Image forming device

Abstract

To reduce the color displacement of each color due to rotation changes between a plurality of photoconductor drums without performing a rotation phase control for making rotation phases agree to each other.SOLUTION: The image formation device includes: a first coupling member provided in a first position of a first drum gear; and a second coupling member provided in a second position of a second drum gear. The second position is displaced from the position of the first position by a predetermined angular difference into a direction opposite to the direction in which the second drum gear rotates.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to an image forming apparatus including a drive unit for driving a plurality of photosensitive drums. [Background technology]

[0002] As an electrophotographic color image forming apparatus, there is a tandem type image forming apparatus having an image forming section for each color independently. In a tandem type image forming apparatus, images are transferred from the photosensitive drums of each image forming section to an intermediate transfer belt so as to be superimposed, and the images are further transferred from the intermediate transfer belt to a recording medium all at once. For this reason, there is a problem that the speed fluctuations of the multiple photosensitive drums and the intermediate transfer belt occur, and when the images are superimposed, the images do not match, and color shift occurs, whereby the colors are misaligned.

[0003] In order to reduce color shifts caused by the speed fluctuations, it is necessary to prevent the speed fluctuations of the multiple photosensitive drums and intermediate transfer belt from occurring on the image.

[0004] A technology for reducing color misregistration caused by speed fluctuations of an intermediate transfer belt is disclosed in Patent Document 1. In Patent Document 1, multiple photosensitive drums are driven by a common drive source, and the photosensitive drums are arranged so that the time interval between adjacent transfer positions of the intermediate transfer belt is equal to an integer multiple of the drive unevenness period of the drive source.

[0005] According to Patent Document 1, the effect of speed fluctuations due to the period of the drive roller of the intermediate transfer belt can be suppressed, but the effect of speed fluctuations of the gears that drive the photosensitive drums cannot be suppressed.

[0006] Patent Document 2 discloses a technique for suppressing the speed fluctuation of a drive gear that drives a photosensitive drum. Patent Document 2 discloses that the phase of one-rotation fluctuation of the drive gear and the coupling is measured for each component, and then the drive gear and the coupling are coupled at a position where the phase of one-rotation fluctuation of the drive gear and the phase of one-rotation fluctuation of the coupling are relatively shifted from the state where they are coincident. Furthermore, it discloses that a composite amplitude obtained by coupling one drive gear and one coupling as described above and combining them is used as a reference, and another drive gear and another coupling are coupled as described above so that the other composite amplitudes coincide with this reference composite amplitude. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 63-11967 [Patent Document 2] Patent No. 5130507 Summary of the Invention [Problem to be solved by the invention]

[0008] However, in Patent Document 2, the drive gear and the coupling are connected with a relative shift to match the composite amplitude, but since their rotation phases do not match, there is a problem that color shifts of each color due to the rotation fluctuations between the multiple photosensitive drums remain. In other words, in Patent Document 2, unless the drive gear and the coupling with the composite amplitude matched and rotation phase control is performed to match each other, there is a problem that color shifts of each color due to the rotation fluctuations between the multiple photosensitive drums remain.

[0009] An object of the present invention is to reduce color misregistration of each color caused by rotational fluctuations among a plurality of photosensitive drums, without performing rotational phase control for matching the rotational phases of the multiple photosensitive drums. [Means for solving the problem]

[0010] A representative configuration of the present invention for achieving the above object is an image forming apparatus including: a transfer body configured to be movable in a moving direction; a first photosensitive drum abutted against the transfer body at a first transfer position; a second photosensitive drum arranged adjacent to the first photosensitive drum in the moving direction and abutted against the transfer body at a second transfer position located downstream of the first transfer position in the moving direction; and a drive unit for driving the first photosensitive drum and the second photosensitive drum, the drive unit including: (i) a drive unit for driving the first photosensitive drum and the second photosensitive drum; (ii) at least one drive transmission gear that receives a drive force from the drive source and rotates; (iii) a first drum gear that meshes with the at least one drive transmission gear, receives a drive force from the at least one drive transmission gear and rotates in a first direction to drive the first photosensitive drum; (iv) a first coupling member that is provided at a first position of the first drum gear in the first direction, rotates together with the first drum gear, and engages with the first photosensitive drum to rotate the first photosensitive drum; and (v) ) a second drum gear that meshes with the at least one drive transmission gear, receives a drive force from the at least one drive transmission gear to rotate in a second direction, and drives the second photosensitive drum; and (vi) a second coupling member that is provided at a second position of the second drum gear in the second direction, rotates together with the second drum gear, and engages with the second photosensitive drum to rotate the second photosensitive drum, wherein, when a position at which the first drum gear meshes with the at least one drive transmission gear is defined as a first meshing position and a position at which the second drum gear meshes with the at least one drive transmission gear is defined as 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, and the second position of the second drum gear is shifted in the opposite direction to the second direction from a position corresponding to the first position by a difference between the first angle and the second angle. Effect of the Invention

[0011] According to the present invention, the phase of the first coupling member and the phase of the second coupling member can be matched at each transfer position, so that color misregistration of each color due to rotational fluctuations between multiple photosensitive drums can be reduced without performing rotational phase control to match the rotational phases of the first and second coupling members. [Brief description of the drawings]

[0012] [Figure 1] 13 is a diagram showing the configuration of a drive unit in which a plurality of drum gears are driven by one drive transmission gear. FIG. [Diagram 2] 6(a) to 6(d) are diagrams illustrating phase alignment of a drum gear and a coupling of a drive unit. [Diagram 3] 1 is a cross-sectional view of an image forming apparatus including a drive unit. [Figure 4] FIG. 1A is a diagram showing a part of the configuration of a drive unit, and FIG. 1B is a diagram showing the gear period when the number of gear teeth is set to an integer multiple. [Diagram 5] 6(a) to 6(c) are diagrams showing the phase matching shapes of the drum gear and the drum coupling. [Figure 6] 13 is a diagram showing a configuration of a drive unit in which a plurality of drum gears are driven by separate drive transmission gears. FIG. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] Preferred embodiments of the present invention will be described in detail below with reference to the drawings. However, the dimensions, materials, shapes, and relative positions of the components described in the following examples may be changed as appropriate depending on the configuration of the device to which the present invention is applied and various conditions, and are not intended to limit the scope of the present invention to these alone.

[0014] Example 1 An image forming apparatus including a drive unit according to Example 1 will be described below. In the following example, a full-color image forming apparatus to which four process cartridges can be detachably attached is exemplified as the image forming apparatus. The number of process cartridges to be attached to the image forming apparatus is not limited to this, and may be set appropriately as required.

[0015] <General Configuration of Image Forming Apparatus> First, a schematic configuration of the image forming apparatus will be described with reference to Fig. 3. Fig. 3 is a cross-sectional view of the image forming apparatus of this embodiment.

[0016] The image forming apparatus 1 is capable of forming a color image on a recording medium S by removably mounting four process cartridges P (PY, PM, PC, PK) (hereinafter referred to as cartridges) of different colors in an apparatus main body 2.

[0017] 3, the side where the device opening / closing door 3 is provided is referred to as the front side (front surface), and the side opposite the front side is referred to as the back side (rear surface). In addition, when looking at the image forming apparatus 1 from the front, the right side is referred to as the drive side, and the left side is referred to as the non-drive side. The figure is a cross section of the image forming apparatus 1 as seen from the non-drive side, with the front side being the non-drive side of the image forming apparatus 1, the back side being the drive side of the image forming apparatus 1, the right side being the front side of the image forming apparatus 1, and the left side being the back side of the image forming apparatus 1.

[0018] In the apparatus main body 2, four cartridges P (PY·PM·PC·PK), namely, a first cartridge PY, a second cartridge PM, a third cartridge PC and a fourth cartridge PK, are arranged in the horizontal direction.

