Photoreceptor drum runout control apparatus

An apparatus for mounting a photoreceptor drum so that the imaging surface thereof remains at a substantially constant distance from an imaging source to minimize image distortion caused by drum runout. The photoreceptor drum is mounted on a rotatable supports and is resiliently urged against a guide. By urging the imaging surface of the photoreceptor drum against a rotatable guide, the imaging surface is maintained a substantially constant distance from the imaging source thereby minimizing lateral magnification error caused by drum runout.

This invention relates generally to a photoreceptor drum mounting 
apparatus, and more particularly concerns a photoreceptor drum runout 
control for multiple imaging stations in a tandem architecture 
electrophotographic printing machine. 
In a typical electrophotographic printing process, a photoconductive member 
is charged to a substantially uniform potential so as to sensitize the 
surface thereof. The charged portion of the photoconductive member is 
exposed to a light image of an original document being reproduced. 
Exposure of the charged photoconductive member selectively dissipates the 
charges thereon in the irradiated areas. This records an electrostatic 
latent image on the photoconductive member corresponding to the 
informational areas contained within the original document. After the 
electrostatic latent image is recorded on the photoconductive member, the 
latent image is developed by bringing a developer material into contact 
therewith. Generally, the developer material comprises toner particles 
adhering triboelectrically to carrier granules. The toner particles are 
attracted from the carrier granules to the latent image forming a toner 
powder image on the photoconductive member. The toner powder image is then 
transferred from the photoconductive member to a copy sheet. The toner 
particles are heated to permanently affix the powder image to the copy 
sheet. 
The foregoing generally describes a typical black and white 
electrophotographic printing machine. With the advent of multicolor 
electrophotographic, it is desirable to use the so-called tandem 
architecture which comprises a plurality of image forming stations. This 
tandem architecture offers a high potential for throughput and image 
quality. One choice of photoreceptors in this tandem engine architecture 
is a drum based photoreceptor architecture used in combination with an 
intermediate transfer belt. To provide durability, it is desirable to use 
a larger diameter drum which affords more than a single pitch so as to 
increase drum life, number of copies per minute and copy volume. A problem 
with a larger drum is that the total indicated runout (being runout caused 
by the eccentricity of the drum) of the drum becomes larger as the 
diameter of the drum increases. This total indicated runout causes image 
registration error at each photoreceptor drum and between the multiple 
photoreceptor drums. 
It is desirable that the multiple drum surfaces in a tandem printing 
machine remain at a constant distance from the imaging source. 
Photoreceptor drum runout causes a change in the conjugate length between 
the imaging source and the photoreceptor surface. This variation in 
conjugate length due to the drum runout can cause a lateral magnification 
error which precludes proper image registration when printing color 
copies. 
The following disclosures may be relevant to various aspects of the present 
invention: 
U.S. Pat. No. 5,038,172 
Patentee--Schreyer 
Issue Date--Aug. 6, 1991 
U.S. Pat. No. 4,922,297 
Patentee--Kondo 
Issue Date--May 1, 1990 
U.S. Pat. No. 4,120,576 
Patentee--Babish 
Issue Date--Oct. 17, 1978 
The relevant portions of the foregoing disclosures may be briefly 
summarized as follows: 
U.S. Pat. No. 5,038,172 discloses a bearing for a photoconductive drum 
which is free of bearing play while providing easy replacement of the 
photoconductive drum. 
U.S. Pat. No. 4,922,297 discloses a support shaft supporting a 
photosensitive drum detachable in the radial direction supported by a 
bearing. A joint integrally rotatably linked to the support shaft and the 
photosensitive drum shaft is disposed so as to be disconnected from either 
of the support shaft or the photosensitive drum drive shaft. 
U.S. Pat. No. 4,120,576 discloses a drum support apparatus including an 
outbound and inbound hub, having outer surfaces adapted for interface 
fitting with the inside surface of a drum. The hubs, while being supported 
on a cantilevered shaft from a frame, have provisions to be secured by the 
tightening of a single nut at the center of a drum. The assembly creates 
an equal distribution of force on the hubs, thus diminishing circular 
runout of the drum. 
In accordance with one aspect of the present invention, there is provided a 
drum mounting apparatus. The apparatus comprises means for guiding the 
drum so as to maintain the surface thereof at a substantially constant 
distance from a pre-selected reference location. Means for resiliently 
urging the drum toward said guiding means to position the drum surface in 
contact therewith are also provided. 
