Abstract:
An image forming apparatus may include a sheet conveying belt that conveys a sheet to a position opposed to an image carrier, and a cleaning unit that cleans the sheet conveying belt. The sheet conveying belt may be driven by a first driving unit such as a first motor while the cleaning unit may be driven a second driving unit such as a second motor. In one or more arrangements, the second driving unit is independent from the first driving unit. Additionally or alternatively, the first driving unit may also be configured to drive an image carrier and/or a developing unit.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a continuation of co-pending U.S. patent application Ser. No. 12/732,418, filed Mar. 26, 2010, which is a divisional application of U.S. patent application Ser. No. 11/476,896, filed on Jun. 29, 2006, now U.S. Pat. No. 7,720,407, which claims priority from Japanese Patent Application No. 2005-189545, filed on Jun. 29, 2005, each of which are incorporated herein by reference in their entirety. 
    
    
     TECHNICAL FIELD 
     Aspects of the present invention relate to an image forming apparatus including a laser printer and more particularly to an image forming apparatus provided with a cleaning unit for cleaning a belt. 
     BACKGROUND 
     Conventionally, an image forming apparatus including a laser printer, which has such a structure that an endless belt is employed for conveying sheets of paper and for conducting intermediate transfer, has been known. In the image forming apparatus having this structure, there is generally provided a cleaning unit including a roller and/or a brush, in order to remove foreign substances such as toner, paper dust, etc., which have adhered to a surface of the belt. In an image forming apparatus disclosed in JP-A-9-152788, for example, a cleaning roller that is in pressure contact with the surface of the belt is driven to rotate so that foreign substances, which have been shifted from the belt to the cleaning roller, is scraped off by a blade. The cleaning unit of this type is usually driven, making use of power of a motor for driving the belt. 
     SUMMARY 
     By the way, with an enhanced request for images of high quality in recent years, uses of polymerized toner as developer has been more and more increased. Because the polymerized toner has excellent fluidity due to its nearly spherical shape and uniform particle diameter, it is possible to obtain enhanced quality of the images. However, there has been a problem that the polymerized toner is unlikely to be removed when it has adhered to the surface of the belt or the like. Under the circumstances, it has been desired that cleaning performance of the cleaning unit to be enhanced. 
     In order to enhance the cleaning performance of the cleaning unit, it would be advantageous to increase contact pressure of the cleaning roller to be exerted on the belt, or to increase contact pressure of the blade to be exerted on the cleaning roller. However, in case where the contact pressures have been increased, a larger load will be required for driving the cleaning unit, which will adversely affect operation of the belt, and irregularity in speed of the belt will be likely to occur. Consequently, there is such a possibility that the quality of the images may be deteriorated. In order to solve such problem, it has been considered that the cleaning unit is driven by employing an exclusive motor separately from the motor for driving the belt. In this case, however, additional components are required, and so, upsizing of the apparatus, an increase of electric power consumption, and increase in cost will be incurred. 
     Aspects of the invention provide an image forming apparatus with which high quality of images can be ensured, while achieving the downsizing, cost reduction of the apparatus, and decrease of electric power consumption. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a sectional view schematically showing a structure of a laser printer according to a first aspect of the invention; 
         FIG. 2  is an explanatory view schematically showing a driving mechanism; 
         FIG. 3  is an explanatory view schematically showing a driving mechanism according to a second aspect; and 
         FIG. 4  is a sectional view schematically showing a structure of a laser printer according to another aspect. 
     
    
    
     DETAILED DESCRIPTION 
     First Aspect 
     Now, a first aspect of the invention will be described referring to  FIGS. 1 and 2 . 
     (Overall Structure of a Laser Printer) 
       FIG. 1  is a sectional side view schematically showing a structure of a laser printer  1  as an image forming apparatus according to this aspect. It is to be noted that a right side in  FIG. 1  is regarded as a front side, in the following description. 
     This laser printer  1  is a color laser printer of a tandem type for direct transfer. The laser printer  1  includes a main body casing  2  in a substantially box-like shape, as shown in  FIG. 1 . A top face cover  3 , which can be opened or closed, is provided on an upper face of the main body casing  2 . By opening the top face cover  3 , it will be possible to exchange a process cartridge  26  and a belt unit  16  in the main body casing  2 . There is further formed, on a surface of the top face cover  3 , a paper discharging tray  5  on which sheets  4  will be stacked after an image forming process has been finished. 
     In a lower part of the main body casing  2 , a paper feeding tray  7  that accommodates a sheet  4  on which an image is formed is mounted. The paper feeding tray  7  can be withdrawn frontward. The paper feeding tray  7  is provided therein with a paper pressing plate  9  that is urged by a spring  8  so as to be tilted in a manner of lifting a leading end side of the sheet  4 . At a position above a leading end of the paper feeding tray  7 , there are provided a pickup roller  10 , which functions as an intermittent motion mechanism and a driven mechanism, and a separating pad  11 , which is urged by a spring (not shown) so as to be in pressure contact with the pickup roller  10 . The pickup roller  10  is in a substantially D-shape in section and has on its outer peripheral part a circumferential face which will come into contact with a surface of the sheet  4  and a non-contact position which will not come into contact with the sheet  4 . The pickup roller  10  will be intermittently driven to rotate. The uppermost sheet  4  on the paper feeding tray  7  will be pressed by the paper pressing plate  9  toward the pickup roller  10 , and will be separated one by one, when the sheet  4  is clamped between the pickup roller  10  and the separating pad  11 , with the rotation of the pickup roller  10 . 
     At a diagonally forward and upward position of the pickup roller  10 , there are provided a paper feeding roller  12 A (the driven mechanism), which is driven to rotate, and a further paper feeding roller  12 B that is faced with the paper feeding roller  12 A and rotated following the rotation of the paper feeding roller  12 A. At a further upward position, there are provided a registration roller  13 A (the intermittent motion mechanism and the driven mechanism), which is intermittently driven to rotate, and a further registration roller  13 B that is faced with the registration roller  13 A and rotated following the rotation of the registration roller  13 A. The sheet  4 , which has been fed from between the pickup roller  10  and the separating pad  11 , will be conveyed to the registration rollers  13 A,  13 B, with the rotations of the paper feeding rollers  12 A,  12 B. The registration rollers  13 A,  13 B will convey the sheet  4  to the belt unit  16  in the downstream, at a predetermined timing, after the skew of the sheet  4  has been corrected. 
     The belt unit  16  is detachably mounted in the main body casing  2 . The belt unit  16  includes a belt supporting roller  17 A which is located rearward and driven to rotate; a further belt supporting roller  17 B which is located forward and rotated following the rotation of the belt supporting roller  17 A; and a conveying belt  18  which is stretched between the two belt supporting rollers  17 A and  17 B. The conveying belt  18  is an endless belt formed of resin material such as polycarbonate. When the rearward belt supporting roller  17 A is driven to rotate, the conveying belt  18  will cyclically move in a counterclockwise direction in  FIG. 1 , thereby to convey the sheet  4 , which is carried on its upper face, rearward. Inside the conveying belt  18 , four transfer rollers  19  that are respectively opposed to photosensitive drums  31  of the process cartridges  26 , which will be described below, are arranged at predetermined intervals in a longitudinal direction, in such a manner that the conveying belt  18  is interposed between the photosensitive drums  31  and the corresponding transfer rollers  19 . The transfer rollers  19  can make follow-up rotations, and transfer biases will be applied between the transfer rollers  19  and the corresponding photosensitive drums  31 , when transfer process is performed. 
     Underneath the belt unit  16 , there is provided a cleaning device  20  (a cleaning unit) for removing toner, paper dust, etc., which have adhered to the transfer belt  18 . The cleaning device  20  includes a cleaning roller  21  having foam material of silicone or urethane provided around a shaft member, which is made of metal and extends in a lateral direction of the transfer belt  18 . This cleaning roller  21  is opposed to a backup roller (a grounding roller)  22  which is made of metal and provided in the belt unit  16  so as to make follow-up rotation, interposing the conveying belt  18 . The cleaning roller  21  will be driven to rotate in an opposite direction to the direction of the conveying belt  18 , while it is in contact with a lower face (an outer face) of the conveying belt  18 , and at the same time, a predetermined bias (removing bias) will be applied between the cleaning roller  21  and the backup roller  22 , whereby the toner or the like on the conveying belt  18  will be electrically removed by the cleaning roller  21 . Moreover, a recovering roller  23  made of metal is provided in contact with the cleaning roller  21 . This recovering roller  23  will be driven to rotate at a different circumferential speed from the cleaning roller  21 , while subjected to the stronger removing bias than to the cleaning roller  21 , whereby the toner or the like which has adhered to a surface of the cleaning roller  21  will be removed. Further, a blade  24  for scraping off the toner or the like adhered to a surface of the recovering roller  23  is provided in contact with the recovering roller  23 . 
