Wire-dot impact printer

Abstract: In a printing head of a wire-dot impact printer a single or plural pinting wires (31) are adapted to be selectively driven upon a print signal against a recording paper through an ink ribbon. The printing head has a core block (1) having a projection (2) for restricting oscillations of printing levers (30), said projection being provided at the inside central portion of said core block (1), and a prin­ ting lever driving coil provided on the peripheral edge portion of the core block (1). A nose block (50) provided with a nose (51) for guiding the printing wires (31) and a disk part (52) opposite to the core block (1) is placed on the core block and the printing levers (30) are pivotally supported therebetween.

Description: The present invention relates to a wire-dot impact printer and is particularly concerned with the structure of the printing head of such printer. A wire-dot impact printing head wherein a pulse current is impressed on a driving coil and a printing lever is magnetically attracted to have a printing wire fixed to it projected from a nose of the printing head to print on a recording paper has already been generalized. Further, a multistage printing head with a plurality of head units stacked one upon the other to provide a high print quality by a high number of printing wires, is disclosed in the document JP-A-40745/1989. The well-known printing head of this type must have driving coils disposed on a nose side for projecting printing wires from a nose end and, therefore, the printing wires become unnecessarily long, the mass increases accordingly rendering a high-speed drive difficult, and further the size of the driving coils must be determined for providing a predetermined impact on the printing wires, thus making it difficult to miniaturize the printing head. On the other hand, this type of printing head is constructed such that a shaft or spindle of each printing lever is supported on a dished yoke forming part of a magnetic circuit, and the top limit of the oscillation of the printing levers is determined by a damper on a push plate positioned opposite to the driving coils. Therefore, it is difficult to precisely control a gap between a magnetic pole and the corresponding printing lever so that a dispersion of the strokes among a plurality of levers may occur. Further, the printing head of this type comprises coil springs for biasing an arm part of each printing lever upward, the coil springs being held in corresponding holes of a spring holding member. Therefore, the shape of such spring holding member becomes complicated and if the inner bottom portion is increasingly worn by a long use, the coil springs vary in height among each other thus causing a dispersion in outputs. The document US-A-4,611,393 discloses a wire-dot impact printer whose wire-dot impact printing head comprises a core block having formed on one side thereof a plurality of magnetic poles arrayed in a ring fashion, a nose block provided with a nose for guiding printing wires and a disk part having a mounting face opposite to the core block, a stopper plate arranged between the core block and the disk part of the nose block, a plurality of printing levers pivotally disposed between said core block and said stopper plate, each printing lever having on one side of its pivot an arm part provided with a printing wire and on the opposite side an armature part facing counter to the printing wire and disposed opposite to a respective one of said magentic poles, and springs for biasing the printing levers into a position, where the side of the respective armature part opposite to the magnetic pole abuts against the stopper plate. Thus, in this prior art the rest position of the printing levers is determined by the stopper plate arranged between the core block and the nose block. The document EP-A-0 259 685 discloses a wire-dot impact printer whose wire-dot impact printing head comprises a core block having formed on one side thereof a plurality of magnetic poles arrayed in a ring fashion, a nose block provided with a nose for guiding printing wires and a disk part having a mounting face opposite to the core block, a plurality of printing levers pivotally disposed on the side of the core block opposite to the nose block, each printing lever being pivotally supported at one end and connected to a printing wire at the other end with an armature part being formed adjacent to said first end, a ring-like stopper provided on the side of the printing levers opposite to the core block, a biasing unit for biasing the printing levers toward said stopper, and a yoke plate disposed on said core block and having positioning slits for positioning the printing levers therein. In this prior art the rest position of the printing levers is determined by the ring-like stopper provided on the side of the printing levers opposite to the core block Still further, in the multistage printing head the individual head units are stacked one upon the other with