Abstract:
A print head for a wire dot-matrix printer includes an element base, and a plurality of print elements engaged with and supported by the element base. Each of the print elements includes a print wire, an actuator moving the print wire, and a base supporting the actuator, the base of each of the print elements having an engagement hole. An engagement member is inserted into the engagement hole of the base of each of the print elements and is engaged with the base of each of the print elements.

Description:
DESCRIPTION OF THE PRIOR ART 
     1. Field of the Invention 
     The present invention generally relates to print heads for wire dot-matrix printers, and more particularly to a print head having a plurality of print elements which respectively have a print wire driven by an actuator using a piezoelectric material. 
     2. Description of the Prior Art 
     In recent wire dot-matrix printers, a piezoelectric material is used for actuators driving print elements in order to increase print speed. A print head is made up of a plurality of print elements fastened to an element base so that print wires respectively provided in the print elements are arranged at predetermined equal intervals. In order to arrange the print wires at the equal intervals, it is necessary to fasten the print elements to the element base with high precision. 
     FIG. 1 is a perspective view of a conventional wire dot-matrix printer of a shuttle bar system type. The wire dot-matrix printer shown in FIG. 1 comprises a shuttle mechanism 11, which is slidably supported at three supporting points 11a and 11b (the remaining supporting point is not illustrated) and reciprocates in directions indicated by arrow D1. A head assembly comprises a plurality of print heads, for example, 13 print heads mounted on the shuttle mechanism 11. A platen 12 is disposed so that it faces the shuttle mechanism 11. Paper 13 is inserted between the shuttle mechanism 11 and the platen 12. The paper is transported in the direction indicated by arrow D2 by means of tractors 14 and 15. 
     FIG. 2 is a front view of the shuttle mechanism 11. The print heads 21, 22, 23 and 24 (the remaining nine print heads are omitted) are obliquely disposed at a fixed angle and fastened to the shuttle mechanism 11. Each of the print heads includes 24 print elements grouped into opposing pairs. FIG. 2 shows the 24 print elements of only the print head 21. 
     FIG. 3A is a perspective view of the print head 21, and FIG. 3B is a perspective view of an element base 32, which is one structural element of the print head 21 and made of a metallic member having a low coefficient of thermal expansion. Each of the other print heads has the same structure as the print head 21. The print head 21 comprises four element assemblies 33, 34, 35 and 36, each having an identical structure. For example, the element assembly 33 has six print elements 31a-31f, which have an identical structure and are spaced apart from each other at fixed intervals. The six print elements 31a-31f are fastened to the element base 32 shown in FIG. 3B. The element base 32 has a comb-shaped structure having seven projections. Each of the six print elements 31a-31f is inserted into a space between two adjacent projections and engaged therewith. The element assemblies 33-36 are located at respective predetermined positions on a module base 37, and are fastened to the module base 37 by means of screws (not shown) or the like. Print wires extending from the element assemblies 33 and 34 are opposite to print wires extending from the element assemblies 35 and 36. The module base 37 has an opening 37A from which the print wires extending from the element assemblies 33-36 project. 
     FIG. 4A is a front view of the print element 31a, and FIG. 4B is a side view of the print element 31a. Each of the other print elements has the same structure as the print element 31a. The print element 31a is made up of an actuator 41, and a base 42 mechanically supporting the actuator 41. The actuator 41 has a piezoelectric element 41a, a joint member 41b, a block 41c, a beam 41d, a print wire 41e, and a spring member 41f. The piezoelectric element 41a has a first end fixed to the base 42, and a second end connected to the joint member 41b. The block 41c is connected to the joint member 41b. The beam 41d is fixed to the block 41c. The print wire 41e is fastened to the beam 41d. The spring member 41f has a first end fastened to the block 41c, and a second end fastened to the base 42. The base 42 has recesses 42a engaged with the element base 32 (FIG. 3B). Lead wires 43 are connected to the piezoelectric element 41a, by which a control voltage is applied thereto. In response to the control voltage, the piezoelectric element 41a is slightly expanded in the leftward direction in FIG. 4B, and the block 41c is pushed in the leftward direction via the joint member 41b. Thereby, the spring member 41f is contracted, and the block 41c, the beam 41d and the print wire 41e are turned in the counterclockwise direction. In this case, the print wire 41e projects leftwards, by approximately 350-400 microns, from a guide 37b (FIG. 4A) of an opening 37a (FIG. 3A) of the module base 37. Then, the print wire 41e presses against a ribbon, paper and platen provided on the left side of the print wire 41e. In this manner, a dot image is printed on the paper. 
