Patent Publication Number: US-11392071-B2

Title: Image forming apparatus

Description:
BACKGROUND 
     Field 
     The present disclosure relates to an image forming apparatus including a driving unit that drives a plurality of photosensitive drums. 
     Description of the Related Art 
     As an electrophotographic color image forming apparatus, there is known a tandem-type image forming apparatus including independent image forming units for respective colors. The tandem-type image forming apparatus transfers images from the respective photosensitive drums of the image forming units onto an intermediate transfer belt so as to superimpose the images, and further transfers the images from the intermediate transfer belt onto a recording medium all at once. The tandem-type image forming apparatus thus has an issue where the occurrence of speed fluctuations of the plurality of photosensitive drums and the intermediate transfer belt causes color misregistration in which the superimposed images are misaligned and the respective colors are misregistered. 
     To reduce the color misregistration caused by the occurrence of the speed fluctuations, it is necessary to prevent the influence of the speed fluctuations of the plurality of photosensitive drums and the intermediate transfer belt from appearing on the image. 
     Japanese Patent Application Laid-Open No. 63-11967 discusses a technique for reducing the color misregistration caused by the speed fluctuation of the intermediate transfer belt. According to the technique discussed in Japanese Patent Application Laid-Open No. 63-11967, a plurality of photosensitive drums is driven by a common driving source, and is spaced at a distance that allows the time interval of when the intermediate transfer belt passes between adjacent transfer positions to be equal to an integral multiple of the cycle of driving unevenness of the driving source. 
     According to Japanese Patent Application Laid-Open No. 63-11967, it is possible to reduce the influence of the speed fluctuation during the cycle of the driving roller that drives the intermediate transfer belt. However, this technique fails to reduce the influence of speed fluctuations of driving gears that drive the photosensitive drums. 
     Japanese Patent No. 5130507 discusses a technique for reducing the speed fluctuations of the driving gears that drive the photosensitive drums. According to the technique discussed in Japanese Patent No. 5130507, after the phases of one-revolution fluctuations of a driving gear and a coupling are measured for each of the components, the driving gear and the coupling are connected to each other at a position where the phase of the one-revolution fluctuation of the driving gear and the phase of the one-revolution fluctuation of the coupling are relatively shifted from each other from an aligned state. Furthermore, Japanese Patent No. 5130507 discusses that using a composite amplitude obtained by connecting one driving gear and one coupling in the above-described manner as a reference, the other driving gears and the other couplings are connected in the above-described manner so that the other composite amplitudes match the reference composite amplitude. 
     However, the technique discussed in Japanese Patent No. 5130507 has an issue where the composite amplitudes are matched by connecting the driving gears and the couplings while relatively shifting them, but the rotational phases are not aligned with one another, thereby not addressing misregistration among the respective colors, which is caused by rotational fluctuations among the plurality of photosensitive drums. More specifically, the technique discussed in Japanese Patent No. 5130507 has an issue where the misregistration among the respective colors caused by the rotational fluctuations among the plurality of photosensitive drums is not addressed unless the driving gears and the couplings with the composite amplitudes matched are further subjected to rotational phase control for aligning the rotational phases with one another. 
     SUMMARY 
     The present disclosure is directed to reducing misregistration among respective colors due to rotational fluctuations among a plurality of photosensitive drums, without performing rotational phase control for aligning the rotational phases with one another. 
     According to an aspect of the present disclosure, an image forming apparatus includes a transfer member configured to move in a movement direction, a first photosensitive drum disposed in contact with the transfer member at a first transfer position, a second photosensitive drum disposed in contact with the transfer member at a second transfer position, wherein the second photosensitive drum is arranged adjacent to and side by side with the first photosensitive drum in the movement direction, and the second transfer position is located downstream of the first transfer position in the movement direction, and a driving unit configured to drive the first photosensitive drum and the second photosensitive drum, wherein the driving unit includes: (i) a driving source, (ii) at least one driving force transmission gear configured to rotate by receiving a driving force from the driving source, (iii) a first drum gear that meshes with the at least one driving force transmission gear, wherein the first drum gear is configured to receive the driving force from the at least one driving force transmission gear to rotate in a first direction and drive the first photosensitive drum, (iv) a first coupling member provided at a first position of the first drum gear in the first direction, wherein the first coupling member is configured to rotate together with the first drum gear, and to rotate the first photosensitive drum while engaging with the first photosensitive drum, (v) a second drum gear that meshes with the at least one driving force transmission gear, wherein the second drum gear is configured to receive the driving force from the at least one driving force transmission gear to rotate in a second direction and drive the second photosensitive drum, and (vi) a second coupling member provided at a second position of the second drum gear in the second direction, wherein the second coupling member is configured to rotate together with the second drum gear, and to rotate the second photosensitive drum while engaging with the second photosensitive drum, wherein, assuming that a position where the first drum gear meshes with the at least one driving force transmission gear is a first meshing position and a position where the second drum gear meshes with the at least one driving force transmission gear is a second meshing position, a first angle from the first meshing position to the first transfer position in the first direction and a second angle from the second meshing position to the second transfer position in the second direction are different from each other, and wherein the second position of the second drum gear is shifted from a position corresponding to the first position of the first drum gear by a difference between the first angle and the second angle in a direction opposite of the second direction. 
     Further features of the present disclosure will become apparent from the following description of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram illustrating a configuration of a driving unit that drives a plurality of drum gears using a single driving force transmission gear. 
         FIGS. 2A to 2D  are diagrams illustrating phase alignment of drum gears and couplings in the driving unit. 
         FIG. 3  is a cross-sectional view illustrating an image forming apparatus including the driving unit. 
         FIG. 4A  is a diagram illustrating a part of the configuration of the driving unit. 
         FIG. 4B  is a diagram illustrating the cycle of each gear for which the number of teeth is adjusted to an integral multiple. 
         FIGS. 5A to 5C  are diagrams illustrating phase alignment shapes of each drum gear and each drum coupling. 
         FIG. 6  is a diagram illustrating a configuration of a driving unit that drives each of a plurality of drum gears using a different driving force transmission gear. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Hereinafter, exemplary embodiments of the present disclosure will be described in detail with reference to the drawings. Dimensions, materials, shapes, and relative arrangement of components according to the exemplary embodiments described below may be changed as appropriate based on the configuration of an apparatus to which any of the exemplary embodiments of the present disclosure is applied and various kinds of conditions, and are not intended to limit the scope of the present disclosure only thereto. 
     Hereinafter, an image forming apparatus including a driving unit according to a first exemplary embodiment will be described. In the following exemplary embodiments, a full-color image forming apparatus to which four process cartridges are detachably attached will be described as an example of an image forming apparatus. The number of process cartridges attached to an image forming apparatus is not limited to four, and may be appropriately set as necessary. 
     &lt;Schematic Configuration of Image Forming Apparatus&gt; 
     First, a schematic configuration of an image forming apparatus according to the present exemplary embodiment will be described with reference to  FIG. 3 .  FIG. 3  is a cross-sectional view illustrating an image forming apparatus  1  as the image forming apparatus according to the present exemplary embodiment. 
     The image forming apparatus  1  can form a color image on a recording medium S in a state where four process cartridges P (PY, PM, PC, and PK) (hereinafter referred to as the cartridges P) for different colors are detachably attached to an apparatus main body  2  thereof. 
     In  FIG. 3 , the side of the image forming apparatus  1  on which an apparatus opening/closing door  3  is provided is defined as the front (front side) and the opposite side of the front side is defined as the back (back side). In addition, when the image forming apparatus  1  is viewed from the front side, the right side and the left side are referred to as the driving side and the non-driving side, respectively. In other words,  FIG. 3  illustrates the cross section of the image forming apparatus  1  viewed from the non-driving side. The front side, the back side, the right side, and the left side of  FIG. 3  correspond to the non-driving side, the driving side, the front side, and the back side of the image forming apparatus  1 , respectively. 
     In the apparatus main body  2 , the four cartridges P (PY, PM, PC, and PK), namely, the first cartridge PY, the second cartridge PM, the third cartridge PC, and the fourth cartridge PK are arranged in the horizontal direction. 
     Each of the first to fourth cartridges P (PY, PM, PC, and PK) is configured similarly to one another, and includes process members that act on a photosensitive drum  4 . In this example, each of the cartridges P includes a charging member, a development member, and a cleaning member, which will be described below, as the process members. Each of the first to fourth cartridges P (PY, PM, PC, and PK) is used for a different color toner. 
     Bias voltages (e.g., charging bias voltage, development bias voltage) are supplied from the apparatus main body  2  to each of the first to fourth cartridges P (PY, PM, PC, and PK). A rotational driving force is transmitted from a driving unit provided in the apparatus main body  2  to each of the first to fourth cartridges P (PY, PM, PC, and PK). A configuration of the driving unit will be described below. 
     Each of the first to fourth cartridges P (PY, PM, PC, and PK) according to the present exemplary embodiment includes a drum unit  8  as a first unit and a development unit  9  as a second unit. The drum unit  8  includes the photosensitive drum  4 , a charging roller  5  as the charging member, and a cleaning blade  7  as the cleaning member. The development unit  9  includes a development roller (a developer bearing member)  6  as the development member and a supply roller  33 . The drum unit  8  and the development unit  9  are joined to each other. 
     The first cartridge PY contains yellow (Y) toner in a development frame body  29  of the development unit  9 , and forms a yellow toner image on the surface of the photosensitive drum  4 . The second cartridge PM contains magenta (M) toner in the development frame body  29  of the development unit  9 , and forms a magenta toner image on the surface of the photosensitive drum  4 . The third cartridge PC contains cyan (C) toner in the development frame body  29  of the development unit  9 , and forms a cyan toner image on the surface of the photosensitive drum  4 . The fourth cartridge PK contains black (K) toner in the development frame body  29  of the development unit  9 , and forms a black toner image on the surface of the photosensitive drum  4 . 
