Patent Publication Number: US-7912417-B2

Title: Image forming apparatus and conveyance device

Description:
BACKGROUND 
     1. Technical Field 
     The present invention relates to an image forming apparatus such as a copier, a facsimile machine or a printer, and a conveyance device used in the apparatus. 
     2. Related Art 
     In this type of image forming apparatus, having a first conveyance roller to convey a recording medium such as a sheet to an image forming part, a forward/reverse-rotatable second conveyance roller positioned between the first conveyance roller and the image forming part, and a contact member in contact with the second conveyance roller to form a nip, a technique of temporarily stopping a lead edge of recording medium during conveyance at the nip, thereby correcting skew of the recording medium is known. 
     SUMMARY 
     According to an aspect of the invention, there is provided an image forming apparatus including: an image forming part; a first conveyance roller that conveys a recording medium to the image forming part; a forward/reverse-rotatable second conveyance roller, located between the first conveyance roller and the image forming part in a conveyance direction of the recording medium; a contact member, in contact with the second conveyance roller, that forms a nip between the second conveyance roller and the contact member; a first conveyance roller driver that rotate-drives the first conveyance roller in the same direction as the conveyance direction of the recording medium; and a drive transmission mechanism that performs drive transmission from the first conveyance roller driver to the second conveyance roller so as to start rotation of the second conveyance roller in a reverse direction of the conveyance direction of the recording medium before a lead edge of the recording medium arrives at the nip. Accordingly, the timing of driving of the first conveyance roller can be accurately determined based on the relation with respect to the timing of driving of the second conveyance roller, and the skew correction of recording medium can be excellently performed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       An exemplary embodiment of the present invention will be described in detail based on the following figures, wherein: 
         FIG. 1  is a cross-sectional view showing a structure of an image forming apparatus according to an exemplary embodiment of the present invention; 
         FIG. 2A  is a significant-part expanded cross-sectional view of the image forming apparatus according to the exemplary embodiment of the present invention showing a status before the start of sheet conveyance; 
         FIG. 2B  is a significant-part expanded cross-sectional view of the image forming apparatus according to the exemplary embodiment of the present invention showing a status during the sheet conveyance; 
         FIG. 3  is a perspective diagram showing a paper feeder employed in the image forming apparatus according to the exemplary embodiment of the present invention viewed from a front position; 
         FIG. 4  is a perspective diagram showing the paper feeder employed in the image forming apparatus according to the exemplary embodiment of the present invention viewed from a rear position; 
         FIG. 5  is a perspective diagram showing a state at start of gear array drive to transmit driving to a paper feed roller and a registration roller of the image forming apparatus according to the exemplary embodiment of the present invention, viewed from a front position; 
         FIG. 6  is a perspective diagram showing a state during the gear-array drive to transmit driving to the paper feed roller and the registration roller of the image forming apparatus according to the exemplary embodiment of the present invention, viewed from a front position; 
         FIG. 7  is a perspective diagram showing a state upon completion of the gear array drive to transmit driving to the paper feed roller and the registration roller of the image forming apparatus according to the exemplary embodiment of the present invention, viewed from a rear position; 
         FIG. 8  is a block diagram showing a controller employed in the image forming apparatus according to the exemplary embodiment of the present invention; and 
         FIG. 9  is a flowchart showing an operation of the image forming apparatus according to the exemplary embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     An exemplary embodiment of the present invention will be described with reference to the drawings. 
       FIG. 1  shows an image forming apparatus  10  according to an exemplary embodiment of the present invention. The image forming apparatus  10  has an image forming apparatus main body  12 . The image forming apparatus main body  12  includes an image forming part  14 , a sheet feeder  54  to feed a sheet to the image forming part  14 , a power source unit  16 , and a controller  200  used as a controller. Further, a sheet discharge part  15 , to which the sheet after image formation is discharged, is provided in an upper part of the image forming apparatus main body  12 . 
     The image forming part  14  is an electrophotographic type unit to form a color image. The image forming part  14  has drum-shaped photoreceptors  22 Y,  22 M,  22 C and  22 B as image holders to hold developing material images, chargers  24 Y,  24 M,  24 C and  24 B as charging units having charging rollers to uniformly charge the respective photoreceptors  22 Y,  22 M,  22 C and  22 B, optical writers  26 Y,  26 M,  26 C and  26 B as latent-image forming units to optically write latent images on the respective photoreceptors  22 Y,  22 M,  22 C and  22 B, developing devices  28 Y,  28 M,  28 C and  28 B as developing units to develop the latent images written on the respective photoreceptors  22 Y,  22 M,  22 C and  22 B with developing material (toner), a transfer unit  42  as a transfer unit to transfer the developing material images formed on the respective photoreceptors  22 Y,  22 M,  22 C and  22 B to a sheet, and cleaners  30 Y,  30 M,  30 C and  30 B as developing-material removal units to remove the developing material remaining on the photoreceptors  22 Y,  22 M,  22 C and  22 B after the transfer of the developing material images by the transfer unit  42 . 
     The optical writers  26 Y,  26 M,  26 C and  26 B respectively have a laser exposure device. The optical writer  26 Y emits a laser beam corresponding to a yellow image to the photoreceptor  22 Y; the optical writer  26 M, a laser beam corresponding to a magenta image to the photoreceptor  22 M; the optical writer  26 C, a laser beam corresponding a cyan image to the photoreceptor  22 C; and the optical writer  26 B, a laser beam corresponding to a black image to the photoreceptor  22 B. In this manner, the optical writers  26 Y,  26 M,  26 C and  26 B respectively write latent images on the photoreceptors  22 Y,  22 M,  22 C and  22 B. 
     Among the units included in the image forming part  14 , the photoreceptor  22 , the charger  24 , the developing device  28  and the cleaner  30  are integrated as a process cartridge  32  used as an exchangeable unit. The process cartridge is attachable/removable to/from the image forming apparatus main body  12 . Further, the process cartridge  32  has a toner cartridge (toner bottle)  34  as a developing material container (exchangeable unit) containing developing material (toner) supplied to the developing devices  28 , and a discharge toner bottle  36  as a developing-material collecting container to collect developing material (toner) removed by the cleaner  30 , attachably/removably or integrally with the process cartridge  32 . 
     In the image forming apparatus main body  12 , the process cartridges  32 Y,  32 M,  32 C and  32 B are arrayed, in this order, from a lower position toward an upper position in  FIG. 1 , along a conveyance belt  46  to be described later. 
     The process cartridge  32 Y is used for image formation with yellow developing material; the process cartridge  32 M, for image formation with magenta developing material; the process cartridge  32 C, for image formation with cyan developing material; and the process cartridge  32 B, for image formation with black developing material. Accordingly, the toner cartridge  34 Y is filled with yellow toner; the toner cartridge  34 M, magenta toner; the toner cartridge  34 C, cyan toner; and the toner cartridge  34 B, black toner. 
     The transfer unit  42  is provided in contact with the photoreceptors  22 Y,  22 M,  22 C and  22 B of the process cartridges  32 Y,  32 M,  32 C and  32 B. The transfer unit  42  has two support rollers  44   a  and  44   b  integrated as a unit, the conveyance belt  46  as a conveyance unit to convey a sheet or image, an attachment roller  48  as an attachment unit to attach the sheet to the conveyance belt  46 , and transfer rollers  50 Y,  50 M,  50 C and  50 B to transfer developing material images formed on the respective photoreceptors  22 Y,  22 M,  22 C and  22 B onto the sheet conveyed with the conveyance belt  46 . 
     The attachment roller  48  is provided in press-contact with the support roller  44   a  via the conveyance belt  46 . The attachment roller  48  receives a voltage applied from the power source unit  16  and electrostatically attaches the sheet to the conveyance belt  46 . 
     A transfer bias is applied to the respective transfer rollers  50 Y,  50 M,  50 C and  50 B, to sequentially transfer developing material images formed on the photoreceptors  22 Y,  22 M,  22 C and  22 B to the sheet conveyed with the conveyance belt  46 , thus a color developing-material image is formed with overlaid four color images, i.e., yellow, magenta, cyan and black developing material images. 
     Further, a fixing device  52  to fix the developing material image, transferred on the sheet by the transfer unit  42 , onto the sheet, is provided in the upper part of the image forming apparatus main body  12 . The fixing device  52 , having a heating roller  52   a  and a pressure roller  52   b , fixes the developing material image to the sheet by heating and pressing the sheet passing between the heating roller  52   a  and the pressure roller  52   b.    
     Further, a conveyance path  60  to convey a sheet supplied from the sheet feeder  54  to the sheet discharge part is provided in the image forming apparatus main body  12 . Along the conveyance path  60 , registration rollers  62  and  76 , the transfer unit  42 , the fixing device  52  and a discharge roller  78  are provided sequentially from the upstream side of sheet conveyance direction. The discharge roller  78  discharges the sheet conveyed from the fixing device  52  to the sheet discharge part  15 . 
       FIGS. 2 to 7  show the details of the sheet feeder  54 . The sheet feeder  54  has a sheet feed cassette  56  containing sheets, and a feed roller  58  to feed a sheet from the sheet feed cassette  56  toward the image forming part  14 . 
     The sheet feed cassette  56 , in which sheets such as normal sheets or OHP sheets are stacked, is provided attachably/removably to/from the image forming apparatus main body  12 . The sheet feed cassette  56  is used as a recording medium container, and also used as a conveyance-subject container. 
     The feed roller  58  has a meniscus core  58   a  fixed to a rotation shaft  59 , and disk-shaped skids  58   b  provided at both ends of the core  58   a . The core  58   a  has a diameter slightly larger than that of the skids  58   b , and eccentric from the rotation shaft  59  by the large-diameter portion. The feed roller  58  is in contact with a handling roller  81  having a member with a high surface frictional force. The skids  58   b  of the feed roller  58  are rotated in contact with the handling roller  81  in accordance with rotation of the rotation shaft  59 . As the core  58   a  having a meniscus shape is eccentric, when the skids  58   b  are rotated, the core  58   a  is rotated intermittently in contact with a sheet on the top of the sheet pile stacked on the sheet feed cassette  56 , thereby the top sheet is fed. At this time, when plural sheets are held between the feed roller  58  and the handling roller  81 , the handling roller  81  is stopped or reverse-rotated, so as to cause slipping between the sheets and feed only the top sheet. 
     When plural sheets have been held between the feed roller  58  and the handling roller  81  and only the top sheet has been fed with the handling roller  81 , a lead edge of the sheet to be fed next is located in a position where the feed roller  58  and the handling roller  81  are in contact with each other, or a position around the contact position. On the other hand, when only the top sheet has been fed with the feed roller  58 , the lead edge of the sheet to be fed next is located at the end of the sheet feed cassette  56 . In this manner, the position of the lead edge of the sheet to be fed is different in accordance with whether or not the previous sheet has been fed with the handling roller  81 . 
     A pair of registration rollers  61  is provided on the downstream side of the feed roller  58  and the handling roller  81 . The pair of registration rollers  61  has a registration roller  62  and a registration roller  76 . The registration roller  62 , as a driving side, rotates in a sheet conveyance reverse direction at predetermined timing, and rotates in the same direction as the sheet conveyance direction at predetermined timing. The registration roller  76 , as a driven side, is rotated in accordance with the rotation of the registration roller  62 . 
     The registration roller  62  has four cores  66  fixed to a rotation shaft  64 , and the cores  66  are provided with rubber members  67  of rubber having a high surface friction force. The cores  66  are rotated in contact with the registration roller  76  in accordance with rotation of the rotation shaft  64 . 
     The registration roller  76 , in press-contact with the cores  66  of the registration roller  62 , is rotated in accordance with driving of the registration roller  62 . The registration roller  76 , of metal, has at least a surface of a material with a surface frictional coefficient lower than that of the surface of the registration roller  62 . 
     The registration roller  62  and the registration roller  76  form a nip N. Guide plates  80   a  and  80   b  to guide a sheet to the nip N are provided upstream of the nip N. 
     As shown in  FIGS. 2A and 2B , the guide plates  80   a  and  80   b  are opposite to each other, with the conveyance path  60  therebetween, upstream of the nip N. As shown in  FIG. 2B , the guide plates  80   a  and  80   b  guide a sheet fed from the feed roller  58  so as to bring the lead edge of the sheet into contact with the registration roller  76 . In this manner, the guide plates  80   a  and  80   b  are used as a guide unit for the registration roller  76 , as one of the registration rollers forming the nip N, at least having a surface frictional coefficient lower than that of the registration roller  62 , to guide a sheet so as to bring the lead edge of the sheet into contact with the registration roller  76 . 
     Further, a sensor  84  to detect timing of arrival of the lead edge of the sheet at the nip N and to detect timing of passing of the trail edge of the sheet through the nip N is provided in a position upstream of the nip N and close to the nip N. 
     The sensor  84  has a movable member  86  rotatably supported with a shaft  88 . As shown in  FIG. 2A , when a sheet is not passed, the movable member  86  is in a position crossing the conveyance path  60 . From this state, when a sheet has been fed with the feed roller  58 , as shown in  FIG. 2B , the lead edge of the sheet pushes the movable member  86 , and against a pressure by a pressure unit having a spring (not shown), rotates the movable member  86  about the shaft  88 , to move the movable member  86  outside the conveyance path  60 . Then the movement of the movable member  86  is detected by an optical sensor (not shown), thus the timing of arrival of the sheet at the nip N is detected. 
     When the trail edge of the sheet has been passed through the position of the sensor  84 , the movable member  86  again moves to the position crossing the conveyance path  60  shown in  FIG. 2A . As the movement is detected by the optical sensor (not shown), the timing of passing of the trail edge of the sheet through the nip N is detected. In this manner, the sensor  84  is used as a detection unit to detect the timing of passing of the trail edge of the sheet through the nip N. 
     A feed roller driving mechanism  100  to rotate-drive the feed roller  58  in the sheet conveyance direction is attached to the feed roller  58 . 
     The feed roller driving mechanism  100  has a driving gear  101  and a driven gear  102 . The driving gear  101  receives drive transmission from a motor  104  used as a drive source and is rotated. The driven gear  102  is a notched gear fixed to one end of the rotation shaft  59 . The driven gear  102  has a notch portion  102   a  having no tooth. When the notch portion  102   a  is opposite to the driving gear  101 , the driving gear  101  runs idle, and the drive from the driving gear  101  is not transmitted to the driven gear  102 . As shown in  FIG. 4 , the driven gear  102  is connected to one end of a pressure unit  106  having an elastic body such as an extension spring, and the driven gear is pressurized in one direction with the pressure unit  106 . Further, as shown in  FIG. 3 , a cylindrical member  110  having a claw  112  is fixed to the rotation shaft  64 , to which the driven gear  102  is fixed, and a movable member  118  of a solenoid  116  is engaged with the claw  112 . Accordingly, when the solenoid  116  is driven from the state shown in  FIG. 3 , the engagement between the movable member  118  and the claw  112  is released, then the driven gear  102  is rotated by the pressure of the pressure unit  106 , and the driving gear  101  engages with a gear portion  102   b  of the driven gear  102 . In this state, drive transmission is performed from the driving gear  101  to the driven gear  102 , and the feed roller  58  starts rotation in the sheet conveyance direction together with the driven gear  102  and the rotation shaft  64 . 
     A drive transmission mechanism  130  to transmit driving to rotate the registration roller  62  in a reverse direction of the sheet conveyance direction from the feed roller driving mechanism  100  to the registration roller  62  is attached to the feed roller driving mechanism  100 . 
     The drive transmission mechanism  130  has a transmission gear  132  and a reverse gear  134 . 
     The transmission gear  132  is a notched gear fixed to the rotation shaft  59  in a position between the driven gear  102  and the cylindrical member  110 . The transmission gear  132  has a notch portion  132   a  having no tooth and a gear portion  132   b  with teeth. As shown in  FIG. 5 , when the notch portion  132   a  is opposite to the reverse gear  134 , the transmission gear  132  runs idle, and drive transmission is not performed from the transmission gear  132  to the reverse gear  134 . As described above, when the engagement between the movable member  118  of the solenoid  116  and the claw  112  of the cylindrical member  110  is released and the driven gear  102 , the rotation shaft  59  and the feed roller  58  start rotation, the transmission gear  132  fixed to the rotation shaft  59  starts rotation. When the transmission gear  132  rotates and the gear portion  132   b  of the transmission gear  132  moves to a position opposite to the reverse gear  134  as shown in  FIG. 6 , the transmission gear  132  engages with the reverse gear  134 , then drive transmission is performed from the transmission gear  132  to the reverse gear  134 , and the reverse gear  134  and the rotation shaft  64  start rotation. As the rotation shaft  64  rotates, the registration roller  62  is rotated in the reverse direction of the sheet conveyance direction. 
     In the drive transmission mechanism  130 , as the transmission gear  132  rotates, the gear portion  132   b  passes through the position opposite to the reverse gear  134 , and when the notch portion  132   a  moves to the position opposite to the transmission gear  132 , the transmission gear  132  runs idle and the drive transmission from the transmission gear  132  to the reverse gear  134  is stopped. Note that the positions and length of the notch portion  132   a  and the gear portion  132   b  of the transmission gear  132  are determined so as to cause the registration roller  62  to start rotation in the reverse direction of the sheet conveyance direction before arrival of the sheet fed with the feed roller  58  at the nip N and to stop the drive transmission to the registration roller  62  after the arrival of the sheet at the nip N. 
     A registration roller driving mechanism  140  to rotate the registration roller  62  in the same direction of the sheet conveyance direction is attached to the registration roller  62 . 
     As shown in  FIG. 7 , the registration roller driving mechanism  140  has a driving gear  142  and a driven gear  144 . The driving gear  142  receives drive transmission from the motor  104  and rotates. The driven gear  144 , engaged with the driving gear  142 , is fixed to the rotation shaft  64  via an electromagnetic clutch  146 . When the driving gear  142  receives the drive transmission from the motor  104  and rotates, the rotation is transmitted to the driven gear  144  and the driven gear  144  starts rotation. Even when the driven gear  144  has started rotation, the rotation of the driven gear  144  is not transmitted to the rotation shaft  64  as long as the electromagnetic clutch  146  is OFF. On the other hand, when the electromagnetic clutch  146  is turned ON, the rotation of the driven gear  144  is transmitted to the rotation shaft  64 , and the rotation shaft  64  and the registration roller  62  are rotated in the sheet conveyance direction. 
     The electromagnetic clutch  146 , to select a state where the drive transmission from the registration roller driving mechanism  140  to the registration roller  62  is possible or a state where the drive transmission is broken, is attached to the registration roller driving mechanism  140 . Further, the registration roller driving mechanism  140  is provided with a torque limiter  150 . When the driving force in the same direction as the sheet conveyance direction and the driving force in the reverse direction of the sheet conveyance direction are simultaneously applied to the registration roller  62 , the torque limiter  150  breaks the driving force in the reverse direction of the sheet conveyance direction to the registration roller  62 . 
     The torque limiter  150  is attached between the driven gear  144  and the rotation shaft  64 . When the driving force from the driven gear  144  in the same direction as the sheet conveyance direction and the driving force from the transmission gear  132  in the reverse direction of the sheet conveyance direction are simultaneously applied to the registration roller  62 , the torque limiter  150  breaks the driving force from the transmission gear  132 , to rotate the registration roller  62  in the sheet conveyance direction with the driving force transmitted from the driven gear  144 . 
       FIG. 8  shows a controller  200  in the image forming apparatus  10 . 
     The controller  200  has a control circuit  202  which inputs an output from the sensor  84 . The control circuit  202  inputs image data via a communication interface  204 . The image forming part  14 , the motor  104 , the solenoid  116  and the electromagnetic clutch  146  are controlled in accordance with an output from the control circuit  202 . 
       FIG. 9  shows a control flow of the controller  200 . 
     When image data is inputted into the control circuit  202  via the communication interface  204  and the control flow is started, the control circuit  202  starts rotation of the motor  104 , and upon reception of drive transmission from the motor  104 , the driving gear  101  and driving gear  142  start rotation. 
     At step S 10 , the control circuit  202  turns the solenoid  116  ON to start sheet feed. When the solenoid  116  is turned ON, the movable member  118  moves to release the engagement between the movable member  118  and the claw  112  of the cylindrical member  110 . When the engagement between the movable member  118  and the claw  112  is released, the driven gear  102  is rotated by pressure of the pressure unit  106 , and the driving gear  101  and the gear portion  102   b  of the driven gear  102  engage with each other. Then, in this state, drive transmission from the driving gear  101  to the driven gear  102  is performed, and the feed roller  58  rotates in the sheet conveyance direction together with the driven gear  102  and the rotation shaft  59 , and sheet feed is started. 
     The position of the lead edge of the sheet fed with the feed roller  58  is different in accordance with whether or not the previous sheet has been fed with the handling roller  81 . That is, the lead edge of the sheet is located in an end of the sheet feed cassette  56  as denoted by A (hereinbelow, a “position A”) in  FIG. 2A , or in a position where the feed roller  58  and the handling roller  81  are in contact with each other as denoted by B (hereinbelow, a “position B”) in  FIG. 2B . The time before the sheet arrives at the nip N differs in accordance with whether the lead edge of the sheet is in the position A or the position B. 
     At the same time of the start of rotation of the feed roller  58  and the rotation shaft  59 , the transmission gear  132  fixed to the rotation shaft  59  starts rotation. Then, as shown in  FIG. 6 , when the gear portion  132   b  of the transmission gear  132  arrives at a position opposite to the reverse gear  134 , the gear portion  132   b  of the transmission gear  132  and the reverse gear  134  engage with each other. The driving is transmitted from the transmission gear  132  to the reverse gear  134 , and the registration roller  62  starts rotation in the reverse direction of the sheet conveyance direction (hereinbelow, “reverse rotation”). Then the transmission gear  132  continues the rotation, and when the gear portion  132   b  passes through the position opposite to the reverse gear  134  and the notch portion  132   a  is opposite to the reverse gear  134 , the drive transmission from the transmission gear  132  to the reverse gear  134  is not performed. The drive transmission to the registration roller  62  is stopped. 
     Note that the position and width of the gear portion  132   b  of the transmission gear  132  are determined so as to start the reverse rotation of the registration roller  62  before the arrival of the sheet fed with the feed roller  58  at the nip N and to stop the drive transmission to the registration roller  62  for the reverse rotation after elapse of sufficient time for sheet skew correction. In this manner, upon arrival of the sheet at the nip N, as the registration roller  61  is reverse-rotating, there is no probability of insertion of the sheet in the nip N in the sheet skew correction by temporary stoppage of the conveyance of the sheet in the nip N. That is, when the lead edge of the sheet arrives at the nip N formed with the pair of registration rollers  61  in a stopped state, the lead edge of the sheet may be inserted between the rollers against a pressure-contact force applied to the two rollers forming the nip. On the other hand, in the present exemplary embodiment, as the pair of registration rollers  61  is reverse-rotating when the lead edge of the sheet arrives at the nip N, the probability of the insertion of the lead edge of the sheet between the rollers can be reduced. 
     Further, as the lead edge of the sheet is not easily inserted between the pair of registration rollers  61 , it is not necessary to bring the registration roller  62  and the registration roller  76  in high-pressure contact for prevention of the insertion of the sheet between the registration rollers  61 . In comparison with the case where the pair of registration rollers  61  are not reverse-rotated, the press-contact force to the registration roller  62  and the registration roller  76  can be reduced. Accordingly, an image formation error caused when a sheet is firmly supported with the pair of registration rollers  61  can be suppressed. That is, when a toner image is transferred onto a sheet with the transfer unit  42 , the trail edge of the sheet is still firmly held between the pair of registration rollers  61 . Accordingly, when the registration roller  62  and the registration roller  76  are in high-pressure contact state, the trail edge of the sheet is stretched, and a sheet conveyance speed in the transfer part is lower than a designed value and an image formation error may occur. In the present exemplary embodiment, however, the pressure force to press the registration roller  62  and the registration roller  76  in contact is comparatively low; the occurrence of such image formation error is suppressed. 
     Next, at step S 12 , a sheet end detection signal is inputted from the sensor  84  into the control circuit  202 . That is, the movable member  86  of the sensor  84  provided to cross the conveyance path  60  is pushed with the lead edge of the being-conveyed sheet, then the movable member  86  is rotated about the shaft  88 , and the sheet end detection signal, generated in accordance with detection of the rotation of the movable member  86  by the optical sensor, is inputted into the control circuit  202 . 
     Next, at step S 14 , if it is determined that predetermined time has elapsed since the detection of the lead edge of the sheet, the control circuit  202  turns the electromagnet clutch  146  ON at step S 16 . When the electromagnet clutch  146  is ON, the driven gear  144  is coupled with the rotation shaft  64 , and the driving of the rotation of the registration roller  62  in the sheet conveyance direction (hereinbelow, “forward rotation”) is transmitted from the driven gear  144  to the rotation shaft  64 . The predetermined time at step S 14  is determined so as to, when a sheet with its lead edge in the position A is fed with the feed roller  58 , stop the reverse-rotation driving force transmission from the transmission gear  132  to the registration roller  62 , and at the same time, start the forward-rotation drive transmission from the driven gear  144  to the registration roller  62  and the rotation shaft  64 . 
     In this manner, the timing to turn the electromagnetic clutch  146  ON is determined so as to, when a sheet with its lead edge in the position A is fed with the feed roller  58 , stop the reverse-rotation driving force transmission from the transmission gear  132  to the registration roller  62 , and at the same time, start the drive transmission to forward-rotate the registration roller  62  to the registration roller  62 . Accordingly, when the lead edge of the sheet is in the position B, the lead edge is detected by the sensor  84  at earlier timing in comparison with the case where the lead edge of the sheet is in the position A, and the timing to start the forward-rotation driving force transmission via the driven gear  144  to the registration roller  62  is earlier. Accordingly, upon start of forward-rotation drive transmission to the registration roller  62 , the reverse-rotation drive transmission from the transmission gear  132  has not been completed. When the electromagnetic clutch  146  is turned ON, the forward-rotation driving and the reverse-rotation driving are simultaneously transmitted to the registration roller  62 . In this case, the reverse-rotation drive transmission via the driven gear  144  is broken by the torque limiter  150 . The registration roller  62  starts forward rotation when the electromagnetic clutch  146  is turned ON. Accordingly, the probability of breakage of the transmission gear  132 , the reverse gear  134 , the driving gear  142 , the driven gear  144  and the like is reduced, and even when the lead edge of the sheet is in the position A, the registration roller  62  starts forward rotation at the same time of completion of reverse rotation. That is, regardless of whether the lead edge of the sheet is in the position A or the position B, when the lead edge of the sheet becomes into contact with the registration roller  62 , the registration roller  62  is reverse-rotating. Predetermined slag is formed in the sheet and skew correction is performed, thereafter, the registration roller  62  starts forward rotation. 
     The timing of turning the electromagnetic clutch  146  ON at step S 16  may be determined so as to start the forward-rotation drive transmission to the registration roller  62  before the completion of the reverse-rotation drive transmission to the registration roller  62  even when the lead edge of the sheet is in the position A. In this case, the registration roller  62  infallibly starts the forward rotation at the same time of the completion of the reverse rotation. The skew of the sheet can be more excellently corrected. 
     When the drive transmission to forward-rotate the registration roller  62  has been performed and the registration roller  62  has started forward rotation, the sheet subjected to the skew correction at the nip N is conveyed to the image forming part  14 , and in the image forming part  14 , yellow, magenta, cyan and black toner images formed on the photoreceptors  22 Y,  22 M,  22 C and  22 B are sequentially transferred with the transfer rollers  50 Y,  50 M,  50 C and  50 B onto the sheet. The sheet where the four color toner images have been transferred is conveyed to the fixing device  52 , and the toner image is fixed to the sheet by the fixing device  52 , then discharged to the sheet discharge part  15  with the discharge roller  78 . The image formation in the image forming part  14  is performed by actuation of the image forming part by the control circuit  202  after elapse of predetermined time from the input of the sheet end detection signal at step S 14 . 
     Next, at step S 18 , a sheet trail edge detection signal is inputted from the sensor  84  into the control circuit  202 . That is, the movable member  86  of the sensor  84 , pushed with the conveyed sheet and moved to a position outside of the conveyance path  60  as shown in  FIG. 2B , moves to a position crossing the conveyance path  60  as shown in  FIG. 2A  by passing of the trail edge of the sheet around the movable member  86 . Then the sheet trail edge detection signal, generated by detection of the movement by the optical sensor, is inputted into the control circuit  202 . 
     If it is determined at step S 20  that predetermined time has elapsed since the trail edge of the sheet has been detected, the control circuit  202  stops the rotation of the motor  104  at step S 22 , and turns the electromagnetic clutch  146  OFF, thereby stops the forward rotation of the registration roller  62 . The predetermined time at step S 20  is determined to be longer than the time between the detection of the trail edge of the sheet by the sensor  84  and the passing of the trail edge of the sheet through the nip N formed with the registration rollers  61 . 
     Next, at step S 24 , the control circuit  202  determines whether or not the sheet subjected to the image formation is a final sheet based on data from the communication interface  204 . If it is determined that the sheet is a final sheet, the control circuit  202  terminates the series of operations. Further, if it determined that the sheet is not a final sheet, the process returns to step S 10 , to repeat the series of image forming operations on the next sheet. 
     As described above, the present invention is applicable to an image forming apparatus such as a copier, a facsimile machine or a printer, and to a conveyance device to convey a conveyance subject such as a sheet. 
     The foregoing description of the exemplary embodiments of the present invention has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in the art. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, thereby enabling others skilled in the art to understand the invention for various embodiments and with the various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents.