Constant force roll assembly

A mounting mechanism for the backup roll of a roll pair wherein a four bar linkage is provided. The backup roll is mounted at one end of a first arm and bears against a mating roll positioned in a stationary location. The first arm extends to a pivot point around which the backup roll rotates to engage the mating roll. A second arm is rigidly connected to the first arm at the pivot point and extends to an end point. A third arm, comprised of a coil spring with a coupler threaded thereto, extends from the end point of the second arm to a stationary point. The fourth arm of the four bar linkage is the ground extending between the stationary points to which the first and third arms are attached. By adjusting the coupler, the number of active coils in the third arm is adjusted thereby adjusting the normal force between rolls to a desired value. By providing the first arm with substantially more length than the second arm, the normal force between rolls is maintained relatively constant over a wide tolerance in the stationary location of the mating roll.

This invention relates to paper feed devices and more particularly relates 
to a mounting mechanism capable of maintaining relatively constant normal 
force between paper feed rolls despite variations in roll position. 
BACKGROUND OF THE INVENTION 
Many types of paper processing machines such as convenience copiers, 
printers, and the like, utilize paper feeding devices to move documents to 
a processing station. Machine operators may feed documents by inserting 
them one at a time into a feeding mechanism or may place a stack of 
documents upon a feed tray from which they are automatically fed. Blank 
sheets to be printed are generally fed to a print receiving station from a 
stack of sheets. 
U.S. Pat. No. 4,052,054 describes a semiautomatic document feed device 
(SADF) for a convenience copier. In that device, the operator inserts a 
document to be copied into a pair of aligner rolls positioned near the 
front of the machine. Those rolls take the document from the operator's 
hand and move it forward to register the leading edge of the document 
against an entry gate. While moving the document forward, the aligner 
rolls simultaneously move the document sideways to a reference edge near 
the front of the machine so that when the document reaches the entry gate 
the side edge of the document is also in proper position. In the SADF 
shown in the above-mentioned patent, a second set of aligner rolls are 
provided near the rear of the machine such that the leading edge of a 
second document to be copied can be positioned at the entry gate in 
side-by-side relationship with a first document. As the second set of 
aligner rolls operate, the second document is side-edge referenced against 
a reference edge at the rear of the machine. When the entry gate drops, 
the aligner rolls are restarted to feed the two sheets to the processing 
station simultaneously. 
In the operation of the device described in the above-named patent, it is 
sometimes desired to copy a wide sheet of paper, for example, a computer 
printout sheet, which comes under the influence of both the front aligner 
rolls and the rear aligner rolls. Since the front aligner rolls attempt to 
move the side edge of the sheet toward a front reference edge and the rear 
aligner rolls attempt to move the sheet toward a rear reference edge, the 
two sets of aligner rolls tend to move the sheet to be copied in opposite 
directions. In the device described in the above-mentioned patent, it is 
desired to move the sheet toward the front reference edge for proper 
positioning, and therefore a slightly higher normal force must be exerted 
between the aligner rolls at the front than is exerted between the aligner 
rolls at the rear. By so doing, the large sheet is caused to slip in the 
nip of the rear aligner rolls and is moved to the front reference edge. 
Another problem faced by the SADF of the above-mentioned patent is that the 
normal force produced between the aligner rolls cannot be too great since 
the rolls feed the leading edge of the paper against the entry gate. If 
the force is too high, the paper may buckle or may be damaged. To prevent 
that result, the normal force between the aligner rolls must be low enough 
to allow the paper to slip between the rolls when the leading edge reaches 
the entry gate. 
The normal force between aligner rolls cannot be too low, however, since 
too low a force would cause the paper to slip prior to the time it reaches 
the entry gate and create a situation in which the paper may not be 
properly registered at the entry gate. 
It has been found that the normal force produced between the aligner rolls 
must be between 20 and 30 grams in order to drive the paper properly. 
Within that narrow range of force, the normal force between the front 
aligner rolls must be greater than the normal force between rear aligner 
rolls. It is desired, therefore, to produce a front aligner roll nominal 
force of 271/2 grams with a tolerance of .+-.2 grams and a nominal rear 
aligner roll force of 221/2 grams with a tolerance of .+-.2 grams. When 
one considers that the weight of a standard paper clip is about one gram, 
one can appreciate the sensitive force adjustment needed to hold within 
the small tolerance levels necessary for the proper operation of the 
device. 
In a roll-mounting device which makes use of a spring to set the required 
force between rolls, most commercially available springs carry about 
.+-.10% tolerance in the spring rate. If one desires a .+-.2 gram 
tolerance in a force of 221/2 grams, it is apparent that the entire 
tolerance figure for the mounting mechanism is more than consumed by the 
tolerance in spring rate. Since tolerances must exist in the mechanism 
itself and in the rolls, the problem is compounded. One should also note 
that a 10% tolerance in spring rate results in greater than 10% variation 
in deflection. 
In the particular paper moving aligner rolls used in the above-named 
patent, another problem is the displacement tolerance provided by the 
nominal positioning of the aligner rolls. Note that one of the aligner 
rolls is mounted in a movable cover, thus the position of this roll is not 
determined until the cover is closed. One may appreciate that a 
significant tolerance must be provided in the positioning of this roll 
from machine to machine to accommodate the nature of a pivoting cover. In 
such an environment, it is essential to provide a roll mounting mechanism 
which is capable of adjusting to a nominal spring rate and capable of 
providing a relatively constant normal force over a relatively wide 
deflection range. Additionally, as outlined above, the roll mounting 
mechanism must achieve these results despite the presence of accumulated 
tolerances in the roll mounting mechanism itself as well as tolerances in 
the spring rate. 
The object of this invention, therefore, is to provide a mechanism for 
mounting a backup roll to mate with a drive roll such that a relatively 
constant normal force is provided between rolls over a wide deflection 
range. 
SUMMARY OF THE INVENTION 
This invention provides a four-bar linkage for mounting the backup roll of 
a roll pair. One bar of the linkage is comprised of a coil spring into 
which a coupler is threaded. The coil spring and the coupler provide one 
arm of the four-bar linkage and exert a force on a second bar which leads 
from one end of the coil spring arm to a pivot point. A third bar of the 
linkage runs from the pivot point to the backup roll shaft and the fourth 
bar is the ground plane represented by the drive roll against which the 
backup roll is positioned, and the ground plane to which the other end of 
the spring arm is connected. With this arrangement, a moment arm is 
created around the pivot point such that a force is exerted through the 
backup roll on the mating drive roll. By adjusting the coupler, the 
desired nominal force is obtained and in the unique design of the mounting 
arrangement, the nominal force remains relatively constant over a wide 
tolerance band in the positioning of the drive roll.

DETAILED DESCRIPTION 
FIG. 1 is a view which corresponds to FIG. 1 of the above-mentioned patent. 
It is a perspective view of a convenience copier which can make 
advantageous use of the instant invention. A processing station comprised 
of a document glass 12 receives a document to be copied. Proper 
positioning of the document requires the leading edge to be placed against 
an exit gate 17 and the side of the document against reference edge 22. To 
move documents into position to be copied, the operator places a document 
on the entry tray 10 such that the leading edge is moved to the left by 
the aligner rolls 28 and 18. Simultaneously, the side of the document is 
moved toward the front of the machine against a front reference edge 22 
such that when the document reaches entry gate 24, the document is 
referenced against edge 22. When entry gate 24 is dropped, the aligner 
rolls feed the document onto the document glass where it comes under the 
influence of document feed belt 14. Feed belt 14 moves the document into 
processing position with the leading edge at the gate 17 and the side edge 
against front reference edge 22. 
If two documents are to be copied simultaneously, a second document is 
inserted by hand near the rear reference edge 26 until it comes under the 
influence of the rear aligner roll pair, rolls 20 and 32. These rolls move 
the leading edge of the document to the entry gate 24 and simultaneously 
move the side of the document toward rear reference edge 26. With 
documents present at both the front and rear reference edges, when the 
entry gate 24 is dropped, the two documents are fed simultaneously onto 
the document glass 12 and into registration with the gate 17. The side 
edge of the front document is positioned against reference edge 22 and the 
side edge of the rear document is positioned against reference edge 26. 
Machine control to set the optical system at proper reduction for two 
sheet copying and for moving the documents onto the document glass at the 
proper time are described in the above-named patent and in manuals 
published for the IBM Series III Model 60 Copier/Duplicator. Since these 
controls do not form a part of this invention, they need not be described 
herein. 
FIG. 1 shows the document cover 16 in opened position to illustrate the 
mounting of rolls 28 and 32 in the movable cover 16. In some arrangements, 
these rolls may be drive rolls and in other arrangements, these rolls may 
be the backup rolls. When cover 16 is closed, these rolls bear against the 
mating rolls 18 and 20. In any event, because of the presence of rolls in 
the movable cover and because of difficulty in holding the exact position 
of these rolls from machine to machine, variations in the exact 
positioning of the roll nip with the cover closed from machine to machine 
are unavoidable. Consequently, an arrangement for the backup roll must be 
provided such that it provides a required normal force against the mating 
roll regardless of variations in the exact positioning of the nip. 
