Abstract:
A lead screw assembly ( 251 ) for preventing axial movement of a lead screw ( 250 ) within a write engine system ( 200 ) includes a threaded shaft ( 252 ) having a ball end ( 263 ) and a first member attached to the ball end ( 263 ). A second member is arranged to be magnetically attracted to the first member and spaced apart from the first member so as to prevent mechanical friction between the first and second members. The first and second members prevent substantial axial movement of the threaded shaft ( 252 ) while it rotates. The first member may comprise a magnet insertably attached to the ball end ( 263 ) such that the ball end ( 263 ) is annularly surrounded by the first member. An end cap ( 268 ) may be attached to provide an axial-stop for the lead screw ( 250 ).

Description:
FIELD OF THE INVENTION  
         [0001]    The invention relates to image processors in general and in particular to image processors utilizing a rotating lead screw for moving a printhead. More particularly, the invention relates to an improvement in the performance, quality and cost of such a lead screw assembly. Still more particularly, the invention relates to an improved lead screw assembly that substantially minimizes shifting or movement.  
         BACKGROUND OF THE INVENTION  
         [0002]    Pre-press color proofing is a procedure long used by the printing industry for creating representative images of printed materials in an effort to lessen the high cost and time required to produce printing plates and to set up a high-speed, high volume, printing presses.  
           [0003]    One such commercially available image processor, as depicted in U.S. Pat. No. 5,268,708, includes half-tone capabilities. Such printing systems are able to form an image on a sheet of thermal print media (TPM) in which dye from a sheet of donor material is transferred to the TPM by applying an adequate amount of thermal energy to the dye material. Generally, the processor is comprised of a material supply carousel and a lathe bed engine writing system. The write engine itself includes an engine frame, translation drive, translation stage member, write-head, image drum and exit port for the TPM and the dye donor sheets.  
           [0004]    In operation, sheets of TPM and dye donor material are transported from the materials carousel and peripherally wrapped around the imaging drum. Once secured, a print engine provides the printing function by exposing the TPM and dye donor material while it rotated past the printhead by means of the rotating imaging drum. The translation drive then traverses the printhead is fixed onto a translation member, axially along the axis of the image drum and in a coordinated motion with the spinning drum. Inevitably, these movements combine to produce the intended image on the thermal print media. The processor repeats these step over again but with different colored dye donor sheets in order to produce the desired image. Once complete, both the TPM and the dye donor sheets are removed from the image drum and transported to their respective external holding trays.  
           [0005]    To allow for movement of the printhead along the imaging drum, the translation stage with the printhead mounted thereon may be coupled to a lead screw nut which in turn is attached to a lead screw having a threaded shaft. An example of such a lead screw assembly is described and disclosed in U.S. Pat. No. 5,771,059, the entirety of which is incorporated herein by reference. The lead screw rests between the two sides of the write engine frame and is supported by a ball and bearing socket and a radial bearing at the drive end. The drive end of the lead screw continues through the radial bearing and is connected to the drive motor that provides rotation of the lead screw.  
           [0006]    A problem associated with such lead screw assemblies is the tolerance between the lead screw and the bearing socket in which it fits. An increased tolerance between the end of the lead screw, which is usually a ball, and the mounting socket could result in the ball releasing from the mounting socket. Alternatively, the epoxy holding the ball in the mounting socket, if assembled improperly, can stick on the ball causing interference with the bearing pocket. This may lead to unwanted axial lateral shifting or movement of the lead screw assembly resulting in an image defect. Other problems include improper seating within the mounting socket or loss of the bond holding the lead screw within the mounting socket.  
           [0007]    Accordingly, a need exists for an improved lead screw assembly that eliminates the problems associated with shifting or movement of the lead screw.  
         SUMMARY OF THE INVENTION  
         [0008]    It is the object of the present invention to provide an improved lead screw assembly.  
           [0009]    Another object of the present invention is to provide a ball end lead screw assembly that overcomes one or more of the problems set forth above.  
           [0010]    Still another object of the present invention is to provide a lead screw assembly that eliminates shifting or movement of the lead screw within the write engine.  
           [0011]    As such, disclosed in one embodiment is an improved lead screw assembly. The lead screw assembly comprises a threaded shaft having a ball end and a first member attached to the ball end. A second member is arranged to be magnetically attracted to the first member and spaced apart from the first member so as to prevent mechanical friction between the first and second members. The first and second members prevent substantial axial movement, shatter, or vibration of the threaded shaft while it rotates.  