     As an exposure unit, a laser scanner unit LB is provided above the first to fourth cartridges P (PY, PM, PC, and PK). The laser scanner unit LB outputs laser light Z corresponding to image information. The output laser light Z passes through an exposure window portion  10  of each of the cartridges P to scan and expose the surface of the photosensitive drum  4 . 
     As a transfer unit, an intermediate transfer belt unit  11  is provided below the first to fourth cartridges P (PY, PM, PC, and PK). The intermediate transfer belt unit  11  includes a driving roller  14 , a tension roller  13 , and an assist roller  15 , and a flexible transfer belt  12  that is stretched across the driving roller  14 , the tension roller  13 , and the assist roller  15 . 
     The surface of the photosensitive drum  4  in each of the first to fourth cartridges P (PY, PM, PC, and PK) is in contact with the outer peripheral surface of the transfer belt  12  serving as a transfer member. A primary transfer roller  16  is provided on the inner side of the transfer belt  12  so as to face each of the photosensitive drums  4 . A primary transfer portion  30  is where the photosensitive drum  4  and the primary transfer roller  16  face each other and the photosensitive drum  4  and the transfer belt  12  are in contact with each other. 
     A secondary transfer roller  17  is brought into contact with the driving roller  14  via the transfer belt  12 . A secondary transfer portion  31  is where the driving roller  14  and the secondary transfer roller  17  face each other and the transfer belt  12  and the secondary transfer roller  17  are in contact with each other. 
     A feeding unit  18  is provided below the intermediate transfer belt unit  11 . The feeding unit  18  includes a feeding tray  19  in which the recording medium S is stacked and accommodated, and a feeding roller  20  which feeds the recording medium S accommodated in the feeding tray  19 . 
     A fixing unit  21  and a discharge unit  22  are provided in the upper left portion of the apparatus main body  2  illustrated in  FIG. 3 . The fixing unit  21  fixes toner images transferred to the recording medium S onto the recording medium S. The discharge unit  22  discharges the recording medium S to a discharge tray  23  provided on the top surface of the apparatus main body  2 . 
     The image forming apparatus  1  according to the present exemplary embodiment has been described to have the configuration in which each of the cartridges P detachably attached to the apparatus main body  2  includes the drum unit  8  (including the photosensitive drum  4 ) and the development unit  9  (including the development roller  6 ) that are joined to each other, but may have a configuration different from this configuration. For example, the image forming apparatus  1  may include at least one photosensitive drum  4  and at least one charging roller  5  in the apparatus main body  2 , and a cleaning unit including the cleaning blade  7  may be detachably attached as the cartridge P to the apparatus main body  2 . 
     &lt;Image Forming Operation&gt; 
     An operation to form a full-color image will be described next. 
     The photosensitive drum  4  in each of the first to fourth cartridges P (PY, PM, PC, and PK) is rotationally driven at a predetermined speed in the direction indicated by a corresponding arrow in  FIG. 3  (i.e., the counterclockwise direction). The transfer belt  12  is also rotationally driven at a speed corresponding to the speed of the photosensitive drum  4  in the forward direction of the rotation of the photosensitive drum  4  (the direction indicated by an arrow C in  FIG. 3 ). 
     The laser scanner unit LB is also driven. In synchronization with the driving of the laser scanner unit LB, the charging roller  5  in each of the cartridges P uniformly charges the surface of the photosensitive drum  4  to a predetermined polarity and potential. The laser scanner unit LB scans and exposes the surface of each of the photosensitive drums  4  with the laser light Z based on an image signal of the corresponding color. Accordingly, an electrostatic latent image based on the image signal of the corresponding color is formed on the surface of each of the photosensitive drums  4 . The formed electrostatic latent image is developed by the development roller  6  that is rotationally driven at a predetermined speed in the direction indicated by a corresponding arrow in  FIG. 3  (i.e., the clockwise direction). 
     In the first cartridge PY, the yellow toner image corresponding to the yellow component of the full-color image is formed on the photosensitive drum  4  by the above-described electrophotographic image forming process operation. The yellow toner image formed on the photosensitive drum  4  is primarily transferred onto the transfer belt  12  by the primary transfer roller  16  at the primary transfer portion  30 . 
     Similarly, in the second cartridge PM, the magenta toner image corresponding to the magenta component of the full-color image is formed on the photosensitive drum  4 . The magenta toner image formed on the photosensitive drum  4  is primarily transferred onto the transfer belt  12  by the primary transfer roller  16  at the primary transfer portion  30 , so as to be superimposed on the yellow toner image that has already been transferred to the transfer belt  12 . 
     Similarly, in the third cartridge PC, the cyan toner image corresponding to the cyan component of the full-color image is formed on the photosensitive drum  4 . The cyan toner image formed on the photosensitive drum  4  is primarily transferred onto the transfer belt  12  by the primary transfer roller  16  at the primary transfer portion  30 , so as to be superimposed on the yellow toner image and the magenta toner image that have already been transferred to the transfer belt  12 . 
     Similarly, in the fourth cartridge PK, the black toner image corresponding to the black component of the full-color image is formed on the photosensitive drum  4 . The black toner image formed on the photosensitive drum  4  is primarily transferred onto the transfer belt  12  by the primary transfer roller  16  at the primary transfer portion  30 , so as to be superimposed on the yellow toner image, the magenta toner image, and the cyan toner image that have already been transferred to the transfer belt  12 . 
     In this manner, the unfixed full-color toner images of the four colors, namely, the yellow color, the magenta color, the cyan color, and the black color are formed on the transfer belt  12 . 
     Meanwhile, sheets of the recording medium S accommodated in the feeding tray  19  are separated and fed one by one by the feeding roller  20 . Each sheet of the recording medium S is guided to the secondary transfer portion  31 , which is the contact portion of the secondary transfer roller  17  and the transfer belt  12 , at a predetermined control timing. At the secondary transfer portion  31 , the toner images of the four colors superimposed on the transfer belt  12  are secondarily transferred onto the recording medium S all at once. 
     The toner images transferred to the recording medium S are fixed onto the recording medium S by the fixing unit  21 . The recording medium S with the images fixed thereon is discharged to the discharge tray  23  on the top surface of the apparatus main body  2  by the discharge unit  22 . 
     &lt;Configuration of Driving Unit&gt; 
     A configuration of a driving unit  50  for driving the plurality of photosensitive drums  4  will be described next. The configuration of the driving unit  50  will be described with reference to  FIGS. 1 to 4A , using a part of the driving unit  50  that drives two of the photosensitive drums  4  adjacent to each other, as an example. 
       FIGS. 1 and 4A  illustrate the driving unit  50  that drives a first photosensitive drum and a second photosensitive drum that is arranged adjacent to and side by side with the first photosensitive drum in the movement direction of the transfer belt  12 . For example, in  FIG. 3 , if the photosensitive drum  4  in the process cartridge PY is assumed to be the first photosensitive drum, the photosensitive drum  4  in the process cartridge PM is the second photosensitive drum. The driving unit  50  illustrated in  FIG. 1  drives the first photosensitive drum that is brought into contact with the transfer belt  12  at a first transfer position  301  (corresponding to the primary transfer portion  30  of  FIG. 3 ) where the toner image is transferred. The driving unit  50  illustrated in  FIG. 1  also drives the second photosensitive drum that is brought into contact with the transfer belt  12  at a second transfer position  302  (corresponding to the primary transfer portion  30  of  FIG. 3 ) located downstream of the first transfer position  301  in the movement direction of the transfer belt  12 . 
     As illustrated in  FIGS. 1 and 4A , the driving unit  50  includes a driving motor  50 M as a driving source, and a driving force transmission gear  52  that rotates by receiving a driving force from the driving motor  50 M. The driving unit  50  further includes drum couplings  71  and  72  and drum gears  511  and  512 . The drum couplings  71  and  72  are drum coupling members that engage with the photosensitive drums  4 . The drum gears  511  and  512  rotationally drive the drum couplings  71  and  72 . The driving force transmission gear  52  transmits the driving force from the driving motor  50 M to each of the drum gears  511  and  512 . The drum gear  511  is a first drum gear that meshes with the driving force transmission gear  52  and is configured to rotate in a first direction by receiving the driving force from the driving force transmission gear  52 , thereby driving the first photosensitive drum. The drum coupling  71  is a first coupling member provided at a first position of the drum gear  511  in the first direction and configured to rotate together with the drum gear  511 . The drum coupling  71  also rotates the first photosensitive drum while engaging with the first photosensitive drum. The drum gear  512  is a second drum gear that meshes with the driving force transmission gear  52  and is configured to rotate in a second direction by receiving the driving force from the driving force transmission gear  52 , thereby driving the second photosensitive drum. The drum coupling  72  is identical to the drum coupling  71  in amplitude variation (speed variation) during one rotation cycle from a reference phase. The drum coupling  72  is a second coupling member provided at a second position of the drum gear  512  in the second direction and configured to rotate together with the drum gear  512 . The drum coupling  72  also rotates the second photosensitive drum while engaging with the second photosensitive drum. 
     &lt;Causes for Occurrence of Color Misregistration&gt; 
     When the toner image formed on each of the photosensitive drums  4  is transferred onto the transfer belt  12  at the primary transfer portion  30  so as to be superimposed on the toner image(s) already transferred to the transfer belt  12 , the toner image may be transferred in a state of being shifted from a predetermined position, thereby causing color misregistration. Causes for the occurrence of color misregistration will be described next. Types of color misregistration include steady color misregistration and non-steady color misregistration. The steady color misregistration and the non-steady color misregistration will be described in this order. 
     The steady color misregistration occurs due to, for example, the shift of the position irradiated with the laser light Z of each color. Thus, in each image forming apparatus  1 , the shift amount of the position irradiated with the laser light Z is detected using a sensor (not illustrated) that detects the position of the toner transferred to the transfer belt  12 , and the irradiation timing of the laser light Z is adjusted, thereby correcting the shift of the position. 
     The non-steady color misregistration occurs due to, for example, the speed fluctuation caused by the eccentricity of the driving roller  14  that drives the transfer belt  12  or the eccentricity of the photosensitive drums  4  and the driving gears that drive the photosensitive drums  4 . 
     &lt;Reduction in Color Misregistration Due to Eccentricity of Transfer Belt Driving Roller&gt; 
     The following configuration is provided to reduce the non-steady color misregistration caused by a driving component of the transfer belt  12 . The plurality of photosensitive drums  4  is driven by the common driving source, and is spaced at a distance that allows the time interval of when the transfer belt  12  passes between the adjacent primary transfer portions  30  to be equal to an integral multiple of the cycle of driving unevenness of the driving source. 
     The configuration will be further described with reference to  FIG. 1 . Assuming that Lst represents the distance (the spacing) between the first and second transfer positions  301  and  302 , which is the spacing between the primary transfer portions  30  adjacent to each other, and Di represents the diameter of the driving roller  14 , the photosensitive drums  4  are arranged to satisfy the following relation.
 
