Abstract:
A photoconductive member for an image forming apparatus includes a hollow, conductive cylindrical drum; an insulating end cap disposed axially within the drum at either end thereof, each end cap including an axial bore; a shaft disposed axially through the drum and the bores, the shaft electrically isolated from the drum by the end caps; and an electrical contact assembly operative to bias the drum, but not the shaft, to an operating voltage.

Description:
BACKGROUND 
   The present invention relates generally to the field of image forming apparatuses and in particular to a photoconductive member biased to an operating voltage and electrically isolated from a mounting shaft. 
   A variety of elements within an electrophotographic image forming apparatus operate at relatively high operating voltages. These voltages are used, for example, to pre-charge a photoconductive member to allow a latent image to be optically formed thereon; to transfer electrically and/or magnetically charged toner particles to the photoconductive member to develop the latent image; and to transfer the developed image from the photoconductive member to a media sheet. Where possible, it is preferable to isolate these voltages to specific operational elements, to reduce the risk of short circuit or electrocution. For example, an operating voltage applied to a photoconductive member is preferably restricted to the photoconductive member itself, and isolated from a metallic housing to which the photoconductive member is mounted. 
   Electrical isolation of various components has traditionally been addressed in the design of removable cartridges in which the components are mounted. For example, a typical prior art electrophotographic image forming apparatus may include one or more removable cartridges, each cartridge containing a reservoir holding a supply of toner, a photoconductive drum for optically forming a latent image and developing the image with the toner, and a developer roller for applying the toner to the photoconductive drum. The image removable cartridge may additionally include various rollers, paddles, augers and blades, as well known in the art. One or more electrical contacts on the cartridge accept an operating voltage, and transfer it to the appropriate component(s). 
   A recent development in the state of the art of electrophotography is the separation of many components traditionally CaCO-located in a single removable container into separate units. In some cases, the components may be mounted to a moveable subunit such as a door, and removed from their operational position whenever the subunit is opened. This requires the provision of precise positioning means, so that the components are returned to a precise operating position each time the subunit is mated to the main housing. One well-known way to repeatedly, precisely locate a cylindrical component is to provide V-shaped receiving voids in a rigid frame, into which fit metallic bearings supporting a shaft that runs through the axis of the cylindrical component. In the event that the cylindrical component must be biased to a high operating voltage, however, prior-art electrical contacts would simultaneously bias the conductive shaft to the operating voltage. This may present an unacceptable hazard where the conductive shaft mounts via conductive bearings to a conductive machine frame. 
   SUMMARY 
   The present invention relates to a photoconductive member for an image forming apparatus. The photoconductive member includes a hollow, conductive cylindrical core; an insulating end cap disposed axially within the core at either end thereof, each end cap including an axial bore; a shaft disposed axially through the core and the bores, the shaft electrically isolated from the core by the end caps; and an electrical contact assembly operative to bias the core, but not the shaft, to an operating voltage. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIG. 1  is a schematic diagram of a representative image forming apparatus having photoconductive members. 
       FIG. 2  is a schematic diagram of a representative image forming apparatus having subunit movable between open and closed positions. 
       FIG. 3  is a partial perspective view of one end of a photoconductive member mounting to a frame. 
       FIG. 4  is an exploded perspective view of the end cap subunit of a photoconductive member. 
       FIG. 5  is a partial section view of one end of a photoconductive member. 
   

   DETAILED DESCRIPTION 
     FIG. 1  depicts a representative image forming apparatus, indicated generally by the numeral  10 . The image forming apparatus  10  comprises a body  12  having a top portion  11 , a subunit  13  and a media tray  14 . The media tray  14  includes a main media sheet stack  16  with a sheet pick mechanism  18 , and a manual input  20 . The media tray  14  is preferably removable for refilling, and located on a lower section of the device  10 . 
   Within the image forming apparatus body  12  and/or in the subunit  13 , the image forming apparatus  10  includes registration rollers  22 , a media sheet transfer belt  24 , one or more removable developer units  26 , a corresponding number of removable photoconductor units  28 , an imaging device  30 , a fuser  32 , reversible exit rollers  34 , and a duplex media sheet path  36 , as well as various rollers, actuators, sensors, optics, and electronics (not shown) as are conventionally known in the image forming apparatus arts, and which are not further explicated herein. 
