PRINT HAMMER ACTUATOR FOR IMPACT PRINTER

Abstract: The present invention relates to a print hammer actuator for an impact printer of the type comprising a print hammer pivot shaft (2), a print hammer (1) pivoted at one end about the print hammer pivot shaft, having an impact mass (3) at the other end, and formed with an energy transfer surface (4), an armature pivot shaft (9), an armature (5) pivoted about the armature pivot shaft, and formed with an energy transfer surface (7) in contact with the hammer energy transfer surface (4), and an electromagnetic coil (13) adapted to cooperate with the armature in order to move the armature so that the armature energy transfer surface (7) transfers energy to the hammer energy transfer surface (4) and moves the hammer, resulting in the impact mass (3) moving into an impact printing position. A print hammer actuator according to the invention is characterised in that the print hammer pivot shaft (2), the armature pivot shaft (9) and the energy transfer surfaces (4, 7) are substantially coplanar during the transfer of energy from the armature energy transfer surface (7) to the hammer energy transfer surface (4).

Description: This invention relates to a print hammer actuator for an impact printer. A great variety of print hammer actuators have been described in the literature, and many print hammer actuators have been deployed in great numbers in a wide variety of computer impact printers. These print hammer actuators must in general be small enough to be replicated for each print position in multiactuator printers, or to allow space for other mechanisms in single actuator printers. The actuator, particularly if replicated, must be inexpensive and trouble free, while maintaining tolerances sufficiently close to nominal to provide good print quality. Speed is required to accomplish printing without smudging. The usual solution is to maintain very high mechanical and electrical standards, and to drive any electromagnetic coils in the actuator with very high power pulses of very short duration. These high power pulses result in a great deal of heat being concentrated in very small coil wires which are crowded into a small volume. This is very wasteful in energy, giving low print energy versus input energy efficiencies with the majority of the input energy being dissipated as heat loss. The need for an inexpensive, trouble free, effective print hammer actuator persists, particularly for printers at the lower end of the product cost spectrum. The following patents and publication are representative of the prior art: US-A-3,164,085 shows a rocking lever two piece print hammer actuator, with the hammer pivot in line with a pair of energy transfer surfaces and an armature pivot, but not with the hammer pivot between the armature pivot and the energy transfer surfaces. This relationship, while permitting energy transfer from the armature to the hammer with a minimum of sliding movement, results in a bellcrank configuration of the hammer. This hammer configuration differs from a simple lever print hammer in that it has greater inherent mass and inherently inferior flight dynamics due to lesser power-to-velocity advantage, suffers greater damping in the bellcrank and greater energy transfer to the pivot shaft. IBM Technical Disclosure Bulletin, Vol. 27, No. 4A, September, 1984, pp. 2090-2092, shows a two-piece print hammer actuator assembly including pivoted armature and a pivoted hammer and a pair of energy transfer surfaces, but does not align the armature pivot, the hammer pivot and the energy transfer surfaces. A number of one-piece print hammer actuators have been deployed, typified by the whipping hammer described in US-A-4,269,117, in which a relatively flexible long print hammer leg and relatively large mass coil leg form an integral hammer and armature. A number of two-piece print hammer actuators have been deployed in which an armature transfers energy to the print hammer directly, usually by a camming action. A number of three-piece print hammer actuators have been deployed, typically having a pushrod, a print hammer, and an armature. The pushrod transfers energy from the armature to the print hammer. The pushrod is subject to sliding friction, is subject to bending, and requires careful assembly. This in general results in a costly assembly. The object of the invention is to provide an improved print hammer actuator for an impact printer. The present invention relates to a print hammer actuator for an impact printer of the type comprising a print hammer pivot shaft, a print hammer pivoted at one end about the print hammer pivot shaft, having an impact mass at the other end, and formed with an energy transfer surface, an armature pivot shaft, an armature pivoted about the armature pivot shaft and formed with an energy transfer surface in contact with the hammer energy transfer surface, and an electromagnetic coil adapted to cooperate with the armature in order to move the armature so that the armature energy transfer surface transfers energy to the hammer energy transfer surface and moves the hammer, resulting in the impact mass moving into an impact printing position. A print hammer actuator according to the invention is characterised in that the print hammer pivot shaft, the armature pivot shaft and the energy transfer surfaces are substantially coplanar during the transfer of energy from the armature energy transfer surface to the hammer energy transfer surface. In a print hammer actuator in accordance with the invention the armature pivot, the print hammer pivot and the energy transfer surfaces are aligned in that sequence and are essentially coplanar. This