Patent Publication Number: US-2007115319-A1

Title: Image forming apparatus, maintenance-recovery mechanism, and suction pump

Description:
TECHNICAL FIELD  
      The present specification describes an image forming apparatus, a maintenance-recovery mechanism, and a suction pump, and more particularly an image forming apparatus, a maintenance-recovery mechanism, and a suction pump for sucking a liquid from a recording head of the image forming apparatus.  
     DISCUSSION OF THE BACKGROUND  
      An image forming apparatus, such as a copying machine, a printer, a facsimile machine, or a multifunction printer including copying, printing, scanning, and facsimile functions, forms an image on a recording medium (e.g., a sheet) according to image data with an ink. The ink is discharged from a nozzle of a recording head. While the recording head moves in a main scanning direction, the recording head discharges the ink onto the sheet to form an image. After the ink is discharged from the nozzle of the recording head, a recording head maintenance mechanism, which is provided in a non-printing area where the recording head does not discharge the ink, wipes the nozzle of the recording head and sucks residue ink having an increased viscosity from the nozzle to prevent the nozzle from being clogged with the ink.  
       FIGS. 1A and 1B  illustrate a suction pump  10   r  of a recording head maintenance mechanism included in one example of a related art image forming apparatus. The suction pump  10   r  applies suction to draw an ink from a nozzle of a recording head (not shown).  FIG. 1A  is a perspective plane view of the suction pump  10   r  during a pumping operation.  FIG. 1B  is a perspective plane view of the suction pump  10   r  during a non-pumping operation. As illustrated in  FIGS. 1A and 1B , the suction pump  10   r  includes a central shaft  11   r  , a rear plate  12   r , a front plate  13   r , pressing rollers  15   ra  and  15   rb , pressing roller shafts  14   ra  and  14   rb , grooves  16   ra  and  16   rb , guides  17   ra  and  17   rb , engaging ends  18   ra  and  18   rb , engaging ends  19   ra  and  19   rb , a housing  21   r , and a tube  20   r.    
      The central shaft  11   r  supports the rear plate  12   r  and the front plate  13   r . The pressing rollers  15   ra  and  15   rb  are provided between the rear plate  12   r  and the front plate  13   r . The pressing roller shafts  14   ra  and  14   rb  support the pressing rollers  15   ra  and  15   rb , respectively. The pressing roller shafts  14   ra  and  14   rb  movably engage with the grooves  16   ra  and  16   rb , respectively. When a motor (not shown) rotates the rear plate  12   r  and the front plate  13   r  in a rotating direction R (depicted in  FIG. 1A ), the pressing roller shaft  14   ra  of the pressing roller  15   ra  moves towards the engaging end  18   ra  along the groove  16   ra  provided on the rear plate  12   r  and along the guide  17   ra  provided on the front plate  13   r . The pressing roller shaft  14   rb  of the pressing roller  15   rb  moves towards the engaging end  18   rb  along the groove  16   rb  provided on the rear plate  12   r  and along the guide  17   rb  provided on the front plate  13   r . Thus, the pressing roller shafts  14   ra  and  14   rb  contact the engaging ends  18   ra  and  18   rb  provided on one end of the guides  17   ra  and  17   rb , respectively, as illustrated in  FIG. 1A . The engaging ends  19   ra  and  19   rb  are provided on another end of the guides  17   ra  and  17   rb , respectively. A distance S from the center of the central shaft  11   r  to the engaging end  18   ra  is greater than a distance T from the center of the central shaft  11   r  to the engaging end  19   r a. Similarly, a distance from the center of the central shaft  11   r  to the engaging end  18   rb  is greater than a distance from the center of the central shaft  11   r  to the engaging end  19   rb.    
      The housing  21   r  includes a concave portion (not shown) which engages with the tube  20   r . The tube  20   r  is set in the concave portion of the housing  21   r  and conveys an ink. One end of the tube  20   r  is connected to a head cap (not shown) which caps the nozzle of the recording head to suck ink from the nozzle. Another end of the tube  20   r  is connected to a collected ink container (not shown). When the rear plate  12   r  and the front plate  13   r  further rotate in the rotating direction R while the pressing roller shafts  14   ra  and  14   rb  contact the engaging ends  18   ra  and  18   rb , respectively, the pressing rollers  15   ra  and  15   rb  alternately press the tube  20   r  towards the concave portion of the housing  21   r  while the pressing rollers  15   ra  and  15   rb  move along the tube  20   r . Namely, the pressing rollers  15   ra  and  15   rb  apply a positive pressure to the tube  20   r . When the rear plate  12   r  and the front plate  13   r  further rotate in the rotating direction R, the pressing rollers  15   ra  and  15   rb  alternately move away from the tube  20   r  and the pressure applied to the tube  20   r  is released. Namely, the pressing rollers  15   ra  and  15   rb  apply a negative pressure to the tube  20   r . Thus, a pumping operation is performed and ink is sucked into the tube  20   r . To stop the pumping operation, the rear plate  12   r  and the front plate  13   r  rotate in a rotating direction U depicted in  FIG. 1B  (i.e., a direction opposite to the rotating direction R depicted in  FIG. 1A ) so as to move the pressing roller shafts  14   ra  and  14   rb  towards the engaging ends  19   ra  and  19   rb  along the grooves  16   ra  and  16   rb  and along the guides  17   ra  and  17   rb , respectively. Thus, the pressing roller shafts  14   ra  and  14   rb  contact the engaging ends  19   ra  and  19   rb , respectively, and thereby the pressing rollers  15   a  and  15   b  do not contact the tube  20   r  while the pressing rollers  15   a  and  15   b  move along the tube  20   r . Namely, the pressing rollers  15   ra  and  15   rb  do not apply a pressure to the tube  20   r . Thus, a pumping operation is not performed.  
      When the dimension tolerance between the dimension of the pressing rollers  15   ra  and  15   rb  and the dimension of the housing  21   r  is not controlled to be within a predetermined range, the pressing rollers  15   ra  and  15   rb  do not move properly. For example, when the rear plate  12   r  and the front plate  13   r  rotate in the rotating direction R (depicted in  FIG. 1A ), the pressing rollers  15   ra  and  15   r b may move to the engaging ends  19   ra  and  19   rb , respectively. As a result, the pressing rollers  15   ra  and  15   rb  may not press the tube  20   r . When the rear plate  12   r  and the front plate  13   r  rotate in the rotating direction U (depicted in  FIG. 1B ), the pressing rollers  15   ra  and  15   rb  may not move to the engaging ends  19   ra  and  19   rb , respectively. As a result, the pressing rollers  15   ra  and  15   rb  may press the tube  20   r . Further, when the pressing rollers  15   ra  and  15   rb  alternately move away from the engaging ends  18   ra  and  18   rb , respectively, an elastic force of the tube  20   r  may bounce the pressing rollers  15   ra  and  15   rb  and an impulsive sound may be generated when the bounced pressing rollers  15   ra  and  15   rb  hit the grooves  16   ra  and  16   rb , respectively.  