a retainer plate of high rigidity being interposed between adjacent head units. Therefore, the printing wires of the stacked units become longer causing a dispersion in printing operation among the units. The present invention is intended to remedy the above problems and has the object of providing a printer having a wire-dot impact printing head allowing a high-speed printing by shortening the length of the printing wires and minimizing the moment of inertia of moving parts and further allowing a miniaturization of the printing head. Another object of the invention is to provide such printer wherein any dispersion of the printing characteristics among the printing wires is substantially reduced and does not increase even after a long use. Still another object of the invention is to provide a multistage printing head having the difference in the length of of the printing wires between different stages kept as little as possible in order to minimize a dispersion of the printing operation among the individual stages. These objects are achieved with a printer as claimed. Preferred embodiments of the present invention will be explained in detail below with references to the accompanying drawings, wherein: Fig. 1 is a sectional view of a typical single-stage wire-dot impact printing head according to the invention, Fig. 2 is an exploded perspective view of the printing head of Fig. 1, Fig. 3 is an explanatory drawing of a main part representing the rest position of a printing lever, Fig. 4 is an exploded view of a spring unit according to one embodiment of the invention, Fig. 5 is a sectional view of a printing head incorporating the spring unit of Fig. 4, Fig. 6 is a sectional view of a typical multistage wire-dot impact printing head according to the invention, Fig. 7 is a plan view of a core block assembly of the printing head, taken from a nose block side, Fig. 8 is a plan view of the nose block of the printing head, taken from the core block side, Fig. 9 is a sectional view of a multistage printing head according to another embodiment of the invention, Fig. 10 is a plan view showing a main part of the printing head in section, and Fig. 11 is a plan view of a printer with the printing head installed thereon. Fig. 11 shows a schematic plan view of a wire-dot impact printer according to the invention. Desired patterns forming for example characters or the like are printed on a recording medium or paper K disposed between a platen P and an ink ribbon R by a printing head H mounted on a carriage C which is supported shiftably in the direction of print columns or printing lines. Figs. 1 and 2 illustrate a single-stage printing head for which a basic conception of the present invention is employed. The printing head roughly comprises a core block 1, a plurality of printing levers 30 each having a respective printing wire 31 secured to its nose, a dished yoke 15 used as part of a magnetic circuit as will be explained later and serving at the same time to support a spindle 32 of each printing lever 30 at a predetermined height, a disk yoke 20 also forming part of a magnetic circuit and further positioning and supporting said plurality of printing levers 30 radially, a spring unit 40 for biasing an intermediate or arm part 33 of each printing lever 30 and a nose block 50 consisting of a nose part 51 and a disk part 52. The core block 1 is molded of a ferromagnetic material such as silicon steel or the like through a lost wax, metal injection or any other process. A trapezoid projection 2 providing a wide plane is integrally formed at a center portion on the front side of the core block 1 opposite to the nose block 50. This projection 2 serves to restrict the oscillation of the printing levers 30 as will be explained later. A plurality of magnetic pole forming cores 3 are protrusively formed in a ring fashion at regular intervals around the projection 2. An outer peripheral flange 4 is protrusively formed outside of said ring of cores 3. The front side and the back side of the core block 1 are precisely ground through simultaneous double grinding and, thus, the upper surface or end face of the outer peripheral flange 4 on the front side of the core block 1, the end face of each core 3 and the said wide plane of the projection 2 are finished to be accurately in the same plane. A coil bobbin 5 is pressed onto each of the cores 3, and the ends of a corresponding coil wound round each coil bobbin 5 is connected to a circuit board 7 disposed on the back of the core block 1. Positioning pins 11, 11 are pressed into the top of the projection 2 and are used for positioning and fixing a doughnut damper 10 with which the arm parts 33 of the printing levers 30 will come into contact to restrict their stroke. Each printing lever 30 comprises the said arm part 33 with the corresponding printing wire 31 fixed at its nose and an armature part 34 to be attracted by a corresponding core 3. A spindle 32 is mounted on a portion of each printing lever 30 near to its armature