Abstract:
In an image forming apparatus, the following inequalities (1) and (2) are satisfied: 
       (L1−L3)×(V2/V1)&lt;L2  (1)
 
       L2&lt;(L1−L3+L4)×(V2/V1)  (2)
 
     where L1 denotes a peripheral length of an image carrier from an exposure position to a transfer position, L2 denotes a peripheral length of a transfer member from a gripping position to the transfer position, L3 denotes a length of a leading-end margin, L4 denotes a peripheral length of the transfer member, when recording media having a maximum transportable size are wrapped around the transfer member, between a trailing end of an image region of the first recording medium and a leading end of an image region of the second recording medium, V1 denotes a peripheral velocity of the image carrier, and V2 denotes a peripheral velocity of the transfer member.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2012-037707 filed Feb. 23, 2012. 
       BACKGROUND 
       [0002]    The present invention relates to an image forming apparatus. 
       SUMMARY 
       [0003]    An image forming apparatus according to an aspect of the present invention includes an image carrier, a surface of which is charged while the image carrier is rotating, an exposing device that faces the image carrier at an exposure position and emits light toward the image carrier to form an electrostatic latent image on the charged surface of the image carrier, a developing member that develops the electrostatic latent image formed on the surface of the image carrier into a toner image, a transfer member that, while rotating, transports a recording medium to a transfer position, at which the transfer member faces the image carrier, and that transfers the toner image formed on the surface of the image carrier to the recording medium, the recording medium being wrapped around an outer peripheral surface of the transfer member, and a gripping member that is disposed on the transfer member, the gripping member gripping a leading end portion of the recording medium having been transported to the rotating transfer member at a gripping position. In the image forming apparatus, the following inequalities (1) and (2) are satisfied: 
         [0000]      ( L 1− L 3)×( V 2 /V 1)&lt; L 2  (1)
 
         [0000]        L 2 &lt;( L 1 −L 3 +L 4)×( V 2 /V 1)  (2)
 
         [0004]    Here, L 1  denotes a peripheral length of the image carrier from the exposure position to the transfer position in a rotating direction of the image carrier. L 2  denotes a peripheral length of the transfer member from the gripping position to the transfer position in the rotating direction of the transfer member. L 3  denotes a length of a margin on a leading-end side of the recording medium in which no toner image is formed. L 4  denotes a peripheral length of the transfer member, when a first recording medium and a second recording medium having a maximum size transportable by the transfer member are wrapped around the transfer member, between a trailing end of an image region of the first recording medium and a leading end of an image region of the second recording medium, the image regions of the recording media each being a region over the entire area of which an image is formable, the first recording medium being positioned on a side that is further upstream than the second recording medium in the rotating direction of the transfer member. V 1  denotes a peripheral velocity of the image carrier. V 2  denotes a peripheral velocity of the transfer member. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]    An exemplary embodiment of the present invention will be described in detail based on the following figures, wherein: 
           [0006]      FIG. 1  is a schematic diagram illustrating the vicinity of a position at which a transfer drum and an image carrier of an image forming apparatus according to an exemplary embodiment perform nipping; 
           [0007]      FIG. 2  is a schematic diagram illustrating the vicinity of the position at which the transfer drum and the image carrier of the image forming apparatus according to the exemplary embodiment perform nipping; 
           [0008]      FIG. 3  is a table illustrating dimensions or other properties of the transfer drum and the image carrier of the image forming apparatus according to the exemplary embodiment; 
           [0009]      FIGS. 4A and 4B  illustrate a leading-end gripper of the image forming apparatus according to the exemplary embodiment that is positioned in a releasing state and in a gripping state, when viewed from a side; 
           [0010]      FIGS. 5A and 5B  illustrate the transfer drum, the leading-end gripper, and a trailing-end gripper of the image forming apparatus according to the exemplary embodiment when viewed in plan and from a side; 
           [0011]      FIGS. 6A and 6B  schematically illustrate the configuration of the trailing-end gripper of the image forming apparatus according to the exemplary embodiment; 
           [0012]      FIGS. 7A and 7B  schematically illustrate the configuration of the transfer drum and the trailing-end gripper of the image forming apparatus according to the exemplary embodiment; 
           [0013]      FIGS. 8A ,  8 B,  8 C, and  8 D illustrate a series of states in which a sheet medium P is wrapped around the transfer drum of the image forming apparatus according to the exemplary embodiment; 
           [0014]      FIGS. 9A ,  9 B,  9 C, and  9 D illustrate a series of states in which a sheet medium P that has been wrapped around the transfer drum of the image forming apparatus according to the exemplary embodiment becomes separated from the transfer drum; 
           [0015]      FIG. 10  schematically illustrates the surroundings of a transfer position Tr of the image forming apparatus according to the exemplary embodiment; and 
           [0016]      FIG. 11  schematically illustrates the image forming apparatus according to the exemplary embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]    An image forming apparatus  10  according to an exemplary embodiment of the present invention will be described referring to  FIGS. 1 to 11 . The arrow UP illustrated in the drawings denotes a vertically upward direction. 
