Friction drive for an electrophotographic print engine

A print engine is provided for electrophotographically transferring an image from an image source to an image-support member. The print engine includes a photoconductive member having a photoconductive surface for storing a latent, electrostatic image. A photoconductive drum charger and an image transfer device generate the latent, electrostatic image on the photoconductive surface. A developer station supplies developer to the latent, electrostatic image as it is being carried by the photoconductive surface to provide a developed image. A carrier member includes an electrically charged support surface for supporting an image-support member which is passed through an image transfer nip. The image transfer nip extends between the photoconductive member and the carrier member. The image-support member and the developed image are simultaneously passed through the image transfer nip so that the developed image is transferred to the image-support member. A mounting assembly is provided for movably supporting the photoconductive member and the carrier member in a relative frictional engagement therebetween, wherein the carrier member is pressed into the photoconductive member so that the support surface of the carrier member will move at substantially the same speed through the image transfer nip as the photoconductive surface of the photoconductive member in response to movement of the photoconductive member.

TECHNICAL FIELD OF THE INVENTION 
The present invention relates in general to print engines, and in 
particular, to a power drive system for a print engine. 
BACKGROUND OF THE INVENTION 
Prior art print engines have been used in both copiers and printers for 
electrophotographic printing. Prior art print engines have included 
photoconductive transfer members and paper carrier members. Either or both 
of these members may be a cylindrical drum or a belt. The belts typically 
extend over cylindrical rollers. The photoconductive transfer member 
generally travels beneath a photoconductive drum charger and then an image 
generator which together place a latent image of electrostatic charge upon 
an exterior surface of the photoconductive transfer member. The surface of 
the photoconductive transfer member then moves the latent image of 
electrostatic charge beneath a developer station. Toner is applied to the 
latent image to provide a developed image. The toner may be black or one 
of multiple colors, depending upon whether a color image or a black and 
white image is being produced. 
After passing through the developing station, the developed image is 
transferred to an image-support member, which is typically a sheet of 
paper, but may also be a sheet of clear plastic such as the type used for 
overhead projector transparencies. The image-support member is secured to 
and carried on the carrier member, which transports the image-support 
member through an image transfer nip. The image-support member and the 
developed image are simultaneously transported through the image transfer 
nip by the carrier member and the photoconductive transfer member, 
respectively. The image-support member and the surface of the 
photoconductive transfer member are usually supported at the image 
transfer nip by counter rotating cylinders which are rotating at precisely 
controlled angular velocities such that the developed image and the 
image-support member will pass through the image transfer nip at precisely 
the same speed and in proper alignment. In some cases, a double transfer 
drum or belt is used in which the carrier member includes an image-support 
member which is permanently mounted to and part of the carrier member. The 
image is typically transferred to a second image-support member, such as a 
sheet of paper. 
The alignment between the latent image and the image-support member at the 
image transfer nip is herein defined by the term "registration." Proper 
registration between the developed image and the image-support member 
becomes even more critical when color images are being produced. 
Typically, for color printing, each of three colors are sequentially 
transferred from the photoconductive transfer member to the image-support 
member as partial images. For each of the three colors, a partial latent 
image of electrostatic charge is first placed on the surface of the 
photoconductive transfer member, developed with a toner of the 
corresponding color and then passed through the image transfer nip with 
the counter rotating image-support member such that the developed partial 
image will be transferred to the image-support member. Additionally, a 
fourth developed partial image may be passed from the photoconductive 
transfer member to the image-support member for black and white portions 
of a composite image being transferred to the image-support member. Thus, 
the image-support member may be passed through the image transfer nip four 
separate times for transfer of the complete, composite color image. Each 
of the partial images must properly register with the other partial images 
for the composite image to be correctly produced. 
Proper registration between the various developed partial images and the 
image-support member has been accomplished by using direct gear drives and 
meshing gears to directly couple together the photoconductive transfer 
member and the carrier member. Spur gears are typically used. Often, the 
photoconductive transfer member and the carrier member are firmly mounted 
to cylinders to provide cylindrical drums. The cylindrical drums are 
aligned with central longitudinal axes of the drums being parallel and 
spaced apart, such that the image-support member will be pressed into the 
surface of the photoconductive transfer member at the image transfer nip. 
Gears are coaxially mounted to each of the cylindrical drums and mesh with 
one another such that, when one of the cylindrical drums turns, the other 
drum will also turn. Typically, the cylindrical drum on which the 
photoconductive transfer member is mounted will be directly driven by a 
drive motor, and the other cylindrical drum, on which the carrier member 
is mounted, will be directly driven by the gears. The cylindrical drum for 
the carrier member is usually pressed toward the cylindrical drum for the 
photoconductive transfer member with sufficient force to cause the gears 
to mesh. 
Several problems have arisen with these types of prior art print engines. 
Often, when the cylindrical drum for the carrier member is pressed toward 
the cylindrical drum for the photoconductive transfer member with 
sufficient force for the gears to properly mesh, the carrier member will 
press the image-support member into the surface of the photoconductive 
member with excessive force, causing a condition known as fine-line 
breakup. Excessive pressure at the image transfer nip causes the lines of 
an image to widen, and often a blank space may appear in the center of the 
lines. Additionally, mechanical gears have a condition known as "gear 
lash" caused by the clearances between the teeth of the intermeshing 
gears. Gear lash causes a banding pattern in which sections of the images 
are compressed and decompressed on the carrier member in a direction in 
which the carrier member is fed through the image transfer nip. Image 
banding patterns generally increase when the cylindrical drums are pushed 
together with insufficient force for the gears to properly mesh. 
Increasing the differential electrical potential between the 
photoconductive transfer member and the carrier member is of little effect 
in correcting fine-line breakup and image banding patterns. 
SUMMARY OF THE INVENTION 
The present invention disclosed and claimed herein comprises a print engine 
for electrophotographically transferring an image from an image source to 
an image-support member. The print engine includes a photoconductive 
transfer member having a photoconductive surface for storing a latent, 
electrostatic image. The latent, electrostatic image is first formed on 
the photoconductive surface of the photoconductive transfer member. A 
developer station supplies developer to the latent, electrostatic image as 
it is being carried by the photoconductive surface of the transfer member 
to provide a developed image. A carrier member includes an electrically 
charged support surface for supporting an image-support member, which is 
passed through an image transfer nip. The image transfer nip extends 
between the photoconductive transfer member and the carrier member. The 
image-support member and the developed image are simultaneously passed 
through the image transfer nip such that the developed image is 
transferred to the image-support member. A mounting assembly is provided 
for movably supporting the photoconductive transfer member and the carrier 
member with a relative frictional engagement therebetween, wherein the 
carrier member is pressed into the photoconductive transfer member so that 
the support surface of the carrier member will move at substantially the 
same speed through the image transfer nip as the photoconductive surface 
of the photoconductive transfer member in response to movement of the 
photoconductive transfer member. 
