Abstract:
An image-forming apparatus includes a main body, a belt unit detachably attachable to the main body, a coupling and a first restricting part. The belt unit includes: a first roller defining an axis extending in an axial direction, a second roller opposing the first roller in a first direction perpendicular to the axial direction, and a belt looped over the first roller and the second roller. The coupling is configured to move in the axial direction to come into engagement with the first roller for inputting a drive force into the first roller. The first restricting part is provided on the main body and is configured to restrict the first roller from moving in a second direction perpendicular to the axial direction and the first direction when the belt unit is attached to the main body.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority from Japanese Patent Application No. 2014-135639 filed Jul. 1, 2014. The entire content of the priority application is incorporated herein by reference. 
     TECHNICAL FIELD 
     The present disclosure relates to an image forming apparatus provided with a belt unit that is detachably mountable in a main body of the image forming apparatus. 
     BACKGROUND 
     In one conventional image-forming apparatus provided with a belt unit that is detachably mountable in a main body of the image-forming apparatus, the belt unit includes a belt, and a drive roller and a follow roller over which the belt is mounted in a taut state (for example, please refer to Japanese Patent Application Publication No. 2011-075618). The main body of this image-forming apparatus is provided with a position-fixing part for fixing the position of the belt unit, and a belt drive gear that inputs a drive force into a drive roller gear provided on the drive roller. This drive force serves to press the belt unit against the position-fixing part. 
     However, changing the distance between the axes of the belt drive gear and the drive roller gear tends to change the conveying speed of the belt. In order to suppress such fluctuations in the conveying speed of the belt, a construction employing a coupling was conceived, whereby the coupling engages the drive roller in its axial direction and inputs a drive force into the drive roller. However, since the drive force inputted by the coupling does not generate a force for pressing the belt unit against the position-fixing part, as does the drive force of the belt drive gear, any disturbance or the like that changes the vertical position of the belt unit could cause the behavior of the belt to become unstable. 
     SUMMARY 
     In view of the foregoing, it is an object of the disclosure to provide an image-forming apparatus capable of ensuring a stable behavior of a belt. 
     In order to attain the above and other objects, the disclosure provides an image-forming apparatus that may include: a main body; a belt unit attachable to the main body; a coupling; and a first restricting part. The belt unit may include a belt, a first roller and a second roller. The first roller defines an axis extending in an axial direction and is configured to rotate about the axis. The second roller opposes the first roller in a first direction perpendicular to the axial direction. The belt is looped over the first roller and the second roller and extends in the first direction. The coupling is configured to move in the axial direction to come into engagement with the first roller for inputting a drive force into the first roller. The first restricting part is provided on the main body and is configured to restrict the first roller from moving in a second direction perpendicular to the axial direction and the first direction when the belt unit is attached to the main body. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The particular features and advantages of the disclosure as well as other objects will become apparent from the following description taken in connection with the accompanying drawings, in which: 
         FIG. 1  is a cross-sectional view showing a general construction of a color printer according to a first embodiment, the color printer having a belt unit; 
         FIG. 2  is a partially enlarged explanatory view showing structures near the belt unit according to the first embodiment; 
         FIG. 3A  is an explanatory view illustrating movement of an interlocking mechanism according to the first embodiment, wherein a retaining member is at a removed position; 
         FIG. 3B  is an explanatory view illustrating movement of the interlocking mechanism according to the first embodiment, wherein the retaining member is at a mounted position; 
         FIG. 4A  is a cross-sectional view taken along a plane AI-AI shown in  FIG. 3A ; 
         FIG. 4B  is a cross-sectional view taken along a plane AII-AII shown in  FIG. 3B ; 
         FIGS. 5A-5C  are explanatory views illustrating how the belt unit according to the first embodiment is attached to a main body of the color printer; 
         FIG. 6  is a cross-sectional view showing a general construction of a color printer according to a second embodiment; 
         FIG. 7A  is an explanatory view illustrating movement of an interlocking mechanism according to the second embodiment, wherein a process unit according to the second embodiment is displaced from its mounted position; 
         FIG. 7B  is an explanatory view illustrating movement of the interlocking mechanism according to the second embodiment, wherein the process unit according to the second embodiment is in its mounted position; 
         FIG. 8  is an explanatory view illustrating a structure of an interlocking mechanism according to a third embodiment, wherein a process unit according to the third embodiment is in an unlocked state; 
         FIG. 9  is an explanatory view illustrating a structure of the interlocking mechanism according to the third embodiment, wherein the process unit according to the third embodiment is in a locked state; 
         FIG. 10A  is a bottom side view of a coupling configured to be engaged with a drum locking rod constituting the interlocking mechanism according to the third embodiment in a state shown in  FIG. 8 ; 
         FIG. 10B  is a bottom side view of the coupling configured to be engaged with the drum locking rod constituting the interlocking mechanism according to the third embodiment in a state shown in  FIG. 9 ; 
         FIG. 11A  is a schematic view showing structures of a first restricting member and a second restricting member according to a first modification of the first embodiment; 
         FIG. 11B  is a schematic view showing structures of a first restricting member and a second restricting member according to a second modification of the first embodiment; and 
         FIG. 11C  is a schematic view showing structures of a first restricting member and a second restricting member according to a third modification of the first embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     1. First Embodiment 
     A color printer  1  according to a first embodiment will be described while referring to  FIGS. 1 through 5C . 
     Directions in the following description will be based on an orientation of the color printer  1  shown in  FIG. 1 . Specifically, the right side of the color printer  1  in  FIG. 1  will be called the “front,” the left side will be called the “rear,” the near side will be called the “left,” and the far side will be called the “right.” Further, the “top” and “bottom” of the color printer  1  will correspond to the vertical direction in  FIG. 1 . 