       Entire Configuration 
       [0018]    As illustrated in  FIG. 11 , the image forming apparatus  10  according to the exemplary embodiment of the present invention includes an image forming unit  12 , a transfer device  14 , a fixing device  16 , a sheet feeding unit  18 , and a controlling unit  20 . The image forming unit  12  forms a toner image. A sheet medium P is a recording medium and is fed to the transfer device  14 , and the transfer device  14  transfers the toner image, having been formed thereon by the image forming unit  12 , to the sheet medium P that is wrapped around the transfer device  14 . The fixing device  16  fixes the toner image, having been formed on the sheet medium P released from the transfer device  14 , onto the sheet medium P. The sheet feeding unit  18  feeds the sheet medium P to the transfer device  14 . The controlling unit  20  controls the entirety of the image forming apparatus  10 . 
       Image Forming Unit 
       [0019]    The image forming unit  12  that forms a toner image will be described first. 
         [0020]    The image forming unit  12  includes an image carrier  22 , on whose surface toner images are sequentially formed while the image carrier  22  is rotating. The image forming unit  12  also includes a charging device  24 , an exposing device  26 , a rotary developing device  28 , and a cleaning device  46 . The charging device  24  charges the surface of the image carrier  22 . The exposing device  26  exposes the charged surface of the image carrier  22  to light to form an electrostatic latent image. The rotary developing device  28  develops the electrostatic latent image, having been formed on the surface of the image carrier  22 , by using a developer into a toner image. The cleaning device  46  cleans remnants remaining on the image carrier  22 . 
       Image Carrier 
       [0021]    The image carrier  22  is disposed so as to rotate in the arrow A direction and includes a negatively charged photosensitive layer  22 A on the surface. The outer diameter of the image carrier  22  is 30 mm, for example. The charging device  24 , the exposing device  26 , the rotary developing device  28 , and the cleaning device  46  are arranged around the image carrier  22  in this order in the arrow A direction. A driving source (not illustrated) that drives the image carrier  22  to rotate at a peripheral velocity V 1  is also provided. 
       Charging Device 
       [0022]    The charging device  24  is a roller-type charging device that is arranged so as to face the image carrier  22 . While the charging device  24  is driven to rotate by the rotating image carrier  22 , the charging device  24  charges the surface of the image carrier  22  by applying a charging bias to the surface from a charging-bias power source, which is not illustrated. 
       Exposing Device 
       [0023]    The exposing device  26  irradiates the surface of the image carrier  22  having been charged by the charging device  24  with light to form an electrostatic latent image. In this exemplary embodiment, the exposing device  26  includes, for example, multiple light emitting diodes (LEDs, which are not illustrated). 
       Rotary Developing Device 
       [0024]    The rotary developing device  28  includes a rotation shaft  28 A and developing members  28 Y,  28 M,  28 C, and  28 K for yellow (Y), magenta (M), cyan (C), and black (K) arranged around the rotation shaft  28 A. The rotary developing device  28  rotates in the arrow C direction around the rotation shaft  28 A. 
         [0025]    In the rotary developing device  28 , each of the developing members  28 Y,  28 M,  28 C, and  28 K is positioned at a position opposite the image carrier  22 . The rotary developing device  28  then applies a developing bias from a developing bias power source, which is not illustrated, to each electrostatic latent image on the image carrier  22  having been formed by the exposing device  26  in order to sequentially develop the electrostatic latent images into toner images of the different colors. 
