WIRE-DOT PRINTING HEAD

Abstract: A wire-dot printing head (1) having a plurality of printing elements each including an armature (3) fixedly holding a printing wire (4) on the free end thereof, a plate spring (2) fixedly holding the armature on the free end thereof, a permanent magnet (5) for attracting the armature (3) to strain the plate spring (2) so that strain energy is stored in the plate spring, and an electromagnet (6) which cancels the magnetism of the permanent magnet (5) when energized to release the armature (3) so that the armature is returned to the free position thereof by the strain energy of the plate spring (2) to advance the printing wire for printing. The plate spring (2) has an operating lug portion (28) which can be bent in either direction from outside, and an adjusting lug portion (29) which is formed integrally with the operating lug portion (28) and is moved in the opposite direction when the operating lug portion (29) is moved in one direction to adjust the position of the base end of the armature so that the strain energy stored in the plate spring is varied.

Description: This invention relates to a wire dot printing head. It is known from US-A-4 225 250 to provide a wire dot printing head with a plurality of printing elements each including permanent magnet means; an electromagnet, an armature to be moved from a printing to a non-printing position by the magnetism of the permanent magnet, a printing wire for effecting printing in response to movement of the armature; and plate spring means for being flexed to store strain energy when the armature is moved to the non-printing position and for discharging the stored strain energy when the electromagnet is energised to release the armature from the non-printing position. To perform printing, the coil of the electromagnet is energised by a drive signal to generate a magnetic flux of a polarity opposite to that produced by the permanent magnet, and thereby the magnetic flux of the permanent magnet is cancelled to release the armature. Then, the strain energy stored in the plate spring drives the armature to project the tip of the print wire against a recording sheet for printing. In such a conventional printing head, it often occurs that the difference between the magnetic attraction and the resilient force (strain energy) of the plate spring, namely, armature holding force, varies for the individual printing elements. When this variation of the mature holding force is particularly large, the armature release timing varies between the printing elements, and hence the printing period of the dot printing head is caused to vary, and in some circumstances prevents the head from being used. Furthermore, if the dot printing head is usable, prints may not come out clearly or may be printed with different densities degrading print quality. To eliminate such drawbacks, it is proposed in US-A-4 692 043 (Ando et al.) to provide a wire dot printing head having a plate spring with adjustment means for the plate spring, comprising a deformable adjustment portion, and an adjusting screw having its tip in contact with the adjustment portion. The distortion of the plate spring is adjusted by operating the adjusting screw. However, such an arrangement entails problems that additional parts, namely, adjusting screws, are necessary for adjusting the distortion of the plate springs, that the adjusting screws must be screwed in place one by one in the manufacturing process, and that the distortion of the plate springs must be adjusted to an optimum distortion by operating the corresponding adjusting screws one by one. Thus such adjusting work must be carried out for each of the plate springs, for example, twenty-four times when the dot printing head has twenty-four wires with twenty-four plate springs, to form a character with a matrix of twenty-four dots. It is an object of the present inveniton to provide a much simpler structure for permitting individual adjustment of the stored spring strain energy for each print element. The invention is characterised in that, to provide said adjustment, the plate spring means has a substantially planar main body with integral operating and adjusting lugs, the adjusting lug being configured to be moved out of the plane of the main body on one side thereof against the armature in response to the operating lug being moved out of the plane of the main body on the other side thereof so as to change the strain energy stored in the plate spring means when the armature is in the non-printing position. Further features and advantages of the invention will become apparent from the following description of an embodiment thereof given by way of example with reference to the accompanying drawings in which: Figure 1 is a sectional view of a wire-dot printing head according to the present invention; Figure 2 is an exploded perspective view showing the principal components of the wire-dot printing head of Figure 1; Figure 3 is a fragmentary perspective view of assistance in explaining the manner of positional adjustment of an armature; and Fig. 4 is a fragmentary sectional view of assistance in explaining the manner of positional adjustment of an armature. A wire-dot printing head 1 embodying the present invention has a plurality of printing elements; however, since the printing elements are substantially the same in construction, a description of only one of the printing elements will be given in detail. Referring to Fig. 1 showing the wire-dot printing head 1, disposed between a guide head 11 and a base plate 12 is a plate spring 2 mounted with an armature 3. A print wire 4 is fixed to one end, namely, the free end 31 of the armature 3. The other end, namely, the base end 32, of the armature 3 is received loosely in a groove formed in an armature yoke 33. A base yoke 52, a permanent magnet 5, a magnetic yoke 51 and a spacer 34 are placed one over another in that order on the base plate 12 under the plate spring 2. An electromagnet 6 comprising a core 61 and an exciting coil 62 is disposed below the plate spring 2 on the base plate 12. A plurality of such printing elements are held fixedly between the guide frame 11 and the base plate 12 in a single unit with clamping springs 13 engaging the guide frame 11 and the base plate 12. When the wire-dot printing head 1 thus constructed is in a nonprinting state, the magnetic flux lines of the permanent magnet 5 form a closed magnetic circuit consisting of the base yoke 52, the base plate 12, the core 61 of the electromagnet 6, the armature 3, the armature yoke 33, the spacer 34, and the magnet yoke 51. The armature 3 is attracted to the core 61 against the resilient force of the plate spring 2 by the magnetic function of the closed magnetic circuit. That is, strain energy is stored in the plate spring 2 while the wire-dot printing head 1 is in a nonprinting state. In printing operation, a drive signal is provided to energize the exciting coil 62 of the electromagnet 6 so as to generate magnetic flux of a polarity opposite to that of the magnetic flux of the permanent magnet 5. Thus, the magnetic flux of the permanent magnet 5 is cancelled to release the armature 3 from the core 61. Then, the armature 3 is turned at the base end 32 thereof by the strain energy of the plate spring 2 to advance the printing wire 4 fixed to the free end 31 of the armature 3 along a guide groove 11a so that the tip of the printing wire 4 is pressed through an ink ribbon, not shown, against a recording sheet, not shown, for printing a dot. Fig. 2 shows the details of the plate spring 2, the armature 3, the printing wire 4, the armature yoke 33 and the spacer 34. The plate spring 2 is formed substantially in the shape of a disk. The plate spring 2 has a planar main body including a tongue portion 22 extending inward (to the right as viewed in Fig. 2) and having elastic leg portions 24, a punched hole 23 defining the inner edges of the elastic leg portions 24, an adjusting lug portion 29 projecting into the punched hole 23 from the rear portion of the plate spring 2, an armature holding portion 25 for fixedly holding the armature 3, extending outward (to the left as viewed in Fig. 2) into the punched hole 23 from the outer end of the tongue portion 22, recesses 26 formed in the outer end (the left end as viewed in Fig. 2) of the plate spring 2, an operating lug portion 28 projecting outward from the outer end of the plate spring 2, and a neck portion 27 formed between the adjusting lug portion 29 and the operating lug portion 28. The operating lug portion 28 is bent perpendicularly to a plane including the plate spring 2 in one direction to bend the neck portion 27 for plastic deformation, so that the adjusting lug portion 29 is moved in the opposite direction. The upper surface of the adjusting lug portion 29 is in contact with the lower surface of the base end 32 of the armature 3. The armature yoke 33 placed on the plate spring 2 has a recess 33a in the outer side thereof, and a groove 33b for loosely receiving the armature 3, substantially in the middle portion hereof. The spacer 34 placed under the plate spring 2 has an inner recess 34a, an outer recess 34b and a pillow portion 34c. The plate spring 2, the armature 3, the armature yoke 33 and the spacer 34 are joined fixedly together by spot-welding at positions indicated by marks "x" in Fig. 2. When the wire-dot printing head 1 is assembled, the recess 33a of the armature yoke 33, the operating lug portion 28 of the plate spring 2, and the recess 34b of the spacer 34 are exposed outside. In adjusting the resilience of the plate spring 2, the operating lug portion 28 of the plate spring 2 is bent up or down with the tip 71 of a screw driver 7 inserted in the recess 33a or 34b as shown in Fig. 3. Referring to Fig. 4, in increasing the printing speed of the printing wire 4, the operating lug portion 28 is bent toward the recess 34b when the tip 71 of the screw driver 7 is inserted in the recess 33a, whereby the adjusting lug portion 29 is turned up on the pillow portion 34c to push up the base end 32 of the armature 3 from a position indicated by dotted lines to a position indicated by solid lines. Consequently, the strain energy stored in the plate spring 2 when the armature 3 is attracted to the core 61 of the electromagnet 6 by the permanent magnet 5 is increased, and hence the printing wire 4 advances at an increased printing speed when released from the core 61. On the other hand, in reducing the printing speed of the printing wire 4, the tip 71 of the screw driver 7 is inserted in the recess 34b, whereby the adjusting lug portion 29 is turned down on the pillow portion 34c to lower slightly the base end 32 of the armature 3. Although the invention has been described in its preferred form with a certain degree of particularity, it is to be understood that many variations and changes are possible in the invention without departing from the scope of the claims.

