Free Flight Head Assembly for Dot Matrix Printers and the Like

The present invention relates to impact type dot matrix print heads and more particularly to impact dot matrix print heads of the free flight, ballistic type in which a novel flexible parasol is utilized to permit armature biasing force to be established and maintained independently of the adjustable spacing between the armature and the free flight print wires.

Dot matrix printers are typically comprised of a plurality of print wires whose printing ends are arranged in linear fashion to selectively print dots at dot positions arranged along an imaginary vertical line. A print wire housing is provided for mounting the print wires in a reciprocating fashion so that their forward ends may impact a paper document, typically through an inked ribbon. The rearward ends of the print wires are reciprocally driven by solenoid assemblies selectively energized to cause only those desired print wires to impact the paper document. In order to be assured that the proper amount of driving force will be imparted by each solenoid to its print wire, critical adjustments are typically required. In an effort to simplify the amount and precision of such adjustments, print heads of the type described in U.S. Pat. No. 3,929,214 have been developed which enable the print wire ends to be adjusted relative to the armatures through one single adjustment operation permitting all of the print wires and armatures to be adjusted simultaneously. However, print heads of this type have the drawback of altering the biasing force imposed upon the armature elements whereupon the appropriate positioning between the armature and the print wire results in a biasing force imposed upon each armature.

The present invention is characterized by providing a novel, free flight type print head assembly which utilizes a biasing member and flexible resilient parasol enabling adjustment of the positioning of the print wires relative to the solenoids and their armatures without in any way affecting the biasing force imposed upon each of the armatures by the aforesaid biasing member when the armatures are in the rest or impact position.

The novel print head assembly of the present invention is comprised of a nose cone section adapted to slidably mount a plurality of elongated print wires which experience reciprocating movement therein during printing. The nose cone is provided at one end with a jewel bearing assembly which maintains the printing tips of the print wires along an imaginary straight line, which assembly is adapted to provide a very low friction bearing. The opposite (rearward) end of the nose cone is provided with a projection which is slidably and adjustably mounted within a bore provided in a parasol member, the adjustment facilitating spacing between the rearward ends of the print wires extending outwardly through openings provided in the rear wall of the nose cone and armature members cooperating therewith.

A pole plate in the form of a circular disc is secured to a flange provided a spaced distance inwardly from the rear end of the nose cone upon which disc are mounted a plurality of solenoid assemblies each including a U-shaped pole piece, a coil and an armature swingably mounted at a heel portion of the pole piece and adapted to be selectively magnetically attracted to the coil.

The aforementioned parasol member serves as an enclosure for the solenoid assemblies and is provided with a plurality of slots which, together with the natural resilience of the material used to form the parasol member, permits the parasol to freely flex enabling displacement of the central portion thereof relative to the outer lip. The parasol member is provided adjacent its interior and exterior portions with means for aligning and slidably guiding the inner and outer ends of the armatures. The central portion of the parasol member is provided with a recess for seating an energy absorbing O-ring. A biasing spring which is generally of a star wheel design is housed within the concave shaped interior of the parasol member so that alternating radially aligned arms thereof engage shoulders provided at spaced intervals along the parasol member while the remaining interspersed radially aligned arms serve as a means for biasing the heels of the armatures towards their associated pole pieces.

The rearward ends of the print wires are each provided with biasing springs which urge the print wires in the rearward direction and toward and against the inwardly directed tips of the radially aligned armatures. When energized, the armatures are magnetically attracted towards their associated solenoid coils to be rapidly accelerated. The armatures push the heads of their associated print wires imparting kinetic energy thereto. The momentum imparted to the print wires cause them to continue travel even after the armature bottoms. A dot is printed on a paper document by the kinetic energy imparted to the print wire whereupon the elasticity of collision occurring during printing causes the print wire to return after impact, being aided thereby through the biasing spring. During return, the print wire end cap strikes the armature breaking any remnance of the magnetic circuit and imparting movement to the armature in the return direction whereupon both the armature and print wire move toward an energy absorbing O-ring which absorbs the remaining kinetic energy of the print wire and armature and brings these elements to rest. Adjustment of the location of the O-ring relative to the pole pieces may be both readily and accurately performed without in any way affecting the biasing forces imparted to the heels of the armature due to the novel construction of the parasol member and the cooperating pole pieces.