 Lst=NπDi ( N :integer)
 
     The satisfaction of the relation expressed by the above-described equation allows the transfer belt  12  to pass between the photosensitive drums  4  at the same speed variation cycle, thereby reducing the color misregistration due to the eccentricity of the driving roller  14  that drives the transfer belt  12 . 
     &lt;Speed Fluctuations Due to Eccentricity of Motor and Gears&gt; 
     Similarly, one of the causes for the non-steady color misregistration is speed fluctuations due to the eccentricity of the motor and the gears that drive the photosensitive drums  4 . More specifically, this is a phenomenon in which, while the gears are rotating, if any of the gears swings to shift the rotational axis of the gear from the center, the rotational speed slows down at a portion where the distance from the center to the surface of the gear is long and speeds up at a portion where the distance from the center to the surface of the gear is short. 
     To reduce the influence of the eccentricity of the motor and the gears, a configuration in which the numbers of teeth of the gears are selected is provided, so that the color misregistration is reduced. 
     The configuration will be described with reference to  FIGS. 4A and 4B , using the rotational fluctuation of the photosensitive drum  4  as an example.  FIG. 4A  illustrates a part of the driving unit  50  that drives the photosensitive drums  4 . The driving unit  50  includes the drum gear  51  ( 511  or  512 ), which drives the photosensitive drum  4 , a stepped gear (driving force transmission gear)  52 , which drives the drum gear  51 , an idler gear  53 , which drives the stepped gear  52 , and a pinion gear  54 , which drives the idler gear  53  and is attached to the driving motor  50 M (the driving source). The stepped gear  52  includes a small gear  52   a  and a large gear  52   b  larger in diameter than the small gear  52   a . The pinion gear  54  attached to the driving motor  50 M meshes with the idler gear  53 . The idler gear  53  meshes with the large gear  52   b  of the stepped gear  52 . The small gear  52   a  of the stepped gear  52  meshes with the drum gear  51 . The driving force from the driving motor  50 M is transmitted to the drum gear  51 , so that the photosensitive drum  4  is rotationally driven. 
     Assume that an exposure position  61  is a position at which the photosensitive drum  4  is irradiated with the laser light Z emitted from the laser scanner unit LB, and the primary transfer portion  30  is a contact portion at which the photosensitive drum  4  is in contact with the transfer belt  12 . The photosensitive drum  4  is rotationally driven by the drum gear  51  to which the driving force is transmitted. The surface of the photosensitive drum  4  is exposed by the laser light Z of the laser scanner unit LB, so that the electrostatic latent image is formed thereon. 
     Assume that, when θrt represents an angle from the exposure position  61  to the primary transfer portion  30  on the drum shaft, t(θrt) represents the time required for the photosensitive drum  4  to rotate by θrt. 
       FIG. 4B  illustrates the speed fluctuations of the stepped gear  52 , the idler gear  53 , and the pinion gear  54  that drive the photosensitive drum  4 , with the elapse of the time t(θrt) during which the photosensitive drum  4  rotates by the angle θrt. In  FIG. 4B , the vertical axis and the horizontal axis represent a speed V and the time t(θrt), respectively. 
     In  FIG. 4B , a stepped gear speed fluctuation  52 A indicates the speed fluctuation of the stepped gear  52 , an idler gear speed fluctuation  53 A indicates the speed fluctuation of the idler gear  53 , and a pinion gear speed fluctuation  54 A indicates the speed fluctuation of the pinion gear  54 . 
     Assume that, in this case, the number of teeth of the drum gear  51  is z 51 , the number of teeth of the small gear  52   a  of the stepped gear  52  is z 52   a , the number of teeth of the large gear  52   b  of the stepped gear  52  is z 52   b , the number of teeth of the idler gear  53  is z 53 , and the number of teeth of the pinion gear  54  is z 54 . In order to set the number of rotations of each of the gears to an integer, the number of teeth of each of the gears is set to satisfy the following relation, using the number of teeth z 51  of the drum gear  51  as a reference.
 