FIG. 2 shows a mounting mechanism for the backup roll capable of doing the 
required job. Backup roll 18 is mounted at an end point 39 of arm 41 and 
forms a roll nip with mating drive roll 28. Arm 41 is pivoted at pivot 
point 42 and is considered to end at that point. Arm 41' extends from 
pivot point 42 to an end point 44. Coupler 43 is connected at end point 44 
to the arm 41' and contains a threaded portion 45 which is screwed into 
one end of spring 46. The opposite end of spring 46 is connected to 
bracket 40 at end point 47. Bracket 40 is stationary thus providing a 
ground plane to which end point 47 is attached. Note that backup roll 18 
bears against drive roll 28 (which acts as a ground point) and by virtue 
of the action of spring 46 provides a force against roll 28. 
The mounting mechanism for backup roll 18 shown in FIG. 2 is capable of 
providing a relatively constant normal force against drive roll 28 over a 
relatively wide range in the vertical positioning of the roll nip. That 
is, when drive roll 28 is brought against backup roll 18 by the closing of 
cover 16 (FIG. 1), the exact position of the nip between the two rolls can 
vary from machine to machine. Despite that variation, the required force 
between the two rolls is relatively constant because of the unique 
geometry of the mounting mechanism. One may observe from FIG. 2 that if 
roll 28 were positioned below the level shown in FIG. 2, (angle .theta. 
decreases) spring 46 would be stretched to a greater degree and therefore 
provide a higher force bearing on the end point 44. Despite the increase 
in the force exerted by spring 46, the output normal force of backup roll 
18 remains relatively constant. This result is obtained since, as the 
angle .theta. decreases (.theta. is the angle of deflection), the length 
of the moment arm through which spring 46 acts around pivot point 42 also 
decreases. As a result, the moment remains relatively constant even though 
spring force increases. Since the sum of the moments at static equilibrium 
around the pivot point 42 equals zero and since the geometry is such that 
the moment arm of backup roll 18 (generally along arm 41) is much longer 
than the opposing moment arm for spring 46, the changes in the angle 
.theta. have a significantly lowered effect on the length of the moment 
arm of the backup roll. As a result, the force exerted by the backup roll 
18 on the mating roll 28 remains relatively constant. The result of the 
application of this geometry to the front aligner rolls is illustrated in 
FIG. 3. 
FIG. 3 shows a family of curves for various spring rates and illustrates a 
desired operating range where .theta. varies from 4.degree.-10.degree.. 
The figure shows that a spring rate (Ks) of 21 provides a relatively 
constant force throughout the operating range. Note that the nominal force 
provided with a spring rate of 21 is approximately 27.8 grams at a 
deflection of 7.degree. while at 4.degree. the force is about 27.75 grams 
and at 10.degree. it is about 27.7 grams. Thus, a spring with the spring 
rate of 21 provides a normal force ranging only about 0.1 grams from the 
nominal force over a deflection angle of 6.degree.. This result is 
achieved despite tolerance variations in the spring rate (compensated for 
by the coupler 43) and despite accumulated tolerances in the mounting 
mechanism. The key considerations in obtaining this result are the 
provision of an arm 41 considerably longer than arm 41' and the use of 
threaded coupler 43 which alters the number of active coils in spring 46. 
FIG. 4 illustrates a family of curves for the rear aligner rolls utilizing 
the mounting arrangement shown in FIG. 2. In FIG. 4, note that with a 
spring rate of 17, a nominal force of about 22.3 grams is provided at a 
deflection of 7.degree.. At a deflection of 4.degree., the force is about 
22.15 grams and at 10.degree. deflection, the force is about 22.2 grams. 
Thus, the normal force between the rear aligner rolls ranges only about 
0.15 grams over a deflection of 6.degree.. 
FIGS. 3 and 4 show that the unusually severe force requirements found in 
the SADF of FIG. 1 are met by the mounting mechanism of FIG. 2. As a 
practical matter, however, whatever device is chosen to meet the force 
requirements, it should also be economical, easy to adjust, once adjusted 
it should maintain its adjustment, and it should require no fine tuning 
from machine to machine. One will appreciate that the mechanism of FIG. 2 
utilizes standard components. It can be adjusted easily by use of the 
coupler, once adjusted the coupler does not change position, and the 
device has been purposely designed to require no fine tuning despite 
variations in roll position from machine to machine. 
While the invention has been particularly shown and described with 
reference to a preferred embodiment thereof, it will be understood by 
those skilled in the art that the foregoing and other changes in form and 
details may be made therein without departing from the spirit and scope of 
the invention.