The wire guide housing design facilitates assembly of its components and precisely locates the wire tips relative to a portion of the mounting surface of the housing, greatly facilitating precision alignment of the printed head in a printer.

The solenoid mounting assembly may be used in any applications requiring multiple solenoids and in which gap adjustment is greatly simplified.

It is therefore one object of the present invention to provide a novel free flight dot matrix print head having a flexible parasol member which serves to provide means for seating an independent biasing spring which biases the heels of the armatures, said parasol being adapted to flex so as to provide for simplified precision adjustment of the positioning of the armature tips without in any way affecting the biasing force applied to the heels of the armatures by said spring.

Another object of the present invention is to provide a free flight print head of the character described hereinabove wherein the free flight arrangement, together with an insulation sheet ("shim") serves to significantly reduce residual magnetism forces in each of the magnetic structures.

Still another object of the present invention is to provide a novel assembly which provides more precise alignment of components as a result of the novel and yet simplified design thereof.

Another object of the invention is to provide a novel mounting assembly for a plurality of solenoids in which the armatures are simultaneously adjustable without changing the biasing forces exerted on the armatures.

The above as well as other objects of the present invention will become apparent when reading the accompanying description and drawings in which:

FIG. 1 shows a side elevational view of a free flight dot matrix print head embodying the principles of the present invention.

FIG. 1a shows a front view of the print head of FIG. 1.

FIG. 1b shows the rear end view of the print head of FIG. 1 having a portion thereof broken away.

FIG. 1c shows a sectional view of the print head of FIGS. 1-1b looking in the direction of arrows C-C of FIG. 1b.

FIG. 1d shows an enlargement of part of FIG. 1c.

FIG. 2 shows a top plan view of the nose cone employed in the print head assembly of FIG. 1.

FIGS. 2a, 2b and 2c are sectional views of the nose cone of FIG. 2 looking in the direction of arrows A--A, B--B and C--C, respectively.

FIG. 2d shows a rear end view of the nose cone of FIG. 2.

FIG. 2e shows a sectional horizontal view of a portion of the housing of FIG. 2.

FIG. 3 shows a sectional view of the parasol of FIGS. 1, 1b and 1c wherein the parasol is shown in the unflexed position.

FIG. 3a shows an interior view of the parasol of FIG. 3.

FIG. 4 shows a plan view of the shim employed in the print head assembly as shown in FIG. 1c.

FIG. 5 shows a plan view of the leaf spring assembly employed in the print head.

FIG. 5a shows an end view of one typical leaf spring arm adapted to bear against the heel of an associated armature.

FIG. 6 shows a plan view of the pole plate assembly of FIG. 1.

FIG. 6a shows a sectional view of one of the pole pieces of FIG. 6 looking in the direction of arrows 6A--6A.

Considering FIGS. 1-1c, there is shown therein a print head assembly designed in accordance with the principles of the present invention and which is adapted to be mounted upon a movable carriage and connected to suitable electrical control means (not shown for purposes of simplicity) through the flexible printed circuit member 11 (FIG. 6) for the purpose of selectively energizing solenoids causing the print wires to impact a ribbon R in order to print dots upon a paper document by transfer of the ink to the paper which is supported by a platen P.sub.L.

FIG. 1c is a sectional view of the free flight print head looking in the direction of arrows C--C of FIG. 1b and is comprised of a nose cone assembly 12, a solenoid and pole plate assembly 13 and a parasol and armature biasing assembly 14.

The nose cone assembly is comprised of a one-piece housing 15 open along one side and having a rear wall 16 with a rearwardly extending projection 17 having a threaded aperture 17a. The nose cone may be covered by mounting the open side against the surface of the movable carriage (not shown). The bottom surface 15k (FIGS. 1, 1c, 2b) and the left hand edge 15m thereof (FIGS. 2, 2e) are precisely aligned relative to the print wire tips in a manner to be more fully described to facilitate mounting and alignment upon a printer. The rear wall 16 is provided with a plurality of tapered openings 16a which gradually increase in size from the inside rear wall to the outside thereof as shown at 16b and further being provided with a large outwardly flaring opening 16c arranged to form a shoulder 16d between the bore portion 16b and the bore portion 16c. Only one such opening has been shown in detail (enlarged inset portion of FIG. 1c), it being understood that a one of such openings 16a are provided for each print wire, as shown best in FIG. 2d.

