Patent Publication Number: US-7594657-B2

Title: Medium pressing guide

Description:
BACKGROUND 
     A drum is sometimes used to transport a medium. Insufficient retention of the medium against the drum during transport may result in incorrect positioning of the medium on the drum or in the medium catching upon structures adjacent drum during its rotation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic illustration of a printing system according to an example embodiment. 
         FIG. 2  is a schematic illustration of another embodiment of the printing system of  FIG. 1  according to an example embodiment. 
         FIG. 3  is a top perspective view of another embodiment of the printing system of  FIG. 1  with portions schematically shown according to an example embodiment. 
         FIG. 4  is an enlarged fragmentary side elevational view of the printing system of  FIG. 3  illustrating a cam follower in a cam engaged state and a guide in a retracted position according to an example embodiment. 
         FIG. 5  is an enlarged fragmentary perspective view of the portion of the printing system of  FIG. 4  according to an example embodiment. 
         FIG. 6  is another enlarged fragmentary perspective view of the portion of the printing system of  FIG. 4  according to an example embodiment. 
         FIG. 7  is an enlarged fragmentary side elevational view of the printing system of  FIG. 3  illustrating the cam follower in a cam engaged state and the guide in a media pressing position according to an example embodiment. 
         FIG. 8  is a sectional view of the printing system of  FIG. 7  illustrating the guide in the media pressing position according to an example embodiment. 
         FIG. 9  is a sectional view of the printing system of  FIG. 7  illustrating the guide in the media pressing position according to an example embodiment. 
         FIG. 10  is an enlarged fragmentary side elevational view of the printing system of  FIG. 3  illustrating the cam follower in a cam disengaged state and the guide in a first retracted position according to an example embodiment. 
         FIG. 11  is a sectional view of the printing system of  FIG. 10  illustrating the guide in the first retracted position according to an example embodiment. 
         FIG. 12  is an enlarged fragmentary side elevational view of the printing system of  FIG. 3  illustrating the cam follower in a second cam disengaged state and the guide in a second retracted position according to an example embodiment. 
         FIG. 13  is a sectional view of the printing system of  FIG. 12  illustrating the guide in the second retracted position according to an example embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE EXAMPLE EMBODIMENTS 
       FIG. 1  schematically illustrates printing system  20  according to an example embodiment. Printing system  20  is configured to print or deposit material onto a medium supported by a drum  22 . As will be described in more detail hereafter, printing system  20  loads media onto the drum such that the media is securely retained against the drum during transport and printing. 
     Printing system  20  generally includes media transport drum  22 , media hold-down mechanism  24 , drum drive  26 , media input  28 , printing mechanism  30 , media eject  32 , media output  34 , load assist system  36  and controller  38 . Media transport drum  22  may comprise a large, generally cylindrical member configured to be rotationally driven about axis  44  and includes a media support surface  46 . Media support surface  46  comprises a generally circumferential surface upon which one or more pieces or sheets  50  of a medium, such as a cellulose-based material, polymer-based material, metallic-based material or combinations thereof, may be held or retained during printing and/or other interaction. In one embodiment, surface  46  may include elongated circumferential grooves or depressions (not shown) to facilitate separation of sheets from surface  46 . In the particular embodiment illustrated, surface  46  is configured to retain at least three 8.5 inch by 11 inch sheets of the medium. In other embodiments, surface  46  may be configured to support a fewer or greater of the same sheets or larger or smaller sheets. 
     Media hold-down mechanism  24  comprises a mechanism configured to hold and retain sheets  50  against surface  46  during rotation of drum  22  about axis  44 . In one embodiment, media hold-down mechanism retains sheets  50  against surface  46  in a continuous fashion as sheets  50  are transported from load assist system  36 , past printing mechanism  30  and to media eject  32 . In other embodiments, this retention may be periodic or discontinuous. In one embodiment, media hold-down mechanism  24  includes perforations or other openings along surface  46  through which a vacuum from a vacuum source is applied to retain sheets  50  against surface  46 . In still other embodiments, media hold-down mechanism  24  may be configured to create electrostatic charge (or force) along surface  46  to retain one or more of sheets  50  against surface  46 . In particular embodiments, media hold-down mechanism  24  may use both electrostatic forces and vacuum forces to hold sheets  50  against surface  46 . Yet in other embodiments, media hold-down mechanism  24  may be configured to retain sheets  50  against surface  46  in other fashions. 
     Drum drive  26  (schematically shown) comprises a device configured to rotationally drive drum  22  about axis  44  so as to move one or more sheets  50  from media input  28  to printing mechanism  30  and ultimately to media eject  32 . In one embodiment, drum drive  26  comprises an electric motor operably coupled to drum  22  by a transmission or other power train. In other embodiments, drum drive  26  may comprise other devices configured to provide torque to rotate drum  22 . 