It is another aspect of the present invention that the surfaces of the 
photoconductive transfer member and the carrier member each have clean, 
dry regions which are disposed outside of an image path for engaging one 
another in the frictional engagement. 
It is another aspect of the present invention that the carrier member 
includes two bands of material having a high coefficient of friction which 
are disposed on opposite sides of the carrier member from one another, 
outward of the image path. 
It is another aspect of the present invention that the photoconductive 
transfer member and the carrier member are each cylindrical drums, the 
photoconductive transfer member having a photoconductive layer which 
defines a circumferentially extending periphery thereof, and the carrier 
member having a resilient layer which defines an exterior 
circumferentially extending periphery of the carrier member.

DETAILED DESCRIPTION OF THE INVENTION 
Referring now to FIG. 1, there is illustrated a schematic diagram of a 
print engine 10 of the preferred embodiment of the present invention. The 
print engine 10 includes a photoconductive drum charger 12 and an image 
generator 14, which together provide an image transfer device of the type 
used in either a printer or a photocopier. The print engine 10 further 
includes a developer station 15. The developer station 15 contains toner 
and is preferably of the type for developing color images. A cylindrical 
photoconductive drum 16 provides a photoconductive transfer member having 
a photoconductive surface. 
The photoconductive drum 16 rotates beneath the photoconductive drum 
charger 12, the image generator 14 and the developer station 15. The 
photoconductive drum charger 12 deposits a substantially uniform blanket 
of electrostatic charge on the surface of the photoconductive drum 16. The 
image generator 14 then generates light of various intensities to 
discharge selected regions of the blanket of electrostatic charge, which 
provides a latent, electrostatic image on the surface of the 
photoconductive drum 16. The print engine 10 further includes a carrier 
member 17, which preferably includes a cylindrical drum 18. The carrier 
drum 18 has a cylindrical exterior support surface. The carrier drum 18 
engages the photoconductive drum 16 at an image transfer nip 20 and at two 
(2) spaced apart frictional engagement nips 21. The photoconductive drum 
16 and the carrier drum 18 are counter rotating members, with 
photoconductive transfer drum 16 rotating in an angular direction 22 and 
carrier member drum 18 rotating in an angular direction 24. 
A cleaning station 26 and a discharge station 28 are disposed aside of 
photoconductive drum 16, after the image transfer nip 20, for removing 
remaining toner and electrostatic charge from the photoconductive surface 
of drum 16. An image-support member handler 30 is provided for feeding 
image-support members, which are typically sheets of paper, along a path 
32 to the carrier drum 18 and then for receiving the image-support members 
from carrier drum 18 along path 34. A diverter 36 is provided for 
selectively operating to remove the image-support members from the carrier 
drum 18 and directing them into the path 34. The print engine 10 further 
includes a programmable controller 38, which controls the operation of 
print engine 10. The programmable controller 38 preferably includes a 
central processing unit, memory storage and input-output channels. 
Referring now to FIG. 2, there is illustrated a partial side view of the 
print engine 10, as would be viewed along section line 2--2 of FIG. 1. The 
photoconductive drum 16 is mounted to a shaft 40 for rotating around a 
longitudinal axis 42. The cylindrical surface of the exterior of the 
photoconductive drum 16 is concentrically disposed around the longitudinal 
axis 42. The shaft 40 is mounted by bearings 44 to a frame 46. A drive 
motor 48 is mechanically coupled to the shaft 40 to provide a direct 
mechanical drive for powering rotation of the shaft 40. The 
photoconductive transfer member 16 is rigidly mounted to and rotates with 
shaft 40. Of course, a gear coupling could also be utilized to 
mechanically couple the shaft 40 to the drive motor 48 for directly 
driving the shaft 40 with drive motor 48. If the rigid coupling of FIG. 2 
is used, rather than a gear coupling, gear backlash is avoided. Two 
frictional engagement regions 47 and 49 (shown in phantom) are provided by 
outwardly disposed, cylindrical regions of photoconductive transfer member 
16, which are flush with and part of the photoconductive surface thereof 
The two friction regions 47 and 49 are preferably the opposite side of the 
photoconductive surface of drum 16, and provide friction drive surfaces. 
The carrier drum 18 is concentrically mounted to a shaft 50 for rotating 
around a longitudinal axis 52. The cylindrical exterior support surface of 
carrier drum 18 extends concentrically around the longitudinal axis 52. 
The longitudinal axis 52 of carrier drum 18 is parallel to the 
longitudinal axis 42 of the photoconductive drum 16. The bearings 54 
rotatably secure the shaft 50 to a mounting assembly 56. The bearings 54 
are secured to the frame 46 by the mounting assembly 56. The mounting 
assembly 56 preferably includes two springs 58 and two control members 60. 
The springs 58 secure the bearings 54 to the control members 60. The 
control members 60 moveably interface the springs 58 to the frame 46. The 
control members 60 are selectively operable for retracting and extending 
in parallel directions to move the position of the shaft 50 of the carrier 
drum 18 with respect to the frame 46, which controls the force with which 
the carrier drum 18 is pressed into the photoconductive image transfer 
member 16. The carrier drum 18 is pressed into the image transfer member 
16 with forces F1 and F2. In the preferred embodiment of the present 
invention, the control members 60 are provided by a manual type of 
adjustment means for varying the forces F1 and F2, which the springs 58 
apply to the bearings 54, urging the shaft 50 toward the shaft 40, and the 
carrier drum 18 against the photoconductive drum 16. 
Two cylindrical bands 62 and 64 are provided by two strips of friction tape 
which are mounted to the cylindrical surface of and circumferentially 
extend around opposite ends of the carrier drum 18 to provide regions of 
high coefficients of friction. The two friction bands 62 and 64 are spaced 
apart so that they define two spaced apart frictional engagement surfaces 
63 and 65, respectively, which are preferably surfaces having higher 
coefficients of friction than the adjacent exterior support surface of the 
carrier drum 18. The friction bands 62 and 64 provide two spaced apart 
circumferentially extending ribs on opposite sides of the image path 68, 
raised above, or offset from, a support surface 75. The bands 62 and 64 
engage photoconductive transfer drum 16 on opposite sides and outside of 
an image path 66. Preferably, the friction bands 62 and 64 are provided by 
a composite material, such as a fiberglass tape, which is relatively 
incompressible as compared to the support surface 75 of the resilient 
layer 76 (shown in FIG. 3) of carrier drum 18. 
