Patent Publication Number: US-9904230-B2

Title: Axially shifting a photoconductive drum using a cam

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
     None. 
     BACKGROUND 
     1. Field of the Disclosure 
     The present disclosure relates generally to electrophotographic imaging devices and more particularly to axially shifting a photoconductive drum using a cam. 
     2. Description of the Related Art 
     During the electrophotographic printing process, an electrically charged rotating photoconductive drum is selectively exposed to a laser beam. The areas of the photoconductive drum exposed to the laser beam are discharged creating an electrostatic latent image of a page to be printed on the photoconductive drum. Toner particles are then electrostatically picked up by the latent image on the photoconductive drum creating a toned image on the photoconductive drum. The toned image is transferred to the print media (e.g., paper) directly by the photoconductive drum in a direct contact imaging system. The toner is then fused to the media using heat and pressure to complete the print. 
     Repeated contact with the media sheets causes wear on the surface of the photoconductive drum, particularly where the edges of the media sheets contact the surface of the photoconductive drum. Excessive wear on the surface of the photoconductive drum may limit the useful life of the photoconductive drum and cause print defects. Accordingly, it is desired to reduce the occurrence of wear on the surface of the photoconductive drum in order extend the useful life of the photoconductive drum. 
     SUMMARY 
     A photoconductor unit for an electrophotographic image forming device according to one example embodiment includes a housing and a photoconductive drum rotatably mounted on the housing. A cam is mounted on the housing and has a cam surface that is positioned to contact a corresponding locating surface. The cam surface has a variable height in an axial direction of the photoconductive drum such that as a position of the cam changes relative to the housing, the photoconductive drum shifts in the axial direction relative to the locating surface. 
     A photoconductor unit for an electrophotographic image forming device according to another example embodiment includes a housing and a photoconductive drum rotatably mounted on the housing. A cam is mounted on the housing coaxial with the photoconductive drum and rotatable independent of the photoconductive drum. The cam and the photoconductive drum have a fixed relationship to one another in an axial direction of the photoconductive drum. The cam has a cam surface on an axial end of the cam that is positioned to contact a locating surface. The cam surface has a variable height in the axial direction of the photoconductive drum such that as a rotational position of the cam changes relative to the housing, the cam and the photoconductive drum shift in the axial direction of the photoconductive drum relative to the locating surface. 
     An image transfer assembly of an electrophotographic image forming device according to one example embodiment includes a photoconductive drum rotatable about an axis of rotation within the image forming device. A cam is connected to the photoconductive drum and rotatable independent of the photoconductive drum. The cam has a cam surface that has a variable height in an axial direction of the photoconductive drum. A locating surface is in contact with the cam surface. As a rotational position of the cam changes relative to the locating surface, the cam shifts in the axial direction of the photoconductive drum relative to the locating surface causing the photoconductive drum to shift in the axial direction relative to the locating surface. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings incorporated in and forming a part of the specification, illustrate several aspects of the present disclosure, and together with the description serve to explain the principles of the present disclosure. 
         FIG. 1  is a block diagram depiction of an imaging system according to one example embodiment. 
         FIG. 2  is a perspective view of a toner cartridge and an imaging unit of an image forming device according to one example embodiment. 
         FIG. 3  is a bottom perspective view of the imaging unit showing a photoconductive drum assembly according to one example embodiment. 
         FIG. 4  is a schematic illustration of a media sheet being fed past and contacting the photoconductive drum. 
         FIGS. 5A-5C  are schematic illustrations of axial movement of the photoconductive drum according to one example embodiment. 
         FIG. 6  is a perspective view of a portion of the imaging unit showing a drive coupler of the photoconductive drum and a corresponding drive coupler of the image forming device according to one example embodiment. 
         FIG. 7  is an exploded view of the imaging unit shown in  FIG. 6  showing a wear member according to one example embodiment. 
         FIGS. 8A-8C  are cross-sectional views illustrating axial shifting of the photoconductive drum shown in  FIGS. 6 and 7  due to frictional contact between the wear member and the drive coupler of the photoconductive drum according to one example embodiment. 
         FIG. 9  is a perspective view of the imaging unit having a portion of the drive coupler removed to illustrate a wear member according to another example embodiment. 
         FIG. 10  is an exploded view of the imaging unit shown in  FIG. 9 . 
         FIG. 11  is a perspective view of the imaging unit showing a ratchet mechanism according to one example embodiment. 
         FIG. 12  is an exploded view of the ratchet mechanism shown in  FIG. 11 . 
         FIGS. 13 and 14  are front and side elevation views, respectively, of a cam of the ratchet mechanism shown in  FIG. 12  according to one example embodiment. 
         FIG. 15  is a perspective view of a datum member of the image forming device according to one example embodiment. 
         FIGS. 16A-16D  are schematic illustrations of the operation between the cam and the datum member shown in  FIGS. 11-15  according to one example embodiment. 
         FIGS. 17A and 17B  are side elevation views illustrating axial movement of the cam and the photoconductive drum relative to the datum member according to one example embodiment. 