     Media input  28  (schematically shown) comprises a mechanism configured to supply and transfer sheets  50  of media to drum  22 . In one embodiment, media input  28  may include a media storage volume, such as a tray, bin and the like, one or more pick devices (not shown) configured to pick a sheet of media from the storage volume and one or more media transfer mechanisms configured to transfer the medium to drum  22 . Media input  28  may have a variety of sizes and configurations. 
     Printing mechanism  30  (schematically shown) comprises a mechanism or device configured to print or otherwise deposit materials upon sheet  50 . In one embodiment, printing mechanism  30  may be configured to print a pattern or image upon sheets  50  as sheets  50  are held against surface  46  of drum  22 . In one embodiment, printing mechanism  30  may be configured to eject fluid ink onto sheets  50  held by drum  22 . In one embodiment, printing mechanism  30  may include one or more print heads carried by one or more carriages that are configured to be scanned across sheets  50  of media held by drum  22  and directions generally along an axis  44 . In other embodiments, printing mechanism  30  may include print heads which substantially extend across a width or dimension of sheets  50  held by drum  22  such as with a page-wide-array printer. In still other embodiments, printing mechanism  30  may comprise other printing devices configured to deposit ink, toner or other materials upon sheets  50  held by drum  22  in other fashions. 
     Media eject  32  (schematically shown) comprises a mechanism configured to eject or separate sheets  50  from surface  46  of drum  22  and to transfer such removed sheets to media output  34 . In one embodiment, media eject  32  includes claw  54  (or several claws  54  aligned along the axis of drum  22 , or at regular or irregular intervals along the width of sheets  50 ) extending into close proximity with surface  46 , wherein the claw  54  assists in lifting and guiding a sheet away from surface  46 . In one embodiment, claw  54  is movable between a media ejecting position (shown) and a retracted position. In the retracted position, claw(s)  54  is separated from surface  46 , permitting the sheet  50  to pass media eject  32  and to continue to be transported about axis  44  for further printing or other interaction. In other embodiments, claw(s)  54  may be stationarily retained in the ejecting position wherein sheets  50  are not permitted to make multiple passes. In other embodiments, claw(s)  54  may be omitted where other mechanisms are used to separate sheet  50  from surface  46  of drum  22 . In one embodiment, such separation of sheet  50  from surface  46  may further be assisted by pressurized air provided through ports (not shown) along surface  46   
     Media output  34  comprises a mechanism or device configured to transport sheets  50  separated from drum  22  by media eject  32  to one or more locations for further interaction with such removed sheets or for output to a user of printing system  20 . For example, in one embodiment, media output  34  may be configured transport such ejected sheets of media to a duplexer and back to media input  28  for two sided printing. In still other embodiments, media output  34  may be configured to transport such ejected sheets for receipt by a user of printing system  20 . 
     Load assist system  36  comprises one or more components configured to selectively urge a sheet  50  being loaded from media input  28  towards surface  46  of drum  22 . In particular, load assist system  36  assists bending of the sheet  50  towards surface  46  about axis  44  such that gaps or spaces between sheet  50  and surface  46  are reduced and such that the sheet is more securely retained against surface  46 . For example, in embodiments were media hold-down mechanism  24  utilizes a vacuum to hold sheets  50  against surface  46 , load assist system  36  reduces potential leaks along the edges of sheet  50  such that the sheet  50  is held with a greater vacuum force. In embodiments where media hold-down mechanism utilizes electrostatic forces, a greater area of sheet  50  is electrostatically held against surface  46 . Because load assist system  36  is selectively movable with respect to drum  22 , system  36  may be moved further away and out of engagement with surface  46  when not being used to reduce wear against surface  46  and to permit sheets  50  to move past system  36  during a second or subsequent pass with a reduced likelihood of system  36  smearing or undesirably contacting deposited material upon sheet  50 . 
     Load assist system  36  includes guide  58  and actuation mechanism  60 . Guide  58  (schematically shown) comprises a structure movable between the first media pressing position (shown in solid lines) and a second retracted position (shown in broken lines). In the media pressing position, guide  58  is configured to press or urge sheet  50  towards surface  46 . In one embodiment, guide  58  is configured to contact a face of sheet  50  such that the opposite face of sheet  50  is in contact with face  46 . In another embodiment, guide  58  may alternatively be configured to move sheet  50  into close proximity to surface  46 , wherein media hold-down mechanism  24  further draws sheet  50  into contact with surface  46  and out of contact with guide  58 . In particular embodiments, guide  58  may be movable between various media pressing positions which have different proximities to surface  46  depending upon a sensed or input thickness or stiffness of sheet  50 . 
     In the retracted position, guide  58  is retracted or withdrawn from surface  46 . Guide  58  is retracted further from surface  46  than when guide  58  is in the media pressing position. In one embodiment, guide  58  is spaced from surface  46  in the retracted position by a distance sufficient such that media held against surface  46  and having a thickness of up to about 1 mm may pass beneath guide  58  without being contacted by guide  58 . In one embodiment, in the retracted position, guide  58  permits access to media input  28  or drum  22  to facilitate removal of media sheet jams or correction of other issues. For example, in one embodiment, guide  58 , in the retracted position, is spaced from surface  46  by a distant radially extending from axis  44  by a distance of at least 30 mm. In one embodiment, guide  58  is movable between a plurality of different retracted positions spaced from surface  46  by different extents. For example, guide  58  may be movable between a first retracted position in which guide  58  is ready to be quickly actuated to the media pressing position just prior to receipt of sheet  50 , a second retracted position sufficiently spaced from surface  46  such that a first media sheet  50  at a first thickness may pass without contact with guide  58 , a third retracted position sufficiently spaced from surface  46  such that a second media sheet  50  having a second greater thickness may pass without contact with guide  58  and a fourth retracted position sufficiently spaced from surface  46  permitting media jams to be cleared or access to portions of drum  22  or media input  28 . 
     In the particular example illustrated, guide  58  pivots between the media pressing position and the retracted position. In other embodiments, guide  58  may alternatively linearly or arcuately move along surface  46 , wherein one or more cams or ramps are employed for moving (raising and lowering) guide  58  towards and away from surface  46  in response to such linear or arcuate translation. 
     Actuation mechanism  60  comprises a mechanism configured to move guide  58  between the one or more media pressing positions and the one or more retracted positions. In one embodiment, actuation mechanism  60  is configured to selectively pivot guide  58  about axis  62 . As noted above, in other embodiments, actuation mechanism  60  may alternatively be configured to raise or lower guide  58  with respect to surface  46  in other fashions. 
     According to one embodiment, actuation mechanism  60  is configured to move guide  58  between the media pressing position and the retracted position based upon an angular positioning of circumstantial surface  46 . In one embodiment, actuation mechanism  60  actuates or moves guide  58  based upon control signals received from controller  38  which are based upon sensed rotation of drum  22 . For example, in one embodiment, encoders or sensors may be associated with drum  22  or drum drive  26 , wherein controller  38  generates control signals controlling the operation of actuation mechanism  60  based upon such sensed values. In another embodiment, actuation mechanism  60  may be directly connected to a sensor which senses angular positioning of drum  22 . In still another embodiment, actuation mechanism  60  may include a cam follower engaged with a cam that rotates with the rotation of drum  22 , wherein actuation mechanism  60  moves guide  58  based upon that interaction of the cam and the cam follower. 
     Controller  38  comprises one or more processing units configured to generate control signals directing the operation of media hold-down mechanism  24 , drum drive  26 , media input  28 , printing mechanism  30 , media eject  32 , media output  34  and actuation mechanism  60 . In embodiments where actuation mechanism  60  is actuated directly and automatically in response to the angular positioning of drum  22 , actuation mechanism  60  may alternatively operate independently of controller  38 . 
     For purposes of this application, the term “processing unit” shall mean a presently developed or future developed processing unit that executes sequences of instructions contained in a memory. Execution of the sequences of instructions causes the processing unit to perform steps such as generating control signals. The instructions may be loaded in a random access memory (RAM) for execution by the processing unit from a read only memory (ROM), a mass storage device, or some other persistent storage. In other embodiments, hard wired circuitry may be used in place of or in combination with software instructions to implement the functions described. For example, controller  38  may be embodied as part of one or more application-specific integrated circuits (ASICs). Unless otherwise specifically noted, the controller is not limited to any specific combination of hardware circuitry and software, nor to any particular source for the instructions executed by the processing unit. 
     In operation, in response to receiving a print command, controller  38  generates control signals directing media input  28  to initiate loading of a sheet  50  onto drum  22 . Controller  38  further generates control signals directing drum drive  26  to rotate from  22  about axis  44 . In one embodiment, media input  28  loads sheets  50  at selected angular locations upon surface  46  or based upon angular positioning of drum  22 . As the leading edge  66  of sheet  50  is moved to position generally opposite to guide  58 , actuation mechanism  60  actuates guide  58  from a retracted position to a media pressing position. As a result, the leading edge  66  of sheet  50  is urged towards surface  46 . This may result in bending of leading edge  66  towards surface  46  and about axis  44 , Guide  58  remains in the pressing position until a trailing edge  68  of sheet  50  has been pressed and potentially bent towards surface  46 , resulting in an entire longitudinal length of sheet  50  being urged into abutting contact with surface  46 . As a result, sheet  50  is more securely retained against surface  46  by media hold-down mechanism  24 . 
     In other embodiments, actuation mechanism  60  may alternatively move guide  58  to a retracted position after the leading edge  66  has been urged against surface  46  and once again move guide  58  back to the media pressing position so as to urge trailing edge  68  against surface  46 . In still other embodiments, actuation mechanism  60  may alternatively move guide  58  to the media pressing position when encountering one of leading edge  66  or trailing edge  68 . 
     Once securely positioned against drum  22 , sheet  50  is further transported by drum  22  to printing mechanism  30 . When sheet  50  is positioned opposite to printing mechanism  30 , as detected by sensors or as determined based upon angular positioning of drum  22 , controller  38  generates control signals causing printing mechanism  30  to deposit material upon sheet  50 . 