part 34. The printing levers are formed thin on one side of the spindle, namely the arm part 33 side, but as thick as possible on the other side of the spindle 32, namely the armature part 34 side. The armature part 34 extend into a direction counter to the printing wire 31 to face the corresponding core 3. The dished yoke 15 is formed of a magnetic material like a doughnut and constitutes a part of a magnetic path along with the disk yoke 20 as mentioned before. The dished yoke 15 is finished by double grinding to have an accurate thickness so as to support the spindles 32 of the printing levers 30 at a predetermined height on the front side of the core block 1. The dished yoke 15 has a plurality of radially extending slits 16 accommodating the printing levers 30 and allowing them to oscillate. The disk yoke 20 has a plurality of radially extending slits 21 of a cross-like shape (see Fig. 2) adapted to the plane shape of the levers 30 including the spindles 32. By these slits 21, the printing levers 30 are positioned and retained opposite to the respective cores 3. The disk yoke is formed somewhat thinner than the outside diameter of the spindles 32 so as to hold the spindles 32 between the dished yoke 15 and a projection 53 of the nose block 50 with the printing levers 30 fitted in the slits 21 (Fig. 3). The dished yoke 15 and the disk yoke 20 are provided with positioning fine holes 17, 22 near the radially outer end or top of the slits 16 and 21, respectively. During the assembly of the printing head the yokes 15, 20 are placed one over the other with each slit 16 being registered with a corresponding slit 21. A dowel 6 protrudent at a front end of each coil bobbin 5 is then fitted into the corresponding positioning fine holes 17, 22, and then the yokes 15, 20 are mounted on the core block 1 with the slits 16, 21 kept in registration with respective cores 3. As mentioned before, the nose block 50 comprises the disk part 52 opposite to the core block 1 and the tubular nose part 51. The disk part 52 has a plurality of said projections 53 forming a ring near the peripheral edge portion of the disk part 52. As mentioned before, the spindles 32 of the printing levers 30 are held between these projections 53 and the dished yoke 15. At the central portion of the disk part 52 a fitting recession 54 for the spring holding member 41 is provided. In this embodiment the spring holding member 41 integrally formed with a guide holder 56 is fitted into the recession 54 from the back side of the disk part 52 toward the nose part 51. In the embodiment the spring holding member 41 has a plurality of spring bearing holes 42 arranged on a circle at positions corresponding to the positions of the arm parts 33 of the printing levers 30. A coil spring 43 is inserted into each of said spring bearing holes 42 to exert a biasing force onto the arm part 33 of the corresponding printing lever 30 urging it into contact with the damper 10. The guide holder 56 has wire guides 57, 59', 59 mounted on its nose and middle portions, operating to guide the printing wires 31 slidably toward a tip of the nose part 51. Fig. 3 shows one printing lever 30, the means for supporting it and the means for electromagnetically driving it, at a rest position, i.e. at the time of non-printing. As shown in Fig. 3, the printing lever 30 is supported at a position of a fixed height a from the front of a core block 1 by the dished yoke 15 which has an accurate thickness. The arm part 33 of the printing lever 30 is urged by the coil spring 43 (not shown in Fig. 3) into contact with the damper 10. In this stage, the printing lever 30 is kept in a precise rest position. This rest position is determined by the core block 1 having its end faces in the same plane as mentioned before, the dished yoke 15 having an accurate thickness, which is positioned thereon and the damper 10. The armature part 34 of the printing lever 30 faces the core 3 through an accurate gap d. If, under this holding or rest state of the printing lever 30, a print signal is generated and a pulse current impressed on the corresponding coil, the armature part 34 is attracted to the core 3 so that the printing lever 30 turns round the spindle 32 by a predetermined angle and projects the printing wire 31 to the lower or front end of the nose part 51 for printing. Fig. 4 shows one preferred example of a spring unit 40 to be installed in the printing head. Fig. 5 shows the printing head with this printing unit 40. The spring unit 40 comprises the tubular spring holding member 41 and a platelike spring bearing seat 46. With a moldable plastic or the like as material, the spring holding member 41 is formed as a cylinder having, a height capable of being fitted into the recession 54 on the front of the disk part 52 of the nose block 50. The spring holding member 41 has a plurality of through holes 42′ for accommodating and holding the coil springs 43 therein. The through holes are provided at regular intervals on a circle concentric to the