[0019] Each of the first to fourth cartridges P (PY, PM, PC, PK) has the same configuration and includes process means that act on the photosensitive drum 4. Here, the process means includes charging means, developing means, and cleaning means, which will be described later. Also, each of the first to fourth cartridges P (PY, PM, PC, PK) contains toner of a different color.

[0020] Moreover, bias voltages (charging bias, developing bias, etc.) are supplied to each of the first to fourth cartridges P (PY, PM, PC, PK) from the apparatus main body 2. A rotational driving force is transmitted to the first to fourth cartridges P (PY, PM, PC, PK) from a driving unit provided in the apparatus main body 2. The configuration of this driving unit will be described later.

[0021] Each of the first to fourth process cartridges P (PY, PM, PC, PK) in this embodiment has a drum unit 8 which is a first unit, and a developing unit 9 which is a second unit. The drum unit 8 has a photosensitive drum 4, a charging roller 5 which serves as a charging means, and a cleaning blade 7 which serves as a cleaning means. The developing unit 9 has a developing roller (developer carrier) 6 which serves as a developing means, and a supply roller 33. The drum unit 8 and the developing unit 9 are connected to each other.

[0022] The first cartridge PY accommodates yellow (Y) toner in the developing frame 29 of the developing unit 9, and forms a yellow toner image on the surface of the photosensitive drum 4. The second cartridge PM accommodates magenta (M) toner in the developing frame 29 of the developing unit 9, and forms a magenta toner image on the surface of the photosensitive drum 4. The third cartridge PC accommodates cyan (C) toner in the developing frame 29 of the developing unit 9, and forms a cyan toner image on the surface of the photosensitive drum 4. The fourth cartridge PK accommodates black (K) toner in the developing frame 29 of the developing unit 9, and forms a black toner image on the surface of the photosensitive drum 4.

[0023] A laser scanner unit LB serving as an exposure means is provided above the first to fourth cartridges P (PY, PM, PC, PK). The laser scanner unit LB outputs a laser beam Z corresponding to image information. The laser beam Z passes through an exposure window 10 of the cartridge P and scans and exposes the surface of the photosensitive drum 4.

[0024] An intermediate transfer belt unit 11 is provided below the first to fourth cartridges P (PY, PM, PC, PK) as a transfer member. This intermediate transfer belt unit 11 has a drive roller 14, a tension roller 13, and an assist roller 15, and a flexible transfer belt 12 is stretched across the intermediate transfer belt unit 11. The transfer belt 12 is configured to be movable in a movement direction (the direction of the arrow C in FIG. 3).

[0025] The surface of the photosensitive drum 4 of each of the first to fourth cartridges P (PY, PM, PC, PK) is in contact with the outer circumferential surface of the transfer belt 12, which is a transfer body. A primary transfer roller 16 is provided on the inside of the transfer belt 12, facing each photosensitive drum 4. The opposing portion between the photosensitive drum 4 and the primary transfer roller 16, that is, the contact portion between the photosensitive drum 4 and the transfer belt 12, is the primary transfer portion 30.

[0026] A secondary transfer roller 17 is in contact with the driving roller 14 via the transfer belt 12. The opposing portion of the driving roller 14 and the secondary transfer roller 17, that is, the contact portion of the transfer belt 12 and the secondary transfer roller 17, is a secondary transfer portion 31.

[0027] A feeding unit 18 is provided below the intermediate transfer belt unit 11. This feeding unit 18 has a feeding tray 19 that holds and accommodates recording media S, and a feeding roller 20 that feeds the recording media S accommodated in the feeding tray 19.

[0028] In the upper left of the device main body 2 in Figure 3, there are provided a fixing unit 21 that fixes the toner image transferred to the recording medium S to the recording medium S, and a discharge unit 22 that discharges the recording medium S to a discharge tray 23 on the top surface of the device main body 2.

[0029] In the image forming apparatus 1 of this embodiment, a cartridge P detachably attached to the apparatus main body 2 is combined with a drum unit 8 including a photosensitive drum 4 and a developing unit 9 including a developing roller 6, but other configurations are also possible. For example, the image forming apparatus 1 may be configured such that at least one or more photosensitive drums 4 and charging rollers 5 are provided in the apparatus main body 2, and a cleaning unit including a cleaning blade 7 is provided as a cartridge P detachably attached to the apparatus main body 2.

[0030] <Image formation operation> The operation for forming a full color image is as follows.

[0031] The photosensitive drums 4 of the first to fourth cartridges P (PY, PM, PC, PK) are rotated at a predetermined speed in the direction of the arrow in Fig. 3 (counterclockwise direction). The transfer belt 12 is also rotated at a speed corresponding to the speed of the photosensitive drums 4 in the forward direction (the direction of the arrow C in Fig. 3) of the rotation of the photosensitive drums 4.

[0032] 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 cartridge 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 photosensitive drum 4 with a laser beam Z in accordance with an image signal for each color. As a result, an electrostatic latent image in accordance with the image signal for the corresponding color is formed on the surface of each photosensitive drum 4. The formed electrostatic latent image is developed by the developing roller 6 which is rotationally driven in the direction of the arrow in FIG. 3 (clockwise direction) at a predetermined speed.

[0033] By the above-described electrophotographic image forming process operation, a yellow toner image corresponding to the yellow component of the full-color image is formed on the photosensitive drum 4 of the first cartridge PY. Then, the yellow toner image formed on the photosensitive drum 4 is primarily transferred onto the transfer belt 12 by the primary transfer roller 16 in the primary transfer unit 30.

[0034] Similarly, a magenta toner image corresponding to the magenta component of the full-color image is formed on the photosensitive drum 4 of the second cartridge PM. Then, the magenta toner image formed on the photosensitive drum 4 is primarily transferred to the primary transfer unit 30 by the primary transfer roller 16 so as to be superimposed on the yellow toner image already transferred onto the transfer belt 12.

[0035] Similarly, a cyan toner image corresponding to the cyan component of the full-color image is formed on the photosensitive drum 4 of the third cartridge PC. Then, the cyan toner image formed on the photosensitive drum 4 is primarily transferred to the primary transfer unit 30 by the primary transfer roller 16 so as to be superimposed on the yellow and magenta toner images already transferred onto the transfer belt 12.

[0036] Similarly, a black toner image corresponding to the black component of the full-color image is formed on the photosensitive drum 4 of the fourth cartridge PK. Then, the black toner image formed on the photosensitive drum 4 is primarily transferred by the primary transfer roller 16 in the primary transfer unit 30 so as to be superimposed on the yellow, magenta, and cyan toner images already transferred onto the transfer belt 12.

[0037] In this manner, an unfixed full-color toner image of four colors, yellow, magenta, cyan, and black, is formed on the transfer belt 12.

[0038] Meanwhile, recording media S accommodated in a feed tray 19 are separated and fed one by one by a feed roller 20. The recording media S are introduced into a secondary transfer section 31, which is a contact portion between a secondary transfer roller 17 and a transfer belt 12, at a predetermined control timing. Then, the four-color superimposed toner images on the transfer belt 12 are secondarily transferred collectively onto the recording media S in the secondary transfer section 31.

[0039] The toner image transferred to the recording medium S is then fixed to the recording medium S by a fixing unit 21. The recording medium S with the fixed image is discharged by a discharge unit 22 onto a discharge tray 23 on the top surface of the apparatus main body 2.

[0040] <Drive unit configuration> Next, the configuration of the drive unit 50 that drives a plurality of photosensitive drums will be described. Here, a part of the drive unit 50 that drives two adjacent photosensitive drums will be described as an example with reference to Figs. 1 to 4.