     SUMMARY  
      This patent specification describes a novel image forming apparatus. One example of a novel image forming apparatus includes a recording head configured to discharge a liquid onto a recording medium and a maintenance-recovery mechanism configured to maintain and recover the recording head. The maintenance-recovery mechanism includes a cap configured to cap the recording head and a suction pump configured to suck a liquid from the recording head through the cap. The suction pump includes a tube, a housing, a pressing member, a rotating member, and a regulating member. The tube is connected at one end of the tube to the cap, and channels the liquid drawn through the cap and the one end of the tube by the suction pump and discharges the liquid at another end of the tube. The housing includes an inner wall having an arc-like shape, the inner wall holding the tube along the inner wall. The pressing member is configured to move along the tube and includes a shaft provided at a center of the pressing member. The rotating member is configured to rotate to move the pressing member and includes a groove engaged with the shaft of the pressing member to guide the pressing member between a first position at which the pressing member presses the tube towards the inner wall of the housing while the pressing member moves along the tube and a second position at which the pressing member does not press the tube. The regulating member is configured to coaxially rotate with the rotating member to regulate movement of the pressing member in a circumferential direction.  
      This patent specification further describes a novel maintenance-recovery mechanism for maintaining and recovering a recording head of an image forming apparatus. One example of a novel maintenance-recovery mechanism includes a cap configured to cap the recording head and a suction pump configured to suck a liquid from the recording head through the cap. The suction pump includes a tube, a housing, a pressing member, a rotating member, and a regulating member. The tube is connected at one end of the tube to the cap, and channels the liquid drawn through the cap and the one end of the tube by the suction pump and discharges the liquid at another end of the tube. The housing includes an inner wall having an arc-like shape, the inner wall holding the tube along the inner wall. The pressing member is configured to move along the tube and includes a shaft provided at a center of the pressing member. The rotating member is configured to rotate to move the pressing member and includes a groove engaged with the shaft of the pressing member to guide the pressing member between a first position at which the pressing member presses the tube towards the inner wall of the housing while the pressing member moves along the tube and a second position at which the pressing member does not press the tube. The regulating member is configured to coaxially rotate with the rotating member to regulate movement of the pressing member in a circumferential direction.  
      This patent specification further describes a novel suction pump for sucking a liquid from a recording head of an image forming apparatus. One example of a novel suction pump includes a tube, a housing, a pressing member, a rotating member, and a regulating member. The tube is connected at one end of the tube to the recording head, and channels the liquid drawn through the one end of the tube by the suction pump and discharges the liquid at another end of the tube. The housing includes an inner wall having an arc-like shape, the inner wall holding the tube along the inner wall. The pressing member is configured to move along the tube and includes a shaft provided at a center of the pressing member. The rotating member is configured to rotate to move the pressing member and includes a groove engaged with the shaft of the pressing member to guide the pressing member between a first position at which the pressing member presses the tube towards the inner wall of the housing while the pressing member moves along the tube and a second position at which the pressing member does not press the tube. The regulating member is configured to coaxially rotate with the rotating member to regulate movement of the pressing member in a circumferential direction.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      A more complete appreciation of the disclosure and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:  
       FIG. 1A  is a perspective plane view of a suction pump during a pumping operation;  
       FIG. 1B  is a perspective plane view of a suction pump during a non-pumping operation;  
       FIG. 2  is a schematic view of an image forming apparatus according to an exemplary embodiment;  
       FIG. 3  is a top view of an image forming unit and a bypass sheet supplier included in the image forming apparatus shown in  FIG. 2 ;  
       FIG. 4A  is a perspective plane view of a suction pump included in the image forming unit shown in  FIG. 3  during a pumping operation;  
       FIG. 4B  is a perspective plane view of a suction pump included in the image forming unit shown in  FIG. 3  during a non-pumping operation;  
       FIG. 5A  is a plane view of a rear plate included in the suction pump shown in  FIGS. 4A and 4B ;  
       FIG. 5B  is a plane view of a front plate included in the suction pump shown in  FIGS. 4A and 4B ;  
       FIG. 5C  is a plane view of a regulating plate included in the suction pump shown in  FIGS. 4A and 4B ;  
       FIG. 6A  is a perspective plane view of a suction pump included in the image forming unit shown in  FIG. 3  during a pumping operation according to another exemplary embodiment;  
       FIG. 6B  is a perspective plane view of a suction pump included in the image forming unit shown in  FIG. 3  during a non-pumping operation according to another exemplary embodiment;  
       FIG. 7  is a perspective plane view of a suction pump included in the image forming unit shown in  FIG. 3  according to yet another exemplary embodiment;  
       FIG. 8  is a perspective plane view of a suction pump included in the image forming unit shown in  FIG. 3  according to yet another exemplary embodiment;  
       FIG. 9  is a perspective plane view of a suction pump included in the image forming unit shown in  FIG. 3  according to yet another exemplary embodiment;  
       FIG. 10  is a sectional side view of a suction pump included in the image forming unit shown in  FIG. 3  according to yet another exemplary embodiment;  
       FIG. 11  is a sectional side view of a suction pump included in the image forming unit shown in  FIG. 3  according to yet another exemplary embodiment;  
       FIG. 12  is a sectional side view of a suction pump included in the image forming unit shown in  FIG. 3  according to yet another exemplary embodiment;  
       FIG. 13  is a sectional side view of a suction pump included in the image forming unit shown in  FIG. 3  according to yet another exemplary embodiment;  
       FIG. 14  is a sectional side view of a suction pump included in the image forming unit shown in  FIG. 3  according to yet another exemplary embodiment;  
       FIG. 15  is a perspective plane view of a suction pump included in the image forming unit shown in  FIG. 3  according to yet another exemplary embodiment; and  
       FIG. 16  is a perspective plane view of a suction pump included in the image forming unit shown in  FIG. 3  according to yet another exemplary embodiment. 
    
    
     DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS  
      In describing exemplary embodiments illustrated in the drawings, specific terminology is employed for the sake of clarity. However, the disclosure of this patent specification is not intended to be limited to the specific terminology so selected and it is to be understood that each specific element includes all technical equivalents that operate in a similar manner.  
      Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, in particular to  FIG. 2 , an image forming apparatus  100  according to an exemplary embodiment is explained.  
       FIG. 2  is a schematic view of the image forming apparatus  100 . As illustrated in  FIG. 2 , the image forming apparatus  100  includes a reader  170 , an image forming unit  110 , a paper tray unit  140 , a sub-scanning direction conveyer  130 , an output conveyer  150 , a duplex unit  160 , a guide  180 , a bypass sheet supplier  190 , and ink cartridges  118 .  
      The image forming apparatus  100  may be a copying machine, a printer, a facsimile machine, and a multifunction printer including copying, printing, scanning, and facsimile functions. In this non-limiting exemplary embodiment, the image forming apparatus  100  functions as a color copying machine for forming a color image on a recording medium.  
      The reader  170  is disposed in an upper portion of the image forming apparatus  100  and above the output conveyer  150 . The reader  170  scans an image on an original sheet to generate image data. The image forming unit  110  forms an image on a recording medium according to the image data generated by the reader  170 . The paper tray unit  140  is attachable to and detachable from the front of the image forming apparatus  100  and loads a recording medium (e.g., sheets P). The sub-scanning direction conveyer  130  turns a direction in which a sheet P fed from the paper tray unit  140  is conveyed by about  90  degrees so that the sheet P opposes the image forming unit  110 , and conveys the sheet P towards the output conveyer  150 . The output conveyer  150  outputs the sheet P or conveys the sheet P to the duplex unit  160 . The duplex unit  160  reverses the sheet P fed from the output conveyer  150  and feeds the sheet P towards the image forming unit  110  so that an image is formed on the other side of the sheet P. The guide  180  is disposed between the paper tray unit  140  and the sub-scanning direction conveyer  130  in a sheet conveyance direction and swings to slack the sheet P fed from the paper tray unit  140  or the. duplex unit  160 . The bypass sheet supplier  190  loads a sheet P such as thick paper and an OHP (overhead projector) transparency. The ink cartridges  118  are attachable to and detachable from the front of the image forming apparatus  100  and contain black, cyan, magenta, and yellow inks, respectively.  
