Patent Publication Number: US-7588248-B2

Title: Sheet conveying device and image forming apparatus

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a sheet conveying device including a sheet reversing mechanism capable of reversing a sheet, and to an image forming apparatus, such as a copy machine, a printer, and a facsimile machine, including the sheet conveying device. 
     2. Description of the Related Art 
     In general, a sheet conveying device for an image forming apparatus including a sheet reversing mechanism uses a switch-back method. In the switch-back method, a sheet is pulled into a reversing conveying path and is then extracted in the opposite direction to reverse the sheet. In the sheet conveying device having the sheet reversing mechanism using the switch-back method, the reversing conveying path dedicated to reversing the sheet and a switch for switching conveying paths are required. This leads to an increase in the size of the sheet conveying device and the image forming apparatus. 
     Accordingly, to reduce the size of the sheet conveying device and the image forming apparatus, a sheet reversing mechanism that reverses a sheet by rotating a pair of planetary rollers while the sheet is nipped therebetween has been suggested. This technique is described in Japanese Patent Laid-Open No. 2001-122479. 
     The structure of known planetary rollers will be described below with reference to  FIGS. 8A to 8C . 
     A pair of planetary rollers  102  and  103  is used to reverse and convey a small, thick sheet, such as a card. The planetary rollers  102  and  103  are provided on a rotating disc  120  having a diameter corresponding to the length of a side of a card-shaped sheet (hereinafter simply called a sheet)  101  in a conveying direction thereof. The planetary rollers  102  and  103  include a sun roller  102  that functions as a driving roller and that is rotated by a driving force, and a satellite roller  103  that functions as a driven roller and that is capable of holding the sheet  101  between the sun roller  102  and the satellite roller  103 . Guides  140  for guiding the sheet  101  toward the planetary rollers  102  and  103  are attached to the rotating disc  120 . The rotating disc  120  is supported such that the rotating disc  120  can rotate around a drive shaft of the sun roller  102 . 
     A pair of supply rollers  121  for conveying the sheet  101  to the position between the sun roller  102  and the satellite roller  103  and guides  140  for guiding the sheet  101  are disposed around the rotating disc  120 . In addition, a card position sensor  131  for detecting the trailing edge of the sheet  101  supplied by the supply rollers  121  and a rotational angle sensor  132  for detecting the rotational angle of the rotating disc  120  are also disposed around the rotating disc  120 . 
     A sheet-reversing operation performed by the known planetary rollers  102  and  103  having the above-described structure will now be described. 
     The sun roller  102  is rotated at the same surface speed (peripheral speed) as that of the supply rollers  121 . When the sheet  101  is supplied, the rotating disc  120  is stopped at the position shown in  FIG. 8A . In this state, the sheet  101  is conveyed by the supply rollers  121  in the direction shown by the arrow X. After the leading edge of the sheet  101  reaches the position where it is nipped between the sun roller  102  and the satellite roller  103 , the sheet  101  is further conveyed while being nipped by the planetary rollers  102  and  103 . Then, when it is detected by the card position sensor  131  that the trailing edge of the sheet  101  has passed a predetermined position and the overall body of the sheet  101  is supported by the rotating disc  120 , the rotating disc  120  starts to rotate together with the sun roller  102  (see  FIG. 8B ). 
     When it is detected by the rotational angle sensor  132  that the rotating disc  120  has rotated about 180° and reached the position shown in  FIG. 8C , the rotating disc  120  stops rotating. At this time, the sun roller  102  continues to rotate, so that the sheet  101  nipped between the sun roller  102  and the satellite roller  103  is ejected in the direction shown by the arrow Y. The supply rollers  121  are rotated in the direction opposite to that in the state shown in  FIG. 8A . Accordingly, the sheet  101  is conveyed by the supply rollers  121  after being reversed by the rotation of the rotating disc  120 . 
     In the above-described known technique, the sheet  101  is rotated around the drive shaft of one of the planetary rollers  102  and  103 , i.e., the sun roller  102 , while the sheet  101  is nipped between the planetary rollers  102  and  103 . Therefore, the sheet  101  cannot be conveyed along a straight conveying path before and after the rotation, but is conveyed along paths that are shifted from each other. In other words, the position at which the sheet  101  is received by the planetary rollers  102  and  103  and the position at which the sheet  101  is ejected therefrom are shifted from each other in the vertical direction in  FIGS. 8A to 8C . Therefore, the sheet conveying performance is degraded and the occurrence of jamming is increased. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a sheet conveying device having a small, simple structure and capable of reversing a sheet such that the sheet can be conveyed on a straight line before and after the reversal, and to an image forming apparatus including the sheet conveying device. 