     As further shown by  FIG. 1 , when printing upon sheet  50  is completed, controller  38  generates control signals directing media eject  32  to move claw(s)  54  to the ejecting position until such a sheet  50  is lifted from surface  46  and ultimately transported to media output  34 . If further printing upon sheet  50  is desired or if other interactions are to be performed on sheet  50 , controller  38  may generate control signals directing media eject  32  to move claw  54  to the withdrawn position, permitting sheet  50  to pass media eject  32  and to be further transported by drum  22  about axis  44 . Once ejected and transported to media output  34 , the printed upon sheet  50  is ready for receipt by a user. 
       FIG. 2  schematically illustrates printing system  120 , another embodiment of printing system  20 . Printing system  120  is similar to printing system  20  except that printing system  120  includes load assist system  136  and controller  138  in lieu of load assist system  36  and controller  38 , respectively. Like printing system  20 , printing system  120  further includes drum  22 , media hold-down mechanism  24 , drum drive  26 , media input  28 , printing mechanism  30 , media eject  32 , and media output  34 , each of which is shown and described with respect to system  20 . 
     Like load assist system  36 , Load assist system  136  comprises one or more components configured to selectively urge a sheet  50  (shown in  FIG. 1 ) being loaded from media input  28  towards surface  46  of drum  22 . In particular, load assist system  36  assists bending of the sheet  50  towards surface  46  about axis  44  such that gaps or spaces between sheet  50  and surface  46  are reduced and such that the sheet is more securely retained against surface  46 . For example, in embodiments were media hold-down mechanism  24  utilizes a vacuum to hold sheets  50  against surface  46 , load assist system  136  reduces potential leaks along the edges of sheet  50  such that the sheet  50  held with a greater vacuum force. In embodiments where media hold-down mechanism utilizes electrostatic forces, a greater area of sheet  50  is electrostatically held against surface  46 . Because load assist system  136  is selectively movable with respect to drum  22 , system  136  may be moved further away and out of engagement with surface  46  when not being used to reduce wear against surface  46  and to permit sheets  50  to move past system  136  during a second or subsequent pass with a reduced likelihood of system  136  smearing or undesirably contacting deposited material upon sheet  50 . 
     Load assist system  136  includes a guide  158  (described above with respect to  FIG. 1 ) and actuation mechanism  160 . Actuation mechanism  160  actuates guide  158  between one or more medium pressing positions and one or more retracted positions. In the example illustrated, actuation mechanism  160  actuates guide  158  based upon the angular positioning of drum  22 . In the example illustrated, actuation mechanism  160  actuates guide  158  based upon the angular positioning of drum  22  automatically in response to rotation of drum  22  without electronic sensing of the rotation of drum  22 . 
     As shown by  FIG. 2 , actuation mechanism  160  includes cam  170 , cam follower  172  and actuator  174 . Cam  170 , schematically shown, comprises one or more cam surfaces operably coupled to (and nominally directly coupled) to drum  22  so as to rotate with drum  22 . In one embodiment, cam  170  rotates about axis  44  with drum  22 . For purposes of this disclosure, the term “coupled” shall mean the joining of two members directly or indirectly to one another. Such joining may be stationary in nature or movable in nature. Such joining may be achieved with the two members or the two members and any additional intermediate members being integrally formed as a single unitary body with one another or with the two members or the two members and any additional intermediate member being attached to one another. Such joining may be permanent in nature or alternatively may be removable or releasable in nature. The term “operably coupled” shall mean that two members are directly or indirectly joined such that motion may be transmitted from one member to the other member directly or via intermediate members. 
     Cam follower  172  comprises one or more members coupled to guide  158  and configured to interact with cam  170  during rotation of cam  170  such that guide  158  automatically moves between a media pressing position and a retracted position in response to interaction between cam  170  and cam follower  172 . For example, upon engaging selected zones or portions of cam  170 , cam follower  172  transmits force to guide  158  to move guide  158  to the retracted position. Upon engaging other selected zones or portions of cam  170 , cam follower  172  will transmit force to guide  158  so as to move and retain guide  158  in the media pressing position. Such movement between the media pressing position and at least one retracted position occurs without other sensing of the rotation of drum  22 . In the particular embodiment illustrated, such movement of guide  158  occurs without other power trains or motors driving movement of guide  158 . As a result, system  120  is less complex and potentially less expensive. 
     In the particular embodiment illustrated, as shown in  FIG. 2 , cam follower  172  is selectively movable between a cam engaged state (shown in solid lines) and a cam disengaged state (shown in broken lines). In the cam engaged state, cam follower  172  is in engagement with cam  170  such that positioning of guide  158  (media pressing position or retracted position) is dependent or may be dependent upon the particular portion of cam  170  in engagement with cam follower  172 . In the cam disengaged state, cam follower  172  is out of engagement with cam  170  such that the positioning of guide  158  is also independent of cam  170 . In the particular example illustrated, when cam follower  172  is in the cam disengaged state, guide  158  is in a retracted position. As a result, regardless of the angular positioning of drum  22  and cam  170 , guide  158  is in a retracted position. In the particular embodiment illustrated, cam follower  172  is further configured such that cam follower  172  may be moved to one of a plurality of cam disengaged states wherein guide  158  also moves to one of a plurality of different retracted positions in response to movement of cam follower  172 . 