Claims: A wire dot printing head with a plurality of printing elements each including permanent magnet means (5), an armature (3) to be moved from a printing to a non-printing position by the magnetism of the permanent magnet, an electromagnet (6) for releasing the armature from the non-printing position, a printing wire (4) for effecting printing in response to movement of the armature; plate spring means (2) for being flexed to store strain energy when the armature is moved to the non-printing position and for discharging the stored strain energy when the electromagnet is energised to release the armature from the non-printing position, and adjustment means for adjusting the strain energy stored in said plate spring means, charaterised in that, to provide said adjustment means, the plate spring means (2) has a substantially planar main body with integral operating and adjusting lugs (28, 29), the adjusting lug (29) being configured to be moved out of the plane of the main body on one side thereof against the armature (3) in response to the operating lug (28) being moved out of the plane of the main body on the other side thereof so as to change the strain energy stored in the plate spring means when the armature is in the non-printing position. A printing head according to claim 1 including a base (12), the electromagnet (6) having a core (61) mounted on the base, the permanent magnet (5) being arranged to produce a closed loop magnetic circuit through the armature (3) the core (61) and the base (12) to hold the armature (3) in the non-printing position. A printing head according to claim 1 or 2, wherein the armature (3) is mounted on a spacer (34) including a recess (34b) to permit adjustment of the operating lug (28). A printing head according to any preceding claim including a yoke (33) overlying the plate spring (2) and spot welded thereto. A printing head according to any preceding claim including a guide head (11) with an opening (11a) through which the print wires (4) extend.

Inventors: ASAKA, TOSHIYUKI, ISHIMIZU, HIDEAKI, KIKUCHI, HIROSHI, TANUMA, JIRO
Applicants: OKI ELECTRIC INDUSTRY COMPANY, LIMITED
Application year: 1988
Main classification: B41J2
IPC codes: B41J2