The housing 15 is substantially hollow and has a first pair of support grooves 18a and 18b arranged on the interior sidewalls thereof and adapted to receive and position the opposite sides of a partition 19 which serves as a guide plate for the print wires as will be more fully described. Partition 19 is provided with double tapered openings 19a somewhat similar to those provided in rear wall 16, for receiving each of the print wires. A second pair of support grooves 20a and 20b are provided downstream of the grooves 18a and 18b and are designed to receive and position the opposite sides of a second partition 21 having a double tapered openings 21a again serving as a guide means for the print wires. The tapered openings serve to facilitate insertion of the print wires and, in both partitions, generally lie along an oval shaped line.

FIGS. 2b and 2c show the details of these grooves and partitions. It should be noted that the grooves 18b and 20b are shorter in length than the grooves 18a and 20a, respectively, and that the partitions 19 and 21 conform to these shapes whereby the shape of the grooves and the conforming shapes of the cooperating sides of the partitions serve to assure proper orientation of the partitions within the grooves. The partitions need not be secured or epoxied into place since they are maintained in place by their cooperating grooves 18a, 18b and 20a, 20b and by the print wires 23. Thus, the partitions serve to maintain relative positioning between and among the print wires but do not and need not precisely position the wires relative to the nose cone.

As can be seen from FIG. 2, the interior sidewalls 15a and 15b of the housing taper very slightly in the region between the pairs of grooves 18a-18b and 20a-20b, the sidewalls being substantially parallel in the immediate region of the groove pairs and then tapering quite significantly at 15c and 15d to form a very narrow passageway at 15e which also tapers, but in a gradual fashion, toward the forward or nose end of the nose cone. The V-shaped taper defined by surfaces 15c-15d facilitates insertion of the print wires into the narrow region 15e. The nose is provided with a first recess 15f and a second recess 15g set within the first recess 15f and which is arranged to receive a low friction jewel bearing member 22 provided with a plurality of closely spaced generally circular shaped openings so as to align the forward ends or tips of the print wires along an imaginary straight line in closely spaced fashion enabling a vertically aligned dot pattern to be formed upon the paper document P. The jewel is mounted as follows:

The springs 25 are placed on the print wires. The partitions 19 and 21 are mounted into their guides 18a, 18b, 20a, 20b. The wires are inserted through the two openings in rear wall 16 and partitions 19 and 21. As the wires are moved forward, walls 15c, 15d guide the wires into the narrow guideway 15e. The width and length of the slot at the outlet end is about equal to that of the openings in the jewel bearing and serves to maintain the wires in the desired alignment. The precision alignment is sufficient to permit the jewel 22 to be placed in position as aligned by the print wire tips. The jewel is held in place by a suitable adhesive (i.e., epoxy). Thus the alignment of the jewel is automatic. The mold from which the housing is cast is machined to very accurately locate the left hand edge 15m and bottom surface 15k relative to one another. The plastic material is chosen to provide excellent dimensional stability and low shrinkage. Preferably the material is a filament reinforced plastic material. The opening of 15e into the nose may be rectangular as shown, may be separate round openings for each wire, and if desired the openings may be staggered, i.e., arranged to lie along an imaginary path that defines a zig-zag pattern. As shown in FIGS. 2 and 2e there is provided a pair of openings 15h and 15j which receive suitable fastening means (not shown) for securing the nose cone assembly to a carriage assembly which, as is well known in the dot matrix line printer art, serves as the means for moving the print head across the paper document during printing (preferably "on the fly"). The carriage mating surfaces which engage surface 15k and edge 15m are machined to a similar level of precision to align the carriage thereon.

FIG. 1c shows two typical print wires 23 and 24 which can be seen to be maintained in a generally curved condition between the jewel bearing 22 provided in the nose and the partitions 21 and 19 positioned rearward of the jewel bearing 22, as well as the rear wall 16. The print wires are generally of constant diameter over their length and are fitted at their rearward ends with end caps such as, for example, the end cap 24a fitted on the rear end of print wire 24. A helical spring 25 has its left-hand end (relative to FIG. 1c) seated within opening 16c and against shoulder 16d and has its opposite end bearing against end cap 24a. As can clearly be seen, the helical spring biases each print wire in a direction shown by arrow 26.

The nose cone housing has an integral flange 27 (FIGS. 1c, 2 and 2d) which flange 27 is provided with a pair of openings 27a and 27b for receiving the fastening members 28 (see FIGS. 1 and 1a) which serve to secure pole plate 29 thereto, pole plate 29 having been referred to hereinabove as part of the solenoid and pole plate assembly 13 and which will be more fully described hereinbelow.

