Abstract:
Printer ( 1 ) feeds paper or other media sheets ( 5 ) to pinch rollers ( 9   a,    9   b ), which further feed the sheets into an imaging station ( 7 ). Pressure rollers ( 9   b ) are held by plate  22  which is biased by a spring ( 33 ) and pivots on extensions ( 37   a   , 37 ) to moves pressure rollers ( 9   b ) into nip relationship with nip roller ( 9   a ). To evenly distribute the forces across pressure rollers ( 9   b ), plate  22  is weakened by depressed lines ( 27   a,    27   b ) located between the spring and the pressure rollers ( 9   b ).

Description:
TECHNICAL FIELD  
       [0001]     This invention relates to apparatus to apply pressure evenly to rollers forming a nip with a drive roller for sheet feed, such as in printers and other imaging devices,  
       BACKGROUND OF THE INVENTION  
       [0002]     Advancing of paper and other sheets through two rollers forming a nip is highly conventional in printers and other imaging devices. Typically one of the rollers is driven from a motor and the second roller turns freely while it applies pressure to keep the rollers nipped against the sheets. Typically, the pressure rollers are supported by cantilevered plate or the like which is rotated toward the nip by a spring. It is known to bevel or weaken the ends of rollers to reduce damaging forces.  
         [0003]     It has been discovered in accordance with this invention that the pressure roller plate may induce significant pressure differences on the pressure roller along it length. The tends to skew sheets being fed. When the second or driven roller is compliant enough to absorb differences in a pressure roller pressures, along its length, such pressure differences have been tolerable. However, reliable, forward sheet feeding can be improved by eliminating such differences. This permits the drive roller to be less compliant, which contributes to the reliable sheet feeding.  
         [0004]     The pressure roller plate of the disclosed embodiment of this invention in general form and in primary function is essentially the same as pressure plates previously employed in printers constituting prior art to this invention. This invention differs in the plate having different bending characteristics between the pressure application point and the part of the plate at which rollers are supported.  
       DISCLOSURE OF THE INVENTION  
       [0005]     This invention includes a pressure plate for supporting one or more pressure rollers. The pressure plate connects to a spring at one location and has pivot locations which contact frame members at opposite locations spaced from the spring-connection location. The plate is weakened at location intermediate the spring-connection location and locations supporting the pressure roller or rollers.  
         [0006]     In an embodiment, the plate is generally rectangular, with a spring-connection location generally central on one side (the spring connection side). Two pivot surfaces are somewhat forward of the spring-connection and are spaced from the spring connection to locations generally near the two sides of the plate leading away from the spring connection side (the lateral sides of the plate). Two rollers are held on the side of the plate opposite the spring-connection location, each roller being positioned generally from the center of the plate to the lateral side of the plate. The two rollers are held in resilient, C-shaped elements partially surrounding their shafts to be parallel to the roller holding side. The plate is of a single material which has a thinned portion beginning generally near the center and connecting to the sides leading from the spring connection. In an embodiment this weakened part is a depression located from generally near the center of one side to generally near the far ends of the other sides in a somewhat V shape when viewed from the top. 
     
    
     DESCRIPTION OF THE DRAWINGS  
       [0007]     The details of this invention will be described in connection with the accompanying drawings, in which  
         [0008]      FIG. 1  is illustrative of a printer or other imaging device employing this invention,  
         [0009]      FIG. 2  is a perspective view from the right rear showing one pressure plate as mounted with associated frame parts and the nip rollers,  
         [0010]      FIG. 3  is a perspective view from the left rear showing one pressure plate a mounted with associated frame parts and the spring for providing nip force,  
         [0011]      FIG. 4  is a cross-section from the left side showing a pivot member and a part of the depressed area,  
         [0012]      FIG. 5  is a top view of an embodiment of the pressure plate,  
         [0013]      FIG. 6  shows actually measured force distributions along the roller without the invention, and  
         [0014]      FIG. 7  shows measured actual force distributions along the roller with this invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0015]      FIG. 1  is illustrative of a printer  1  in which this invention would be employed. Printer  1  may be a standard inkjet printer in most respects. As such it has a bottle printhead  3  which jets dots of ink through nozzles not shown, which are located above a sheet  5  of paper or other media at a imaging station  7   
         [0016]     Imaging station  7  is located past nip rollers  9   a ,  9   b  (see  FIG. 2 ) which grasp paper  5  in the nip of rollers  9   a ,  9   b  and move it under printhead  3 . Nip rollers  9   a ,  9   b  are stopped normally several times to permit printhead  3  to partially image sheet  5  by moving across sheet  5  (in and out of the view of  FIG. 1 ) while expelling dots in the desired pattern. In a draft mode the number of such intermittent stops may be only two, while in a quality mode that number may be five or more.  
         [0017]     With reference to  FIG. 2  pressure roller  9   b  is held against drive roller  9   a  by pressure plate  22 . With reference to  FIG. 1  nip roller  9   a ,  9   b  push paper through the imaging station  7  where they enter exits rollers  11   a ,  11   b ,  11   c , and  11   d . This invention is to biasing the nip rollers  9   a ,  9   b  employing pressure plate  22 .  
