Wire-dot printer having printing wire driving device and manufacturing method thereof

Abstract: A lever (130) for driving printing wires (198) has its outer end hooked to form a rotation support portion (196). A frame (100) for enclosing driving coils (130) and others has a yoke (150) having projections same in number as that of levers disposed on its end surface. The lever has the hooked supporting point locked by a recession formed bet­ ween the yoke projections and an end surface (114) of the frame and fixed by a retainer spring (210). When an exci­ ting current flows to the coils, the printing wires are struck on a platen round the supporting point. Since the lever is not provided with a rotating shaft member, the size in a cross direction is minimized. Accordingly, the number of levers enclosable in the frame is multiplied to enhance a packaging density.

Description: The present invention relates to a wire-dot printer having a printing wire driving device or printing head for printing a predetermined dot matrix pattern by means of printing wires which are impacted via an ink ribbon onto a recording medium placed on a platen by energizing driving coils. The present invention further relates to a method for manufacturing such printing wire driving device. A printing wire driving device for printing patterns by means of printing wires requires to have a multiplicity of for example 24 or 48 sets of a driving coil, a lever and a printing wire enclosed in a frame as a fundamental head constituent. Therefore, it is a big problem to harmonize an enhancement of the integration density with an easiness of assembly. One method to solve this problem is disclosed in US-A-4,767,227, where printing wires of a printing wire driving device are fixed to one end of levers, a rotating shaft is mounted to each lever which is to be attracted by means of a driving coil and recessions are formed for fixing the rotating shafts on a yoke. A problem inherent in this known printing wire driving device is that since the yoke requires to have sufficient space for accommodating the rotating shafts of the levers, the number of units including driving coil, lever and printing wire, which can be arranged on one plane is limited and, thus, the integration density cannot be enhanced. Further, since a rotating shaft has to be mounted on each lever, the number of manufacturing steps increases and the assembling work becomes complicated to incur a rise of the cost. In order to increase the number of printing wires to enhance the printing quality a multistage frame is required resulting not only in a large overall size of the printing wire driving device but also in different lengths of driving wires. Due to such different lengths the flexure of the driving wires at the time of impact varies with the consequence of a difference in the print density. The document GB-A-2 175 853 discloses a wire-dot printer having a printing wire driving device comprising a frame consisting of a magnetic material and an outer peripheral wall and a plurality of core portions disposed radially inside of said outer peripheral wall at regular intervals in a circumferential direction, the end faces of said outer peripheral wall and core portions being in the same plane, a plurality of driving coils each surrounding a corresponding core portion, a first yoke consisting of a magnetic material and having radially extending projections, which is placed on the outer peripheral wall of said frame such that recessions formed between said projections are registered with the core portions, a plurality of levers having their respective radially outer side hooked to form a rotation support portion, their central portion adapted to act as an armature portion and a respective printing wire driven by their radially inner end portions, wherein the rotation support portion engages a corresponding recession of said first yoke, a supporting point retainer spring having radial spring arms each engaging the radially outer side of a respective lever for resiliently pressing the rotation support portion thereof toward the first yoke and the frame, a nose for guiding said printing wires to a front end, and a lever holder for resiliently pressing the first yoke to the frame and the supporting point retainer spring to the levers. It is an object of the present invention to provide a printing wire driving device for a wire-dot printer, capable of enhancing the integration density of printing wires, simplifying the assembling work and further reducing the manufacturing costs at the same time. A further object of the invention is to provide a printing wire driving device having a short reset time after printing. Another object of the invention is to provide a printing wire driving device using levers that can be manufactured by forging and press working. Another object of the invention is to provide a printing wire driving device with a multiplicity of printing wires having uniform motional characteristics. Another object of the invention is to provide a printing wire driving device including a positioning structure allowing a simple and precise positioning of each member in a layer-built structure. Another object of the invention is to provide a printing wire driving device exhibiting a smooth movement of moving members such as lever, printing wire and the like. Another object of the invention is to provide a printing wire driving device having a fixing structure capable of a simple mounting of a radiator for radiating heat generated by the printing wire driving device. Another object of the invention is to provide a printing wire driving device which allows an adjustment of the stroke of the printing wires through a simple work. Still another object of the invention is to provide a manufacturing method for a printing wire driving device which is capable of simplifying the manufacturing process. These objects are achieved with a wire-dot printer having a printing wire driving device and a manufacturing method thereof, respectively, as claimed. Further objects and effects of the invention will become apparent from the following detailed description of preferred embodiments in conjunction with the accompanying drawings, wherein: Fig. 1 is a sectional view taken on line I - I in Fig. 11, representing one example of a typical printing wire driving device according to the invention, Figs. 2 and 3 are exploded perspective views of the printing wire driving device shown in Fig. 1, Fig. 4 is a partially sectional view indicating portions where to apply and charge a lubricant, Fig. 5 is a top view representing a