center of the cylinder. Two fixing dowels 44 for fixing the spring bearing seat 46 and three positioning dowels 45 to be fitted in positioning holes 55 provided in the fitting recession 54 of the disk part 52 are projectingly provided on the fitting side end face at regular intervals each. The spring bearing seat 46 is formed of a material with a high wear resistance such as, for example, carbon steel having its hardness enhanced by quenching and tempering after press working, ceramics, FRP having a high wear resistance or the like as a platelike member whose outer diameter substantially corresponds to that of the spring holding member 41. The spring bearing seat 46 has a plurality of guide holes 47 provided at the center thereof for slidably guiding the printing wires 31. Further, the spring bearing seat 46 has insertion holes 48 for the fixing dowels 44 and notches 49 on the outer peripheral portion, through which the positioning dowels 45 pass. The spring unit 40 constructed as mentioned above is assembled by bringing the spring bearing seat 46 into contact with the end face on the fitting side of the spring holding member 41 at first such that the dowels 44, 45 enter into the insertion holes 48 and notches 49, respectively. Then the protruding end of the dowels 44 is caulked. The assembled spring unit 40 is then fitted into the fitting recession 54 of the disk part 52 with the dowels 45 entering into the positioning holes 55, and firmly fixed. Finally the coil springs 43 are fitted into the through holes 42′ along with a high viscous oily liquid, and then the individual components of the printing head are built up bringing the coil springs 43 into contact with the arm parts 33 of the printing levers 30. The advantage of a spring unit 40 having such a construction is that the spring holding member 41 which is complicated in shape but almost free from load working and the spring bearing seat 46 simple in shape but repeatedly subjected to load working can be molded separately of an easily moldably plastic material and a material with a high wear resistance, respectively. Since a wear of the bottom portion of the spring insertion holes and a wear of the wire guide holes 47 can be prevented, a fluctuation of the load working on the coil springs 43 and an energy loss due to a bend of the printing wires 31 may be prevented. Figs. 6 to 8 show a typical wire-dot impact printing head according to the invention which is constructed as a multistage type. As will be understood from a comparison with the single-stage printing head of Fig. 1, the multistage printing head is different with respect to the back side of the core blocks 1 of each stage. That is, the core block 1 of each stage has a fitting recession 8 at a central portion of its back side, i.e. the central portion of the side facing the core block 1 of the next stage, said fitting recession accommodating the spring holding member of the next stage. Further, surrounding the fitting recession 8 each core block 1 has a plurality of projections 9 acting like the projections 53 of the disk part 52 of the nose block 50 for supporting the spindles 32 of the printing levers 30 of the next stage. The projections 9 are formed at regular intervals in a ring outside of the fitting recession 8. Accordingly, for assembling these head units, first the spring holding member 41 of the first stage is fitted into the fitting recession 54 provided at the central portion of the inner side of the disk part 52 of the nose block 50 as in the case of the single-stage printing head. Next, the disk yoke 20 is placed on the disk part 52, the printing levers 30 are fitted into respective slits 21 of the disk yoke 20 and then the dished yoke 15 is placed thereon. The spindles 32 of the printing levers 30 are held between the projections 53 of the disk part 52 and the dished yoke 15. Then the core block 1 of the first stage is placed on the dished yoke 15. The head units of the second and third stages are built up thereafter according to the same procedure. The embodiment shown in Figs. 9 and 10 is constructed so as to place each core block 1 of a multi-stage printing head into position by means of common rods 58. In this embodiment two diametrically opposite positioning rods 58 are provided on the disk part 52 of the nose block while one circular hole 12 and one or elliptic hole 13 are provided in each core block 1 corresponding to the positioning rods 58. The positioning rods 58 are fitted into the holes 12, 13 in the core blocks 1 and, thus, the printing head is built up accurately as a whole. In the printing head constructed as mentioned above, the printing levers 30 of the first stage are positioned directly on the nose block 50 while each printing lever 30 of the next stages is positioned directly on the core block 1 of the pre-stage. Therefore, the difference in length of the printing wires 31 of each stage is substantially reduced and a dispersion of the printing operation between the individual stages can be minimized.