 Zdr×θrt=z 52 a  
 
 z 52 b= 2× z 53=6× z 54
 
     In this case, since the stepped gear  52  (with the number of teeth z 52   a  of the small gear  52   a  and the number of teeth z 52   b  of the large gear  52   b ) is an integrated gear, the rotation amount of the small gear  52   a  and the rotation amount of the large gear  52   b  are equal to each other. 
     The rotation amount corresponding to Zdr×θrt is defined to be f(Zdr×θrt). In this case, similarly, the rotation amount of the small gear  52   a  having the number of teeth z 52   a  can be expressed as f(z 52   a ). The small gear  52   a  and the large gear  52   b  are integrated as the stepped gear  52 , and this means that the rotation amount f(z 52   b ) of the large gear  52   b  having the number of teeth z 52   b  is equal to the rotation amount f(z 52   a ) of the small gear  52   a , i.e., f(z 52   a )=f(z 52   b ). 
     Thus, when all the rotation amounts of the gears are expressed as an equation, the following relation is satisfied among the rotation amounts of the gears.
 
 f ( Zdr×θrt )= f ( z 52 a )= f ( z 52 b )=2× f ( z 53)=6× f ( z 54)
 
     The relation among the actual numbers of teeth of the respective gears is as follows. 
     The number of teeth z 51  (Zdr) of the drum gear  51  is 103, and the angle θrt from the exposure position  61  to the primary transfer portion  30  is 178.25° (51/103×360 degrees). Accordingly, the rotation amount of the drum gear  51  is the rotation amount corresponding to Zdr×θrt=51 teeth. 
     The number of teeth z 52   a  of the small gear  52   a  of the stepped gear  52  is 51. Thus, the rotation amount of the small gear  52   a  is the rotation amount corresponding to Zdr×θrt=1×z 52   a  teeth. The number of teeth z 52   b  of the large gear  52   b  of the stepped gear  52  is 72. Since the large gear  52   b  is integrated with the small gear  52   a , the rotation amount of the large gear  52   b  is equal to the rotation amount of the small gear  52   a  (i.e., the rotation amount corresponding to the number of teeth z 52   b =the rotation amount corresponding to the number of teeth z 52   a ). 
     The number of teeth z 53  of the idler gear  53  is 36. Thus, the rotation amount of the idler gear  53  is the rotation amount corresponding to Zdr×θrt=z 52   a =z 52   b= 2×z 53 . 
     The number of teeth z 54  of the pinion gear  54  is 12. Thus, the rotation amount of the pinion gear  54  is the rotation amount corresponding to Zdr×θrt=z 52   a =z 52   b= 2×z53=6×z 54 . 
     In this manner, the relationship among the gears is such that, when the drum gear  51  rotates from the exposure position  61  to the primary transfer portion (the transfer position)  30 , each of the gears  52 ,  53 , and  54  in the preceding stage rotates the integer number of times. 
       FIG. 4B  illustrates the rotational fluctuations at this time. When the drum gear  51  rotates from the exposure position  61  to the primary transfer portion  30 , i.e., when the time t(θrt) has elapsed, each of the stepped gear  52 , the idler gear  53 , and the pinion gear  54  rotates the integer number of times. Accordingly, the respective fluctuations of the three gears  52 ,  53 , and  54  during one rotation are in phase, and the speed fluctuations of the motor and the gears are in phase at the exposure position  61  and the transfer position  30 . Thus, the present configuration can reduce the color misregistration caused by the speed fluctuations due to the eccentricity of the motor and the gears. 
     &lt;Reduction in Color Misregistration Caused by Rotational Fluctuations Due to Degrees of Precision and Eccentricity of Drum Gears&gt; 
     Next, the rotational fluctuation due to the eccentricity of the drum gear  51  that drives the photosensitive drum  4  will be described. Similarly to the gears described so far, the drum gear  51  is also subjected to a rotational fluctuation due to the degree of precision and the eccentricity. The driving unit  50  used in the image forming apparatus  1  including the plurality of photosensitive drums  4  includes the drum gears  51  that drive the photosensitive drums  4  as many as the number of photosensitive drums  4 . In such a configuration, it is desirable that the drum gears  51  driving the respective photosensitive drums  4  have the same shape in order to reduce color misregistration due to an error in the meshing and transmission of the drum gears  51 . In the present exemplary embodiment, the drum gears  51  that drive the respective photosensitive drums  4  are molded with the same mold cavity. 
     Using the drum gears  51  of the same shape allows the degree of precision and the eccentricity to be kept constant among the drum gears  51  that drive the respective photosensitive drums  4 . Thus, it is desirable to use the drum gears  51  molded with the same mold cavity, as the drum gears  51  that drive the respective photosensitive drums  4 . 
     Phase alignment among the respective drum gears  51  will be described with reference to  FIGS. 1 to 2D .  FIG. 1  illustrates a part of the driving unit  50  that drives the adjacent two photosensitive drums  4  using the respective drum gears  511  and  512 , and drives the drum gears  511  and  512  using the same driving force transmission gear  52 . 
     Assume that Lst (unit: mm) represents the distance between the first and second transfer positions  301  and  302  adjacent to each other, Vi (unit: mm/sec) represents the speed of the transfer belt  12 , and Rd (unit: rps) represents the rotational speed of the photosensitive drum  4 . In this case, in order to drive the drum gear  512  at the same meshing position as that of the drum gear  511 , the phases of the drum gears  511  and  512  adjacent to each other need to be aligned in the following manner. More specifically, the phase of the drum gear  512  needs to be aligned with a phase obtained by rotating the drum gear  512  by an angle θst in the opposite direction (the clockwise direction in  FIG. 1 ) of the rotational direction, using a second meshing position K 2  of the drum gear  512  as a reference. In this case, the angle θst by which the drum gear  512  rotates before the image formed on the first transfer position  301  reaches the second transfer position  302  can be expressed by the following equation.
 
θ st=Lst/Vi·Rd× 360
 
     The phase alignment between the adjacent drum gears  511  and  512  will be described in further detail. As described above, the drum gears  511  and  512  are molded with the same cavity of the same mold. Thus, the phase of the tooth of the drum gear  512  corresponding to the tooth of the drum gear  511  that meshes with the driving force transmission gear  52  at the first meshing position K 1  is aligned with the phase obtained by rotating the drum gear  512  by the angle θst in the opposite direction (the clockwise direction in  FIG. 1 ) of the rotational direction, using the second meshing position K 2  as the reference. The position of the tooth of the drum gear  512  corresponding to the tooth of the drum gear  511  that meshes with the driving force transmission gear  52  at the first meshing position K 1  is indicated by a broken line circle in  FIG. 1 . 
       FIG. 2A  illustrates how the speed of the drum gear  511  fluctuates during one rotation cycle of the drum gear  511  from the first meshing position K 1 .  FIG. 2B  illustrates how the speed of the drum gear  512  fluctuates during one rotation cycle of the drum gear  512  from the second meshing position K 2 . 
     In this example, the angular difference between the first meshing position K 1  and the second meshing position K 2  can be expressed as the angle Est. Using the drum gears  511  and  512  of the same shape allows the speed fluctuations during one rotation cycle to be brought into phase at the respective meshing positions K 1  and K 2  with the driving force transmission gear  52 . Thus, the present configuration can reduce the color misregistration caused by the rotational fluctuations due to the degrees of precision and the eccentricity of the drum gears. 
     &lt;Reduction in Color Misregistration Due to Speed Fluctuations of Drum Couplings&gt; 
     The color misregistration may occur due to the shift of the first and second transfer positions  301  and  302  between the photosensitive drums  4  and the transfer belt  12 , caused by the speed fluctuations of the drum couplings  71  and  72  that drive the photosensitive drums  4  while engaging with the photosensitive drums  4 . In other words, to reduce the color misregistration caused by the drum couplings  71  and  72 , the phase alignment needs to be performed using the first and second transfer positions  301  and  302  as references. 
     The phase alignment between the drum couplings  71  and  72  will be described with reference to  FIG. 1 . First, the angular relationship required for the phase alignment will be described based on the relationship among the first and second transfer positions  301  and  302  and the rotational centers of the respective gears  511 ,  512 , and  52 . Then, the phase alignment between the drum couplings  71  and  72  will be described. 
     &lt;Relational Expression for Angles&gt; 
     First, the angular relationship required for the phase alignment will be described with reference to  FIG. 1 , based on the relationship among the first and second transfer positions  301  and  302 , a rotational center  52   c  of the driving force transmission gear  52  that meshes with the drum gears  511  and  512 , and rotational centers  511   c  and  512   c  of the drum gears  511  and  512 . 
     Assume that θ 1  represents an angle formed in the rotational direction between a line connecting the rotational center  511   c  of the drum gear  511  and the rotational center  52   c  of the driving force transmission gear  52 , and a line connecting the rotational center  511   c  of the drum gear  511  and the first transfer position  301 . In  FIG. 1 , the angle θ 1  is a first angle from the first meshing position K 1  of the drum gear  511  with the driving force transmission gear  52  to the first transfer position  301  in the rotational direction of the drum gear  511 . 
     Similarly, assume that θ 2  represents an angle formed in the rotational direction between a line connecting the rotational center  512   c  of the drum gear  512  and the rotational center  52   c  of the driving force transmission gear  52 , and a line connecting the rotational center  512   c  of the drum gear  512  and the second transfer position  302 . In  FIG. 1 , the angle θ 2  is a second angle from the second meshing position K 2  of the drum gear  512  with the driving force transmission gear  52  to the second transfer position  302  in the rotational direction of the drum gear  512 . The angles θ 1  and θ 2  are different from each other. More specifically, the first angle from the first meshing position K 1  of the drum gear  511  to the first transfer position  301  in the rotational direction and the second angle from the second meshing position K 2  of the drum gear  512  to the second transfer position  302  in the rotational direction are different from each other. 
     In addition, assume that θ 3  represents an angle formed between a line connecting the rotational center  52   c  of the driving force transmission gear  52  and the rotational center  511   c  of the drum gear  511 , and a line connecting the rotational center  52   c  of the driving force transmission gear  52  and the rotational center  512   c  of the drum gear  512 . 
     Furthermore, assume that θ 4  represents an angle formed between a line connecting the rotational center  511   c  of the drum gear  511  and the first transfer position  301 , and a line connecting the first transfer position  301  and the second transfer position  302 . Assume that θ 5  represents an angle formed between a line connecting the rotational center  512   c  of the drum gear  512  and the second transfer position  302 , and the line connecting the first transfer position  301  and the second transfer position  302 . 
     Assuming that the first and second transfer positions  301  and  302  are the same position on each of the photosensitive drums  4 , the following relational expression (1) is satisfied.
 
