Note: Descriptions are shown in the official language in which they were submitted.
2015~21
PATENT
P~-188429
~IFFERENTIAL FRICTION MEDIA DRIVE
TECHNICAL FIE$D
The present invention relates to medium drive mecha-
10nisms for computer hard copy printers. The predominant
current usage of the improved medium drive ~echanism is as
a means for causing a medium such as paper to move through
the printer and past the printing area in a thermal ink-jet
printer.
BACKGROUND ART
A num~er of various means have ~een employed to move
a medium such as paper through computer hard copy printers.
20The first of these was the traditional typewriter roller
arrangement. This arrangement proved unsatisfactory for
automated computer printers, however, because it was not
well suited for automatically accepting single sheets of
medium as they were fed into the printer.
25Other methods that have been tried have included trac-
tor feed mechanis~s wherein drive sproc~ets engage holes in
the medium. This method works ~ell for form-feed type doc-
uments and other ~edia; however, it is not adaptable for
use with single sheets of media.
30Current drive mechanis~s for ther~al ink-jet printers
frequently include roller mechanis~s wherein the medium is
passed between two opposing rollers as in the rollers of an
old ~ashioned washing machine. This arrangement works well
because it provides a positive feed ~or the mediuc and al-
3Slows the medium to exit the drive area in a straight line.
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However, the problem with this arrangement is that there is
frequently insufficie~t room to ea~ily include a pair o~
opposed rollers. This problem is particularly acute as re-
gards the top roller, as it may interfere with the movement
of the printhead if it is placed too near.
It is desirable to place the drive mechanism as close
to the printhead as possible, because the less distance the
medium has to travel from the drive mechanism to ~he print-
head, the less chance there is of it buckling in that in-
terval. Further, if the printhead could be placed nearerto the drive area, necessary top and bottom margins could
be greatly reduced. This is considered to be a highly de-
sirable feature. Therefore, printer designers have been
faced with the dilemma of having to place the drive mecha-
lS nism as close as possible to the printhead to avoid buc~-
ling and to reduce necessary top and bottom margin sizes,
and also having to remove the drive mechanism from the
printhead sufficiently far to provide room for a top rol-
ler. This dilemma has been compounded by the fact that the
height of the printhead above the medium must also be mini-
mized, as this is an important factor in print quality.
Obviously, it would be desirable to provide a means
for driving media through printers which did not include a
top roller and therefore could be placed as close as is de-
sired to the printhead. However, previous attempt~ to de-
sign such a mechanism have been less than successful. For
instance, if the top roller is replaced with a flat sur-
face, the medium has tended to feed improperly and to jam
in the drive mechanism unless an additional pair of drive
rollers is provided, thereby allowing for a clearance be-
tween the drive roller and the flat surface.
To the inventor's knowledge, no prior art mechanism
for driving medium through a printer has successfully met
desirable size restrictions so as to reduce the overall
size of the printer and also to allow the drive mechanism
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to be placed very close to the printhead. All successful
medium drive mechanisms to date have used either a combina-
tion of rollers or other complex mechanisms in order to
avoid the problems of mediu~ buckling, and also to avoid
the problems of mediu~ misfeed and jamming, and further to
cause the medium to exit the drive mechanism in a straight
line parallel to the printhead. Further, no prior art
mechanism for driving medium through a printer has allowed
for producing top and bottom margins as small as 2 to 3
millimeters.
DISC~OSURE OF INVENTION
This invention relates to a medium drive mechanism for
a computer hard copy printer having an extremely low fric-
tion surface opposed to a high friction drive surface.
This combination provides a positive medium drive formerly
available only with dual roller drive systems.
Briefly, the preferred embodiment of the present in-
vention is a medium drive mechanism constructed such that
a high friction rubber drive roller imparts a motive force
to a sheet print medium. The medium is held in place
against the rubber drive roller by means of a guide plate
including a structural backing mechanism and an extremely
low friction surface. In the preferred embodiment of the
present invention, the low friction surface is constructed
of ultra-high molecular weight (UHMW) polyethylene. Al-
though other very low friction materials such as Teflon
could be used in the manufacture of the inventive drive
mechanism, ~HNW polyethylene has been found by the inventor
to provide the best combination of low ~riction properties
and wear resistance. It has been found by the inventor
that drive mechanism$ so constructed are no more prone to
~amming or medium misalignment than are dual roller drive
mechanisms.
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The superior drive qualities of the inventive drive
mechanism are largely due to the fact that the sum of forc-
es on the medium produced by the inventive drive actually
produces a greater total force in the intended drive direc-
tion than have simil~r prior art drive mechanisms.
The inventive medium drive mechanism subassembly may
be placed closer to the printhead in a printer assembly
than is possible with a dual roller medium drive mechanism,
because the guide surface does not interfere with the move-
ment of the printhead as does a top roller. Of course, the
greater distance the medium has to travel after leaving the
drive mechanism and before getting to the printing area of
a printer, the greater the liXelihood that buckling or oth-
er medium misalignment may occur. But the primary advan-
lS tages of the inventive drive mechanism are that the amount
of necessary top and bottom margins are greatly reduced,
and that high image_quality may be maintained by allowing
placement of the printhead close to the medium and to the
drive assembly.
Therefore, the inventive method is conducive to the
highest print quality possible, since medium orientation to
the printhead is optimized. That placement of the drive
mechanism close to the printhead is desirable has long been
known. However, prior attempts to design mechanisms which
could be placed closer to the printhead have failed because
all such attempts have resulted in mechanisms that were
more prone to jamming or other problems than were the con-
ventional dual roller drive mechanisms. Therefore, prior
to the present invention, dual roller mechanis~s have been
incorporated into ink-~et printer assemblies.
Another desirable factor of the present invention is
that a drive mechanism constructed according to the inven-
tion may be made smaller than conventional mediu~ drive
mechanisms, there~y facilitating a reduction in overall
printer size.
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An advantage of the present invention is that the size
of top and bottom margins on the print medium may be de-
creased.
Another advantage of the present invention is that a
printer medium drive mechanism may be placed horizontally
closer to an ink-jet printer printhead.
Yet another advantage of the present invention is that
image quality is improved by allowing placement of the
printhead vertically closer to the medium.
A further advantage of the present invention is that
a tendency of medium to jam in the drive mechanism is re-
duced.
A still further advantage of the present invention is
that the unsupported area between a drive mechanism and a
printhead wherein a medium might buckle is reduced.
Yet another advantage of the present invention is that
overall printer size may be reduced.
These and other objects and advantages of the present
invention will become clear to those sXilled in the art in
view of the description of the best presently known mode of
carrying out the invention and the industrial applicability
of the preferred embodiment as described herein and as il-
lustrated in the several figures of the drawing.
3RIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a diagrammatic representation of a side view
of a portion of a printer including the drive mechanism o~
the present invention:
FIG. 2 is a diagrammatic representation similar to
FIG. 1, showing the low friction surface o~ the present in-
vention replaced with a prior art pinch roller; and
FIG. 3 is a representation of the various forces ex-
erted on a print ~edium by a drive mechanism.
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BE~T MODES FOR CARRYI~G QUT INVENTION
The best presently known mode for carrying out the in-
vention is a thermal ink-jet printer incorporating a con-
ventional printer per. and pen traversing mechanism, but al-
so using a medium drive mechanism in accordance with the
present invention, wherein motive force is transferred to
the medium via a high friction surface and the medium is
held against the high friction surface by an opposing low
friction surface. The medium drive mechanism of the pres-
ent invention is more compact then comparable prior art
drive mechanisms, and also provides for more accurate me-
dium placements than has been considered to be feasible us-
ing compact drive mechanisms. In the best presently known
embodiment of the present invention, the friction differen-
tial necessary to the succe~s of the invention is achieved
by use of an ultrahigh molecular weight (UHMW) polyethylene
tape on the low friction suxface. The sum of the resultant
forces produced on the medium by the inventive drive mecha-
nism is greater in a desired drive direction than thatwhich is produced by comparable prior art drive mechanisms.
The predominant expected usage of the inventive medium
drive mechanism is in thermal ink-jet printers, wherein
clearance of the drive mechanism to the printhead pen is
critical, and especially in applications, wherein the
height O~r the combined assembly is a concern, such as in a
portable or small desk top units, and in applications
wherein minimal top and bottom margins are required.
A portion of the thermal ink-jet printer assembly o~
the presently preferred embodiment o~ the present invention
is illustrated in diagrammatic form in a side view in FIG.
1 and is designated therein by the general reference char-
acter lO. In most of its substantial components, the
printer lO does not differ signi~icantly from conventional
thermal ink-jet printers.
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The conventional elements of the printer 10 include a
printhead 12 and a printhead traversing mechanism (not
shown) for moving the printhead 12 across a piece of medium
14, thus positioning the printhead 12 for printing upon the
medium 14.
The medium 14 is moved through the printer by means of
a high friction drive surface 16. In the presently pre-
ferred embodiment of the present invention, the drive sur-
face 16 is made of high friction rubber and constitutes the
surface 16 of a drive roller 18. A drive plate 20 is pro-
vided to hold the medium 14 firmly against the drive sur-
face 16. A means for pressing the drive plate 20 toward
the drive surface 16 in a force direction 22 is provided in
the presently preferred embodiment of the invention by con-
structing the drive plate 20 of spring steel and tensioning
it against the drive roller 18 in force direction 22.
The tangent point 24 of the drive roller 18 to the
drive plate 20, according to the present invention, trans-
fers motive force to the medium 14 in a medium advance di-
rection 26 as the drive roller 18 rotates in a rotationaldirection 28. Therefore, normal force in force direction
22 must be sufficiently great to prevent slipping and mis-
handling of the medium 14. The actual amount of necessary
normal force in force direction 22 may vary depending upon
the type of medium 14 used and other variables, but this
can be guickly and easily adjusted according to the exact
application to which the present invention is applied.
Contrary to previous beliefs about printer drive mech-
anisms, the inventor has found that sufficient normal force
in force direction 22 may be applied to prevent slippage
and resultant ~amming of the medium 14 when tangent point
24 is the point of transfer of motive force from the high
friction drive surface 16 to the medium 14. In t~s best
presently known embodiment of the present invention, an ul-
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trahigh molecular weight (UHMW) polyethylene tape 30 is ap-
plied to the drive plate 20.
Using the UHMW polyethylene tape 30, friction differ-
ential as between th~ first junction 32 of the high fric-
5tion drive surface 16 to the medium 14, and the second
junction 34 of drive plate 20 to the medium 14, sufficient
to prevent problems previously believed to be unavoidable
in such an application, may be created. UHMW polyethylene
tape may be obtained from 3M Company, Industrial Tape Divi-
10sion, St. Paul, Minnesota, under their part number 5425.
The tape is provided with a solvent-resistant acrylic adhe-
sive backing (not shown) for adhering the UHMW polyethylene
tape 30 to the drive plate 20.
As opposed to prior art mechanisms, as depicted in
15FIG. 2, wherein a pinch roller 44 is used to reduce drag,
the UHMM polyethylene tape of the presently preferred embo-
diment of the present invention allows the printhead 12
(FIG. l and FIG. 2) to be placed closer to the tangent
point 24 while not necessitating raising the printhead 12
20higher above the medium. ~his is an important advantage in
thermal ink-jet printers, since maintaining the printhead
12 at a minimal height above the medium 14 is necessary to
print quality, and since placement of the printhead 12
close to the drive tangent point 24 allows printing on a
25greater portion of the medium 14, thereby reducing the size
of top and bottom margins (not shown).