     As shown in  FIG. 1 , the color printer  1  includes a main body  10 . Within the main body  10 , primarily provided are a sheet-feeding section  2  for supplying sheets P of paper to be printed, an image-forming section  3  for forming images on the sheets P supplied by the sheet-feeding section  2 , and a paper-discharging section  4  for discharging sheets P from the main body  10  after the image-forming section  3  has formed an image thereon. 
     The sheet-feeding section  2  is provided in a bottom portion of the main body  10 . The sheet-feeding section  2  primarily includes a paper tray  21  configured to accommodate sheets P of paper, and a sheet-feeding mechanism  22  for supplying the sheets P from the paper tray  21  to the image-forming section  3 . The sheet-feeding mechanism  22  is configured to separate the sheets P in the paper tray  21  and supply the sheets P one at a time to the image-forming section  3 . 
     The image-forming section  3  primarily includes an exposure unit  30 , an image-forming unit  40 , a belt unit  50 , a belt cleaner  60 , and a fixing unit  70 . 
     The exposure unit  30  is provided in an upper portion of the main body  10 . Although not shown in the drawings, the exposure unit  30  includes a laser light-emitting unit, a polygon mirror, lenses, reflecting mirrors, and the like. 
     The image-forming unit  40  is arranged between the sheet-feeding section  2  and the exposure unit  30 . The image-forming unit  40  primarily includes four process units  41 , and a retaining member  42  for retaining the four process units  41  at positions juxtaposed in the front-rear direction. The retaining member  42  can be moved between a mounted position inside the main body  10  (the position shown in  FIGS. 1 and 3B ) and a removed position displaced from the mounted position (the position shown in  FIG. 3A , for example). 
     Each process unit  41  includes a photosensitive drum  43  (as an example of a photosensitive member), a charger  44 , and a developing roller  46 , as well as a supply roller, a thickness-regulating blade, and a toner-accommodating portion for which reference numerals have been omitted. 
     The belt unit  50  is provided between the sheet-feeding section  2  and the image-forming unit  40 . The belt unit  50  can be mounted in and removed from the main body  10 . The belt unit  50  primarily includes a drive roller  51  (as an example of a first roller), a follow roller  52  (as an example of a second roller), an endless conveying belt  53  mounted over the drive roller  51  and follow roller  52  in a taut state, and four transfer rollers  54 . 
     As shown in  FIG. 2 , the conveying belt  53  (as an example of a belt) is stretched over the drive roller  51  and follow roller  52  in the front-rear direction (as an example of a first direction). The conveying belt  53  has a top surface  53 A, and a bottom surface  53 B. The top surface  53 A extends in the front-rear direction and contacts each of the photosensitive drums  43 . The bottom surface  53 B also extends in the front-rear direction and contacts a cleaning roller  61  of the belt cleaner  60  described later. The transfer rollers  54  are arranged on the inside of the loop formed by the conveying belt  53  at positions for pinching the conveying belt  53  against corresponding photosensitive drums  43 . 
     As shown in  FIG. 1 , the belt cleaner  60  is disposed beneath the belt unit  50 . The belt cleaner  60  includes the cleaning roller  61 , a recovery roller  62 , a blade  63 , and a waste box  64 . As described above, the cleaning roller  61  is in contact with the bottom surface  53 B of the conveying belt  53  and is configured to rotate so as to move in a direction opposite that the bottom surface  53 B moves. A backup roller  56  is disposed inside the loop defined by the conveying belt  53  at a position for confronting the cleaning roller  61 . 
     The fixing unit  70  is disposed to the rear of the image-forming unit  40  and belt unit  50 . The fixing unit  70  includes a heating roller  71 , and a pressure roller  72  disposed in confrontation with the heating roller  71  and contacting the heating roller  71  with pressure. 
     In an image-forming operation of the image-forming section  3 , the charger  44  applies a uniform charge to the surface of the photosensitive drum  43 , after which the exposure unit  30  irradiates laser beams (indicated by chain lines in  FIG. 1 ) in a high-speed scan to expose the surfaces of corresponding photosensitive drums  43  and form electrostatic latent images thereon. The supply rollers of the process units  41  then supply toner from the corresponding toner-accommodating portions to the corresponding developing rollers  46 , and the toner on the surfaces of the developing rollers  46  is maintained at a uniform thickness by the corresponding thickness-regulating blades. 
     The toner carried on the surfaces of the developing rollers  46  is then supplied to the electrostatic latent images formed on the corresponding photosensitive drums  43  to produce toner images on the photosensitive drums  43 . Thereafter, a sheet P supplied onto the conveying belt  53  is conveyed between the photosensitive drums  43  and conveying belt  53  (transfer rollers  54 ), whereby the toner images formed on the photosensitive drums  43  are respectively transferred to and superposed on the sheet P. After the toner image has been transferred onto the sheet P, the sheet P is conveyed between the heating roller  71  and pressure roller  72  of the fixing unit  70  to thermally fix the toner image to the sheet P. 
     The paper-discharging section  4  includes a discharge path  81  that extends upward from the exit point of the fixing unit  70  and subsequently frontward, and a plurality of conveying rollers  82  for conveying the sheets P. After a toner image is thermally fixed to a sheet P in the fixing unit  70 , the conveying rollers  82  convey the sheet P along the discharge path  81  and discharge the sheet P onto a discharge tray  12  provided on the top of the main body  10 . 
     A front cover  11  is provided on a front end portion of the main body  10  and serves as a front wall that can be opened and closed. When the front cover  11  is open, an operator can pull the retaining member  42  out from the main body  10  to the removed position in order to remove the process units  41 . By removing the retaining member  42  from the main body  10 , the operator can also remove the belt unit  50  from the main body  10 . 
     Next, a detailed structure near the belt unit  50  will be described. 
     As shown in  FIG. 2 , the belt unit  50  includes a frame  55  that supports the drive roller  51 , follow roller  52 , and conveying belt  53  described above. 