     Actuator  174  comprises a device configured to selectively actuate or move cam follower  172  between the cam engaged state and at least one cam disengaged state. In the example illustrated, actuator  174  is configured to pivot a portion of cam follower  172  about axis  173 . In another embodiment, actuator  174  may be configured to move cam follower  172  in other fashions. 
     In one embodiment, actuator  174  comprises a motor and power train configured to transmit torque to cam follower  172  to move cam follower  172 . In one embodiment, actuator  174  is configured to move cam follower  172  between the states depending upon a direction of torque supplied by the motor of actuator  174 . In other embodiments, actuator  174  may alternatively be other power sources such as hydraulic, pneumatic or electrical power sources. For example, a solenoid may alternatively be used to move cam follower  172 . 
     Controller  138  is similar to controller  38  in that controller  138  comprises one or more processing units which generate control signals directing the operation of media hold-down mechanism  24 , drum drive  26 , media input  28 , printing mechanism  30 , media ejector  32 , media output  34  (shown in  FIG. 1 ) and actuation mechanism  160 . In embodiments where cam follower  172  remains in engagement with cam  170 , actuation mechanism  160  may alternatively operate independently of controller  138 . Controller  138  is different from controller  38  in that controller  138  may operate following a different set of control instructions contained in a memory. In particular, controller  138  is configured to generate control signals directing actuator  174  to move cam follower  172  between the cam engaged state and one or more cam disengaged states based upon a sensed or known angular positioning of drum  22 , or based upon other sensed information, such as the occurrence of a media jam or based upon commands received from a user. 
     In operation, according to one embodiment, controller  138  generates control signals directing actuator  174  to move cam follower  172  to a cam engaged state as or just prior to the loading of a sheet  50  by media input  28  (shown in  FIG. 1 ). Shortly thereafter, a first portion of cam  170  is rotated into engagement with cam follower  172 . Such interaction causes cam follower  172  to move and thereby causes guide  158  to move from a retracted position to a media pressing position (shown in solid lines). As a result, the sheet  50  of media is pressed towards the surface  46 , enhancing the ability of media hold-down mechanism  24  to retain a sheet  50  against surface  46 . After the trailing edge  68  of sheet  50  has passed guide  158  a different second portion of cam  170  may be rotated into engagement cam follower  172 . In other embodiments, the second portion of cam  170  is rotated into engagement with cam follower before the trailing edge  68  of sheet  50  has passed guide  58 . This interaction causes movement of cam follower  172  and responsive movement of guide  158  to a first retracted position, ready for assisting with loading of a subsequent sheet  50 . 
     In certain circumstances, it may be desirable to move a particular sheet through multiple passes across printing mechanism  30  for enhanced resolution or application of multiple overlying layers of print material. In such an example scenario, controller  138  may generate control signals causing actuator  174  to move cam follower  172  to a cam disengaged state. As a result, guide  158  remains in the retracted position as a second portion of cam  170  moves beneath cam follower  172 . Consequently, a sheet may be moved past guide  158  without interaction with guide  158 . Thus, there is a reduced likelihood of guide  158  smearing or otherwise undesirably contacting material deposited upon sheet  50 . 
     In certain circumstances, sheet  50  may become jammed or access to media input  28  or portions of drum  22  may be desired. In one embodiment, controller  138  may additionally be configured to generate control signals causing actuator  174  to move cam follower  172  to an additional retracted state in which cam follower  172  is spaced further from cam  170 . Because cam follower  172  is coupled to guide  158 , such movement of cam follower  172  results in guide  158  being moved to a retracted position even further away from surface  46 . In other embodiments, controller  138  may alternatively be configured to generate control signals causing actuator  174  to move cam follower  172  to a fewer or greater of such states or positions. 
       FIG. 3  illustrates printing system  220 , another embodiment of printing system  20 . Printing system  220  is configured to print or deposit material onto a medium supported by a drum  22 . As will be described in more detail hereafter, printing system  220  loads media onto the drum  22  such that the media is more securely retained against the drum during transport and printing. 
     Printing system  220  generally includes frame  221 , media transport drum  22 , media hold-down mechanism  24  (shown in  FIG. 1 ), drum drive  26 , media input  28 , printing mechanism  30 , media eject  32 , media output  34 , load assist system  236  and controller  238 . Drum  22 , media hold-down mechanism  24 , drum drive  26 , media input  28 , printing mechanism  30 , media eject  32  and media output  34  are each described above with respect to system  20 . Frame  221  comprises one or more structures proximate to drum  22  configured to support components of printing system  220 . Although illustrated as including two parallel plates, frame  221  may have various other sizes and configurations and may support fewer or additional components of printing system  220 . 