θ4+θ5=180  (1)
 
     The following relational expression (2) is satisfied based on a sum of interior angles of a hexagon formed by the first and second transfer positions  301  and  302 , the rotational centers  511   c  and  512   c  of the two drum gears  511  and  512 , and the rotational center  52   c  of the driving force transmission gear  52 .
 
360−θ1+θ2+θ3+θ4+θ5=540  (2)
 
     Based on the above-described expressions (1) and (2), the following relational expression (3) is satisfied as the relationship among the above-described angles.
 
θ1−θ2=θ3  (3)
 
&lt;Phase Alignment Between Drum Couplings&gt;
 
     Next, the phase alignment between the drum couplings  71  and  72  will be described with reference to  FIGS. 1 to 2D . 
     As described above, to reduce the color misregistration due to the speed fluctuations of the drum couplings  71  and  72  that drive the photosensitive drums  4  while engaging with the photosensitive drums  4 , the phases of the drum couplings  71  and  72  need to be aligned using the first and second transfer positions  301  and  302  as the references. The configuration according to the present exemplary embodiment will be described next with reference to a comparative example. 
     As the comparative example,  FIG. 2D  illustrates, with a broken line, the speed fluctuation of the drum coupling  72  in a configuration where the drum gear and the drum coupling are integrated and are driven with only one phase. In the comparative example, the position of the drum coupling  72  relative to the drum gear  512  in the rotational direction of the drum gear  512  is the same as the position of the drum coupling  71  relative to the drum gear  511  in the rotational direction of the drum gear  511 . 
     In the comparative example, the phase of the drum coupling  72  is also similar to the phase of the drum gear  512  as indicated by the broken line in  FIG. 2D . In other words, the drum coupling  72  and the drum gear  512  are in a similar phase relationship to the relationship between the phase of the drum gear  511 , which is adjacent to the drum gear  512  at the upstream position in the movement direction of the transfer belt  12 , and the phase of the drum coupling  71 , which engages with the drum gear  511 . This causes a difference between the phase of the speed fluctuation of the drum coupling  71  of the drum gear  511  at the first transfer position  301  and the phase of the speed fluctuation of the drum coupling  72  of the drum gear  512  at the second transfer position  302 , resulting in the color misregistration. 
     To address this, in the present exemplary embodiment, the phase alignment between the drum couplings  71  and  72  is performed so as to bring the speed fluctuations of the drum couplings  71  and  72  into phase at the first and second transfer positions  301  and  302 . More specifically, the phase of the position of attaching one drum coupling to one of adjacent drum gears is shifted by a predetermined angular difference, using the position of attaching another drum coupling to the other drum gear as a reference. 
     First, to reduce the color misregistration due to the rotational fluctuations of the drum gears  511  and  512 , the phase of the drum gear  512  is aligned with the phase delayed by the rotational angle θst relative to the drum gear  511  as described above, so that the phase of the drum gear  512  is aligned with the phase of the drum gear  511 . More specifically, assuming that the drum gear  511  has the first tooth and the drum gear  512  has the second tooth corresponding to the first tooth of the drum gear  511 , when the first tooth of the drum gear  511  is located at the first meshing position K 1 , the second tooth of the drum gear  512  is located at the position shifted from the second meshing position K 2  by the rotational angle θst in the opposite direction of the rotational direction (second direction) of the drum gear  512 . For example, when the first tooth of the drum gear  511  is located at the first meshing position K 1  illustrated in  FIG. 1  (the position indicated by the black circle), the second tooth of the drum gear  512  is located at the position shifted from the second meshing position K 2  by the rotational angle θst in the opposite direction of the second direction (i.e., the position indicated by the broken line circle). 
     The first transfer position  301  is located at a position shifted by the angle θ 1  from the first meshing position K 1  of the drum gear  511 . Similarly, the second transfer position  302  is located at a position shifted by the angle θ 2  from the second meshing position K 2  of the drum gear  512 .  FIGS. 2A to 2D  illustrate the first meshing position K 1  of the drum gear  511 , the second meshing position K 2  of the drum gear  512 , the first transfer position  301 , and the second transfer position  302  with vertical dot-dot dashed lines. 
     Next, the position of attaching the drum coupling  72  to the drum gear  512  is shifted by the predetermined angular difference θ 1 −θ 2 , using the position of attaching the drum coupling  71  to the drum gear  511  as the reference, in order to align the phases of the drum couplings  71  and  72  at the first and second transfer positions  301  and  302 . 
     Assume that, with respect to a first portion of the drum coupling  71 , a portion of the drum coupling  72  corresponding to the first portion is a second portion. Assume further that, in  FIG. 1 , when the first tooth of the drum gear  511  is located at the first meshing position K 1 , the first portion of the drum coupling  71  is located at the first transfer position  301  indicated by a black triangle. 
     In the comparative example, the position of attaching the drum coupling  71  to the drum gear  511 , and the position of attaching the drum coupling  72  to the drum gear  512  are the same. In this configuration, when the second tooth of the drum gear  512  (corresponding to the first tooth of the drum gear  511 ) is located at the second meshing position K 2 , the second portion is located at a position shifted by the angle θ 3  from the second transfer position  302  through which the second portion has passed. As a result, for example, when the speed of the drum coupling  71  is reduced at the first transfer position  301  as illustrated in  FIG. 2C , the speed of the drum coupling  72  is increased at the second transfer position  302  as indicated by the broken line in  FIG. 2D . 
     On the other hand, in the configuration according to the present exemplary embodiment, the position of attaching the drum coupling  72  to the drum gear  512  is shifted from the position of attaching the drum coupling  71  to the drum gear  511  by the angle θ 3  in the opposite direction of the rotational direction of the drum gear  512 . Accordingly, when the second tooth corresponding to the first tooth is located at the second meshing position K 2 , the second portion is located at the second transfer position  302 . As a result, for example, when the speed of the drum coupling  71  is reduced at the first transfer position  301  as illustrated in  FIG. 2C , the speed of the drum coupling  72  is also reduced at the second transfer position  302  as indicated by a solid line in  FIG. 2D . Therefore, the color misregistration can be reduced. 
     In the present exemplary embodiment, the drum gears  511  and  512  are the same drum gears molded with the same mold as described above, and each have two attachment positions (attachment portions) for attaching the drum couplings  71  and  72  at different phases. More specifically, as illustrated in  FIG. 5A , the drum gears  511  and  512  each have a first position (a first attachment position) A, and a second position (a second attachment position) B shifted from the first position A by the above-described predetermined angular difference θ 3  (=θ 1 −θ 2 ), as coupling attachment positions. 
     At the first position A, the drum coupling  71  is attached to the drum gear  511  in such a manner that the reference phase for the speed fluctuation of the drum coupling  71  matches the first meshing position K 1  (illustrated in  FIG. 2C ) of the drum gear  511  with the driving force transmission gear  52 . 
     The second position B is different from the position corresponding to the first position A. At the second position B, the drum coupling  72  is attached to the drum gear  512 . In this case, the reference phase for the speed fluctuation of the drum coupling  72  is shifted by the difference between the angles θ 1  and θ 2  in the opposite direction of the rotational direction of the drum gear  512 , compared to when the drum coupling  72  is attached at the first position A of the drum gear  512 . More specifically, the phase of the drum coupling  72  attached at the second position B of the drum gear  512  illustrated in  FIG. 2B  is shifted from the position indicated by the broken line illustrated in  FIG. 2D  to the position indicated by the solid line illustrated in  FIG. 2D . The drum couplings  71  and  72  are connected to the drum gears  511  and  512 , respectively by being selectively attached at the first position A or the second position B. 
     The drum gears  511  and  512  have the same shape. As illustrated in  FIG. 5A , each of the drum gear  511  and the drum gear  512  has the two attachment positions (the first position A and the second position B). Each of the drum gear  511  and the drum gear  512  has the first position (the first attachment portion) A and the second position (the second attachment portion) B. In each of the drum gears  511  and  512 , the second position B is located at the position shifted from the first position A by the difference between the angles θ 1  and θ 2  in the opposite direction of the rotational direction of each of the drum gears  511  and  512 . 