     The frame  55  rotatably supports the drive roller  51  through bearings  51 A serving as examples of a first restricted part. The drive roller  51  has a shaft  51 B that protrudes outward (rightward and leftward) in the left-right direction from the frame  55 . Each of the bearings  51 A has an outer circumferential surface  51 C as an example of a cylindrical surface that has a circular shape concentric with the drive roller  51 . That is, each bearing  51 A defines an axial center coincident with the axis of the drive roller  51  (axis of the shaft  51 B of the drive roller  51 ). 
     The frame  55  rotatably supports the follow roller  52  through bearings  52 A such that the follow roller  52  can move in the front-rear direction. Compressed springs  13  provided in the main body  10  urge the follow roller  52  forward so that the follow roller  52  can be moved away from the drive roller  51  to apply a suitable tension to the conveying belt  53 . 
     Pins  55 A as examples of a second restricted part and a third restricted part are provided on the belt unit  50  so as to protrude outward in respective left and right directions. The pins  55 A are positioned closer to the follow roller  52  side of the frame  55  than the drive roller  51  side. 
     A handle  57  is provided on the belt unit  50 , and specifically is fixed to a front end portion of the frame  55 . The user can grip the handle  57  when mounting the belt unit  50  in and removing the belt unit  50  from the main body  10 . 
     Provided in the main body  10  are a first restricting member  100  as an example of a first restricting part, a pair of (left and right) second restricting members  110 , and a pair of (left and right) pressing members  120  as examples of a first pressing member and a second pressing member. 
     The first restricting member  100  is provided on the left side of the drive roller  51  of the belt unit  50  mounted in the main body  10 . The first restricting member  100  is configured to restrict the position of the drive roller  51  vertically through the bearing  51 A of the drive roller  51 . This vertical direction is an example of a second direction that is orthogonal to both the left-right and front-rear directions. 
     The first restricting member  100  has a base part  101  that is elongated vertically, an upper part  102  that extends forward from a top edge of the base part  101 , and a lower part  103  that extends forward from a bottom edge of the base part  101 . The lower part  103  extends much farther forward than the upper part  102 . The first restricting member  100  is open on the front side. 
     The upper part  102  and lower part  103  define a gap therebetween slightly larger than the outer diameter of the bearing  51 A so as to fit around the outer circumferential surface  51 C of the bearing  51 A. By fitting the bearing  51 A between the upper part  102  and lower part  103 , the outer circumferential surface  51 C of the bearing  51 A can contact a bottom surface  102 A of the upper part  102  and a top surface  103 A of the lower part  103 . Thus, the bottom surface  102 A of the upper part  102  and the top surface  103 A of the lower part  103  oppose each other with the axis of the drive roller  51  interposed therebetween. The bottom surface  102 A and top surface  103 A are examples of a pair of restricting surfaces. 
     The second restricting members  110  are positioned to be spaced away from the first restricting member  100  in the front-rear direction. The second restricting members  110  are configured to restrict movement of the belt unit  50  in both front-rear direction and vertical direction. The second restricting members  110  are disposed closer to the follow roller  52  of the belt unit  50  than to the drive roller  51  in the front-rear direction. Each second restricting member  110  integrally includes a rear restricting part  111  as an example of a second restricting part, and a bottom restricting part  112  as an example of a third restricting part. 
     The rear restricting part  111  is elongated vertically and positioned to contact a rear surface of the corresponding pin  55 A. The bottom restricting part  112  extends forward from a bottom edge of the corresponding rear restricting part  111  and is positioned to contact a bottom surface of the corresponding pin  55 A. 
     The pressing members  120  are provided in front of the corresponding pins  55 A and are capable of pivoting about respective pivot shafts  121 . Each pressing member  120  includes a first arm  122  extending upward from the pivot shaft  121 , and a second arm  123  extending downward from the pivot shaft  121 . 
     The first arm  122  is configured to contact an upper front portion of the corresponding pin  55 A in order to press the pin  55 A in a direction diagonally downward and rearward. The second arm  123  has a bottom end that is engaged with one end of a spring  124 . 
     The other end of the spring  124  is engaged with the main body  10 . The spring  124  constantly urges the second arm  123  forward. Consequently, the first arm  122  is constantly urged rearward by the spring  124 . With this configuration, the pressing member  120  presses the corresponding pin  55 A toward the second restricting member  110 , and specifically toward the rear restricting part  111  and bottom restricting part  112 . 
     The first arm  122  has a top endface  122 A that slopes downward toward the rear. When mounting the belt unit  50  in the main body  10 , the pins  55 A coming downward contact the top endfaces  122 A of the first arms  122 , causing the first arms  122  to move forward out of the path of the pins  55 A. 
     Further, as shown in  FIGS. 3A and 4A , a coupling  130  is provided in the main body  10  for inputting a drive force into the drive roller  51 . The coupling  130  is provided leftward of the drive roller  51  and is capable of engaging the shaft  51 B of the drive roller  51  from the left side. 
     As shown in  FIG. 4A , an output-side coupling  51 D is provided on a left end of the shaft  51 B in the drive roller  51 . The coupling  130  is capable of advancing toward the output-side coupling  51 D and retracting from the output-side coupling  51 D in the left-right direction. Specifically, the coupling  130  can move between a retracted position (the position shown in  FIG. 4A ) separated from the output-side coupling  51 D and a connected position (the position shown in  FIG. 4B ) connected to the output-side coupling  51 D. 
     The coupling  130  includes a shaft  130 B, a flange  130 C and an input part  130 D. The flange  130 C protrudes radially outward from a right end of the shaft  130 B (the bottom end in the drawing). The input part  130 D protrudes rightward (downward in the drawing) from the flange  130 C. 