      The reader  170  includes an exposure glass  177 , optical scanning systems  173  and  176 , a lens  178 , and a scanning element  179 . The optical scanning system  173  includes a light source  171  and a mirror  172 . The optical scanning system  176  includes mirrors  174  and  175 .  
      An original sheet bearing an image is placed on the exposure glass  177  facing down. The optical scanning systems  173  and  176  move to scan the image on the original sheet. The light source  171  emits light onto the original sheet placed on the exposure glass  177 . The mirror  172  deflects the light reflected by the original sheet towards the mirror  174 . The mirror  174  further deflects the light deflected by the mirror  172  towards the mirror  175 . The mirror  175  further deflects the light deflected by the mirror  174  towards the lens  178 . The lens  178  emits the light deflected by the mirror  175  towards the scanning element  179 . The scanning element  179  converts the light into an image signal. The image signal is digitized and processed to generate image data.  
      The image forming apparatus  100  can also receive image data sent from an information processing device (e.g., a personal computer), an image scanning device (e.g., an image scanner), or a capturing device (e.g., a digital camera) via a cable or a network. The image forming apparatus  100  processes the received image data to form an image according to the processed image data.  
      The image forming unit  110  includes a guide rod  111 , a carriage  112 , recording heads  116 , and sub tanks  117 . The guide rod  111  supports the carriage  112  together with a stay (not shown) in a manner that the carriage  112  can move in a main scanning direction. The carriage  112  carries the recording heads  116 . The recording heads  116  discharge liquid drops onto a sheet P sent from the paper tray unit  140  or the bypass tray supplier  190  according to the image data generated by the reader  170 . The sub tanks  117  are mounted on the carriage  112  and contain inks to be supplied to the recording heads  116 .  
      The paper tray unit  140  includes a paper tray  141 , a feeding roller  142 , a friction pad  143 , a bypass tray  146 , a bypass tray roller  147 , a conveying roller  148 , a feeding motor  149 , and a registration roller pair  144 . The paper tray  141  loads sheets P. The feeding roller  142  and the friction pad  143  feed the sheets P from the paper tray  141  one by one towards the registration roller pair  144 . The bypass tray  146  loads sheets P. The bypass tray roller  147  feeds the sheets P from the bypass tray  146  one by one towards the registration roller pair  144 . The conveying roller  148  feeds a sheet P fed from another paper tray (not shown), which can be optionally attached to a lower portion of the image forming apparatus  100 , or the duplex unit  160  towards the registration roller pair  144 . The feeding motor  149  includes an HB (hybrid) type stepping motor and rotatably drives the rollers for feeding the sheet P towards the sub-scanning direction conveyer  130 , such as the feeding roller  142 , the registration roller pair  144 , the bypass tray roller  147 , and the conveying roller  148 , via an electromagnetic clutch (not shown). The registration roller pair  144  temporarily stops feeding the sheet P fed by the feeding roller  142 , the bypass tray roller  147 , or the conveying roller  148 .  
      The sub-scanning direction conveyer  130  includes a conveying belt  133 , a conveying roller  131 , a tension roller  132 , a charging roller  134 , a guide  135 , two pressing rollers  136 , two spur rollers  137 , and a separating nail  138 .  
      The conveying belt  133  has an endless belt-like shape and is looped over the conveying roller  131  and the tension roller  132 . The conveying roller  131 , serving as a driving roller, rotates the conveying belt  133 . The rotating conveying belt  133  rotates the tension roller  132 . A high voltage power source (not shown) applies a high, alternating voltage to the charging roller  134 , so that the charging roller  134  charges a surface of the conveying belt  133 . The guide  135  opposes the image forming unit  110  and guides the rotating conveying belt  133 . The pressing rollers  136  oppose the conveying roller  131  via the conveying belt  133  and press the sheet P conveyed on the conveying belt  133  towards the conveying belt  133 . The spur rollers  137  press the sheet P bearing an image formed by the image forming unit  110  and conveyed on the conveying belt  133  towards the conveying belt  133 . The separating nail  138  separates the sheet P bearing the image from the conveying belt  133 .  
      A sub-scanning direction motor (not shown) rotatably drives the conveying roller  131  via a timing belt (not shown) and a timing roller (not shown) so that the rotating conveying roller  131  rotates the conveying belt  133  in a rotating direction A. The conveying belt  133  includes two layers, that is, a front layer which attracts the sheet P and a back layer which forms a medium resistive layer or a grounded layer. The front layer includes a resin material for which resistance control is not performed [e.g., an ETFE (ethylene tetrafluororethylene) material]. The back layer includes a material common to the front layer, for which resistance control is performed by using a carbon. However, the conveying belt  133  may include one layer or three or more layers.  
      The sub-scanning direction conveyer  130  further includes a cleaner (not shown) and a discharging brush (not shown). The cleaner is disposed between the tension roller  132  and the charging roller  134  in the rotating direction A and removes paper dust or the like adhered to the surface of the conveying belt  133 . The discharging brush discharges the surface of the conveying belt  133 .  
      The sub-scanning direction conveyer  130  further includes a rotary coder (not shown). The rotary coder includes a high-resolution code hole (not shown) and a transmission photo sensor (not shown). The high-resolution code hole is attached to a shaft (not shown) of the conveying roller  131 . The transmission photo sensor detects a slit (not shown) formed in the high-resolution code hole.  
      The sub-scanning direction conveyer  130  further includes a linear encoder (not shown) and a joint sensor (not shown). The linear encoder includes a linear scale (not shown) and a reflection photo sensor (not shown). The linear scale is formed on an inner circumferential surface of the conveying belt  133 , which contacts an outer circumferential surface of the conveying roller  131 . The reflection photo sensor detects the linear scale. The linear scale can be formed in a stripe pattern by evaporating aluminum onto the inner circumferential surface of the conveying belt  133  and then irradiating a laser beam. The linear scale is disposed on a portion of the inner circumferential surface of the conveying belt  133  where the guide  135  may not disturb the detection by the reflection photo sensor. The joint sensor is adjacent to the reflection photo sensor and detects a joint of the linear scale provided on the inner circumferential surface of the conveying belt  133 .  
      The output conveyer  150  includes three conveying rollers  153   a ,  153   b , and  153   c , three spur rollers  152   a ,  152   b , and  152   c , a lower guide  154 , an upper guide  155 , a first output path  156 , a reverse roller pair  157 , an output roller pair  158 , an output tray  151 , a second output path  197 , a straight output tray  198 , and a switching mechanism  159 .  
      The conveying rollers  153   a ,  153   b , and  153   c  convey the sheet P separated from the conveying belt  133  by the separating nail  138  towards the switching mechanism  159 . The spur rollers  152   a ,  152   b , and  152   c  oppose the conveying rollers  153   a ,  153   b , and  153   c , respectively, and convey the sheet P towards the switching mechanism  159 . The lower guide  154  and the upper guide  155  guide the sheet P nipped and conveyed by the conveying rollers  153   a ,  153   b , and  153   c  and the spur rollers  152   a ,  152   b , and  152   c . The first output path  156  is provided on a downstream side from the switching mechanism  159  relative to the sheet conveyance direction. The first output path  156  leads the sheet P to the output tray  151  so that the sheet P is reversed and output onto the output tray  151  facing down. The reverse roller pair  157  and the output roller pair  158  are disposed on the first output path  156  and feed the sheet P towards the output tray  151 . The output tray  151  receives the sheet P fed by the output roller pair  158 . The second output path  197  is provided on a downstream side from the switching mechanism  159  relative to the sheet conveyance direction. The second output path  197  leads the sheet P to the straight output tray  198 . The straight output tray  198  receives the sheet P fed by the conveying roller  153   c  and the spur roller  152   c . The switching mechanism  159  moves to guide the sheet P towards the first output path  156 , the second output path  197 , or the duplex unit  160 .  