Referring now to FIG. 3, there is illustrated a sectional view of the 
photoconductive transfer drum 16 and the carrier drum 18, taken along 
section line 3--3 of FIG. 2. The photoconductive transfer member 16 is 
preferably a cylindrical drum, having a hollow, aluminum, cylindrical core 
70, which is preferably 0.6 inches thick. Photoconductive transfer drum 16 
includes an outer periphery which defines a photoconductive surface 72. 
Photoconductive surface 72 is provided by an organic photoconductor which 
is twenty micrometers thick and extends around the outer cylindrical 
surface of cylindrical core 70. The outside diameter of the 
photoconductive surface 72 of the photoconductive transfer member drum 16 
preferably measures 2.756 inches. 
The carrier member 17 preferably includes the cylindrical drum 18 having a 
hollow, conductive, cylindrical core 74 with a wall thickness of 0.25 
inches, measured in a radially extending direction. Conductive core 74 may 
be formed of a metal, such as aluminum, or another type of material, such 
as a conductive plastic. Cylindrical drum 18 further includes the 
resilient layer 76 which extends around the cylindrical core 74 and which 
is preferably provided by butadine acrylonitrile, having a hardness of 25 
shore A durometer and a thickness of 0.125 inch. The overall outside 
diameter of the exterior surface 75 of the resilient layer 76 of the 
carrier drum 18 is preferably 5.512 inches. It should also be noted that 
the materials and the hardnesses of the materials for the cylindrical core 
74 and the resilient layer 76 may vary according in various embodiments of 
the present invention. The carrier member 18 also includes an 
image-supporting member 78, which is depicted in FIG. 3 as a sheet of 
paper which is passing through the image transfer nip 20. A periphery of 
cylindrical drum 18 includes the frictional surfaces 63 and 65 of the 
friction bands 62 and 64, and the exterior surface 75 of the resilient 
layer 76. 
Referring now to FIG. 4, there is illustrated a partial sectional view of 
the photoconductive transfer member drum 16 and the carrier drum 18, taken 
along section line 4--4 of FIG. 1. It depicts the image-support member 78 
being carried through the image transfer nip 20 by the carrier drum 18. 
The friction band 62 is also depicted. The friction band 62 is being 
squeezed between the photoconductive transfer drum 16 and the carrier drum 
18, such that it is compressed in the radial direction between the drums 
16 and 18, and expanded outward in a lateral direction between the drums 
16 and 18. The total normal force F.sub.N applied to press carrier drum 18 
into transfer drum 16 applies a normal pressure to compress the 
image-support member 78, the friction band 62 and the friction band 64 
(shown in FIG. 2) between the photoconductive image transfer drum 16 and 
the carrier drum 18. A normal pressure is herein defined to be the force 
resulting from the normal force F.sub.N acting to press drums 16 and 18 
together. 
Applying the combined normal force F.sub.N to press the carrier drum 18 
into the photoconductive drum 16 results in a deflection which is herein 
defined to be a change in a distance 80 across the resilient layer 76 and 
the friction bands 62 and 64 in a radial direction. The distance 80 is 
used herein as the initial, undeflected distance of the region of the 
print engine 10, which is deflected when acted upon by the compression 
normal force F.sub.N. Substantially all of the deflection from compression 
caused by the force F.sub.N acting across the distance 80 occurs across 
the friction bands 62 and 64, and the portion of the resilient layer 76 
which extends underneath the friction bands 62 and 64, that is, the 
portion of the resilient layer 76 which is disposed at the frictional 
engagement nips 21. The portion of the resilient layer 76 which is 
disposed between the friction bands 62 and 64, that is, the portion of the 
resilient layer 76 which is disposed at the image transfer nip 20, will 
preferably not be deflected until an image-supporting member 78 is 
disposed within the image transfer nip 20. The distance 80 extends 
perpendicular to the longitudinal axes 42 and 56. 
The print engine 10 should be constructed and operated such that the 
frictional engagement between the photoconductive transfer drum 16 and 
carrier drum 18 provides an acceptable drive coupling. A frictional 
engagement of an acceptable drive coupling is herein defined as that which 
is provided by a normal force F.sub.N which is sufficient to assure that 
acceptable tracking will result between the two drums 16 and 18. A 
sufficient normal force F.sub.N for acceptable tracking is herein defined 
as a force of a minimum value which presses the photoconductive drum 16 
and the carrier drum 18 together such that there is substantially no 
misregistration in four sequential image transfers. Thus, when the 
sufficient normal force F.sub.N is applied to press photoconductive drum 
16 and the carrier drum 18 together, the frictional engagement of the 
acceptable drive coupling will be provided such that there be no 
appreciable slippage between the carrier drum 18 and the photoconductive 
drum 16 at the image transfer nip 20. 
As used herein, a normal force F.sub.N refers to the vector component of 
the total of forces F.sub.1 +F.sub.2, which press the photoconductive drum 
16 and carrier drum 18 together. The normal force F.sub.N is the vector 
component of the total of forces F.sub.1 +F.sub.2, which extends 
perpendicular to the engaged portions of the surface of the 
photoconductive drum 16 and carrier drum 18 at the frictional engagement 
nip 21. In general, for the print engine 10 having two counter rotating, 
cylindrical drums 16 and 18 which define the image transfer and frictional 
engagement nips 20 and 21, the normal force F.sub.N will extend through of 
the longitudinal axes 42 and 56 of the cylindrical drums 16 and 18, 
respectively. In the preferred embodiment, the normal force F.sub.N 
component of the total of forces F.sub.1 +F.sub.2, which press the 
photoconductive drum 16 into the carrier drum 18, extends through the axes 
42 and 52, perpendicular to axes 42 and 52. 
In the preferred embodiment, during operation of the friction drive print 
engine 10, the normal force F.sub.N is applied to the carrier drum 18, 
pushing it towards the photoconductive drum 16, such that the fraction 
bands 62 and 64 will press into the cylindrical photoconductive the 
surface 72 of the photoconductive drum 16. As the normal force F.sub.N 
increases to a sufficient level, an acceptable drive coupling is achieved 
between the photoconductive drum 16 and the carrier drum 18. When the 
sufficient level of force is present, the drive coupling is such that 
acceptable registration, that is tracking, is maintained between both 
drums 16 and 18. 
