Armature of printing head for use in wire printer

Abstract: An armature device of a printing head for use in a wire printer includes a stylus (2) having a free end to hit a recording medium for printing, an armature (1) connected to the stylus (2) for reciprocating the stylus (2) in a longitudinal direction (A, B) thereof, a permanent magnet (4) for generating a magnetic flux, a core (3) for conducting the magnetic flux into the armature (1) to form a magnetic path, a coil winding (5) wound around the core (3) for substantially canceling, when powered, the magnetic flux, and a spring member (6) having a free (62) end supporting the armature (1) for biasing the armature (1) by a spring force in a direction to separate the armature (1) from the core (3). The armature (1) has generally an elongated shape, in an end portion (80) of which a groove (11) is formed to extend in a direction substantially perpendicular to the longitudinal direction of the armature (1). The end portion (80) of the armature (1) has a first portion (19/80) forming one wall of the groove (11) in the longitudinal direction of the armature (1) and a second portion (80/19) forming the other wall of the groove (11) in the longitudinal direction. The one wall (a/b) is higher than the other wall (b/a). The stylus (2) has an end portion (21) opposing to the free end thereof and housed in the groove (11), with an end surface facing a bottom of the groove (11), to be bonded on the groove (11).

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a mechanism of a wire printing head of a stylus or wire printer and in particular, to a mechanism for driving an armature thereof. Description of the Prior Art Conventionally, various types of printing heads including those of plunger type and clapper type have been employed in stylus printers. Because of a satisfactory high-speed response, such wire printers of spring charge type have been primarily put to practice. The stylus printer of the spring charge type inlcudes a printing head having an armature, a printing wire, a leaf spring, a permanent magnet, a core, a coil winding, and a stylus having a free end. The armature is provided with a length of wire fixed thereto so as to be swingably supported by the leaf spring providing a bias thereto. Resisting against an elastic force of the bias plate spring, the permanent magnet attracts the armature toward the core. In a printing operation, the coil winding disposed around the core is excited to produce a magnetic flux in a direction opposite to one associated with the permanent magnet. The armature is thus released from the magnetic force of the permanent magnetic so as to cause the free end of the stylus to hit a platen. The armature includes an end portion such that a width of the armature becomes smaller in a portion nearer to the end portion. The printing wire is bonded or brazed on the end portion. However, a recent improvement in the printing speed tends to increase a load imposed on the brazed portion between the armature and the printing wire. For a higher printing speed, the armature is required to have a reduced weight. In consequence, an area of the bonded portion also develops a tendency to decrese. With the armature of the conventional wire printing head, the printing wire is simply fixed to the end of the armature. Consequently, a sufficient bonding strength of the linkage required at a printing operation is not easily obtained between the armature and the printing wire. During the operations in which the printing impact is repeatedly exerted, the printing wire may be separated from the armature in some cases. EP-A-0 188 672 discloses an armature device according to the preamble of present claim 1 where the armature has generally an elongated shape and in an end portion of which a hole or a slit is formed which slit extends in a direction substantially perpendicular to the longitudinal direction of said armature so that a pair of walls is arranged in the longitudinal direction of said armature whereby one wall is higher than the other due to the conical shape of the end portion of said armature. The stylus of this armature device has an end portion opposed to its free end and housed in said slit and bonded to said slit. It is therefore an object of the present invention to provide an armature mechanism of a wire printing head in which a shearing force acting upon the printing wire is mitigated to prevent the wire from being easily removed from the end of the armature, thereby enabling a stable printing to be conducted. This object is solved by the features of claim 1. The objects and features of the present invention will