         [0026]    These developing members  28 Y,  28 M,  28 C, and  28 K contain developers of corresponding colors. 
       Cleaning Device 
       [0027]    The cleaning device  46  recovers toner remaining on the surface of the image carrier  22  without being transferred to the sheet medium P by the transfer device  14 , which will be described below, or other extraneous matters from the surface of the image carrier  22 . The cleaning device  46  according to the exemplary embodiment is a blade-type cleaner. 
         [0000]    Transfer device 
         [0028]    Now, description will be given on the transfer device  14  around which a sheet medium P is wrapped and that transfers a toner image having been formed thereon by the image forming unit  12  to the wrapped sheet medium P. 
         [0029]    The transfer device  14  includes a transfer drum  30 , a leading-end gripper  32 , and a trailing-end gripper  34 . The transfer drum  30  is taken as an example of a transfer member around which a sheet medium P, to which a toner image on the image carrier  22  is transferred, is wrapped. The leading-end gripper  32  is taken as an example of a leading-end gripping member that grips a leading end portion of the sheet medium P that is wrapped around the transfer drum  30 . The trailing-end gripper  34  is taken as an example of a trailing-end controlling member that controls the position of a trailing end portion of the sheet medium P. 
         [0030]    The transfer device  14  also includes a sheet sensor  36  that detects a sheet medium P passing thereby, a driving motor M 1  (see  FIGS. 7A and 7B ) that drives the transfer drum  30  to rotate, and a power source  48  that applies a transfer bias, which is a voltage of a polarity opposite to that of the toner, to the transfer drum  30 . 
       Transfer Drum 
       [0031]    The transfer drum  30  arranged so as to face the image carrier  22  includes a rotation shaft  30 A, a drum-shaped base portion  30 B, and an elastically deformable elastic layer  30 C that is formed around the outer peripheral surface of the base portion  30 B. The outer diameter of the transfer drum is 119.4 mm, for example. 
         [0032]    The elastic layer  30 C, from a leading end to a trailing end of the elastic layer  30 C in a direction in which the sheet medium P is transported, contiguously lies on the outer periphery of the drum-shaped base portion  30 B. A portion of the transfer drum  30 , around which even a maximum-size sheet medium P is not wrapped, is a cutout region  30 D in which the elastic layer  30 C is absent such that a part of the periphery of the elastic layer  30 C is cut out. 
         [0033]    The dimensions of the components and the positional relationships between the components are determined such that the transfer drum  30  and the image carrier  22  do not contact each other when the cutout region  30 D of the transfer drum  30  faces the image carrier  22 . A dielectric substance, such as a dielectric sheet, is not attached to the outer peripheral surface of the elastic layer  30 C, and thus wrapping of a sheet medium P around the transfer drum  30  does not involve the use of electrostatic attraction. 
         [0034]    As illustrated in  FIG. 10 , at a transfer position Tr at which the transfer drum  30  and the image carrier  22  face each other to transfer a toner image to the sheet medium P, the elastic layer  30 C of the transfer drum  30  is pressed by the image carrier  22 . The elastic layer  30 C is then pressed by the image carrier  22  down to a compressed circumference NL illustrated in  FIG. 10  with the two-dot chain line. 
         [0035]    At the transfer position Tr, transporting of the sheet medium P that is nipped by the transfer drum  30  and the image carrier  22  is performed dominantly by using electrostatic attraction of the image carrier  22 . 
         [0036]    As illustrated in  FIGS. 7A and 7B , a gear  30 E is mounted on an end portion of the rotation shaft  30 A of the transfer drum  30  and engages with a gear  30 F mounted on an output shaft of the driving motor M 1 , which drives the transfer drum  30  to rotate. The transfer drum  30  is driven to rotate by the driving force of the driving motor M 1  at a peripheral velocity V 2 , which is lower than a peripheral velocity V 1  of the image carrier  22 . 
       Sheet Sensor 
       [0037]    As illustrated in  FIG. 11 , the sheet sensor  36  is arranged so as to face the outer peripheral surface of the transfer drum  30 . The sheet sensor  36  irradiates the sheet medium P, which is transported while being wrapped around the transfer drum  30 , with infrared light, and detects the sheet medium P passing thereby using the reflected light. 
