Sheet transport system

An apparatus which transports a sheet through a transfer station to transfer an image from an image receiving member thereto. A stationary member has at least a portion thereof positioned closely adjacent the image receiving member to define a transfer zone therebetween. A substantial portion of the sheet is vacuum tacked to the transfer member. The sheet is moved relative to the transfer member to advance through the transfer zone so that the image is transferred from the image receiving member to the sheet.

This invention relates generally to an electrophotographic printing 
machine, and more particularly concerns an apparatus for transporting a 
sheet into registration with a toner image developed on a photoconductive 
member. 
In the process of electrophotographic printing, a photoconductive member is 
uniformly charged and exposed to a light image of an original document. 
Exposure of the photoconductive member records an electrostatic latent 
image corresponding to the informational areas contained within the 
original document. After the electrostatic latent image is recorded ono 
the photoconductive surface, the latent image is developed by bringing a 
developer material into contact therewith. This forms a powder image on 
the photoconductive member which is subsequently transferred to a copy 
sheet and permanently affixed thereto in image configuration. 
Multi-color electrophotographic printing is similar to foregoing process of 
black and white printing. However, rather than forming a single latent 
image on the photoconductive surface, successive latent images 
corresponding to different colors are recorded thereon. Each single color 
electrostatic latent image is developed with toner of a color 
complimentary in color thereto. The latent images may be developed with a 
liquid or dry developer material. This process is repeated a plurality of 
cycles for differently colored images and their respective complementarily 
colored toner. Each single color toner image is transferred to the copy 
sheet in superimposed registration with the prior toner image. This 
creates a multi-layered toner image on the copy sheet. Thereafter, the 
multilayered toner image is permanently fused to the copy sheet creating a 
color copy. 
Hereinbefore, the toner images have been transferred to the copy sheet by 
an electrical field created by a corona generating device of the type 
disclosed in U.S. Pat. No. 2,836,725 issued to Vyverberg in 1958. A corona 
generator of this type induces transfer to the copy sheet by spraying a 
corona discharge having a polarity opposite to that of the toner on the 
photoconductive surface. This causes the toner to be electrically 
transferred to the copy sheet. However, in transferring multiple toner 
images, each toner image must be in superimposed registration with one 
another in order to produce a color copy which is not blurred. In lieu of 
utilizing a corona generating device, an electrically biased transfer roll 
may be used. The electrically biased transfer roll generates a high 
voltage discharge in the proximity of the surface of the copy sheet, or it 
may be applied by means of a conductive cylinder in contact with the copy 
sheet, as disclosed in U.S. Pat. No. 2,807,233 issued to Fitch in 1957. 
The copy sheet is interposed between the conductive roller and the 
photoconductive surface. A charge of opposite polarity from the toner is 
deposited on the back side of the copy sheet which attracts the toner 
thereto. Frequently, when only the lead edge of the copy sheet is secured 
to the cylinder, the trail edge flips causing the toner image to be 
disturbed. In either case, the copy sheet must be advanced in a 
recirculating path and the toner images transferred in registration with 
one another. Various approaches have been devised to move the copy sheet 
in a recirculating path, the following patents appear to be relevant: 
U.S. Pat. No. 3,547,535 Patentee: McLean et al. Issued: Dec. 15, 1970. 
U.S. Pat. No. 3,677,643 Patentee: Sagawa Issued: July 18, 1972. 
U.S. Pat. No. 4,550,999 Patentee: Anderson Issued: Nov. 5, 1985. 
U.S. Pat. No. 4,552,448 Patentee: Davidson Issued: Nov. 12, 1985. 
U.S. Pat. No. 4,687,323 Patentee: Fujii Issued: Aug. 18, 1987. 
U.S. Pat. No. 4,739,361 Patentee: Roy et al. Issued: Apr. 19, 1988. 
U.S. Pat. No. 4,740,813 Patentee: Roy Issued: Apr. 26, 1988. 