minimises the sliding action of contacting surfaces during energy transfer by having the energy transfer surface on the armature and print hammer follow similarly convex circumferences of epitangent circles, the radii of which are centered at the armature pivot and the print hammer pivot respectively. The circular paths traversed by the energy transfer surfaces on the armature and the print hammer remain tangent over their short travel during operation, minimising sliding contact between the armature and the print hammer energy transfer surfaces. As a result of this configuration there is a minimisation of moving parts and a minimisation of wear in the print hammer actuator. In order that the invention may be more readily understood, an embodiment will now be described with reference to the accompanying drawings in which: FIG 1 is a simplified diagram of the armature and print hammer of a print hammer actuator according to the invention, FIG 2 is a diagram of the pivot and energy transfer surface geometries of the actuator illustrated in FIG. 1, FIG 3 is a graph of the print hammer velocity ratio as a function of the armature legs length ratio at a constant electrical input energy of the actuator illustrated in FIG. 1, FIG 4 is a simplified diagram of the print hammer actuator illustrated in FIG. 1 showing relationships of interest in the graph of FIG 3, FIG 5 is a detail diagram of the preferred embodiment of the print hammer actuator illustrated in FIG. 1, FIG 6 is an axonometric diagram of a bank of print hammer actuators of the type illustrated in FIG. 1, and FIG 7 and FIG 8 are respectively front and side elevation views of a double bank of print hammer actuators of the type illustrated in FIG. 1. FIG 1 shows the basic elements and relationships of the components of a print hammer actuator for an impact printer. A print hammer 1, pivoted on a print hammer pivot shaft 2, carries an impact mass 3 at its printing face at its distal end relative to the pivot shaft 2, and carries an energy transfer surface 4 located at approximately the mid point between the shaft 2 and the impact mass 3. The energy transfer surface 4, which preferably is of increased area and increased mass as compared with the portion of the print hammer on which it is located, is smoothed and hardened so as to minimise the destructive wear it will experience in use. Impact energy to drive the hammer is developed, separately from the printer hammer 1, by an armature 5, which is pivoted on an armature pivot shaft 9 and which carries at its distal end 6 relative to the shaft 9 an energy transfer surface 7 adapted to cooperate with the energy transfer surface 4 of the print hammer. In operation, the armature 5 receives energy by the energisation of a coil 13, causing counter-clockwise motion (as viewed in FIG. 1) of the armature 5 about its pivot 9, and imparting kinetic energy to its energy transfer surface 7. As the armature energy transfer surface 7 comes into contact with print hammer energy transfer surface 4, the armature 5 and print hammer 1 will be accelerated together. Print hammer impact mass 3 is thus accelerated to the velocity required to attain good print quality when impacting a print medium 8. Armature 5 stops when it strikes an armature stop 10, after which print hammer 1 continues in pivoted free flight during print medium impact and rebound. Armature 5 is a two-leg bellcrank. It includes a relatively short, relatively large mass ferromagnetic coil leg 12, which enters coil 13 for efficient energy transfer, and also a relatively small mass, relatively long transfer leg 11, which carries at its end the rounded, mass-controlled energy transfer member 6. Energy transfer surface 7 has as its contact surface a convex polyurethane polymer insert, in the range .75-1.00 millimeter thickness, moulded at the tip. Armature pivot shaft 9 extends parallel to print hammer pivot shaft 2, both shafts lying essentially in a common plane with energy transfer surfaces 4 and 7 during transfer of energy from the armature 5 to the print hammer 1 by contact of the energy transfer surfaces 4 and 7. Print hammer energy transfer surface 4 and armature energy transfer surface 7 are in intimate contact except during the short interval of print hammer free flight and a portion of the hammer rebound cycle. FIG 2 illustrates the operation in more detail. This relationship of the energy transfer surfaces 4 and 7 and the pivot shafts 9 and 2 is important in minimising the wear of the energy transfer surfaces by minimising the sliding action between armature energy transfer surface 7 and print hammer energy transfer surface 4 during energy transfer. The geometries are such that the armature energy transfer surface 7 and the print hammer energy transfer surface 4 are pivoted so that they move in similarly convex epitangent circular arcs 22 and 23 (arc 2, 4 and arc 9, 6 in FIG. 2). The epitangent circular arcs remain continuously tangent (or approximately so) at the point of contact of the two energy transfer surfaces 7, 4, as the point of contact moves a very short distance along a path essentially parallel to line 24. Since the pivot shafts and the energy transfer surfaces are aligned, there is very little dynamic change in relative position of the energy transfer surfaces during the period of motion during which energy transfer takes place. The energy transfer thus is accomplished with minimal sliding action between the energy transfer surfaces of the armature and the print hammer, and even that sliding action is ameliorated by the curvature of the energy transfer surface 7. The print hammer, during operation, is accelerated from rest to its