As can best be seen from FIGS. 1c and 2, the centrally located projection 17 extending rearwardly from the rear end thereof is designed to be slidably received within an opening provided in the parasol 35, as will be more fully described to accurately align the nose cone and its print wire end caps relative to the armature assembly in a manner to be more fully described hereinbelow.

The pole plate and solenoid assembly is comprised, as was mentioned hereinabove, of pole plate 29, shown best in FIG. 6, the pole plate being a substantially circular disc having a plurality of openings 29a, one of which is shown in FIG. 6a, each of said openings being adapted to receive a pole piece 30 having a base portion 30a, an arm 30b of substantially rectangular cross-section and an arm 30c of a substantially circular cross-section and extending upwardly from base portion 30a, the pole piece having a generally U-shaped configuration. As shown best in FIG. 6, each pole piece has its arm 30c of generally circular shaped cross-section arranged to lie closer to the central axis of disc 29 and having its arm 30b of generally rectangular shaped cross-section arranged closer to the outer periphery of the disc. As will be noted, each arm 30b is provided with a notch having a ledge 30b-1 provided for a purpose to be more fully described.

The pole pieces are mounted to the pole plate as follows:

Pole pieces 30 are formed of a metal which is harder than the pole plate 29. The plate 29 is supported on a hard rigid block placed against surface 29b. The left hand ends of the pole pieces are inserted in a jig J having precision formed recesses. The opening 29a in the plate is initially of a straight-through shape as shown by dotted lines 29a' and conforming to the shape of the end face 30f. The right hand ends of all of the pole pieces 30 (only one of which is shown in FIG. 6a) are simultaneously pressed into their associated openings (see opening 29a') causing the shoulders 30g forming one surface of V-shaped indentations 30h to cause a "cold flow" of the metallic material of plate 29 into the indentations as a result of displacement of the metal by shoulder 30g, resulting in the final shape 29a. The soft plate compensates for any possible differences in the lengths of the pole pieces so that the left hand ends of the pole pieces are all accurately aligned with their faces 30d and 30e coplanar. The conforming shapes of the end face 30f and openings 29a assure proper alignment of each pole piece 30 in plate 29.

Each arm 30c is adapted to receive a solenoid coil 31 wound upon a bobbin 32 having a circular shaped bore and which is fitted upon the arm 30c. The wires of each coil are led out to an associated terminal provided on the flexible circuit element 11, the connection of two such wires for the solenoid 31' (wires W1 and W2) being shown as connected to the terminals T1 and T2 on the flexible circuit element 11.

Although the armatures can clearly be said to be part of the solenoid assemblies, the manner in which they are mounted and the manner in which they function will be described after first considering the parasol and armature biasing assembly 14.

The parasol and armature biasing assembly is comprised of a parasol member 35 having a central portion 36 and an outer annular portion. Considering FIGS. 3 and 3a, the central hub portion 36 is provided with an opening 36a for receiving projection 17 of the nose cone housing in the manner shown, for example, in FIG. 1c. The central portion, as can best be seen in FIG. 3a, has a substantially oval or elliptical shaped flange 37 provided with a plurality of slots 37a at spaced intervals therearound and each adapted to slidably receive the forward end of an associated armature element 38 which, from a consideration of FIG. 1b, can be seen to be comprised of a heel portion or outer end 38a having a pair of notches 38b, 38b and tapers to form a tip portion or inner end 38c of reduced width, which tip portion is slidably received within one of the aforementioned slots 37a.

The inner wall 37b of oval shaped flange 37 receives an energy absorbing resilient compressible O-ring 39 which is force-fittingly received therein and is adapted to absorb kinetic energy from the armatures in a manner to be more fully described.

The outer portion of parasol 35 is provided with an oval shaped continuous flange or outer peripheral ring or annular projection 40, whose major diameter is just slightly greater than its minor diameter, and having a plurality of pairs of projections 41, one such pair being comprised of a first projection 42 and a second projection 43, each projecting inwardly from flange 40 and each provided with transversely aligned armature locating flanges 42a and 43a extending toward one another and having semicircular or rounded free ends or tips which are adapted to be slidably inserted within the notches 38b, 38b of an associated armature, as shown best, for example, in FIG. 1b. A similar pair of flanges is provided for each armature.