         FIGS. 18A and 18B  are schematic illustrations of an actuator of the image forming device that axially shifts the photoconductive drum according to one example embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, reference is made to the accompanying drawings where like numerals represent like elements. The embodiments are described in sufficient detail to enable those skilled in the art to practice the present disclosure. It is to be understood that other embodiments may be utilized and that process, electrical, and mechanical changes, etc., may be made without departing from the scope of the present disclosure. Examples merely typify possible variations. Portions and features of some embodiments may be included in or substituted for those of others. The following description, therefore, is not to be taken in a limiting sense and the scope of the present disclosure is defined only by the appended claims and their equivalents. 
     Referring now to the drawings and particularly to  FIG. 1 , there is shown a block diagram depiction of an imaging system  20  according to one example embodiment. Imaging system  20  includes an image forming device  22  and a computer  24 . Image forming device  22  communicates with computer  24  via a communications link  26 . As used herein, the term “communications link” generally refers to any structure that facilitates electronic communication between multiple components and may operate using wired or wireless technology and may include communications over the Internet. 
     In the example embodiment shown in  FIG. 1 , image forming device  22  is a multifunction machine (sometimes referred to as an all-in-one (AIO) device) that includes a controller  28 , a print engine  30 , a laser scan unit (LSU)  31 , an imaging unit  200 , a toner cartridge  100 , a user interface  36 , a media feed system  38 , a media input tray  39  and a scanner system  40 . Image forming device  22  may communicate with computer  24  via a standard communication protocol, such as for example, universal serial bus (USB), Ethernet or IEEE 802.xx. Image forming device  22  may be, for example, an electrophotographic printer/copier including an integrated scanner system  40  or a standalone electrophotographic printer. 
     Controller  28  includes a processor unit and associated electronic memory  29 . The processor may include one or more integrated circuits in the form of a microprocessor or central processing unit and may be formed as one or more Application-specific integrated circuits (ASICs). Memory  29  may be any volatile or non-volatile memory or combination thereof, such as, for example, random access memory (RAM), read only memory (ROM), flash memory and/or non-volatile RAM (NVRAM). Memory  29  may be in the form of a separate memory (e.g., RAM, ROM, and/or NVRAM), a hard drive, a CD or DVD drive, or any memory device convenient for use with controller  28 . Controller  28  may be, for example, a combined printer and scanner controller. 
     In the example embodiment illustrated, controller  28  communicates with print engine  30  via a communications link  50 . Controller  28  communicates with imaging unit  200  and processing circuitry  44  thereon via a communications link  51 . Controller  28  communicates with toner cartridge  100  and processing circuitry  45  thereon via a communications link  52 . Controller  28  communicates with fuser  37  and processing circuitry  46  thereon via a communications link  53 . Controller  28  communicates with media feed system  38  via a communications link  54 . Controller  28  communicates with scanner system  40  via a communications link  55 . User interface  36  is communicatively coupled to controller  28  via a communications link  56 . Controller  28  processes print and scan data and operates print engine  30  during printing and scanner system  40  during scanning. Processing circuitry  44 ,  45 ,  46  may provide authentication functions, safety and operational interlocks, operating parameters and usage information related to imaging unit  200 , toner cartridge  100  and fuser  37 , respectively. Each of processing circuitry  44 ,  45 ,  46  includes a processor unit and associated electronic memory. As discussed above, the processor may include one or more integrated circuits in the form of a microprocessor or central processing unit and may be formed as one or more Application-specific integrated circuits (ASICs). The memory may be any volatile or non-volatile memory or combination thereof or any memory device convenient for use with processing circuitry  44 ,  45 ,  46 . 
     Computer  24 , which is optional, may be, for example, a personal computer, including electronic memory  60 , such as RAM, ROM, and/or NVRAM, an input device  62 , such as a keyboard and/or a mouse, and a display monitor  64 . Computer  24  also includes a processor, input/output (I/O) interfaces, and may include at least one mass data storage device, such as a hard drive, a CD-ROM and/or a DVD unit (not shown). Computer  24  may also be a device capable of communicating with image forming device  22  other than a personal computer such as, for example, a tablet computer, a smartphone, or other electronic device. 
     In the example embodiment illustrated, computer  24  includes in its memory a software program including program instructions that function as an imaging driver  66 , e.g., printer/scanner driver software, for image forming device  22 . Imaging driver  66  is in communication with controller  28  of image forming device  22  via communications link  26 . Imaging driver  66  facilitates communication between image forming device  22  and computer  24 . One aspect of imaging driver  66  may be, for example, to provide formatted print data to image forming device  22 , and more particularly to print engine  30 , to print an image. Another aspect of imaging driver  66  may be, for example, to facilitate collection of scanned data from scanner system  40 . 
     In some circumstances, it may be desirable to operate image forming device  22  in a standalone mode. In the standalone mode, image forming device  22  is capable of functioning without computer  24 . Accordingly, all or a portion of imaging driver  66 , or a similar driver, may be located in controller  28  of image forming device  22  so as to accommodate printing and/or scanning functionality when operating in the standalone mode. 
     Print engine  30  includes laser scan unit (LSU)  31 , toner cartridge  100 , imaging unit  200  and fuser  37 , all mounted within image forming device  22 . Imaging unit  200  is removably mounted in image forming device  22  and includes a developer unit  202  that houses a toner sump and a toner development system. In one embodiment, the toner development system utilizes what is commonly referred to as a single component development system. In this embodiment, the toner development system includes a toner adder roll that provides toner from the toner sump to a developer roll. A doctor blade provides a metered uniform layer of toner on the surface of the developer roll. In another embodiment, the toner development system utilizes what is commonly referred to as a dual component development system. In this embodiment, toner in the toner sump of developer unit  202  is mixed with magnetic carrier beads. The magnetic carrier beads may be coated with a polymeric film to provide triboelectric properties to attract toner to the carrier beads as the toner and the magnetic carrier beads are mixed in the toner sump. In this embodiment, developer unit  202  includes a magnetic roll that attracts the magnetic carrier beads having toner thereon to the magnetic roll through the use of magnetic fields. Imaging unit  200  also includes a photoconductor unit  204  that houses a photoconductive drum and a waste toner removal system. 
     Toner cartridge  100  is removably mounted in image forming device  22  in a mating relationship with developer unit  202  of imaging unit  200 . An outlet port on toner cartridge  100  communicates with an inlet port on developer unit  202  allowing toner to be periodically transferred from toner cartridge  100  to resupply the toner sump in developer unit  202 . 
     The electrophotographic printing process is well known in the art and, therefore, is described briefly herein. During a printing operation, laser scan unit  31  creates a latent image on the photoconductive drum in photoconductor unit  204 . Toner is transferred from the toner sump in developer unit  202  to the latent image on the photoconductive drum by the developer roll (in the case of a single component development system) or by the magnetic roll (in the case of a dual component development system) to create a toned image. The toned image is then transferred to a media sheet received by imaging unit  200  from media input tray  39  for printing. In one example embodiment, toner is transferred directly to the media sheet by the photoconductive drum. Toner remnants are removed from the photoconductive drum by the waste toner removal system. The toner image is bonded to the media sheet in fuser  37  and then sent to an output location or to one or more finishing options such as a duplexer, a stapler or a hole-punch. 
     Referring now to  FIG. 2 , toner cartridge  100  and imaging unit  200  are shown according to one example embodiment. Imaging unit  200  includes developer unit  202  and photoconductor unit  204  mounted on a common frame or housing  206 . Developer unit  202  includes a toner inlet port  208  positioned to receive toner from toner cartridge  100 . As discussed above, imaging unit  200  and toner cartridge  100  are each removably installed in image forming device  22 . Imaging unit  200  is first slidably inserted into image forming device  22 . Toner cartridge  100  is then inserted into image forming device  22  and onto housing  206  in a mating relationship with developer unit  202  of imaging unit  200  as indicated by the arrow A shown in  FIG. 2 , which also indicates the direction of insertion of imaging unit  200  and toner cartridge  100  into image forming device  22 . This arrangement allows toner cartridge  100  to be removed and reinserted easily when replacing an empty toner cartridge  100  without having to remove imaging unit  200 . Imaging unit  200  may also be readily removed as desired in order to maintain, repair or replace the components associated with developer unit  202 , photoconductor unit  204  or housing  206  or to clear a media jam. 
     While the example embodiment shown in  FIG. 2  illustrates a single toner cartridge  100  and corresponding imaging unit  200 , it will be appreciated that a multicolor image forming device  22  may include multiple toner cartridges  100  and corresponding imaging units ix)  200 . Further, although in the example embodiment shown in  FIG. 2  toner is transferred directly from toner cartridge  100  to imaging unit  200 , toner may alternatively pass through an intermediate component such as a chute or duct that connects toner cartridge  100  with its corresponding imaging unit  200 . 
     The configurations and architecture of toner cartridge  100  and imaging units  200  shown in  FIG. 2  are meant to serve as examples and are not intended to be limiting. For instance, although the example image forming devices discussed above include a pair of mating replaceable units in the form of toner cartridge  100  and imaging unit  200 , it will be appreciated that the replaceable unit(s) of the image forming device may employ any suitable configuration as desired. For example, in one embodiment, the main toner supply for image forming device  22  and the components of imaging unit  200  are housed in a single replaceable unit. In another embodiment, the main toner supply for image forming device  22  and developer unit  202  are provided in a first replaceable unit and photoconductor unit  204  is provided in a second replaceable unit. In another embodiment, the main toner supply for image forming device  22  is provided in a first replaceable unit, developer unit  202  is provided in a second replaceable unit and photoconductor unit  204  is provided in a third replaceable unit. One skilled in the art will appreciate that many other combinations and configurations of toner cartridge  100  and imaging unit  200  may be used as desired. 
     With reference to  FIG. 3 , imaging unit  200  is shown including a photoconductive drum assembly  250  including a photoconductive drum  255  rotatably mounted on housing  206  between opposed side walls  206   a ,  206   b  about an axis of rotation  256 . When imaging unit  200  is inserted into image forming device  22 , photoconductive drum  255  is paired with a transfer roll (not shown) forming a toner transfer nip therebetween for use in transferring toner to a sheet of print media passing through the transfer nip. In the example shown, a media sheet M is fed in a media feed direction MFD and passes through the toner transfer nip to receive toner from the surface of photoconductive drum  255 . Photoconductive drum  255  has an axial length including an imaging region  255   a  at a central portion thereof and non-imaging regions  255   b ,  255   c  at end portions thereof. Media sheet M contacts the imaging region  255   a  of photoconductive drum  255  as media sheet M passes through the toner transfer nip. The physical roughness of media sheet M may wear the surface of photoconductive drum  255  throughout the imaging region  255   a  contacted by media sheet M. The areas where the edges E 1 , E 2  of media sheet M contact photoconductive drum  255  typically cause significantly more wear on the surface of photoconductive drum  255  than the area of imaging region  255   a  between edges E 1 , E 2 . In particular, as photoconductive drum  255  rotates, media sheet edges E 1 , E 2  may create relatively deep scratches or form wear rings on the surface coating of photoconductive drum  255  over time that may extend around its entire circumference. For example, in  FIG. 4  showing a simplified illustration of media sheet M being fed in the media feed direction MFD and contacting photoconductive drum  255 , wear marks W 1 , W 2  are formed on opposed end regions of the surface of photoconductive drum  255  due to repeated contact between the surface of photoconductive drum  255  and edges of media sheets being fed through the transfer nip, such as edges E 1 , E 2  of media sheet M. 
     According to example embodiments of the present disclosure, the additional wear in the regions where edges of the media sheet contact photoconductive drum  255  may be reduced by shifting photoconductive drum  255  axially, perpendicular to the media feed direction MFD. In particular, a shifting mechanism is provided to translate an operating position of photoconductive drum  255  within image forming device  22  axially relative to its axis of rotation  256 . By axially moving photoconductive drum  255 , wear on the surface of photoconductive drum  255  caused by the edges of the media sheet is spread out over a relatively wider area at each end of photoconductive drum  255  instead of being concentrated at a single location at each end of photoconductive drum  255 . Spreading the wear incurred on the surface of photoconductive drum  255  aids in extending the useful life of photoconductive drum  255 . 
     As an example,  FIGS. 5A-5C  illustrate schematic representations of photoconductive drum  255  movable along its rotational axis  256 , perpendicular to media feed direction MFD, and media sheet M passing through photoconductive drum  255 . Media sheet M is provided to illustrate the location of media sheet edges relative to the surface of photoconductive drum  255  as media sheets are fed through the toner transfer nip. In  FIG. 5A , photoconductive drum  255  is at an initial position in image forming device  22  with initial edge wear boundaries W 1 , W 2  corresponding to the location of edges E 1 , E 2  of media sheet M. In order to substantially reduce wear at the initial edge wear boundaries W 1 , W 2 , photoconductive drum  255  is axially shifted, perpendicular to the media feed direction MFD, such as shown in  FIGS. 5B and 5C . In  FIG. 5B , photoconductive drum  255  is axially shifted in a first direction  258   a  such that edge wear boundaries W 1 , W 2  are shifted laterally from respective edges E 1 , E 2  of media sheet M by a distance D 1 . In  FIG. 5C , photoconductive drum  255  is axially shifted in a second direction  258   b  such that media sheet edges E 1 , E 2  are spaced apart from the initial edge wear boundaries W 1 , W 2  by a distance D 2 . By axially moving photoconductive drum  255  between the positions shown in  FIGS. 5B and 5C , location of the media sheet edges relative to the surface of photoconductive drum  255  are shifted such that the media sheet edges do not contact and apply stress concentration on the same respective regions of the photoconductive drum surface as media sheets pass through the toner transfer nip. Instead, wear is spread out over a wider area, such the areas defined by distances D 1  and D 2 , which extends the useful life of photoconductive drum  255 . In one example embodiment, photoconductive drum  255  is moved gradually between the positions shown in  FIGS. 5B and 5C . In another example embodiment, photoconductive drum  255  is moved between the positions illustrated in  FIGS. 5B and 5C  and discrete positions intermediate those illustrated in  FIGS. 5B and 5C . 
     Referring now to  FIG. 6 , photoconductive drum assembly  250  includes a drive coupler  220  that is positioned to mate with a corresponding drive coupler  120  in image forming device  22 . When imaging unit  200  is installed in image forming device  22 , drive coupler  220  is engaged with drive coupler  120  and receives rotational and axial force therefrom for rotating and axially biasing photoconductive drum  255  in a direction indicated by the arrow B shown in  FIG. 6 , which is also perpendicular to the media feed direction MFD. Drive coupler  120  is biased toward drive coupler  220  in order to ensure reliable contact between the two to permit the transfer of rotational force from drive coupler  120  to drive coupler  220 . For example, in the embodiment illustrated, a biasing spring  125  biases drive coupler  120  toward drive coupler  220 . The bias applied to drive coupler  120  presses drive coupler  120  axially against the axial end surface of drive coupler  220  in order to maintain contact between drive coupler  120  and drive coupler  220 . 
       FIG. 7  illustrates an exploded view of an end portion of photoconductive drum  255 . As shown, side wall  206   a  of housing  206  includes an opening  208 . Provided in opening  208  is a bushing  230  which is fixedly mounted on side wall  206   a  and arranged to receive and rotatably support a shaft end  260  of photoconductive drum  255  via an opening  232 . Side wall  206   a  includes retainers  209  which secure bushing  230  on side wall  206   a . Drive coupler  220  is mounted on shaft end  260  extending through opening  232  and rests within a socket  234  of bushing  230 . Splines  262  are provided on shaft end  260  to seat drive coupler  220  onto shaft end  260  and cause photoconductive drum  255  to rotate when drive coupler  220  is driven to rotate. 
     In one example embodiment shown, a raised wear surface or member  240  is provided between drive coupler  220  and bushing  230 . In the example shown, raised wear member  240  is provided as a wear ring integrally formed as part of bushing  230  and protrudes from an inner surface  236  of socket  234 . Raised wear member  240  is positioned to receive frictional contact from drive coupler  220  in the axial bias direction B. Raised wear member  240 , although shown as having an annular shape surrounding shaft end  260 , may have other forms or shapes, such as, for example, one or more posts or pegs. As drive coupler  220  and photoconductive drum  255  rotate, bushing  230  including raised wear member  240  remains stationary relative to housing  206  and the frictional contact between drive coupler  220  and raised wear member  240  gradually wears away raised wear member  240  in the axial bias direction B. The wearing away of wear member  240  in the axial bias direction B gradually shifts the position of photoconductive drum  255  axially in the axial bias direction B relative to housing  206 , which occupies a fixed position in image forming device  22 . In this embodiment, wear member  240  is made of softer material than drive coupler  220  such that drive coupler  220  wears at a much slower rate, or not at all, relative to wear member  240 . 
     With reference to  FIGS. 8A-8C , axial shifting of photoconductive drum  255  due to frictional contact between raised wear member  240  and drive coupler  220  is shown according to one example embodiment. Photoconductive drum  255  is axially movable between an initial axial position (shown in  FIG. 8A ) and a final axial position (shown in  FIG. 8C ), perpendicular to the media feed direction MFD. The initial axial position corresponds to a position of photoconductive drum  255  prior to the first use thereof and the final axial position corresponds to a position at which photoconductive drum  255  stops and no longer moves axially after photoconductive drum  255  has been used in image forming device  22  for some time. In  FIG. 8A , photoconductive drum  255  is at its initial axial position relative to housing  206  with raised wear member  240  having an initial thickness T 1  in the axial direction and engaging a first contact surface  221  of drive coupler  220 . As shown, first contact surface  221  of drive coupler  220  is spaced from inner surface  234  by a gap defined by thickness T 1 . As drive coupler  220  is axially biased against raised wear member  240  in the bias direction B when drive coupler  220  receives rotational and axial force from drive coupler  120 , frictional engagement between raised wear member  240  and drive coupler  220  wears away raised wear member  240  and gradually reduces the thickness T of wear member  240 . In  FIG. 8B , the thickness of raised wear member  240  has been reduced to an intermediate thickness T 2 . With the axial thickness T of raised wear member  240  being reduced and drive coupler  220  receiving continued axial bias from drive coupler  120 , drive coupler  220  is pushed closer to bushing  230  in the axial bias direction B. Since drive coupler  220  is coupled to shaft end  260  of photoconductive drum  255 , the shift in axial position of drive coupler  220  pushes photoconductive drum  255  in the axial bias direction B thereby shifting the axial position of photoconductive drum  255  relative to housing  206 . The wear rate of wear member  240  and, in turn, the rate of shifting of photoconductive drum  255  may vary based on the material selection of wear member  240 , the axial load applied to drive coupler  220  and the speed at which photoconductive drum  255  is rotated during operation. 
     In one example embodiment, bushing  230  includes a stop  236  that locates drive coupler  220  in its final position shown in  FIG. 8C . That is, when raised wear member  240  has worn to an extent that a second contact surface  223  of drive coupler  220  contacts stop  236 , stop  236  blocks drive coupler  220 , and consequently photoconductive drum  255 , from axially moving further in the bias direction B. The depth of stop  236  in the axial direction may be selected such that photoconductive drum  255  does not move beyond the operating window for the imaging process. In one example embodiment, photoconductive drum  255  is shifted axially about 1-2 mm from its initial position to its final position. 
     In one alternative example embodiment, the wear member may be provided as a separate component that is positioned between bushing  230  and drive coupler  220 . For example,  FIGS. 9-10  show a dedicated spacer or washer  240 ′ disposed between bushing  230  and drive coupler  220  that serves as the wear member. As with raised wear member  240 , washer  240 ′ is positioned to receive frictional contact from drive coupler  220  in the axial bias direction B on photoconductive drum  255  such that as photoconductive drum  255  rotates, frictional contact on washer  240 ′ gradually wears away washer  240 ′ in the axial bias direction B resulting in the gradual shifting of photoconductive drum  255  in the axial bias direction B. When washer  240 ′ has worn beyond a predetermined point, the second contact surface  223  of drive coupler  220  contacts stop  236  of bushing  230  thereby limiting further axial movement of drive coupler  220  and consequently photoconductive drum  255 . 
     The above example embodiments show a wear surface or member positioned between bushing  230  and drive coupler  220 . However, it will be appreciated that a wear member may be provided elsewhere in photoconductive drum assembly  250 . Further, although the example embodiments include a wear member in frictional contact with drive coupler  220 , the wear member may be in frictional contact with other components of photoconductive drum assembly  250  (e.g., with photoconductive drum  255 ). For example, a wear member may instead be positioned at an axial end of photoconductive drum  255  opposite shaft end  260  thereof. Alternatively, a wear member may be formed as part of or attached to drive coupler  220  and biased against bushing  230 . 
     The wear member may be composed of any suitable material based on the desired wear rate. Example materials include graphite, polytetrafluoroethylene (e.g., Teflon™ sold by Chemours™), thermoplastic elastomers such as polyester (e.g., Hytrel® sold by DuPont™) Preferably, the wear member has a low coefficient of friction and a consistent, predictable wear rate. It is also preferred that debris generated by the wearing away of the wear member does not contaminate or damage the electrophotographic components of image forming device  22 . 
     The configurations for axially moving the position of photoconductive drum  255  are not limited to the example embodiments illustrated. Other configurations may be implemented as desired. For example, image forming device  22  may include features that shift or vary the position of imaging unit  200  relative to image forming device  22  along axis of rotation  256  or that shift or vary the position of photoconductive drum  255  relative to housing  206  along axis of rotation  256 . 
     With reference to  FIG. 11 , there is shown an adjustment mechanism  300  for periodically shifting the position of imaging unit  200  within image forming device  22  along axis of rotation  256 , perpendicular to the media feed direction MFD, according to one example embodiment. Adjustment mechanism  300  includes a datum member  310  provided within an interior of image forming device  22  and a ratchet mechanism  340  provided in imaging unit  200 . In the example shown, datum member  310  is integrated within a housing of image forming device  22  and ratchet mechanism  340  is rotatably mounted on imaging unit  200  adjacent to bushing  230  and positioned to engage datum member  310  when imaging unit  200  is installed in image forming device  22 . In this example embodiment, ratchet mechanism  340  operates as a rotating mechanism that includes a cam  345  having a cam surface  347  ( FIG. 12 ) for causing imaging unit  200  to move between a plurality of positions in a direction parallel to the axis of rotation  256  of photoconductive drum  255 . 
       FIG. 12  illustrates an exploded view of ratchet mechanism  340 . As shown, cam  345  is positioned between an axial end  261  of photoconductive drum  255  and bushing  230 . Bushing  230  includes a rear journal portion  238  that passes through an opening  349  provided in cam  345  to rotatably secure cam  345  in imaging unit  200 . Cam  345  is rotatable relative to bushing  230  and has a rotational axis that is coaxial with the axis of rotation  256  of photoconductive drum  255 . Cam  345  may be retained on side wall  206   a  by retainers or hook features (not shown) provided in side wall  206   a . Shaft end  260  of photoconductive drum  255  passes through cam  345  and bushing  230  via openings  232 ,  349  and is received by drive coupler  220  which is seated within socket  234  of bushing  230 . Cam  345  is rotatable relative to housing  206  independent of drive coupler  220  and photoconductive drum  255 . In the example embodiment illustrated, cam  345  is rotatable in a single direction. In other embodiments, cam  345  is rotatable in two directions. 
     With reference to  FIGS. 13-14 , cam  345  includes a plurality of teeth  350  radially extending outward therefrom with each tooth  350  having an engaging surface  351  and a sliding surface  352 . In the embodiment illustrated, each time imaging unit  200  is inserted into image forming device  22 , one of the teeth  350  contacts datum member  310  to rotate cam  345  a predetermined amount. In  FIG. 15 , datum member  310  is shown including a locating surface  315  and a rail  320  projecting from locating surface  315  in the axial direction of photoconductive drum  255 . Rail  320  generally has a triangular profile formed by an abutment surface  322  and a ramped surface  324 . Abutment surface  322  is engageable by a tooth  350  of cam  345  during insertion of imaging unit  200  into image forming device  22  which causes cam  345  to rotate in one direction. On the other hand, ramped surface  324  allows imaging unit  200  to be removed from image forming device  22  without causing cam  345  to rotate. 
     For example,  FIGS. 16A-16D  illustrate interaction between cam  345  and datum member  310  during insertion and removal of imaging unit  200  from image forming device  22 . Locating surface  315  has been omitted to more clearly illustrate the operation between rail  320  and a tooth  350 - 1  of cam  345 .  FIG. 16A  shows engaging surface  351 - 1  of tooth  350 - 1  contacting abutment surface  322  of datum member  310  as imaging unit  200  is inserted into image forming device  22 . As imaging unit  200  is further advanced towards its final position in image forming device  22 , contact between tooth  350 - 1  and abutment surface  322  urges cam  345  to rotate clockwise as viewed in  FIG. 16B  until imaging unit  200  reaches its final position within image forming device  22 , shown in  FIG. 16C . When imaging unit  200  is removed from image forming device  22 , cam  345  maintains its rotational position as shown in  FIG. 16D  due to the position and angle of sliding surface  352 - 2  of tooth  350 - 2  relative to ramped surface  324 . Sliding surface  352 - 2  of tooth  350 - 2  may or may not ride up ramped surface  324  upon removal of imaging unit  200  from image forming device  22 . Upon reinsertion of imaging unit  200  into image forming device  22 , the engaging surface  351 - 2  of tooth  350 - 2  contacts abutment surface  322  causing cam  345  to once again rotate clockwise as viewed in  FIGS. 16A-16D . With each subsequent insertion of imaging unit  200  into image forming device  22 , cam  345  is cycled to its next rotational position. In one example embodiment, the rotational position of cam  345  sets the axial position of photoconductive drum  255  relative to datum member  310  as described in greater detail below. 
     With reference back to  FIG. 14 , cam surface  347  has an uneven surface profile relative to an imaginary plane that is perpendicular to the axis of rotation  256  for contacting locating surface  315  of datum member  310 . In the example shown, cam surface  347  has a substantially continuous tapered surface on an inner axial side of cam  345  such that cam surface  347  has a variable height in the axial bias direction B. However, it will be appreciated that cam surface  347  may have other forms or shapes that provide an uneven cam surface profile. For example, cam surface  347  may have discrete indexed surfaces or steps instead of being a continuous surface as shown. Cam surface  347  is positioned to abut locating surface  315  of datum member  310  such that changing the rotational position of cam surface  347  shifts the position of imaging unit  200  relative to datum member  310  along axis of rotation  256 . For example,  FIGS. 17A-17B  illustrate interaction between cam surface  347  of cam  345  and locating surface  315  of datum member  310 . Rail  320  of datum member  310  has been omitted in  FIGS. 17A-17B  to more clearly illustrate the positioning of cam surface  347  relative to locating surface  315 . 
     In  FIG. 17A , cam  345  is at a first rotational position in which a first point P 1  of cam surface  347  contacts locating surface  315 . In this first rotational position, cam  345  is displaced by a predetermined distance D 1  from datum member  310  defined by the height H 1  of first point P 1  contacting locating surface  315 . Displacement of cam  345  moves imaging unit  200  perpendicular to the media feed direction MFD thereby axially shifting photoconductive drum  255 . In  FIG. 17B , cam  345  is at a second rotational position whereby cam  345  has been rotated 180° relative to the first rotational position shown in  FIG. 17A . In this second rotational position, a second point P 2  of cam surface  347 , which has a height H 2  less than the height H 1  of first point P 1 , contacts locating surface  315  causing cam  345  to be displaced by a predetermined distance D 2  from datum member  310  that is less than distance D 1 . Accordingly, as the rotational position of cam  345  changes relative to datum member  310 , a point of contact between cam surface  347  and locating surface  315  changes such that the distance from cam  345  to datum member  310  changes as the rotational position of cam  345  changes as defined by the height of the region of cam surface  347  contacting locating surface  315 . In this manner, rotation of cam  345  moves imaging unit  200  perpendicular to the media feed direction MFD thereby axially shifting photoconductive drum  255 . 
     Each tooth  350  of cam  345  provides a corresponding rotational position of cam  345 . In the example illustrated, cam  345  includes six teeth  350  such that when imaging unit  200  is inserted into image forming device  22 , one of the teeth  350  of cam  345  contacts the abutment surface  322  of rail  320  and causes cam  345  to rotate 60°. The uneven profile of cam surface  347  changes the axial position of photoconductive drum  255  each time imaging unit  200  is inserted into image forming device  22 . Since each tooth  350  of cam  345  provides a corresponding rotational position of cam  345 , each tooth  350  defines an extent of travel by photoconductive drum  255  in the axial direction. When, for example, imaging unit  200  is removed from image forming device  22  and thereafter reinserted, the axial position of photoconductive drum  255  is adjusted accordingly as a result of cam  345  undergoing rotational movement in response to contact between datum member  310  and a tooth  350  of cam  345 . While the illustrated example embodiment shows cam  345  having six teeth  350 , it will be appreciated that cam  345  may include any number of teeth to define a plurality of axial positions for photoconductive drum  255 . It will also be appreciated that each tooth  350  of cam  345  may provide a unique axial position of photoconductive drum  255  relative to all other teeth  350  or some teeth  350  of cam to  345  may provide the same axial position of photoconductive drum  255 . Further, the amount of shifting of photoconductive drum  255  for each rotational position may be adjusted by modifying the profile of cam surface  347  as desired. 
     Although the example embodiment illustrates rotation of cam  345  upon insertion of imaging unit  200  into image forming device  22 , rotation of cam  345  may be triggered by any suitable means. For example, cam  345  may be rotated upon the removal of imaging unit  200  from image forming device  22  or upon the insertion of toner cartridge  100  into image forming device  22 . In another embodiment, cam  345  is rotated upon the closing of a door in image forming device  22  that permits access to imaging unit  200 . For example, a plunger or other projection extending from an internal portion of the door may contact a tooth  350  of cam  345  (or another engagement member of cam  345 ) to rotate cam  345 . In other embodiments, cam  345  is rotated at predetermined intervals by an electromechanical device, such as a solenoid or motor in image forming device  22 . Although the example embodiment illustrated includes a rotatable cam  345 , the cam may take other suitable paths of motion (e.g., translating) as desired. 
     In the above example embodiment, locating surface  315  is provided as part of the image forming device  22  in which imaging unit  200  is installed. In other embodiments, cam surface  347  contacts a fixed locating surface on housing  206  of imaging unit  200 . In these embodiments, an engagement member, such as a feature similar to rail  320 , is provided in image forming device  22  to contact and rotate cam  345  upon insertion of imaging unit  200  into image forming device  22 . Drive coupler  120  axially biases cam  345  in the axial bias direction B such that cam surface  347  remains in contact with the locating surface on housing  206 . As a rotational position of cam  345  changes relative to housing  206 , cam  345  shifts in the axial direction of photoconductive drum  255  relative to housing  206  causing photoconductive drum  255  to shift in the axial direction relative to the locating surface on housing  206 . In this way, photoconductive drum  255  is axially shifted without shifting the entire imaging unit  200  relative to image forming device  22 . 
     Referring now to  FIGS. 18A-18B , another example embodiment of a system for axially shifting photoconductive drum  255  is illustrated. In this embodiment, image forming device  22  includes an actuator  400  that is operative to engage an exposed portion of imaging unit  200  to move imaging unit  200  along axis of rotation  256  and thereby shift an axial position of photoconductive drum  255  relative to its axis of rotation  256 . For example, the exposed portion of imaging unit  200  may be a feature projecting from housing  206  or a portion of housing  206 . In the example shown, actuator  400  includes a plunger  405  that is movable by a solenoid  410  to engage an exposed portion  207  of side wall  206   a . It will be appreciated, however, that actuator  400  may take other suitable shapes or forms. Solenoid  410  is communicatively coupled to and activated by controller  28  to linearly move plunger  405  toward or away from exposed portion  207  as indicated by arrow  406 . Plunger  405  has a tapered edge  407  that engages exposed portion  207  such that when exposed portion  207  of side wall  206   a  is in contact with tapered edge  407 , linear motion of plunger  405  in the direction  406  is translated into reciprocating motion  210  of housing  206  along axis of rotation  256 . For example, in  FIG. 18A , plunger  405  is shown at an initial position prior to engaging exposed portion  207  of side wall  206   a . As plunger  405  is moved toward and engages side wall  206   a , the tapered edge  407  exerts an actuation force on side wall  206   a  against the biasing force of spring  125 , causing imaging unit  200  to shift in a direction opposite the bias direction B as shown in  FIG. 18B . 
     Photoconductive drum  255  may be shifted periodically by actuator  400  based on any desired condition or time interval. Photoconductive drum  255  may be axially shifted based on operating parameters and usage information related to image forming device  22  or imaging unit  200 . For example, photoconductive drum  255  may be shifted based on the number of pages printed, the number of revolutions of photoconductive drum  255 , etc. In this manner, photoconductive drum  255  may be shifted automatically without user intervention. 
     The configurations for actively shifting photoconductive drum  255  in the axial direction by an actuator mechanism of image forming device  22  are not limited to the example embodiments illustrated in  FIGS. 18A-18B . Other configurations are possible. For example, actuator  400  may include a drive mechanism other than a solenoid, such as a motor. Further, an engagement member other than plunger  405  may be used as desired. For example, a solenoid or motor may move an indexing mechanism (such as cam  345  discussed above) or an engagement member that physically pushes or pulls imaging unit  200  a predetermined amount. In other embodiments, a shim may engage and disengage from between a portion of imaging unit  200  (e.g., bushing  230  or photoconductive drum  255 ) and a reference surface in image forming device  22  in order to shift the position of housing  206  within image forming device  22 . While the example embodiment illustrated includes actuator  400  shifting the position of housing  206  within image forming device  22 , other embodiments include actuator  400  shifting photoconductive drum  255  relative to housing  206 . For example, actuator  400  may engage and disengage a shim from between bushing  230  and photoconductive drum  255  in order to shift photoconductive drum  255  relative to housing  206 . 
     Accordingly, photoconductive drum  255  is shifted axially in order to distribute the wear on the surface of photoconductive drum  255  caused by the edges of the media sheet to help extend the useful life of photoconductive drum  255 . 
     The foregoing description illustrates various aspects and examples of the present disclosure. It is not intended to be exhaustive. Rather, it is chosen to illustrate the principles of the present disclosure and its practical application to enable one of ordinary skill in the art to utilize the present disclosure, including its various modifications that naturally follow. All modifications and variations are contemplated within the scope of the present disclosure as determined by the appended claims. Relatively apparent modifications include combining one or more features of various embodiments with features of other embodiments.