As the normal force F.sub.N continues to increase, the image-support member 
78 on the outer surface of the resilient layer 76 of the carrier drum 18 
contacts the surface 72 of the photoconductive drum 16, and pressure 
builds in the image transfer nip 20 where the photoconductive transfer 
member extends in contact with both the carrier member 18 and the 
photoconductive drum 16. Preferably, the cylindrical surface 72 of the 
photoconductive transfer member 16 will not touch the cylindrical surface 
of the resilient layer 76 at the image paths 66 and 68 such that pressure 
will only be present in the image transfer nip 20 when the image-support 
member 78 is disposed therein. However, when an image-support member 78 is 
in the image transfer nip 120, given some range of values for the modulus 
of elasticity and the thickness of the combination of materials being 
compressed in the image transfer nip 20, the pressure in the image 
transfer nip 20 for a particular range of normal forces F.sub.N could stay 
below, reach or exceed the pressure limit for fine line breakup. 
Given some set of parameters for the thickness and width of the friction 
bands 62 and 64, the modulus of elasticity and thickness of the materials 
of the image transfer nip 20, then an acceptable drive coupling can be 
provided between the carrier member 18 and the photoconductive drum 16. 
Once these parameters are known, the minimum normal force F.sub.N 
sufficient for providing an acceptable drive coupling can be empirically 
determined. The following sets forth a predictive model that determines 
the range of normal forces F.sub.N required for both providing an 
acceptable drive coupling and for avoiding fine line breakup. From such a 
predictive model, an upper and lower range of the modulus of elasticity 
and the thickness of the materials of the image transfer nip 20, and the 
widths and thicknesses of friction bands 62 and 64 can be defined for 
various drum-to-drum contact architectures using the minimum and maximum 
values for the normal force F.sub.N. 
As the carrier drum 18 and photoconductive drum 16 are pressed together 
with a normal force F.sub.N of increasing value, the central axes 42 and 
52 of the carrier drum 18 and the photoconductive drum 16, respectively, 
will be pressed closer together. This change in distance between the 
central axes 42 and 52 of the photoconductive drum 16 and the carrier drum 
18, respectively, will cause the deflection of the distance 80 of the 
friction bands 62 and 64, and the portion of the resilient layer 76 
disposed underneath the friction bands 62 and 64. An effective, composite 
modulus of elasticity for the carrier drum 18 and the photoconductive drum 
16 configuration can be determined according to the equation for Young's 
modulus of elasticity, which defines the pressure on a compressed surface 
as: 
##EQU1## 
where: F.sub.N =normal force applied 
A=area being compressed 
.DELTA.l=compression deflection 
lo=uncompressed thickness across distance 80 
For the image transfer nip 20 and the frictional engagement nip 21,"A" and 
".DELTA.l" will increase as the normal force F.sub.N being applied to each 
of the nips 20 and 21 increases, with the values of the increases 
depending upon the effective composite modulus of elasticity of the 
composite of materials involved in compression between the photoconductive 
drum 16 and the carrier drum 18. 
Referring now to FIG. 5, there is illustrated a graph having two curves 
which depict the surface area of the image transfer nip 20 and the surface 
area of the frictional engagement nip 21 versus the total normal force 
F.sub.N applied to press the carrier drum 18 and the photoconductive drum 
16 together. A curve 82 represents the graph of the surface area of the 
image transfer nip 20 versus the total normal force F.sub.N applied to 
press the carrier drum 18 and the photoconductive drum 16 together with 
the image-support member 78 disposed within the image transfer nip 21. A 
curve 84 represents a graph of the surface area of the frictional 
engagement nip 21 versus the total normal force F.sub.N applied to press 
the carrier drum 18 and the photoconductive drum 16 together with the 
image-support member 78 disposed within the image transfer nip 21. 
Referring now to FIG. 6, there is illustrated a graph of the deflection of 
the distance 80 between the photoconductive drum 16 and the carrier drum 
18 of the print engine 10 versus the normal force F.sub.N applied to push 
the carrier drum 18 into the photoconductive drum 16. 
The graphs of FIGS. 5 and 6 were empirically determined by deflecting the 
carrier drum 18 into the photoconductive drum 16 using a series of known 
forces, and then measuring the resulting deflections and surface areas 
created by the interferences between the carrier drum 18, the 
photoconductive drum 16 and the image-support member 78 in the image 
transfer nip 20, and between the carrier drum 18, the photoconductive drum 
16, and the friction bands 62 and 64 of the frictional engagement nip 21. 
Next, the portion of the normal force F.sub.N which contributes to the 
deflection and area of the image transfer nip 20 was determined by 
deflecting a prior art carrier drum into a prior art photoconductive drum 
of a prior art print engine with several known values for the normal force 
F.sub.N, and measurement of the resultant areas and deflections of the 
transfer nip, as depicted in FIG. 7, which is discussed below. Then, the 
measured resultant areas and deflections were correlated to the image 
transfer nip 20 areas and deflections for the present invention to 
determine the portion of the total normal force F.sub.N which is being 
applied to the image transfer nip 20. Then, the portion of the total 
normal force F.sub.N which was being applied at the frictional engagement 
nip 21 was equal to the total normal force F.sub.N minus the portion of 
the total normal force F.sub.N applied at the image transfer nip 20. 
Referring now to FIG. 7, there is illustrated a graph of the compressed 
area of the image transfer nip between the photoconductive drum and the 
carrier drum of a prior art print engine versus the normal force F.sub.N 
applied for the deflections. The areas of FIG. 7 can be correlated to the 
numerically equivalent areas of FIGS. 5 and 6 to determine the amount of 
that portion of the total normal force F.sub.N of FIGS. 5 and 6 which is 
applied across the image transfer nip 20. The value for the portion the 
normal force F.sub.N applied across the image transfer nip 20, for a 
particular resultant area, is the normal force give in FIG. 7. The 
remainder of the total normal force F.sub.N contributes to the frictional 
engagement applied across the friction bands 62 and 64 at friction nip 21, 
to deflect the friction bands 62 and 64. These values are shown in tabular 
form in Table I. 