Positioned between projections of adjacent pairs, for example, pairs 41 and 41' and between pairs 41 and 41", there is provided a ledge or step 44 and 45 respectively, each being adapted to serve as a seat for one of the flat fingers of leaf spring 46. Considering FIGS. 1b, 5 and 5a, the leaf spring 46 can be seen to be comprised of a plurality of fingers 47 each having a tapered shape, being wider at its inner end and narrower at its free end 47a, for example. Interspersed between each of the fingers 47 are fingers 48 which, as can best be seen from FIG. 5a, is provided with a substantially V-shaped bend 48a so that the knee 48b of the V-shaped bend is positioned to rest upon the heel portion of an associated armature as can best be seen in FIG. 1c wherein knee 48b is shown as resting upon the heel portion 38a, which heel portion is immediately behind the pair of notches 38b, 38b.

The parasol is provided with a plurality of flexure slots 49, 50 and 51, each of which is comprised of an outer arcuate shaped slot 49a, 50a and 51a, an inner arcuate shaped slot 49c, 50c and 51c and a diagonally aligned intermediate and substantially linear slot 49b, 50b and 51b with the intermediate slots joining the inner and outer slot portions in the manner shown best in FIG. 1b so as to cooperatively form three substantially similar shaped slot arrangements. The parasol is formed of a suitable plastic material which inherently has a high degree of natural resiliency. This fact further coupled with the slot arrangements as shown in FIG. 1b permits the portion of the parasol between the central ring portion 36 and the outer ring portion 40 to be adapted to flex. The nature of the flexure may best be understood from a consideration of FIGS. 3 and 1c wherein FIG. 3 shows the parasol assembly 35 in its unflexed condition. When assembling the parasol into the print head, the O-ring 39 is mounted in the manner shown in FIG. 1c. Thereafter, the leaf spring is positioned within the concave interior portion of the parasol so that the free ends of each of the tapered fingers 47 rest upon an associated ledge (see ledges 44 and 45, for example) and so that the remaining interspersed fingers 48 are positioned between each associated pair of guide projections. The armatures are then positioned upon the leaf spring so that each pair of slots 38b, 38b are positioned between a pair of guide projections 41 and so that the slots are received by projections 42a and 43a, for example, (see FIG. 3a).

After this subassembly is completed, the parasol, together with the O-ring and leaf spring and armature elements, is positioned so as to receive the projection 17 within central opening 36a. The angular orientation of the nose cone relative to the parasol is such as to position the tip 38c of each armature immediately above and in fact upon a head 24a of one of the print wires. Note especially FIGS. 1b and 1c in this regard. As can best be seen from FIG. 1c, the projections 42a and 43a, for example, rest upon a shoulder 30b-1 (see FIGS. 1c and 6a) provided in the arm 30b of each U-shaped pole piece so that the location of the outer ring 40 of the parasol is fixed relative to the pole pieces. The oval shape of ring 37 and the similar oval arrangement of openings in the guide plates 19 and 21 greatly reduce the amount of curvature imparted to each print wire in bringing the wires into the desired linear alignment at the bearing 22.

The fastening member 50 is then threaded into the threaded aperture 17a provided in projection 17 and is tightened or otherwise adjusted so as to adjust the gap or displacement D between the confronting end faces 30d of pole piece arms 30c and the adjacent surface of the tip 38c of each armature 38. Helical springs 25 serve to urge each armature tip against O-ring 39. It can further be seen that the biasing force exerted upon the heel of each armature element is totally independent of the adjustment of the central portion 36 of parasol 35 regardless of the actual positioning of the central portion. The adjustment is facilitated by the natural resiliency of the material from which the parasol is formed together with the aforementioned slot arrangements 49, 50 and 51 shown, for example, in FIG. 1b, FIG. 1c clearly showing the amount of flexure experienced by the parasol 35 as a result of appropriate adjustment, to thereby simultaneously adjust the air gaps of all of the solenoid assemblies.

