Dot Printer Head

1. Field of the Invention

This invention relates to a dot printer having a needle which is driven to print combinations of dots each defining a particular letter or figure. More particularly, it relates to the head of any such dot printer.

2. Description of the Prior Art

There is known a dot printer in which armatures are actuated upon energization of coils to strike needles against a platen for printing dots. The armatures are driven by a mechanism which is shown by way of example in FIGS. 1 and 2. Each of the coils 3 is disposed about one of a plurality of cores 2 each forming an integral part of a yoke 1. Each of the armatures 5 is supported rotatably about a pivot 6 to cause the needle 4 to strike against the platen when the coil 3 is energized. Each armature 5 has a pair of recesses 8. A guide 7 is provided between every two adjoining armatures 5 and has one end disposed in one of the recesses 8 of one armature, while the other end of the guide 7 is disposed in one of the recesses 8 of the other armature. The guides 7 are located in a plane facing the yokes 1. If the coils 3 are energized, the armatures 5 are attracted toward the cores 2 to drive the needles 4. A magnetic flux travels from the cores 2 to the yokes 1 through the armatures 5 and from the yokes 1 to the cores 2. In order to perform effective printing, it is necessary to increase the attracting force of the cores 2 by applying a small magnetomotive force. This necessitates an increase in the surface area S of the region in which each armature 5 faces the corresponding yoke 1. This in turn requires an increase in the radial width 1.sub.3 of the yoke 1. The distance between the pivot 6 and the center of the core 2 is shown at 1.sub.1, and the distance between the pivot 6 and the free end of the armature 5 at which it hits the needle 4 is shown at 1.sub.2. If the distance 1.sub.3 is increased, it is necessary to increase the distance 1.sub.1 resulting in the necessity for an increased air gap G to maintain an appropriate stroke A.sub.s for the movement of the free end of the armature 5. An increase in the air gap G, however, results in the failure of the core 2 to produce a satisfactorily large attracting force. Moreover, an increase in the distance 1.sub.1 means a lower lever ratio 1.sub.2 /1.sub.1 and an increase in the equivalent mass of the armature 5. This disables fast printing and gives rise to an increase in power consumption. These problems are worsened by the presence of the recesses 8 in the plane facing the yoke 1. If the recesses 8 are taken into account, an increase in the surface area S calls for a greater increase in the distances 1.sub.3 and 1.sub.1 and gives rise to a greater increase in the air gap G.

It is an object of this invention to increase the force for attracting the armatures of a dot printer.

It is another object of this invention to reduce the equivalent mass of the armatures.

It is still another object of this invention to achieve fast printing.

It is a further object of this invention to reduce the power consumption of a dot printer.

It is a still further object of this invention to provide a simple structure for supporting the armatures of a dot printer.

Other objects of this invention will become apparent from the following description.

FIG. 1 is a fragmentary side elevational view, partly in section, of a known armature, yoke and core arrangement in a dot printer;

FIG. 2 is a fragmentary front elevational view of the device shown in FIG. 1;

FIG. 3 is a horizontal sectional view of an apparatus embodying this invention;

FIG. 4 is a fragmentary enlarged side elevational view, partly in section, of the armature, yoke and core arrangement shown in FIG. 3;

FIG. 5 is a fragmentary enlarged front elevational view of the apparatus shown in FIG. 3;

FIG. 6 is a fragmentary horizontal sectional view of another armature, yoke and core arrangement embodying this invention; and

FIG. 7 is a fragmentary front elevational view of the arrangement shown in FIG. 6.

An apparatus embodying this invention is shown in FIGS. 3 to 5. A pair of needle guides 12 and 13 are secured to a guide frame 10 for supporting a plurality of needles 11 slidably. A plurality of cores 16 are disposed in an annular array on a circular yoke 14 screwed to the guide frame 10. Each core 16 forms an integral part of the yoke 14. A coil 15 surrounds each core 16. The yoke 14 has an annular projection 17. A plurality of armatures 18 face the cores 16 and the projection 17. Each armature 18 has a free end to which one of the needles 11 is secured. Each armature 18 is formed intermediate the ends thereof with a shoulder 19 defining a portion of reduced thickness which extends toward the needle 11. The shoulder 19 has a corner which contacts the yoke projection 17 and thereby defines a pivot 20 about which the armature 18 is rotatable. The projection 17 and each armature 18 have therebetween a small surface S of contact which defines a magnetic path. Accordingly, the outer peripheral edge of the projection 17 and the pivot 20 have a small distance 1.sub.3 therebetween, and the center of the core 16 and the pivot 20 have, therefore, a small distance 1.sub.1 therebetween.

Each armature 18 has a circular opening 21 to which the pivot 20 is tangential. A pole 22 formed from a magnetic material is welded or otherwise secured to the yoke projection 17 and extends through the opening 21. The pole 22 has a circular cross section and is tapered. The pole 22 can, of course, be formed as an integral part of the yoke 14. A plurality of armature guide members 23 are formed on the guide frame 10 for restricting the direction in which each armature 18 is rotated. An armature spring 24 is provided for urging each armature 18 into its original position in which its free end rests on a stop member 25.

If a particular coil 15 is energized, a magnetic flux travels along a magnetic path A defined by the core 16, yoke 14 and armature 18, and a magnetic path B defined by the core 16, yoke 14, pole 22 and armature 18. The core 16 attracts the armature 18 and thereby enables the needle 11 to strike against the platen to effect the printing of a dot. As the yoke 14 and the pole 22 have a large area of contact therebetween, and as the armature 18 and the pole 22 have a large area in which the outer surface of the pole 22 faces the armature surface defined by the opening 21, the yoke 14 and the armature 18 have only a small amount of magnetic resistance therebetween, though the area S in which the armature 18 faces the yoke projection 17 may be small. This enables a reduction in the distances 1.sub.3 and 1.sub.1 and thereby in the air gap G between the core 16 and the armature 18, as shown in FIG. 4. This ensures a reduction in the magnetic resistance between the core 16 and the armature 18 and enables the core 16 to exert a strong attracting force on the armature 18. A reduction in the distance 1.sub.1 gives rise to a higher 1.sub.2 /1.sub.1 ratio in which 1.sub.2 stands for the distance between the needle 11 and the pivot 20, and thereby a reduction in the equivalent mass of the armature 18. The apparatus is, therefore, suitable for fast printing and achieves a reduction in power consumption.

Another embodiment of this invention is shown in FIGS. 6 and 7. In this embodiment the cores disposed radially inwardly of the pivots, while the cores 16 are disposed radially outwardly of the pivots in the apparatus of FIGS. 3 to 5. A yoke 26 includes a plurality of cores 16 disposed in an annular array. Each core 16 is associated with a coil 15, and a plurality of equally spaced apart supporting walls 27 are each located radially outwardly of one of the cores 16. Each armature 28 faces one of the cores 16 and one of the walls 27. Moreover, each armature 28 has an inner end on which the rear end 29 of a needle 11 abuts. Each needle 11 is urged toward its original position by an armature spring (not shown). The armature 18 has at its outer end a shoulder 30 which spaces it apart from the supporting wall 27. The wall 27, and the shoulder 30 have a corner of contact which defines a pivot 20 about which the armature 28 is rotatable. The shoulder 30 has a rectangular opening 31 having a side located in the plane in which the pivot 20 lies. A projection 32 formed from a magnetic material extends through the opening 31 and forms an integral part of the yoke 26. The projection 32 reduces the magnetic resistance between the supporting wall 27 and the armature 28. This enables a reduction in the distance 1.sub.3 between the pivot 20 and the inner edge of the wall 27, the area S in which the armature 28 faces the wall 27 and thereby the distance 1.sub.1 between the center of the core 16 and the pivot 20 as compared with the distance 1.sub.2 between the needle 11 and the pivot 20. Therefore, the air gap G is sufficiently small to reduce the equivalent mass of the armature 28.