     The four process cartridges  26  corresponding to four colors, namely, magenta, yellow, cyan, and black, are detachably mounted above the belt unit  16 , in parallel in the longitudinal direction. Further above the process cartridges  26 , there is provided a scanner device  27 , which is integrally fitted to the top face cover  3 . The scanner device  27  irradiates, by rapid scanning, laser beams L corresponding to the respective colors based on predetermined image data, onto the surfaces of the corresponding photosensitive drums  31 . 
     Each of the process cartridges  26  includes a cartridge frame  30  in a frame-like shape; the photosensitive drum  31 ; and a scorotron type charger  32  which are provided in a lower part of the cartridge frame  30 , and a developing cartridge  34  which is detachably fitted to the cartridge frame  30 . 
     Each of the photosensitive drums  31 , which functions as an image carrier, includes a grounded drum body made of metal; and a photosensitive layer formed of polycarbonate or the like, which can be positively charged, is covered over the drum body. The photosensitive drum  31  will be driven to rotate in a clockwise direction in the drawing. 
     The scorotron type charger  32  is arranged above the photosensitive drum  31  at a diagonally rearward position in a manner opposed to the photosensitive drum  31 , keeping a predetermined distance so as not to come into contact with the photosensitive drum  31 . This scorotron type charger  32  will generate corona discharge from an charging wire of tungsten or the like, thereby to positively charge the surface of the photosensitive drum  31  uniformly. 
     Each of the developing cartridges  34  is in a substantially box-like shape and includes a toner containing room  38  in an upper part thereof. A supply roller  39 , a developing roller  40 , and a layer thickness regulating blade  41  are provided in a lower part thereof. As the developers, non-magnetic toners of mono-component type for positive charging of the respective colors, namely, yellow, magenta, cyan and black are contained in the respective toner containing rooms  38 . As the toners, polymerized toner which is obtained by copolymerizing polymeric monomer, for example, styrene monomer such as styrene, acryl monomer such as acrylic acid, alkyl(C1-C4)acrylate, alkyl(C1-C4)methacrylate, by known polymerization method such as suspension polymerization is employed. The polymerized toner of this type is in a substantially globular shape having an average particle diameter of about 6 to 10 μm, and has excellent fluidity, whereby formation of an image of high quality can be achieved. Moreover, the toner is blended with coloring agent such as carbon black, wax and so on, and added with outer admixture such as silica, for enhancing the fluidity. Further, the toner containing rooms  38  are respectively provided with agitators  42  which are driven to rotate thereby to agitate the toners. 
     The supply roller  39  is constructed by covering a roller shaft made of metal with electrically conductive foam material, and will be driven to rotate in a counterclockwise direction in the drawing. The developing roller  40  is constructed by covering a roller shaft made of metal with electrically conductive rubber material, and will be driven to rotate in a counterclockwise direction in the drawing. The toner discharged from the toner containing room  38  will be supplied to the developing roller  40  with the rotation of the supply roller  39 , and positively charged by friction between the supply roller  39  and the developing roller  40 . The toner supplied to a surface of the developing roller  40  will intrude between the layer thickness regulating blade  41  and the developing roller  40  with the rotation of the developing roller  40 . Then, the toner will be further sufficiently charged there by friction, and carried on the developing roller  40  as a thin layer having a certain thickness. 
     To begin with, the surface of the photosensitive drum  31  will be positively charged uniformly by the scorotron type charger  32 , while it rotates, and thereafter, will be exposed to light by rapid scanning of the laser beam from the scanner device  27 , whereby an electrostatic latent image corresponding to the image to be formed on the sheet  4  will be formed. 
     Then, the toner which is carried on the developing roller  40  and positively charged will be supplied to the electrostatic latent image which has been formed on the surface of the photosensitive drum  31 , when the toner arrives at a position opposed to the photosensitive drum  31  and comes into contact therewith, with the rotation of the developing roller  40 . In this manner, the electrostatic latent image on the photosensitive drum  31  will be converted into a visual image, and a toner image by reverse development will be carried on the surface of the photosensitive drum  31 . 
     Thereafter, the toner images carried on the surfaces of the respective photosensitive drums  31  will be sequentially transferred to the sheet  4  by the transfer biases of negative polarity which are applied to the transfer rollers  19 , while the sheet  4  conveyed by the conveying belt  18  passes respective transfer positions between the photosensitive drums  31  and the transfer rollers  19 . Then, the sheet  4  to which the toner images have been transferred in this manner will be conveyed to a fixing device  43 . 