Referring now to F~G. 3, wherein is depicted a drive
force 46, a drag force 48, and two normal forces 50 acting
on a print medium 14, it can be seen that the sum of forces
30~6, 48, and 50 will influence movement of the medium 14 in
the medium advance direction 26. The normal forces 50 are
produced, in the best presently known emkodiment of the
present invention, by tensioning of the drive plate 20 to-
ward the drive roller 18. Since the drive plate 20 and the
35drive roller 18 are immobile in the plane of the normal
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forces 50, the two normal forces 50 are necessarily equal
in magnitude and opposite in direction. The drive force 44
in medium advance direction 26 acting on the medium 14 is
a function of the product of the normal force 50 and a
friction factor between the high friction drive surface 16
and the medium. The drag force 46 acting in a direction
opposite to medium advance direction 26 is a function of
the product of the normal force 50 and a friction factor
between the low friction surface 30 and the medium 14. As
can be appreciated by one skilled in the art, greatly re-
ducing the friction factor between the medium 14 and the
drive plate 20 by addition of the low friction surface 30,
causes the ratio of the drive force 44 to the drag force 46
to be greatly increased. The sum of forces acting on the
medium 14 in a plane parallel to the medium drive direction
26 is the drive force 44 minus the drag force 46. The nor- -
mal force 50 can be increased, in accordance with the pres-
ent invention, to a value necessary to provide sure and
positive handling of various types of medium 14 without
slippage or other mishandling of the medium 14.
various modifications may be made to the invention
without altering its value or scope. For example, the high
friction drive surface 16 may be part of a flat sliding
plate, rather than a roller. Another conceivable altera-
tion would be to use another very low friction surfacingmaterial as a substitu~e for the UHMW polyethylene tape 30.
All of the above are only some of the examples of
available embodiments of the present invention. Those
skilled in the art will readily observe that numerous other
modifications, alterations, and adaptations may be made
without departing from the spirit and scope of the inven-
tion. Accordingly the above disclosure is not intended as
limiting and the appended claims are to be intarpreted as
encompassing the entire scope o~ the invention.
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INDUSTRIAL APPLICABILITY
Thermal ink-jet printers have found wide acceptance in
the marketplace. Among the many desirable qualities of
thermal ink-jet printers are the high print quality and the
ease of use of these printers. The present invention has
been found to enhance both of these qualities. The print
quality of thermal ink-jet printers is enhanced through use
of the drive mechanism of the present invention by virtue
of the fact that the printhead may be placed at a height
which is relatively close to the medium as compared with
prior art arrangements, and because the height relationship
of the medium to the printhead is held relatively constant
by virtue of the fact that the medium is supported close to
the printhead by the inventive drive assembly.
Since printing is accomplished in a thermal ink-jet
printer by ejecting droplets of ink onto the medium, any
vari~tions in the plane of the medium relative to the sur-
face of the printhead from which the ink is ejected will
result in a reduction of print quality. Since the market-
place for thermal ink-jet printers is quite competitive and
since one of the major differentiating factors of such
printers is the print quality, any innovation such as the
present invention which will enhance quality will greatly
enhance the utility of the printer in the marketplace.
Furthermore, since the printhead may be placed very
close to the drive assembly, according to the present in-
vention, the top and bottom margins (unprinted areas) may
be greatly reduced. These margins are a function of the
distance from the drive assembly to the printhead, since
the medium must necessarily be supported in the drive as-
sembly when printing is being accomplished, ~nd since the
absolute minimum margin would be the distance from the
printhead to point at which the medium is supported by the
drive assembly. It is thought that this factor alone will
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provide a major benefit to the users of printers employing
the inventive drive mechanism.
Ease of use of thermal ink-jet printers is greatly
reduced by any jamming or mishandling of medium within the
printer. Minor variations in medium handling will result
in print being improperly placed upon the medium. Major
mishandling problems require operator intervention and are
considered to be extremely detrimental to user satisfac-
tion. For this reason, dual roller type medium drive mech-
anisms have been used in spite of the fact that they are
less than ideal for other reasons discussed herein.
However, the inventive mechanism has been found to be
as conducive to error-free medium handling as have prior
art dual roller mechanisms. Therefore, it is anticipated
that medium drive ~echanisms according to the present in-
vention will greatly enhance the usability of thermal
ink-jet printers.
Another factor which may increase the acceptance and
usefulness of the inventive drive mechanism is that the
need for smaller printers is increasing. In fact, portable
printers to accompany portable computers may be a future
application.
Since the medium drive mechanisms of the present in-
vention may bs readily constructed and are easily incorpo-
rated into printer designs, it is expected that they will
be accepted in the industry as substitutes for conventional
medium drive mechanisms.
For these and other reasons, it is expected that the
utility and industrial applicability of the invention will
both sign$ficant in scope and long lasting in duration. `
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