     Load assist system  236  is similar to load assist system  136  shown and described with respect to  FIG. 2 . Load assist system  236  includes guide bar  257 , guide  258  and actuation mechanism  260 . Guide bar  257  comprises an elongate shaft, rod or cylinder operably coupled between actuation mechanism  260  and guide  258 . Guide bar  257  supports guide  258  such that rotation of guide bar  257  results in pivoting of guide  258 . In the particular embodiment illustrated, guide bar  257  is further configured to assist in engaging and directing a sheet of media towards surface  46  prior to the sheet of media being encountered by guide  258 . In one embodiment, guide bar  257  is spaced from circumferential surface  46  by less than or equal to about 2 mm. 
     Like guide  58 , guide  258  comprises a structure movable between the first media pressing position (shown in  FIGS. 7-9 ) and one or more retracted positions (shown in FIGS.  4  and  10 - 13 ). As shown in  FIG. 3 , in the particular embodiment illustrated, guide  258  continuously and without interruption extends axially across substantially an entirety of those portions of surface  46  adapted to bear against, contact and support a medium. In one embodiment, guide  258  continuously and without interruption axially extends across substantially all of surface  46  and drum  22 . In other embodiments, guide  258  may extend across lesser portions of drum  22 . In other embodiments, guide  258  may include multiple segments or fingers axially spaced from one another along drum  22 . In such an embodiment, the multiple segments or fingers could be independently actuated. 
     According to one embodiment, guide  258  comprises a band or strip of material configured to flex upon engaging a sheet of medium which is also in contact with surface  46 . As a result, guide  258  assumes a slightly bent or arcuate shape to conform to surface  46  which is curved. Consequently, a greater portion of a sheet  50  may be concurrently contacted by both surface  46  and guide  58  for enhanced pressing of the sheet against surface  46 . In other embodiments, guide  258  may be inflexible. 
     According to one embodiment, guide  258  comprises a strip of thin sheet metal. In other embodiments, guide  258  may comprise other somewhat flexible materials. As shown in  FIG. 3 , portions of guide  258  proximate to actuation mechanism  260  are stiffened by one or more stiffening supports  300 . In other embodiments, such supports  300  may be omitted. 
     Actuation mechanism  260  actuates or moves guide  258  between the media pressing position and one or more retracted positions. In particular, actuation mechanism  260  moves guide  258  based upon angular positioning of drum  22 . Like actuation mechanism  160  (shown in  FIG. 2 ), actuation mechanism  260  includes cam  270 , cam follower  272  and actuator  274 . 
     Cam  270  comprises one or more cam surfaces directly coupled to drum  22  so as to rotate with drum  22  about axis  44 . In the example illustrated, cam  270  is mounted to drum  22  on an axial end of drum  22 . In other embodiments, cam  270  may be joined to drum  22  in other fashions or may be integrally formed as part of a single unitary body with drum  22 . In still other embodiments, cam  270  may be provided at other locations with respect to drum  22 . 
     As shown by  FIG. 3 , cam  270  comprises a substantially circumferential surface axially extending from surface  46 . Cam  270  includes zones or portions  304  and zones or portions  306  (sometimes referred to as the actuation zones). Portions  304  and  306  are configured to interact with cam follower  272 . Portion  304  is configured to interact with cam follower  272  such that guide  258  is moved to or retained in a retracted position. Portion  306  is configured to interact with cam follower  272  such that guide  258  is moved to or retained in a media pressing position. In the example illustrated, portion  304  comprises generally continuous convex arcuate portions extending between portions  306 . Portion  306  comprise craters or cavities into which cam follower  272  moves when encountering portions  306 . 
     In the particular example embodiment illustrated, portion  306  are located so as to overlap and extend adjacent to those portions of surface  46  against which the leading edge and the trailing edge of a sheet are to be loaded. According to one embodiment, portion  306  are located such that guide  258  is moved to the media pressing position so as to contact surface  46  approximately 6 mm ahead of an oncoming leading edge  66  of a sheet  50  (shown in  FIG. 1 ). In the particular example illustrated, each portion  306  has a circumferential length of approximately 2.6 cm. In other embodiments, portions  306  and portions  304  may have other circumferential extents. In addition, cam  270  may have a greater or fewer of such portions  306 . 
     Cam follower  272  interacts with cam  270 . In the example illustrated, cam follower  272  comprises a rotational tire or wheel  310  which rolls against cam  270  during rotation of drum  22  and cam  270 . In other embodiments, cam follower  272  may comprise other structures configured to slide or bear against cam  270  during rotation of drum  22 . Cam follower  272  is operably coupled to guide  258  such that movement of cam follower  272  may also result in movement of guide  258 . 
     Actuator  274  comprises a device configured to move cam follower  272  between a cam engaged state and one or more cam disengaged states. In the embodiment illustrated, when actuator  274  moves cam follower  272  between different cam disengaged states, actuator  274  also moves guide  258  between different retracted positions. Actuator  274  moves cam follower  272  between the cam engaged state and one or more cam disengaged states in response to control signals from controller  238 . 