construction of the base plate of a frame, Fig. 6 is a perspective view, partly in section, representing one example of a coil, Fig. 7 is a perspective view representing one example of a spring holder, Figs. 8(a) and (b) are a front view and a side view, respectively, representing a construction of a lever, Fig. 9 is sectional perspective view representing an intermediate state of the assembly, Fig. 10 is a sectional view of a core and an outer peripheral wall portion according to another example of the frame, Fig. 11 is a perspective view showing the outer appearance of the printing wire driving device according to the invention, Fig. 12 is an exploded perspective view showing a mounting construction of a guard cover, Fig. 13 is a perspective view representing the relation between a nose and a circuit substrate, Fig. 14 is a side view indicating a process for mounting the printing wire driving device on a carriage, Figs. 15(a) to (d) are illustrations representing various steps of the operation of the printing wire driving device according to the invention, Fig. 16 is a top view representing the arrangement of the levers, Fig. 17 is a sectional view indicating a flexure of a printing wire from the end of a lever to a rear guide plate, Figs. 18(a) to (c) are illustrations indicating driving methods applicable to the invention, Fig. 19 (a) is a diagram indicating a relation between a number of concurrently driven printing wires and the printing force when the printing wire driving device is driven according to the methods illustrated by Figs. 18(a) and 18(b), Fig. 19 (b) is a diagram indicating the relation between the number of concurrently driven printing wires and the printing force obtained with a preferred driving method of the printing wire driving device according to the invention, Fig. 20 is a perspective exploded view for explaining a method of determining an excitation polarity of the coils, Fig. 21(a) and (b) are illustrations for explaining another possibility of modifying the excitation polarity of the coils, Figs. 22 and 23 are side views representing other examples of levers applicable to the present invention, Fig. 24 is a perspective exploded view representing a method for setting damper elements, Fig. 25 represents a method for setting a fixing spring to a lever holder and illustrating the pressure exerted by the spring to the lever holder and Fig. 26 is a plan view of a printer provided with a printing wire type printing head. Fig. 26 shows a schematic plan view of a wire-dot printer according to the invention. Desired patterns forming for example characters or the like are printed on a paper K disposed between a platen P and an ink ribbon R by a printing wire type printing head H mounted on a carriage C which is supported shiftably in the direction of print columns or printing lines. The printing head H is shown in Fig. 1. Generally it comprises a circuit substrate 340 to which coils 130 are fixed and which is used for connecting the printing head to an external control device, a frame 100 enclosing the coils 130 and constituting a part of a magnetic circuit, levers 190 consisting of a magnetic material and having an outer side hook shaped to form a rotation support portion 194, a central portion forming a projected armature portion 196 and an inner end to which a printing wire 198 is fixed, a first yoke 150 and a second yoke 200 for rotatably supporting the levers 190 in cooperation with the frame 100, a supporting point retainer spring 210 for resiliently and continuously pressing the outer end portion of the levers 190 into a corner formed by an end face of an outer peripheral wall 102 of the frame 100 and a recession 152 (Fig. 3) of the first yoke 150, a lever holder 220 pressing the retainer spring 210 and functioning as a part of the case of the printing head, a nose 240 for guiding the printing wires 198, and a fixing spring 230 for fixing the lever holder 220 on the nose 240. The frame 100 consists of a magnetic material and is formed like a cup having the ring shaped outer peripheral wall 102 and a ring shaped inner peripheral wall 104 standing upright on a base plate 112. A through hole 106 is provided in the base plate 112 and inside of the inner peripheral wall 104 (Fig. 2). A portion of the nose 240 and a spring holder 170 extend into the through hole 106. As is best shown in Fig. 2, a plurality of cores 110 (24 in this embodiment) are circumferentially arranged at regular intervals within a space 108 formed between the inner side of the outer peripheral wall 102 and the outer side of the inner peripheral wall 104. The outer peripheral wall 102, the inner peripheral wall 104 and the cores 110 have the same height so that their end faces 114, 116 and 118, respectively, are in the same plane, this plane providing a reference plane for other components to be assembled on the frame 100. A lubricant 113 is applied to the end faces 118 of the cores 110 (Fig. 4) in order to decrease friction and wear resulting from a contact with the armature portion 196 of respective levers 190. The coils 130 are wound on respective bobbins 132 and are inserted into the space 108 between the outer peripheral wall 102 and the inner peripheral wall 104 such that each core 110 is surrounded by a respective coil 130. Through holes 120, 122 are, as shown in Fig. 5, provided in the base plate 112 of the frame 100 to allow the terminals of the coils 130 to pass through and to be connected to the circuit substrate 340 as shown in Fig. 1. Fig. 6 represents one example of a coil 130 wound around the bobbin 132. The bobbin is formed of an electrically insulating material such as a macromolecular material or the like in a tube like shape to be insertable into the frame 100 as mentioned above. The bobbin 132 has such a height that it projects above the end face 118 of the core 110 forming a space 136 above the end face of the core as shown in Fig. 9. The armature portion 196 of the respective lever 190 is accommodated in this space 136. Two cylindrical legs 138, 140 are provided on the lower side of the bobbin 132 and terminals 142, 144 inserted in the legs project through the through holes 120, 122 (Fig. 5) of the frame base plate 112 to be soldered to the circuit substrate 340. The two ends 143 and 146 of the coil 130 wound on the bobbin 132 are connected to the terminals 142, 144, respectively. The radially extending walls 132a of the bobbin 132 are preferably thinner than the walls 132b extending in the circumferential direction in order to allow a multiplicity of coils to be efficiently arrayed in the circumferential