      The duplex unit  160  includes a vertical conveyer  161   a  and a horizontal conveyer  161   b . The vertical conveyer  161   a  includes a vertical path  160   c . The horizontal conveyer  161   b  includes a horizontal path  160   a , a switchback path  160   b , and a switching board  166 . The vertical path  160   c  includes an entrance roller pair  161  and a conveying roller pair  162 . The horizontal path  160   a  includes five conveying roller pairs  163   a ,  163   b ,  163   c ,  163   d , and  163   e . The switchback path  160   b  includes three conveying roller pairs  165   a ,  165   b , and  165   c  and an exit roller pair  164 .  
      The vertical conveyer  161   a  conveys the sheet P guided by the switching mechanism  159  towards the horizontal conveyer  161   b . The horizontal conveyer  161   b  conveys the sheet P conveyed from the vertical conveyer  161   a  towards the conveying roller  148 . The vertical path  160   c  leads the sheet P downward towards the horizontal path  160   a . The horizontal path  160   a  leads the sheet P towards the switchback path  160   b . The switchback path  160   b  switches back the sheet P and leads the sheet P towards the conveying roller  148 . The switching board  166  swings to switch between a position illustrated in the solid line and a position illustrated in the broken line. When the switching board  166  is positioned at the position illustrated in the solid line, the sheet P is fed from the horizontal path  160   a  towards the switchback path  160   b . When the switching board  166  is positioned at the position illustrated in the broken line, the sheet P is fed from the switchback path  160   b  towards the conveying roller  148 .  
      The entrance roller pair  161  feeds the sheet P guided by the switching mechanism  159  downward to the conveying roller pair  162 . The conveying roller pair  162  further feeds the sheet P towards the conveying roller pair  163   a . The conveying roller pairs  163   a ,  163   b ,  163   c ,  163   d , and  163   e  feed the sheet P towards the exit roller pair  164 . The exit roller pair  164  feeds the sheet P towards the conveying roller pairs  165   c ,  165   b , and  165   a . The exit roller pair  164  and the conveying roller pairs  165   c ,  165   b , and  165   a  also serve as reverse rollers. The conveying roller pairs  165   c ,  165   b , and  165   a  feed the sheet P towards the exit roller pair  164 . The exit roller pair  164  feeds the sheet P towards the conveying roller  148 . The conveying roller  148  feeds the sheet P towards the registration roller pair  144 .  
      The sheet P fed from the paper tray  141 , the bypass tray  146 , or the duplex unit  160  is further fed by the registration roller pair  144  towards the sub-scanning direction conveyer  130 . When the sheet P is nipped by the registration roller pair  144  and by the conveying belt  133  and the pressing roller  136 , the guide  180  swings in a direction B to slack the sheet P so as to prevent the sheet P from being tensioned backward.  
      When the registration roller pair  144  feeds the sheet P towards the sub-scanning direction conveyer  130 , the guide  180  swings in the direction B to slack the sheet P and guides the sheet P towards the sub-scanning direction conveyer  130 . When the sheet P reaches the sub-scanning direction conveyer  130 , the guide  180  swings back to the original position to become ready for slacking the next sheet P.  
      The bypass sheet supplier  190  includes a bypass tray  191 , a shaft  192 , an opening  195 , and a shutter  196 . The bypass tray  191  is openable from and closable to one side of the image forming apparatus  100 . To insert a sheet P into the bypass tray  191 , the bypass tray  191  is opened to an open position illustrated in the chain double-dashed line. The shaft  192  supports the bypass tray  191  in a manner that the bypass tray  191  is openable from and closable to the image forming apparatus  100 . The opening  195  is disposed on a downstream side from the bypass tray  191  relative to the sheet conveyance direction and is opened and closed by the shutter  196 . The shutter  196  is disposed on an upstream side from the sub-scanning direction conveyer  130  relative to the sheet conveyance direction. The shutter  196  closes to regulate the sheet P inserted into the bypass tray  191  in a sub-scanning direction and opens to send the sheet P towards the sub-scanning direction conveyer  130  through the opening  195 .  
       FIG. 3  is a top view of the image forming unit  110  and the bypass sheet supplier  190  of the image forming apparatus  100  (depicted in  FIG. 2 ). As illustrated in  FIG. 3 , the bypass sheet supplier  190  further includes side fences  193  and  194 . The image forming unit  110  further includes a timing belt  115 , a main scanning motor  113 , a driving pulley  114   a , a driven pulley  114   b , a maintenance-recovery mechanism  119 , and an idle discharge receiver  124 . The recording heads  116  include liquid drop discharging heads  116   k   2 ,  116   k   1 ,  116   c ,  116   m , and  116   y . The maintenance-recovery mechanism  119  includes moisture retention caps  120 , a sucking cap  121 , a suction pump  30 , an idle discharge receiver  123 , and a wiper blade  122 . The moisture retention caps  120  include moisture retention caps  120   k   2 ,  120   k   1 ,  120   c ,  120   m , and  120   y . The idle discharge receiver  124  includes openings  125 . The openings  125  include openings  125   k   2 ,  125   k   1 ,  125   c ,  125   m , and  125   y . The side fences  193  and  194  are disposed on the bypass tray  191  and regulate the sheet P in the main scanning direction (i.e., directions C). The side fences  193  and  194  are interlocked via a rack (not shown) and a pinion (not shown) to simultaneously slide in directions D. Specifically, the side fences  193  and  194  simultaneously move closer to each other or simultaneously move away from each other. However, as an alternative, one of the side fences  193  and  194  may be fixed and disposed on the bypass tray  191 , and the other may regulate the sheet P to contact or touch the one of the side fences  193  and  194 .  
      The timing belt  115  is looped over the driving pulley  114   a  and the driven pulley  114   b . The main scanning motor  113  drives the driving pulley  114   a . The driving pulley  114   a  rotates the timing belt  115 . The rotating timing belt  115  rotates the driven pulley  114   b . The rotating timing belt  115  moves the carriage  112  supported by the guide rod  111  and the stay in the main scanning direction (i.e., the directions C).  
      The recording heads  116  are mounted on the carriage  112  and discharge liquid droplets by a shuttle method. Specifically, while the sheet P is conveyed on the conveying belt  133  rotating in a direction E (i.e., the sub-scanning direction), the recording heads  116  on the carriage  112  moving in the directions C discharge liquid droplets onto the sheet P. However, the recording heads  116  may be configured to discharge liquid droplets by a line method in which the recording heads  116  discharge liquid droplets without moving in the main scanning direction.  
      The liquid drop discharging heads  116   k   2  and  116   k   1  discharge a black ink. The liquid drop discharging heads  116   c ,  116   m , and  116   y  discharge cyan, magenta, and yellow inks, respectively. The black, cyan, magenta, and yellow inks are supplied from the sub tanks  117  (depicted in  FIG. 2 ).  