[0041] The drive unit 50 shown in FIG. 1 and FIG. 4(a) illustrates a drive unit that drives a first photosensitive drum and a second photosensitive drum arranged next to the first photosensitive drum in the moving direction of the transfer belt 12. For example, in FIG. 3, if the photosensitive drum 4 of the process cartridge PY is the first photosensitive drum, the photosensitive drum 4 of the process cartridge PM is the second photosensitive drum. The drive unit 50 shown in FIG. 1 drives the first photosensitive drum abutted against the transfer belt 12 at a first transfer position 301 (a position corresponding to the primary transfer section 30 shown in FIG. 3) where a toner image is transferred. The drive unit 50 shown in FIG. 1 also drives the second photosensitive drum abutted against the transfer belt 12 at a second transfer position 302 (a position corresponding to the primary transfer section 30 shown in FIG. 3) located downstream of the first transfer position 301 in the moving direction of the transfer belt 12.

[0042] As shown in FIG. 1 and FIG. 4(a), the drive unit 50 has a motor 50M serving as a drive source and a drive transmission gear 52 that rotates by receiving a drive force from the motor. The drive unit 50 further has couplings 71 and 72 that are drum coupling members that engage with the photosensitive drums, and drum gears 511 and 512 that rotate and drive the couplings 71 and 72. The drive transmission gear 52 transmits drive from the motor to the drum gears 511 and 512. The drum gear 511 is a first drum gear that meshes with the drive transmission gear 52, receives a drive force from the drive transmission gear 52, rotates in a first direction, and drives the first photosensitive drum. The drum coupling 71 is provided at a first position of the drum gear 511 in the first direction, and rotates together with the drum gear 511. The drum coupling 71 is also a first coupling member that engages with the first photosensitive drum to rotate the first photosensitive drum. The drum gear 512 is a second drum gear that meshes with the drive transmission gear 52, receives a drive force from the drive transmission gear 52, rotates in a second direction, and drives a second photosensitive drum. The drum coupling 72 has the same change in amplitude (speed) per one rotation period from a reference phase as the drum coupling 71. The drum coupling 72 is provided at a second position of the drum gear 512 in the second direction, and rotates together with the drum gear 512. The drum coupling 72 is a second coupling member that engages with the second photosensitive drum to rotate the second photosensitive drum.

[0043] <Explanation of the cause of color misregistration> Here, we will explain the cause of color misregistration, which occurs when the toner image formed on the photosensitive drum 4 is transferred onto the transfer belt 12 in a superimposed manner at the primary transfer unit 30, and the toner images are transferred to the transfer belt 12 out of their predetermined positions. There are two types of color misregistration: steady color misregistration and non-steady color misregistration. We will explain steady color misregistration and non-steady color misregistration in order.

[0044] The steady color shift occurs due to a shift in the irradiation position of the laser light of each color, etc. Therefore, for each image forming device 1, a sensor (not shown) that detects the position of the toner transferred onto the transfer belt 12 detects the amount of deviation in the irradiation position of the laser light, and correction is performed by adjusting the irradiation timing of the laser light.

[0045] The non-stationary color shift occurs due to speed fluctuations caused by eccentricity of the drive roller 14 of the transfer belt 12, the photosensitive drum 4, and the drive gear that drives the photosensitive drum 4, and the like.

[0046] <Reduction of color misalignment caused by eccentricity of the transfer belt drive roller> Of the non-stationary color shifts, the following configuration is used to reduce color shifts due to the drive components of the transfer belt 12. A plurality of photosensitive drums 4 are driven by a common drive source, and the photosensitive drums 4 are arranged so that the passing time interval of the transfer belt 12 passing adjacent primary transfer units 30 is a distance interval equal to an integer multiple of the drive unevenness period of the drive source.

[0047] This will be explained with reference to Figure 1. When the distance (spacing) from transfer position 301 to transfer position 302, which is the interval between adjacent primary transfer units 30, is Lst and the diameter of the drive roller 14 is Di, the photosensitive drum 4 is disposed so that the following relationship holds:

[0048] Lst=NπDi(N: integer)

[0049] When the above relationship is satisfied, the speed change period of the transfer belt 12 when passing each photosensitive drum 4 becomes the same, so that color shift due to eccentricity of the drive roller 14 of the transfer belt 12 can be suppressed.

[0050] <Speed ​​fluctuation due to eccentricity of motor and gear> Similarly, one of the causes of non-stationary color shift is speed fluctuation due to eccentricity of the motor and gear that drive the photosensitive drum 4. When the gear rotates and the rotation axis of the gear oscillates so as to deviate from the center, the rotation speed becomes slower at points where the distance from the center to the gear surface is long, and the rotation speed becomes faster at points where the distance from the center to the gear surface is short.

[0051] Color shift is suppressed by selecting the number of gear teeth to reduce the effects of eccentricity in the motor and gears.

[0052] The rotational fluctuation of the photosensitive drum 4 will be described with reference to Figs. 4(a) and 4(b). Fig. 4(a) is a diagram showing a part of a drive unit 50 that drives the photosensitive drum 4. A drum gear 51 that drives the photosensitive drum 4, a stepped gear 52 that drives the drum gear 51, an idler gear 53 that drives the stepped gear 52, and a pinion gear 54 attached to a drive motor (drive source) that drives the idler gear 53 are arranged. The stepped gear 52 has a small gear 52a and a large gear 52b having a larger diameter than the small gear 52a. The pinion gear 54 of the drive motor meshes with the idler gear 53, the idler gear 53 meshes with the large gear 52b of the stepped gear 52, and the small gear 52a of the stepped gear 52 meshes with the drum gear 51. The driving force from the drive motor is transmitted to the drum gear 51, and the photosensitive drum 4 is driven to rotate.

[0053] The position where the photosensitive drum 4 is irradiated with the laser light Z emitted by the laser scanner unit LB is defined as an exposure position 61, and the contact portion (abutment portion) where the photosensitive drum 4 contacts the transfer belt 12 is defined as a primary transfer portion 30. The photosensitive drum 4 is rotated by a driving force transmitted to a drum gear 51. The photosensitive drum 4 is exposed to the laser light Z from the laser scanner unit LB, and an electrostatic latent image is formed on the photosensitive drum 4.

[0054] When the angle from the exposure position 61 to the primary transfer portion 30 on the drum axis is θrt, the time taken for the photosensitive drum 4 to rotate by θrt is expressed as t(θrt).

[0055] 4(b) is a diagram showing the speed fluctuations of the stepped gear 52, idler gear 53, and pinion gear 54 that drive the photosensitive drum 4 after the photosensitive drum 4 has rotated θrt and time t(θrt) has elapsed. In FIG. 4(b), the vertical axis represents the speed V, and the horizontal axis represents the time t(θrt).

[0056] In FIG. 4(b), the speed fluctuation of stepped gear 52 is assumed to be stepped gear speed fluctuation 52A, the speed fluctuation of idler gear 53 is assumed to be idler gear speed fluctuation 53A, and the speed fluctuation of pinion gear 54 is assumed to be pinion gear speed fluctuation 54A.

[0057] In this case, the number of teeth of the drum gear 51 is z51, the number of teeth of the small gear 52a of the stepped gear 52 is z52a, the number of teeth of the large gear 52b of the stepped gear 52 is z52b, the number of teeth of the idler gear 53 is z53, and the number of teeth of the pinion gear 54 is z54. In order to make the number of rotations of the gears an integer, the number of teeth of each gear is set to satisfy the following relationship based on the number of teeth of the drum gear.

[0058] Zdr×θrt=z52a z52b=2×z53=6×z54

[0059] At this time, since the stepped gear 52 (the number of teeth of the small gear 52a is z52a and the number of teeth of the large gear 52b is z52b) is an integral gear, they rotate by the same amount.