direction. The thinner the walls 132a are, the higher the space efficiency becomes. It is also possible to use a coil bobbin having a portion of one or both walls 132a removed like a window. As shown in Fig. 3, the first yoke 150 made of a magnetic material is substantially ring shaped provided with tooth like projections 154 projecting radially outward from the outer periphery of the ring at regular intervals. Recessions 152 are defined between adjacent projections 154. The first yoke 150 is disposed on the end face 114 of the outer peripheral wall 102 of the frame 100 in such a way that the recessions 152 are radially aligned with respective cores 110. A ring plate 160 of a wear resisting material is placed between the end face 114 and the first yoke 150. A lubricant 158 (Fig. 4) is applied to the surfaces of these recessions 152 and the ring plate 160 for smoothing the movement of the levers 190 and decreasing their wear at the same time. Normally the wear resisting material has a low magnetic permeability so that its insertion results in an increased magnetic resistance. However, since the ring plate 160 used in the present invention is extremely thin, namely has a thickness between 30 to 160 µm at most, the increase of the magnetic resistance or a magnetic saturation caused by the ring plate 160 can be neglected. As shown in Fig. 7, the spring holder 170 has a body 172 adapted to be inserted into the through hole 106 of the frame 100. The body 172 is provided with an extension 173 at its lower end portion for positioning a rear end guide plate 260 (Fig. 2). Spring accommodating holes 174 are provided on the upper side of the body 172 at positions corresponding to the positions of the cores 110 in an assembled state. A reset spring 176 is provided in each of the spring accommodating holes 174 for pushing a corresponding lever 190 into a reset position against a damper member 410 (Fig. 1). Axially extending projections 178 are provided at the upper side of the body 172 to partly surround the spring accommodating holes 174 at the outer periphery side and to form guide grooves 180 between each other for guiding an inner end or nose portion of the respective lever 190. A plurality of these projections (three projections in the shown embodiment) are further extended to function as guide pieces 182 for the damper member 410 (Fig. 24). The levers 190 have their outer end portion formed like a hook as shown in Fig. 8(b), providing a rotation support portion 194 with a circular arc shaped portion 192 formed at the nose of the hook. The central portion of each lever 190 which is disposed opposite to a core 110 in the assembled state, is projected to form the armature portion 196. The printing wire 198 is fixed to the inner end portion of the lever. The rotation support portion 194 has the inside formed somewhat in an acute angle to be positioned by three wall surfaces 152a, 152b, 152c (Fig. 9) of the recession 152 of the first yoke 150 and the wear resisting ring plate 160. The nose portions of the levers 190 are respectively fitted in the grooves 180 of the spring holder 170, come into contact with a respective reset spring 176 and are resiliently pressed against the damper member 410. As described above, ring plate 160 is interposed between the outer peripheral wall 102 of the frame 100 and the first yoke 150. The rotation support portion 194 of each lever 190 lies on the ring plate 160 as shown in Figs. 4 and 9. Instead of providing ring plate 160 a wear resisting layer 504 may be formed on the end face 114 of the outer peripheral wall 102 of the frame 100 as shown in Fig. 10. A similar wear resisting layer 506 may be formed on the end face 118 of each core 110. The wear resisting layer 506 serves to decrease a wear of the core 110 that might occur due to the contact with the armature portion 196. The second yoke 200 consists of a magnetic material and is formed like a daisy as shown in Fig. 3. The second yoke 200 has a ring portion 202 and branches 204 extending radially outwardly from the lower end face of the ring portion 202 (Figs. 3 and 9). The branches are arranged at the same regular intervals as the projections 154 of the first yoke 150 to coincide with the latter when the second yoke 200 is placed on the first yoke 150. The ring portion 202 is fitted onto an outer peripheral wall portion 181 of the spring holder 170 in this embodiment (Fig. 9). Thus, the levers 190 will be further guided between side walls 205 of the branches 204. In other words each lever 190 is guided in the lateral (circumferential) direction by side walls 205 of branches 204 and side walls 152a, 152c defining the recessions 152 of the first yoke 150. The smaller the gap between the lever 190 and the corresponding walls, the higher the magnetic efficiency is. However, if these gaps are very small and the relative position between the first yoke and the second yoke is out of order, it will be impossible to insert the levers 190. In order to avoid this problem the gap between the corresponding wall faces of the first yoke and the second yoke on the one hand and the lever 190 on the other hand may be somewhat larger than would be optimum with respect to the magnetic efficiency. In the described embodiment the ring portion 202 of the second yoke 200 is disposed at the radially inner ends of the branches 204. However, it is apparent that a similar effect would be obtained if the ring portion 202 was disposed at the radially outer ends of the branches 204. As shown in Fig. 3 the supporting point retainer spring 210 has an inner ring portion 212 from the outer periphery of which spring arms 214 extend slantwise radially outwardly. In the assembled state the outer end portions of the spring arms 214 come into contact with an upper end of the rotation support portion 194 of respective levers 190 as is best shown in the lower portion of Fig. 1. Thus, the circular arc shaped portion 192 of the rotation support portion 194 of each lever 190 is pressed against the wall surface 152b of the recession 152 of the first yoke 150 and the wear resisting plate 160 (or the wear resisting layer 504 in case of the above mentioned alternative). The lever holder 220 is shown in Fig. 3 as being of a cup-like form. It is made of a resilient material such as a macromolecular material or the like. In the assembled state, the end face of an outer peripheral wall 224 is in contact with the second yoke 200, namely the outer portions of its branches 204, whereas a cover plate 226 of the lever holder 220 is in contact with the ring portion 212 of the supporting point retainer spring 210 (see Fig. 1 in addition to Fig. 3). The lever holder 220 is resiliently fixed to a projection 244 of the nose 240 by means of the fixing spring 230. The spring 230 acts on a central portion of the lever holder 220 via a reinforcing plate 400. The lever holder 220 is thereby deflected to come into contact with the projections 178 provided on the spring holder 170 and is thus placed in position axially. According to this construction, the holding or rest position of the printing levers 190 is determined by the position of the damper member 410 provided on the lever holder 220. Therefore the holding position of the printing levers 190 can be kept accurate, thus minimizing any dispersion of the stroke among the levers (Fig. 1, Fig. 24). According to the invention, the levers 190 have their rotation support portion 194 pushed into the recession 152 of the first yoke 150 and against the wear resisting ring plate 160 by the supporting point retainer spring 210. The center of rotation of the levers is formed here and, therefore, a special rotating shaft is not required. This allows a reduced thickness W (Fig. 8(a)) of the levers 190 and a correspondingly high number of levers to be arrayed on the frame 100 to a high density. Further, since according to the invention a rotating shaft for the levers 190 is not required, they can be manufactured through forging and presswork only leading to a reduction of manufacturing cost. The nose 240 comprises a cylindrical portion 249 fitted into the through hole 106 of the frame 100, a pedestal 246 in contact with the base plate 112 of the frame 100 and two legs 242 projecting downwardly beyond the frame 100 as shown in Figs. 1 and 13. The thickness of the legs in the axial direction is greater than the height of soldered portions 342 where a connector 350 is soldered to the circuit substrate 340. At the front end at the nose 240 a front guide plate 250 is provided through which guide holes 252 (Fig. 2) for guiding and aligning the front ends of the printing wires 198 are perforated. A rear guide plate 260 having guide holes 262 for the printing wires 198 arrayed along a circle according to the array of the inner end portions of the levers 190, is disposed at the rear end of the nose nearest to the levers 190, i.e. in the cylindrical portion 249 of the nose 240. In addition to these guide plates 250 and 260, a cup-like guide member 270 and intermediate first, second and third guide plates 280, 290 and 300 are disposed in this order between the rear guide plate 260 and the front guide plate 250. A plurality of guide holes 272, 282, 292, 302 is perforated in each of the guide member 270 and the guide plates 280, 290 and 300, respectively, each of these guide holes lying on a straight line by which corresponding guide holes 252 of the front guide plate 250 and 262 the rear guide plate 260 are connected. Thus, the guide holes of the rear guide plate 260, the guide member 270, the guide plates 280, 290, 300 and the front guide plate 250 define a plurality of straight line passages for the plurality of printing wires 198. Lubricants 274′, 306 such as grease or the like are enclosed in a space 275 defined by the cuplike guide member 270 and the rear guide plate 260, and also in a space 304 defined by the front guide plate 250 and the guide plate 300 (Fig. 4). Thereby the friction between the printing wires 198 and the guide holes is minimized and a smooth movement of the printing wires ensured. The printing wire driving device 310 thus assembled has a heat sink member 320 installed on its outer periphery (Fig. 11). The heat sink member 320 functions as heat radiating means efficiently radiating the joulean heat generated by the coils 130. Spring arms 232 are formed on the fixing spring 230 to project outwardly and are used to lock the heat sink member 320 (Fig. 1). By the elastic contact between the spring arms 232 and the heat sink member 320 a heat conduction path is established and the heat sink member 320 is grounded through the fixing spring 230 at the same time. Further, when installed on a portion that may be touched by a user, namely a carriage 360 (Fig. 14), the heat sink member 320 has a guard cover 330 installed on its top portion. The guard cover 330 is made of a bad heat conductor such as plastic or the like. As shown in Fig. 12, the guard cover 330 has a multiplicity of air vents 328 perforated therein and two legs 332 extended so as to catch a side portion of the heat sink member 320. When the legs 332 are inserted between fins 324 of the heat sink member 320, locking claws 334 formed on the end portions of the legs 332 engage with locking projections 326 on the side of the heat sink member. The printing wire driving device 310 is connected to an external driving circuit which is not illustrated, by means of the connector 350 on the circuit substrate 340 and is then fixed on the carriage 360 as shown in Fig. 14. Since the legs 242 of the nose 240 are formed thicker than the soldered portions 342 of the circuit substrate 340, as mentioned earlier, the soldered portions 342 are protected by the legs 242 and, hence, will be prevented from being short-circuited by the carriage 360 consisting of a metallic material. Next, the operation of the printing head constructed as explained above will be described with reference to Figs. 15(a) to 15(c). As long as no exciting current is applied to a coil 130, the radially inner portion of the corresponding lever 190 is pressed against the damper member 410 by the reset spring 176 while its outer or rear end is pushed by a corresponding spring arm 214 of the supporting point retainer spring 210, holding the rotation support portion 194 against the vertical wall 152b of the recession 152 of the first yoke 150 and the wear resisting ring plate 160. In this position of the lever 190, a gap δ will be formed between its armature portion 196 and the end face 118 of the corresponding core 110 (Fig. 15(a)). When an exciting current is applied to the coil 130 in such state, the lever 190 is exposed to a magnetic attraction force by the core 110 and turns, against the force of the reset spring 176, toward the core 110 around the rotation support portion 194. The ends of the printing wires 198 fixed to the respective levers 190 define a circle 198a in the plan view as shown in Fig. 16. The guide holes 262 of the rear guide plate 260 are arrayed on a circle 265 whose diameter is smaller than that of the circle 198a. Therefore, a portion 199 of each printing wire 198 between the corresponding lever 190 and the corresponding guide hole 262 is slanted by the same angle Θ as shown in Fig. 17. Accordingly, the reaction force arising from such deformation of the printing wires 198 will take the same effect on all levers 190 and, hence, all levers 190 will have the same kinematic characteristic with respect to the magnetic attraction force of the corresponding coil 130. It has been confirmed that if the route of the printing wires 198 from the rear guide plate 260 to the front guide plate 250 is linear, the kinematic characteristic of the printing wires will be almost constant irrespective of an angle of the route. When a lever 190 is attracted to the core 110 and the corresponding printing wire 198 shifted, the printing wire strikes a recording medium (normally a piece of paper) and a platen through an ink ribbon to form a dot on the surface of the recording medium. The distance between the printing end of the printing wires 198 and the platen in the rest position of the printing wires is set so long that there is a gap α between the end face 118 of the core 110 and the armature portion 196 at the time of impact, as shown in Fig. 15(b). Therefore, the magnetic flux density will not increase unnecessarily. If the gap between the printing end of the printing wires 198 and the platen is set as mentioned above, when the flow of current to a coil 130 is stopped in this stage, the lever 190 will move further due to its inertia. A rear corner 195 of the armature portion 196 comes then into contact with the core 110, and the lever 190 turns around this point of contact as pivot, while its rotation support portion 194 is lifted against the elastic force of the spring arm 214 (this situation is shown in Fig. 15(c)). When the end face of the armature portion 196 fully contacts the end face 118 of the core 110, the lever 190 will be stopped (Fig. 15(d)). In this stage, the lever 190 is subject to a reaction force F exerted by the reset spring 176 and a reaction force F1 from the spring arm 214 of the supporting point retainer spring 210 urging it into the original or rest position. In a first stage of the lever's movement induced by these reaction forces, the lever will turn around the contact point 195 (Fig. 15(c)), i.e. a point close to the center of the lever resulting in a low moment of inertia. Therefore, the lever is turned at a high speed due to the reaction forces F, F1 until its rest position is reached where it comes into contact with the damper member 410 (Fig. 15(a)). Tests revealed that the moment of inertia with the contact point 195 as the pivot was reduced by 20 to 50% as compared with that when the lever turned around the rotation support portion 194, and the reset time of the lever 190 was shortened by 20 to 50% as compared to the prior art, accordingly. Thus, a high-speed driving of the printing wires 198 may be realized. In the printing wire driving device according to the invention, to attract the levers 190 to the respective cores 110 of the frame 100, all coils may either be excited to establish magnetic fields of the same polarity os shown in Fig. 18(a) or to exhibit magnetic fields of alternately inversed polarities as shown in Fig. 18(b). With these driving methods, however, the printing force exerted on an individual printing wire decreases with an increasing number of concurrently driven printing wires as shown in Fig. 19(a). In this Fig., I denotes the printing force applied to a printing wire when the driving method according to Fig. 18(a) is used, and II denotes the corresponding printing force, when the driving method according to Fig. 18(b) is used. Because of this decrease of the printing force, when many printing wires are to be driven concurrently, the coils 130 are preferably divided into groups of three adjacent coils each. The coils of one group (for instance C2 in Fig. 18(c)) are excited to establish magnetic fields of the same polarity, whereas the coils of the adjacent groups (C1, C3 in Fig. 18(c)) are excited to establish magnetic fields of the opposite polarity. This driving method minimizes the mutual magnetic interference among the levers 190 resulting in an almost uniform printing force regardless of the number of concurrently driven printing wires. In Fig. 19(b) I represents the printing force exerted on the printing wire positioned at the center of each group, whereas II represents the corresponding printing force exerted on the printing wires positioned at the opposite ends of each group. Needless to say, that the polarity of the magnetic field established by the coils 130 can be simply and arbitrarily set by changing the position (and thus the direction of the exciting current flow) in which the coils are connected to a wiring pattern 392 of the circuit substrate 340 as shown in Fig. 20, or by changing the turning directions R, L of the windings forming the coils 130 as shown in Fig. 21. Fig. 22 represents another example of the lever 190 applicable to this invention. In this case, the end face of the armature portion 196′ is formed with a slope γ so as to bring the front corner 195′ of the armature portion 196′ into contact with the end face 118 of the core when the printing wire 198 strikes the platen. Fig. 23 represents a further example of the lever 190 applicable to the invention. In this case, the armature portion 196″ of the lever has a triangular shape with the vertex of the triangle forming the point or line which will be brought into contact with the end face 118 of the core 110 leaving a gap between the remaining portions of the armature portion and the core at the time of impact. With the alternative structures of the lever 190 shown in Figs. 22 and 23, the center of rotation at the time of resetting will be at the armature portion 196′, 196″ and, therefore, the reset time will be shortended for the same reason as explained above. Turning to Fig. 1 again, a ring felt 370 impregnated with a lubricant is inserted in a space between the retainer spring 210 and the second yoke 200. The lubricant from the ring felt 370 spreads to the contact points between the spring arms 214 of the retainer spring 210 and the levers 190, as well as to the contact points between the rotation support portions 194 and the first yoke 150 and the wear resisting ring plate 160. Therefore, a wear or friction of these contact points will be minimized. The assembly of the printing head according to the invention will be described next. Reference is made to Fig. 2 to explain the assembly of the nose. The front portion of the nose 240, i.e. the portion that will face the platen in a printer, has an L shape in section. The guide holes 252 in the front guide plate 250 are