       FIGS. 4-6  illustrate portions of system  220 , including cam follower  272  and actuator  274 , in detail. As shown by  FIGS. 4-6 , cam follower  272  includes wheel  310 , arm  312  and pin  314 . Wheel  310  rotates along cam  270  and is rotationally supported by arm  312 . Arm  312  is rotationally journaled to frame  221  so as to rotate about axis  262 . As shown by  FIG. 5 , arm  312  is connected to guide bar  257  which is connected to guide  258 . Arm  312  further includes a support surface  316  in abutment with stiffening support  300 . As a result, arm  312  may apply a greater torque to media guide bar  257  to pivot guide  258  about axis  262 . Pin  314  projects from arm  312  and interacts with portions of actuator  260 , facilitating movement of arm  312  and wheel  310  of cam follower  272  between the cam engaged state and cam disengaged states. 
     Actuator  274  comprises a mechanism configured to selectively move wheel  310  of cam follower  272  between the cam engaged state and cam disengaged states. In the particular example illustrated, actuator  274  includes rotational structure  320 , bias  322 , drive train  324  and rotary actuator  326 . Rotational structure  320  comprises a structure rotationally coupled to frame  221  and journaled guide bar  257  along an axis  262 . In other words, rotational structure  320  is configured to rotate about axis  262  relative to guide bar  257  and relative to frame  221 . Rotational structure  320  is configured to be operably coupled to rotary actuator  326  so as to be rotationally driven in either direction about axis  262  by rotary actuator  326 . In the particular embodiment illustrated, rotational structure  320  includes a gear having teeth  323  configured to be in engagement with drive train  324 . In other embodiments, rotational structure  320  may be configured to be operably coupled to rotary actuator  326  in other fashions. For example, in other embodiments, rotational structure  320  may comprise a pulley, wherein torque is transmitted to rotational structure  324  by a belt or rotational structure  320  may comprise a sprocket, wherein torque is transmitted to rotational structure  320  by a chain. In still other embodiments, rotational structure  320  may be rotationally driven in either direction by means of a linear actuator pivotally connected to a portion of rotational structure  320 . In yet another embodiment, the rotational structure may be driven by a connecting rod, such as is used with servo motors. 
     As shown by  FIG. 4 , rotational structure  320  includes a detent in the form of a slot  330  receiving pin  314 , serving as a projection. Slot  330  is sufficiently sized such that arm  312  of cam follower  272  may pivot about axis  262  when wheel  310  is in engagement with portion  306  a sufficient extent to rotate media guide bar  257  and guide  258  about axis  262  to press a sheet  50  a sufficient distance towards surface  46  of drum  22 . In one embodiment, slot  330  extends approximately 30 degrees about axis  262  and has a length of about 8 mm while portions  306  have a depth of about 7 mm. In other embodiments, these dimensions may vary. 
     In addition to permitting pivoting arm  312  and wheel  310  when cam follower  272  is in the cam engaged state as shown in  FIG. 4 , slot  330  further provides a driving surface  332  configured to abut and engage pin  314  upon rotation of rotational structure  320  in a counter-clockwise direction (as seen in  FIG. 4 ). During such rotation, surface  332  drives pin  314  to pivot arm  312  and wheel  310  of cam follower  272  about axis  262  to move cam follower  272  to a cam disengaged state. Depending upon the extent to which rotational structure is rotated in the counter-clockwise direction as seen in  FIG. 4 , wheel  310  may be pivoted to a multitude of different cam disengaged states. Likewise, guide  258  may also be moved to any of a multitude of retracted positions having different spacings with respect to surface  46 . 
     Although rotational structure  320  is illustrated as including a slot  330  receiving pin  314 , in other embodiments, arm  312  and rotational structure  320  may be similarly engaged in other fashions. For example, arm  312  may alternatively include a slot while rotational structure  320  includes a pin. In another embodiment, arm  312  and rotational structure  320  may alternatively include mutually opposing surfaces, such as mutually opposing tabs. In still other embodiments, arm  312  and rotational structure  320  may have other configurations providing similar interactions. 
     As shown by  FIGS. 5 and 6 , bias  322  comprises a biasing structure configured to resiliently urge arm  312  towards a cam engaged state. In the particular embodiment illustrated, bias  322  comprises a torsion spring coupled between rotational structure  322  and pin  314 . In other embodiments, bias  322  may comprise other resiliently biasing mechanisms or springs. 
     Drive train  324  is configured to transmit torque from rotary actuator  326  to rotational structure  320 . In the particular embodiment illustrated, drive train  324  includes a worm gear  334 , a helical gear  336  and a spur gear  338 . Worm gear  334  is connected to an output shaft of rotary actuator  326 . Helical gear  336  is in engagement with worm gear  334  and is fixed to rotate with spur gear  338 . Spur gear  338  is in engagement with teeth  323  of rotational structure  320 . In other embodiments, drive train  324  may have other configurations such as belt and pulley or chain and sprocket arrangements depending upon corresponding alternative configurations of rotational structure  320 . 