[0060] The amount of rotation when Zdr×θrt is defined as f(Zdr×θrt). In this case, the amount of rotation of the small gear 52a with the number of teeth z52a can be written as f(z52a). Since the small gear 52a is a stepped gear 52 that is an integral gear with the large gear 52b, the amount of rotation f(z52b) of the large gear 52b with the number of teeth z52b is the same as the amount of rotation of the small gear 52a, f(z52a)=f(z52b).

[0061] Therefore, if we express everything in an equation, the relationship between the gear rotation amounts is as follows.

[0062] f(Zdr×θrt)=f(z52a)=f(z52b)=2×f(z53)=6×f(z54)

[0063] The actual number of teeth of each gear is as follows:

[0064] The number of teeth z51 (Zdr) of the drum gear 51 is 103 teeth, and the angle θrt from the exposure position 61 to the primary transfer unit 30 is θrt = 51 / 103 × 360 degrees = 178.25 degrees. Therefore, the rotation amount of the drum gear 51 is Zdr × θrt = 51 tooth rotations.

[0065] The number of teeth z52a of the small gear 52a of the stepped gear 52 is 51. Therefore, the amount of rotation of the small gear 52a is Zdr×θrt=1×z52a tooth rotation. The number of teeth z52b of the large gear 52b of the stepped gear 52 is 72. Since the large gear 52b is an integral gear with the small gear 52a, the amount of rotation of the large gear 52b is the same as that of the small gear 52a (z52b tooth rotation=z52a tooth rotation).

[0066] The number of teeth z53 of the idler gear 53 is 36. Therefore, the rotation amount of the idler gear 53 is Zdr×θrt=z52a=z52b=2×z53 tooth rotations.

[0067] The number of teeth z54 of the pinion gear 54 is 12. Therefore, the amount of rotation of the pinion gear 54 is Zdr×θrt=z52a=z52b=2×z53=6×z54 tooth rotations.

[0068] In this way, when the drum gear 51 rotates from the exposure position 60 to the primary transfer portion (transfer position) 30, the preceding gears 52, 53, and 54 rotate an integer number of times.

[0069] The rotational fluctuation at this time is shown in Figure 4 (b). When the drum gear 51 rotates from the exposure position 61 to the primary transfer unit 30, that is, when time t (θrt) has elapsed, the stepped gear 52, idler gear 53, and pinion gear 54 rotate an integer multiple. Therefore, the one-rotation fluctuation of each of these three gears has the same phase, and the speed fluctuation of the motor and gears has the same phase at the exposure position and the transfer position. This makes it possible to suppress color misregistration caused by speed fluctuations due to eccentricity of the motor and gears.

[0070] <Reduction of color misalignment caused by rotational fluctuations due to drum gear accuracy and eccentricity> Next, the rotational fluctuation due to eccentricity of the drum gear 51 that drives the photosensitive drum 4 will be described. The drum gear 51 also has rotational fluctuation due to precision and eccentricity, like the gears described above. A drive unit used in an image forming apparatus having a plurality of photosensitive drums 4 has the same number of drum gears 51 that drive the photosensitive drums 4 as the number of photosensitive drums 4. In such a configuration, in order to reduce color shift due to meshing transmission error of the drum gear 51, it is preferable that the drum gears 51 that drive each photosensitive drum 4 have the same shape. In this embodiment, drum gears 51 molded in the same cavity of a mold are used as the drum gears 51 that drive each photosensitive drum 4.

[0071] By using drum gears 51 having the same shape, the gear precision and eccentricity become the same for the drum gears 51 that drive the respective photosensitive drums 4. Therefore, it is preferable to use drum gears 51 molded in the same cavity of a mold as the drum gears 51 that drive the photosensitive drums 4.

[0072] The phase alignment of each drum gear 51 will be described with reference to Fig. 1 and Fig. 2. Fig. 1 shows a part of a drive unit 50 in which two adjacent photosensitive drums are driven by drum gears 511 and 512, respectively, and driven by the same drive transmission gear 52.

[0073] The distance between adjacent transfer positions 301, 302 is Lst [mm], the speed of the transfer belt 12 is Vi [mm / sec], and the rotation speed of the photosensitive drum 4 is Rd [rps]. Then, in order to drive the drum gear 512 at the same meshing position as the drum gear 511, the phases of the adjacent drum gears need to be matched as follows. That is, the meshing position K2 of the drum gear 512 is used as a reference, and the drum gear 512 needs to be matched to a phase rotated by an angle θst in the opposite direction to the rotation direction (clockwise direction in FIG. 1). At this time, the angle θst through which the drum gear rotates before the image formed at the transfer position 301 reaches the transfer position 302 can be expressed by the following formula.

[0074] θst=Lst / Vi·Rd×360

[0075] The phase alignment of adjacent drum gears (drum gear 511, drum gear 512) will be described in more detail. As described above, the drum gears 511, 512 are gears molded in the same cavity of the same mold. Therefore, in the drum gear 512, the tooth corresponding to the tooth of the drum gear 511 that meshes with the drive transmission gear 52 at meshing position K1 is aligned with a phase rotated by an angle θst in the opposite direction to the rotation direction (clockwise direction in FIG. 1) with respect to the meshing position K2. The position of the tooth of the drum gear 512 that corresponds to the tooth of the drum gear 511 that meshes with the drive transmission gear 52 at this meshing position K1 is indicated by a dashed circle in FIG. 1.

[0076] Fig. 2(a) shows the state where the speed of the drum gear 511 fluctuates from the meshing position K1 in one rotation period of the drum gear, and Fig. 2(b) shows the state where the speed of the drum gear 512 fluctuates from the meshing position K2 in one rotation period of the drum gear.

[0077] At this time, the angle difference between meshing position K1 and meshing position K2 can be expressed as angle θst. By making drum gear 511 and drum gear 512 the same type of gear, it is possible to align the speed fluctuation in one rotation period with the same phase at the meshing position with drive transmission gear 52. Therefore, it is possible to reduce color shift caused by rotation fluctuation due to gear precision and eccentricity.

[0078] <Reduction of color misalignment caused by speed fluctuation of drum coupling> Furthermore, color misregistration may occur due to deviation of the primary transfer positions 301, 302 between the photosensitive drum 4 and the transfer belt 12 caused by speed fluctuations of the drum couplings 71, 72 that engage with the photosensitive drum 4 to drive the photosensitive drum 4. In other words, the color misregistration caused by the coupling needs to be adjusted in phase based on the transfer positions 301, 302.

[0079] 1, the phase alignment of the drum couplings 71 and 72 will be described. First, the angular relationship required for phase alignment will be described based on the relationship between the rotation centers of the primary transfer positions 301 and 302 and the gears 511, 512, and 52. Next, the phase alignment of the drum couplings 71 and 72 will be described.

[0080] <Angle relationship> First, with reference to FIG. 1, an angular relationship required for phase alignment will be described based on the relationship between the primary transfer positions 301, 302, the rotation center 52c of the drive transmission gear 52 with which the drum gears 511, 512 mesh, and the rotation centers of the drum gears 511, 512.

[0081] A position where the drum gear 511 meshes with the drive transmission gear 52 is defined as a first meshing position K1. A position where the drum gear 512 meshes with the drive transmission gear 52 is defined as a second meshing position K2.

[0082] An angle θ1 is defined as an angle formed in the rotation direction when a rotation center 511c of the drum gear 511 is connected to a rotation center 52c of the drive transmission gear 52 and a rotation center 511c of the drum gear 511 is connected to the primary transfer position 301. In Fig. 1, the angle θ1 is a first angle from the first meshing position K1 to the first transfer position 301 in the rotation direction (first direction) of the drum gear 511.