     The fixing device  43  is arranged in a rearward area of the conveying belt  18  in the main body casing  2 . This fixing device  43  includes a heating roller  44  which is provided with a heat source such as a halogen lamp and driven to rotate; and a pressurizing roller  45  which is arranged below the heating roller  44 . The pressurizing roller  45  is opposed to the heating roller  44  so as to pressurize it, and driven to rotate following the rotation of the heating roller  44 . In this fixing device  43 , the sheet  4  carrying thereon the toner images of the four colors will be heated, while the sheet  4  is clamped between the heating roller  44  and the pressurizing roller  45  and conveyed. In this manner, the toner images will be fixed on the sheet  4 . 
     At a diagonally rearward and upward position of the fixing device  43 , there are arranged a conveying roller  46 A which is driven to rotate; and a further conveying roller  46 B which is rotated following the rotation. Further above the conveying rollers  46 A,  46 B, there are arranged a paper discharging roller  47 A which is driven to rotate; and a further paper discharging roller  47 B which is rotated following the rotation. The sheet  4  which has been fixed by heating will be conveyed to the paper discharging rollers  47 A,  47 B by means of the conveying rollers  46 A,  46 B, and discharged from the paper discharging rollers  47 A,  47 B onto the above described paper discharging tray  5 . It is to be noted that the sheet conveying mechanism in this aspect means those components for conveying the sheet  4 , separately from the conveying belt  18 , specifically, the pickup roller  10 , the paper feeding rollers  12 A,  12 B, the registration rollers  13 A,  13 B, the fixing device  43 , the conveying rollers  46 A,  46 B, and the paper discharging rollers  47 A,  47 B. 
     (Driving Mechanism) 
       FIG. 2  is an explanatory view schematically showing a driving mechanism  90 , which constitutes a part of a driving system in the laser printer  1 . The driving mechanism  90  includes a driving motor  91  (a second driving unit), as a power source. The driving mechanism  90  has a driving gear  91 A that is meshed with a relay gear  92 . Further, a recovering gear  93  that is connected to the recovering roller  23  via a shaft body  93 A, and a cleaning gear  94  that is connected to the cleaning roller  21  via a shaft body  94 A are successively meshed with this relay gear  92 . The driving gear  91 A is also meshed with a pair of relay gears  95 ,  96 , successively. The relay gear  96  is meshed with a registration gear  97  that is connected to the registration roller  13 A via a shaft body  97 A. On a halfway of the shaft body  97 A, there is provided a registration clutch  98  that can switch transmission and interruption of the power. The registration gear  97  is meshed with a relay gear  99 , and a pickup gear  100  that is connected to the pickup roller  10  via a shaft body  100 A is meshed with this relay gear  99 . On a halfway of the shaft body  100 A, there is provided a pickup clutch  101  that can switch transmission and interruption of the power. Further, the pickup gear  100  is meshed with a relay gear  102 , and a paper feeding gear  103  that is connected to the paper feeding roller  12 A via a shaft body  103 A is meshed with the relay gear  102 . 
     It is to be noted that other driven components (the belt supporting roller  17 A, the photosensitive drum  31 , the developing roller  40 , the supply roller  39 , the agitator  42 , and so on) in the laser printer  1  are so constructed as to be driven by a first driving unit  1 A (see  FIG. 2 ) which is independent from the driving mechanism  90 . 
     When printing process has started, the driving motor  91  will be driven, and the gears  93 ,  94 ,  97 ,  100 ,  103  will be respectively driven to rotate in directions of arrow marks in  FIG. 2 . While the driving motor  91  is driven, the power of the driving motor  91  is always transmitted to the cleaning roller  21 , the recovering roller  23 , and the paper feeding roller  12 A so that these rollers can be always driven to rotate. On the other hand, the pickup roller  10  and the registration roller  13 A will be intermittently driven, because the power of the driving motor  91  will be transmitted or interrupted by means of the clutches. Operations of the belt supporting roller  17 A and the photosensitive drum  31  by the first driving unit  1 A will be also started. 