     The drum coupling  71  is attached at the first position (the first attachment portion) A of the drum gear  511 . The drum coupling  72  is attached at the second position (the second attachment portion) B of the drum gear  512 . 
     With this configuration, as illustrated in  FIGS. 2A and 2C , the drum coupling  71  engages with the drum gear  511  in such a manner that the reference phase for the speed fluctuation of the drum coupling  71  matches the first meshing position K 1  of the drum gear  511 . In this case, the tooth of the drum gear  511  located at the first meshing position K 1  is referred to as a first reference tooth, and the tooth of the drum gear  512  corresponding to the first reference tooth (the tooth located at the same position as that of the first reference tooth in the rotational direction of the drum gear  512 ) is referred to as a second reference tooth. As illustrated in  FIGS. 2B and 2D , when the second reference tooth of the drum gear  512  is located at the second meshing position K 2 , the reference phase for the speed fluctuation of the drum coupling  72  is shifted from the second meshing position K 2  by the difference between the angles θ 1  and θ 2  in the opposite direction of the rotational direction, relative to the drum gear  512 . 
     In other words, when the first position A matches the position of the first reference tooth in the drum gear  511 , in the drum gear  512 , the second position B is located at the position shifted from the second reference tooth by the difference between the angles θ 1  and θ 2  in the opposite direction of the rotational direction (second direction). 
     The drum couplings  71  and  72 , which are molded from the same mold, have the same speed fluctuation during one rotation cycle from the above-described reference phase. In addition, the reference phase of the drum coupling  72  (the first transfer position  301  illustrated in  FIG. 2D ) corresponds to the reference phase of the drum coupling  71  (the first meshing position K 1  illustrated in  FIG. 2C ) adjusted to the first meshing position K 1  of the drum gear  511 . Furthermore, as described above, the tooth of the drum gear  512  corresponding to the tooth of the drum gear  511  that meshes with the driving force transmission gear  52  at the first meshing position K 1  is indicated by the broken line circle in  FIG. 1 . 
     In this manner, the drum gears  511  and  512  according to the present exemplary embodiment each have the plurality of phases for engaging the drum couplings  71  and  72 . More specifically, the drum gears  511  and  512  each have the plurality of attachment positions for attaching the drum couplings  71  and  72  in such a manner that the drum couplings  71  and  72  are shifted from each other by the predetermined angular difference. With this configuration, the drum gears  511  and  512  allow the drum couplings  71  and  72  to be attached at different phases, thereby making it possible to align the phase of the drum coupling  71  at the first transfer position  301  and the phase of the drum coupling  72  at the second transfer position  302  with each other. As a result, the speed fluctuation of the drum coupling  71  at the first transfer position  301  and the speed fluctuation of the drum coupling  72  at the second transfer position  302  can be brought into phase without implementation of rotational phase control for aligning the rotational phases with each other. Thus, the color misregistration due to the speed fluctuations of the drum couplings  71  and  72  can be reduced. Furthermore, the misregistration among the respective colors due to the rotational fluctuations among the plurality of photosensitive drums  4  can be reduced. 
     &lt;Attachment Positions of Drum Gear and Shape of Coupling&gt; 
     Next, the shape for attaching the drum coupling  71  or  72  to the drum gear  51  will be described with reference to  FIGS. 5A to 5C . Since the drum gears  511  and  512  have the same shape, the drum gears  511  and  512  will be collectively described as the drum gear  51  in the following description. In addition, since the drum couplings  71  and  72  have the same shape, the drum couplings  71  and  72  will be collectively described as the drum coupling  70  in the following description. However, the drum couplings  71  and  72  may not necessarily have the same shape at a portion not relating to the function for driving the photosensitive drum  4  as long as the drum couplings  71  and  72  have the same shape at a portion relating to the function for driving the photosensitive drum  4 . Similarly, the drum gears  511  and  512  may not necessarily have the same shape at a portion not relating to the function for driving the photosensitive drum  4  as long as the drum gears  511  and  512  have the same shape at a portion relating to the function for driving the photosensitive drum  4 . 
     Furthermore, even when there is a slight shape difference due to a dimensional tolerance at a portion relating to the function for driving the photosensitive drum  4 , the drum couplings  71  and  72  can still be defined to have the same shape and the drum gears  511  and  512  can still be defined to have the same shape. For example, the drum couplings  71  and  72  and the drum gears  511  and  512  may include a portion that has a dimensional tolerance of −0.5 mm to +0.5 mm for the position or the dimension or has a dimensional tolerance of −3° to +3° for the angle. 
     It is desirable that the drum gears  511  and  512  are approximately exactly the same. For example, it is desirable that the drum gears  511  and  512  are molded from the same mold cavity. Furthermore, it is desirable that the drum couplings  71  and  72  are approximately exactly the same. For example, it is desirable that the drum couplings  71  and  72  are molded from the same mold cavity. In the present exemplary embodiment, the drum gears  511  and  512  and the drum couplings  71  and  72  are manufactured by resin molding. 
     A case where the drum gears  51  ( 511  and  512 ) mesh with the driving force transmission gear  52  at different angles and there are two driving force transmission points will be described. More specifically, a configuration of the driving unit  50  in which the two drum gears  51  ( 511  and  512 ) adjacent to each other mesh with the same single driving force transmission gear  52  will be described as an example. In other words, the configuration of the driving unit  50  including, as at least one driving force transmission gear, the single driving force transmission gear  52  that meshes with both the drum gears  511  and  512  will be described. 
     The drum coupling  70  includes engagement portions  70   g  ( 70   g   1  and  70   g   2 ) that engage with the photosensitive drum  4 . The photosensitive drum  4  detachably engages with the engagement portions  70   g . The speed of the photosensitive drum  4  may fluctuate due to variations in the positions of the engagement portions  70   g  in the rotational direction of the drum coupling  70 . The drum gear  51  includes a positioning portion  51   a  that positions a positioning portion  70   a  of the drum coupling  70 . At one of the attachment positions (the first position A), the drum gear  51  is provided with first and second driving force transmission surfaces  51   b  and  51   c  for driving the drum coupling  70 . The second driving force transmission surface  51   c  is provided at a phase opposite to the phase of the first driving force transmission surface  51   b . At the other attachment position (the second position B) having the phase difference of θ 1 −θ 2 , the drum gear  51  is provided with first and second driving force transmission surfaces  51   d  and  51   e  for driving the drum coupling  70 . The second driving force transmission surface  51   e  is provided at a phase opposite to the phase of the first driving force transmission surface  51   d.    
     Providing the drum gear  51  with the two attachment positions at different phases allows the drum couplings  70  ( 71  and  72 ) to be attached to the drum gears  51  while shifting the phases from each other by the difference in the angle from the meshing position to the primary transfer position. The two attachment positions (the first position A and the second position B) provided to each of the drum gears  51  are arranged in such a manner that the second position B is shifted from the first position A by the predetermined angular difference (θ 1 −θ 2 ) in the opposite direction of the rotational direction of the drum gear  51 . Thus, assuming that the rotational direction of the drum gear  51  is the counterclockwise direction in  FIG. 5A , the first position A corresponds to the attachment position on the downstream side in the rotational direction, and the second position B corresponds to the attachment position on the upstream side in the rotational direction. 
     &lt;Shape for Preventing Erroneous Attachment with Phase Difference of 180°&gt; 
     The drum couplings  70  ( 71  and  72 ) as the coupling members each include a first protrusion portion  70   d , which has a first width in the rotational direction, and a second protrusion portion  70   e , which has a second width narrower than the first width in the rotational direction. The first protrusion portion  70   d  is provided in a manner protruding outward from the positioning portion (the outer peripheral surface)  70   a . The second protrusion portion  70   e  is provided at a position opposite to the first protrusion portion  70   d  via the rotational center of the drum coupling  70 , and is provided in a manner protruding outward from the positioning portion (the outer peripheral surface)  70   a.    