TABLE I 
______________________________________ 
Total Friction Band 
Image Transfer 
Normal force F.sub.N 
Nip Normal force 
Nip Normal force 
(lbs) (lbs) (lbs) 
______________________________________ 
0.000 0.000 0.000 
0.903 0.903 0.000 
1.255 1.255 0.000 
2.255 1.995 0.260 
3.155 2.745 0.410 
4.155 3.355 0.800 
5.055 4.055 1.000 
6.055 4.865 1.190 
6.955 5.485 1.470 
8.485 6.185 2.300 
9.485 6.835 2.650 
10.385 7.285 3.000 
12.935 8.445 4.490 
14.835 8.780 6.055 
17.299 8.814 8.485 
20.277 8.227 12.050 
______________________________________ 
Knowing the area, deflection and the portion of the normal force F.sub.N 
applied to the image of transfer nip 20, the modulus of elasticity and 
pressure in the image transfer nip 20 is determined. These values are 
shown in Table II. 
TABLE II 
______________________________________ 
Image Transfer Image 
Nip Normal 
Image Transfer 
Image Image Transfer 
Force Nip Transfer Nip 
Transfer 
Nip 
(lbs) deflection area Pressure 
Modulus 
______________________________________ 
349.61 
0 0 0 0 0 
0.8 0.00045 0.521 1.534 450.07 
1 0.00057 0.629 1.590 368.17 
1.19 0.00067 0.700 1.700 334.93 
1.47 0.00082 0.765 1.922 309.33 
2.3 0.00118 0.901 2.553 285.56 
2.65 0.00133 0.955 2.775 275.40 
3.1 0.00151 1.025 3.024 264.38 
______________________________________ 
The results listed in Table II are plotted as a function of deflection in 
FIGS. 8-10. 
Referring now to FIG. 8, there is illustrated a graph of the compressed 
surface area of the image transfer nip 20 versus the deflection of the 
distance 80 between the photoconductive drum 16 and the carrier drum 18 
for the preferred print engine 10 of the present invention. 
Referring now to FIG. 9, there is illustrated a graph of the portion of 
total normal force F.sub.N which is applied across the image transfer nip 
20 to press the photoconductive drum 16 and the carrier drum 18 together, 
versus the deflection of the distance 80 of the image transfer nip 20 in 
the direction of the normal force F.sub.N. 
Referring now to FIG. 10, there is illustrated a graph of the pressure 
applied across the image transfer nip 20 versus the deflection of the 
distance 80 of the photoconductive drum 16 and the carrier drum 18. 
Referring now to FIG. 11, there is illustrated a graph of the calculated, 
effective, composite modulus of elasticity of the materials of the image 
transfer nip 20 deflected by pressing the photoconductive drum 16 and the 
carrier drum 18 together, versus the deflection across the distance 80. 
From extensive testing of various toner and developer materials, it is 
found that fine line breakup will most likely occur when, for the friction 
band and photoconductive drum 16 material used, the normal force F.sub.N 
exceeds 6.05 total lbs, or 1.19 pounds of normal force across the image 
transfer nip 20. At 1.19 lbs of normal force across the image transfer nip 
20, the pressure in the image transfer nip 20 is 1.7 lbs/in.sup.2. This 
sets the maximum pressure desired in the pressure in the image transfer 
nip 20 for acceptable image quality, in which fine line breakup is 
avoided, at 1.7 lbs/in.sup.2. In some cases, this value of maximum 
pressure may be reduced to 1.5 lbs/in.sup.2. Thus, the maximum pressure 
allowable in the image transfer nip 20 of any given modulus and thickness 
is preferably 1.5 to 1.7 lbs/in.sup.2. 
Given the maximum pressure constraint for the image transfer nip 20, if the 
values for the thickness or the composite modulus of elasticity of the 
materials of the image transfer nip 20 are changed, the pressure will 
change for the same normal force being applied. A mathematical model is 
constructed to simulate these responses in the following. The normal force 
applied is constant regardless of the modulus of elasticity or thickness 
of the materials of nips 20 and 21. For this model, the component of the 
normal force F.sub.N applied to the image transfer nip 20 varies from 0.8 
to 3.1 lbs., representing the usable range in the working machine 10. The 
resultant area of the image transfer nip 20 is a function of the geometry 
of the photoconductive drum 16 and the carrier drum 18 for a given 
deflection across the distance 80. A plot of the area of the image 
transfer nip 20 as a function of the deflection across the distance 80 can 
be determined by correlating the plots of FIGS. 6 and 7 for the prior art 
print engine. A simple but well correlated function to this relationship 
is as follows: 
##EQU2## 
Referring now to FIG. 12, there is illustrated a graph of the compressed 
surface area of the image transfer nip 20 verses the deflection of the 
distance 80. The plotted curve represents the compressed surface area of 
the image transfer nip 20 and was calculated as a function of the 
deflection of the distance 80 according to the above equation relating the 
compressed surface area to the deflection across the distance 80. The 
plotted points were empirically determined and are shown to provide a 
comparison of the values provided by the model to actual measured values. 
Deflection may be found in terms of Young's modulus of elasticity by 
substituting the above area equation for the area term, resulting in the 
following equation: 
EQU deflection=F*lo/M*A=F*lo/M*26.4!.sup..667 (3) 
With the photoconductive drum 16 and the carrier drum 18, the surfaces 
forming the image transfer nip 20 are radiused and the deflection across 
the distance 80 varies with position across the nip 20 for any single 
normal force applied. The deflection across the distance 80 is actually 
the maximum deflection obtained. The averaged deflection would be lower. 
The modulus of elasticity calculated from empirical data is found to also 
vary as the normal force is changed. In other words, the modulus of 
elasticity should stay constant for a given material, thickness and 
deflection. For the image transfer nip 20 this is not the case. As a 
result, a function to simulate the change in the effective Modulus of 
elasticity of the image transfer nip 20 with the normal force is developed 
based on inputting a single modulus value (SMV) for the resilient layer 76 
of the carrier drum 18, where substantially all of the deflection occurs 
in print engine 10 of the preferred embodiment. The single modulus value 
is placed in the offset term of the equation, which creates a Y direction 
shift as the SMV is changed. This creates the proper functional movement 
as different photoconductive drum 16 materials, having different 
photoconductive drum 16 modulus, are evaluated. The equation is given as 
follows: 
EQU Modulus=428((Force * 10)-7).sup.-.15 +(SMV-287) (4) 
Referring now to FIG. 13, there is illustrated a graph of the effective 
modulus of elasticity of the image transfer nip 20 verses the portion of 
the normal force F.sub.N being applied across the image transfer nip 20. 
