is limited in its upper position by the adjusting screw 424. Referring to Figure 13 illustrating the inked ribbon cartridge pedestal 410 in detail, the inked ribbon cartridge pedestal 410 comprises upper and lower pedestals 411 and 412. The lower pedestal 412 has a shaft 428 provided thereon for mounting a gap changeover lever 427 around which shaft 428 the gap changeover lever 427 is made rotatable. The tip end of the gap changeover lever 427 is fan-shaped and has teeth formed therein. The teeth are engaged with the gear 425 in a state where the inked ribbon cartridges pedestal 410 is mounted on the base plate 7. A projection 430 is provided on the upper pedestal 411, which has a concave position thereon adapted to fit the lug 429 of the gap changeover lever 427. The method of adjustment of the platen gap will be described. First, the gear 425 is fitted movably up and down to the adjusting screw 424 is energised upwards by the coil spring 426 and thereby engaged with the gap changeover lever 427. Thereupon, the gear 425 is forced down against the force of the coil spring 426, whereby the gear 425 is released from engagement with the gap changeover lever 427. Thereafter, turning the adjusting screw 424 clockwise or counterclockwise, the rear part of the base plate 7 is moved vertically. Consequently, the carriage 4 is rotated clockwise or counterclockwise around the guide shaft 3 together with the printing head 6, whereby the platen gap is altered to a desired adjustment. Namely, initial setting of the amount of the platen gap is effected, which provides a predetermined reference. This is conducted after allowing lug 429 of the gap changeover lever 427 to come on the GAP1 side of Figure 12. Here, the gear 425 is returned to the original state (i.e. that state in which the gear 425 engaged with the gap changeover lever 427). Hereby, a prescribed adjustment of the platen gap can be achieved. Moreover, provided that the amount of the platen gap is needed to be adjusted, the lug 429 of the gap changeover lever 427 is brought to the GAP2 side. Thereupon, the adjusting screw 424 is rotated via the gear 425 whereby the carriage 4 is rotated around the guide shaft together with the printing head 6 so that a proper adjustment of the platen gap can be obtained in response to the thickness of the printing paper to be used. Although in the above description, the gap changeover lever 427 is assumed to take two fixed states, any number of such fixed states may be allowed. Some embodiments of the platen gap adjusting mechanism have been described wherein a motor is carried on the carriage for moving the carriage, and the pinion mounted on the output shaft of the motor is engaged with a rack provided on the guide shaft in parallel thereto. It is however a matter of course that the present invention is not limited to such an arrangement. Namely, fixing the carriage on a wire or a belt without carrying the motor on the carriage and moving the wire of belt by means of a motor may allow the same platen gap adjustment.

Claims: A printer including a platen (1) for supporting a printing paper and having a longitudinal axis, a printing head (6) mounted in opposition to said platen so as to form a platen gap therebetween, and means for adjusting the size of said platen gap, said means for adjusting comprising: a guide shaft (3) mounted to extend parallel to said axis of said platen; a carriage (4) supporting said printing head (6) and including a carriage frame (5) mounted on said guide shaft (3) for movement axially therealong and for movement rotatably thereabout and a base plate (7) having a first end integral with said carriage frame and a second end directed away therefrom; a guide plate (20) fixed at a position below said second end of said base plate (7) and extending parallel to said guide shaft (3); said printer being characterised in that said means for adjusting further comprises: a parallel link mechanism (17) including a fixing member (173) attached to a lower surface of said second end of said base plate (7) and a slider member (171) connected to said fixing member, said slider member having therein a slot (170) within which fits said guide plate (20), whereby upon movement of said carriage axially of said guide shaft, said parallel link mechanism and said second end of said base plate are guided by said guide plate (20); adjusting screw means (19), threaded through a hole in said second end of said base plate and having a tip end in engagement with said slider member (171), for, upon rotation within said hole, moving said slider member and said second end of said base plate relatively toward or away from each other and thereby moving said carriage and said printing head rotatably about said guide shaft to increase or decrease said platen gap; and said parallel link mechanism (17) including means for, upon said moving said slider member (171) and said second end of said base plate (7) relative to each other, preventing said slider member (171) from becoming inclined relative to said guide plate (20) and thereby maintaining a parallel relation therebetween, said means for preventing comprising flexible means(174,175) connecting said slider member to said fixing member. A printer as claimed in Claim 1, wherein said flexible means (174,175) comprises at least one deflectable plate-shaped member connecting said slider member to said fixing member. A printer as claimed in Claim 2, wherein said flexible means (174,175) comprises two parallel plate-shaped members. A printer as claimed in Claim 2, wherein said fixing member (173), said slider member (171) and said at least one plate-shaped member are integrally formed of molded resin. A printer as claimed in Claim 1, further comprising a gear (425) mounted on said adjusting screw means (19) for axial movement relative thereto, a changeover lever (427) mounted on said carriage (4) for rotation about a pivot axis and having an end with gear teeth for meshing with said gear, and spring means (426) for urging said gear axially of said adjusting screw means to a position relative thereto whereat said gear meshes with said gear teeth, whereat rotation of said changeover lever about said pivot axis rotates said gear and said adjusting screw means and thereby adjusts said platen gap. A printer as claimed in Claim 5, wherein said carriage (4) further includes a pedestal (410) mounted on said base plate (7), said pedestal having a shaft defining said pivot axis about which rotates said changeover lever (427), and an ink ribbon cartridge (400) mounted on said pedestal.

Inventors: HAYASHI, KUNIHARA OKI ELECTRIC INDUSTRY CO., LTD., ITAYA, TAKASHI OKI ELECTRIC INDUSTRY CO., LTD., KIKUCHI, HIROSHI OKI ELECTRIC INDUSTRY CO., LTD., NAKANO, TOSHIHIRO OKI ELECTRIC INDUSTRY CO., LTD., OTA, YUKIO OKI ELECTRIC INDUSTRY CO., LTD., WATANABE, SHYOICHI OKI ELECTRIC INDUSTRY CO., LTD.
Applicants: OKI ELECTRIC INDUSTRY COMPANY, LIMITED
Application year: 1986
Main classification: B41J25
IPC codes: B41J25