provided in plural rows (two rows in the embodiment), according to an array of the dots. The front guide plate 250 has a lubricant feed port 258 perforated in a portion 256 externally exposed in the assembled state. The intermediate third guide plate 300 is inserted at a predetermined distance from the front guide plate 250 to form the space 304 used for keeping a lubricant therein. The second guide plate 290 and the first guide plate 280 are inserted toward the pedestal 246 in that order. The cuplike guide member 270 charged with a lubricant such as grease or the like is inserted into the cylindrical portion 249 where the pedestal 246 is joined to the latter. The rear guide plate 260 is then inserted to cover the open end of the cuplike guide member 270. These guide plates 250, 300, 290, 280 are held in position by grooves 241, 243, 245, 247, respectively, formed on the nose 240. The guide member 270 is positioned by a recession 274 engaging a projection (not indicated) within a through hole 248 of the cylindrical portion 249 of the nose 240. The rear guide plate 260 is positioned by through holes 264 engaging pins 276 of the guide member 270. The coils 130 are inserted into the frame 100 with each coil surrounding a corresponding core 110 and their terminals 142, 144 projecting through the through holes 120, 122 in the base plate of the frame. The circuit substrate 340 is attached to the base plate of the frame 100 with the insulating plate 380 being interposed. The terminals 142, 144 of the coils 130 projecting from the circuit substrate 340 are soldered to the predetermined conductive pattern 392 of the circuit substrate 340. Thus, the frame 100, the coils 130, the insulating plate 380 and the circuit substrate 340 are assembled to an integral unit. The cylindrical portion 249 of the nose 240 is inserted into the through hole 106 of the frame 100 of this unit. A projection 247′ provided on the pedestal 246 of the nose 240 passes a through hole 394 of the circuit substrate 340 to engage with a positioning hole 123 (Fig. 5) of the frame 100, thereby positioning the frame 100 and the nose 240 with respect to each other. The spring holder 170 having the reset springs 176 inserted into the spring accommodating holes 174 before, is set in the through hole 106 of the frame 100 to build up a frame unit 450 (Fig. 3). The cuplike guide member 270 enclosed in the nose 240 and the rear guide plate 260 are retained by the extension 173 of the spring holder 170. The further assembling steps will be described with reference to Fig. 3. The wear resisting ring plate 160 is placed on the end surface 114 of the outer peripheral wall 102 of the frame 100 of the frame unit 450, and the first yoke 150 is placed on the ring plate 160 with positioning holes 156 of the first yoke 150 being registered with positioning holes 162 of the ring plate 160. The ring plate may be guided by the inner side of the heat sink member 320. Spaces for accommodating the levers 190 are defined by the guide grooves 180 of the spring holder 170 and the recessions 152 of the first yoke 150. In this stage, the levers 190 are dropped and their printing wires 198 inserted into the guide holes 262, 272, 282, 292, 302 and 252 of the corresponding guide plates to extend along a straight line from the guide holes 262 to the guide holes 252. The levers 190 are inserted into the guide grooves 180 of the spring holder 170 at their nose portions and supported on the reset springs 176. When then the second yoke 200 is placed on the first yoke 150 with positioning holes 206 registered with the positioning holes 156 of the first yoke 150, the levers 190 will be caught in respective gaps between the branches 204. Thus, the levers 190 are pushed by the ring portion 202 of the second yoke 200 to have their rotation support portions 194 inserted into respective recessions 152 of the first yoke 150 onto the wear resisting ring plate 160. In this state, a damper spacer 412, a damper rubber 414 and a damper spacer 416 which constitute the damper member 410 are in that order inserted in a manner to be guided by the guide pieces 182 of the spring holder 170, and positioned on the nose or inner end portion of the levers 190 (Fig. 24). The levers 190 are in this state guided by the side walls of the branches 204 of the second yoke 200 and the guide grooves 180 of the spring holder 170 and are arrayed at regular intervals in the circumferential direction. Now the ring felt 370 impregnated with a lubricant is placed thereon. When the supporting point retainer spring 210 is placed on the second yoke 200 with its positioning holes 216 engaging the positioning pins 184 of the spring holder 170 (Fig. 7), the tips of the spring arms 214 are positioned between the branches 204 of the second yoke 200 to come into contact with the upper ends of the rotation support portions 194 of the levers 190. The ring felt 370 is then surrounded by the second yoke 200 and the supporting point retainer spring 210 and, thus, kept from coming off. Next, the lever holder 220 is installed with positioning pins 222 formed on its outer peripheral wall 224 engaging the positioning holes 162, 156 and 206 of the wear resisting ring plate 160, the first yoke 150 and the second yoke 200, respectively. Positioning holes 228 of the cover plate 226 of the lever holder 220 come into engagement with the positioning pins 184 of the spring holder 170. Thus, the lever holder 220 has an end face of its outer peripheral wall 224 brought into contact with the branches 204 of the second yoke 200 and its cover plate 226 brought into contact with the end faces of the projections 178 of the spring holder 170. The engagement of the positioning pins 222 with the positioning holes 162, 156 and 206 provides the positioning in the circumferential direction. The tips of the guide pieces 182 of the spring holder 170 are fitted into holes 229 of the lever holder 220 and will, therefore, not come into contact with the cover plate 226. Now, the reinforcing plate 400 is placed on the surface of the lever holder 220. As shown in Fig. 25, the fixing spring 230 has legs 236 expanded somewhat outside. When the fixing spring 230 is placed on the lever holder 220, upper portions of those legs 236 are accommodated in recessions 227 of the cover plate 226, and a window 238 of each leg 236 is caught on a corresponding projection 244 of the nose 240. A central portion of the cover plate 226 is urged by a rib portion 234 of the fixing spring 230 and the reinforcing plate 400 toward the nose so that the spring arms 214 of the supporting point retainer spring 210 