     FIG. 5A is a perspective view of the piezoelectric element 41a, which is made up of a piezoelectric block 51, two electrodes 52A and 52B 
     (FIG. 5B), leaf springs 53, two blocks 54 and 55, and a flexible printed wiring board 56. The piezoelectric block 51 comprises a plurality of (200, for example) piezoelectric layers stacked in the direction indicated by arrow D3. The electrodes 52A and 52B are formed on opposite sides of the piezoelectric block 51. The control voltage is applied to each of the piezoelectric layers in the direction D3. The leaf springs 53 permanently exert compression forces on the piezoelectric block 51 in the directions D3 in order to protect it. The blocks 54 and 55 are provided on respective ends of the block 51 and support the leaf springs 53. 
     FIG. 5C is a plan view of the flexible printed wiring board 56, which has an approximately L-shaped structure. FIG. 5D is a cross-sectional view of the flexible printed wiring board 56. As shown in FIG. 5D, the board 56 comprises a first insulating layer 57, a patterned conductive layer 58 made of, for example, copper, and a second insulating layer 59. A circular soldering window 56a for soldering is formed in the second insulating layer 59, so that the conductive layer 58 is partially exposed. The circular soldering window 56a is used for connecting one of the two lead wires 43 (FIG. 4B) respectively formed by the patterned conductive layers to the electrode 52A. A circular soldering window 56b is used for connecting the other lead wire 43 to the electrode 52B. A part of the printed wiring board 56 is wrapped around the block 51, as shown in FIG. 5A. 
     However, the print heads have the following disadvantages. The print elements 31a-31f are fastened to the element bases 32 by using an adhesive or a bonding material. Hence, it is not possible to detach the print elements 31a-31f from the element bases 32. If a failure has occurred in one of the print elements and the defective print element must be replaced with a new one, it is necessary to replace the element assembly in which the defective print element is provided with a new element assembly irrespective of whether or not the other print elements provided in the same element assembly normally operate. This is not economical. 
     Further, it is very difficult, because of insufficient flexibility of the flexible printed wiring board 56, to wrap the board 56 around a part of the piezoelectric block 51 so that it is completely in contact with the piezoelectric block body 51. Hence, it is very difficult to position the wiring board 56, and soldering failure may occur. 
     SUMMARY OF THE INVENTION 
     It is a general object of the present invention to provide a print head for a wire dot-matrix printer, in which the above disadvantages are eliminated. 
     A more specific object of the present invention is to provide a print head for a wire dot-matrix printer in which each print element is replaceable. 
     The above objects of the present invention are achieved by a print head for a wire dot-matrix printer comprising: an element base; a plurality of print elements engaged with and supported by the element base, each of the print elements including a print wire, an actuator moving the print wire, and a base supporting the actuator, the base of each of the print elements having an engagement hole; and an engagement member inserted into the engagement hole of the base of each of the print elements and engaged with the base of each of the print elements. 
     The above objects of the present invention are also achieved by a print head for a wire dot-matrix printer comprising: a module base; and a plurality of element assemblies supported by the module base. Each of the element assemblies comprises: an element base supported by one of the element assemblies; a plurality of print elements engaged with and supported by the element base, each of the print elements including a print wire, an actuator moving the print wire, and a base supporting the actuator, the base of each of the print elements having an engagement hole; and an engagement member inserted into the engagement hole of the base of each of the print elements and engaged with the base of each of the print elements. 
     The above objects of the present invention are also achieved by a print head for a wire dot-matrix printer comprising: a shuttle base; and a plurality of head units. Each of the head units comprises: a module base; and a plurality of element assemblies supported by the module base. Each of the element assemblies comprises: an element base supported by one of the element assemblies; a plurality of print elements engaged with and supported by the element base, each of the print elements including a print wire, an actuator moving the print wire, and a base supporting the actuator, the base of each of the print elements having an engagement hole; and an engagement member inserted into the engagement hole of the base of each of the print elements and engaged with the base of each of the print elements. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects, features and advantages of the present invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a perspective view of a conventional shuttle bar system type wire dot-matrix printer; 
     FIG. 2 is a front view of a shuttle mechanism employed in the printer shown in FIG. 1; 
     FIG. 3A is a perspective view of a print head used in the printer shown in FIG. 1; 
     FIG. 3B is a perspective view of an element base used in the printer shown in FIG. 1; 
     FIG. 4A is a front view of a print element used in the printer shown in FIG. 1; 
     FIG. 4B is a side view of the print element shown in FIG. 1; 
     FIG. 5A is a perspective view of a piezoelectric actuator; 
     FIG. 5B is a perspective view of a piezoelectric body of the actuator shown in FIG. 5A; 
     FIG. 5C is a plan view of a flexible printed wiring board used in the actuator shown in FIG. 5A; 
     FIG. 5D is a cross-sectional view of the wiring board shown in FIG. 5C; 
     FIG. 6 is a perspective view of part of a print head according to an embodiment of the present invention; 
     FIG. 7A is a front view of a print element used in the embodiment of the present invention shown in FIG. 6; 
     FIG. 7B is a side view of the print element shown in FIG. 7A; 
     FIG. 8 is a perspective view of a print head having a plurality of element assemblies, each having the structure shown in FIG. 6; 
     FIG. 9A is a front view of a variation of the print element shown in FIG. 6; 
     FIG. 9B is a side view of the variation shown in FIG. 6; 
     FIG. 10A is a plan view of a flexible printed wiring board used in embodiments of the present invention; and 
     FIG. 10B is a perspective view of a piezoelectric actuator used in the embodiments of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 6 is a perspective view of part of a print head according to an embodiment of the present invention. In FIG. 6, parts that are the same as those shown in the previously described figures are given the same reference numbers. 
     The part of the print head shown in FIG. 6 comprises an element base 103, six print elements 101 and 102 (only two print elements are illustrated for the sake of convenience), and an engagement member 104. The print element 101 comprises a piezoelectric actuator 101a, and a base 101b supporting the piezoelectric actuator 101a. An engagement hole 101c is formed in the base 101b, and connects two opposite sides of the base 101b to each other in the direction substantially perpendicular to the direction in which the print wire 41e (FIG. 7A) of the print element 101 extends. 
     The print element 102 comprises a piezoelectric actuator 102a, and a base 102b supporting the piezoelectric actuator 102a. An engagement hole 102c is formed in the base 102b, and connects two opposite sides of the base 102b to each other in the direction substantially perpendicular to the direction in which the print wire of the print element 102 extends. Each of the remaining four print elements is configured in the same manner as the print element 101 or 102. 
     The element base 103 has seven projections 103d. The two recesses 42a (FIGS. 7A and 7B) of each print element engage two adjacent projections 103d. Three of the seven projections 103d respectively have holes 103a, 103b and 103c. A screw 105 penetrates the hole 103a. Similarly, two screws (not shown for the sake of convenience) penetrate the holes 103b and 103c, respectively. 
     The engagement member 104 is a plate-shaped member having a length and width such that it is suitable to pass through the engagement holes 101c and 102c of the six print elements. The engagement member 104 has a property of resilience, and is made of, for example, a metallic material such as stainless steel or SPCC. It is also possible to make the engagement member 104 of resin. The engagement member 104 has six engagement portions 104a-104f, and three screw holes 104g, 104h and 104i. The screw 105 penetrates a hole of the element base 103 and engages the screw hole 104g of the engagement member 104 when the engagement member 104 is placed in position. Further the engagement member 104 has indentations 104j and 104k. The indentations 104j are located between the screw holes 104g and 104hand the indentations 104k are located between the screw holes 104k and 104i. The width of the engagement member 104 measured at the indentations 104j and 104k is less than that measured at other portions thereof. 
     The piezoelectric actuators 101a and 102a can be configured as shown in FIGS. 5A through 5D. However, as will be described in detail later, it is preferable to use an improved piezoelectric actuator. 
     The six print elements 101 and 102 are fastened to the element base 103 by means of the engagement member 104 and the three screws 105 in the following process. The print elements 101 and 102 are inserted into spaces, each formed between two adjacent projections 103d of the element base 103. In this state, the engagement member 104 is inserted into the engagement holes 101c and 102c of the print elements 101 and 102 and is positioned so that the engagement portions 104a-104fengage the inner walls of the engagement holes 101c and 102c of the six print elements 101 and 102. Then, the three screws 105 are respectively inserted into the holes 103a, 103b and 103c, and are turned such that the three screws 105 engage the screw holes 104g104h and 104i. As the screws 105 are turned, the engagement member 104 is elastically deformed at the positions of the indentations 104j and 104kand the engagement portions 104a-104fcome into strong contact with the inner walls of the engagement holes 101c and 102c of the six print elements. Hence, all the print elements are tightly fastened to the element base 103 at the respective given positions. It will be noted that the indentations 104j and 104k of the engagement member 104 facilitate elastic deformation thereof. However, it is possible to omit the indentations if the engagement member without any indentations can be deformed by being screwed in. It can be seen from the above that one or more print elements can be easily replaced by a new one or more print elements. 
     FIG. 8 is a perspective view of a print head including four element assemblies 131, 132, 133 and 134, each of which is configured as shown in FIG. 6. In practice, a plurality of print heads, each having the structure shown in FIG. 8, are installed in a printing machine in the same manner as shown in FIG. 1. 
     FIGS. 9A and 9B show a variation of the aforementioned piezoelectric actuator 101. A base 101h having an engagement cutout portion 101i is used in lieu of the base 101b having the engagement hole 101c. The engagement member 104 shown in FIG. 6 can be easily inserted into the cutout portion 101i from the bottom side of the base 101h. 
     FIG. 10A shows an improvement in the flexible printed wiring board 56 shown in FIG. 5C. An improved flexible printed wiring board 156 shown in FIG. 10A, which has the same layer structure as shown in FIG. 5D, has cutout portions 156c, 156d, 156e and 156f. As shown in FIG. 10B, the cutout portions 156c and 156d are positioned at an edge of the piezoelectric block 51 when the flexible wiring board 156 is placed in position and wrapped around the block. Similarly, the cutout portions 156e and 156f are positioned at another edge of the piezoelectric block 51 when the flexible wiring board 156 is placed in position and wrapped. The cutout portions 156c-156f make it easy to bend the flexible wiring board 156, so that the board 156 can be brought into close contact with the piezoelectric block 51. Hence, it is easy to position the flexible wiring board 156 and prevent soldering failure. The cutout portions 156c-156f shown in FIG. 10A have a semi-oval shape. It is also possible to use cutout portions having a semi-circular shape, a rectangular shape or a triangle shape. 
     The flexible wiring board 156 has oval soldering windows 156a and 156b formed in the second insulating layer 59 (FIG. 5D). The major axis of each of the oval windows 156a and 156b extends in the direction perpendicular to the longitudinal direction of the electrode 52A. Thereby, it is possible to compensate for positional errors of the electrodes 52A and 52B and the flexible wiring board 156. It is also possible to employ a shape, such as a rectangular shape having the longitudinal direction perpendicular to the longitudinal direction of the electrodes 52A and 52B. The flexible wiring boards are not limited to L-shaped structures. Alternatively, flexible wiring boards having an extension extending in the direction perpendicular to the longitudinal direction of the electrodes 52A and 52B can be used. For example, such flexible wiring boards may have I-shaped or T-shaped structures. In this case, it is preferable to form cutout portions as shown in FIGS. 10A and 10B and to employ windows for soldering as shown in FIG. 10A. 
     The present invention can be applied to not only line printing machines but also serial printing machines. It is also possible to use an element base which is made of resin and has hooks capable of engaging the element base 103. 
     The present invention is not limited to the specifically disclosed embodiments, and variations and modifications may be made without departing from the scope of the present invention.