     In the drum coupling  70 , the first protrusion portion  70   d  includes a first driving force reception surface  70   b  that receives the driving force while being in contact with the first driving force transmission surface  51   b  or  51   d  of the drum gear  51 . Also, in the drum coupling  70 , the second protrusion portion  70   e  includes a second driving force reception surface  70   c  that receives the driving force while being in contact with the second driving force transmission surface  51   c  or  51   e  of the drum gear  51 , at a position opposite to the first driving force reception surface  70   b  via the rotational center of the drum coupling  70 . 
     Each of the drum gears  51  ( 511  and  512 ) has the first position A and the second position B that is shifted from the first position A by the predetermined angular difference in the rotational direction. One of the attachment positions of the drum gear  51  (the first position A) is provided with an attachment groove including a first groove portion  51   b   1  and a second groove portion  51   c   1 . The other attachment position of the drum gear  51  (the second position B) is provided with an attachment groove including a first groove portion  51   d   1  and a second groove portion  51   e   1 . The attachment groove including the first groove portion  51   b   1  and the second groove portion  51   c   1 , and the attachment groove including the first groove portion  51   d   1  and the second groove portion  51   e   1  have the same shape. 
     The first groove portions  51   b   1  and  51   d   1  of the attachment positions each have a width allowing engagement of the first protrusion portion  70   d  in the rotational direction. The second groove portions  51   c   1  and  51   e   1  of the attachment positions are provided at the positions opposite to the first groove portions  51   b   1  and  51   d   1 , respectively, via the rotational center of the drum gear  51 , and each have a width narrower than the width of each of the first groove portions  51   b   1  and  51   d   1  and allowing engagement of the second protrusion portion  70   e  in the rotational direction. 
     In the drum gear  51 , the first groove portions  51   b   1  and  51   d   1  include the first driving force transmission surfaces  51   b  and  51   d , respectively, each of which transmits the driving force while being in contact with the first driving force reception surface  70   b . Also, in the drum gear  51 , the second groove portions  51   c   1  and  51   e   1  include the second driving force transmission surfaces  51   c  and  51   e , each of which transmits the driving force while being in contact with the second driving force reception surface  70   c , at the positions opposite to the first driving force transmission surfaces  51   b  and  51   d  via the rotational center of the drum gear  51 , respectively. 
     As described above, each of the drum couplings  70  includes the first protrusion portion  70   d , which has the first width in the rotational direction, and the second protrusion portion  70   e , which has the second width narrower than the first width in the rotational direction. In addition, each of the drum gears  51  includes the first groove portions  51   b   1  and  51   d   1 , each of which has the width allowing the engagement of the first protrusion portion  70   d  in the rotational direction, and the second groove portions  51   c   1  and  51   e   1 , each of which has the width allowing the engagement of the second protrusion portion  70   e , at the respective attachment positions (the first position A and the second position B). Each of the second groove portions  51   c   1  and  51   e   1  of the drum gear  51  is narrower in width in the rotational direction than each of the first groove portion  51   b   1  and  51   d   1  of the drum gear  51 . 
     In other words, the drum gear  51  is configured in such a manner that only the second protrusion portion  70   e  of the drum coupling  70 , which is narrower in width in the rotational direction than the first protrusion portion  70   d  of the drum coupling  70 , can be attached to each of the second groove portions  51   c   1  and  51   e   1  of the drum gear  51 . 
     Thus, if the drum coupling  70  is rotated by 180° before being attached to the drum gear  51 , interference occurs between the first protrusion portion  70   d  of the drum coupling  70  and the second groove portion  51   c   1  or  51   e   1  of the drum gear  51 , thereby resulting in attachment failure. Accordingly, the drum coupling  70  can be prevented from being attached at a wrong phase shifted by 180° with respect to each of the attachment positions of the drum gear  51 . 
     &lt;Prevention of Erroneous Attachment with Phase Difference of θ 1 −θ 2 &gt; 
     Next, the shape for preventing erroneous attachment due to a difference in the phase angle of the drum coupling  70  will be described. 
     Each of the drum gears  51  includes a first phase hole  51   f  and a second phase hole  51   g  for phase determination. The first phase hole  51   f  is provided at a position distant from the rotational center of the drum gear  51  by a first radius R 1 . The second phase hole  51   g  is provided at a position distant from the rotational center of the drum gear  51  by a second radius R 2  different from the first radius R 1 . The first phase hole  51   f  and the second phase hole  51   g  in each of the drum gears  51  are pin insertion holes. 
     Each of the drum couplings  70  includes a groove hole  70   f  and the positioning portion (the outer peripheral surface)  70   a . The groove hole  70   f  is provided at a position distant from the rotational center of the drum coupling  71  by a third radius R 3 . The positioning portion (the outer peripheral surface)  70   a  is provided at a position distant from the rotational center of the drum coupling  71  by a fourth radius R 4 . The distance of the radius R 3  at which the groove hole  70   f  is provided is shorter than each of the first radius R 1  and the second radius R 2 . The distance of the radius R 4  at which the positioning portion  70   a  is provided is longer than each of the first radius R 1  and the second radius R 2 . 
     In this case, the relation of the radii R 3 &lt;R 1 &lt;R 2 &lt;R 4  is satisfied as the relation among the distances of the first phase hole  51   f  and the second phase hole  51   g  of the drum gear  51  and the distances of the groove hole  70   f  and the positioning portion  70   a  of the drum coupling  70  from the rotational centers. 
     When the drum coupling  70  is attached to the first and second driving force transmission surface  51   b  and  51   c  at one of the attachment positions (the first position A) at the phase for driving the drum coupling  70 , the drum coupling  70  is attached with a phase determination pin (not illustrated) inserted in the second phase hole  51   g  (refer to  FIG. 5B ). At this time, if the drum coupling  70  is to be attached to the first and second driving force transmission surfaces  51   d  and  51   e  at the other attachment position (the second position B) at the phase shifted by the angular difference of θ 1 −θ 2 , interference occurs between the phase determination pin inserted in the second phase hole  51   g  and the drum coupling  70 , thereby resulting in attachment failure. Thus, the drum coupling  70  is prevented from being attached to the drum gear  51  at a wrong phase. 
     When the drum coupling  70  is attached to the first and second driving force transmission surfaces  51   d  and  51   e  at the other attachment position (the second position B) at the phase for driving the drum coupling  70 , the drum coupling  70  is attached with a phase determination pin (not illustrated) inserted in the first phase hole  51   f  (refer to  FIG. 5C ). At this time, if the drum coupling  70  is to be attached to the first and second driving force transmission surfaces  51   b  and  51   c  at one of the attachment positions (the first position A) at the phase shifted by the angular difference of θ 1 −θ 2 , interference occurs between the phase determination pin inserted in the first phase hole  51   f  and the drum coupling  70 , thereby resulting in attachment failure. Thus, the drum coupling  70  is prevented from being attached to the drum gear  51  at a wrong phase. 
     As illustrated in  FIG. 5B , the drum coupling  71  does not overlap the second phase hole (second hole)  51   g  and overlaps the first phase hole (first hole)  51   f  in a state where the drum coupling  71  is attached at the first position A of the drum gear  511 . As illustrated in  FIG. 5C , the drum coupling  72  does not overlap the first phase hole  51   f  and overlaps the second phase hole  51   g  in a state where the drum coupling  72  is attached at the second position B of the drum gear  512 . 
     As described above, according to the present exemplary embodiment, the misregistration among the respective colors due to the rotational fluctuations among the plurality of photosensitive drums can be reduced without the implementation of the rotational phase control for aligning the rotational phases of the drum gears with one another. Furthermore, when the drum couplings are attached to the drum gears at different attachment positions, the drum couplings can be attached at the respective attachment positions without mistake. 