     During standby for supply of the sheet, the pickup roller  10  rests in a phase where the non-contact position is opposed to a surface of the sheet  4 . The pickup roller  10  will be intermittently driven, according to a paper feeding command, so as to stop when the pickup clutch  101  is turned off, after one rotation of the paper feeding roller  12 A. The sheet  4  at the uppermost position on the paper feeding tray  7  will be separated one by one between the pickup roller  10  and the separating pad  11 , as described above, and thereafter, will be conveyed to the registration roller s 13 A,  13 B by means of the paper feeding rollers  12 A,  12 B. The registration rollers  13 A,  13 B are initially stopped, by turning off the register clutch  98 . When a leading end of the sheet  4  which has been fed from the paper feeding rollers  12 A,  12 B are butted against a nip part between the registration rollers  13 A,  13 B in the stopped state, a loop is formed in the sheet  4 , whereby the skew of the sheet  4  will be corrected. Then, the registration rollers  13 A,  13 B will be started to rotate by turning on the register clutch  98 , and the sheet  4  will be fed out toward the conveying belt  18 . 
     After the leading end of the sheet  4  has been fed from the registration rollers  13 A,  13 B, a laser beam will be emitted from the scanner device  27  to the photosensitive drum  31  for a first color, and writing of an image (exposure to light) will be started. When the sheet  4  has arrived at the transfer position between the photosensitive drum  31  and the transfer roller  19 , the toner image on the photosensitive drum  31  will be transferred onto the sheet  4 . While the toner images having the respective colors are sequentially transferred onto the sheet  4  in this manner, the next sheet  4  will be fed out from the pickup roller  10 , in case where continuous printing is conducted. After the transfer of the image to the sheet  4  by the photosensitive drum  31  for the fourth color has been completed, the sheet  4  will be fed out from the conveying belt  18  to the fixing device  43 , as described above, and thereafter, will be discharged onto the paper discharging tray  5 , by way of the conveying rollers  46 A,  46 B and the paper discharging rollers  47 A,  47 B. 
     The image forming operation in this aspect includes operations on a sheet of paper  4 , from a start of exposure of light by the scanner device  27  to the photosensitive drum  31  for the first color until completion of the transfer to the sheet  4  by the photosensitive drum  31  for the fourth color. During this image forming operation, the pickup roller  10  and the registration roller  13 A will be intermittently driven, accompanying the motions for feeding out the next sheet  4  or so. In the pickup roller  10  and the registration roller  13 A, variations of load torque may occur, following discontinuous motions such as the intermittent motions. In the paper feeding rollers  12 A,  12 B too, variations of load torque may occur, following discontinuous motions such as departure of a backward end of the sheet  4  from the nip position and intrusion of a forward end of the next sheet  4  into the nip position. However, because the driving motor  91  that drives these rollers  10 ,  12 A,  13 A is independently constructed from the first driving unit  1 A that drives the conveying belt  18  and the photosensitive drum  31 , the conveying belt  18  and the photosensitive drum  31  will be stably driven, without being affected by the variations of the load torque. 
     (Effects of this Aspect) 
     As described above, according to the aspect, the driving motor  91  for driving the cleaning device  20  drives also the pickup roller  10 , the paper feeding roller  12 A and the registration roller  13 A. Therefore, as compared with the case where the cleaning device  20  is provided with an exclusive driving unit, downsizing and cost reduction of the image forming apparatus, and decrease of the electric power consumption can be achieved. In the paper feeding roller  12 A, the load torque may considerably vary, at the time of intrusion and departure of the sheet  4 , and in the pickup roller  10  and the registration roller  13 A, the load torque may considerably vary, on occasion of the intermittent motions. Even though irregular rotation may occur in the driving motor  91  with such variations, there will be no functional trouble in the cleaning device  20 , because the cleaning device  20  is such a part that relatively high driving accuracy may not be required. Moreover, because the driving motor  91  for driving the cleaning device  20 , and the rollers  10 ,  12 A,  13 A is constructed independently from the first driving unit  1 A for driving the conveying belt  18 , the irregular rotation of the driving motor  91  will not be transmitted to the conveying belt  18 , and therefore, deterioration of the image quality can be prevented. 
     Moreover, because the cleaning device  20  is provided with the cleaning roller  21 , cleaning performance can be enhanced, as compared with the case where a brush or blade is employed. 
     Further, the image quality can be improved by using the polymerized toner as the developer. The polymerized toner that has adhered to the conveying belt  18  or so is unlikely to be removed, as compared with the pulverized toner. However, according to the aspect, it is possible to effectively remove the toner on the conveying belt  18 , because the cleaning performance of the cleaning device  20  can be enhanced, by increasing the contact pressure of the cleaning roller  21  with respect to the conveying belt  18 , or by other means. 