[0083] Similarly, the angle formed in the rotation direction when the rotation center 512c of the drum gear 512 is connected to the rotation center 52c of the drive transmission gear 52 and the rotation center 512c of the drum gear 512 is connected to the primary transfer position 302 is defined as θ2. In addition, in Fig. 1, the angle θ2 is a second angle from the second meshing position K2 to the second transfer position 302 in the rotation direction (second direction) of the drum gear 512. The angles θ1 and θ2 are configured such that the angles from the meshing positions to the transfer positions in the rotation direction of each drum gear are different.

[0084] The angle formed by the line connecting the rotation center 52c of the drive transmission gear 52 and the rotation center 511c of the drum gear 511 and the line connecting the rotation center 52c of the drive transmission gear 52 and the rotation center 512c of the drum gear 512 is defined as θ3.

[0085] Moreover, the angle formed by the line connecting the rotation center 511c of the drum gear 511 and the primary transfer position 301 and the line connecting the primary transfer position 301 and the primary transfer position 302 is defined as θ4. The angle formed by the line connecting the rotation center 512c of the drum gear 512 and the primary transfer position 302 and the line connecting the primary transfer position 301 and the primary transfer position 302 is defined as θ5.

[0086] If the primary transfer positions 301 and 302 are the same positions on each photosensitive drum, the following relationship holds:

[0087] θ4+θ5=180 (Formula 1)

[0088] From the sum of the interior angles of a pentagon formed by the primary transfer positions 301 and 302, the rotation centers 511c and 512c of the two drum gears 511 and 512, and the rotation center 52c of the drive transmission gear 52, the following relationship holds.

[0089] 360-θ1+θ2+θ3+θ4+θ5=540 (Formula 2)

[0090] From the above formula 1 and formula 2, the relationship of the above angles is as follows:

[0091] θ1-θ2=θ3 (Equation 3)

[0092] <Drum coupling phase alignment> Next, the phase alignment of the drum coupling will be described with reference to Figs.

[0093] As described above, in order to reduce color misregistration caused by speed fluctuations of the drum couplings 71 and 72 that engage with the photosensitive drum 4 to drive the photosensitive drum 4, it is necessary to align the phases of the drum couplings based on the transfer positions 301 and 302. The configuration of this embodiment will be described below using a comparative example.

[0094] As a comparative example, the speed fluctuation of the drum coupling in a configuration in which the drum gear and the drum coupling are driven integrally in only one phase is shown by a dashed line in Fig. 2(d). In the comparative example, the position of the drum coupling 72 relative to the drum gear 512 in the rotation 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 rotation direction of the drum gear 511.

[0095] In the comparative example, as shown by the dashed line in Fig. 2(d), the phase of the drum coupling 72 is the same as that of the drum gear 512. That is, the phase relationship is the same as the phase relationship between the drum gear 511 at the adjacent upstream position in the moving direction of the transfer belt 12 and the drum coupling 71 engaged therewith. Therefore, the phase of the speed fluctuation of the drum coupling 71 at the primary transfer position 301 of the drum gear 511 and the phase of the speed fluctuation of the drum coupling 72 at the primary transfer position 302 of the drum gear 512 differ from each other, causing color misregistration.

[0096] Therefore, in this embodiment, the phases of the drum couplings 71 and 72 are matched in order to make the speed fluctuations of the drum couplings 71 and 72 in the same phase at the primary transfer positions 301 and 302. That is, the phase of the mounting position of the drum coupling relative to one of the adjacent drum gears is shifted by a predetermined angle difference with respect to the mounting position of the drum coupling relative to the other drum gear.

[0097] First, in order to reduce color shift due to rotational fluctuation of the drum gear, as described above, the drum gear 512 is phase-aligned with respect to the drum gear 511 with a phase delayed by a rotation angle θst, and the phase of the drum gear 512 is aligned with respect to the drum gear 511. That is, consider a case where the drum gear 511 has a first tooth, and the drum gear 512 has a second tooth corresponding to the first tooth of the drum gear 511. In this case, when the first tooth of the drum gear 511 is at the meshing position K1, the second tooth of the drum gear 512 is at a position shifted by θst from the meshing position K2 in the opposite direction to the rotational direction (second direction) of the drum gear 512. For example, when the first tooth of the drum gear 511 is at the position K1 (position of the black circle) in FIG. 1, the second tooth of the drum gear 512 is at a position shifted by θst from the meshing position K2 in the opposite second direction (position of the broken circle).

[0098] The primary transfer position 301 of the drum gear 511 is at a position +θ1 from the meshing position K1. Similarly, the primary transfer position 302 of the drum gear 512 is at a position +θ2 from the meshing position K2. In Figures 2(a), (b), (c), and (d), the meshing position K1 of the drum gear 511, the meshing position K2 of the drum gear 512, the primary transfer position 301 of the drum gear 511, and the primary transfer position 302 of the drum gear 512 are indicated by vertical dashed double-dashed lines.

[0099] Next, in order to align the phases of the drum coupling 71 and the drum coupling 72 at the primary transfer position, the mounting position of the drum coupling 72 relative to the drum gear 512 is shifted by a predetermined angle difference θ1-θ2 based on the mounting position of the drum coupling 71 relative to the drum gear 511.

[0100] With respect to the first portion of the drum coupling 71, the portion of the drum coupling 72 that corresponds to the first portion is defined as the second portion. In Fig. 1, when the first tooth of the drum gear 511 is at the meshing position K1, the first portion of the drum coupling 71 is at the position (301) indicated by a black triangle.

[0101] In the comparative example, the attachment position of the drum coupling 71 relative to the drum gear 511 is the same as the attachment position of the drum coupling 72 relative to the drum gear 512. 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 meshing position K2, the second portion is located at a position passing the primary transfer position 302 by θ3. As a result, for example, as shown in FIG. 2(c), when the speed of the drum coupling 71 is decelerating at the primary transfer position 301, the speed of the drum coupling 72 is increasing at the primary transfer position 302, as shown by the dashed line in FIG. 2(d).

[0102] On the other hand, the mounting position of the drum coupling 72 relative to the drum gear 512 is shifted by θ3 in the opposite direction to the rotation direction of the drum gear 512 with respect to the mounting position of the drum coupling 71 relative to the drum gear 511. In this case, when the second tooth corresponding to the first tooth is located at the meshing position K2, the second part can be located at the primary transfer position 302. As a result, for example, as shown in FIG. 2(c), when the speed of the drum coupling 71 is decelerating at the primary transfer position 301, the speed of the drum coupling 72 is also decelerating at the primary transfer position 302 as shown by the solid line in FIG. 2(d). This can reduce color misregistration.

[0103] In this embodiment, the drum gears are the same drum gears molded in the same mold as described above, and have two mounting positions (mounting parts) with different phases for mounting the drum coupling. Specifically, as shown in Fig. 5(a), each drum gear has a first position (first mounting position) A and a second position (second mounting position) B, which is shifted from the first position A by the predetermined angle difference θ3 (=θ1-θ2), as mounting positions for the coupling.

[0104] At the first position A, the drum coupling 71 is attached to the drum gear 511 so that the phase serving as the reference for the speed fluctuation of the drum coupling 71 (position K1 shown in FIG. 2(c)) coincides with the meshing position K1 of the drum gear 511 with the drive transmission gear 52.

[0105] The second position B is a position 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. At this time, the phase serving as a reference for the speed fluctuation of the drum coupling 72 is shifted in the opposite direction to the rotation direction of the drum gear 512 by the difference between the angle θ1 and the angle θ2 compared to the case where the drum coupling 72 is attached to the first position A of the drum gear 512. That is, the phase of the drum coupling 72 attached to the second position B of the drum gear 512 shown in FIG. 2(b) is shifted from the broken line position to the solid line position shown in FIG. 2(d). The drum couplings 71 and 72 are connected to the drum gears 511 and 512 by selectively attaching them to the first position A or the second position B of the drum gears 511 and 512.