      The sub tanks  117  include five tanks. Two of the five tanks contain a black ink. The other three tanks contain cyan, magenta, and yellow inks, respectively. The ink cartridges  118  (depicted in  FIG. 2 ) include four ink cartridges, respectively, containing black, cyan, magenta, and yellow inks. A black ink is supplied from one ink cartridge containing the black ink to two tanks for containing the black ink. Cyan, magenta, and yellow inks are supplied from the other three ink cartridges containing the cyan, magenta, and yellow inks, respectively, to the other three tanks for containing the cyan, magenta, and yellow inks, respectively.  
      Multiple types of the recording heads  116 , including piezo, thermal, and electrostatic types, can be used. The piezo type recording head uses a piezoelectric element as a pressure generator (e.g., an actuator) for applying a pressure to ink in an ink flow route (e.g., a pressure generating room) to deform a vibration board forming walls of the ink flow route, so that a changed volume of the ink flow route discharges an ink droplet. The thermal type recording head uses a heat generating resistance body to generate a bubble by boiling ink in an ink flow route, so that a pressure of the bubble discharges an ink droplet. The electrostatic type recording head uses a vibration board forming walls of an ink flow route and an electrode, which oppose each other, so that the vibration board deformed by an electrostatic force generated between the vibration board and the electrode changes a volume of the ink flow route and discharges an ink droplet.  
      The maintenance-recovery mechanism  119  is disposed in a non-printing area, where the recording heads  116  do not discharge an ink droplet, near one end of the guide rod  111  in the main scanning direction. The maintenance-recovery mechanism  119  maintains nozzles of the recording heads  116  and recovers residue ink from the recording heads. The five moisture retention caps  120   k   2 ,  120   k   1 ,  120   c ,  120   m , and  120   y  cap nozzles of the recording heads  116   k   2 ,  116   k   1 ,  116   c ,  116   m , and  116   y , respectively. The sucking cap  121  caps a nozzle of the recording heads  116  to suck a liquid droplet. The suction pump  30  is connected to the sucking cap  121  and applies suction to draw a liquid droplet from the nozzle of the recording heads  116 . The idle discharge receiver  123  receives a liquid droplet which is discharged during idle discharge and is not used for printing. The wiper blade  122  wipes the nozzles of the recording heads  116 .  
      The idle discharge receiver  124  is disposed in another non-printing area near another end of the guide rod  111  in the main scanning direction. The openings  125  receive liquid droplets which are discharged from the recording heads  116  during idle discharge and are not used for printing. The openings  125   k   2 ,  125   k   1 ,  125   c ,  125   m , and  125   y  receive the liquid droplets discharged from the recording heads  116   k   2 ,  116   k   1 ,  116   c ,  116   m , and  116   y , respectively.  
       FIGS. 4A and 4B  illustrate a perspective plane view of the suction pump  30  (depicted in  FIG. 3 ). As illustrated in  FIGS. 4A and 4B , the suction pump  30  includes a tube  20 , a housing  21 , a rear plate  31 , a front plate  36 , a regulating plate  37 , and pressing rollers  15   a  and  15   b . The rear plate  31  includes a central shaft  32  and grooves  35   a  and  35   b . The pressing rollers  15   a  and  15   b  include pressing roller shafts  14   a  and  14   b , respectively.  
      One end of the tube  20  is connected to the sucking cap  121  (depicted in  FIG. 3 ), which caps the nozzle of the recording heads  116  (depicted in  FIG. 3 ), to suck ink from the nozzle. Another end of the tube  20  is connected to a collected ink container (not shown). The sucked ink is conveyed in the tube  20  and is discharged into the collected ink container. The housing  21  includes an inner wall having an arc-like shape and configured to hold the tube  20 . Specifically, the housing  21  holds the tube  20  along its inner wall. The rear plate  31  is disposed behind the front plate  36 . The front plate  36  is disposed behind the regulating plate  37 . The regulating plate  37  is disposed in front of the front plate  36  and regulates movement of the pressing rollers  15   a  and  15   b  in a circumferential direction. The central shaft  32  is disposed on a center of the rear plate  31 . The grooves  35   a  and  35   b  engage with the pressing roller shafts  14   a  and  14   b , respectively. The pressing rollers  15   a  and  15   b  are provided between the rear plate  31  and the front plate  36  and rotatably move along the tube  20  held by the housing  21 . The pressing roller shafts  14   a  and  14   b  support the pressing rollers  15   a  and  15   b , respectively.  
       FIG. 5A  illustrates the rear plate  31 . The groove  35   a  includes engaging ends  33   a  and  34   a . The groove  35   b  includes engaging ends  33   b  and  34   b . The engaging end  33   a  is formed at a pumping position on the groove  35   a  in a manner that a distance H is provided between the engaging end  33   a  and the center of the central shaft  32 . Similarly, the engaging end  33   b  is formed at a pumping position on the groove  35   b  in a manner that the distance H is provided between the engaging end  33   b  and the center of the central shaft  32 . The engaging end  34   a  is formed at a non-pumping position on the groove  35   a  in a manner that a distance I is provided between the engaging end  34   a  and the center of the central shaft  32 . Similarly, the engaging end  34   b  is formed at a non-pumping position on the groove  35   b  in a manner that the distance I is provided between the engaging end  34   b  and the center of the central shaft  32 . The distance H is greater than the distance I.  
       FIG. 5B  illustrates the front plate  36 . The front plate  36  includes guides  7   a  and  7   b . The guide  7   a  includes engaging ends  8   a  and  9   a . The guide  7   b  includes engaging ends  8   b  and  9   b . The guides  7   a  and  7   b  guide the pressing rollers  15   a  and  15   b  (depicted in  FIGS. 4A and 4B ), respectively. The engaging ends  8   a  and  8   b  are provided on one end of the guides  7   a  and  7   b  in the circumferential direction, respectively. The engaging ends  9   a  and  9   b  are provided on another end of the guides  7   a  and  7   b  in the circumferential direction, respectively.  
       FIG. 5C  illustrates the regulating plate  37 . The regulating plate  37  includes regulating ends  38   a ,  38   b ,  39   a , and  39   b . The regulating ends  38   a  and  39   a  contact the pressing roller shaft  14   a  (depicted in  FIGS. 4A and 4B ) to regulate movement of the pressing roller  15   a  (depicted in  FIGS. 4A and 4B ). The regulating ends  38   b  and  39   b  contact the pressing roller shaft  14   b  (depicted in  FIGS. 4A and 4B ) to regulate movement of the pressing roller  15   b  (depicted in  FIGS. 4A and 4B ).  
      Referring to  FIGS. 4A and 4B , operations of the suction pump  30  will be further described. As illustrated in  FIGS. 4A and 4B , the central shaft  32  rotatably supports the rear plate  31 , the front plate  36 , and the regulating plate  37 . The pressing rollers  15   a  and  15   b  are provided between the rear plate  31  and the front plate  36 . The grooves  35   a  and  35   b  engage with the pressing roller shafts  14   a  and  14   b  in a manner that the pressing roller shafts  14   a  and  14   b  move in the grooves  35   a  and  35   b  so as to guide the pressing rollers  15   a and  15   b , respectively.  
