Print Head

This invention relates to a print head mounted in an impact type dot matrix printer, and more particularly to an air cooling means for the interior of the print head.

In a conventional print head, when each solenoid mounted within the print head has been energized, each of armatures having a printing wire is swung to perform the printing operation with the end of the printing wire. However the generation of heat in the solenoid caused the temperature within the print head to rise, which in turn increased the electrical resistance of coils, so that the driving properties of the solenoids was reduced and therefore performance was deteriorated. In order to release such heat, the print head in the prior art had numerous cooling fins attached to the side face thereof, which resulted in a larger size and heavier weight of the print head. This further required a wider space for a printer to mount the print head therein and resulted in a larger-sized apparatus, and hence it was difficult to move the print head in reciprocating motion at a higher speed and smoothly, thus making the printing operation at a higher speed impossible. There is also known a print head in which a cooling fan is mounted to cool solenoids by the air introduced into the printing head. However such print head also was larger in size and heavier in weight.

It is therefore to provide a print head which is reduced in number of parts and in weight, and which ensures effective cooling of the interior of the print head to thereby improve the performance of response of the armatures.

According to the invention, there is provided a print head, wherein the outer face of the print head along the direction of movement thereof is formed so as to cause the faster flow of air along said outer face when the print head is moved, and said outer face is provided with ventilating hole means which communicates the interior of the print head and the outside. With the construction above, the air elevated in temperature within the print head may be discharged through the ventilating means out of the print head due to the difference in pressure.

Other objects and aspects of the invention will become apparent from the following description of embodiments with reference to the accompanying drawings.

FIG. 1 is a perspective view, with parts partially broken away, of a print head of the first embodiment of the invention;

FIG. 2 is a schematic sectional view of the print head in FIG. 1;

FIG. 3(A) is a perspective view illustrating the rear side of the print head;

FIG. 3(B) is a schematic sectional view of the print head;

FIG. 4 is a perspective view of an actuator mounted in the print head;

FIG. 5 is a sectional view taken along the line V--V of FIG. 4;

FIG. 6 is a fragmentary sectional view of the print head showing a state in which the cooling operation is effected by the air;

FIG. 7 is a perspective view, with parts partially broken away, of a print head of the second embodiment of the invention;

FIG. 8 is a fragmentary vertical sectional view of the print head in FIG. 7;

FIG. 9 is a fragmentary sectional view of the print head in FIG. 7; and

FIG. 10 is a cross-sectional view of an outer yoke containing therein comb-like portions with solenoid coils illustrating a state in which the cooling operation is effected by the air.

The first embodiment of the invention will be explained with reference to FIGS. 1 to 6.

As shown in FIGS. 1 and 2, a yoke member 1 which constitutes a magnetic circuit is formed of Co.Fe alloy called "Permendur" having a property of highly saturated magnetic flux density, and consists of a substantially cylindrical outer yoke 3 and an inner yoke 2 located in spaced relation to the outer yoke 3 and having numerous comb-like portions 4 formed integrally on the same circumference at suitable pitches. Solenoid coils 5 are wound round the comb-like portions 4 to constitute an electromagnetic device. A rear casing 6 is glued to the rear side of the outer yoke 3, and an annular permanent magnet 7 and a holding member 8 are glued to the front side of the outer yoke 3. On the other hand, a plurality of actuators 9 are each secured by screws to the end portion at the outer peripheral side of the rear face of a front yoke 10 in radially corresponding relation to each of the comb-like portions 4. The front yoke 10 with each of the actuators 9 secured thereto by screws is located relative to the holding member 8 by pins (not shown) projecting from the holding member 8, and each of the actuators 9 is magnetically attracted on the member 8 by magnetic force of the permanent magnet 7. A plurality of projections 10a extending between armatures 21 to be explained later are each integrally formed on the rear face of the front yoke 10 to form a magnetic circuit leading to the actuator 9 and the yoke 4. A circular opening 14 is formed in the central portion of the front yoke 10, and a rectangular guide 11 having numerous guide holes 11a arranged in the form of rhomb is mounted on the front side of the front yoke 10, and serves to guide slidably the end portion at the printing side of each of printing wires 28 bonded by brazing to the free end of the actuators 9 and extending substantially linearly. The opening 14 is constituted with the guide 11 attached thereto such that the opening 14 communicates with the interior of the print head and allows to introduce the air therein. Further, the electrical lead end of each of the solenoid coils 5 wound round the comb-like portions 4 is soldered on a flexible cable 13 attached to a base plate 12. The electrical wire of the flexible cable 13 is connected to a solenoid driving means (not shown) within the printer proper, whereby exciting current is selectively supplied to each of the solenoid coils 5.

Referring to FIGS. 4 and 5, each of the actuators 9 comprises a holder 20, an armature 21 arranged in opposed relation to the yoke 4, a torsion spring 22 which supports the armature 21 for swinging movement relative to the holder 20, and a wire holder 27. The holder 20 is fixedly secured at its base end to the end portion at the outer peripheral side of the rear face of the front yoke 10, and is formed at its top end portion with bifurcated portions 23a, 23b. These bifurcated portions 23a, 23b are provided with through-holes 24a, 24b, into which both axial end portions of the torsion spring 22 are fitted and fixed. Further, these bifurcated portions 23a, 23b are formed with holes 25a, 25b for solder material communicating with the through-holes 24a, 24b, and these solder holes are filled with solder material such as palladium solder, silver solder, gold solder or the like in the form of pellet or rod when bonding the holder 20 to the torsion spring 22 by brazing.

Each of the armatures is constructed of Co.Fe alloy called "Permendur" having a property of highly saturated magnetic flux and constitutes a magnetic circuit together with the comb-like portion 4 and the projection 10a. The armature 21 is formed with a through-hole 26 at the opposite end portion 21a positioned between the bifurcated portions 23a and 23b, into which the axial central portion of the torsion spring 22 is fitted and fixed by brazing. Further, a wire holder 27 is bonded by brazing to the top end portion of the armature 21. A printing wire 28 shaped in tapered form from the end at the printing side to the base end is bonded by brazing to the top end portion of the wire holder 27. The armature 21 is also formed at the opposite end portion 21a with solder hole 21b communicating with the through-hole 26, and the solder hole 21b is filled with solder material when bonding the armature 21 and the bifurcated portion 23a, 23b by brazing. The armature 21 is formed at its front end side with drill holes 21c, which eliminates excessive material portions unnecessary for the magnetic circuit leading to the projection 10a to thereby reduce the mass. Further, the wire holder 27 is also formed with holes 27a to reduce the mass.

The above-mentioned torsion springs are made of maraging steel consisting of high nickel steel containing extremely low carbon, or JIS (Japanese Industrial Standard) 15-7 PH, 17-7 PH stainless steel of percipitation hardening type martensite system. The torsion spring 22 is formed with larger diameter portions 22a-22c at both the end portions and the central portions in the axial direction which are fitted into and fixed to the inner walls of the through holes 24a, 24b and 26 by brazing. Moreover, the intermediate portions 22d, 22e interposed between these large diameter portions 22a-22c are formed in the concave section such that they become progressively greater in diameter with the approach to the larger diameter portions 22a-22c. This makes it difficult for torsional stress actuating at the time of printing operation to concentrate in the boundary portions between the larger diameter portions 22a-22c and the intermediate portions 22d, 22e.

The construction as described above does not constitute part of the invention. Now, air cooling means of the print head according to the invention will be explained below.

As shown in FIGS. 3 (A) and (B), the rear face of the rear casing 6 constitutes an outer face along the direction of movement of the print head. Formed integrally on the rear face of the rear casing according to the invention to cause the faster flow of air along the direction of movement of the print head is a projecting portion 6 which increases progressively in amount of projection in the direction of movement of the print head as shown with arrow marks of full lines. And, the projecting portion 6 is formed at its top portion 6b with a ventilating hole 6c which communicates with the interior of the print head.

The operation of the air cooling means of the print head according to the invention will be explained below. In FIG. 6, assuming that the rear face of the rear casing 6 is kept at a suitable distance from the wall face of the body frame 30 (not shown), when the print head is moved in reciprocating motion along said wall face, the speed of flow of the air passing the top portion 6b of the projecting portion 6 is higher than that at the other portion 6a of the projecting portion 6 due to the fact that the sectional area of the space through which the print head runs is the minimum at the top portion 6b. With such higher speed, the pressure at the top portion 6b becomes lower than that at the other portions, and the air which has been introduced through the opening 14 and increased in temperature due to the generation of heat in the solenoid coil 5 with the printing operation, is discharged through the ventilating hole 6c out of the print head because of the above-mentioned difference in pressure. On the other hand, with the discharge of the warm air, the cold air is introduced through the opening 14. As a result, the flow of the air as shown in the arrow marks of the full lines in FIG. 6 is produced, whereby rise in temperature within the print head can be prevented.

Now, the second embodiment of cooling means of the print head according to the invention will be explained below with reference to FIGS. 7-10. As best seen in FIGS. 7 and 10, according to the invention an outer yoke 103 is provided with ventilating holes 103a, 103b and 103c, 103d at either left and right side faces facing the direction of movement of the print head and at the upper and lower side faces which constitute the outer faces along the direction of movement of the print head, respectively.

The operation of the second embodiment will be explained below. In FIG. 10, when the print head is moved in the direction of arrow mark of full line, the air is introduced with movement of the print head into the interior of the print head through the ventilating hole 103a provided at the side faces of the outer yoke 103 toward the direction of movement of the print head. Due to the face that the space through which the print head runs is the minimum in sectional area at the top and bottom sides of the outer yoke 3, the flows of the air passing the top and bottom sides of the outer yoke 3 become faster than those of the air passing the other portions, whereby the top and bottom sides become lower in pressure than the other portions. Accordingly, the air which has been introduced through the ventilating holes 103a, flowing as shown with arrow marks of full lines and increased in temperature due to the generation of heat of the solenoids 105 with the printing operation, is discharged through the ventilating holes 103c, 103d out of the print head because of the above-mentioned difference in pressure. As a result, the flows of the air are produced as shown with arrow marks of full lines in FIG. 6, whereby rise in temperature within the print head can be prevented. Further, when the print head is moved in the direction of arrow marks of dotted line in FIG. 6, the air which has been introduced through the ventilating holes 103b with movement of the print head, flowing as shown with dotted lines in FIG. 6 and increased in temperature due to the generation of heat of the solenoid coils 105, is discharged through the ventilating holes 103c, 103d out of the print head.

In the print head according to the invention, the difference in pressure between the outer face of the print head along the direction of movement thereof and other portions of the same causes the warm air within the print head to be discharged out of the print head, so that rise in temperature within the print head is prevented and the deterioration of driving properties of the solenoids is also prevented.