The curve representing the effective modulus of elasticity was calculated 
according to the Equation 4, as a function of the normal force being 
applied at the image transfer nip 20. A single value for the modulus of 
elasticity for the resilient layer 76 of the carrier drum 18 was used in 
the Equation 4. The plotted points were empirically determined. 
At this point, the model is complete. The modulus of elasticity, the 
thickness and the normal force may be determined. The effective Modulus of 
the image transfer nip 20 is determined according to Equation 4. 
Deflection across the distance 80 is determined by Equation 3. Area of the 
image transfer nip 20 is determined according to Equation 2. The model 
results which may be determined for an equivalent modulus of elasticity 
for the image transfer nip 20 are calculated for various values of the 
thickness and the modulus of elasticity of the resilient layer 76 of the 
carrier member 18. 
Referring now to FIG. 14, there is illustrated a graph of the portion of 
the normal force F.sub.N applied across the image transfer nip 20 versus 
the deflection across the distance 80 of the carrier drum 18 and the 
photoconductive drum 16, for a carrier member 18 which includes a 
resilient exterior layer 76 having modulus of elasticity of 297.0 and a 
thickness of 0.125 inches. The plotted curve representing the portion of 
the normal force F.sub.N being applied across the image transfer nip 20 
was calculated as a function of the deflection, and the plotted points 
were empirically determined. 
Referring now to FIG. 15, there is illustrated a graph of the pressure 
applied across the image transfer nip 20 versus the deflection of the 
distance 80 of the image transfer nip 20, for a carrier drum 18 which 
includes a resilient exterior surface layer 76 having a modulus of 
elasticity of 287.0 and a thickness of 0.125 inches. The plotted curve 
representing the pressure across the image transfer nip 20 was calculated 
as a function of the deflection according to the model, and the plotted 
points were empirically determined. 
Referring now to FIG. 16, there is illustrated a graph of the effective 
modulus of elasticity of the image transfer nip 20 versus the deflection 
of the distance 80 of the image transfer nip 20, for a carrier drum 18 
having a resilient layer 76 with a modulus of elasticity of 287.0 and a 
thickness 0.125 inches. The plotted curve representing the effective 
modulus of elasticity was calculated as a function of the deflection of 
the distance 80 according to the model, and the plotted points were 
empirically determined. 
Referring now to FIG. 17, there is illustrated a graph of the squeezed area 
of the image transfer nip 20 versus the deflection across the distance 80 
of the image transfer nip 20, for a carrier drum 18 having a resilient 
layer 76 with a modulus of elasticity of 287.0 and a thickness 0.125 
inches. The plotted curve representing the squeezed area of the image 
transfer nip 20 was calculated as a function of the deflection across the 
distance 80 according to the model. The plotted points were empirically 
determined. 
Referring now to FIGS. 18 and 19, there are illustrated examples of various 
combinations of various values for the modulus of elasticity and the 
thickness of the resilient layer 76 of the carrier member necessary to 
limit the maximum pressure for fine line breakup of 1.7 lb/in.sup.2. 
Thicknesses for the resilient layer 78 ranged from 0.0625 inches to 0.5 
inches. FIG. 18 illustrates the preferred modulus of elasticity of the 
resilient layer 76 of 287.0, and FIG. 19 illustrates an alternative 
Modulus of elasticity of 187.0. 
Referring now to FIG. 18, there is illustrated a graph of the pressure 
versus the portion of the normal force F.sub.N applied across the image 
transfer nip 20, with an image-support member 78 disposed therein. Curve 
102 represents a resilient layer 76 of the carrier member 18 having a 
thickness of 0.0625 inches. Curve 104 represents the resilient layer 96 
having a thickness of 0.125 inches. Curve 106 represents a thickness of 
0.1875 inches for the resilient layer 76. Curve 108 represents a thickness 
for the resilient layer 76 of 0.25 inches. Curve 110 represents a 
thickness for the resilient layer 76 of 0.3125 inches. Curve 112 
represents a thickness for the resilient layer of 0.375 inches. Curve 114 
represents a thickness for the resilient layer 76 of 0.4375 inches. Curve 
116 represents a thickness for the resilient layer 76 of 0.5 inches. Curve 
118 represents a plot of the maximum pressure which can be applied across 
the image transfer nip 20 without incurring fine line breakup. 
Referring now to FIG. 19, there is illustrated a graph of the pressure 
applied across the image transfer nip 20 versus the portion of the normal 
force F.sub.N applied across the image transfer nip 20, for various 
thicknesses of the resilient layer 76 of the carrier drum 18 of an 
alternative material having a modulus of elasticity of 186.83. Curve 120 
represents a resilient layer 76 having a thickness of 0.0625 inches. Curve 
122 represents a resilient layer 76 having a thickness of 0.125 inches. 
Curve 124 represents a resilient layer 76 having a thickness of 0.1875 
inches. Curve 126 represents a thickness of the resilient layer 76 of 0.25 
inches. Curve 128 represents a thickness of the resilient layer 76 of 
0.1325 inches. Curve 130 represents a thickness of the resilient layer 76 
of 0.375 inches. Curve 132 represents a thickness of the resilient layer 
76 of 0.4375 inches. Curve 134 represents a thickness of the resilient 
layer 76 of 0.5 inches. Curve 136 represents the maximum pressure across 
the image transfer nip 20 which can be applied without incurring fine line 
breakup, that of 1.7 pounds per square inch. 
There is a minimal normal force F.sub.N required in order to facilitate 
drive between the carrier drum 18 and the photoconductive drum 16. The 
minimal acceptable normal force F.sub.N is that which does not contribute 
to misregistration in 4 color prints. On the working machine, this force 
F.sub.N was empirically found to be between approximately 2.0-3.0 lbs. 
This is obtained from the normal force F.sub.N difference between carrier 
drum 18 and photoconductive drum 16 first making contact and reaching 
acceptable drive conditions. In this instance, an acceptable drive is 
obtained before the image-support member 78 and the carrier drum 18 make 
contact. The limit of acceptable drive for any photoconductive drum 16 is 
defined according to the following parameters. 
friction band thickness, 
friction band width, 
effective modulus of elasticity, and 
thickness of the image transfer nip 20. 
Using Equation 1 and setting a minimum value for the normal force F.sub.N 
defines the usable range on these parameters for any change desired in the 
photoconductive drum 16 construct. The equation becomes 
##EQU3## 
where: M=effective modulus of elasticity 
A=related to friction band width 
.DELTA.l =related to friction band thickness 
lo=friction nip thickness 
At a minimum, the friction band thickness allows an acceptable friction 
band nip drive pressure to be reached at or before the image-support 
member 78 contacts the photoconductive surface of the photoconductive drum 
16. In the preferred embodiment, the acceptable friction band drive nip 21 
pressure is reached before the carrier drum image path 68 is pressed into 
and contacts the photoconductive drum image path 66. The minimum normal 
force F.sub.N required for a given effective modulus and thickness, and a 
particular friction band width and thickness, ranges from approximately 
2.0 to 3.0 lbs. 