The pertinent portions of the foregoing patents may be briefly summarized 
as follows: 
U.S. Pat. No. 3,547,535 discloses a vacuum drum for use in a photocopier 
that holds down a document on the surface of the drum. The drum is 
rotatably supported on a fixed hollow shaft having ball bearing assemblies 
at either end. A hollow mounting drum on one end has a flow passage for a 
vacuum source while an annular gear wheel rotates the drum. 
U.S. Pat. No. 3,677,643 describes a drum having a multiplicity of orifices 
on the surface with the air pressure in orifices being reduced to attract 
a sheet of paper to the drum surface. 
U.S. Pat. No. 4,550,999 discloses a rotatable drum for use in an 
electrophotographic copier. The drum has a plurality of holes connected to 
a vacuum pump for securing a leading edge of a sheet onto the drum. The 
pump assembly is attached to the drum but does not rotate with it. 
U.S. Pat. No. 4,552,448 describes a sheet transport which advances a sheet 
into registration with successive toner powder images developed on a 
photoconductive drum. The sheet gripper is moved in a recirculating path 
and a pin extending outwardly therefrom mates with a hole in the 
photoconductive drum to register successive toner images with one another 
for transfer to the copy sheet. Corona generators spray ions onto the back 
side of the copy sheet to attract successive toner images of different 
colors thereto in superimposed registration with one another to form a 
multi-color copy. 
U.S. Pat. No. 4,687,323 discloses a rotary suction drum having a number of 
pores on the drum surface. The pores are in communication with the suction 
source by ducts. The reduced pressure on the drum in the region of the 
pores attacts a sheet of of paper to the drum surface. 
U.S. Pat. Nos. 4,739,361 and 4,740,813 describe a transfer roller apparatus 
for photocopiers which have vacuum pumped orifice holes for tacking the 
leading edge of a document to the peripheral surface of a drum. 
In accordance with one aspect of the present invention, there is provided 
an apparatus for transporting a sheet through a transfer station to 
transfer an image from an image receiving member thereto. The apparatus 
includes a stationary transfer member having at least a portion thereof 
positioned closely adjacent the image receiving member to define a 
transfer zone therebetween. Means are provided for vacuum tacking a 
substantial portion of the sheet to the transfer member. Means move the 
sheet relative to the transfer member to advance the sheet through the 
transfer zone to transfer the image from the receiving member to the 
sheet. 
Pursuant to another aspect of the present invention, there is provided an 
electrophotographic printing machine of the type having a toner image 
developed on a moving photoconductive member with a sheet being advanced 
into registration with the toner image developed on the photoconductive 
member. A stationary transfer member has at least a portion thereof 
positioned closely adjacent the photoconductive member to define a 
transfer zone therebetween. Means are provided for vacuum tacking a 
substantial portion of the sheet to the transfer member. Means move the 
sheet relative to the transfer member to advance the sheet through the 
transfer zone to transfer the toner image from the photoconductive member 
to the sheet.

While the present invention will be described hereinafter in conjunction 
with a preferred embodiment thereof, it will be understood that it is not 
intended to limit the invention to this embodiment. On the contrary, it is 
intended to cover all alternatives, modifications and equivalents as may 
be included within the spirit and scope of the invention as defined by the 
appended claims. 
For a general understanding of the features of the present invention, 
reference is made to the drawings. In the drawings, like reference 
numerals have been used throughout to designate identical elements. FIG. 1 
schematically depicts the various components of an illustrative 
electrophotographic printing machine having the sheet transport system of 
the present invention therein. It will become evident from the following 
discussion that this system is equally well suited for use in a wide 
variety of printing machines and is not necessarily limited in its 
application to the particular printing machine described herein. 
Inasmuch as the art of electrophotographic printing is well known, the 
various processing stations employed in the FIG. 1 printing machine will 
be shown hereinafter schematically and their operation described briefly 
with reference thereto. 
As shown in FIG. 1, the illustrative electrophotographic printing machine 
employs a drum 10 having a photoconductive surface 12 adhering to a 
conductive substrate. Preferably, the photoconductive surface comprises a 
selenium alloy with the conductive substrate being an electrically 
grounded aluminum alloy. Drum 10 moves in the direction of arrow 14 to 
advance successive portions of the photoconductive surface sequentially 
through the various processing stations disposed about the path of 
movement thereof. 