[0106] The drum gears 511 and 512 are gears of the same shape. As shown in Fig. 5(a), each of the drum gears 511 and 512 has two mounting positions (a first position A and a second position B). Each of the drum gears 511 and 512 has a first position (first mounting portion) A and a second position (second mounting portion) B. In each of the drum gears 511 and 512, the second position B is shifted from the first position A by the difference between the angle θ1 and the angle θ2 in the direction opposite to the rotational direction of the drum gears 511 and 512.

[0107] A drum coupling 71 is attached to a first position (first attachment portion) A of the drum gear 511. A drum coupling 72 is attached to a second position (second attachment portion) B of the drum gear 512.

[0108] As a result, as shown in Fig. 2(a) and Fig. 2(c), the drum coupling 71 is engaged with the drum gear 511 so that the phase serving as a reference for the speed fluctuation of the drum coupling 71 coincides with the meshing position K1 of the drum gear 511. At this time, the tooth of the drum gear 511 located at the meshing position K1 is referred to as the 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 the first reference tooth in the rotation direction of the drum gear 512) is referred to as the second reference tooth. As shown in Fig. 2(b) and Fig. 2(d), when the second reference tooth of the drum gear 512 is located at the second meshing position K2, the phase serving as a reference for the speed fluctuation of the drum coupling 72 is shifted from the meshing position K2 in the opposite direction to the rotation direction by the difference between the angle θ1 and the angle θ2.

[0109] In other words, when the first position A of the drum gear 511 is the same as the position of the first reference tooth, the second position A of the drum gear 512 is a position shifted from the second reference tooth in the opposite direction to the rotational direction (second direction) by the difference between the angles θ1 and θ2.

[0110] Since the drum couplings 71 and 72 are formed in the same mold, the speed fluctuations in one rotation period from the reference phase are the same. The reference phase of the drum coupling 72 (position 301 shown in FIG. 2(d)) corresponds to the reference phase of the drum coupling 71 (position K1 shown in FIG. 2(c)) that is aligned with the meshing position K1 of the drum gear 511. As described above, the teeth of the drum gear 512 that correspond to the teeth of the drum gear 511 that mesh with the drive transmission gear 52 at the meshing position K1 are indicated by dashed circles in FIG. 1.

[0111] In this way, the drum gears 511 and 512 according to this embodiment have a plurality of phases for engaging the drum couplings 71 and 72. That is, each drum gear 511 and 512 has a plurality of mounting positions for mounting the drum couplings 71 and 72 with a predetermined angle difference therebetween. As a result, the drum gears 511 and 512 can assemble the drum couplings 71 and 72 with different phases, and 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 can be matched. Therefore, even if the rotation phase control for matching the rotation phases is not performed, the speed fluctuations of the drum couplings at the primary transfer positions 301 and 302 can be made to have the same phase. Therefore, the color shift caused by the speed fluctuations of the drum couplings 71 and 72 can be reduced, and the color shift of each color caused by the rotation fluctuations between the multiple photosensitive drums can be suppressed.

[0112] <Drum gear mounting position and coupling shape> Next, the shapes of the drum couplings 71 and 72 attached to the drum gear 51 will be described with reference to Fig. 5. Since the drum gears 511 and 512 have the same shape, they will be described as the drum gear 51 here. Since the drum couplings 71 and 72 have the same shape, they will be described as the drum coupling 70 here. Here, the drum couplings 71 and 72, and the drum gears 511 and 512 do not have to have the same shape in parts not related to the function of driving the photosensitive drum 4, as long as they have the same shape in parts related to the function of driving the photosensitive drum 4.

[0113] Furthermore, parts related to the function of driving the photosensitive drum 4 can be called the same shape even if their shapes differ slightly due to dimensional tolerances. For example, some parts may allow dimensional tolerances of -0.5 mm to +0.5 mm for position and dimensions, and -3° to +3° for angles.

[0114] It is preferable that the drum gears 511 and 512 are nearly identical, and are preferably molded in the same cavity of a mold, for example. It is also preferable that the drum couplings 71 and 72 are nearly identical, and are preferably molded in the same cavity of a mold, for example. In this embodiment, the drum gears 511, 512, drum couplings 71 and 72 are made by molding resin.

[0115] A case will be described in which the drum gear 51 (511, 512) has a different meshing angle with the drive transmission gear 52 and has two drive transmission points. That is, a configuration will be described in which two adjacent drum gears 51 (511, 512) mesh with the same drive transmission gear 52. That is, a configuration will be described in which the drive unit has, as at least one drive transmission gear, one drive transmission gear 52 that meshes with both the drum gear 511 and the drum gear 512.

[0116] The drum coupling 70 has an engagement portion 70g (70g1, 70g2) that engages with the photosensitive drum 4. The photosensitive drum 4 is detachably engaged with the engagement portion 70g. The position of the engagement portion 70g in the rotation direction of the drum coupling 70 fluctuates, which may cause the speed of the photosensitive drum 4 to fluctuate. The drum gear 51 has a positioning portion 51a that positions the positioning portion 70a of the drum coupling 70. The drum gear 51 is provided with a drive transmission surface 51b that drives the drum coupling 70 and a drive transmission surface 51c in a phase opposite to the drive transmission surface 51b at one mounting position. The drum gear 51 is provided with another drive transmission surface 51d that drives the drum coupling 70 and a drive transmission surface 51e in a phase opposite to the drive transmission surface 51d at the other mounting position having a phase difference of θ1-θ2.

[0117] By providing the drum gear 51 with two sets of mounting positions with different phases, the drum coupling 70 (71, 72) can be assembled to the drum gear 51 by changing the phase by the angle difference from the meshing position to the primary transfer position. The two mounting positions (first position A and second position B) of each drum gear 51 are such that the second position B is shifted from the first position A by a predetermined angle difference (θ1-θ2) in the opposite direction to the rotation direction of the drum gear. Therefore, if the rotation direction of the drum gear 51 is the counterclockwise direction in FIG. 5(a), the mounting position downstream in the rotation direction is the first position A, and the mounting position upstream of that is the second position B.

[0118] <180° phase misalignment prevention design> Next, a shape for preventing an incorrect assembly in the phase assembly of the drum coupling to the drum gear will be described with reference to FIG.

[0119] Each of the drum couplings 70 (71, 72) is provided with a first protruding portion 70d having a first width in the rotational direction and a second protruding portion 70e having a second width in the rotational direction that is narrower than the first width. The first protruding portion 70d is provided so as to protrude outward from the positioning portion (outer peripheral surface) 70a. The second protruding portion 70e is provided at a position facing the first protruding portion 70d across the rotation center of the drum coupling, and is provided so as to protrude outward from the positioning portion (outer peripheral surface) 70a.

[0120] In the drum coupling 70, the first protrusion 70d has a first driven transmission surface 70b that abuts against the drive transmission surface 51b or 51d of the drum gear 51 to receive the transmission of driving force. The second protrusion 70e has a second driven transmission surface 70c that abuts against the drive transmission surface 51c or 51e of the drum gear 51 to receive the transmission of driving force, at a position facing the first driven transmission surface 70b across the rotation center of the drum coupling.

[0121] Each drum gear 51 (drum gear 511, drum gear 512) has a first position A and a second position B that is different from the first position A by a predetermined angle difference in the rotation direction. One mounting position (first position A) of the drum gear 51 has a mounting groove including a first groove portion 51b1 and a second groove portion 51c1. The other mounting position (second position B) of the drum gear 51 has a mounting groove including a first groove portion 51d1 and a second groove portion 51e1. The mounting groove including the first groove portion 51b1 and the second groove portion 51c1 and the mounting groove including the first groove portion 51d1 and the second groove portion 51e1 have the same shape. The first groove portions 51b1 and 51d1 at each mounting position have a width that allows the first protrusion 70d to engage in the rotation direction. In addition, the second groove portions 51c1, 51e1 of each mounting position are provided at positions opposite the first groove portions 51b1, 51d1, respectively, via the rotation center of the drum gear, and are narrower in width than the first groove portions 51b1, 51d1 in the rotational direction, and have a width that allows the second protrusion portion 70e to engage.