      As illustrated in  FIG. 4A , when a motor (not shown) rotates the rear plate  31 , the front plate  36 , and the regulating plate  37  in a rotating direction F to perform a pumping operation, the pressing roller shaft  14   a  moves towards the engaging end  33   a  (depicted in  FIG. 5A ) along the groove  35   a  (depicted in  FIG. 5A ) provided on the rear plate  31  (depicted in  FIG. 5A ) and along the guide  7   a  (depicted in  FIG. 5B ) provided on the front plate  36  (depicted in  FIG. 5B ). The pressing roller shaft  14   b  moves towards the engaging end  33   b  (depicted in  FIG. 5A ) along the groove  35   b  (depicted in  FIG. 5A ) provided on the rear plate  31  (depicted in  FIG. 5A ) and along the guide  7   b  (depicted in  FIG. 5B ) provided on the front plate  36  (depicted in FIG. .  5 B). The regulating ends  38   a  and  38   b  (depicted in  FIG. 5C ) contact the pressing roller shafts  14   a  and  14   b  to keep the pressing roller shafts  14   a  and  14   b  to contact the engaging ends  33   a  and  33   b  (depicted in  FIG. 5A ), respectively. Thus, the pressing roller shafts  14   a  and  14   b  keep contacting the engaging ends  33   a  and  33   b , respectively. When the rear plate  31 , the front plate  36 , and the regulating plate  37  further rotate in the rotating direction F while the pressing roller shafts  14   a  and  14   b  contact the engaging ends  33   a  and  33   b , respectively, the pressing rollers  15   a  and  15   b  alternately press the tube  20  towards the inner wall of the housing  21  while the pressing rollers  15   a  and  15   b  move along the tube  20 . Namely, the pressing rollers  15   a  and  15   b  apply a positive pressure to the tube  20 . When the rear plate  31 , the front plate  36 , and the regulating plate  37  further rotate in the rotating direction F while the pressing roller shafts  14   a  and  14   b  contact the engaging ends  33   a  and  33   b , respectively, the pressing rollers  15   a  and  15   b  alternately move away from the tube  20  and the pressure applied to the tube  20  is released. Namely, the pressing rollers  15   a  and  15   b  apply a negative pressure to the tube  20 . Thus, a pumping operation is performed and ink is drawn by suction into the tube  20 .  
      As illustrated in  FIG. 4B , when the rear plate  31 , the front plate  36 , and the regulating plate  37  rotate in a rotating direction G (i.e., a direction opposite to the rotating direction F) to stop the pumping operation, the pressing roller shaft  14   a  moves along the groove  35   a  and the guide  7   a  (depicted in  FIG. 5B ) towards the engaging end  34   a  (depicted in  FIG. 5A ). The pressing roller shaft  14   b  moves along the groove  35   b  and the guide  7   b  (depicted in  FIG. 5B ) towards the engaging end  34   b  (depicted in  FIG. 5A ). Thus, the pressing roller shafts  14   a  and  14   b  contact the engaging ends  34   a  and  34   b , respectively. The regulating ends  39   a  and  39   b  (depicted in  FIG. 5C ) contact the pressing roller shafts  14   a  and  14   b  to keep the pressing roller shafts  14   a  and  14   b  to contact the engaging ends  34   a  and  34   b , respectively. As a result, the pressing rollers  15   a  and  15   b  do not contact the tube  20 . Namely, the pressing rollers  15   a  and  15   b  do not apply a pressure to the tube  20 . Thus, a pumping operation is not performed.  
      When the rear plate  31 , the front plate  36 , and the regulating plate  37  rotate in the rotating direction F while the pressing roller shafts  14   a  and  14   b  contact the engaging ends  33   a  and  33   b , respectively (i.e., while the pressing rollers  15   a  and  15   b  are at a pumping position), the pressing rollers  15   a  and  15   b  press the tube  20  when the pressing rollers  15   a  and  15   b  move along the tube  20 . Namely, the pressing rollers  15   a  and  15   b  apply a positive pressure to the tube  20 . When the rear plate  31 , the front plate  36 , and the regulating plate  37  rotate in the rotating direction G while the pressing roller shafts  14   a  and  14   b  contact the engaging ends  34   a  and  34   b , respectively (i.e., while the pressing rollers  15   a  and  15   b  are at a non-pumping position), the pressing rollers  15   a  and  15   b  do not contact the tube  20  while the pressing rollers  15   a  and  15   b  move along the tube  20 , and thereby do not press the tube  20 . Namely, the pressing rollers  15   a  and  15   b  apply a negative pressure to the tube  20 . Thus, ink is drawn by suction through one end of the tube  20  from a nozzle of the recording heads  116  (depicted in  FIG. 3 ) and is discharged into a collected ink container (not shown) through another end of the tube  20 .  
      As described above, in the suction pump  30  in the embodiment of  FIGS. 4A and 4B , the regulating plate  37  holds each of the pressing rollers  15   a  and  15   b  at a predetermined position (i.e., the pumping or non-pumping position) in the circumferential direction defined for each of pumping and non-pumping operations. Namely, each of the pressing rollers  15   a  and  15   b  does not move to a position other than the predetermined position defined for each of pumping and non-pumping operations. As a result, pumping and non-pumping operations can be performed with improved reliability.  
       FIGS. 6A and 6B  illustrate a perspective plane view of a suction pump  40   a  according to another exemplary embodiment. The same reference numerals are assigned to the elements common to the suction pump  30  in the embodiment of  FIGS. 4A and 4B . As illustrated in  FIGS. 6A and 6B , the suction pump  40   a  includes a rear plate  12 , a front plate  13 , a regulating plate  41 , the pressing rollers  15   a  and  15   b , the housing  21 , and the tube  20 . The rear plate  12  includes a central shaft  11  and grooves  16   a  and  16   b . The front plate  13  includes guides  17   a  and  17   b . The guide  17   a  includes engaging ends  18   a  and  19   a . The guide  17   b  includes engaging ends  18   b  and  19   b . The regulating plate  41  includes grooves  42   a  and  42   b . The pressing rollers  15   a  and  15   b  include the pressing roller shafts  14   a  and  14   b , respectively.  
      The rear plate  12  is disposed behind the front plate  13 . The front plate  13  is disposed behind the regulating plate  41 . The regulating plate  41  is disposed in front of the front plate  13  and regulates movement of the pressing rollers  15   a  and  15   b  in a circumferential direction. The pressing rollers  15   a  and  15   b  are provided between the rear plate  12  and the front plate  13  and rotatably move along the tube  20  held by the housing  21 . The central shaft  11  rotatably supports the rear plate  12 , the front plate  13 , and the regulating plate  41 . The pressing roller shafts  14   a  and  14   b  support the pressing rollers  15   a  and  15   b , respectively. The grooves  16   a  and  16   b  engage with the pressing roller shafts  14   a  and  14   b  in a manner that the pressing roller shafts  14   a  and  14   b  move in the grooves  16   a  and  16   b  so as to guide the pressing rollers  15   a  and  15   b , respectively. The guides  17   a  and  17   b  guide the pressing rollers  15   a  and  15   b , respectively. The engaging ends  18   a  and  18   b  are provided on one end of the guides  17   a  and  17   b  in the circumferential direction, respectively. The engaging ends  19   a  and  19   b  are provided on another end of the guides  17   a  and  17   b  in the circumferential direction, respectively. The grooves  42   a  and  42   b  are provided on the regulating plate  41  and engage with the pressing roller shafts  14   a  and  14   b , respectively. The grooves  42   a  and  42   b  guide the pressing roller shafts  14   a  and  14   b  to regulate movement of the pressing rollers  15   a  and  15   b  in the circumferential direction, respectively.  