Claims: A wire-dot impact printer using a wire-dot impact printing head, comprising: a core block (1) having a printing lever oscillation restricting projection (2) formed at the central portion on one side thereof and a plurality of magnetic poles (3) arrayed in a ring fashion to surround said projection (2), a nose block (50) provided with a nose (51) for guiding printing wires (31) and a disk part (52) having a mounting face opposite to the core block (1), a plurality of printing levers (30) pivotally disposed between said core block (1) and said disk part (52), each printing lever (30) having on one side of a pivotal part (32) an arm part (33) provided with a printing wire (31), and on the opposite side an armature part (34) facing counter to the printing wire (31), and disposed opposite to a respective one of said magnetic poles (3), a biasing unit (40) for biasing the printing levers (30) toward said projection (2), and a disk shaped yoke (20), disposed on said core block, having positioning slits (21) for positioning the printing levers (30) therein. The printer as defined in claim 1, wherein the end faces of said magnetic poles (3), a top of said printing lever oscillation restricting projection (2) and an upper surface of an outer peripheral flange (4) are formed to be in the same plane, a dished yoke (15) having a predetermined thickness for determining a working height of the printing levers (30) and a platelike yoke (20) having positioning slits (21) for accommodating the printing levers (30) are disposed between said outer peripheral flange (4) and the mounting face of said disk part (52), and a damper (10) to be contacted by a nose portion of the printing levers (30) is placed on said printing lever oscillation restricting projection (2). The printer as defined in claim 1 or 2, wherein a recession (54) for enclosing said biasing unit (40) is provided at the central portion of the mounting face of said disk part (52) of said nose block (50), and a projection (53) for holding the pivotal part (32) of said printing levers (30) thereon in cooperation with said dished yoke (15) is provided on a peripheral edge portion of the mounting face of the disk part (52). The printer as defined in any of the preceding claims, wherein said printing levers (30) have their arm part (33) formed thin and their armature part (34) formed thick. The printer as defined in any of the preceding claims, wherein said biasing unit (40) comprises a spring holding member having a hole at a central portion through which a pluraltiy of printing wires pass and a plurality of through holes (42′) for enclosing respective coil springs (43) for biasing said printing levers, said through holes (42′) being arranged around said hole, and a platelike spring bearing member (46) formed of a wear resisting material and fixed to one end of said spring holding member (41). The printer as defined in claim 5, wherein said spring holding member (41) is molded from a moldable material. The printer as defined in any claims 5 and 6, wherein a plurality of guide holes (47) for guiding the printing wires is provided centrally of said spring bearing member. The printer as defined in claim 7, wherein said plurality of guide holes (47) is provided in said platelike spring bearing member (46). The printer as defined in any of claims 5 to 8, wherein at least one projection (44) is provided on an end portion of said spring holding member (41), said spring bearing member (46) being placed in position and fixed through said projection (44). The printer as defined in any of the preceding claims, wherein a plurality of said core blocks (1) are stacked one upon the other, a printing lever pivotal part holding projection (9) being provided at the peripheral edge portion on the other side, opposite to said one side of each core block (1), and a plurality of said printing levers being disposed between said core blocks (1) in addition to those disposed between said disk part (52) and a first of said core blocks (1). The printer as defined in claim 10, wherein a recession (54, 8) for enclosing a printing lever biasing unit (40) is provided at the central portion of said opposite side of each core block (1). The printer as defined in claim 10 or 11, wherein at least two positioning rods (58) are provided on a peripheral edge portion of the mounting face of said disk part (52), and a corresponding rod insertion part (12, 13) is provided on the peripheral edge portion of each core block (1), and each core block (1) is placed in position and built up on said disk part (52) through said rod (58).

Inventors: SATO TAKUMI SEIKO EPSON CORPOR, SATO, TAKUMI SEIKO EPSON CORPORATION
Applicants: SEIKO EPSON CORP, SEIKO EPSON CORPORATION
Application year: 1989
Main classification: B41J2
IPC codes: B41J2