         [0038]    The sheet sensor  36  is disposed on a side that is further upstream, in the direction in which the sheet medium P is transported, than a stand-by position of the trailing-end gripper  34  (the position of the trailing-end gripper  34  illustrated in  FIG. 11 ), which will be described below, and on a side that is further downstream, in the direction in which the sheet medium P is transported, than a feeding-sheet position Pa at which a sheet medium P is fed to the transfer drum  30 . Since a leading end portion of a sheet medium P is gripped by the leading-end gripper  32  at the feeding-sheet position Pa, the feeding-sheet position Pa is also referred to as a gripping position Pa. 
       Leading-End Gripper 
       [0039]    As illustrated in  FIGS. 5A and 5B , the leading-end gripper  32  that grips the leading end portion of the sheet medium P wrapped around the transfer drum  30  is attached to the transfer drum  30 , and is disposed in the cutout region  30 D.  FIG. 5A  is a development drawing in which the outer periphery of the transfer drum  30  is developed. 
         [0040]    As illustrated in  FIGS. 4A and 4B , the leading-end gripper  32  includes a pressing plate  32 A and a shaft member  32 B. The pressing plate  32 A presses the leading end portion of the sheet medium P against the elastic layer  30 C. The shaft member  32 B causes the pressing plate  32 A to rotate such that a leading end portion of the sheet medium P is griped or released. 
         [0041]    The pressing plate  32 A extends in a direction of a rotation axis of the transfer drum  30  (or may simply be referred to as a “drum axis direction”, below). For example, the pressing plate  32 A is formed by bending a stainless steel plate, and has a single bent portion when viewed in the drum axis direction. 
         [0042]    An axis direction of the shaft member  32 B is along the drum axis direction. The shaft member  32 B, which is cylindrical, is secured to a first end portion of the pressing plate  32 A. Accordingly, when the shaft member  32 B is rotated, the leading-end gripper  32  moves so as to switch between a gripping state, in which a second end portion of the pressing plate  32 A grips the leading end portion of the sheet medium P (see  FIG. 4B ), and a releasing state, in which the second end portion releases the leading end portion of the sheet medium P (see  FIG. 4A ). 
         [0043]    As illustrated in  FIGS. 4A and 4B , a locus of the leading-end gripper  32  that moves so as to switch between the gripping state and the releasing state is formed on the inner side of the compressed circumference NL, and thus the leading-end gripper  32  does not contact the image carrier  22 . In other words, the leading-end gripper  32  is located outside a region within which the image carrier  22  compresses the elastic layer  30 C, and thus when the leading-end gripper  32  has been moved to the transfer position Tr, the leading-end gripper  32  is separated from the image carrier  22 . 
       Trailing-End Gripper 
       [0044]    As illustrated in  FIGS. 5A and 5B , the trailing-end gripper  34  is stretched across the transfer drum  30  in the drum axis direction, and rotates around the rotation shaft  30 A independently of the transfer drum  30 . 
         [0045]    As illustrated in  FIGS. 7A and 7B , the trailing-end gripper  34  includes a sheet controlling portion  34 A extending in the drum axis direction, and holding portions  34 B that hold both end portions of the sheet controlling portion  34 A. The sheet controlling portion  34 A stops the trailing end portion of the sheet medium P from moving. 
         [0046]    The sheet controlling portion  34 A is made of a film-formed resin material and is elastically deformable. Examples of the resin material include polyethylene terephthalate (PET), polyimide, and fluorocarbon resins. 
         [0047]    The holding portions  34 B extend in the radial direction of the transfer drum  30  (also simply referred to as a “drum radial direction”, below). The trailing-end gripper  34  also includes wedge-shaped shifting members  34 C, whose movement in the drum axis direction causes the sheet controlling portion  34 A to move in the drum radial direction via the holding portions  34 B. 
         [0048]    As illustrated in  FIGS. 6A and 6B , gears  34 E are attached to the rotation shaft  30 A via bearings  34 D, and supporting portions  34 F extending in the drum radial direction are attached to the gears  34 E. Each holding portion  34 B is disposed so as to be movable with respect to a corresponding one of the supporting portions  34 F in the drum radial direction. A spring member  34 G is interposed between each holding portion  34 B and a corresponding supporting portion  34 F, the spring member  34 G urging the holding portion  34 B in a radially inward direction. The trailing end gripper  34  also includes stopper portions  34 J that control the positions of the holding portions  34 B when being contacted by the holding portions  34 B having been urged by the spring members  34 G in the radially inward direction. 