     A drive gear  131  is coaxially provided on a left end (the top end in the drawing) of the shaft  130 B for receiving a drive force from a motor (not shown). The shaft  130 B is inserted into the drive gear  131  and rotates together with the same. The shaft  130 B can move relative to the drive gear  131  in the left-right direction. A compressed spring  15  is provided in the main body  10  for urging the shaft  130 B rightward (toward the drive roller  51 ). The flange  130 C has an annular shape and is capable of contacting a sliding member  142  described later. 
     The input part  130 D is configured to input the drive force transmitted from the drive gear  131  into the drive roller  51  via a joint  132 . The input part  130 D has a smaller diameter than the shaft  130 B and flange  130 C. 
     The joint  132  functions to transmit the drive force from the drive gear  131  to the drive roller  51  via the coupling  130  while the shaft  130 B of the coupling  130  (input side) is offset from the shaft  51 B of the drive roller  51  (output side). The joint  132  includes an Oldham member configured to be able to transmit the drive force even when the shaft  130 B and the shaft  51 B are in a decentered state. More specifically, as shown in  FIGS. 3A and 4A , the joint  132  includes a disc-shaped base part  132 A, protruding parts  132 B protruding rightward from the base part  132 A, and an engaging part (not shown) that engages with the right end of the input part  130 D. The protruding parts  132 B are capable of engaging with the output-side coupling  51 D on the shaft  51 B of the drive roller  51 . An elongate hole  132 C extending in substantially the front-rear direction is formed in the base part  132 A. 
     As shown in  FIG. 3A , an interlocking mechanism  140  as an example of a first interlocking mechanism is provided in the main body  10 . The interlocking mechanism  140  is configured to advance and retract the coupling  130  in the left-right direction in association with movement of the retaining member  42 . 
     The interlocking mechanism  140  includes a side frame  141 , a sliding member  142 , and a pivot member  143 . The side frame  141  is disposed on the left side of the conveying belt  53 . An opening  141 A is formed in the side frame  141  at a position corresponding to the coupling  130 . The opening  141 A has a larger diameter than the flange  130 C. 
     A pair of sliding rails  141 B is provided on the side frame  141 . The sliding rails  141 B are provided one above and one below the opening  141 A and extend in the front-rear direction. A guiding slot  141 C is formed in the side frame  141  frontward of the opening  141 A and extends substantially in the front-rear direction. 
     The sliding member  142  has a plate shape. The sliding member  142  has top and bottom edges supported by the pair of sliding rails  141 B so that the sliding member  142  can slide in the front-rear direction along the sliding rails  141 B. In other words, the sliding member  142  can move between a forward position (the position shown in  FIG. 3A ) at which the retaining member  42  is in its removed position, and a rearward position (the position shown in  FIG. 3B ) at which the retaining member  42  is in its mounted position. 
     An elongate hole  142 A is formed in the sliding member  142  at a position corresponding to the coupling  130 . The elongate hole  142 A is elongated in the front-rear direction and has a vertical width that is slightly smaller than the flange  130 C. A first contact part  142 B is provided on an upper end of the sliding member  142  at a position for contacting the front end of the retaining member  42 . 
     As shown in  FIG. 4A , a protruding wall  142 C is provided to protrude leftward from a left surface  142 F of the sliding member  142 . The protruding wall  142 C is formed along a rear half of the elongate hole  142 A, i.e., along a rear half of an upper edge, a rear half of a lower edge and a rear edge of the elongate hole  142 A. The protruding wall  142 C is positioned to contact the upper and lower peripheral portions of the flange  130 C. Specifically, the protruding wall  142 C includes a pair of (upper and lower) second contact parts  142 D sloping leftward toward the rear, and a pair of (upper and lower) third contact parts  142 E each extending rearward from the rear edge of the corresponding second contact part  142 D. 
     When the sliding member  142  is in its forward position ( FIG. 4A ), the third contact parts  142 E are in contact with the flange  130 C, forcing the coupling  130  leftward against the urging force of the compressed spring  15 . At this time, the coupling  130  is in its retracted position. 
     The pivot member  143  is provided on a front end portion of the sliding member  142 . The pivot member  143  includes a pivot shaft  143 A, a first pivot arm  143 B extending rearward from the pivot shaft  143 A and a second pivot arm  143 C extending frontward from the pivot shaft  143 A. The pivot member  143  is configured to pivotally move about the pivot shaft  143 A. 
     The first pivot arm  143 B has a distal end that is configured to engage in a groove  42 A formed in a lower rear end portion of the retaining member  42 . The second pivot arm  143 C has a distal end that is engaged in the guiding slot  141 C. A coil spring  144  is provided on and around the pivot shaft  143 A constituting the pivot member  143 . The coil spring  144  urges the first pivot arm  143 B clockwise in  FIG. 3 . 
     When the sliding member  142  moves rearward, the pivot member  143  is caused to pivot counterclockwise in  FIG. 3A  while the distal end of the second pivot arm  143 C moves along the guiding slot  141 C against the biasing force of the coil spring  144 . 
     Next, operations performed with the color printer  1  having the above structure will be described. 
     To mount the belt unit  50  in the main body  10 , the user first opens the front cover  11  and, with the retaining member  42  removed from the main body  10 , inserts the belt unit  50  into the main body  10  as illustrated in  FIG. 5A . At this time, the user inserts the drive roller  51  side of the belt unit  50  first into the main body  10 , with the handle  57  positioned higher than the drive roller  51 . The user places the bearing  51 A of the drive roller  51  on top of the lower part  103  of the first restricting member  100  and slides the belt unit  50  rearward. Since the lower part  103  extend farther forward than the upper part  102 , the user can easily place the bearing  51 A on the lower part  103  to smoothly slide the belt unit  50  rearward along the lower part  103 . 