     Rotary actuator  326  comprises a source of torque for driving worm gear  334 . In the embodiment illustrated, rotary actuator  326  comprises an electric motor. Rotary actuator  326  is configured to selectively provide torque in either direction in response to control signals from controller  238 . 
     Controller  238  is similar to controller  138 . Controller  238  comprises one or more processing units configure to generate control signals directing the operation of media hold-down mechanism  24  (shown in  FIG. 1 ), drum drive  26 , media input  28 , printing mechanism  30 , media eject  32 , media output  34  and actuator  274  of load assist system  236 . 
     FIGS.  4  and  7 - 13  illustrate operation of load assist system  236 .  FIG. 4  illustrates load assist system  236  in a media guide activated state, wherein cam follower  272  is in the cam engaged state and wherein guide  258  is in a retracted position. In the follower releasing state of rotational structure  320  illustrated in  FIG. 4 , surface  332  is out of contact with pin  314  and is spaced from surface pin  314  such that arm  312  is free to rotate in a clockwise direction as seen in  FIG. 4  upon wheel  310  dropping into portion  306  of cam  270 . 
       FIGS. 7-9  illustrate repositioning of cam  270  such that guide  258  is moved to the media pressing position. As shown by  FIG. 7 , sheet  50  is loaded onto drum  22  at a location such that wheel  310  of cam follower  272  engages portion  306  just prior to arrival of leading edge  66  of sheet  50 . As a result of wheel  310  engaging portion  306 , bias  322  urges arm  312  and guide  258  in a clockwise direction as seen in  FIG. 7  to move guide  258  towards surface  46 . In one embodiment, cam follower wheel  310  engages portion  306  and lowers guide  258  into contact with surface  340  approximately 6 mm ahead of an oncoming sheet  50  as seen in  FIG. 8 . In the embodiment illustrated, wheel  310  remains in portion  306  for approximately 1 inch after leading edge  66  of sheet  50  has passed tip  340  of media guide  258 . In other embodiments, these relationships may be varied. 
     As shown in  FIG. 9 , in those embodiments in which surface  46  includes vacuum ports  350  (one of which is shown) having a circumferential length L and through which a vacuum is applied by a vacuum source  352  (schematically shown), guide  258  has a tip  340  separated from axis  262  by distance greater than or equal to a circumferential length L. In one embodiment, the combination of media guide bar  257  and guide  258  force the first 32 mm of sheet  50  into contact with surface  46 . As a result, sheet  50  is pressed against and across port  350  so as to seal the channel or port  350  below surface  46 . This enhances the hold-down vacuum force created by vacuum source  352  and ports  350 . As a result, sheet  50  is more securely retained and held against surface  46  after loading. 
       FIGS. 10 and 11  illustrate actuator  274  moving cam follower  272  to a first cam disengaged state. In particular, as shown in  FIG. 10 , controller  238  (shown in  FIG. 3 ) generates control signals directing rotary actuator  326  (shown in  FIG. 6 ) to drive or rotate rotational structure  320  counter-clockwise from the follower releasing state shown in  FIG. 4  to the follower driving state shown in  FIG. 10 . When in the follower driving state in which surface  332  contacts pin  314 , rotation of rotational structure  320  in the counter-clockwise direction rotates arm  312  in the counter-clockwise direction about axis  262 . As a result, wheel  310  is lifted to a cam disengaged state. This also results in arm  312  engaging and pivoting guide  258  about axis  262  to a first retracted position. As shown by  FIG. 10 , cam follower wheel  310  remains relatively close to cam  270 , ready to quickly engage upon loading of another sheet  50 . As shown by  FIG. 1 , in the first retracted position, guide  258  is sufficiently spaced from surface  46  such that sheet  50  may pass beneath guide  258  without contact with guide  258 . 
       FIGS. 12 and 13  illustrate actuator  274  moving cam follower  272  and media guide  258  to a second retracted position spaced further from surface  46  or other user interactions. In particular, controller  238  (shown in  FIG. 3 ) generates control signals causing Rotary actuator  326  to drive rotational structure  320  further counter-clockwise from the position shown in  FIG. 10  to the position shown in  FIG. 12 . Such rotation moves wheel  310  and guide  258  further away from surface  46 , facilitating paper or media jam removal. To return cam follower  272  to the first cam disengaged state or the cam engaged state described above and to also move guide  258  towards the first retracted position or towards a media pressing position, controller  238  generates control signals causing rotary actuator  326  to drive rotational structure  320  in a clockwise direction. 
     Although the present disclosure has been described with reference to example embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the claimed subject matter. For example, although different example embodiments may have been described as including one or more features providing one or more benefits, it is contemplated that the described features may be interchanged with one another or alternatively be combined with one another in the described example embodiments or in other alternative embodiments. Because the technology of the present disclosure is relatively complex, not all changes in the technology are foreseeable. The present disclosure described with reference to the example embodiments and set forth in the following claims is manifestly intended to be as broad as possible. For example, unless specifically otherwise noted, the claims reciting a single particular element also encompass a plurality of such particular elements.