come to push the rotation support portion 194 of the levers 190 to the first yoke 150 and the wear resisting ring plate 160 at a uniform and constant pressure. The fixing spring 230 contacts the cover plate 226 of the lever holder 220 only through the rib portion 234 formed at a central portion thereof (Fig. 25), and the central portion of the lever holder 220 is subjected to a uniform pressure and uniformly pushed to the positioning pins 184 of the spring holder 170 irrespective of some strain of the fixing spring 230. A shallow ring shaped recession 225 is provided in the inside surface of the lever holder 220 near to its central portion (Fig. 17). The recession 225 is formed to provide some clearance with respect to the ring portion 212 of the supporting point retainer spring 210. Thus, the supporting point retainer spring 210 has a strain of its ring portion 212 absorbed by the clearance, and the spring arms 214 will push the levers 190 with a constant force, accordingly. The cuplike guide member 270 and the rear guide plate 260 are positioned in the through hole 248 and held by the bottom of the spring holder 170 which is pressed by the cover plate 226 of the lever holder 220. Being resiliently pressed against the outer peripheral wall 224 of the lever holder 220, the ring plate 160, the first yoke 150 and the second yoke 200 will be placed in position by the positioning pins 222. After these assembling steps the branches 204 of the second yoke 200 and the projections 154 of the first yoke 150 are in contact with each other, and a magnetic flux generated by a coil 130 passes through a first magnetic circuit comprising the core 110 of the frame 100, the armature portion 196 of the lever 190, the rotation support portion 194, the outer peripheral wall 102, the base plate 112 and the core 110, and through a second magnetic circuit comprising the core 110, the armature portion 196 of the lever 190, the branches 204 of the second yoke 200, the projection 154 of the first yoke 150, the outer peripheral wall 102 of the frame 100 and the base plate 112 of the frame 100. Due to this magnetic flux, the lever 190 is efficiently attracted to the core 110. When the printing head has been assembled as explained above, a lubricant such as grease or the like is fed into the space 304 between the front guide plate 250 and the third guide plate 300 from the feed port 258 of the front guide plate 250 by means of a syringe or the like. As described above, each member to be placed between the frame 100 and the lever holder 220 is dropped and set and, therefore, an automatic assembling can be used for most of the assembling process. If during inspection after the assembly the stroke of the printing wires has to be adjusted, the fixing spring 230 will be demounted from the nose 240. Then the lever holder 220 is demounted and the stroke adjusted by simply changing the number of spacers 416.

Claims: A wire-dot printer having a printing wire driving device comprising: (a) a frame (100) consisting of a magnetic material and having an inner peripheral wall (104) and an outer peripheral wall (102) and a plurality of core portions (110) disposed between said inner and outer peripheral walls (104, 102) at regular intervals in a circumferential direction, the end faces (116, 114, 118) of said inner and outer peripheral walls and core portions being in the same plane, (b) a plurality of driving coils (130) each surrounding a corresponding core portion (110), the upper ends of said driving coils projecting beyond the end faces (118) of the core portions (110), (c) a first yoke (150) consisting of a magnetic material and having radially extending projections (154), which is placed on the outer peripheral wall (102) of said frame (100) such that recessions (152) formed between said projections (154) are registered with the core portions (110), (d) a plurality of levers (190) having their respective radially outer side hooked to form a rotation support portion (194), their central portion adapted to act as an armature portion (196) and a respective printing wire (198) fixed to their radially inner end portion, wherein the rotation support portion engages a corresponding recession (152) of said first yoke (150), (e) a second yoke (200) consisting of a magnetic material and having branches (204) extending radially from a ring portion (202), said branches (204) being placed on said projections (154) of said first yoke (150), (f) a supporting point retainer spring (210) having radial spring arms (214) each engaging the radially outer side of a respective lever (190) for resiliently pressing the rotation support portion thereof toward the first yoke (150) and the frame (100), (g) a nose (240) for guiding said printing wires (198) to a front end, and (h) a lever holder (220) for resiliently pressing the first yoke (150) and the second yoke (200) to the frame (100) and the supporting point retainer spring (210) to the levers (190). The printer as defined in claim 1, wherein said lever holder (220) is fixed to the nose (240). The printer as defined in claim 2, characterized by a fixing spring (230) for fixing said lever holder (220) on the nose (240). The printer as defined in any of claims 1 to 3, further comprising a spring holder (170) inserted into a through hole (106) provided in a center portion of said frame (100) inside of said inner peripheral wall (104), a plurality of grooves (180) being formed in said spring holder (170) at positions opposite to respective core portions (110), a reset spring (176) being inserted into each groove (180) at a center portion thereof for urging said levers (190) into a reset position, wherein the spring holder (170) is resiliently pressed by said lever holder (220) to the frame (100). The printer as defined in claim 4, wherein said spring holder (170) is provided with a positioning pin (182) engaging a positioning hole (229) in said lever holder (220). The printer as defined in any of claims 4 and 5, wherein said fixing spring (230) has a rib (234) on a surface opposite to a cover (226) of the lever holder (220), said rib projecting toward the lever holder. The printer as defined in any of claims 4 to 6, wherein a reinforcing plate (400) is interposed between said fixing spring (230) and said lever holder (220). The printer as defined in claims 4 to 7, wherein the fixing spring (230) has an outwardly projecting claw formed on a leg (232), radiating means (320) being fixed thereon through said claw. The printer as defined in any of claims 4 to 8, wherein a radially inner end portion of each lever (190) is inserted into a