         [0049]    In this configuration, when the controlling unit  20  controls a solenoid, which is not illustrated, to move the wedge-shaped shifting members  34 C in the drum axis direction and insert each of the wedge-shaped shifting members  34 C between one of the holding portions  34 B and a corresponding stopper portion  34 J, the holding portions  34 B are moved in a radially outward direction. With this operation, the sheet controlling portion  34 A switches to the releasing state, in which the sheet controlling portion  34 A becomes separated from the elastic layer  30 C to release the trailing end portion of the sheet medium P (see  FIGS. 6A and 7A ). 
         [0050]    On the other hand, when the controlling unit  20  controls a solenoid, which is not illustrated, to move the wedge-shaped shifting members  34 C in the drum axis direction and pull out each of the wedge-shaped shifting members  34 C from between one of the holding portions  34 B and a corresponding stopper portion  34 J, the holding portions  34 B are moved in a radially inward direction. With this operation, the sheet controlling portion  34 A switches to the controlling state in which the sheet controlling portion  34 A brings the sheet medium P into contact with the elastic layer  30 C such that the sheet medium P contiguously lies on the elastic layer  30 C (see  FIGS. 6B and 7B ). 
         [0051]    As illustrated in  FIGS. 7A and 7B , a driving motor M 2  that drives the trailing-end gripper  34  to rotate around the rotation shaft  30 A is provided, and a gear  34 H that is mounted on an output shaft of the driving motor M 2  engages with one of the gears  34 E. 
         [0052]    As described above, since the trailing-end gripper  34  is disposed as a body that is separate from the transfer drum  30 , the position of the trailing-end gripper  34  is changeable with respect to the transfer drum  30 . 
         [0053]    When the leading-end gripper  32  grips the leading end portion of the sheet medium P, the leading-end gripper  32  does not allow the sheet medium P to move in the transporting direction and stops the sheet medium P from being separated from the transfer drum  30 . On the other hand, when the trailing-end gripper  34  controls the trailing end portion of the sheet medium P, the trailing-end gripper  34  allows the sheet medium P to move in the transporting direction but stops the sheet medium P from being separated from the transfer drum  30 . 
       Fixing Device 
       [0054]    The fixing device  16  that fixes a toner image formed on a sheet medium P onto the sheet medium P will be described now. 
         [0055]    As illustrated in  FIG. 11 , the fixing device  16  includes a heating roller  16 A and a pressurizing roller  16 B. The heating roller  16 A includes a heating source (not illustrated) and a rotating force is transmitted to the heating roller  16 A. The pressurizing roller  16 B is in contact with the heating roller  16 A with pressure. 
         [0056]    When a sheet medium P holding a toner image is nipped between and transported by the heating roller  16 A and the pressurizing roller  16 B, the toner image is melted and pressurized and is thus fixed onto the sheet medium P. 
         [0057]    Discharging rollers  44  are disposed on a side that is further downstream than the fixing device  16  in the direction in which the sheet medium P is transported. The discharging rollers  44  discharge the sheet medium P, having a toner image fixed thereon, to a discharge portion  42  formed on an upper surface of an apparatus body  10 A. 
       Sheet Feeding Unit 
       [0058]    Now, the sheet feeding unit  18  that feeds a sheet medium P to the transfer device  14  will be described. 
         [0059]    The sheet feeding unit  18  is disposed at a lower portion in the apparatus body  10 A of the image forming apparatus  10  and includes a sheet containing member  18 A, a pick-up roller  18 B, separation rollers  18 C, and a leading-end sensor  18 D. The sheet containing member  18 A contains sheet media P. The pick-up roller  18 B picks up the sheet media P from the sheet containing member  18 A. The separation rollers  18 C separate closely attached sheet media P from each other. The leading-end sensor  18 D detects the leading end portion of a sheet medium P passing thereby. 
         [0060]    The sheet feeding unit  18  also includes multiple transporting rollers  18 E. Each sheet medium P is transported by the transporting rollers  18 E along a transport path  40 . 