[0122] In the drum gear 51, the first grooves 51b1, 51d1 have first drive transmission surfaces 51b, 51d that contact the first driven transmission surface 70b to transmit the driving force. The second grooves 51c1, 51e1 have second drive transmission surfaces 51c, 51e that contact the second driven transmission surface 70c to transmit the driving force, at positions facing the first drive transmission surfaces 51b, 51d, respectively, through the rotation center of the drum gear.

[0123] As described above, each drum coupling 70 has a first protruding portion 70d having a first width in the rotation direction at each mounting position, and a second protruding portion 70e having a second width narrower than the first width in the rotation direction. Each drum gear 51 has a first groove portion 51b1, 51d1 having a width with which the first protruding portion 70d can be engaged in the rotation direction, and a second groove portion 51c1, 51e1 having a width with which the second protruding portion 70e can be engaged in the rotation direction at each mounting position. The second groove portions 51c1, 51e1 of the drum gear 51 have a narrower width in the rotation direction than the first groove portions 51b1, 51d1 of the drum gear 51.

[0124] In other words, only the second protrusion 70e, which has a narrower width in the rotational direction than the first protrusion 70d of the drum coupling 70, can be assembled into the second groove portions 51c1, 51e1 of the drum gear 51.

[0125] Therefore, when attempting to rotate the drum coupling 70 by 180 degrees and assemble it to the drum gear 51, assembly is not possible because the width of the second groove portions 51c1, 51e1 of the drum gear 51 interferes with the width of the first protrusion portion 70d of the drum coupling 70. This makes it possible to prevent the drum coupling 70 from being mounted in an incorrect phase by 180 degrees relative to each mounting position of the drum gear 51.

[0126] <Prevention of θ1-θ2 phase mismatch errors> Next, a description will be given of a configuration for preventing misassembly due to differences in the phase angle of the coupling.

[0127] Each drum gear 51 has a first phase hole 51f provided at a position of a first radius R1 from the rotation center of the drum gear 51 and a phase hole 51g provided at a position of a second radius R2 different from the first radius R1 from the rotation center of the drum gear 51 for phase determination. The first phase hole 51f and the second phase hole 51g of each drum gear 51 are holes for attaching a pin.

[0128] In addition, each drum coupling 70 has a groove hole 71f provided at a position of a third radius R3 from the rotation center of the drum coupling 71 and a positioning portion (outer peripheral surface) 70a provided at a position of a fourth radius R4 from the rotation center of the drum coupling 71. The distance of the radius R3 where the groove hole 71f is provided is shorter than the first radius R1 and the second radius R2. The distance of the radius R4 where the positioning portion 70a is provided is longer than the first radius R1 and the second radius R2.

[0129] At this time, the relationship of the distances from the rotation centers of the first phase hole 51f and the second phase hole 51g of the drum gear 51, the groove hole 71f and the positioning portion 70a of the drum coupling 70 is a relationship of radius R3 < R1 < R2 < R4.

[0130] When assembling the drum coupling 70 in the phase of driving the drum coupling 70 on the drive transmission surfaces 51b and 51c at one mounting position, it is assembled with a phase determining pin (not shown) attached to the phase hole 51g (see Fig. 5(b)). At this time, when trying to attach the drum coupling 70 to the drive transmission surfaces 51d and 51e at the other mounting position where the phase differs by the angle difference θ1 - θ2, the phase determining pin attached to the phase hole 51g interferes with the drum coupling 70 and cannot be assembled. Therefore, the phase of the drum coupling 70 cannot be misassembled with respect to the drum gear 51.

[0131] Next, when assembling the drum coupling 70 in the phase for driving the drive transmission surfaces 51d, 51e at the other mounting position, the phase determination pin (not shown) is attached to the phase hole 51f (see FIG. 5(c)). At this time, if the drum coupling 70 is to be attached to the drive transmission surfaces 51b, 51c at one mounting position with a phase difference of only the angle difference θ1-θ2, the phase determination pin attached to the phase hole 51f interferes with the drum coupling 70 and the drum coupling 70 cannot be assembled. Therefore, the drum coupling 70 is not assembled in the wrong phase with respect to the drum gear 51.

[0132] As shown in Fig. 5(b), when the drum coupling 71 is attached to the first position A of the drum gear 511, the drum coupling 71 does not overlap with the phase hole (second hole) 51g, but overlaps with the phase hole (first hole) 51f. As shown in Fig. 5(c), when the drum coupling 72 is attached to the second position B of the drum gear 512, the drum coupling 72 does not overlap with the phase hole 51f, but overlaps with the phase hole 51g.

[0133] As described above, according to this embodiment, it is possible to reduce color shifts of each color due to the rotational fluctuation between multiple photosensitive drums, even without performing rotation phase control to match the rotation phases of the drum gears as in the previous embodiment. Furthermore, when couplings are attached to different mounting positions of the drum gears, the drum couplings can be attached to each mounting position of the drum gears without making a mistake.

[0134] Example 2 In the first embodiment, the at least one drive transmission gear that rotates by receiving a drive force from a drive source is meshed with the drum gears that drive adjacent photosensitive drums, and the drive transmission gear is the same gear (one gear). In contrast, in the second embodiment, the at least one drive transmission gear is meshed with each of the drum gears that drive adjacent photosensitive drums. Note that other configurations are the same as those in the above-mentioned embodiments, and therefore description thereof will be omitted here.

[0135] A case where each drum gear meshes with a different drive transmission gear will be described with reference to Fig. 6. Fig. 6 is a schematic diagram showing a part of a drive unit according to a second embodiment. In the drive unit according to this embodiment, a first drum gear 513 meshes with a first drive transmission gear 523, and is driven to rotate by receiving a drive force from the first drive transmission gear 523. A second drum gear 514 adjacent to the first drum gear 513 meshes with a second drive transmission gear 524 different from the first drive transmission gear 523, and is driven to rotate by receiving a drive force from the second drive transmission gear 524.

[0136] The first drive transmission gear 523 and the second drive transmission gear 524 mesh with the same idler gear 531, and are rotated by receiving a driving force from the idler gear 531. The idler gear 531 meshes with a pinion gear 541 attached to a motor (not shown) serving as a drive source, and are rotated by receiving a driving force from the pinion gear 541.

[0137] The angle formed in the rotation direction between a line connecting the rotation center 513c of the first drum gear 513 and the rotation center 523c of the first drive transmission gear 523 and a line connecting the rotation center 513c of the first drum gear 513 and the primary transfer position 303 is defined as θ6.

[0138] Similarly, the angle formed in the rotation direction between a line connecting the rotation center 514c of the second drum gear 514 and the rotation center 524c of the second drive transmission gear 524 and a line connecting the rotation center 514c of the second drum gear 514 and the primary transfer position 304 is defined as θ7.

[0139] The second drum gear 514 is disposed so that, when the first tooth of the first drum gear 513 is at meshing position K3, the second tooth corresponding to the first tooth is positioned at a phase shifted by angle θst in the opposite direction to the rotation direction from meshing position K4 with the second drive transmission gear 524. When the position where the drum coupling 73 is attached to the first drum gear 513 is defined as the first position, the drum coupling 74 is attached at a position shifted in phase by an angle difference θ6-θ7 from the position corresponding to the first position.