           [0012]    The lead screw assembly may further comprise a write engine frame adapted for housing the threaded shaft such that the shaft is firmly secured as it rotates. A motor is mounted on the engine frame, the motor having an output shaft attached to the opposite end of the threaded shaft and adapted for rotating the threaded shaft.  
           [0013]    The first member may comprise a magnet mounted and attached to the ball end such that the ball end is annularly surrounded by the first member. Also, an end cap may be attached to the frame such that the cap provides an axial-stop for the lead screw. In one embodiment, the end cap comprises a circular flat surface and a shaped circular surface opposite the flat surface such that the shaped circular surface is adapted for receiving the ball end of the shaft and eliminate axial movement the shaft as it rotates.  
           [0014]    Further disclosed is a print engine system having an improved lead screw assembly for improving an image generating process. The system comprises a print head and a lead screw nut coupled to the print head by means of a translation stage. A threaded shaft is insertably coupled to the lead screw nut and adapted to cause the lead screw nut to move the print head mounted on the translation stage axially along the threaded shaft. The print head is substantially stabilized as the nut moves axially along the shaft while the print head generates an image.  
           [0015]    The system may also comprises a write engine frame adapted for housing the threaded shaft such that the shaft is firmly secured as it rotates. If so configured, a motor is provide and is mounted on the engine frame, the motor having an output shaft attached to the opposite end of the threaded shaft. The motor is adapted for rotating the threaded shaft.  
           [0016]    The threaded shaft may further comprise a ball end and a first member mounted to and attached to the ball end. A second member is magnetically attracted to the first member and spaced apart from the first member so as to prevent mechanical friction between the first and second members while the shaft rotates.  
           [0017]    According to one embodiment, the second member may further comprise an end cap attached to the write engine frame for providing an axial stop for the thread shaft. The end cap may further comprise a circular flat surface and a shaped circular surface opposite the flat surface. The shaped surface is adapted for receiving the ball end of the shaft so as to substantially diminish the axial movement of the shaft as it rotates.  
           [0018]    The invention can be used in any image processing apparatus that uses thermal print media and dye donor materials or other similar materials using colorant.  
           [0019]    An advantage of the present invention is that it simplifies the manufacture the lead screw assembly.  
           [0020]    Another advantage of the present invention that it provides a better quality lead screw assembly.  
           [0021]    Still another advantage of the present invention that it provides a lower cost lead screw assembly.  
           [0022]    Although not described in detail, it would be obvious to someone skilled in the art that this invention could be used in other imaging applications where a lead screw is used for printhead positioning. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]    [0023]FIG. 1 is a side view in vertical cross section of an image processing apparatus in which the improved lead screw assembly of the present invention may be used.  
         [0024]    [0024]FIG. 2 is perspective view of the lathe bed scanning subsystem or write engine of the present invention.  
         [0025]    [0025]FIG. 3 is a top view in horizontal section of a prior art lead screw assembly.  
         [0026]    [0026]FIG. 4 is a top view in horizontal section of the lead screw assembly according to the present invention. 
     
    
       [0027]    References in the detailed description correspond to like references in the figures unless otherwise indicated.  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0028]    Referring to FIG. 1, therein is illustrated an image processing apparatus  10  according to the present invention having an image processor housing  12  which provides a protective cover. The hinged image processor door  14  is attached to the front portion of the image processor housing  12  permitting access to two sheet material trays consisting of a lower sheet material tray  50   a  and upper sheet material tray  50   b . The lower sheet material tray  50   a  and upper sheet material tray  50   b  are positioned in the interior portion of the image processor housing  12  for supporting thermal print media  32 , thereon. Only one of the sheet material trays will dispense the thermal print media  32  to create an intended image thereon.  
         [0029]    The alternate sheet material tray either holds an alternative type of thermal print media  32  or functions as a back up sheet material tray. In this regard lower sheet material tray  50   a  includes a lower media lift cam  52   a  used to lift the lower sheet material tray  50   a  and ultimately the thermal print media  32 , upwardly toward lower media roller  54   a  and upper media roller  54   b  which, when both are rotated, permits the thermal print media  32  to be pulled upwardly towards a media guide  56 . The upper sheet material tray  50   b  includes a upper media lift cam  52   b  for lifting the upper sheet material tray  50   b  and ultimately the thermal print media  32  towards the upper media roller  54   b  which directs it towards the media guide  56 .  
         [0030]    The movable media guide  56  directs the thermal print media  32  under a pair of media guide rollers  58  which engage the thermal print media  32  for assisting the upper media roller  54   b  in directing it onto the media staging tray  60 . The media guide  56  is attached and hinged to the write engine frame (shown in FIG. 2) at one end, and is uninhibited at its other end for permitting multiple positioning of the media guide  56 . The media guide  56  then rotates the uninhibited end downwardly, as illustrated in the position shown. The direction of rotation of the upper media roller  54   b  is reversed for moving the thermal print media  32  resting on the media staging tray  60  under the pair of media guide rollers  58 , upwardly through an entrance passageway  204  and up to the imaging drum  300 .  
         [0031]    A roll of dye donor material  34  is connected to the media carousel  100  in a lower portion of the image processor housing  12 . Typically, four rolls are used, but only one is shown for clarity. Each roll includes a dye donor material  34  of a different color, typically black, yellow, magenta and cyan, or other colorant. These dye donor materials  34  are ultimately cut into dye donor sheet materials  36  and passed to the imaging drum  300  for forming the medium from which dyes imbedded therein are passed to the thermal print media  32  resting thereon. In this regard, a media drive mechanism  110  is attached to each roll of dye donor material  34 , and includes three media drive rollers  112  through which the dye donor material  34  of interest is metered upwardly into a media knife assembly  120 . After the dye donor material  34  reaches a predetermined position, the media drive rollers  112  cease driving the dye donor material  34 .  
         [0032]    The two media knife blades  122  positioned at the bottom portion of the media knife assembly  120  cut the dye donor material  34  into dye donor sheet materials  36 . The lower media roller  54   a  and the upper media roller  54   b  along with the media guide  56  then pass the dye donor sheet material  36  onto the media staging tray  60  and ultimately to the imaging drum  300 . Once the thermal print media  32  is moved into position, a magnetic load roller (not shown) is moved into contact with thermal print media  32  against the imaging drum  300 . The imaging drum  300  has a ferrous coating that attracts the magnetic load roller to it with the magnetic load roller aligning its self to the imaging drum  300 . The imaging drum  300  is the rotated counter clock wise with the load roller engaged until the magnetic load roller is at the end of the thermal print media  32 .  
         [0033]    In operation, the imaging drum  300  is reversed until the load roller is passed the opposite end of the thermal print media  32 , and over the embedded magnets (not shown) in the imaging drum  300 . The opposing force of the embedded magnets in the imaging drum  300  and roller  350  force the load roller away from the surface of the imaging drum  300 . Once the thermal print media  32  is in place the dye donor sheet material  36  is positioned on the imaging drum  300  in registration with the thermal print media  32  using the same process as described above for loading the thermal print media  32  to the imaging drum  300 . The dye donor sheet material  36  now rests atop the thermal print media  32  with a narrow gap between the two created by micro-beads imbedded in the surface of the thermal print media  32 .  
         [0034]    A laser assembly  400  includes a quantity of laser diodes  402  in its interior, the laser diodes  402  are connected via fiber optic cables  404  to a distribution block  406  and ultimately to the printhead  500 . The printhead  500  directs thermal energy received from the laser diodes  402  causing the dye donor sheet material  36  to pass the desired color across the gap to the thermal print media  32 . As shown more clearly in FIG. 2, the printhead  500  attaches to the lead screw  250 . This is done by way of the lead screw drive nut  254  and drive coupling  256  permitting axial movement along the longitudinal axis of the imaging drum  300  for transferring the data to create the intended image onto the thermal print media  32 .  
         [0035]    For writing, the imaging drum  300  rotates at a constant velocity. The printhead  500  begins at one end of the thermal print media  32  and traverse the entire length of the thermal print media  32  for completing the transfer process for the particular dye donor sheet material  36  resting on the thermal print media  32 . After printhead  500  completes the transfer process for a dye donor sheet material  36  resting on the thermal print media  32 . The dye donor sheet material  36  is then removed from the imaging drum  300  and transferred out the image processor housing  12  via a skive or ejection chute  16 . The dye donor sheet material  36  eventually comes to rest in a waste bin  18  for removal by the user. The above described process is then repeated for the other rolls of dye donor materials  34 .  
         [0036]    After the color from all four sheets of the dye donor sheet materials  36  have been transferred. The dye donor sheet material  36  is removed from the imaging drum  300 . The thermal print media  32  with the intended image thereon is then removed from the imaging drum  300  and transported via a transport mechanism  80  out of the image processor housing  12  and comes to rest against a media stop  20 .  
         [0037]    Referring again to FIG. 2, therein is illustrated a perspective view of the write engine subsystem  200  of the image processing apparatus  10 , including the imaging drum  300 , printhead  500  and lead screw  250  mounted in the write engine frame  202 . The imaging drum  300  is mounted for rotation about an axis X in the write engine frame  202 . The printhead  500  is movable with respect to the imaging drum  300 , and is arranged to direct a beam of light to the dye donor sheet material  36 . The beam of light from the printhead  500  for each laser diode  402  is modulated individually by modulated electronic signals from the image processing apparatus  10 , which are representative of the shape and color of the original image so that the color on the dye donor sheet material  36  is heated to cause volatilization only in those areas in which its presence is required on the thermal print media  32  to reconstruct the shape and color of the original image.  
         [0038]    The printhead  500  is mounted on a movable translation stage member  220  which, in turn, is supported for low friction movement on translation bearing rods  206  and  208 . The translation bearing rods  206  and  208  are sufficiently rigid so as not sag or distort between mounting points and are arranged as parallel as possible with the axis X of the imaging drum  300  with the axis of the printhead  500  perpendicular to the axis X of the imaging drum  300  axis. The front translation bearing rod  208  locates the translation stage member  220  in the vertical and the horizontal directions with respect to axis X of the imaging drum  300 . The rear translation bearing rod  206  locates the translation stage member  220  only with respect to rotation of the translation stage member  220  about the front translation bearing rod  208  so that there is no over-constraint condition of the translation stage member  220 , which might cause it to bind, chatter, or otherwise impart undesirable vibration or jitters to the printhead  500  during the generation of an intended image.  
         [0039]    Referring to FIG. 3, a prior art lead screw assembly  251  is shown which includes an elongated, threaded shaft  252  which is attached to the linear drive motor  258  on its drive end and to the write engine frame  202  by means of a radial bearing  272 . A lead screw drive nut  254  includes grooves in its hollowed-out center portion  70  for mating with the threads of the threaded shaft  252 . Permitting the lead screw drive nut  254  axial movement along the threaded shaft  252  as the threaded shaft  252  is rotated by the linear drive motor  258 . The lead screw drive nut  254  is integrally attached to the printhead  500  through the lead screw coupling  256  (not shown) and the translation stage member  220  at its periphery such that as the threaded shaft  252  is rotated by the linear drive motor  258  moving the lead screw drive nut  254  axially along the threaded shaft  252  which, in turn, moves the translation stage member  220  and ultimately the printhead  500  axially along the imaging drum  300 .  
         [0040]    As illustrated in FIG. 3, an annular-shaped axial load magnet  260   a  is integrally attached to the driven end of the threaded shaft  252 , and is in a spaced apart relationship with another annular-shaped axial load magnet  260   b  attached to the write engine frame  202 . The axial load magnets  260   a  and  260   b  are preferably made of rare-earth materials such as neodymium-iron-boron.  
         [0041]    A generally circular-shaped boss  262  forms part of the threaded shaft  252  and rests in the hollowed-out portion of the annular-shaped axial load magnet  260   a , and includes a generally V-shaped surface  271  which forms a mounting socket for receiving a ball bearing  264 . A circular-shaped insert  266  is placed in the hollowed-out portion of the other annular-shaped axial load magnet  260   b . As shown, the insert  266  includes a circular-shaped surface  265  which forms a bearing socket at one end of the assembly  251  for receiving ball bearing  264 , and a flat surface  267  at its other end for receiving an end cap  268  placed over the annular-shaped axial load magnet  260   b , which is attached to the lathe bed-scanning frame  202  for protectively covering the annular-shaped axial load magnet  260   b  and providing an axial stop for the lead screw  250 . The circular shaped insert  266  is preferably made of material such as Rulon J or Delrin AF, both well known in the art.  
         [0042]    The lead screw assembly  251  operates as follows. The linear drive motor  258  is energized and imparts rotation to the lead screw  250 , as indicated by the arrow  1000 , causing the lead screw drive nut  254  to move axially along the threaded shaft  252 . The annular-shaped axial load magnets  260   a  and  260   b  are magnetically attracted to each other, which prevents axial movement of the lead screw  250 . The ball bearing  264 , however, permits rotation of the lead screw  250  while maintaining the positional relationship of the annular-shaped axial load magnets  260 , i.e., slightly spaced apart, which prevents mechanical friction between them while obviously permitting the threaded shaft  252  to rotate.  