         [0061]    In this manner, each sheet medium P is transported along the transport path  40  from the sheet containing member  18 A to the feeding-sheet position Pa, which is positioned on a side that is further upstream than the transfer position Tr in the direction of rotation of the transfer drum  30 . 
       Operations of Entire Configuration 
       [0062]    Now, operations of the entire configuration will be described. 
         [0063]    Firstly, color image data that has been formed by a personal computer or the like, which is not illustrated, is input to an image signal processor (not illustrated) as red (R), green (G), and blue (B) data, for example, and is then subjected to image processing. The image data that has been subjected to image processing is converted into four-color gradation data for yellow (Y), magenta (M), cyan (C), and black (K), which is output to the exposing device  26 , so that an image forming operation is started. 
         [0064]    With the start of the image forming operation, the image carrier  22  and the transfer drum  30  start rotating together. Here, the peripheral velocity V 1  of the image carrier  22  is higher than the peripheral velocity V 2  of the transfer drum  30 . For example, the peripheral velocity V 1  of the image carrier  22  is approximately 0.5% to 1% higher than the peripheral velocity V 2  of the transfer drum  30 . 
         [0065]    At this time, the leading-end gripper  32  and the trailing-end gripper  34  are in the releasing state. 
         [0066]    While the leading-end gripper  32  rotates together with the transfer drum  30 , the trailing-end gripper  34  remains stationary at the stand-by position without rotating together with the transfer drum  30 . 
         [0067]    The photosensitive layer  22 A of the rotating image carrier  22  is charged by the charging device  24 . The exposing device  26  then irradiates the image carrier  22  with light so that an electrostatic latent image for a first color (yellow, for example) based on the image information is formed on the image carrier  22 . 
         [0068]    Meanwhile, the rotary developing device  28  rotates so that a developing member containing a toner of the color corresponding to the electrostatic latent image to be formed on the image carrier  22  (the yellow developing member  28 Y, if the corresponding color is yellow) is positioned at a position opposite the image carrier  22 . 
         [0069]    Thereafter, the developing member  28 Y develops the electrostatic latent image on the image carrier  22  to form a toner image on the image carrier  22 . This toner image is transported toward the transfer position Tr, at which the toner image faces the transfer drum  30 , with the rotation of the image carrier  22 . 
         [0070]    With the start of the image forming operation, feeding of a sheet medium P is also started. Specifically, sheet media P that are picked up from the sheet containing member  18 A by the pick-up roller  18 B are separated by the separation rollers  18 C. The separated sheet media P are forwarded to the transport path  40  by the transporting rollers  18 E. The leading-end sensor  18 D then detects the leading end portion of each sheet medium P passing thereby and transmits a detection signal to the controlling unit  20 . 
         [0071]    The controlling unit  20  that has received the detection signal controls transportation of the sheet medium P on the basis of the detection signal such that the sheet medium P arrives at the feeding-sheet position Pa at the same time as when the leading-end gripper  32  arrives at the feeding-sheet position Pa (see  FIG. 8A ). 
         [0072]    Here, at the time of feeding the sheet medium P, information on the size of the sheet medium P that has been detected by a sheet-size sensor (not illustrated) is transmitted to the controlling unit  20 . 
         [0073]    As illustrated in  FIG. 8B , the leading-end gripper  32  that has been in the releasing state switches to the gripping state at the same time as when the leading end portion of the sheet medium P arrives at the feeding-sheet position Pa. The leading end portion of the sheet medium P is thus gripped by the leading-end gripper  32 . 
         [0074]    The leading-end gripper  32  gripping the sheet medium P then passes a position opposite the stationary trailing-end gripper  34 . The leading-end gripper  32  having passed the trailing-end gripper  34  then moves toward the transfer position Tr while gripping the sheet medium P. 
         [0075]    The sheet medium P that has passed the transfer position Tr while being gripped by the leading-end gripper  32  is consequently wrapped around the transfer drum  30  while being gripped by the leading-end gripper  32 , as illustrated in  FIG. 8C . 
         [0076]    The toner image of the first color (yellow, for example) formed on the image carrier  22  is transferred to the sheet medium P on the transfer drum  30  at the transfer position Tr at which the image carrier  22  and the transfer drum  30  face each other. Part of toner remaining on the image carrier  22  after the transfer is recovered from the image carrier  22  by the cleaning device  46  (see  FIG. 10 ). 