     According to an aspect of the present invention, a sheet conveying device includes a sheet reversing unit configured to reverse a sheet. The sheet reversing unit includes a pair of rollers configured to be rotatable while being in pressure contact with each other, a connecting mechanism configured to support the pair of rollers such that the pair of rollers are capable of rotating together, a roller driving mechanism configured to transmit a rotational driving force to at least one of the pair of rollers, and a roller-pair rotating mechanism configured to rotate the pair of rollers supported by the connecting mechanism together. The connecting mechanism supports the pair of rollers such that the pair of rollers is capable of rotating around a nip portion of the pair of rollers that are in pressure contact with each other. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a sectional view of an image forming apparatus according to an embodiment of the present invention. 
         FIG. 2  is a side view of a reversing roller unit according to the embodiment of the present invention. 
         FIG. 3  is a front view of the reversing roller unit according to the embodiment of the present invention. 
         FIG. 4  is a control block diagram according to the embodiment of the present invention. 
         FIG. 5  is a flowchart of an operation according to the embodiment of the present invention. 
         FIGS. 6A to 6E  are diagrams illustrating the operation of the reversing roller unit according to the embodiment of the present invention. 
         FIGS. 7A and 7B  are diagrams illustrating the structure of a part of an image forming apparatus including the reversing roller unit according to the embodiment of the present invention. 
         FIGS. 8A to 8C  are diagrams illustrating an example of a known reversing roller unit. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     An embodiment of the present invention will be described below with reference to the drawings. 
     A sheet conveying device according to the embodiment of the present invention will be described in detail below. First, the overall schematic structure of an image forming apparatus including the sheet conveying device according to the present embodiment will be described. Then, the structure and operation of a reversing path unit included in the sheet conveying device according to the present embodiment will be described in detail. 
     The overall structure of the image forming apparatus will be described with reference to the schematic sectional view shown in  FIG. 1 . In  FIG. 1 , the image forming apparatus includes a main body  1 . An original plate  2  made of a transparent glass plate and a plate cover  3  are fixed at an upper section of the main body  1 . The main body  1  of the image forming apparatus is connected to an automatic document feeder disposed in the upper section thereof. An original O is fed by the automatic document feeder, and is stopped at a predetermined position on the original plate  2  such that an image side thereof faces downward. An optical system placed below the original plate  2  includes a lamp  4  for illuminating the original O, reflective mirrors  5 ,  6 , and  7  for guiding an optical image of the illuminated original O to an image memory  9 , and an imaging lens  11 . The lamp  4  and the reflective mirrors  5 ,  6 , and  7  move in the direction shown by the arrow a at a predetermined speed, thereby scanning the original O. 
     An image forming unit includes a photosensitive drum  12 , a charger  13 , a developing unit  14 , a transfer charger  19 , a detach charger  20 , and a cleaner  26 . The charger  13  is provided to uniformly charge the surface of the photosensitive drum  12 . A scanner  8  emits, via a mirror  10 , an optical image for forming an electrostatic latent image on the surface of the photosensitive drum  12  charged by the charger  13 . The developing unit  14  is provided to develop the electrostatic latent image to form a toner image that is to be transferred onto a sheet S. The transfer charger  19  is provided to transfer the toner image on the surface of the photosensitive drum  12  onto the sheet S. The detach charger  20  is provided to remove the sheet S onto which the toner image is transferred from the photosensitive drum  12 . The cleaner  26  is provided to remove the toner remaining on the photosensitive drum  12  after the toner image is transferred. As shown in  FIG. 1 , a central processing unit (CPU)  27  is provided as a control unit for controlling the components included in the apparatus. 
     A case in which, for example, an image is copied on one side of the sheet S and the sheet S is ejected with the side on which the image is copied facing upward will be described. When a copy start button (not shown) is pressed, a sheet S on a feed tray  32  is fed by a sheet feeding unit  36  including feed rollers  36 A,  36 B, and  36 C, and is conveyed by a plurality of pairs of conveying rollers  38 . Then, the tilt of the sheet S is corrected by a pair of registration rollers  45 . Alternatively, a sheet S on a feed tray  33  is fed by a sheet feeding unit  37  and is conveyed to the registration rollers  45  by the pairs of conveying rollers  38 . A sheet may also be fed from a manual feed tray  40 . A sheet S on the manual feed tray  40  is fed by a feed roller  42  and is conveyed to the registration rollers  45  by the pairs of conveying rollers  44 . 
     After the sheet S reaches the registration rollers  45  and the tilt thereof is corrected by the registration rollers  45 , the sheet S is conveyed to the image forming unit at a predetermined timing. In the image forming unit, the toner image is transferred onto the sheet S. A belt conveyor unit  21  for conveying the sheet S on which the toner image is transferred and a fixing unit  22  for fixing the image on the sheet S conveyed by the belt conveying unit  21  as a permanent image are disposed on the downstream of the image forming unit. The sheet S having the image fixed thereon by the fixing unit  22  passes through a pair of inner ejection rollers  23 , guided by a first switch  51 , and is ejected from the main body  1  of the image forming apparatus through an ejection conveying path  49 . The ejected sheet S is placed on an ejection tray  25  provided outside the main body  1  of the image forming apparatus. 
     A reversing roller unit  55 , which functions as a sheet reversing unit that characterizes the present invention, will be described with reference to  FIGS. 3 and 6 . In the present embodiment, the reversing roller unit  55  is used as a pair of duplex reversing rollers  55 X and a pair of ejection reversing rollers  55 Y. 
       FIGS. 2 and 3  show an example of the structure of the reversing roller unit  55 .  FIG. 2  is a side view of the reversing roller unit  55 , and  FIG. 3  is a front view thereof. 
     Reversing rollers  55   a  and  55   b  are structured such that rotating shafts of the rollers  55   a  and  55   b  are rotatably supported by connecting units  61 , which function as a connecting mechanism, provided on the left and right sides such that the rollers  55   a  and  55   b  are in pressure contact with each other. Thus, the reversing rollers  55   a  and  55   b  are rotatably supported while being in pressure contact with each other. 
     A roller-pair rotating mechanism for rotating the reversing rollers  55   a  and  55   b  together will now be described. A rotating gear  58  is fixed at the center of one of the connecting units  61 , and is rotatably supported by a shaft  58   a . The other connecting unit  61  is rotatably supported by another shaft  58   a . In addition, as shown in  FIG. 3 , the shaft  58   a  of the rotating gear  58  is disposed on a line extending parallel to the axial lines of the roller shafts of the reversing rollers  55   a  and  55   b  from nip portions N provided between the reversing rollers  55   a  and  55   b . Accordingly, the reversing rollers  55   a  and  55   b  are supported such that the reversing rollers  55   a  and  55   b  can be rotated around the nip portions N in the side view. The number of pairs of the reversing rollers  55   a  and  55   b  is set to one or more depending on maximum and minimum widths of the sheets that can be conveyed. In the present embodiment, two pairs of reversing rollers  55   a  and  55   b  are provided. 
     The rotating gear  58  meshes with a rotation drive gear  59  to which a rotational driving force is transmitted from a roller-pair rotating motor M 2 , which is a stepper motor. A driving force is input to the rotating gear  58  from the rotation drive gear  59  through a clutch (not shown) or the like, and thus the overall body of the reversing rollers  55   a  and  55   b  can be rotated. 
     Next, a roller driving mechanism for rotating each of the reversing rollers  55   a  and  55   b  will be described. 
     The reversing rollers  55   a  and  55   b  are coaxially integrated with reversing roller gears  57   a  and  57   b , respectively. The reversing roller gears  57   a  and  57   b  are arranged to be capable of selectively meshing with a reversing-roller drive gear  60  to which a rotational driving force is transmitted from a roller driving motor M 1 , which is a stepper motor. When the reversing-roller drive gear  60  meshes with the reversing roller gear  57   a , the reversing rollers  55   a  are rotated by a driving force transmitted thereto, and the reversing rollers  55   b  are rotated by the rotation of the reversing rollers  55   a.    
     Similarly, when the reversing-roller drive gear  60  meshes with the reversing roller gear  57   b , the reversing rollers  55   b  are rotated by the driving force and the reversing rollers  55   a  are rotated by the rotation of the reversing rollers  55   b . When the overall body of the reversing rollers  55   a  and  55   b  is rotated, the reversing-roller drive gear  60  is separated from both the reversing roller gear  57   a  and the reversing roller gear  57   b . Therefore, rotation of the reversing rollers  55   a  and  55   b  is stopped. 
     A trailing-edge detection sensor  52  (see in  FIG. 6 ) that detects the trailing edge of the sheet S being conveyed is disposed on the upstream of the reversing rollers  55   a  and  55   b  in the sheet conveying direction. 
       FIG. 4  is a control block diagram according to the present invention. A detection signal obtained from the trailing-edge detection sensor  52  is input to the CPU  27 . The CPU  27  controls the operations of the roller driving motor M 1  and the roller-pair rotating motor M 2 . 
     The operation of the reversing roller unit  55  will be described with reference to  FIGS. 5 and 6A  to  6 E.  FIG. 5  is a flowchart of the operation and  FIGS. 6A to 6E  are diagrams illustrating the operation. 
     In Step  1 , the driving force of the roller driving motor M 1  is transmitted from the reversing-roller drive gear  60  to the reversing roller gear  57   a . Accordingly, as shown in  FIG. 6A , the reversing rollers  55   a  and  55   b  are rotated and the sheet S is conveyed leftward in the figure. Then, when the trailing edge of the sheet S is detected by the trailing-edge detection sensor  52  in Step  2 , the driving force of the roller-pair rotating motor M 2  is transmitted to the rotating gear  58  through the rotation drive gear  59 , so that the connecting units  61  are rotated in Step  3 . Accordingly, as shown in  FIG. 6B , the reversing rollers  55   a  and  55   b  rotate (revolve) around the nip portions N thereof. In  FIG. 6B , the overall body of the reversing rollers  55   a  and  55   b  is rotated clockwise. 
     While the reversing rollers  55   a  and  55   b  are rotating (revolving) as shown in  FIGS. 6C and 6D , the reversing-roller drive gear  60  meshes with neither of the reversing roller gears  57   a  and  57   b . The reversing rollers  55   a  and  55   b  start to rotate (revolve) before the trailing edge of the sheet S passes through the nip portions N between the reversing rollers  55   a  and  55   b . Therefore, the driving force is applied to neither of the reversing rollers  55   a  and  55   b , and the reversing rollers  55   a  and  55   b  are rotated while the sheet S is nipped therebetween. 
     If it is determined that the overall body of the reversing rollers  55   a  and  55   b  is rotated by 180° as shown in  FIG. 6E  in Step  4 , the driving force applied to the rotating gear  58  from the rotation drive gear  59  is canceled to stop the reversing rollers  55   a  and  55   b  in Step  5 . The rotation of the reversing rollers  55   a  and  55   b  may be stopped at 180° using a sensor (not shown) or be automatically stopped at 180° by setting a step number of the stepper motor. 
     In this state, the reversing-roller drive gear  60  meshes with the reversing roller gear  57   b  so that the driving force is transmitted. Accordingly, each of the reversing rollers  55   a  and  55   b  is rotated around the center thereof and the sheet S is conveyed in the same direction as the direction shown in  FIG. 6A . Thus, the sheet is conveyed along the same plane before and after the reversal. 
     A sheet guide  54  is disposed on the downstream of the reversing rollers  55   a  and  55   b  in the sheet conveying direction. The sheet guide  54  serves to prevent flopping of the sheet S in the reversing process, thereby increasing the conveying stability. The sheet guide  54  may be a roller, a spur roller, or a plate on which a sheet having a high sliding performance is attached. 
     In addition, the rotating direction of the rotation drive gear  59  can be selectively switched so that the rotating (revolving) direction of the reversing rollers  55   a  and  55   b  can be switched between positive and negative directions depending on the direction in which the sheet S is curled. For example, in  FIG. 3 , if the sheet S is curled upward (i.e., if the vertical position of the sheet at the leading end is higher than that at an intermediate position) when the sheet S reaches the reversing rollers  55   a  and  55   b , the reversing rollers  55   a  and  55   b  are rotated counterclockwise. Accordingly, curling of the sheet S can be corrected by winding the sheet S around the reversing rollers  55   b  in a direction opposite to the direction in which the sheet S is curled. 
     An example in which the reversing roller unit  55  is used in an image forming apparatus will be described. Referring to  FIGS. 7A and 7B , the structure of the reversing roller unit  55  is applied to the duplex reversing rollers  55 X and the ejection reversing rollers  55 Y. The duplex reversing rollers  55 X are provided at an intermediate position (position near the fixing unit  22 ) of a duplex conveying path  50  for conveying the sheet S having an image formed on one side thereof to the image-forming unit again to form images on both sides. Trailing-edge detection sensors  52   a  and  53   a  are respectively placed in front of and behind the nip portion of the duplex reversing rollers  55 X in the sheet-conveying direction. 