   The internal components of the developer units  26  and photoconductor units  28  are briefly described (these components are not all explicitly depicted in the drawings). Each developer unit  26  is a removable cartridge that includes a reservoir holding a supply of toner, paddles to agitate and move the toner, a toner adder roll for adding toner to a developer roll  27 , a developer roll  27  for applying toner to develop a latent image on a (separate) photoconductive drum, and a doctor blade to regulate the amount of toner on the developer roll  27 . Each photoconductor unit  28  is a separate removable cartridge that includes a photoconductive (PC) drum  29 . The PC drum  29  may comprise, for example, a hollow aluminum cylindrical drum coated with one or more layers of light-sensitive organic photoconductive materials. The photoconductor unit  28  also includes a charge roll for applying a uniform electrical charge to the surface of the PC drum  29 , a photoconductor blade for removing residual toner from the PC drum  29 , and an auger to move waste toner out of the photoconductor unit  28  into a waste toner container (not shown). 
   Each developer unit  26  mates with a corresponding photoconductor unit  28 , with the developer roll  27  of the developer unit  26  developing a latent image on the surface of the PC drum  29  of the photoconductor unit  28  by supplying toner to the PC drum  29 . In a typical color printer, three or four colors of toner—cyan, yellow, magenta, and optionally black—are applied successively (and not necessarily in that order) to a print media sheet to create a color image. Correspondingly,  FIG. 1  depicts four pairs of developer units  26  and photoconductor units  28 . 
   The operation of the image forming apparatus  10  is conventionally known. Upon command from control electronics, a single media sheet is “picked,” or selected, from either the primary media stack  16  or the manual input  20 . Alternatively, a media sheet may travel through the duplex path  36  for a two-sided print operation. Regardless of its source, the media sheet is presented at the nip of a registration roller  22 , which aligns the sheet and precisely controls its further movement into the print path. 
   The media sheet passes the registration roller  22  and contacts the transport belt  24 , which carries the media sheet successively past the photoconductor units  28 . At each photoconductor unit  28 , a latent image is formed by the imaging device  30  and optically projected onto the PC drum  29 . The latent image is developed by applying toner to the PC drum  29  from the developer roll  27  of the corresponding developer unit  26 . The toner is subsequently deposited on the media sheet as it is conveyed past the photoconductor unit  28  by the transport belt  24 . 
   The toner is thermally fused to the media sheet by the fuser  32 , and the sheet then passes through reversible exit rollers  34 , to land facedown in the output stack  35  formed on the exterior of the image forming apparatus body  12 . Alternatively, the exit rollers  34  may reverse motion after the trailing edge of the media sheet has passed the entrance to the duplex path  36 , directing the media sheet through the duplex path  36  for the printing of another image on the back side thereof. 
     FIG. 2  depicts an image forming apparatus  10  wherein a top cover  11  is opened, and a subunit  13  is separated from the main housing  12  by pivoting about a hinge point  15 . At least the media sheet transport belt  24  and the photoconductor units  28  are mounted to the subunit  13 . In this manner, a user may access both the developer units  26  and photoconductor units  28 , such as for removal and replacement. 
   Accurate positioning of the PC drums  29  is critical to high quality printing. To ensure accurate positioning of the PC drums  29 , V-blocks  40  are cut into the metal framework  42  of the housing  12  of the image-forming apparatus  10 . A steel shaft  44  running through each PC drum  29  is precisely located within a corresponding V-block  40  by a ball bearing assembly  46 . Alternatively, other metallic bearings  46  may be used. 
   During operation, the PC drum  29  is charged to an operating voltage, such as −200V. However, because the steel shaft  44  is electrically connected to the metal frame  42  via ball bearings  46 , the −200V supplied to the PC drum  29  must be electrically isolated from the steel shaft  44 , for user safety considerations. 
   According to the present invention, an electrical connection is established between a biasing contact  48  disposed on part of the photoconductor unit  28  (not depicted in  FIG. 3 ) and the photoconductive drum  29 . The biasing contact  48  is biased to an operating voltage by an appropriate power supply  47  and electrical conductor  49 . The actual configuration of the power supply  47  and its electrical connection to the biasing contact  48  is not material to the present discussion. An external electrical connector  50  forms electrical contact with the biasing contact  48 , and transfers the charge to the PC drum  29 , as described below with reference to  FIGS. 4 and 5 . 