         [0077]    Thereafter, the sheet sensor  36  detects the trailing end portion of the sheet medium P passing thereby. The controlling unit  20  that has received a signal from the sheet sensor  36  sends an instruction to the trailing-end gripper  34 . 
         [0078]    The trailing-end gripper  34  having received the instruction switches from the releasing state to the controlling state to control the trailing end portion of the sheet medium P. The trailing-end gripper  34  that has switched to the controlling state starts rotating together with the transfer drum  30 . In other words, the sheet controlling portion  34 A of the trailing-end gripper  34  moves at the same velocity as the peripheral velocity V 2  of the transfer drum  30 . 
         [0079]    As illustrated in  FIG. 8D , the trailing-end gripper  34  rotating together with the transfer drum  30  passes the transfer position Tr while controlling the trailing end portion of the sheet medium P. 
         [0080]    Likewise, forming and developing of latent images for a second and subsequent colors (magenta and cyan, for example), which precede a final color (black, for example), and transferring of toner images corresponding to the latent images is repeated in accordance with the above-described procedure. 
         [0081]    As illustrated in  FIGS. 9A ,  9 B, and  9 C, in the case of transferring a toner image of a final color (black, for example), the leading-end gripper  32  switches from the gripping state to the releasing state at the transfer position Tr, unlike in the case of transferring a toner image of a color that precedes the final color. 
         [0082]    As illustrated in  FIG. 9D , when the leading-end gripper  32  releases the leading end portion of the sheet medium P on which multiple toner images are formed, the leading end portion becomes separated from the transfer drum  30  due to having been nipped by the elastic layer  30 C and the image carrier  22 . 
         [0083]    The sheet medium P whose leading end portion is separated from the transfer drum  30  is transported toward the fixing device  16  illustrated in  FIG. 11 . 
         [0084]    As the sheet medium P is transported further, the trailing-end gripper  34  that controls the trailing end portion of the sheet medium P arrives at the stand-by position. At the stand-by position, the trailing-end gripper  34  switches from the controlling state to the releasing state to release the trailing end portion of the sheet medium P. The trailing-end gripper  34  that has switched to the releasing state stops at the stand-by position. 
         [0085]    The toner images on the sheet medium P having been transported to the fixing device  16  are fixed onto the sheet medium P by the fixing device  16 . As the sheet medium P is transported further, the sheet medium P becomes separated from the transfer drum  30 . The sheet medium P is finally discharged to the discharge portion  42  by the discharging rollers  44 . 
       Configuration of Related Portion 
       [0086]    Now, a relationship between L 1  and the length of each component will be described, where L 1  denotes the peripheral length of the image carrier  22  from an exposure position Ro to the transfer position Tr. 
         [0087]    For easy understanding of the relationship between the lengths of components,  FIGS. 1 and 2  schematically illustrate simple forms of the components and exclude unrelated components. 
         [0088]    In  FIGS. 1 and 2 , a sheet medium P 1  is a first sheet medium P that is firstly wrapped around the transfer drum  30  and to which first toner images are to be transferred. A sheet medium P 2 , on the other hand, is a sheet medium P that is wrapped around the transfer drum  30  after the sheet medium P 1  has been wrapped and to which second toner images are to be transferred after all the first toner images are transferred to the sheet medium P 1 . 
         [0089]    In  FIG. 2 , the sheet medium P 2  (indicated by the two-dot chain line in  FIG. 2 ) is located on a side that is further upstream than the gripping position Pa in the direction in which the sheet medium P is transported. Thus, the sheet medium P 2  is not actually wrapped around the transfer drum  30  at this time. However, for easy understanding of the positional relationship between the sheet medium P 1  and the sheet medium P 2  on the transfer drum  30 , the sheet medium P 2  is illustrated by the two-dot chain line assuming that the sheet medium P 2  is wrapped around the transfer drum  30 . 
         [0090]    In  FIGS. 1 and 2 , for easy comparison between the peripheral length of the transfer drum  30  and the peripheral length of the image carrier  22 , these peripheral lengths are illustrated without considering the peripheral velocity of the image carrier  22 , which is the peripheral velocity V 1 , and the peripheral velocity of the transfer drum  30 , which is the peripheral velocity V 2 . 