         [0018]     The printer of  FIG. 1  has a paper tray  13  located on the bottom Tray  13  constitutes a bin in which a stack of paper or other media sheets  5  are held to be imaged. Having tray  13  located on the bottom of printer  1  permits a large stack of sheets  5  to be in the printer  1 . This spaces the tray  13  from the print stations  7 , the distance from pick roller  15   a  of tray  13  to nip rollers  9   a ,  9   b  being longer than the length of some media sheets  5  to be printed. Pick roller  15   a  is a part of an autocompensating swing mounted system  15 .  
         [0019]     A C-shaped paper guide  17  is made up of rear guide surface  17   a  and spaced, generally parallel, front guide surface  17   b . Both surfaces have spaced ridges (not shown), as is common. Guide  17  directs a sheet  5  to nip rollers  9   a ,  9   b . Intermediate in guide  17  is drive roller  19   a , which is a part of an autocompensating swing-mounted system  19 . Sensor arm  21  is moved by a sheet  5  to detect the sheet  5  at system  19 .  
         [0020]     Pick roller  15   a  at tray  13  and drive roller  19   a  combine to move sheets  5  from tray  13  to nip rollers  9   a ,  9   b . Drive roller  19   a  is effective to move short media into rollers  9   a ,  9   b , when pick roller  15   a  is no longer in contact with the sheet  5 .  
         [0021]     Operational control is by electronic data processing apparatus, shown as element C in  FIG. 1 . Such control is now entirely standard. A standard microprocessor may be employed, although an Application Specific Integrated Circuit (commonly known as an ASIC) is also employed, which is essentially a special purpose computer, the purpose being to control all actions and timing of printer  1 . Electronic control is so efficient and versatile that mechanical control by cams and relays and the like is virtually unknown in imaging. However, such control is not inconsistent with this invention.  
         [0022]      FIG. 2  shows the pressure plate  22  supporting pressure rollers  9   b  against a drive roller  9   a . In a full implementation of the embodiment shown, three such pressure plates  22  would be located and held across the frame shown, thereby supporting six rollers  9   b . The pressure plates would be located on each side of opening  25  through which a paper-presence sensor arm (not shown) protrudes in the completed assembly.  
         [0023]     Frame  23  where it supports plate  22  is of thin, rigid sheet metal. It has tabs  23   a  folded to form ledges on which plate  22  pivots as will be described. Plate  9   a  is a molded, generally rectangular single piece of rigid polycarbonate which is generally of at least one thickness, except at depressed lines  27   a  and  27   b . Depressed lines  27   a  and  27   b  are to weaken plate  22  for bending.  
         [0024]      FIG. 2  shows a small part of a locating ridge  29  extending through a mating slot in frame  23 .  FIG. 3  shows ridge  29  from the opposite side. Ridge  29  in frame  23  holds plate  22  from moving laterally with respect frame  23 . Upward extensions  31   a  and  31   b , on opposite lateral sides of plate  22 , position plate  22  against frame  23 .  
         [0025]     Spring  33  is connected between frame  23  and a spring connecting bar  35  which is generally central on the side of plate  22  spaced from the side holding rollers  21 . Also shown in  FIG. 3 , but one being best shown in  FIG. 4 , are rounded, pivot extensions  37   a ,  37   b . These contact tabs  23   a  of frame  23 . Pivot extensions  37   a ,  37   b  are located on opposite sides of spring connecting bar  35  near the lateral ends of the spring connecting side of plate  22 .  
         [0026]      FIG. 4 , being a cross-section, shows one location of depression line  27   a .  FIG. 4  shows C-shaped, resilient holding part  41   a  which holds a shaft of roller  9   a  by flexing to receive the shaft as is entirely standard. Corresponding part shown are labeled as  41   b  and  41   c  in  FIG. 5 .  
         [0027]      FIG. 5  is a top view of plate  22  alone and therefore shows lines  27   a  and  27   b  extending from the roller holding side of plate  22  to the lateral sides of plate  22 . Ribs  39   a ,  39   aa ,  39   aaa ,  39   b ,  39   bb ,  39   bbb , as well as central, generally semicircular raised part  41  are for stiffening. Such subject matter is not generally novel with respect to this invention. Depressed channels  27   a ,  27   b  are for weakening and are novel with this invention.  
         [0028]     This invention recognizes that spring  33  operates to bow the center of plate  22  upward, thereby providing the highest forces of the lateral sides of plate  22 . By weakening plate  22  between the center of plate  22  and the roller holding part, the plate tends to flatten, which more evenly distributes the forces on rollers  9   b.    
         [0029]      FIG. 6  and  FIG. 7  show actual measurements of two comparable systems, one,  FIG. 7 , employing this invention and one,  FIG. 6 , not employing this invention. The horizontal axis of each is in nominal, linear units which extend across the entire length of a roller comparable to roller  9   b . The vertical axis of each is in nominal, linear units of measured force. The improved distribution of forces shown in  FIG. 7  is very evident.  
         [0030]     Clearly, the pressure plate can take many forms which are pivoted by spring action or the like and which are subject to being bowed by that spring action. The weakening can be in any manner including employing more still material near the spring connection and less stiff material between the more stiff material and the pressure rollers.