In addition to the above arrangement, there is provided a shim 51' as shown, for example, in FIGS. 1b and 4, which is formed of a suitable insulation material, said shim being provided with a plurality of radially aligned fingers 52 each having a portion 52a of reduced width at its extremity due to the notched corners 52a-1 and 52a-2. The central portion of the shim is provided with an opening 51a to receive the central ring projection 37 of parasol 35 (note FIG. 3a). The diameter of opening 51a is at least equal to the major diameter of the oval shaped ring 37 so as to be positioned at any angular position relative to the oval shaped projection. However, each of the fingers 52 are positioned so as to be in alignment with each armature 38. The notched free ends 52a are adapted to be positioned between each pair of inwardly directed transversely aligned projections 42a and 43a (see FIG. 3a) so as to permit each reduced width portion 52a to be sandwiched between the heel 38b of the armature and the free end face 30e of pole piece 30b (see FIG. 5). From a consideration of FIG. 1b, it can be seen that each finger of the shim is also positioned between the end face 30d of each pole piece (see FIG. 5a) and the associated armature 38. This shim thereby provides a bearing surface at the pivot of each armature, i.e., where corner 30b-2 is located (see FIG. 6a) and also provides a sacrificial material such that the shim may be removed and readily replaced when worn while at the same time minimizing any wearing of the armatures and pole pieces. In addition thereto, the shim cushions the impact of the armature against the pole 30d during operation, as will be more fully described and further, due to its insulation nature, minimizes remnance (i.e., residual magnetism) during operation. Also, during impact, the shim minimizes any metal distortion caused by the impact of the armature upon the pole piece. The minimum air gap provided by the shim also lessens the magnetomotive force caused by remnance of the magnetic circuit.

The operation of the novel structure of the present invention is as follows:

Dependent upon the particular dots to be printed, certain of the coils 31 are energized. However, prior to energization, the armatures are maintained a spaced distance from the end faces of their associated pole pieces 30c as a result of the helical springs 25 which urge the heads 24a of the print wires 24 rearwardly thereby urging the tips of the armatures rearwardly and against O-ring 39.

Upon energization of the coil (or coils), the magnetic flux developed exerts an attractive force upon the associated armature causing the armature to be drawn toward the associated pole piece. The fingers 48 of spring 46 have their knees 48c bearing down upon the heels of the armatures and thereby serve to cause the armature heels to bear against the pivotal mount provided by pole piece arm 30b. As the armatures are accelerated in moving toward their associated solenoid coils, they develop kinetic energy, which kinetic energy is also imparted to the print wire as each actuated armature tip is accelerated, thereby causing the print wire to move together with and in the same direction as its associated armature tip. The momentum of the print wire causes it to continue to travel in the print direction even after the armature bottoms, i.e. impacts against the face 30d of pole piece 30c (see FIG. 5a). The print wire is now in free flight.

A dot is printed by the kinetic energy imparted to the print wire as a result of the print wire impacting against the ribbon R and paper document P supported by a suitable backing surface or platen P.sub.L.

The elasticity of the collision during the printing operation causes the print wire to return, the solenoid coil having been deenergized either at or prior to this time. The return of the print wire is aided by the associated helical spring 25. The cap or button 24a strikes the armature tip 38c. In the event that residual magnetism serves to maintain the armature attracted to the pole piece, the kinetic energy imparted to the armature is used to break the remnance of the magnetic circuit and thereby start the armature in motion in the rearward direction. The armature and print wire both move toward O-ring 39 which absorbs the remaining kinetic energy of the print wire 24 and armature 38 so as to rapidly return the print wire and armature tips to the rest condition in readiness for the next operation. The leaf spring arms 48 serve to maintain the heel of each armature against its associated pole piece as well as serving to urge the tip away from pole piece arm 30a when each solenoid coil is deenergized.

The solenoid mounting assembly 13 may be employed in any applications which employ a plurality of solenoids. The armatures may be utilized to drive members other than print wires while retaining the advantages of ease of adjustment of the gaps of the armature tips and their associated pole pieces. If desired the biasing forces of the individual fingers may be independently controlled by altering the depths of the ledges 45 (FIG. 3a) as well as the bases of slots 37c for independently adjusting the gaps between the armature tips and their associated pole pieces.

It can be seen from the foregoing description that the present invention provides a novel arrangement in which the force imparted to the heel of each armature is constant and remains constant regardless of the relative and adjustable positioning of the central portion 37 of the parasol member 35 relative to the print wire and the caps 24a. Each of the pole pieces positively position outer ring 40 of the parasol member without the need for any adjustment whatsoever. The design of the components is such as to greatly facilitate both assembly and disassembly and the nature of the components is such as to be inexpensive and yet provide tight tolerances therefor.