High speed printer hammer assembly

A high speed printer is provided having a plurality of aligned hammer assemblies for directing hammers at a moving character band for printing characters responsive to the hammer's driving against the character band. The hammer assembly has a striker arm pivotly mounted to a striker assembly frame and which acts as an armature when driven by a pair of magnets. The magnets are attached to the striker assembly frame on opposite sides of the striker arm and on opposite sides of the pivot point for the striker arm so that simultaneously actuating both magnets pulls the striker arm for actuating the hammer. Each magnet has a single coil, with one of the magnets having a reverse polarity from each adjacent magnet on an adjacent assembly, while the other magnet has the same polarity to avoid cross-talk and to increase the speed of the printer. A pressure-sensitive tape is bonded to the striker armature to provide a residual air gap between the armature and magnet poles and improvements are made in the hammer guide combs, return plunger, and the platen.

BACKGROUND OF THE INVENTION 
The present invention relates to printing hammer assemblies for high speed 
line printers which utilize a fast moving, steel character band and high 
speed hammer assemblies for hitting the band at the location of the 
appropriate characters on the band to print upon the band and especially 
to a combination of techniques for improving the operation and speed of 
the hammer assembly. 
In the impact printing field, a wide variety of printing techniques have 
been used in the past including those employed in the ubiquitous 
typewriter: the drum printer, the wheel printer, and the chain or belt 
printer. In the present type of printer, an endless, steel character band 
having various characters of the alphabet as well as numbers embossed or 
raised upon the band, is rotated between a drive pulley and an idler 
pulley. As the band is driven at high speeds adjacent a platen, a bank of 
parallel hammers is driven at a high speed at the moment the particular 
desired character is passing on the band to print the character upon the 
paper. The print hammer actuators are typically electromagnetically 
actuated, such as by solenoids, which magnets are energized by the driving 
circuit with each pass of the character band. The hammer assemblies need 
to be spaced close to each other so that a large bank of hammers can fire 
as the band is passing; and each hammer must respond rapidly in view of 
the fast moving band which might otherwise smear the character if the 
hammer were operating at too slow a speed. Accordingly, the present 
invention is directed towards a hammer assembly of a high speed printer 
designed for increasing the speed of the actuators with an increased speed 
of the character band. To accomplish this, a number of techniques have 
been utilized to produce a printer that can print in excess of 2000 lines 
per minute with a 48 character set. 
In the past, a variety of print hammer assemblies for high speed printers 
have been provided, and some of these may be seen in the following U.S. 
Pat. Nos. 3,144,821, to Drejza; 3,719,139, to Niccolai; 3,289,575, to 
Wassermann; 3,285,164, to Makavazos; 3,584,574, to Smith; 3,726,213, to 
Herbert; 3,707,122, to Cargill; 3,285,166, to Helms; 3,745,495, to Chai; 
3,748,613, to Venker; 3,314,359, to Martin; 3,166,010, to Fradkin; 
3,734,013, to Belser; 3,172,352, to Helms; 3,449,639, to Brown; 3,592,311, 
to Chou; 3,656,425, to Albo; 3,460,469, to Brown; 3,659,238, to Griffing; 
3,630,142, to Fulks; and 3,502,190, to Smith. 
In addition to these assemblies, one U.S. Pat. No. 3,285,165, to Richter, 
teaches a print hammer control apparatus for a high speed printer in which 
a plurality of print hammer actuators respond to a signal applied thereto 
for energizing adjacently flanking pairs of actuators with signals of 
substantially the opposite polarity. In this patent, each magnet has a 
pair of windings as in a typical hammer actuator format and cross-talk 
between adjacent print hammer actuators is controlled by the application 
of a field of opposite polarity adjacent each magnet. This is accomplished 
in one case by auxillary coils located between the hammer assemblies and 
in a second embodiment utilizing NRC circuit connection, connecting 
adjacent coils of opposite polarity. The present invention advantageously 
handles cross-talk by the placing of two magnets adjacent a striker with 
only one coil per magnet in the striker assembly, and thereafter having 
one of the magnets in each assembly of opposite polarity from the adjacent 
magnet while maintaining the second magnet of the same polarity in 
adjacent hammer assemblies, so that like magnets have cross-talk while 
unlike magnets of adjacent hammer assemblies have cross-talk substractive 
and effect a cancellation by equal amounts of additive and substractive 
cross-talk. 