nominal print impact velocity before going into free flight, as a result of energy applied at its energy transfer surface 4. Print hammer 1, when accelerated by energy applied at its energy transfer surface 4, achieves pivoted free flight when the armature 5 is stopped by impact with armature stop 10 prior to print impact. This pivoted free flight travel is controlled to correspond to the desired performance. The impact mass 3 of print hammer 1 strikes print medium 8 for printing. Print medium 8 may be any combination of paper and inked ribbon or equivalent, which prints by impact, compressing the ribbon and paper between the type and the platen. FIGs 3 and 4 illustrate the relationship between the ratio of the length of the two armature legs and the print hammer velocity. The print hammer length ratio a/b is fixed in the preferred embodiment by the choice of a proven print hammer. The print hammer velocity at distance b is essentially twice that at distance a. The armature legs length ratio is selected for optimum performance. Also the mass in armature leg c is minimised, while still maintaining the required mechanical requirements, with respect to the mass in armature leg d for optimum performance. The armature is made of a soft magnetic material such as 1010. The armature leg length ratio shown in the graph (FIG 3) is r=c/d. The overall inertia of the armature may be selected for wearability and other parameters, with the selection of three values of normalised armature inertia shown in the graph as separate lines (I a =.4, I= a .5, I a =.6). As one might expect, as the armature inertia decreases, other things being equal, print hammer velocity increases. Velocity being an important parameter of impact force, it is desirable to optimise velocity. Selecting an armature leg length ratio (c/d) of approximately 2:1 optimises velocity over a range of armature inertias. Also, reducing the hammer mass while maintaining the same ratio a/b and ratio c/d will increase hammer velocity, decrease print energy, reduce contact time and reduce flight time. FIG 5 is a detailed diagram of the preferred print hammer actuator assembly. Print hammer 1, in common with other print hammers as required for multiposition printing (see FIG 6), is carried on print hammer pivot shaft 2. Armature 5, similarly carried on armature pivot shaft 9, is held juxtaposed with its related print hammer by a subassembly of the print hammer actuator assembly. The entire print hammer actuator assembly may be formed in mirror image as shown in FIG 7, with replicated print hammer pivot shaft and armature pivot shaft, and with alternate print hammers interspersed in well known fashion, to achieve a very compact multiactuator assembly. Components 1 to 14 have been previously described with reference to FIG 1; components 15 to 21 complete the print hammer actuator assembly of the preferred embodiment. There are two major subassemblies, the stator block subassembly of components 5 to 7, 9, 11 to 18 and the locating plate subassembly of components 1 to 4, 10, 19 to 21 which are secured together by screws (not shown) in a print hammer actuator assembly means. Armature 5 is held in the rest position by a permanent magnet 15. The magnetic force is maximum at the rest position and minimal at the instant of print impact because distances are greatest at that instant. An armature guide assembly screw 16, an armature backstop adjustment screw 17 and an armature backstop 18 perform their named functions. The backstop 18 is of a material such as polyurethane. A print hammer return spring 19, which rests against an extension 20 of the print hammer 1, provides restoring energy to the print hammer to return it after print impact. This holds the print hammer against the armature energy transfer surface 7 in the rest position. Additional restoring energy is provided through rebound after print impact. In FIG 5, note that pivot shafts 2 and 9, while still essentially coplanar with energy transfer surface 4 during energy transfer, do not have their axes precisely aligned with the contact surface of the energy transfer surface. Tradeoffs in manufacturability may require slight modifications from the optimum. Print hammer pivot shaft 2 is mounted in a channel in a locating plate 21; the axis of print hammer pivot shaft 2 defines a print hammer centre of rotational travel. Each print hammer 1, having an impact face end 3 and a pivoted end 20, is mounted on the print hammer pivot shaft 2, at its pivoted end, in a suitable recess in locating plate 21. This recess may be closely configured so as to hold the print hammer against lateral movement on print hammer pivot shaft 2. Similarly, armature pivot shaft 9 is mounted against plate 21, with the armatures closely confined in cutaways to prevent lateral movement. Armature pivot shaft 9 is essentially coplanar with and parallel to print hammer pivot shaft 2. In operation, it is very desirable to make the energy transfer to accelerate the print hammer with very little sliding contact between the armature and the print hammer; otherwise wear would occur. In the arrangement described there is minimal sliding contact because the armature 5 and the print hammer 1 are pivoted in line with the armature energy transfer surface 7, thus ensuring that the pivoted members (armature 5 and print hammer 1) have minimum relative motion even though moving in circumferential arcs of differing radii. The fixed pivots being in line with the energy transfer surface during the energy transfer limits the relative (sliding) motion. In addition, the armature energy transfer surface 7 is curved, permitting a slight rolling during the very small