Thus, the maximum pressure allowable in the image transfer nip for any 
given effective modulus of elasticity and thickness is preferably 1.5-1.7 
lbs/in.sup.2. The minimum normal force F.sub.N required for a given 
frictional engagement nip modulus and thickness, and friction band width 
and thickness ranges from approximately 2.0 to 3.0 lbs. Referring again to 
Table II, the image transfer nip portion of the normal force F.sub.N is 
approximately 1.19 lbs for a maximum image transfer nip normal pressure of 
1.7 lbs/in.sup.2. Thus, if a friction drive region is not present, the 
recommended maximum image transfer nip pressure will be exceeded if the 
recommended friction drive normal pressure is applied across the image 
transfer nip. However, in the present invention the friction bands 62 and 
64 are relatively incompressible as compared to the resilient layer 76, 
and in the preferred embodiment the surfaces 63 and 65 of the friction 
bands 62 and 64, respectively, are offset to extend outward of the surface 
75 of the resilient layer 76. Thus, as shown in Table I, a friction band 
normal force of 4.865 lbs can be provided when a normal force of 1.19 lbs 
is applied across the image transfer nip. 
In operation, the photoconductive drum charger 12 and the image generator 
14 together form a latent image of electrostatic charge on the 
photoconductive surface 72 of the photoconductive drum 16. The 
photoconductive surface 72 moves the latent image adjacent to the 
developer station 15. Toner, or developer, is then drawn onto the latent 
image by the electrostatic charge on the photoconductive surface 72 to 
provide a developed image. At the same time as the latent image is being 
formed on the exterior surface of the photoconductive member 16, the 
image-support member 78 is being fed by the image-support member handler 
30 along the path 32 to the carrier drum 18. The exterior surface of the 
resilient layer 76 of the carrier drum 18 is charged with a negative 
potential such that the image-support member 78 will adhere to the 
resilient layer 76. The photoconductive transfer member 16 and the carrier 
drum 18 are counter rotating in the angular directions 22 and 24, 
respectively, so that the developed image will be passed through the image 
transfer nip 20 at the same time as the image-support member 78 which is 
adhered to the surface of the carrier drum 18. The developed image will 
then be transferred to the image-support member 78 carried on the exterior 
surface 75 of the carrier drum 18. In the preferred embodiment, 
image-support member 78 is a sheet of paper or a transparency sheet 
supported on carrier drum 18. 
If a color image is being transferred, the above process will occur 
multiple times to sequentially place toner of the various colors on the 
same image-support member. Typically, the colors magenta, cyan and yellow 
are used for the toner. After the photoconductive surface 72 of the 
photoconductive drum 16 passes through the image transfer nip 20, the 
surface 72 will pass across cleaning station 26 and discharge station 28 
so that the developer and static charge remaining on the exterior 
photoconductive surface 72 of the photoconductive drum 16 will be removed 
prior to passing beneath the image generator 12. Once the developed image 
is fully transferred to the image-support member 78, which is adhered to 
the carrier drum 18, the carrier member diverter 36 will be actuated to 
remove the image-support member 78 from the surface of the carrier drum 
18. The image-support member 78 will then travel along the path 34 back to 
the paper handler 30. Operation of the print engine 10 is preferably 
controlled by the programmable controller 38, which preferably includes a 
central processing unit and memory storage. 
Transferring a color image from the photoconductive drum 16 to the 
image-supporting member 78 requires the proper alignment between the 
carrier drum 18 and the photoconductive drum 16 to insure that the 
developed image properly registers with the image-support member 78 as 
they are being passed together through the image transfer nip 20. This 
requires that the support surface 75 of the carrier drum 18 travel in the 
same direction and at substantially the same velocity through the image 
transfer nip 20 as that which the photoconductive surface 72 of 
photoconductive transfer member 16 is traveling. It should be noted that 
in the preferred print engine 10, the surface of the image-support member 
78, which directly engages photoconductive surface 72, preferably travels 
at precisely the same speed as photoconductive surface 72 at image 
transfer nip 20. 
The drive motor 48 is mechanically coupled directly to the shaft 40 of the 
photoconductive transfer member 16. The carrier drum 18 is powered by a 
frictional coupling created by a normal force F.sub.N being applied to the 
shaft 50 to push the carrier drum 18 into the exterior surface 72 of the 
photoconductive transfer member 16. The normal force F.sub.N is provided 
by the mounting assemblies 56, which are adjustable by the control means 
60 to vary the forces F.sub.1 and F.sub.2, which are applied to opposite 
sides of shaft 50 for pressing the carrier drum 18 into the 
photoconductive transfer member 16. The normal force F.sub.N is the 
component of the vector sum of forces F.sub.1 and F.sub.2, and is 
perpendicular to the linear direction in which the photoconductive 
transfer member, drum 16 and the carrier drum 18 are moving through the 
image transfer nip 20. 
The control members 60 are adjusted so that the springs 58 together press 
against opposite sides of the shaft 50 with a minimum combined normal 
force F.sub.N of approximately 2.0 to 3.0 pounds to assure that the 
carrier drum 18 will move the through image transfer nip 20 at 
substantially the same linear speed as the photoconductive surface 72 of 
the photoconductive transfer member 16. The friction bands 62 and 64 will 
be pressed into frictional engagement with the exterior surface of the 
photoconductive surface 78 of the photoconductive transfer member 16, each 
of the bands 62 and 64 being pressed into surface 78 with one-half of the 
total normal force F.sub.N. As discussed above, the modulus of elasticity, 
the thickness and the width of the photoconductive surface 72, resilient 
layer 76 and the friction bands 62 and 64 will be preferably selected so 
that the photoconductive surface 72 and the resilient surface 76 will not 
squeeze the image-support member 78 therebetween with a pressure of 
substantially more than 1.7 pounds per square inch. 
Referring now to FIG. 20, there is illustrated a side view of a schematic 
diagram depicting an alternative print engine 82 of the present invention. 
The alternative print engine 82 includes a photoconductive drum 84 and a 
carrier drum 86 which are mounted for rotating about longitudinal axes 88 
and 90, respectively. The photoconductive drum 84 is powered to rotate 
about the longitudinal axis 88 by a drive motor 92, which is mechanically 
directly coupled to the photoconductive drum 84. The photoconductive drum 
84 includes two frictional drive regions which define two clean, 
frictional drive surfaces 94 and 96, which are preferably flush with and 
spaced apart on opposite sides of the image transfer path 102. The carrier 
drum 86 includes two frictional drive regions which define two clean, 
frictional drive surfaces 98 and 100, which are preferably flush with and 
spaced apart on opposite sides of an image-support member path 104. The 
clean drive surfaces 94 and 96 are provided by the same peripherally 
extending exterior surface at which the exterior surface of the 
photoconductive drum 84 is provided along the image path 82. Similarly, 
the clean, drive, frictional engagement surfaces 98 and 100 are provided 
by the same peripherally extending surfaces of the carrier drum 86 with 
its resilient surface, at which the exterior surface of the drum 86 is 
provided along the image-support member path 104. 
Drive surfaces 94, 96, 98 and 100 define frictional engagement bands which 
are integrally formed into the exterior surfaces of drums 84 and 86, 
respectively. The two friction drive regions of carrier drum 86 which 
define drive surfaces 98 and 100 have a larger modulus of elasticity than 
the modulus of elasticity for the peripheral portion of carrier drum 86 
between drive surfaces 98 and 100 which defines the image support member 
path 104, such that a sufficient normal force may be applied across the 
friction drive regions without the normal pressure within the image 
transfer nip exceeding the recommended maximum pressure of 1.7 
lbs/in.sup.2. Preferably, the width of the image transfer nip defined by 
the image transfer path 102 and the image-support path 104 will be 
substantially wider than the combined widths of the two friction drive 
regions defining drive surfaces 98 and 100. It should also be noted that 
in other embodiments, the drive regions of photoconductive drum 84 may 
have a modulus of elasticity which significantly larger than the modulus 
of elasticity of the photoconductive region therebetween such that a 
sufficient normal force F.sub.N may be applied without exceeding the 
recommended maximum nip pressure of 1.7 lbs/in.sup.2. Materials having a 
higher modulus of elasticity than others will typically be harder than the 
materials having a lower modulus of elasticity. 
In operation, the carrier member 86 and the photoconductive drum 84 are 
pressed together with a sufficient normal force F.sub.N so that a 
frictional engagement will occur between the clean, frictional drive 
surfaces 94 and 96 of photoconductive drum 84 and the clean, frictional 
drive surfaces 98 and 100 of carrier drum 86, such that rotation of the 
photoconductive drum 84 will cause rotation of the carrier drum 90. The 
sufficient normal force F.sub.N of two to three pounds must be provided as 
discussed above such that the exterior surface of the carrier drum will 
rotate at the same speed as the exterior surface of photoconductive drum 
84 as it passes through the image transfer nip, while excessive pressure, 
above 1.7 lbs/in.sup.2, is not applied so that fine-line breakup will not 
occur. 
Referring now to FIG. 21, there is illustrated a side view of a schematic 
diagram depicting a print engine 106 of a second alternative embodiment of 
the present invention. The print engine 106 includes an image transfer 
drum 108 having a photoconductive exterior surface and a carrier drum 110 
having a circumferentially extending resilient surface which are rotatably 
mounted for rotating about longitudinal axes 112 and 114, respectively. 
The photoconductive transfer member 108 is powered to concentrically 
rotate around the longitudinal axis 112 by a drive motor 116, which is 
directly mechanically coupled to the photoconductive drum 108. Two raised 
frictional engagement bands 118 and 120 are provided by a friction tape 
which extends circumferentially around the photoconductive drum 108 on 
opposite sides of an image transfer surface 122, which is disposed within 
a central region of the circumferentially extending exterior surface of 
the photoconductive drum 108. Friction bands 118 and 120 are not flush 
with the photoconductive surface of drum 108. The friction bands 118 and 
120 of the photoconductive drum 108 are also spaced apart and frictionally 
engage the carrier drum 110 on opposite sides of the image-support member 
path 124. The photoconductive drum 108 and the carrier drum 110 are 
pressed toward one another with the friction bands 118 and 120 
therebetween such that rotation of the image transfer drum 108 will cause 
rotation of the carrier drum 110 at the same speed through an image 
transfer nip defined by the exterior surfaces of the photoconductive drum 
108 and the carrier drum 110. 
In other embodiments of the present invention, the frictional engagement 
bands may be disposed on both the photoconductive member and the carrier 
member to provide a frictional engagement for driving one of these members 
in response to relative movement of the other member. Additionally, 
frictional engagement bands may be provided by regions of clean drive 
surfaces, without friction tape, which are pressed into one another to 
provide a frictional engagement for driving one of the carrier member and 
the photoconductive transfer member in response to rotation of the other 
of the members. These regions of clean surfaces may be raised from the 
surfaces of the image path and image-support member path, as shown in FIG. 
21, or they may be flush with the surfaces of the image path and 
image-support member path, as shown in FIG. 20. In still other embodiments 
of the present invention, such as where belts are used to provide one or 
both of a carrier surface and a photoconductive surface, and the belts 
extend over rollers or guide plate surfaces at image transfer nips, 
frictional engagement bands may be provided on such surfaces to provide 
the frictional engagement therebetween for driving one surface in response 
to movement of the other surface. 
In summary, a print engine is provided having a photoconductive image 
transfer member and a carrier member which are pressed together into a 
frictional engagement so that the carrier member will be driven by 
rotation of the photoconductive member. Banding caused by gear lash is 
avoided by such frictional engagement-type of drive. Preferably, two 
cylindrically disposed frictional engagement bands extend around the 
circumferential periphery of the carrier member, spaced apart on opposite 
sides of an image transfer path and an image-support member path. 
Sufficient force is provided so that as the image-support member is 
rotated multiple times around the carrier member, it will register with 
various ones of the developed images being transferred from the 
photoconductive transfer member to the image-support member so that 
various color components of the image are properly aligned. Additionally, 
the materials from which the frictional engagement bands, the 
photoconductive member and carrier member are formed are preferably 
selected, as well as the dimensions thereof, so that excessive force will 
not be applied to the image-support member as it is passing through the 
image transfer nip so that fine-line breakup is avoided. 
Although the preferred and several alternative embodiments have been 
described in detail, it should be understood that various changes, 
substitutions and alterations can be made therein without departing from 
the spirit and scope of the invention as defined by the appended claims.