         [0091]    Each of the sheet media P 1  and P 2  is of a maximum size transportable by the transfer drum  30  (legal size, for example, in the exemplary embodiment). 
         [0092]    The peripheral length of the transfer drum  30  from the gripping position Pa to the transfer position Tr is denoted by L 2 . The length of a leading-end margin on a leading-end side of the sheet medium P in which no toner image is formed is denoted by L 3 . The peripheral length of the transfer drum  30  from a limit position, up to which a toner image is formable, on the trailing-end side of the sheet medium P 1  to a limit position, up to which a toner image is formable, on a leading-end side of the sheet medium P 2  (this peripheral length is also referred to as an inter-image distance) is denoted by L 4  (the distance between the sheet media+the leading-end margin+the trailing-end margin). The peripheral velocity of the image carrier  22  is denoted by the peripheral velocity V 1 , and the peripheral velocity of the transfer drum  30  is denoted by the peripheral velocity V 2 . Under the above conditions, the dimensions of components are determined such that the following inequalities (1) and (2) are satisfied: 
         [0000]      ( L 1− L 3)×( V 2 /V 1)&lt; L 2  (1)
 
         [0000]        L 2 &lt;( L 1 −L 3 +L 4)×( V 2 /V 1)  (2)
 
         [0093]    The length of the leading-end margin on the leading-end side of the sheet medium P in which a toner image is not formed is a length between the limit position, up to which a toner image is formable, on the leading-end side of the sheet medium P and the leading end of the sheet medium P. 
         [0094]      FIG. 3  illustrates examples of dimensions of the components with which the inequalities (1) and (2) are satisfied when, for example, the peripheral velocity V 1  is 100.0 mm/s and the peripheral velocity V 2  is 99.5 mm/s. 
       Operations of Configuration of Related Portion 
       [0095]      FIG. 1  illustrates a state where an exposure for forming a toner image that is to be transferred to a sheet medium P 1  is started, and  FIG. 2  illustrates a state where the exposure for forming a toner image that is to be transferred to a sheet medium P 1  is finished. 
         [0096]    As illustrated in  FIG. 1 , when the inequality (1) is satisfied, the leading end of the sheet medium P 1  arrives at a position that is L 1 −L 3  away from the transfer position Tr at the time of starting an exposure for forming a first toner image that is to be transferred to a sheet medium P 1  (at the exposure start). In other words, the leading end portion of the sheet medium P 1  is gripped at this time (the leading end of the sheet medium P 1  has passed the gripping position Pa). That is, the exposure is started after the leading end portion of the sheet medium P 1  is gripped. 
         [0097]    As illustrated in  FIG. 2 , when the inequality (2) is satisfied on the other hand, the leading end of the sheet medium P does not arrive at the gripping position Pa by the completion of the exposure. In other words, the exposure for forming a final toner image to be transferred to the sheet medium P 1  is complete before the sheet medium P 2  is gripped. That is, the exposure is finished before the leading end portion of the sheet medium P 2  is gripped (completion of exposure). 
         [0098]    As described above, when the configuration satisfies the inequalities (1) and (2), the exposure is started after the leading end portion of a first (preceding) sheet medium P 1  is gripped, and the exposure is complete before the leading end portion of a second (subsequent) sheet medium P 2  is gripped. In other words, while the exposure is performed, gripping of the leading end portion of the sheet medium P is not performed. This suppresses image defects due to vibration, which occurs through the gripping of the leading end portion of the sheet medium P, being transmitted to the exposure position Ro of the image carrier  22 . 
         [0099]    In the above case, a sheet medium P of a maximum size is taken into consideration. Accordingly, image defects due to vibration, which occurs through the gripping of the leading end portion of the sheet medium P, being transmitted to the exposure position Ro of the image carrier  22  are also prevented from occurring in sheet media P of any size. 
         [0100]    Although the present invention has been described in detail on the basis of a specific exemplary embodiment, it is obvious to those skilled in the art that the present invention is not limited to the exemplary embodiment and that various other exemplary embodiments may be made within the scope of the invention. Although the positional relationships are described by taking the inequalities (1) and (2) as examples in the above exemplary embodiment, other configurations which are based on other formulae are also acceptable if the configurations consequently satisfy these inequalities (1) and (2). 
         [0101]    The foregoing description of the exemplary embodiment 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 embodiment was 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.