become more apparent from the consideration of the following detailed description taken in conjunction with the accompanying drawings in which: FIG. 1 is a partly cross-sectional view schematically showing an embodiment of an armature mechanism employed in a printing head of a wire printer in accordance with the present invention; FIG. 2 is a perspective view showing an armature of the embodiment of FIG. 1; FIG. 3 is a magnified side view showing an end portion of the armature of the embodiment; FIG. 4 is an enlarged side view showing, like FIG. 3, an armature for use in an alternative embodiment in accordance with the present invention; and FIG. 5 is a perspective view showing, in a similar fashion as for FIG. 2, an example of an armature for use in a prior art stylus printer. DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows a cross-sectional view of an embodiment of a printing head adpted in a wire or stylus printer in accordance with the present invention. The armature mechanism of the embodiment shown in this figure includes an armature 1 having an end on which a printing wire 2 is fixed. The wire 2 hits a printing medium, not shown, via an ink ribbon, not shown, to print a dot forming part of a letter or figure to be printed on the printing medium. The armature 1 is swingably supported on a yoke 71 by means of a leaf spring 6. The yoke 71 is a member which forms, together with other yokes 72, 73 and 74, a magnetic path of a permanent magnet 4 disposed to attract the armature 1 to a core 3. The yokes 71-74 are each formed with a magnetic substance so as to be assembled together, as shown in the figure, to have generally a contour of a cornered letter C when viewed from a side thereof. On the lowest yoke 74 is attached, as can be seen from FIG. 1, the core 3 also formed with a magnetic material. On side surfaces of the core 3 are wound a coil winding 5. The permanent magnet 4 is supported on the yoke 75 as shown in the figure to constantly generate a magnetic field attracting the armature 1 toward the core 3. The coil winding 5 is wound around the core 3 in a direction such that the coil winding 5, when powered, produces a magnetic flux substantially canceling the magnetic flux generated by the permantent magnet. The armature 1 is supported by the plate spring 6 in a cantilever fashion to be apart from the core 3, namely, to be pushed upward by the spring 6. The armature 1 is swingable about a support section 61 of the spring 6. When the coil winding 5 is not powered, owing to the magnetic attractive force caused by the flux path produced by the permanent magnet 4, the armature is attracted, resisting against the elastic force of the leaf spring 6, toward an end surface 31 of the core 3. The armature 1 comprises, as shown in FIG. 2, a portion 18 having a large thickness and a portion 19 extending therefrom and having a reduced thickness. The thick portion 18 is supported on the free end 61 of the plate spring 6. The printing wire 2 is fixedly attached onto the end portion 19 of the armature 1. FIG. 3 shows a magnified view of the brazed portion. As shown in FIG. 3, in the proximity of an edge 80 of the end portion 19 of the armature 1, a groove 11 is formed to fixedly bond an end portion of the printing wire 2 thereto. The groove 11 extends in a direction substantially perpendiucular to a longitudingal direction of the end portion, namely, to a plane of the sheet of the figure. The groove 11 has open ends. The armature end portion 19 has a thickness of about 0.2 millimeters (mm) to about 0.3 mm in an example. Since the edge 80 has a thickness substantially comparable thereto, the groove or depressed portion 11 is short in a running direction thereof and is deep in a depth direction. The groove 11 is configured to form a step between the end portion 80 and the support portion 19. The printing wire 2 has a fixed end 21, which is inserted into the groove 11 to be fixed by use of silver solder 13 and 14 to the armature 1. The fixed end 21 of the wire 2 has also a diameter of about 0.2 mm to about 0.3 mm. In this situation, as shown in the figure, assuming that the depths of the armature 1 on the respective sides of the support portion 19 and the end portion 80 are respectively a and b, that a discrepancy therebetween is c, and that the width of the groove 11 in the longitudinal direction of the armature 1 is d; when the armature 1 and the printing wire 2 are bonded by use of the silver solder 13 and 14, a length of a + b + d is subjected to the brazing. In consequence, a bonding or brazing area is increased as compared with the prior art. FIG. 5 shows a perspective view of a conventional armature mechanism in which a printing wire 2 is fixedly bonded to an armature 1. In this mechanism, an end portion 19a of the armature 1 has an edge 80a terminating in a simple flat plane. To the flat plane is brazed a fixed portion 21 of the wire 2 with silver solder. Consequently, it is difficult to develope a bonding strength sufficient to overcome the printing impact and hence a satisfactory reliability is not attained. If it is desired only to increase the bonding strength between the printing wire 2 and the armature 1, the depth a of the groove 11 on the side of the support portion 19 could be set to be substantially equal to the depth b thereof on the side of the end portion 80 without disposing the step 12. In this case, however, a shearing force applied to the printing wire 2 due to the printing impact would be concentrated on one of the bonding end portions, which would possibly cause the printing wire 2 to be broken at a position associated with the bonding end portion. In consequence, in accordance with the present invention, the step 12 is formed to dispose two bonding end portions 13 and 14, thereby reducing a chance of the break of the printing wire 2. FIG. 4 shows a side view of a primary section of the armature used in a wire printing head as an alternative embodiment in accordance with the present invention. In the figure, similar components or structural elements are designated by the same reference numerals, and redundant description will be avoided for simplicity. In this embodiment, an armature 1 has a groove 11 of which a depth a thereof on the side of a support portion 19 is smaller than a depth b thereof on the side of an end portion 80, thereby forming a step 15. Namely, the step 15 is disposed on the side of the support end 62 of the armature 1 with respect to a printing wire 2. In the embodiment shown in FIG. 3, the step 12 is located on the side of the free end 80 of the armature 1. In the structure of FIG. 4, the depth b of an end portion 80 contributes much more to enhancing the bonding strength of the printing wire 2. However, since the end portion 80 has an increased mass forming the greater depth b at the top end side, the increased mass may hinder a high-speed driving of the armature 1 in some cases. Returning to FIG. 1, in operation, when the coil winding 5 is not excited, the magnetic flux of the permanent magnet 4 passes through the yokes 71, 72 and 73, the armature 1, the core 3, and the yokes 74 and 75. As a result, the magnetic attractive force exerts on the armature 1, which is therefore attracted toward the core 3 in a direction of an arrow A while distorting the leaf spring 6. Subsequently, when the coil 5 is excited in this state, the magnetic flux produced by the coil winding 5 cancels the flux generated by the permanent magnet 4 so as to release the plate spring 6 from the distorted or biased state. This drives in the direction of an arrow B the printing wire 2 fixedly attached to the end 80 of the armature 1. The free end thereof then hits via the ink ribbon, not shown, the printing medium, not shown. A printing head assembly of a stylus printer has a plurality of such armature mechanisms. Free ends of the printing wires 2 of the armature mechanisms are linearly arranged to form a dot-matrix printing line. The coil windings 5 of the armature mechanisms are selectively excited while moving relatively the printing head or the platen, not shown, in a direction perpendicular to the line, thereby printing a print line. In accordance with the present invention, in the proximity of an end of the armature is disposed a groove having a step formed by a discrepancy in height between opposite walls of the groove on the top end side and the support side of the armature. The printing wire is fixed on the groove, which increases the bonding area between the groove and the printing wire as compared with the printing mechanism of the prior art. Consequently, in a printing operation by use of the wire printing head, a sufficient bonding force, even for when an impact is applied to the end portion of the armature, is obtained by the structure of the present invention when compared with that of the conventional printing head, thereby preventing the printing wire from being separeted from the armature. In addition, due to the step, the shearing force caused to be applied by the printing impact to the printing wire is not concentrated onto a bonding end portion. This prevents the breakage of the wire at the bonding end portion. In consequence, also in a high-speed printing operation of the printing head of the stylus printer, a stable printing is conducted.