         [0043]    A problem associated with prior art lead screw assemblies, such as lead screw assembly  251 , is the tolerance between the lead screw, such as lead screw  252  and the sockets in which the ball, such as ball bearing  264 , fits. An increased tolerance between the end of the lead screw and the socket could result in the ball releasing from the socket. Alternatively, the epoxy holding the ball in the socket can stick on the ball causing interference with the bearing socket. This may lead to unwanted axial lateral shifting or movement of the lead screw assembly. Other problems include improper seating or loss of the bond holding the lead screw within the socket. The present invention provides an improved lead screw assembly that eliminates these problems and is suitable for use in any imaging application where a lead screw is used for printhead positioning.  
         [0044]    Turning to FIG. 4, therein is shown the improved lead screw assembly, denoted generally as  510 , of the invention. In particular, the improved lead screw assembly  510  has an annular-shaped axial load magnet  260   a  integrally attached to the driven end of the threaded shaft  252 , which provides a first member coupled to the ball shaped boss  262  and is in a spaced apart relationship with end cap  268  attached to the write engine frame  202 . The axial load magnet  260   a  is preferably made of rare-earth materials such as neodymium-iron-boron. The generally circular-shaped boss  262  is part of the threaded shaft  252  and rests in the hollowed-out portion of the annular-shaped axial load magnet  260   a , and includes a ball end  263  for receiving end cap  268 . The end cap  268  provides a second member that couples to the boss  262  and includes a circular shaped surface  265  for receiving ball end  263  of the boss  262 , and a flat surface  267  at its other end, which is attached to the write engine frame  202 . In this way, the end cap  268  provides an axial stop for the lead screw  250 .  
         [0045]    The lead screw assembly  251  operates as follows. The linear drive motor  258  is energized and imparts rotation to the lead screw  250 , as indicated by the arrow  1000 , causing the lead screw drive nut  254  to move axially along the threaded shaft  252 . The annular-shaped axial load magnet  260   a  is magnetically attracted to end cap  268 , which prevents axial movement of the lead screw  250 . The ball end  263 , however, permits rotation of the lead screw  250  while maintaining the positional relationship of the annular-shaped axial load magnet  260  slightly spaced apart from end cap  268 , which prevents mechanical friction between them while obviously permitting the threaded shaft  252  to rotate.  
         [0046]    Therefore, the ball end  263  of the lead screw  250  is maintained within the socket provided by the circular surface  265  of end cap  268  that eliminates shifting or motion of the lead screw  250  as it rotates. The circular surface  265  can be coated with a bearing material, such as Rulon J or Delrin AF, to create a magnetic attraction between the end cap  268  and the ball end  263 . The ball end can be made of Nickel Teflon or other similar material and the lead screw assembly  510  is pre-loaded into the socket formed by circular-shaped surface  265 . The lead screw  250  may be furnished with a lubricant, such as Nickel Teflon, that has a low coefficient of friction, thereby facilitating loading of the lead screw assembly  510  and rotation of the lead screw  250 . In this way, the lead screw assembly  510  maintains a substantially uniform tolerance during positioning of the printhead  500  with less shifting or motion of the lead screw  250  and improved performance.  
         [0047]    The invention has been described with reference to the preferred embodiments thereof. It will be appreciated and understood that variations and modifications can be effected within the scope of the invention as described herein above and as defined in the appended claims by a person of ordinary skill in the. In general, the invention is applicable to any imaging apparatus that uses a lead screw for printhead positioning.  
                                             PARTS LIST                                    10.   Image processing apparatus           12.   Image processor housing           16.   Ejection chute           18.   Waste bin           20.   Media stop           32.   Thermal print media           34.   Dye donor material           36.   Dye donor sheet materials           50a.   Material tray           50b.   Material tray           52.   Media lift cam           54b.   Media roller           56.   Media guide           58.   Media guide rollers           60.   Media staging tray           70.   Center portion           80.   Transport mechanism           100.   Media carousel           110.   Media drive mechanism           112.   Media drive rollers           120.   Media knife assembly           122.   Media knife blades           200.   Write engine subsystem           202.   Write engine frame           204.   Entrance passageway           206.   Translation bearing rod           208.   Translation bearing rod           220.   Translation stage member           250.   Lead screw           251.   Lead screw assembly           252.   Threaded shaft           254.   Lead screw drive nut           256.   Coupling           258.   Linear drive motor           260a.   Axial load magnet           260b.   Axial load magnet           262.   Boss           263.   Ball end           264.   Ball bearing           265.   Circular-shaped surface           266.   Insert           267.   Flat surface           268.   End cap           271.   V-shaped surface           272.   Radial bearing           300.   Imaging drum           350.   Roller           400.   Laser assembly           402.   Laser diodes           404.   Fiber optic cables           406.   Distribution block           500.   Printhead           510.   Improved lead screw assembly           1000.   Arrow