     The user slides the drive roller  51  rearward until the bearing  51 A is in the mounted position interposed between the upper part  102  and lower part  103 , as illustrated in  FIG. 5B . In this state, the vertical position of the belt unit  50  on the drive roller  51  side is restricted. Since the vertical position of the drive roller  51  is stabilized in this configuration, the behavior of the conveying belt  53  when operating is also stable. 
     After the bearing  51 A is in its mounted position, the user lowers the handle  57  and, hence, lowers the front end of the belt unit  50 , as illustrated in  FIG. 5C . During this operation, the belt unit  50  is pivoted downward into its mounted position about the bearing  51 A interposed between the upper part  102  and lower part  103 . 
     As the front end of the belt unit  50  is lowered, the pins  55 A contact the top endfaces  122 A on the corresponding first arms  122  of the pressing members  120 , pushing the first arms  122  forward to place the pins  55 A on top of the corresponding second restricting members  110 . Once the pins  55 A are positioned on the second restricting members  110  in this way, the springs  124  pulling the corresponding second arms  123  forward cause the first arms  122  to press against the corresponding pins  55 A in a direction diagonally downward and rearward, thereby pressing the pins  55 A against the second restricting members  110 . Through this configuration, the front-rear and vertical position of the belt unit  50  are restricted on the follow roller  52  side, inhibiting the belt unit  50  from pivotally moving about the bearings  51 A. 
     To remove the belt unit  50  from the main body  10 , the user lifts the handle  57  upward and, hence, lifts the front end of the belt unit  50  upward while pivoting the belt unit  50  about the bearings  51 A, as illustrated in  FIG. 5B . Next, the user slides the bearings  51 A forward along the lower parts  103 , as illustrated in  FIG. 5A  and removes the belt unit  50  from the main body  10 . 
     To mount the retaining member  42  in the main body  10 , the user moves the sliding member  142  from its forward position to its rearward position, with the retaining member  42  abutted against the first contact part  142 B, as shown in  FIG. 3A . While the sliding member  142  is moving rearward, the second pivot arm  143 C of the pivot member  143  moves along the guiding slot  141 C. At this time, the pivot member  143  pivotally moves counterclockwise in  FIG. 3A  against the urging force of the coil spring  144  until the first pivot arm  143 B of the pivot member  143  becomes engaged in the groove  42 A, as shown in  FIG. 3B . At this time, the retaining member  42  is in its mounted position, and the sliding member  142  is in its rearward position. 
     Further, when the sliding member  142  begins moving from its forward position toward its rearward position, the third contact parts  142 E slide past the flange  130 C of the coupling  130 , and the flange  130 C is caused to slide rightward along the second contact parts  142 D due to the urging force of the compressed spring  15 , as illustrated in  FIG. 4A . The flange  130 C thus gradually moves rightward (toward the drive roller  51 ) from its retracted position along the second contact parts  142 D. Once the sliding member  142  moves to its rearward position shown in  FIG. 4B , the flange  130 C is in contact with the left surface  142 F of the sliding member  142  and, hence, is disposed in its contact position. At this time, the coupling  130  is engaged with the output-side coupling  51 D of the drive roller  51 . 
     Thus, by moving the coupling  130  rightward in conjunction with the operation for mounting the retaining member  42  (photosensitive drums  43 ) in the main body  10 , the interlocking mechanism  140  can engage the coupling  130  with the output-side coupling  51 D of the drive roller  51 . 
     To remove the retaining member  42  from the main body  10 , the user moves the retaining member  42  forward from the position shown in  FIG. 3B . At this time, the first pivot arm  143 B engaged in the groove  42 A moves forward, causing the sliding member  142  to move from its rearward position toward its forward position. Consequently, the second pivot arm  143 C pivots clockwise along the guiding slot  141 C, causing the first pivot arm  143 B to disengage from the groove  42 A, as shown in  FIG. 3A . After the retaining member  42  is removed from the main body  10 , the sliding member  142  is maintained in its forward position by the urging force of the coil spring  144 . 
     Further, when the sliding member  142  begins moving from its rearward position toward its forward position, the flange  130 C of the coupling  130  moves gradually leftward (toward the main body  10 ) from its contact position shown in  FIG. 4B  against the urging force of the compressed spring  15  while sliding along the second contact parts  142 D. When the sliding member  142  moves to the forward position, the flange  130 C contacts the third contact parts  142 E, as shown in  FIG. 4A , thereby arriving at its retracted position. 
     Hence, by moving the coupling  130  leftward in association with the operation for removing the retaining member  42  (the photosensitive drums  43 ) from the main body  10 , the interlocking mechanism  140  can disengage the coupling  130  from the output-side coupling  51 D of the drive roller  51 . 
     With the above-described configuration of the first embodiment, the following operational and technical advantages can be achieved. 
     Since the first restricting member  100  restricts the vertical position of the belt unit  50  on the drive roller  51  side, any disturbances or the like are unlikely to cause fluctuations in the vertical position of the drive roller  51 . Accordingly, this construction stabilizes the behavior of the conveying belt  53  while reducing any occurrence of positional fluctuation in images. 
     Since the bearings  51 A of the drive roller  51  are interposed between the bottom surface  102 A of the upper part  102  and the top surface  103 A of the lower part  103 , the position of the drive roller  51  does not change either up or down. 
     Further, since the first restricting member  100  contacts the bearing  51 A when the belt unit  50  is being mounted, the belt unit  50  can be pivoted into its mounted position. 
     By leaving the front end of the first restricting member  100  open, the belt unit  50  can be easily mounted into the front end of the first restricting member  100 . 
     Since the lower part  103  extends farther forward than the upper part  102 , the belt unit  50  can be easily placed on the first restricting member  100  during mounting. 