   The external electrical connector  50  is an integral part of the PC drum brake  52 . The brake  52  counters rotational forces imparted to the PC drum  29  by the corresponding developer roller  27  contacting it, which rotates at a slightly higher speed than the PC drum  29 . This tends to accelerate the rotational speed of the PC drum  29 , a tendency that the PC drum brake  52  counters, such that the PC drum  29  actually rotates at a speed determined by its own drive mechanism. 
   The PC drum brake  52  is disposed about an annular conductive hub  54 . The annular conductive hub  54  is electrically conductive, and is preferably formed from a conductive plastic. Alternatively, the annular conductive hub  54  may be formed from any suitable material, as well known in the art. The annular conductive hub  54  includes at least one protrusion  56 , directed toward the interior of the PC drum  29 . 
   The PC drum brake  52  is disposed over the annular conductive hub  54 , and the assembly of the two is disposed within an insulating end cap  58 . The insulating end cap  58  is an electrical insulator, and may be formed of any suitable material, such as rubber, plastic, and the like, as known in the art. The insulating end cap  58  comprises an outer annular ring  60 , an inner annular ring  62  and a floor  64  forming a cylindrical chamber, in which the assembly comprising PC drum brake  52  and annular conductive hub  54  is disposed. The inner annular wall  62  defines a bore  66 , through which the steel shaft  44  is disposed. Disposed opposite the floor  64  from the inner annular wall  62 , and protruding into the interior region of the PC drum  29 , is an interior annular wall  63 , which may be segmented, as shown in  FIG. 4 . The interior annular wall  63  shares the through bore  66  with the inner annular wall  62 . 
   At least one protrusion  56  of the annular conductive hub  54  protrudes through the floor  64  of the insulating end cap  58 , and forms an electrical connection to a generally disc-shaped internal electrical contact  68 . The internal electrical contact  68  is electrically conductive, and contains a large bore  70  formed in the central region thereof. When assembled, the interior annular wall  63  of the insulating end cap  58  may protrude through the bore  70  in the internal electrical contact  68 . The interior annular wall  63  assists in the capture of the steel shaft  44  as it passes through the PC drum  29 , and may additionally electrically isolate the shaft  44  from the internal electrical contact  68 . Disposed around the periphery of the internal electrical contact  68  is a plurality of points or protrusions  72 . The points  72  extend slightly outward of the inner diameter of the PC drum  29 , and thus form physical and electrical contact to the interior surface of the PC drum  29  when the internal electrical contact  68  is disposed within the PC drum  29 . 
   The entire end cap  58  and electrical connector assembly according to the present invention is preferably assembled and then press fitted into at least one end of the PC drum  29 , as depicted in section view in  FIG. 5 . Note that the outer annular wall  60  of the insulating end cap  58  need not be flush with the PC drum  29  as shown; rather, it may include a shoulder and extend at least partially externally to the PC drum  29 . In operation, the external electrical contact  50  (contacting a biasing contact  48 ) biases the PC drum brake  52  to an operating voltage. The PC drum brake  52 , in physical and electrical contact with annular conductive hub  54 , biases the annular conductive hub  54  to the operating voltage. Both the PC drum brake  52  and annular conductive hub  54  are electrically isolated from the steel shaft  44  by the inner annular ring  62  of the insulating end cap  58 . Both elements are additionally electrically isolated from the PC drum  29  by the outer annular ring  60  of the insulating end cap  58 . At least one protrusion  56  extends from the annular conductive hub  54  through the floor  64  of the insulating end cap  58 , making physical and electrical contact to the internal electrical contact  68 . The internal electrical contact  68  is electrically isolated from the steel shaft  44  by the internal annular ring  63  of the insulating end cap  58 . The points  72  disposed around the periphery of the internal electrical contact  68  are press fitted into physical and electrical contact with the interior surface of the PC drum  29 , biasing the PC drum  29  to its operating voltage. 
   In this manner, the PC drum  29  is biased to an operating voltage, such as for example, −200V, while the steel shaft  44 , located in a V-block  40  of the metal frame  42  via ball bearings  46 , is insulated from the operating voltage. This prevents the metal frame  42  from becoming an electrocution hazard to the user. 
   Although the present invention has been described herein with respect to particular features, aspects and embodiments thereof, it will be apparent that numerous variations, modifications, and other embodiments are possible within the broad scope of the present invention, and accordingly, all variations, modifications and embodiments are to be regarded as being within the scope of the invention. The present embodiments are therefore to be construed in all aspects as illustrative and not restrictive and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein.