PRINT WIRE SOLENOID

Abstract: Print wire solenoid for use in a wire matrix print head including a housing member (22) enclosing a coil assembly (46, 58, 66, 74) which in turn includes an elongated plunger (78) slidably mounted within the coil assembly (46, 58, 66, 74) for reciprocal movement between a home and actuated position. A print wire (84) affixed to one end of the plunger (78) is moved to a printing position upon movement of the plunger (78) to the actuated position. A compression spring (94) mounted within the coil assembly (46, 58, 66, 74) normally urges the plunger (78) to the home position where it engages an impact disc (76) composed of an elastomeric material. The impact disc (76) together with a cap member (34) are held in a position adjacent the plunger by a retainer spring (32) removably secured to the housing member (22). The retainer spring (32) is held in a bowed configuration by the cap member (34), thereby preloading the cap member (34) and the impact disc (76) for help in absorbing the rebound movement of the plunger (78) while retaining the cap member (34) and the impact disc (76) together with the coil assembly (46, 58, 66, 74) in correct registration within the housing member (22). The retainer spring (32) is easily disengaged from the housing member (22) allowing for the quick disassembly of the solenoid.

Description: PRINT WIRE SOLENOID Technical Field The present invention relates to a print wire solenoid for use in dot matrix printing in which a print wire is propelled against a printing medium by an asso¬ ciated plunger-type solenoid actuator. Background Art Prior art print heads having plunger-type solenoids are generally constructed having- a cap member mounted at one end of the solenoid housing which is crimped to the housing to hold the internal elements of the solenoid together. An example of this type of construction may be found in United States Patent No. 4,272,748. Problems found with this type of construc- tion include excessive cap member wear due to repeated impact of the returning plunger, and uncontrolled vari¬ ation in the length of movement of the plunger as a result of the crimping of the cap member. United States Patent No. 4,014,425 discloses another plunger-type solenoid having a rubber disc positioned between the plunger and an anvil or closure member for damping of the plunger. This arrangement alleviates somewhat closure member wear, but there is still insufficient absorption of the impact of the rebounding plunger upon striking the rubber disc and the closure member. Both above constructions have the additional disadvantage that the solenoid cannot be readily disassembled once the crimping of the cap member or the insertion of the anvil or closure member has occurred. Disclosure of the Invention It is an object of the present invention to provide an improved print wire solenoid constructed to absorb the plunger rebound in a more effective manner and to enable easy assembly and disassembly. O PI Thus, according to the invention, there is provided a print wire solenoid for use in a wire matrix print head including a housing member; a coil assembly contained within said housing member; a plunger mounted within said coil assembly for reciprocal movement between a home and an actuated position; spring means for urging said plunger towards its home position; a print wire attached to said plunger and arranged to be moved to a printing position when, upon energization of said coil assembly, the plunger is moved to its actuated position against the action of said spring means; a closure member for retaining said coil assembly within said housing member; and a resilient impact member positioned between said closure member and said plunger, character- ized by resilient means releasably secured to said housing member and arranged to hold said closure member in position and to urge said closure member against said resilient impact member. Brief Description of the Drawings One embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Fig. 1 is a perspective view of the print wire solenoid of the present invention showing the mounting of the retainer spring on the solenoid housing member; Fig. 2 is a longitudinal cross-sectional view of the preferred embodiment of the present invention taken along its central axis; Fig. 3 is an enlarged partial longitudinal cross-sectional view of the solenoid taken along its central axis at 90° rotation from the view shown in Fig. 2; Fig. 4 is a side view of a cylindrical align¬ ment tool used in affixing the print wire to the plunger; Figs. 5A-5C inclusive show waveforms repre¬ senting oscilloscope traces of the coil current and print wire motion during solenoid actuation. Fig. 6 is a plan view of the flat retainer spring used in mounting the cap member to the solenoid. Best Mode for Carrying Out the Invention Referring to Fig. 1, there is shown a per- 5 spective view of the print wire solenoid of the present invention generally indicated by the numeral 20 and whose construction includes a cylindrical housing member 22 having a pair of oppositely-located slots 23 and 24 positioned adjacent the peripheral edge 26 of the 10 housing member 22. The slot 23 includes a notch portion 25 while the slot 24 has a notch opening 28 which extends through the edge 26 of member 22. A corner-notched re¬ tainer spring 32 (Fig. 6) is fed through slot 23 so as to engage slot 24 and the notch portion 25. The spring 1.5 32 is oriented in a bowed configuration (Fig. 3) due to the location of the slot 24 and the notch portion 25 with respect to an elevated curved central portion 31 (Fig. 3) of a groove 30 provided in the upper surface of a cap member 34. The cap member includes a pair of 0 circular cut-out portions 36, each of which accommodates a metal pin 38 comprising an electrical terminal of the coil windings located within the housing member 22. Referring now to Fig. 2, there is shown a longitudinal cross-sectional view of the solenoid 20 5 which also includes a cylindrical shape print wire guide member 40 which may be constructed of a hard plastic material such as acetal resin sold under the trademark "Delrin" manufactured by E. I. duPont de Nemouis and Co. of Wilmington, Delaware. The guide member 40 includes 0 an opening 42 and an annular end flange portion 44 for use in mounting the guide member 40 to the forward end portion 41 of a solenoid core member 46 by snapping the flange portion 44 into an annular groove 48 of the core member 46. 5 The solenoid core member 46 is constructed of a suitable magnetic flux-carrying material such as steel and includes an annular flange portion 50 to which is staked or crimped one end of the housing member 22, an annular groove 52 forwardly of said portion 50 for use in mounting the solenoid member 20 in a correspondingly- shaped opening of a panel support or the like, and a stepped hole 54 extending through the core member along its longitudinal axis. The core member 46 further includes a cylindrical shaped rear end portion 56 through which the hole 54 extends and on which is seated a plastic bobbin generally indicated by the numeral 58 and which is comprised of a front wall portion 60 and an axially spaced rear wall portion 62 joined by a tubular center section 64 having a recessed edge 100 which seats against the top surface of the end portion 56. Wound on the center section 64 is typically four hundred and seventy five (475) turns of electric magnet wire comprising the coil windings 66. The rear wall portion 62 of the bobbin 58 includes a pair of rear- wardly extending, tubular portions 68 (Figs. 1 and 2) through which the wire leads (not shown) from the coil windings 66 are passed so as to be contacted by the pressed-in metal terminal pins 38 for coupling the coil windings to the external drive electronics which supply the appropriate pulses to the coil windings 66 for energizing the solenoid 20. Slidably positioned on an outer end portion 70 of the bobbin 58 and engaging a step portion 72 of the rear wall portion 62 of the bobbin is a circular pole member 74 (Figs. 2 and 3) composed of a suitable mag- netic flux-carrying material such as steel through which extends the path of magnetic flux generated by the energizing of the coil windings 66. The pole member 74 includes a pair of cut-out portions (not shown) similar ' to the cut-out portions 36 (Fig. 1) of the cap member 34 for accommodating the tubular portions 68. Covering the outer surface of the pole member 74 is a thin circular shaped impact disc 76 composed of an elastomeric material such as polyurethane which is engaged by the rear end of a plunger 78 slidably mounted within the core portion 80 of the bobbin member 58. The employment of polyurethane as the material for the disc 76 reduces the wear on the cap member 34 caused by the impact of the rebounding plunger 78. The plunger 78 is composed of a suitable magnetic material such as steel. Secured to the rear end portion 82 of the plunger 78 is a tungsten print wire 84. An interposer member 96, made of a plastic material such as nylon and positioned adjacent the forward end of the plunger 78, serves to limit plunger motion and provides a seat 83 (Fig. 3) for a return spring 94 yieldingly urging such plunger 78 in a rear¬ ward direction against the impact disc 76. An important feature of the present invention is the construction of the plunger 78 and the means provided for securing the print wire 84 to the plunger. The plunger 78 is of hollow construction for purposes of weight reduction and has the opening of its cavity 81 (Figs. 2 and 3) orientated towards the front of the plunger so as to establish a pole face area distribution providing for maximum magnetic force. A small hole 92 is provided in the rear end portion 82 of the plunger 78 to receive the end of the print wire 84 prior to a brazing or welding operation. This hole is shown in Figs. 2 and 3 as a blind hole, but may be a through hole. The print wire 84 is held centered in precise axial alignment with the plunger 78 during the brazing or welding operation, by piloting means such as provided by the alignment tool 86 shown in Fig. 4. As illustrated, the alignment tool 86 is of cylindrical shape stainless steel and has a central hole 90 extending the length of the tool into which the print wire 84 is slidably inserted. A forward portion 88 of the tool ~86 has a diameter which allows the tool to be slidably inserted into the cavity 81 of the plunger 78, OMPI fa tyy, V/ΪPO _ > following which the end of the wire 84 is inserted into the mating hole 92 in the rear end of the plunger. The wire is then secured to the plunger by the brazing or welding operation. By such process, the tendency for the print wire to be deflected off-center by previous assembly procedures involving plastic injection insert molding is eliminated. When the alignment tool 86 is withdrawn from the plunger the print wire 84 retains its concentric alignment with the plunger 78. If the mating hole 92 extends through the rear end portion 82 (Fig. 3) of the plunger 78, the rear surface 85 thereof is machined flat, if necessary, after the print wire 84 has been brazed to the plunger 78 to provide a relatively flat surface for abutting against the impact disc 76. With the rear surface 85 (Fig. 3) of the plunger 78 being solid metal and flat, it provides a maximum area for spreading the rebound impact load of the plunger over the impact disc 76 so as to minimize wear of the impact disc 76 and/or the cap member 34 and thus prevent such wear from being a limit¬ ing factor in the life of the solenoid. The interposer member 96 (Figs. 2 and 3) located within the core portion 80 of the bobbin member 58, made of the hard plastic material such as nylon, serves to limit the forward motion of the plunger 78 while providing a seat portion 83 for one end of the return spring 94. The other end of the spring 94 ex¬ tends within the rear end of the hole 54 of core member 46 and, as shown in Fig. 2, engages a hard plastic core member 98 which is positioned within the hole 54 to slidably receive the print wire 84 therethrough and provide a forward seat for the spring 94. The core member 98 is seated against an annular shoulder 102 formed by the inner diameter of the hole 54. Such a construction simplifies the manufacture of the core member 46 by permitting a larger diameter forward end 103 of the hole 54 than would be the case if the forward OMPI spring seat were to be provided as an integral part of the core member 46. Referring again to Figs. 1, 2 and 3, there is shown mounted adjacent the outer surface of the impact disc 76 the cap member 34 which may be die-cast and composed of a suitably dense material such as a zinc alloy and which includes the groove 30 (Fig. 1) in its outer surface to accommodate the retainer spring 32. As previously described, this groove has a curved and elevated central portion 31 (Fig. 3). Due to the ele¬ vation of such portion 31 with respect to the location of the slot 24 and notch 25, the spring 32 will be bowed by the surface of such portion 31 when installed in the groove 30, thereby applying a force on the cap member 34 which in turn preloads the impact disc 76, the pole member 74, and the bobbin 58. This arrangement provides effective absorption of the rebound energy of the plunger 78 in its return impact against the combination of disc 76 and the spring loaded cap 34. . Instrumented testing has verified the desired near dead-beat character of this return impact phenom¬ enon. The polyurethane disc 76 cushions the return impact of the plunger 78 and prevents wear between the plunger and the cap. The disc 76 also limits the elas- ticity of impact in order to control the subsequent division of momentum between the plunger and the cap. The role of the cap in this respect is to provide suf¬ ficient mass in the correct ratio to the mass of the plunger for further controlling the rebound phenomenon in conjunction with the impact disc. The resulting improved rebound control is illustrated by the waveforms of Figs. 5A-5C inclusive representing oscilloscope traces of print wire motion. Fig. 5A shows the current (i) pulse in the case of a 325 microsecond duration (t) supplied from a 28 volt source. Figs. 5B and 5C show print wire displacement (d) as measured by coupled LED/phototransistor instrumentation. Reading from left- to-right, the first loop 114 in Fig. 5B defines the print wire travel from rest position to impact on media and return. The second loop 116 and an attenuated third loop 118 indicate excessive plunger rebound ac¬ tion. This rebound action interferes with the cyclic operation of the print solenoid and has even been re¬ sponsible for the occurrence of a secondary impact of the print wire with the media. In Fig. 5C, where the second loop 116 is greatly attenuated and the third loop 118 is entirely absent, there is illustrated a waveform taken for a unit which includes the spring loaded cap 34 and disc 76 arranged as described above, and showing the virtual dead-beat return impact charac¬ teristics which is the object of the present invention. In assembling the solenoid 20, the housing member 22 is attached to the core member 46 "by staking the housing member 22 to the flange 50 of the core member, after which the wire guide member 40 is snap- fitted to the forward end of the core member 46. The bobbin 58 is then slidably inserted into the housing member 22 and onto the rear end portion 56 of the core member 46. The pole member 74 is then mounted on the end portion 70 of the bobbin 58. With the print wire 84 concentrically aligned and affixed to the plunger 78, the interposer member 96 together with the compression spring 94 and the core member 98 are then assembled on the print wire 84 of the plunger 78, after which the plunger assembly is mounted within the core portion 80 of the bobbin 58. After locating the plunger assembly within the bobbin 58, the impact disc 76 and the cap member 34 are positioned on the pole member 74 after which the retainer spring 32 is installed by feeding it through the slot 23 of the housing member 22 and directing the notched end thereof into the oppositely located slot 24. In its installed condition the deflected spring 32 is detented at both ends by its corner notches from moving out of engagement with the slot 24 at one end and with the notch 25 of slot 23 at the other end. Mounting of the retainer spring 32 in this manner will bow the spring, thereby preloading the cap member 34 and retaining the solenoid assembly in correct registration. To disassemble the solenoid 20, the near end of the spring 32, as viewed in Fig. 1, is first depressed so as to take it out of detent with the notch portion 25, and the spring is then returned through slot 23 by pressing downwardly on the spring forward end and push- ing on the far end of the spring 32 at notch 24. The notch opening 28 in the edge 26 of the housing member 22 facilitates both the installation and the removal process by providing access for a tool or fingernail or the like for pressing and pushing on the end of the spring 32. The remaining components of the solenoid 20 can then be removed in the reverse order of assembly. It will be seen from this construction that the assembly and dis¬ assembly of the solenoid 20 is extremely simple and can normally be accomplished by hand without the use of any tools. In the operation of the solenoid 20, ener¬ gizing pulses transmitted to the coil windings 66 re¬ sults in the generation of a magnetic flux along the path indicated by the arrows 112 (Fig. 2) which includes the air gap 108 located between the interposer member 96 and the rear end portion 56 of the solenoid core member 46. This generation of magnetic flux results in the attraction of the plunger 78 to the end portion 56 of the core member 46. The air gap 108 defines the maximum extent of movement of the plunger 78 to an actuated position and may be in the order of .020 inch. The movement of the plunger drives the print wire 84 into engagement with the record medium and results in de¬ flection of the compression spring 94. Upon the de- energizing of the coil windings 66, the spring 94 returns the plunger 78 to its home position against the impact disc 76 where the rebound energy is absorbed by such impact disc 76, the cap member 34 and the retainer spring 32.

Claims: CLAIMS : 1. A print wire solenoid (20) for use in a wire matrix print head including a housing member (22); a coil assembly (46, 58, 66, 74) contained within said housing member (22); a plunger (78) mounted within said coil assembly (46, 58, 66, 74) for reciprocal movement between a home and an actuated position; spring means (94) for urging said plunger (78) towards its home position; a print wire (84) attached to said plunger (78) and arranged to be moved to a printing position when, upon energization of said coil assembly (46, 58, 66, 74), the plunger (78) is moved to its actuated position against the action of said spring means (94); a closure member (34) for retaining said coil assembly (46, 58, 66, 74) within said housing member (22); and a resilient impact member (76) positioned between said closure member (34) and said plunger (78), characterized by resilient means (32) releasably secured to said housing member (22) and arranged to hold said closure member (34) in position and to urge said closure member (34) against said resilient impact member (76). 2. A solenoid according to claim 1, charac¬ terized in that said resilient means is in the form of a bowed elongated spring member (32) the ends of which respectively detachably engage opposing portions of said housing member (22). 3. A solenoid according to claim 2, charac¬ terized in that said closure member (34) has an arcuate surface arranged to engage the concave surface of said spring member (32). 4. A solenoid according to either claim 2 or 3, characterized in that said housing member comprises a hollow cylindrical member (22), the ends of said spring member (32) respectively engaging in opposed slots (24, 25) formed in the wall of said cylindrical member (22). 5. A solenoid according to claim 4, charac¬ terized by a cut (28) in the edge of said cylindrical member (22) opening into one of said slots (24) to provide access to that end of said spring member (32) engaged in said one of said slots (24) and to facilitate the removal of said spring member (32) by pushing said end. 6. A solenoid according to claim 1, charac¬ terized in that said plunger (78) includes an axially extended opening (81) which terminates in an end portion (82) to which the print wire (84) is secured. 7. A solenoid according to claim 6, charac¬ terized in that said end portion (82) of said plunger (78) includes an axially extending aperture (92) in which the end of the print wire (84) is secured.

Inventors: REECE JOHN WILLIAM, REECE, JOHN, WILLIAM
Applicants: NCR CO, NCR CORPORATION
Application year: 1984
Main classification: B41J7
IPC codes: B41J2