Dot matrix print head

A dot matrix print head comprises a print wire unit in which a plurality of print wires are slidably mounted, an electromagnet unit having a plurality of electromagnets, and an armature unit secured to the print wire unit and to the electromagnet unit. The armature unit has a cover, a plurality of springs and a plurality of armatures. The armatures are held in the cover by a sub yoke secured to the cover.

BACKGROUND OF THE INVENTION 
The present invention relates to a print head for a dot matrix printer of 
the type in which a plurality of print wires are selectively driven 
through armatures by electromagnets to perform printing. 
A print head of the above described type is disclosed in U.S. Pat. No. 
4,204,778. In a print head, a plurality of armatures (clappers) are 
securely mounted on a leaf spring and a projecting iron plunger is 
provided on each armature so as to be attracted to a core of an 
electromagnet. The plunger is projected through an opening of a yoke plate 
to the core. Therefore, as assembled, the yoke plate is mounted on the 
electromagnet unit and the leaf spring with armatures is secured to a 
cover (stopper plate) by a washer. Then the armatures secured to the leaf 
spring are disposed on the yoke plate such that each of the plungers is 
inserted into the respective opening of the yoke plate, and then the cover 
is secured to a print wire guide body (nose) by a screw. 
Consequently, a large number of processes is required to assemble such a 
complex structure, and workability for the assembly is very low. Further, 
if the plunger is not accurately positioned on the armature, the plunger 
is not properly located in the opening of the yoke plate, and may come in 
contact with the wall of the opening. Accordingly, high accuracy in the 
manufacturing of components is required. 
SUMMARY OF THE INVENTION 
The object of the present invention is to provide a dot matrix print head 
wherein the number of the processes for assembling the head is reduced 
while increasing the workability of the assembly. 
In accordance with the present invention, a print wire unit, electromagnet 
unit and armature unit are independently manufactured respectively, and 
armatures do not remove from the unit. Accordingly, these units are 
manufactured at the same time on respective assembling lines and the units 
are assembled to a complete print head. 
In an aspect of the present invention, a bent end portion of each armature 
has a smaller width than a notch, and the armature has a pair of shoulders 
at both sides of the bent end portion, the lower edge of each shoulder 
being engaged with the surface of a sub yoke, whereby the armature is 
rotated about the lower edges as a fulcrum. 
These and other objects and features of the present invention will become 
more apparent from the following description with reference to the 
accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIGS. 1 to 3, a print head according to the present invention 
comprises a print wire unit 10, an electromagnet unit 20 secured to the 
print wire unit 10, and an armature unit 30 mounted on the electromagnet 
unit 20. 
The print wire unit 10 comprises a print wire guide body 12 and a plurality 
of print wires 11 disposed in the body 12. The wire guide body 12 has a 
post 12a, a hollow wire guide nose 12b extending from the post 12a, and a 
circular flange 12c horizontally provided on the wire guide nose 12b. A 
shoulder 12d formed at a portion between the post 12a and the guide nose 
12b has a plurality of holes for slidably guiding print wires 11. The wire 
guide nose 12b is provided with an end guide plate 13 and an intermediate 
guide plate 14, both of which are secured to the inside wall thereof for 
slidably guiding print wires. Each of the print wires 11 is slidably 
supported on the shoulder 12d, intermediate guide plate 14 and end guide 
plate 13. Print wires 11 are circularly disposed in a rear portion of the 
print head, surrounding the post 12a, and arranged on the straight in the 
end guide plate 13, at the front end of the nose 12b. Each of print wires 
11 has an impact head 15 secured to the tip end thereof and a compression 
spring 16 disposed between the impact head 15 and shoulder 12d. Thus, when 
the print wire 11 is mounted on the wire guide body 12, the print wire 11 
is biased to the rear portion of the print head. 
The electromagnet unit 20 comprises a cylindrical main yoke 21 having a 
central opening 21c for receiving the print wire unit 10 and a plurality 
of cores 22 circularly disposed and secured to a base plate 21a. A coil 
bobbin 24 having a coil 23 is attached to the core 22 so that an 
electromagnet is formed. The core 22 is formed such that the top end 
thereof is projected from the coil bobbin 24 and slightly projected from 
an annular sub yoke plate 33 which will be hereinafter described. 
The armature unit 30 comprises a cylindrical cover 31 to be engaged with 
the main yoke 21, a plurality of armatures 32 for impacting the print 
wires 11 by attracting force of the electromagnet, and sub yoke plate 33 
disposed between the armature 32 and coil bobbin 24. A pair of clamps 36, 
each having a U-shaped section comprising an upper hook portion 36a and a 
lower hook portion 36b with a female thread, are provided for firmly 
securing the print head. The cover 31 has a central hole for receiving the 
wire guide body 12, and small hollows 31d formed on the periphery of the 
inside of a base plate 31a, each corresponding to core 22 for receiving an 
armature loading spring 34. Further, the cover 31 has an annular shoulder 
31c formed on the inner edge of a cylindrical side frame 31b for receiving 
the sub yoke 33. The armature 32 comprises an operating body portion 32a, 
and bent end portion 32b formed on an outer end portion of the operating 
portion 32a. The bent end portion has a smaller width than the operating 
body portion 32a and an engaging portion 32c formed on the end thereof to 
be engaged with an armature loading spring 34. The sub yoke plate 33 is 
formed with a plurality of notches 33a at the periphery thereof 
corresponding to the bent end portion 32b of the armature 32. As shown in 
FIG. 2, the notch 33a has a larger width than the bent end portion 32b, 
and shoulders 32d of the armature 32 formed between the operating body 
portion 32a and bent end portion 32b are positioned on the sub yoke at 
both sides. Accordingly, a lower portion of the bent portion 32b engages 
with the notch 33a, so that the armature 32 can be pivotally rotated about 
the lower edge of each shoulder 32d as a fulcrum. Further, the sub yoke 33 
has a plurality of apertures 33b corresponding to the cores 22 to permit 
the top end of the core 22 to project therefrom, and a central opening 33c 
for print wires 11. 
Thus, when one of electromagnets is energized, armature 32 is attracted, 
rotating about the lower edge of the shoulder 32d, to push the wire to a 
platen (not shown) against springs 16 and 34, to print a dot. 
In assembling, respective units 10, 20 and 30 are previously assembled. 
That is, the impact head 15 is secured to each print wire 11 and the 
spring 16 is engaged with the wire. Then the print wires 11 are inserted 
into holes at the shoulder 12d, intermediate guide plate 14 and end guide 
plate 13. Thus, the print wire unit 10 is assembled. The electromagnet 
unit 20 is assembled in such a way that coil bobbins 24 having coils 23 
are secured to the cores 22 secured to the main yoke 21 while the cover 31 
is turned upside-down, and each armature loading spring 34 is inserted 
into the housing 31d. Then, the engaging portion 32c of the armature 32 is 
engaged with the projected end of the spring 34. A bumper cushion 35 made 
of rubber is disposed between the base plate 31a and the inner end of the 
armature 32. The sub yoke 33 is mounted on the armature 32 such that the 
lower portion of the bent end portion 32b is inserted into the notch 33a 
and the peripheral edge is force fitted with shoulder 31c of the cover 31. 
Thus, the armature unit 30 is independently assembled. 
In order to assemble the print head, the opening 21c of the main yoke 21 is 
engaged with the guide nose 12b of the guide body 12 and the base plate 
21a is mounted on the flange 12c. Thus, the base plate 21a is force fit to 
the guide body 12 at the hole 21c or secured to the flange 12c by screws 
(not shown). Then, the armature unit 30 is mounted on the electromagnet 
unit 20, in such a manner that the side frame 31b of the cover 31 is 
engaged with an upstanding plate 21b of the yoke 21, and the sub yoke 33 
is mounted on bobbins 24 so as to dispose each aperture 33b of the sub 
yoke 33 in coaxial relation with the corresponding core 22. Finally, the 
upper hook portion 36a of each clamp 36 is engaged with an engaging 
portion 31g formed on the base plate 31a and the lower hook portion 36b is 
secured to an engaging portion formed on the flange 12c by a screw 36c. 
From foregoing, it will be understood that the present invention provides a 
print head comprising an independent armature unit, an electromagnet unit 
and a print wire unit, and hence the armature unit can be easily mounted 
on the electromagnet unit without other parts or tools for assembly 
process. Accordingly, the assembly can be done for a short time. 
While the invention has been described in conjunction with preferred 
specific embodiments thereof, it will be understood that this description 
is intended to illustrate and not limit the scope of the invention, which 
is defined by the following claims.