corresponding groove (180) of said spring holder (170). The printer as defined in any of the preceding claims, wherein a lubricant layer is provided on the rotation support portion (194) of each lever (190) and the recessions (152) of the first yoke (150). The printer as defined in any of the preceding claims, wherein a lubricant layer is provided on the end face (118) of each core portion (110). The printer as defined in any of the preceding claims, wherein a ring plate (160) of a wear resisting material is interposed between the end face (114) of the outer peripheral wall (102) of said frame (100) and the first yoke (150). The printer as defined in any of the preceding claims, wherein a wear resisting metallic layer (504 in Fig. 10) is formed on the end face (114) of the outer peripheral wall (102) of said frame (100). The printer as defined in any of the preceding claims having a plurality of guide means (250, 260, 270, 280, 290, 300) for guiding said printing wires (198), in said nose (240). The printer as defined in claim 14, wherein said guide means includes at least a first guide means (260) disposed in said nose (240) on the side of the levers (190) and a second guide means (250) disposed at the nose front end side, said first guide means having printing wire guide holes (262) arrayed on a substantially circular line, and said second guide means (250) having printing wire guide holes arrayed in one or more rows. The printer as defined in claim 15, wherein said first guide means includes a cuplike portion (270) and a platelike portion (260) both forming a lubricant receptacle. The printer as defined in any of claims 14 to 16, wherein said second guide means (250) is L-shaped, forms a lubricant enclosing space (304) together with a third guide means (300) disposed next to it, and is provided with a lubricant feed port (258) at an externally exposed portion. The printer as defined in any of the preceding claims, wherein said driving coils (130) are grouped into sets of three adjacent coils each, the coils of one set being adapted to generate a magnetic field of a first polarity when energized, whereas the coils of the adjacent sets are adapted to generate a magnetic field of a second polarity opposite to the first polarity when energized. The printer as defined in claim 18, wherein the magnetic field polarities are determined by the winding directions of the coils (130). The printer as defined in claim 18, wherein the magnetic field polarities are determined by the wiring of the coils (130). The printer according to any of the preceding claims, wherein the lever (190) comes into contact with the corresponding core portion (110) at a rear end corner or projection of said armature portion (196′, 196″), if the gap between teh tip of the printing wire (198) and a platen is longer than a preset value. The printer as defined in any of the preceding claims, wherein said first and second yokes (150, 200) are positioned by means of positioning pins (222) provided on the lever holder (220) and adapted to engage positioning means (156, 206) of said first and second yokes. The printer as defined in any of the preceding claims, wherein said driving coils (130) are wound around respective bobbins (132), said bobbins having radially extending walls (132a) and circumferentially extending walls (132b) the former (132a) being thinner than the latter (132b). A method for manufacturing a printing wire driving device of a wire-dot printer comprising: (a) mounting a nose (240) on a frame (100) consisting of a magnetic material and having an inner peripheral wall (104) and a outer peripheral wall (102) and a plurality of core portions (110) disposed between said inner and outer peripheral walls at regular intervals in a circumferential direction, the end faces (116, 114, 118) of said inner and outer peripheral walls and said core portions being in the same plane, with a circuit substrate (340) and an insulating plate (380) being interposed between a bottom portion of said frame and said nose, (b) inserting driving coils (130) into said frame (100) so that each core portion (110) is surrounded by a corresponding driving coil, the upper ends of the driving coils projecting from the end face of the core portions, (c) assembling a frame unit (450) by setting a spring holder (170) into a through hole (106) in a center portion of said frame (100) inside of the inner peripheral wall (104), said spring holder having grooves (180) formed at positions opposite to the core portions and springs inserted in a center portion of each groove, (d) placing a first yoke (150) on the outer peripheral wall (102) of said frame (100) such that recessions (152) of said first yoke are registered with said core portions (110), (e) inserting levers (190) whose outer side is hooked to form a rotation support portion (194), whose central portion is projected to form an armature portion, (196) and which have a respective printing wire (198) fixed to an inner end portion, the printing wires being inserted into guide holes of the nose (240), (f) placing a second yoke (200) on the first yoke (150) such that branches (204) radially extending from a ring portion (202) of said second yoke are registered with projections (154) of the first yoke (150), (g) positioning and placing spring arms (214) of a supporting point retainer spring (210) between the branches of said second yoke, and (h) pressing the supporting point retainer spring (210) resiliently and fixing a lever holder (220) on the nose (240). The method as defined in claim 24, characterized by the additional step of placing a damper member (410) on a central portion of said spring holder (170) between steps (f) and (g). The process as defined in any claims 24 and 25, wherein a fixing spring (230) is used for pressing the supporting point retainer spring (210) and fixing the lever holder (220). The method as defined in any of claims 25 and 26, wherein said damper member includes a plurality of pieces, the stroke of the printing wires (198) being adjusted by selecting the number of pieces. The method as defined in claim 26, comprising the step of inserting radiating means (320) on the outside of said fixing spring (230). The method as defined in claim 28, including the step of inserting a guard cover (330) at the outside of said radiating means (320).

Inventors: ASADA TAKASHI, KOSHIISHI OSAMU, NAKAZAWA YASUHIKO, SHIMOMURA MASAKI, ASADA, TAKASHI, KOSHIISHI, OSAMU, NAKAZAWA, YASUHIKO, SHIMOMURA, MASAKI
Applicants: SEIKO EPSON CORP, SEIKO EPSON CORPORATION
Application year: 1989
Main classification: B41J2
IPC codes: B41J2