SUMMARY OF THE INVENTION 
The present invention relates to a high speed line printer having a 
plurality of lined hammer assemblies for directing hammers at a moving 
character band for printing characters responsive to the hammer's driving 
against the character band. The hammer assembly has a striker assembly 
frame and an armature striker arm pivotly attached thereto for driving a 
hammer to print a character when actuated by the striker. The striker is 
driven by a pair of magnets located on either side of the striker armature 
arm with one magnet above the pivot point and one below the pivot point so 
as to simultaneously pull the striker bar. Each magnet has only a single 
coil and one of each pair of magnets has a polarity opposite that of an 
adjacently mounted magent on an adjacent hammer assembly while the other 
magnet has the same polarity as each adjacent magnet in each adjacent 
hammer assembly so as to reduce cross-talk between closely mounted hammer 
assemblies, and to increase the speed of the line printer. An improved 
platen working adjacent the rotating character band is provided along with 
improvements in the hammer assembly.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to FIG. 1, a hammer assembly 10 for a high speed line printer 
is mounted adjacent a platen assembly 11 which has a high speed endless 
character band 12 rotating thereby between a drive roller and an idler 
roller (not illustrated). The hammer assembly 10 has a striker assembly 13 
which drives a striker armature arm 14. The striker arm 14 is mounted to 
the striker assembly framework 15 by pin 16, so as to pivot on the pin 16, 
which allows the striking portion 17 of the striker arm 14 to drive a 
push-rod 18, which in turn, pushes a hammer 20 which hammer surface 21 is 
driven towards the character band 12 moving past the platen 11. Paper 22 
rides between the character band and the hammer so that the hammer 21 is 
actuated in the proper sequence for a predetermined character on the 
character band 12 passing in front of the hammer and prints the character 
on the paper. The striker assembly 13 has an adjustable stop 23 holding 
the striker arm 14 in position. A pair of electromagnet coils 24 and 25 
are mounted to U-shaped magnetic cores 26 and 27 adjacent the striker arm 
14. The coil 24 is mounted on the magnetic core 26 on one side of the 
striker arm 14 and above the pivot point 16 while coil 25 of magnetic core 
26 is mounted on the opposite side of the striker arm 14 and below the 
pivot point 16 so that simultaneous actuation of the coils 24 and 25 will 
pull striker armature arm 14 towards each magnet thereby simultaneously 
driving striker arm 14. The striker arm maintains an air gap 28 and an air 
gap 30 between the striker arm which acts as the armature for both magnets 
and the tips of the cores 26 and 27. This residual air gap is assured with 
a pressure-sensitive tape bonded to the striker armature 14 to provide the 
residual air gap between the armature and the magnet poles in the striker 
assembly for the hammer bank 10. An air gap of between 0.003-0.005 inches 
must be provided between the armature and magnet poles so that the 
armature will release when the magnet coil is deenergized. This gap is 
called the residual air gap. Once the gap has been established, it is 
important that the gap remain constant within 0.0035-0.0045 inches for at 
least 100 million cycles of operation of the electromagnet. A 0.002 inch 
thick Kapton polyimide film backed with a 0.002 inch thick acrylic 
adhesive forms the air gap tape. The tape is applied to both air gap 
surfaces of the armature or striker arm 14. Pressure is applied to the 
tape and the tape armature is baked in an oven to cure the acrylic 
adhesive. The curing of the adhesive prevents the squeezing out of place 
of the tape by repeated operations of the electromagnets. Following the 
curing of the adhesive, the change in the air gap is in order of 0.0002 
inch in one hundred million cycles. Curing a ploymer surface on the 
armature overcomes the problems of line printers which use a long tape 
which is passed through the air gaps of all electromagnets and which has a 
transport mechanism which continuously exposes a fresh section of residual 
air gap tape to the electromagnet poles. It thus prevents a change in the 
air gap due to the compacting of the tape. Thus, the present system 
eliminates the transport mechanism and tape replacement. 