Pursuant to another aspect of the present invention, there is provided An 
electrophotographic printing machine having a drum mounting apparatus. The 
drum mounting apparatus comprises means for guiding the drum so as to 
maintain the surface thereof at a substantially constant distance from a 
pre-selected reference location. Means for resiliently urging the drum 
toward said guiding means to position the drum surface in contact 
therewith are also provided.

While the present invention will be described in connection with a 
preferred embodiment thereof, it will be understood that it is not 
intended to limit the invention to that 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 
references are made to the drawings. In the drawings, like reference 
numerals have been used throughout to designate identical elements. 
Referring now to FIG. 3, an intermediate belt designated generally by the 
reference numeral 10 is mounted rotatably on the machine frame. Belt 10 
rotates in the direction of arrow 12. Four imaging reproducing stations 
indicated generally by the reference numerals 14, 16, 18 and 20 are 
positioned about the periphery of the belt 10. Each image reproducing 
station is substantially identical to one another. The only distinctions 
between the image reproducing stations is their position and the color of 
the developer material employed therein. For example, image reproducing 
station 14 uses a black developer material, while stations 16, 18 and 20 
use yellow, magenta and cyan colored developer material. Inasmuch as 
stations 14, 16, 18 and 20 are similar, only station 20 will be described 
in detail. 
At station 20, a drum 22 having a photoconductive surface deposited on a 
conductive substrate rotates in direction of arrow 21. Preferably, the 
photoconductive surface is made from a selenium alloy with the conductive 
substrate being made from an electronically grounded aluminum alloy. Other 
suitable photoconductive surfaces and conductive substrates may also be 
employed. Drum 22 rotates in the direction of arrow 21 to advance 
successive portions of the photoconductive surface through the various 
processing stations disposed about the path of movement thereof. 
Initially, a portion of the photoconductive surface of drum 22 passes 
beneath a corona generating device 26. Corona generating device 26 charges 
the photoconductive surface of the drum 22 to a relatively high, 
substantially uniform potential. 
Next, the charged portion of the photoconductive surface is advanced 
through the imaging station. At the imaging station, an imaging unit 
indicated generally by the reference numeral 80, records an electrostatic 
latent image on the photoconductive surface of the drum 22. Imaging unit 
80 includes a raster output scanner. The raster output scanner lays out 
the electrostatic latent image in a series of horizontal scan lines with 
each line having a specified number of pixels per inch. Preferably, the 
raster output scanner employs a laser 82 which generates a modulated beam 
of light rays which are scanned across the drum 22 by rotating a polygon 
mirror 84. Alternatively, the raster output scanner may use light emitting 
diode array write bars. In this way, an electrostatic latent image is 
recorded on the photoconductive surface of the drum 22. 
Next, a developer unit indicated generally by the reference numeral 30 
develops the electrostatic latent image with a cyan colored developer 
material. Image reproducing stations 14, 16 and 18 use black, yellow and 
magenta colored developer materials respectively. The latent image 
attracts toner particles from the carrier granules of the developer 
material to form a toner powder image on the photoconductive surface of 
drum 22. After development of the latent image with cyan toner, drum 22 
continues to move in direction of arrow 21 to advance the cyan toner image 
to a transfer zone 32 where the cyan toner image is transferred from drum 
22 to intermediate belt 10 by an intermediate transfer device such as a 
biased transfer roll 24. 
At transfer zone 32, the developed powder image is transferred from 
photoconductive drum 22 to intermediate belt 10. Belt 10 and drum 22 have 
substantially the same tangential velocity in the transfer zone 32. Belt 
10 is electrically biased to a potential of sufficient magnitude and 
polarity by biased transfer roll 24 to attract the developed powder image 
thereto from drum 22. Preferably, belt 10 is made from a conductive 
substrate with an appropriate dielectric coating such as a metalized 
polyester film. 
After the cyan toner image is transferred to the belt 10 at reproducing 
station 20, belt 10 advances the cyan toner image to the transfer zone of 
reproducing station 18 where a magenta toner image is transferred to belt 
10, in superimposed registration with the cyan toner image previously 
transferred to belt 10. After the magenta toner image is transferred to 
belt 10, belt 10 advances the transferred toner images to reproducing 
station 16 where the yellow toner image is transferred to belt 10 in 
superimposed registration with the previously transferred toner images. 
Finally, belt 10 advances the transferred toner images to reproducing 
station 14 where the black toner image is transferred thereto in 
superimposed registration with the previously transferred toner images. 
After all of the toner images have been transferred to belt 10 in 
superimposed registration with one another to form a multicolor toner 
image, the multicolor toner image is transferred to a sheet of support 
material, e.g., a copy paper at the transfer station. 