     In the first exemplary embodiment, the single (same) driving force transmission gear  52  that meshes with the drum gears  51  that drive the photosensitive drums  4  adjacent to each other has been described as an example of at least one driving force transmission gear configured to rotate by receiving the driving force from the driving source. In a second exemplary embodiment, a configuration in which different driving force transmission gears mesh with the respective drum gears that drive the photosensitive drums adjacent to each other, as the above-described at least one driving force transmission gear will be described. The other configuration is similar to that according to the first exemplary embodiment, and thus a description thereof will be omitted. 
     The case where each of the drum gears meshes with a different driving force transmission gear will be described with reference to  FIG. 6 .  FIG. 6  illustrates a schematic configuration of a part of a driving unit according to the present exemplary embodiment. In the driving unit according to the present exemplary embodiment, a first drum gear  513  meshes with a first driving force transmission gear  523 , and is rotationally driven by receiving a driving force transmitted from the first driving force transmission gear  523 . In addition, a second drum gear  514  adjacent to the first drum gear  513  meshes with a second driving force transmission gear  524  different from the first driving force transmission gear  523 , and is rotationally driven by receiving a driving force transmitted from the second driving force transmission gear  524 . 
     Furthermore, the first driving force transmission gear  523  and the second driving force transmission gear  524  mesh with a single (same) idler gear  531 , and are rotationally driven by receiving a driving force transmitted from the idler gear  531 . The idler gear  531  meshes with a pinion gear  541  attached to a motor (not illustrated) serving as the driving source, and is rotationally driven by receiving a driving force transmitted from the pinion gear  541 . 
     Assume that θ 6  represents an angle formed in the rotational direction between a line connecting a rotational center  513   c  of the first drum gear  513  and a rotational center  523   c  of the first driving force transmission gear  523 , and a line connecting the rotational center  513   c  of the first drum gear  513  and a primary transfer position  303 . 
     Similarly, assume that θ 7  represents an angle formed in the rotational direction between a line connecting a rotational center  514   c  of the second drum gear  514  and a rotational center  524   c  of the second driving force transmission gear  524 , and a line connecting the rotational center  514   c  of the second drum gear  514  and a primary transfer position  304 . 
     The second drum gear  514  is arranged in such a manner that, when the first tooth of the first drum gear  513  is located at a meshing position K 3 , the second tooth of the second drum gear  514  corresponding to the first tooth is located at a phase shifted by the angle θst, in the opposite direction of the rotational direction, from a meshing position K 4  with the second driving force transmission gear  524 . In addition, assuming that a drum coupling  73  is attached to the first drum gear  513  at a first position, a drum coupling  74  is attached to the second drum gear  514  at a position shifted in phase from a position corresponding to the first position by an angular difference of θ 6 −θ 7 . 
     According to the present exemplary embodiment, even when each of the drum gears meshes with a different preceding-stage driving force transmission gear, the phases of the couplings can be aligned at the transfer positions and therefore the color misregistration can be reduced, similarly to the above-described first exemplary embodiment. 
     While in the present exemplary embodiment, the configuration not having &lt;Shape for Preventing Erroneous Attachment with Phase Difference of 180°&gt; or &lt;Prevention of Erroneous Attachment with Phase Difference of θ 1 −θ 2 &gt; according to the above-described first exemplary embodiment has been described, the configuration is not limited thereto. The configuration according to the present exemplary embodiment may have &lt;Shape for Preventing Erroneous Attachment with Phase Difference of 180°&gt; and/or &lt;Prevention of Erroneous Attachment with Phase Difference of θ 1 −θ 2 &gt;, similarly to the first exemplary embodiment. 
     A third exemplary embodiment will be described. While in the first and second exemplary embodiments, the configuration in which the drum gear and the drum coupling are separate members and the drum coupling is connected to the drum gear has been described, the configuration is not limited thereto. For example, the drum gear and the drum coupling may be integrally molded and configured as a gear molded with the phases shifted on the mold. 
     For example, the shape of the molded gear in which the drum coupling and the drum gear are integrally molded is molded with two parts in the axial direction, i.e., a recessed cavity and a protruding core. In the case of a helical gear, the tooth profile portion of the molded gear is molded in such a manner that a mold for molding the tooth profile portion is extruded while being rotated. At this time, phase alignment is performed using a return mechanism in such a manner that the shape of the tooth profile is located at the same position at each time of molding in order to make identical the phase relationship between the shapes of the attachment portion for attaching the coupling to the gear and of the phase determination hole, and the tooth portion of the gear. 
     In addition, a mold having the shape of the coupling and a mold having the shape of the gear can be attached while being rotated relative to each other, and are provided with a positioning hole for determining the phases of the molds in the rotational direction. Furthermore, the position of the pin for the phase determination hole that determines the phase of the molded gear can be changed between two positions. More specifically, when the gear is molded with a first phase, the pin is provided in the phase determination hole located at a distance corresponding to a first radius from the rotational center. On the other hand, when the gear is molded with a second phase shifted from the first phase by the predetermined phase difference (angular difference), the pin is provided in the phase determination hole located at a distance corresponding to a second radius, which is different from the first radius, from the rotational center. In this way, the phase determination hole can be provided to the gear. 
     With this method, the gears including the couplings having two attachment phases can be molded using the mold having one tooth profile. Furthermore, the difference between the two types of phases of the coupling in the molded gear can be distinguished based on the phase determination hole. Thus, even when the drum gear and the drum coupling are integrally molded, the molded gear can be attached so as to change the phase depending on the position at which the gear meshes with the preceding-stage driving force transmission gear, as described in the first exemplary embodiment. 
     While in the above-described exemplary embodiments, the printer has been described as an example of the image forming apparatus, the image forming apparatus is not limited thereto. For example, the exemplary embodiments of the present disclosure may be applied to other image forming apparatuses such as a copying machine, a facsimile apparatus, and a multifunction peripheral having a combination of these functions. Furthermore, while in the above-described exemplary embodiments, the image forming apparatus, which uses the intermediate transfer member, transfers the toner images for the respective colors onto the intermediate transfer member so as to superimpose the toner images sequentially, and transfers the toner images borne on the intermediate transfer member onto the recording medium all at once, has been described as an example, the image forming apparatus is not limited thereto. The exemplary embodiments of the present disclosure may also be applied to an image forming apparatus that uses a recording medium bearing member and transfers the toner images for the respective colors onto a recording medium borne on the recording medium bearing member so as to superimpose the images sequentially. Similar advantageous effects can be achieved by applying any of the exemplary embodiments of the present disclosure to these image forming apparatuses. The exemplary embodiments of the present disclosure may also be applied to a manufacturing method for manufacturing the image forming apparatus described in the exemplary embodiments. 
     According to the exemplary embodiments of the present disclosure, the phase of the first coupling member and the phase of the second coupling member can be aligned at the respective transfer positions. Therefore, the misregistration among the respective colors due to the rotational fluctuations among the plurality of photosensitive drums can be reduced without the implementation of the rotational phase control for aligning the rotational phases with one another. 
     While the present disclosure has been described with reference to exemplary embodiments, it is to be understood that the disclosure is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Application No. 2020-119169, filed Jul. 10, 2020 and Japanese Patent Application No. 2021-θ86109, filed May 21, 2021, each of which is hereby incorporated by reference herein in their entirety.