The hammer assembly 10 has a hammer module framework 31 having a hammer 
module mounting bar 32 and a hammer return spring and a cylindrical 
plunger 33 with a rounded tip and having the plunger adjacent a tab 34 on 
one end of the hammer 20. The hammer 20 is pivotedly mounted to hammer 
pivot shaft 35 mounted to the hammer frame 31 and rides between tines or 
teeth in a guide comb 36 which maintains the hammer in horizontal 
alignment and looks similar to a hair comb with protruding tines. The 
guide comb 36 is attached to a guide comb mounting bar 37 which is 
attached to the hammer assembly framework 32 and impression control 
support assembly 38. The forms compressor 39 is attached to the frame 38 
and compresses the forms on paper being printed upon. The hammer 20 also 
works in conjunction with an impression control pad 40 having an 
impression surface 49 which is adjusted to receive a downwardly extending 
protrusion 42 of the hammer 20 to limit the thrust and travel of the 
hammer 20. The hammer return spring and plunger 33 immediately return the 
hammer following actuation, which in turn, throws a push-rod 18 back and 
the striker arm 14 away from the magnets 26 and 27. The hammer return 
plunger is made of a polyimide resin (Polymer SP-211) having a 
fluorocarbon resin added (10% polytrifluorochloroethylene) thereto, such 
as vespel, which provides a plastic material which is self-lubricating but 
long-lasting. Similarly, the platen surface 41 is made of the same 
material which ideally provides a long-lasting platen which has some 
resilience while providing self-lubrication and has been shown to out-last 
metal platens and plungers where different stresses are provided by a fast 
moving steel band being driven thereagainst by the hammer. The platen 11 
is attached by an adhesive to a platen support 42 which is in turn 
attached to a support member 43 by a series of screws 44 which allows the 
platen surface 41 to be quickly changed following wear. The platen is 
mounted in a pair of ribbon guides 45 having a rounded surface which 
allows the ribbon paper to ride thereover and over the character band 12 
but inside of the hammer 20. The guide comb teeth 36 are made of 
polyphenylene sulfide resin having pelletized glass material (40%), used 
as a filler and thereby providing a self-lubricating comb which can 
withstand high temperatures in guiding the hammer. The use of these new 
materials in the hammer assembly and platen advantageously reduces wear on 
the hammers and teeth, and increases the reliability and operation of the 
hammer assembly over that of the conventional metal components commonly 
used for similar purposes; and specifically avoids variations in hammer 
speed caused by friction and frictional wear and thereby provides hammer 
flight-time stability. 
Turning now to FIGS. 2 through 4, the operation of the magnets 26 and 27 in 
their coils 24 and 25 is illustrated in which the armature and striker bar 
14 is mounted to its pivot point 16. The magnetic cores 26 and 27 are 
mounted as illustrated in FIG. 1 having electromagnetic coils 24 and 25. 
Only one coil 24 is used on the magnet 26 and only one coil 25 is used on 
the magnet 27. In addition, the cores 50 and 51 of FIG. 3 have coils 52 
and 53 mounted in the same manner, and each pair of magnet assemblies of 
FIGS. 2 and 3 would be mounted adjacent each other. However, the magnets 
26 and 50 are of the same polarity even though adjacent, while the magnets 
27 and 51 are of opposite polarity, so that facing adjacent magnets 
provide a pair of magnetic poles as illustrated in FIG. 4. This 
arrangement provides magnets 26 and 50 with a cross-talk additive while 
the magnets 27 and 51 have a cross-talk substractive as relates to 
adjacent hammer assemblies. The mounting of adjacent hammer assemblies in 
this way has the cross-talk cancelling each other with the single armature 
experiencing equal amounts of additive and substractive cross-talk. Odd 
numbered magnetic coils may be connected for a series additive and 
even-numbered magnetic coils may be connected for series opposing 
cross-talk. This assembly advantageously eliminates cross-talk while 
maintaining a simplified magnet aramture design for driving closely 
mounted adjacent hammer assemblies thereby improving the speed of 
operation of the hammers which is further enhanced by the improvements in 
the hammer assembly 10 and platen assembly 11. It will, of course, be 
clear that a plurality of hammer assemblies 10 are mounted side by side 
along the front of the character band 12, but that since each hammer 
assembly must drive a hammer in front of the character band, the space 
available for the hammer assemblies is limited and thereby brings the 
magnetic structures closely adjacent each other and thereby allowing 
cross-talk between the hammer assembly and magnets interfering with the 
fast response and operation of the magnets. The present invention, 
however, is not to be construed as limited to the particular forms 
described herein, which are to be considered illustrative rather than 
restrictive.