[0140] According to this embodiment, even if the drive transmission gear in the preceding stage with which the drum gear meshes is a different gear, it is possible to align the phase of the coupling at the transfer position, as in the above-described first embodiment, thereby suppressing color misalignment.

[0141] In this embodiment, a configuration that does not have the <180° phase difference misassembly prevention shape> or <θ1-θ2 phase difference misassembly prevention> of the above-mentioned embodiment 1 is illustrated, but the present invention is not limited to this. In this embodiment, too, a configuration that has the <180° phase difference misassembly prevention shape> or <θ1-θ2 phase difference misassembly prevention> may be used as in the first embodiment.

[0142] Example 3 In the first and second embodiments, the drum gear and the drum coupling are separate bodies, and the drum coupling is connected to the drum gear, but the present invention is not limited to this. For example, the drum gear and the drum coupling may be integrally molded, and the gears may be molded with a phase shift in a mold.

[0143] For example, a molded gear formed by integrally molding a drum coupling and drum gear is formed by two parts: a concave cavity and a convex core in the axial direction. In the case of a helical gear, the teeth of the molded gear are formed by rotating and extruding the mold that forms the teeth. At this time, the phase of the shape of the teeth is adjusted using a return mechanism so that the shape of the coupling attachment part and phase setting hole and the phase relationship of the gear teeth are the same for each molding.

[0144] Furthermore, the mold having the coupling shape and the mold having the gear shape can be attached by rotating relative to each other, and a positioning hole is provided to determine the phase of the rotation direction between the molds. Also, the position of the pin for the phasing hole that determines the phase of the molded gear can be changed in two places. When molding in a first phase, a pin is provided in the phasing hole whose distance from the center of rotation is a first radius. When molding in a second phase that is different from the first phase by a predetermined phase difference (angle difference), a pin is provided in the phasing hole whose distance from the center of rotation is a second radius different from the first radius. This allows the gear to have a phasing hole.

[0145] This allows a gear with a coupling that has two mounting phases to be molded using a die with one tooth type. In addition, the phase determination hole makes it possible to distinguish the difference between the two types of phases of the coupling of the molded gear. This allows the molded gear to be mounted with its phase changed depending on the meshing position with the drive transmission gear in the previous stage, as explained in Example 1, even if the drum gear and drum coupling are integrally molded products.

[0146] In the above-mentioned embodiment, a printer is exemplified as an image forming apparatus, but the present invention is not limited thereto. For example, other image forming apparatuses such as a copier or a facsimile machine, or other image forming apparatuses such as a multifunction machine combining the functions of these, may be used. In addition, an image forming apparatus that uses an intermediate transfer body, transfers toner images of each color to the intermediate transfer body in a sequentially overlapping manner, and transfers the toner images carried on the intermediate transfer body to a recording medium all at once is exemplified, but the present invention is not limited thereto. An image forming apparatus that uses a recording medium carrier, transfers toner images of each color to the recording medium carried on the recording medium carrier in a sequentially overlapping manner. By applying the present invention to these image forming apparatuses, similar effects can be obtained. The present invention can also be applied to a manufacturing method for manufacturing the image forming apparatus shown in the embodiment. [Explanation of symbols]

[0147] 1. Image forming device 2. Device body 4. Photosensitive drum 11. Intermediate transfer belt unit 12 Transfer belt 17. Secondary transfer roller 30...Primary transfer section 31. Secondary transfer section 50. Drive unit 51 (511, 512, 513, 514) ... Drum gear 52...Drive transmission gear (step gear) 53... Idle gear 54...Pinion gear 61. Exposure position 70(71,72)...Drum coupling 301, 302, 303, 304...Primary transcription position

Claims

1. A transfer body configured to be movable in a moving direction; a first photosensitive drum contacted with the transfer body at a first transfer position; a second photosensitive drum arranged adjacent to the first photosensitive drum in the moving direction and contacted with the transfer body at a second transfer position located downstream of the first transfer position in the moving direction; a drive unit that drives the first photosensitive drum and the second photosensitive drum; An image forming apparatus comprising: The drive unit includes: (i) a drive source; and (ii) at least one drive transmission gear that rotates by receiving a drive force from the drive source; (iii) a first drum gear that meshes with the at least one drive transmission gear, receives a drive force from the at least one drive transmission gear, rotates in a first direction, and drives the first photosensitive drum; (iv) a first coupling member that is provided at a first position of the first drum gear in the first direction, rotates together with the first drum gear, and engages with the first photosensitive drum to rotate the first photosensitive drum; and (v) a second drum gear that meshes with the at least one drive transmission gear, receives a drive force from the at least one drive transmission gear, rotates in a second direction, and drives the second photosensitive drum; (vi) a second coupling member that is provided at a second position of the second drum gear in the second direction, rotates together with the second drum gear, and engages with the second photosensitive drum to rotate the second photosensitive drum; and having When a position where the first drum gear meshes with the at least one drive transmission gear is defined as a first meshing position, and a position where the second drum gear meshes with the at least one drive transmission gear is defined as a second meshing position, a first angle from the first meshing position to the first transfer position in the first direction is different from a second angle from the second meshing position to the second transfer position in the second direction; an image forming apparatus characterized in that, in the second drum gear, the second position is shifted from a position corresponding to the first position by an amount equal to the difference between the first angle and the second angle in a direction opposite to the second direction.

2. 2. The image forming apparatus according to claim 1, wherein the first drum gear and the second drum gear have the same shape.

3. the first drum gear has first teeth that mesh with the at least one drive transmission gear; the second drum gear has second teeth meshing with the at least one drive transmission gear, the second teeth corresponding to the first teeth; 3. An image forming apparatus as described in claim 1 or 2, characterized in that when the angle by which the second drum gear rotates until the toner image transferred to the transfer body at the first transfer position reaches the second transfer position is defined as angle θst, the second tooth is positioned at a position shifted from the second meshing position by the angle θst in the opposite direction to the second direction when the first tooth is at the first meshing position.

4. Each of the first drum gear and the second drum gear has a first mounting portion and a second mounting portion, The second mounting portion of the first drum gear is positioned at a position shifted by the difference in the opposite direction to the first direction with respect to the first mounting portion of the first drum gear, and the second mounting portion of the second drum gear is positioned at a position shifted by the difference in the opposite direction to the second direction with respect to the first mounting portion of the second drum gear, 4. The image forming apparatus according to claim 1, wherein the first coupling member is attached to the first mounting portion of the first drum gear, and the second coupling member is attached to the second mounting portion of the second drum gear.

5. 5. The image forming apparatus according to claim 1, wherein the at least one drive transmission gear includes a drive transmission gear that meshes with both the first drum gear and the second drum gear.

6. 5. The image forming apparatus according to claim 1, wherein the at least one drive transmission gear includes a first drive transmission gear that meshes with the first drum gear, and a second drive transmission gear that meshes with the second drum gear.

7. each of the first coupling member and the second coupling member includes a first protruding portion having a first width in a rotational direction of the first coupling member and the second coupling member, and a second protruding portion having a second width in the rotational direction that is narrower than the first width; Each of the first drum gear and the second drum gear has an attachment groove including a first groove portion having a width with which the first protrusion can be engaged, and a second groove portion narrower than the first groove portion and having a width with which the second protrusion can be engaged, 6. The image forming apparatus according to claim 1, wherein each of the first drum gear and the second drum gear has an attachment groove disposed at the first position and an attachment groove disposed at the second position.

8. Each of the first drum gear and the second drum gear has a first hole and a second hole, the first coupling member overlaps with the first hole of the first drum gear and does not overlap with the second hole of the first drum gear; 8. The image forming apparatus according to claim 1, wherein the second coupling member overlaps with the second hole of the second drum gear and does not overlap with the first hole of the second drum gear.

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