period of mutual motion of similarly convex arcs during a period passing through tangency. The period of tangency is lengthened by the rolling of the energy transfer surface 7. The two arcs may be described as being "similarly convex and epitangent." The armature pivot shaft 9, the print hammer pivot shaft 2 and the armature energy transfer surface 7 in the arrangement described are all aligned substantially coplanar, which allows energy transfer from armature to print hammer without harmful sliding between the armature 5 and the print hammer 1. The relationships of the length of the legs of the armature and the print hammer are selected for optimum performance. The energy transfer surfaces of the armature and the print hammer are located at about one half the distance from the print hammer pivot shaft 2 to the distal print hammer face end, and the armature leg 11 to armature coil leg 12 length ratio is about 2.0, optimised for velocity across a range of armature inertia choices. The relationship between the pivot shaft positions and the lengths of the energy transfer armature leg and the position of the energy transfer surface on the hammer is such that energy transfer surfaces 4 and 7 are epitangent; that is, the energy transfer surfaces of the armature and hammer move in similarly convex circular arcs, essentially tangent to the same line at the same point, during energy transfer, thus providing for minimum sliding contact between the energy transfer surfaces during energy transfer. The armature leg lengths are optimised for velocity at the armature inertia selected. FIGs 6, 7 and 8 show the relationships of the print hammers 1 to a single print hammer shaft 2, the armatures 5 and a single armature shaft 9, in a multiactuator printer. As shown in FIGs 7 and 8, the print hammers may be replicated in two banks, interleaved so as to form a very compact print unit for a multiactuator printer.

Claims: 1. A print hammer actuator for an impact printer comprising a print hammer pivot shaft (2), a print hammer (1) pivoted at one end about said print hammer pivot shaft, having an impact mass (3) at the other end, and formed with an energy transfer surface (4), an armature pivot shaft (9), an armature (5) pivoted about said armature pivot shaft, and formed with an energy transfer surface (7) in contact with said hammer energy transfer surface (4), and an electromagnetic coil (13) adapted to cooperate with said armature in order to move said armature so that said armature energy transfer surface (7) transfers energy to said hammer energy transfer (4) surface and moves said hammer (1), resulting in said impact mass (3) moving into an impact printing position, characterised in that said print hammer pivot shaft (2), said armature pivot shaft (9) and said energy transfer surfaces (4, 7) are substantially coplanar during the transfer of energy from said armature energy transfer surface (7) to said hammer energy transfer surface (4). 2. A print hammer actuator as claimed in Claim 1 characterised in that said armature energy transfer surface (7) and said hammer energy transfer surface (4) move in epitangent similarly convex circular arcuate paths during energy transfer. 3. A print hammer actuator as claimed in Claim 1 or Claim 2 characterised in that said armature energy transfer surface is convex. 4. A print hammer actuator as claimed in any one of the preceding claims characterised in that said armature (5) has two legs (11, 12) extending from said armature pivot shaft (9), a first relatively short, relatively large mass coil leg (11) adapted to cooperate with said electromagnetic coil (13), and a second relatively long, relatively small mass leg (12) formed with said armature energy transfer surface (7). 5. A print hammer actuator as claimed in any one of the preceding claims characterised in that said hammer moves in free flight into said impact printing position. 6. A print hammer actuator as claimed in any one of the preceding claims characterised in that one of said energy transfer surfaces is formed from a polymer. 7. A print hammer actuator as claimed in Claim 6 characterised in that said polymer is a moulded insert on said armature. 8. A plurality of print hammer actuators for an impact printer comprising a common print hammer pivot shaft (2), a plurality of print hammers (1) each pivoted at one end about said print hammer pivot shaft (2), having an impact mass (3) at the other end, and formed with an energy transfer surface (4), a common armature pivot shaft (9), a plurality of armatures (5) each pivoted about said common armature pivot shaft (9), and formed with an energy transfer surface (7), each armature energy transfer surface (7) being in contact with the energy transfer surface (4) of an associated print hammer (1), and a plurality of electromagnetic coils (13) each adapted to cooperate with a respective one of said armatures (5) in order to move said respective armature so that the energy transfer surface (7) on said armature transfers energy to the energy transfer surface (4) of the associated print hammer (1) and moves said print hammer, resulting in the impact mass (3) on said print hammer moving into an impact printing position, characterised in that said common print hammer pivot shaft (2), said common armature pivot shaft (9) and all said energy transfer surfaces (4, 7) are substantially coplanar during the transfer of energy from each armature energy transfer surface (7) to the associated hammer energy transfer surface (4).

Inventors: WALLACE, JOSEPH EUGENE, WANG, HAN CHUNG
Applicants: INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1986
Main classification: B41J9
IPC codes: B41J9