     The ejection reversing rollers  55 Y are provided at an intermediate position (position near the fixing unit  22 ) of an ejection conveying path  49  for ejecting the sheet S having an image formed on one side thereof in such a manner that the image faces downward. Trailing-edge detection sensors  52   b  and  53   b  are placed in front of and behind the nip portion of the ejection reversing rollers  55 Y in the sheet-conveying direction. 
     An operation of forming images on both sides of the sheet by the image forming unit (duplex copying) will be described below with reference to  FIG. 7A . The numbers with parentheses show the order in which the sheet is conveyed. Processes from feeding of the sheet S to fixing is similar to those in the above-described single-sided image forming operation, and explanations thereof are thus omitted. 
     The first switch  51  is rotated by a drive unit (not shown) to a position shown in  FIG. 7A  in advance. Therefore, the sheet S having an image formed on one side thereof is conveyed toward the duplex reversing rollers  55 X (( 1 )→( 2 )). Then, when the trailing edge of the sheet S is detected by the trailing-edge detection sensor  52   a  disposed on the upstream of the duplex reversing rollers  55 X, the duplex reversing rollers  55 X are rotated by 180° (( 3 )). While the duplex reversing rollers  55 X are being rotated, the driving force is not applied to the reversing rollers  55   a  and  55   b , so that the duplex reversing rollers  55 X can be rotated while the sheet S is nipped therebetween. After the sheet S is rotated by 180° and reversed by the duplex reversing rollers  55 X, the sheet S is conveyed toward a plurality of pairs of rollers  56  provided on the duplex conveying path  50  (( 4 )). 
     Then, when the trailing edge of the sheet S is detected by the trailing-edge detection sensor  53   a  disposed on the downstream of the duplex reversing rollers  55 X, the operation of the motors and the like is stopped so that the next sheet S can be received. The sheet S ejected from the duplex reversing rollers  55 X is conveyed toward the image forming unit again by the plurality of pairs of rollers  56  (( 5 )). The following processes are similar to those in the single-sided image forming operation, and explanations thereof are thus omitted. 
     Next, an operation of ejecting the sheet S having an image formed on one side thereof such that the image faces downward (reversed ejection) will be described with reference to  FIG. 7B . Processes from feeding of the sheet S to fixing is similar to those in the above-described single-sided image forming operation, and explanations thereof are thus omitted. 
     The first switch  51  is rotated by a drive unit (not shown) to a position shown in  FIG. 7B  in advance. Therefore, the sheet S having an image formed on one side thereof is conveyed toward the ejection reversing rollers  55 Y (( 1 )→( 2 )). Then, when the trailing edge of the sheet S is detected by the trailing-edge detection sensor  52   b  disposed on the upstream of the ejection reversing rollers  55 Y, the ejection reversing rollers  55 Y are rotated by 180° (( 3 )). While the ejection reversing rollers  55 Y are being rotated, the driving force is not applied to the ejection reversing rollers  55 Y, so that the ejection reversing rollers  55 Y can be rotated while the sheet S is nipped therebetween (( 4 )). 
     After the sheet S is rotated by 180° and reversed by the ejection reversing rollers  55 Y, the sheet S is ejected to the ejection tray  25  such that the side on which the image is formed faces downward. Then, when the trailing edge of the sheet S is detected by the trailing-edge detection sensor  53   b  disposed on the downstream of the ejection reversing rollers  55 Y, the operation of the motors and the like is stopped so that the next sheet S can be received. 
     Thus, by using the reversing roller unit according to the present invention, the sheet conveyed to the reversing roller unit can be reversed and ejected along a path that is aligned with (on the same straight line as) the path along which the sheet has been conveyed to the reversing roller unit. Therefore, compared to the known structure, the sheet conveying performance can be increased and the occurrence of jamming can be reduced. In addition, the size of the sheet conveying device and the image forming apparatus can be reduced. 
     Although gear drive units using stepper motors as drive sources have been described in the present embodiment, the present invention is not limited to this. For example, the reversing roller unit  55  may also be driven using DC motors, belts, etc. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all modifications, equivalent structures and functions. 
     This application claims the benefit of Japanese Application No. 2006-339862 filed Dec. 18, 2006, which is hereby incorporated by reference herein in its entirety.