At the transfer station, a copy sheet is moved into contact with the 
multicolor toner image on belt 10. The copy sheet is advanced to transfer 
station from a stack of sheets 34 mounted on a tray 36 by a sheet feeder 
38 or from either a stack of sheets 40 on tray 42 or a stack of sheets 44 
on a tray 46 by either sheet feeder 48 or sheet feeder 50. The copy sheet 
is advanced into contact with the multicolor image on belt 10 beneath 
corona generating unit 52 at the transfer station. Corona generating unit 
52 sprays ions on to the back side of the sheet to attract the multicolor 
image to the front side thereof from belt 10. After transfer, the copy 
sheet passes under a second corona generating unit 53 for detack and 
continues to move in the direction of arrow 54 to a fusing station. The 
fusing station includes a fuser assembly generally indicated by the 
reference numeral 56, which permanently affixes the transferred toner 
image to the copy sheet. Preferably, fuser assembly 56 includes a heated 
fuser roll 58 and a backup roller 60 with the toner image on the copy 
sheet contacting fuser roller 58. In this manner, the toner image is 
permanently affixed to the copy sheet. After fusing, the copy sheets are 
then fed either to an output tray 62 or to a finishing station, which may 
include a stapler or binding mechanism. 
Referring once again to reproducing station 20, invariably, after the toner 
image is transferred from drum 22 to belt 10, some residual particles 
remain adhering thereto. These residual particles are removed from the 
drum surface 22 at the cleaning station 27. Cleaning station includes a 
rotatably mounted fibrous or electrostatic brush in contact with the 
photoconductive surface of drum 22. The particles are cleaned from the 
drum 22 by rotation of the brush in contact therewith. 
Belt 10 is cleaned in a like manner after transfer of the multicolor image 
to the copy sheet. Subsequent to cleaning, a discharge lamp (not shown) 
floods the photoconductive surface of drum 22 to dissipate any residual 
electrostatic charge remaining thereon prior to the charging thereof for 
the next successive imaging cycle. 
It is believed that the foregoing description is sufficient for the 
purposes of the present application to illustrate the general operation of 
a tandem printing machine. 
Referring now to the specific subject matter of the present invention, 
FIGS. 1 and 2 depict the photoreceptor drum runout control apparatus in 
greater detail. With reference to FIG. 1, there is shown a single 
photoreceptor drum 22 illustrative of each of the four drums illustrated 
in FIG. 3. A typical imaging unit, generally indicated by reference 
numeral 80, including a laser 82, a polygon 84 and a modulator (not shown) 
is also illustrated. It is critical that the conjugate length (represented 
by D) between the polygon 84 and the photoreceptor surface 23 remains 
constant. A change in the conjugate length of the distance between the 
polygon 84 and the photoreceptor surface 23, causes a lateral 
magnification error to be introduced to the image. This is especially 
critical in the foregoing type of multicolor printing machine as the four 
different color toner images must be positioned superimposed in 
registration with one another. The lateral magnification error introduced 
by even a small amount of drum runout, which causes the conjugate length 
between the polygon 84 and the photoreceptor surface 23 to vary, can cause 
unacceptable lateral magnification error. The photoreceptor drum 22 is 
supported axially by bearings 70. Each of these bearings 70 is 
spring-biased by spring 72 so as to urge the photoreceptor drum 22 in a 
direction toward the scanning beam reflected by the polygon 84. 
Photoreceptor surface 23 is urged against guide bearings 76 mounted to the 
frame 78 of the machine. The photoreceptor drum 22 is constantly 
spring-biased so that the surface 23 is constantly riding against the 
guide bearings 76, causing the conjugate length D between polygon 84 and 
photoreceptor surface 23 to remain constant. By utilizing this 
configuration for each photoreceptor drum in the machine, the change in 
lateral magnification error on each and between successive drums is 
minimized. 
In recapitulation, it is evident that there is provided a drum runout 
control apparatus which maintains a substantially constant conjugate 
distance between a polygon and the drum surface so as to minimize lateral 
magnification error. The photoreceptor drum is mounted on a rotatable 
supports and is resiliently urged against a guide. By urging the imaging 
surface of the photoreceptor drum against a rotatable guide, the imaging 
surface is maintained a substantially constant distance from the imaging 
source thereby minimizing lateral magnification error caused by drum 
runout. 
It is, therefore, apparent that there has been provided in accordance with 
the present invention, a photoreceptor drum runout control apparatus that 
fully satisfies the aims and advantages hereinbefore set forth. While this 
invention has been described in conjunction with a specific 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 broad scope of the appended claims.