     The second restricting members  110  can restrict the position of the belt unit  50  in both vertical and front-rear directions. Further, owing to their position toward the front end of the belt unit  50 , the bottom restricting parts  112  of the second restricting members  110  can restrict the belt unit  50  from pivotally moving about the drive roller  51 . Also, the pressing member  120  is applied with a smaller force from the rotational moment generated by rotation of the drive roller  51 . 
     Further, a drive force can be made transmittable to the belt unit  50  when mounting the retaining member  42  (photosensitive drums  43 ), and the coupling  130  can be disengaged when removing the retaining member  42  in order to remove and replace the belt unit  50 . 
     Further, since the cleaning roller  61  can rotate such that its point of contact with the conveying belt  53  moves (rearward) opposite the moving direction of the conveying belt  53  (forward), the belt unit  50  can easily be pushed rearward, i.e., the pins  55 A can easily be pushed against the rear restricting parts  111 . 
     Still further, the joint  132  (with an Oldham coupling structure) is disposed between the coupling  130  and the shaft  51 B of the drive roller  51 . Hence, positional offset between the shaft  130 B of the coupling  130  and the shaft  51 B of the drive roller  51  is allowable when mounting the belt unit  50 . 
     2. Second Embodiment 
     A color printer  201  according to a second embodiment will be described while referring to  FIGS. 6 through 7B , wherein like parts and components are designated by the same reference numerals as the first embodiment to avoid duplicating description. 
     As shown in  FIG. 6 , the color printer  201  of the second embodiment is not provided with the retaining member  42  of the first embodiment, but is provided with four LED units  230  instead of the exposure unit  30  of the first embodiment. Each of the LED units  230  has a plurality of LEDs for exposing the corresponding photosensitive drums  43 . Further, four supporting frames  245  are provided in the main body  10  for supporting corresponding process units  41 ′. Each of the process units  41 ′ is detachably mountable in the main body  10 , independently. 
     In the second embodiment, the rearmost process unit  41 ′ includes a process frame  41   a  whose rear end portion is formed with a groove  41   b , as shown in  FIGS. 7A and 7B . 
     Further, as shown in  FIG. 7A , instead of the interlocking mechanism  140  of the first embodiment, an interlocking mechanism  240  is provided in the main body  10  as another example of the first interlocking mechanism. The interlocking mechanism  240  according to the second embodiment is provided at a position corresponding to the rearmost process unit  41 ′ (and the coupling  130 ) and is configured to advance and retract the coupling  130  in the left-right direction in conjunction with the operations to mount and remove the rearmost process unit  41 ′ (the rearmost photosensitive drum  43 ) relative to the main body  10 . The structure of the coupling  130  is identical to that of the first embodiment. 
     Specifically, the interlocking mechanism  240  includes a side frame  241 , a sliding member  242 , and a pivot member  243 . The side frame  241  has a structure approximately equivalent to the side frame  141  of the first embodiment, but is provided with a pair of sliding rails  241 B each sloping downward toward the rear. A guiding slot  241 D is also formed in the side frame  241  at a position obliquely above and forward of the opening  141 A. 
     As in the first embodiment, the sliding member  242  is configured to slidingly move along the sliding rails  241 B in a direction diagonally downward and rearward. The sliding member  242  can move between an upper position (the position shown in  FIG. 7A ) when the rearmost process unit  41 ′ is in its removed position, and a lower position (the position shown in  FIG. 7B ) when the rearmost process unit  41 ′ is in its mounted position. Further, the sliding member  242  includes a first contact part  242 B configured to contact a bottom portion of the process frame  41   a  constituting the rearmost process unit  41 ′. 
     The pivot member  243  has a rotational shaft  243 A provided on an upper end portion of the sliding member  242 , and an arm  243 B extending upward from the rotational shaft  243 A. The arm  243 B has a distal end that is engageable with the groove  41   b  of the process frame  41   a . The distal end of the arm  243 B is also engaged with the guiding slot  241 D. The pivot member  243  is configured to pivot counterclockwise in  FIG. 7A  while the sliding member  242  moves diagonally downward and rearward. A spring (not shown) urges the pivot member  243  clockwise in  FIGS. 7A and 7B . 
     Next, operations performed on the color printer  201  having the above construction will be described. 
     To mount the (rearmost) process unit  41 ′ in the main body  10 , the user places the process frame  41   a  in contact with the first contact part  242 B and moves the sliding member  242  with the process unit  41 ′ from its upper position to its lower position, as shown in  FIG. 7A . As the sliding member  242  moves diagonally downward and rearward, the arm  243 B of the pivot member  243  moves along the guiding slot  241 D, causing the pivot member  243  to pivot counterclockwise in  FIG. 7A  against the urging force of the spring (not shown) until the arm  243 B becomes engaged in the groove  41   b  of the process frame  41   a , as shown in  FIG. 7B . Subsequently, the process unit  41 ′ is placed in its mounted position, while the sliding member  242  is placed in its lower position. 
     When removing the (rearmost) process unit  41 ′ from the main body  10 , the user begins to lift the rearmost process unit  41 ′ diagonally upward and forward from the position shown in  FIG. 7B  to the position shown in  FIG. 7A . At this time, the arm  243 B engaged in the groove  41   b  moves diagonally upward and forward together with the process unit  41 ′, as the sliding member  242  moves from its lower position to its upper position. As the process unit  41 ′ moves upward, the arm  243 B pivots clockwise in  FIG. 7B  along the guiding slot  241 D and disengages from the groove  41   b , as illustrated in  FIG. 7A . Once the rearmost process unit  41 ′ is removed, the sliding member  242  is maintained in its upper position by the urging force of the spring (not shown). 
     3. Third Embodiment 
     A interlocking mechanism  350  according to a third embodiment will now be described while referring to  FIGS. 8 through 10B , wherein like parts and components are designated by the same reference numerals as the first and second embodiments to avoid duplicating description. 