Claims: An armature device of a printing head for use in a wire printer comprising: a stylus (2) having a free end to hit a recording medium for printing; an armature (1) connected to said stylus (2) for reciprocating said stylus (2) in a longitudinal direction (A, B) of said stylus; a permanent magnet (4) for generating a magnetic flux; a core (3) for conducting the magnetic flux into said armature (1) to form a magnetic path; a coil winding (5) wound around said core (3) for substantially cancelling, when powered, the magnetic flux; and a spring member (6) having a free end (62) supporting said armature (1) for biasing said armature (1) by spring force in a direction to separate said armature (1) from said core; characterized in that said armature (1) has generally an elongated shape, in an end portion (80) of which a groove (11) is formed to extend in a direction substantially perpendicular to a longitudinal direction of the elongated shape and to have a pair of walls which are arranged with respect to the longitudinal direction of the elongated shape and extend substantially perpendicularly to the longitudinal direction of the elongated shape, said groove (11) being also defined by a bottom wall contiguous to the pair of walls, said end portion (80) of said armature (1) including a first portion (19/80) forming one of the walls and a second portion (80/19) forming the other of the walls, the one wall (a/b) being higher than the other wall (b/a) by a length which is defined by the height of a step formed by an extension of an edge line of the first portion on the side of said stylus extending toward the free end thereof against an extension of an edge line of the second portion on the side of said stylus extending toward the free end thereof, said stylus (2) having an end portion (21) opposing said free end thereof and housed in said groove (11), with an end surface facing the bottom wall of said groove (11), to be bonded in said groove (11) to be supported by said armature. A device in accordance with claim 1, characterized in that the one wall of said groove (11) which is formed by said first portion is located on the side of said spring member (6) with respect to the longitudinal direction of the elongated shape, the other wall which is formed by said second portion being located apart from said spring member (6) with respect to the longitudinal direction of the elongated shape. A device in accordance with claim 2, characterized in that said armature (1) comprises a third portion (18) which is supported on said spring member (6) and continuous to the first portion (19) in the longitudinal direction of the elongated shape, the first portion (19) being thinner than the third portion (18) with respect to a direction in which the pair of walls extend. A device in accordance with claim 1, characterized in that the one wall of said groove (11) which is formed by said first portion is located apart from said spring member (6) with respect to the longitudinal direction of the elongated shape, the other wall which is formed by said second portion being located on the side of said spring member (6) with respect to the longitudinal direction of the elongated shape. A device in accordance with claim 4, characterized in that said armature (1) comprises a third portion (18) which is supported on said spring member (6) and continuous to the second portion (19) in the longitudinal direction of the elongated shape, the second portion (19) being thinner than the third portion (18) with respect to a direction in which the pair of walls extend. A device in accordance with claim 1 CHARACTERIZED IN THAT said device further comprises a yoke (71-75) for supporting said core (3) and conducting the magnetic flux in cooperation with said core (3), thereby forming a magnetic path, said armature (1) being located between a portion (72) of said yoke and said core (3). A device in accordance with claim 6, CHARACTERIZED IN THAT said permanent magnet (4) is supported on said yoke (71, 75). A device in accordance with claim 6, CHARACTERIZED IN THAT said spring member (6) has a fixed end (61) supported on said yoke (71). A device in accordance with claim 1, characterized in that said groove (11) has opposite ends open in the directions substantially perpendicular to the longitudinal direction of the elongated shape. A device in accordance with claim 3 or 5, characterized in that said first and second portions are substantially as thick as the diameter of said stylus (2). A printing head assembly for use in a wire printer comprising a plurality of armature devices in accordance with claim 1, CHARACTERIZED IN THAT said free ends of said styluses (2) form a print line constituting a dot-matrix to be printed.

Inventors: ANDOU HIROKAZU OKI ELECTRIC IN, IKEDA KIYOSHI OKI ELECTRIC IND, MUTO EISAKU OKI ELECTRIC IND C, ANDOU, HIROKAZU OKI ELECTRIC IND. CO., LTD., IKEDA, KIYOSHI OKI ELECTRIC IND. CO., LTD., MUTO, EISAKU OKI ELECTRIC IND. CO., LTD.
Applicants: OKI ELECTRIC IND CO LTD, OKI ELECTRIC INDUSTRY CO., LTD.
Application year: 1989
Main classification: B41J2
IPC codes: B41J2