      The engaging end  18   a  is formed at a pumping position on the groove  16   a  in a manner that a distance J is provided between the engaging end  18   a  and the center of the central shaft  11 . Similarly, the engaging end  18   b  is formed at a pumping position on the groove  16   b  in a manner that the distance J is provided between the engaging end  18   b  and the center of the central shaft  11 . The engaging end  19   a  is formed at a non-pumping position on the groove  16   a  in a manner that a distance K is provided between the engaging end  19   a  and the center of the central shaft  11 . Similarly, the engaging end  19   b  is formed at a non-pumping position on the groove  16   b  in a manner that the distance K is provided between the engaging end  19   b  and the center of the central shaft  11 . The distance J is greater than the distance K.  
      As illustrated in  FIG. 6A , when a motor (not shown) rotates the rear plate  12 , the front plate  13 , and the regulating plate  41  in a rotating direction L to perform a pumping operation, the pressing roller shaft  14   a  moves towards the engaging end  18   a  along the groove  16   a  provided on the rear plate  12  and along the guide  17   a  provided on the front plate  13 . The pressing roller shaft  14   b  moves towards the engaging end  18   b  along the groove  16   b  provided on the rear plate  12  and along the guide  17   b  provided on the front plate  13 . Thus, the pressing roller shafts  14   a  and  14   b  contact the engaging ends  18   a  and  18   b , respectively. The grooves  42   a  and  42   b  guide the pressing roller shafts  14   a  and  14   b  to regulate movement of the pressing rollers  15   a  and  15   b  in the circumferential direction, respectively. When the rear plate  12 , the front plate  13 , and the regulating plate  41  further rotate in the rotating direction L while the pressing roller shafts  14   a  and  14   b  contact the engaging ends  18   a  and  18   b , respectively (i.e., while the pressing rollers  15   a  and  15   b  are at the pumping position), the pressing rollers  15   a  and  15   b  alternately press the tube  20  towards the inner wall of the housing  21  while the pressing rollers  15   a  and  15   b  move along the tube  20 . Namely, the pressing rollers  15   a  and  15   b  apply a positive pressure to the tube  20 . When the rear plate  12 , the front plate  13 , and the regulating plate  41  further rotate in the rotating direction L while the pressing roller shafts  14   a  and  14   b  contact the engaging ends  18   a  and  18   b , respectively, the pressing rollers  15   a  and  15   b  alternately move away from the tube  20  and the pressure applied to the tube  20  is released. Namely, the pressing rollers  15   a  and  15   b  apply a negative pressure to the tube  20 . Thus, a pumping operation is performed and ink is drawn by suction into the tube  20 .  
      As illustrated in  FIG. 6B , when the rear plate  12 , the front plate  13 , and the regulating plate  41  rotate in a rotating direction M (i.e., a direction opposite to the rotating direction L) to stop the pumping operation, the pressing roller shaft  14   a  moves along the groove  16   a  and the guide  17   a  towards the engaging end  19   a . The pressing roller shaft  14   b  moves along the groove  16   b  and the guide  17   b  towards the engaging end  19   b . The grooves  42   a  and  42   b  guide the pressing roller shafts  14   a  and  14   b  to regulate movement of the pressing rollers  15   a  and  15   b  in the circumferential direction, respectively. Thus, the pressing roller shafts  14   a  and  14   b  contact the engaging ends  19   a  and  19   b  respectively (i.e., the pressing rollers  15   a  and  15   b  are at the non-pumping position) and thereby the pressing rollers  15   a  and  15   b  do not contact the tube  20 . Namely, the pressing rollers  15   a  and  15   b  do not apply a pressure to the tube  20 . Thus, a pumping operation is not performed.  
      As described above, in the suction pump  40   a  in the embodiment of  FIGS. 6A and 6B , the regulating plate  41  holds each of the pressing rollers  15   a  and  15   b  at a predetermined position (i.e., the pumping or non-pumping position) in the circumferential direction defined for each of pumping and non-pumping operations. The central shaft  11  rotatably supports the regulating plate  41 . Thus, neither of the pressing rollers  15   a  and  15   b  moves to a position other than the predetermined position defined for each of pumping and non-pumping operations. As a result, pumping and non-pumping operations can be performed with improved reliability.  
       FIG. 7  is a perspective plane view of a suction pump  40   b  according to another exemplary embodiment. As illustrated in  FIG. 7 , the suction pump  40   b  includes the elements common to the suction pump  40   a  in the embodiment of  FIGS. 6A and 6B , but further includes a rotation suppressor  43 .  
      The rotation suppressor  43  contacts an outer plane surface of the rear plate  12  and/or the front plate  13  and suppresses rotation of the rear plate  12  and/or the front plate  13  by friction. The rotation suppressor  43  includes a viscoelastic material and may be attached to the housing  21 , for example.  
      In the suction pump  40   b  in the embodiment of  FIG. 7 , the rotation suppressor  43 , which suppresses rotation of the rear plate  12  and/or the front plate  13 , causes the regulating plate  41  to start rotating earlier than at least one of the rear plate  12  and the front plate  13 . As a result, the suction pump  40   b  can provide improved reliability.  
       FIG. 8  is a perspective plane view of a suction pump  40   c  according to another exemplary embodiment. As illustrated in  FIG. 8 , the suction pump  40   c  includes the elements common to the suction pump  40   a  in the embodiment of  FIGS. 6A and 6B , but further includes a rotation suppressor  44 .  
      The rotation suppressor  44  contacts an inner plane surface of the rear plate  12  and/or the front plate  13  and suppresses rotation of the rear plate  12  and/or the front plate  13  by friction. The rotation suppressor  44  includes a viscoelastic material and may be attached to the housing  21 , for example.  
      In the suction pump  40   c  in the embodiment of  FIG. 8 , the rotation suppressor  44 , which suppresses rotation of the rear plate  12  and/or the front plate  13 , causes the regulating plate  41  to start rotating earlier than at least one of the rear plate  12  and the front plate  13 . As a result, the suction pump  40   c  can provide improved reliability.  
       FIG. 9  is a perspective plane view of a suction pump  40   d  according to another exemplary embodiment. As illustrated in  FIG. 9 , the suction pump  40   d  includes the elements common to the suction pump  40   a  in the embodiment of  FIGS. 6A and 6B , but further includes a rotation suppressor  45 .  
      The rotation suppressor  45  is disposed between the central shaft  11  and the rear plate  12  and/or the front plate  13  in a manner that the rotation suppressor  45  contacts the central shaft  11 . Thus, the rotation suppressor  45  suppresses rotation of the rear plate  12  and/or the front plate  13  by friction. The rotation suppressor  45  includes a viscoelastic material.  
      In the suction pump  40   d  in the embodiment of  FIG. 9 , the rotation suppressor  45 , which suppresses rotation of the rear plate  12  and/or the front plate  13 , causes the regulating plate  41  to start rotating earlier than at least one of the rear plate  12  and the front plate  13 . As a result, the suction pump  40   d  can provide improved reliability.  
       FIG. 10  is a sectional side view of a suction pump  40   e  according to another exemplary embodiment. As illustrated in  FIG. 10 , the suction pump  40   e  includes the elements common to the suction pump  40   a  in the embodiment of  FIGS. 6A and 6B , but further includes a rotation suppressor  46 .  
      The rotation suppressor  46  contacts a plane surface of the rear plate  12  and suppresses rotation of the rear plate  12  by friction. The rotation suppressor  46  includes a viscoelastic material. The rotation suppressor  46  may contact a plane surface of the front plate  13  so as to suppress rotation of the front plate  13 .  