Initially, a portion of the photoconductive surface passes through charging 
station A. At charging station A, a corona generating device indicated 
generally by the reference numeral 16, charges the photoconductive surface 
to a relatively high, substantially uniform potential. 
Next, the charged portion of the photoconductive surface is advanced 
through imaging station B. At imaging station B, a filtered light image of 
an original document is projected onto the charged portion of 
photoconductive surface 12. A moving lens system and a color filter 
mechanism, indicated generally by the reference numeral 18, move in a 
timed relationship with drum 10 to scan successive incremental areas of 
the original document disposed upon a transparent platen. Lamps, located 
beneath the platen, illuminate successive incremental areas of the 
original document. A suitable moving lens system is described in U.S. Pat. 
No. 3,062,108 issued to Mayo in 1952. Similarly, U.S. Pat. No. 3,775,006 
issued to Hartman et al. in 1973 discloses a suitable filter mechanism. 
Finally, U.S. Pat. No. 3,592,531 issued to McCrobie in 1971 discloses a 
suitable type of lens. The foregoing elements cooperate with one another 
to produce a single color light image of the original document which is 
projected onto the charged portion of the photoconductive surface 12 
selectively dissipating the charge thereon to record a single color 
electrostatic latent image. One skilled in the art will appreciate that in 
lieu of the foregoing optical system, a modulated beam of energy, i.e. a 
laser beam, or other suitable device, such as light emitting diodes, may 
be used to irradiate the charged portion of the photoconductive surface so 
as to record selected information thereon. Information from a computer may 
be employed to modulate the laser beam. The modulated laser beam forms a 
light image corresponding to one of the colors desired in the copy. Thus, 
the information initially recorded on the photoconductive surface by the 
laser beam corresponds to the information desired to be reproduced in one 
color. The foregoing process is repeated for each color. 
After the electrostatic latent image is recorded on the photoconductive 
surface, drum 10 advances the electrostatic latent image to development 
station C. At development station C, three individual developer units, 
generally designated by the reference numerals 20, 22, and 24, 
respectively, render successive electrostatic latent images visible. A 
suitable development station for use in a color electrophotographic 
printing machine is disclosed in U.S. Pat. No. 3,854,449 issued to 
Davidson in 1974. Each of the developer units described therein is of a 
type referred to in the art as "magnetic brush developer units". In 
general, a magnetic brush developer unit employs a developer mix of 
ferromagnetic carrier granules having toner particles triboelectrically 
attracted thereto. This brush of developer mix is brought into contact 
with the latent image recorded on the photoconductive surface 12. Toner 
particles are attracted from the carrier granules to the latent image by 
the greater electrostatic force thereof. Thus, the latent image is 
developed or rendered visible by the toner particles. Developer units 20, 
22 and 24, respectively, contain different colored toner particles. Each 
of the toner particles contained in the respective developer unit 
corresponds to the complement of the single color light image transmitted 
through each of the differently colored filters of the filter mechanism 
used in optical system 18. As an illustration, a latent image formed by a 
green filtered light image is developed with green absorbing magenta toner 
particles. Similarly, latent images formed by blue and red images are 
developed with yellow and cyan toner particles, respectively. If desired, 
a fourth developer unit having black toner particles may be used as well. 
One skilled in the art will appreciate that a liquid developer material 
may be used in lieu of the dry developer material previously described. A 
liquid developer material is a mixture of toner particles in a liquid 
carrier. The toner particles are of the appropriate color with the liquid 
carrier being substantially colorless. 
Drum 10 then advances the toner image to transfer station D. At transfer 
station D, a copy sheet 26 is moved into contact with the toner image. The 
copy sheet may be plain paper or a sheet of thermoplastic material, 
amongst others. The copy sheet is advanced from a stack of copy sheets 26 
disposed upon tray 28. The sheet moves in the direction of arrow 30 to 
transfer station D. At transfer station D, the copy sheet is secured to a 
rotating sheet gripper 32 which moves the sheet through transfer zone 34 
about stationary transfer drum 36. Transfer drum 36 is the same diameter 
as photoconductive drum 10. Sheet gripper rotates, in the direction of 
arrow 38, at the same angular velocity as that of drum 10. Copy sheet 26 
is vacuum tacked to the exterior surface of drum 34. Thus, copy sheet 26 
moves in a recirculating path in synchronism with the rotation of drum so 
that successive electrostatic latent images may be transferred thereto. 
Further details of the sheet transport and transfer station will be 
described hereinafter with reference to FIGS. 2 through 9, inclusive. 
After the last toner imaged has been transferred to the copy sheet, the 
copy sheet advances onto conveyor 40. Conveyor 40 advances the copy sheet 
with the toner image adhering thereto, in the direction of arrow 42, to 
fusing station E. 
Fusing station E includes a fuser assembly, indicated generally by the 
reference numeral 44, which permanently affixes the transferred toner 
image to the sheet. One suitable type of fusing apparatus is described in 
U.S. Pat. No. 3,907,492 issued to Draugelis et al. in 1975. After fusing, 
the copy sheet is advanced in the direction of arrow 46 to catch tray 48 
for subsequent removal from the printing machine by the operator. 
After the image is transferred from the photoconductive surface to the copy 
sheet, drum 10 rotates the photoconductive surface to a cleaning station 
(not shown). At the cleaning station, a pre-clean corona generating device 
neutralizes the charge on the photoconductive surface and that of the 
residual toner particles. This enables a fibrous brush in contact with the 
photoconductive surface to remove the residual particles adhering to the 
photoconductive surface. A suitable brush cleaning system is described in 
U.S. Pat. No. 3,590,412 issued to Gerbasi in 1971. 
It is believed that the foregoing description is sufficient for purposes of 
the present invention to illustration the general operation of an 
electrophotographic printing machine incorporating the features of the 
present invention therein. 
Referring now to the specific subject matter of the present invention, FIG. 
2 illustrates transfer station D in greater detail. As shown thereat, 
transfer drum 36 includes a stationary cylinder 50 mounted on a hollow 
stationary axle 52. Cylinder 50 has a multiplicity of perforations or 
holes 54 therein. The holes 54 in cylinder 50 are connected via suitable 
ducts passing through the hollow axle to a vacuum blower. In this way, the 
pressure at the surface of drum 50 is reduced. Electrical wires and the 
vacuum supply can be distributed through the hollow axle and applied to 
cylinder 50 as required, since both are stationary. This causes the story 
sheet to be vacuum tacked to the exterior surface of cylinder 50 as it is 
being moved relative thereto by sheet gripper 32 so as to insure that the 
trailing edge of the sheet does not flip and disturb the toner image as 
the copy sheet is being recirculated. Sheet gripper 32 includes a gripper 
bar 56 supported at opposed ends by drive arms 58. Drive arm 58 is mounted 
on pulley 60. Pulley 60 is mounted rotatably on stationary axle 52. Belt 
62 couples pulley 64 to pulley 60. The shaft of pulley 64 is connected to 
a direct drive motor. In this way, gripper bar 56 rotates relative to 
cylinder 50 at substantially the same angular velocity as that of drum 10. 
Registration pins 66 extend outwardly from either end of gripper bar 56. 
Pins 66 mate with holes in the non-image area of drum 10. This insures 
that successive toner images are transferred to the copy sheet in 
superimposed registration with one another. The gripper bar is opened by a 
cam as it rotates into the sheet feed area. The copy sheet is fed into the 
gripper bar against a stop. A second cam closes the gripper bar and clamps 
the copy sheet. The gripper bar pulls the copy sheet around the stationary 
cylinder. The holes in the cylinder provide a vacuum to hold the copy 
sheet against the cylinder as the gripper bar drags the copy sheet into 
the transfer zone. The vacuum also controls trail edge flip. 
Turning now to FIG. 3, there is shown an elevational view of the sheet 
transport system and the transfer system. As depicted thereat, a pair of 
rollers 70 are mounted rotatably on cylinder 50 in transfer zone 34. 
Rollers 70 are made from an electrically conductive material. These 
rollers are electrically biased to a suitable magnitude and polarity to 
attract the toner image from the photoconductive surface of drum 10 to the 
copy sheet passing through transfer zone 34. As pulley 64 moves belt 62, 
pulley 60 rotates arm 58 to move gripper bar 56 with the copy sheet 
secured thereto through a recirculating path about the periphery of 
cylinder 50. The copy sheet is vacuum tacked to the cylinder. Inasmuch as 
the cylinder is stationary, the gripper bar 56 drags the copy sheet over 
the surface of the cylinder. This insures that the trailing edge of the 
copy sheet does not flip disturbing the toner image transferred thereto. 
Referring now to FIG. 4, there is shown further details of the sheet 
gripper 32. Gripper bar 56 is mounted, at opposed ends, on drive arms 58 
which extend outwardly from pulleys 60. Pulleys 60 are journalled for 
rotation by being mounted, on opposed ends, on bearings 68. Gripper bar 56 
has a plurality of fingers 75 which are mounted pivotably on a rod and 
held in the closed position by springs. As drive arm 58 rotates gripper 
bar 56, the fingers engage a first cam which open the fingers to receive 
the copy sheet. A second cam closes the fingers. The first cam re-opens 
the fingers when the copy sheet is released from the gripper bar after the 
requisite number of toner images have been transferred thereto in 
superimposed registration with one another. 
As shown in FIGS. 5 and 6, rod 72 supporting the fingers 75 of gripper bar 
56 are mounted in slots 76 of drive arm link 73. Drive arm link 73 is 
spring loaded in drive arms 58. In this way, the gripper bar is spring 
loaded and mounted so that its distance from the center of rotation can 
vary. The radius varies to compensate for changes in the radius required 
at the transfer zone, i.e. change in the radius of drum 10. 
FIGS. 7, 8 and 9 depict registration pins 66 in greater detail. As shown 
thereat, pin 66 is mounted on rod 72. Rod 72 is loaded between two springs 
in slot 76 to compensate for the tolerances of the the registration pin. 
Rod 72 of the gripper bar is mounted on drive arm link 73 in slot 76. 
There two springs and adjusting screws on either side thereof to center 
the rod with the arm. This further insures that the registration pins have 
sufficient movement to compensate for any tolerances or misalignments when 
mating with the registration holes in photoconductive drum 10. The drive 
rotating pulley 64 is a servo motor with an encoder. The electronics mate 
the registration pins with the registration holes in the photoconductive 
drum, the geometry of the registration pins locate the gripper bar in the 
same position for each transfer cycle, thereby achieving accurate 
registration for each toner image being transferred to the copy sheet. The 
length of the sheet path and the circumference of cylinder 50 are the same 
as that of drum 10. This resolves many servo timing problems. The drive 
gear ratio is an integer. This makes encoder phase locking easier. In this 
way, the small number of moving parts enables the device to be more 
accurate, reliable and lower in cost than those hereinbefore used. 
In recapitulation, the sheet transport system of the present invention 
recirculates the copy sheet and registers the copy sheet relative to the 
photoconductive drum to insure that successive toner images are 
transferred thereto in superimposed registration with one another. The 
moving copy sheet is vacuum tacked to a cylinder so that the trail edge of 
the copy sheet is vacuum tacked to a cylinder so that the trail edge of 
the copy sheet cannot flip upward. This insures that the toner image is 
not disturbed during the transfer of successive different color toner 
images to the copy sheet. 
It is, therefore, apparent that there has been provided, in accordance with 
the present invention, a sheet transport system for use in a transfer 
station that fully satisfies the aims and advantages hereinbefore set 
forth. While this invention has been described in conjunction with a 
preferred embodiment thereof, it is evident that many alternatives, 
modifications, and variations will be apparent to those skilled in the 
art. Accordingly, it is intended to embrace all such alternatives, 
modifications and variations that fall within the spirit and scope of the 
appended claims.