     In the third embodiment, the color printer  201  of the second embodiment shown in  FIG. 6  is provided with the interlocking mechanism  350  shown in  FIG. 8  in place of the interlocking mechanism  240  of the second embodiment. The interlocking mechanism  350  is an example of a second interlocking mechanism. 
     The interlocking mechanism  350  includes a drum locking rod  351 , and four sets of a pivot member  352 , a spring support member  353 , a locking member  354  and a torsion coil spring  355 , each set being disposed at a position corresponding to each of the photosensitive drums  43 . Hereinafter, descriptions will be made only for the above-identified members constituting the one set corresponding to the rearmost photosensitive drum  43 . The components constituting each of the other three sets corresponding to the remaining three photosensitive drums  43  have the same structures as those for the set corresponding to the rearmost photosensitive drum  43 , and hence detailed descriptions therefor are omitted. 
     The drum locking rod  351  is disposed at a position corresponding to the top surface of the conveying belt  53  shown in  FIG. 6  and is arranged to extend in the front-rear direction. A groove  351 A is formed in a top surface of the drum locking rod  351  at a position corresponding to the photosensitive drum  43 . A rack gear  351 B is provided along a bottom edge of the drum locking rod  351  at a front end thereof. The rack gear  351 B is engaged with a gear  16  (provided on the main body  10 ) into which a motor (not shown) is configured to input a drive force. By driving the gear  16  to rotate, the drum locking rod  351  can be slidingly moved between a rearward position (the position shown in  FIG. 8 ) and a forward position (the position shown in  FIG. 9 ). 
     As shown in  FIG. 10A , a contact member  351 C is provided on the drum locking rod  351  at a position corresponding to the coupling  130  located near a rear end of the drum locking rod  351 , so as to be capable of contacting the flange  130 C of the coupling  130 . The contact member  351 C includes a second contact part  351 D that slopes leftward toward the front from the rear end of the drum locking rod  351 , and a third contact part  351 E that extends forward from a front end of the second contact part  351 D. 
     As in the first embodiment, the coupling  130  can move between the retracted position (the position shown in  FIG. 10A ) retracted from the shaft  51 B (not shown) of the drive roller  51 , and the contact position (the position shown in  FIG. 10B ) for contacting the shaft  51 B. 
     When the drum locking rod  351  is in its rearward position, the third contact part  351 E is in contact with the flange  130 C as shown in  FIG. 10A . Hence, the coupling  130  is moved leftward against the urging force of the compressed spring  15  and is in its retracted position. 
     As shown in  FIG. 8 , the pivot member  352  is configured to pivot about a pivot shaft  352 A supported by the supporting frame  245  (see  FIG. 6 ). The pivot member  352  is arranged at a position corresponding to the groove  351 A in the drum locking rod  351 . The pivot member  352  includes a body part  352 B forming a general circular shape about the pivot shaft  352 A, and an arm  352 C that extends approximately downward from the body part  352 B. 
     The body part  352 B has an upper portion from which a first cam part  352 D and a second cam part  352 F protrude leftward. An engaging part  352 E is provided on a distal end of the arm  352 C to protrude leftward therefrom. The engaging part  352 E is configured to engage in the groove  351 A of the drum locking rod  351 . 
     The spring support member  353  is disposed above the pivot member  352  for supporting the torsion coil spring  355 . The spring support member  353  includes a spring retaining part  353 A, a spring anchoring part  353 B, and a fourth contact part  353 C. The spring support member  353  is capable of pivoting about the spring retaining part  353 A supported by a side frame or the like constituting the main body  10 . 
     The spring retaining part  353 A is formed in a cylindrical shape. The spring anchoring part  353 B extends rearward from the spring retaining part  353 A. The fourth contact part  353 C protrudes downward from the rear end of the spring anchoring part  353 B and is configured to abut on the first cam part  352 D from the rear side thereof. 
     The locking member  354  is capable of pivotally moving coaxially with the spring retaining part  353 A. The locking member  354  functions to lock a position of a shaft  43   a  of the photosensitive drum  43 . More specifically, the locking member  354  is configured to lock a shaft retaining part  41   c  provided on a process frame  41   f  of each process unit  41 ″. The shaft retaining part  41   c  is configured to hold the shaft  43   a  of the photosensitive drum  43 . The shaft retaining part  41 C includes a contact surface  41   d  that protrudes upward from an upper end of the shaft retaining part  41   c . The locking member  354  is configured to restrict displacement of the shaft  43   a  of the photosensitive drum  43  by locking and unlocking the shaft retaining part  41 C. The locking member  354  includes a locking part  354 A that extends upward from the spring retaining part  353 A, and a fifth contact part  354 B that extends downward from the spring retaining part  353 A. 
     The locking part  354 A is configured to contact the contact surface  41   d  of the shaft retaining part  41   c . The fifth contact part  354 B is positioned to contact the second cam part  352 F of the pivot member  352 . 
     The torsion coil spring  355  functions to urge the locking member  354  toward the contact surface  41   d  on the shaft retaining part  41   c . The torsion coil spring  355  includes a coiled part  355 A, a first arm  355 B, and a second arm  355 C. 
     The coiled part  355 A is retained on the spring retaining part  353 A. The first arm  355 B extends upward from the coiled part  355 A and is anchored by the locking part  354 A for urging the locking part  354 A counterclockwise in  FIG. 8 . The second arm  355 C extends rearward from the coiled part  355 A and is anchored by the spring anchoring part  353 B for urging the spring anchoring part  353 B clockwise in  FIG. 8 . 
     Next, the operations performed on the color printer  201  having the interlocking mechanism  350  will be described. 
     To lock the process unit  41 ″ (the photosensitive drum  43 ) in position, the gear  16  is rotated to move the drum locking rod  351  from its rearward position toward its forward position, as illustrated in  FIG. 8 . As the drum locking rod  351  moves forward, the arm  352 C engaged in the groove  351 A moves forward, causing the pivot member  352  to pivot clockwise in  FIG. 8  about the pivot shaft  352 A. This pivotal movement causes the first cam part  352 D to move rearward to make contact with the fourth contact part  353 C of the spring support member  353 , as shown in  FIG. 9 . 
     Contact from the first cam part  352 D pushes the fourth contact part  353 C rearward, causing the spring support member  353  to pivot counterclockwise in  FIG. 9 . Due to the torsion coil spring  355 , the locking member  354  also pivots counterclockwise as the spring support member  353  pivots, thereby bringing the locking part  354 A into contact with the contact surface  41   d  on the shaft retaining part  41   c  to lock the shaft retaining part  41   c.    
     Further, when the drum locking rod  351  begins to move from its rearward position toward its forward position, as illustrated in  FIG. 10A , the flange  130 C of the coupling  130  moves off the third contact part  351 E and gradually moves rightward from its retracted position while sliding on and along the second contact part  351 D. When the drum locking rod  351  is moved to its forward position, as illustrated in  FIG. 10B , the flange  130 C leaves the second contact part  351 D and is placed in its contact position. 
     In other words, the interlocking mechanism  350  engages the coupling  130  with the drive roller  51  by moving the coupling  130  in the left-right direction (rightward) in conjunction with the locking operation of the locking member  354 . 
     To unlock the process unit  41 ″, the gear  16  is rotated such that the drum locking rod  351  moves from its forward position toward its rearward position, as illustrated in  FIG. 9 . As the drum locking rod  351  moves rearward, the arm  352 C engaged in the groove  351 A also moves rearward, causing the pivot member  352  to pivot counterclockwise in  FIG. 9  about the pivot shaft  352 A. This pivotal movement causes the first cam part  352 D to move forward so as to separate from the fourth contact part  353 C of the spring support member  353 , as illustrated in  FIG. 8 . 
     When the first cam part  352 D no longer contacts the fourth contact part  353 C, the second cam part  352 F contacts the fifth contact part  354 B, causing the spring support member  353  and locking member  354  to pivot clockwise in  FIG. 8  so that the locking member  354  separates from the contact surface  41   d  on the shaft retaining part  41   c  to realize unlocking of the shaft retaining part  41   c  from the locking member  354 . 
     Further, when the drum locking rod  351  begins to move from its forward position toward its rearward position, as illustrated in  FIG. 10B , the flange  130 C of the coupling  130  moves gradually leftward from its contact position while sliding on and along the second contact part  351 D. When the drum locking rod  351  moves to its rearward position shown in  FIG. 10A , the flange  130 C is placed in its retracted position in contact with the third contact part  351 E. 
     Hence, the interlocking mechanism  350  disengages the coupling  130  from the drive roller  51  by moving the coupling  130  in the left-right direction (leftward) in association with the unlocking operation of the locking member  354 . 
     4. Variations and Modifications 
     In the embodiments described above, the conveying belt  53  is mounted over the drive roller  51  and follow roller  52  in a tensioned state, but the conveying belt  53  may be placed in a tensioned state by the drive roller  51  and two or more follow rollers. Further, instead of the conveying belt  53  for conveying sheets P, a transfer belt or a photosensitive belt for carrying toner images and transferring the images onto paper may be used as an example of the belt. 
     Various modifications can be made to the first restricting member  100  and second restricting members  110  of the depicted embodiments. 
       FIG. 11A  shows a first restricting member  100 A according to a first modification. In the first restricting member  100 A, both of the upper part  102  and lower part  103  have tapered distal ends. More specifically, the upper part  102  has a sloped surface  102 B that slopes upward toward the front from the front edge of the bottom surface  102 A. The lower part  103  has a sloped surface  103 B that slopes downward toward the front from the front edge of the top surface  103 A. 
     This structure would facilitate insertion of the bearing  51 A into the gap between the upper part  102  and lower part  103 . 
       FIG. 11B  shows a first restricting member  100 B according to a second modification. In the first restricting member  100 B, the base part  101  has a front surface  101 A that serves as the second restricting part, instead of the rear restricting part  111  of the second restricting member  110  of the depicted embodiments. Further, In this case, a second restricting member  110 B having no rear restricting part is provided in place of the second restricting member  110 . Thus, the second restricting member  110 B of the second modification only includes the bottom restricting part  112 . In this case, the bearings  51 A of the drive roller  51  serve as the first restricted part and the second restricted part. 
       FIG. 11C  shows a first restricting member  100 C according to a third modification. In place of the first restricting member  100  of the depicted embodiments that is open only on the front end thereof, the first restricting member  100 C of the third modification is open on both front and rear ends thereof. 
     In the embodiments described above, only one first restricting member  100  is provided on the left side (coupling  130  side) of the drive roller  51 . However, instead of the one first restricting member  100 , one pair of first restricting members  100  may be provided to interpose the drive roller  51  therebetween in the left-right direction for further improved positioning of the belt unit  50  relative to the main body  10 . 
     In the embodiments described above, the second restricting members  110  are disposed at positions corresponding to the pins  55 A of the frame  55  constituting the belt unit  50 , but the second restricting members  110  may be disposed at positions corresponding to the follow roller  52  instead. In this case, the second restricting members may be configured to restrict the positions of the bearings  52 A of the follow roller  52 . 
     Further, while pressing members  120  are provided in the embodiments described above, these components may be omitted. 
     While the interlocking mechanisms  140 ,  240 ,  350  are provided in the he first to third embodiments described above, these components may be omitted. 
     While the color printer  1  is exemplified as an example of the disclosure in the embodiments, the disclosure may be applied to another type of image-forming apparatus, such as a copying machine or a multifunction peripheral. 
     While the description has been made in detail with reference to specific embodiments thereof, it would be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the scope of the above described embodiments.