      In the suction pump  40   e  in the embodiment of  FIG. 10 , the rotation suppressor  46 , which suppresses rotation of the rear plate  12  or the front plate  13 , causes the regulating plate  41  to start rotating earlier than at least one of the rear plate  12  and the front plate  13 . As a result, the suction pump  40   e  can provide improved reliability.  
       FIG. 11  is a sectional side view of a suction pump  40   f  according to another exemplary embodiment. As illustrated in  FIG. 11 , the suction pump  40   f  includes the elements common to the suction pump  40   a  in the embodiment of  FIGS. 6A and 6B , but further includes a case  47 .  
      The case  47  contacts and covers an outer circumferential edge of the rear plate  12  and suppresses rotation of the rear plate  12  by friction caused between the case  47  and the housing  21  (depicted in  FIGS. 6A and 6B ), for example. The case  47  includes a viscoelastic material such as silicone oil. A viscoelastic force of the viscoelastic material suppresses rotation of the rear plate  12 . The case  47  may contact and cover an outer circumferential edge of the front plate  13  so as to suppress rotation of the front plate  13 .  
      In the suction pump  40   f  in the embodiment of  FIG. 11 , the case  47 , which suppresses rotation of the rear plate  12  or the front plate  13 , causes the regulating plate  41  to start rotating earlier than at least one of the rear plate  12  and the front plate  13 . As a result, the suction pump  40   f  can provide improved reliability.  
       FIG. 12  is a sectional side view of a suction pump  40   g  according to another exemplary embodiment. As illustrated in  FIG. 12 , the suction pump  40   g  includes the elements common to the suction pump  40   a  in the embodiment of  FIGS. 6A and 6B , but further includes a rotation suppressor  48 .  
      The rotation suppressor  48  contacts an outer circumferential edge of the regulating plate  41  and suppresses rotation of the regulating plate  41  by friction caused between the rotation suppressor  48  and the housing  21  (depicted in  FIGS. 6A and 6B ), for example. The rotation suppressor  48  includes a viscoelastic material.  
      In the suction pump  40   g  in the embodiment of  FIG. 12 , the rotation suppressor  48 , which suppresses rotation of the regulating plate  41 , causes the rear plate  12  and the front plate  13  to start rotating earlier than the regulating plate  41 . As a result, the suction pump  40   g  can provide improved reliability.  
       FIG. 13  is a sectional side view of a suction pump  40   h  according to another exemplary embodiment. As illustrated in  FIG. 13 , the suction pump  40   h  includes the elements common to the suction pump  40   a  in the embodiment of  FIGS. 6A and 6B , but further includes a rotation suppressor  49 .  
      The rotation suppressor  49  contacts a plane surface of the regulating plate  41  and suppresses rotation of the regulating plate  41  by friction caused between the rotation suppressor  49  and the housing  21  (depicted in  FIGS. 6A and 6B ), for example. The rotation suppressor  49  includes a viscoelastic material. To further suppress rotation of the regulating plate  41 , a rib may be formed on the plane surface of the regulating plate  41  in a manner that the rib forms a concentric circle in which the central shaft  11  is the center.  
      In the suction pump  40   h  in the embodiment of  FIG. 13 , the rotation suppressor  49 , which suppresses rotation of the regulating plate  41 , causes the rear plate  12  and the front plate  13  to start rotating earlier than the regulating plate  41 . As a result, the suction pump  40   h  can provide improved reliability.  
       FIG. 14  is a sectional side view of a suction pump  40   i  according to another exemplary embodiment. As illustrated in  FIG. 14 , the suction pump  40   i  includes the elements common to the suction pump  40   a  in the embodiment of  FIGS. 6A and 6B , but further includes a case  50 .  
      The case  50  contacts an outer circumferential edge of the regulating plate  41  and suppresses rotation of the regulating plate  41  by friction caused between the case  50  and the housing  21  (depicted in  FIGS. 6A and 6B ), for example. The case  50  includes a viscoelastic material such as silicone oil. A viscoelastic force of the viscoelastic material suppresses rotation of the regulating plate  41 .  
      In the suction pump  40   i  in the embodiment of  FIG. 14 , the case  50 , which suppresses rotation of the regulating plate  41 , causes the rear plate  12  and the front plate  13  to start rotating earlier than the regulating plate  41 . As a result, the suction pump  40   i  can provide improved reliability.  
       FIG. 15  is a perspective plane view of a suction pump  40   j  according to another exemplary embodiment. As illustrated in  FIG. 15 , the suction pump  40   j  includes the elements common to the suction pump  40   a  in the embodiment of  FIGS. 6A and 6B , but further includes a rotation suppressor  51 .  
      The rotation suppressor  51  is disposed between the central shaft  11  and the regulating plate  41  in a manner that the rotation suppressor  51  contacts the central shaft  11 , and suppresses rotation of the regulating plate  41  by friction. The rotation suppressor  51  includes a viscoelastic material.  
      In the suction pump  40   j  in the embodiment of  FIG. 15 , the rotation suppressor  51 , which suppresses rotation of the regulating plate  41 , causes the rear plate  12  and the front plate  13  to start rotating earlier than the regulating plate  41 . As a result, the suction pump  40   j  can provide improved reliability.  
       FIG. 16  is a perspective plane view of a suction pump  40   k  according to another exemplary embodiment. As illustrated in  FIG. 16 , the suction pump  40   k  includes the elements common to the suction pump  40   a  in the embodiment of  FIGS. 6A and 6B , but further includes impact absorbers  52   a  and  52   b.    
      The impact absorbers  52   a  and  52   b  are disposed on the engaging ends  19   a  and  19   b , respectively. When the pressing rollers  15   a  and  15   b  pressing the tube  20  alternately move away from the tube  20  while the suction pump  40   k  performs a pumping operation, an elastic force of the tube  20  bounces the pressing rollers  15   a  and  15   b . The bounced pressing rollers  15   a  and  15   b  hit the engaging ends  19   a  and  19   b , respectively, and generate an impulsive sound. The impact absorbers  52   a  and  52   b  relieve an impact caused by the bounced pressing rollers  15   a  and  15   b.    
      In the suction pump  40   k  in the embodiment of  FIG. 16 , the impact absorbers  52   a  and  52   b  relieve an impact caused by the bounced pressing rollers  15   a  and  15   b  to suppress an impulsive sound caused by the pressing rollers  15   a  and  15   b  hitting the engaging ends  19   a  and  19   b , respectively. As a result, the suction pump  40   k  can provide improved reliability.  
      Instead of providing the impact absorbers  52   a  and  52   b , the elements included in the rear plate  12  and the front plate  13 , a portion of the rear plate  12  and/or the front plate  13  contacting the pressing roller shafts  14   a  and  14   b  (depicted in  FIGS. 6A and 6B ) and/or the pressing rollers  15   a  and  15   b , the pressing roller shafts  14   a  and  14   b , and/or the pressing rollers  15   a  and  15   b  may include a material having a low degree of hardness, gin content, or elasticity, such as an elastomer and/or a resin containing an elastomer.  
      Numerous additional modifications and variations are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the disclosure of this patent specification may be practiced otherwise than as specifically described herein. For example, elements and/or features of different illustrative embodiments may be combined with each other and/or substituted for each other within the scope of this disclosure and appended claims.  
      This patent specification is based on Japanese patent application No. 2005-338368 filed on Nov. 24, 2005 in the Japan Patent Office, the entire contents of which are hereby incorporated herein by reference.