     (Second Aspect) 
       FIG. 3  is an explanatory view schematically showing a driving mechanism  105  in a second aspect. The second aspect is different from the first aspect in the structure of the driving system, but a general structure of other components is substantially the same as in the first aspect. Therefore, the members having the same functions as in the first aspect will be denoted with the same reference numerals, and the description will partly be omitted. 
     A driving mechanism  105  includes a driving motor  91  as a power source, in the same manner as in the first aspect. The driving motor  91  has the driving gear  91 A, which is meshed with the recovering gear  93  and the cleaning gear  94  via the relay gear  92 . A pair of relay gears  106 ,  107  are successively meshed with the cleaning gear  94 , and a fixing gear  108  which is connected to the heating roller  44  (the driven mechanism) of the fixing device  43  via a shaft body  108 A is meshed with the relay gear  107 . It is to be noted that the other driven components in the laser printer  1  (the belt supporting roller  17 A, the photosensitive drum  31 , the developing roller  40 , the supply roller  39 , the agitator  42 , and so on) are driven by the first driving unit  1 A, which is independent from the driving mechanism  105 . 
     According to this aspect, in the fixing device  43 , the load torque of the heating roller  44  may considerably vary particularly at the time of intrusion and departure of the sheet  4  to and from the nip position during the image forming operation. However, the variation will not affect the performance of the cleaning device  20 . Moreover, because this variation of torque will not be transmitted to the first driving unit, it is possible to drive the conveying belt  18  with high accuracy. 
     (Other Aspects) 
     In the above described aspects, the example in which the invention is applied to a color laser printer of a so-called tandem type for direct transfer has been described. However, it is possible to apply the invention to a color laser printer of a tandem type for intermediate transfer, as shown in  FIG. 4 . In the following description, the members having the same functions as in the first aspect will be denoted with the same reference numerals, and the description will be partly omitted. 
     A laser printer  110  in this aspect is provided with an intermediate transfer belt  111 , in place of the conveying belt  18  in the first aspect. The intermediate transfer belt  111  is stretched around three belt supporting rollers  112 ,  113 ,  114 , and will be cyclically rotated in a counterclockwise direction in the drawing, when the belt supporting roller  112  at a front side is driven to rotate. Below the belt supporting roller  114  at a lower side, a secondary transfer roller  115  is provided so as to be opposed to the roller  114 , interposing the intermediate transfer belt  111 . In this laser printer  110 , the toner images formed on the four photosensitive drums  31  will be once transferred to the intermediate transfer belt  111  for the respective four colors, and thereafter, the toner images transferred to the intermediate transfer belt  111  will be transferred onto the sheet  4 , while the sheet  4  passes the pressure contact position between the secondary transfer roller  115  and the intermediate transfer belt  111 . 
     In the laser printer  110  of the tandem type for the intermediate transfer too, a second driving unit (not shown) for driving the cleaning device  20  drives also the pickup roller  10 , the paper feeding roller  12 A, the registration roller  13 A, and the fixing device  43 , in the same manner as in the above described aspects. At the same time, this second driving unit is independently constructed from a first driving unit, which is a driving source for the belt supporting roller  112  and the photosensitive drums  31 . Accordingly, substantially the same effects as in the above described aspects can be obtained. 
     The invention is not limited to the aspects which have been described in the above description referring to the drawings, but, for example, the following aspects are also included in a technical scope of the invention. Further, it is possible to carry out various modifications besides the following aspects, within a scope not deviating from the gist of the invention. 
     (1) In the above-described aspects, the examples in which the invention has been applied to the laser printers of the tandem type for direct transfer and of the tandem type for intermediate transfer have been described. However, it is possible to apply the invention to an image forming apparatus of a so-called four cycle system in which developer images are sequentially formed on a surface of a single image carrier by means of developing devices for respective colors, and these developer images are sequentially superposed on a transferred member such as a sheet which is conveyed on a belt or an intermediate transfer belt and transferred, thereby to form a multi-colored image. 
     (2) In the above described aspects, the example in which a part of the driven components of the sheet conveying mechanism is driven by the second driving unit for driving the cleaning unit has been described. However, all the driven components of the sheet conveying mechanism may be driven by the second driving unit, or the mechanisms for conducting intermittent motions or discontinuous motions except the sheet conveying mechanism may be driven by the second driving unit.