Image forming device

Abstract

To suppress occurrence of banding in an image forming apparatus that can perform color printing and monochrome printing.SOLUTION: An image forming apparatus 1 comprises: first to third developing rollers that carry a color developer; a fourth developing roller that carries a black developer; first to fourth photoconductor drums that correspond to the first to fourth developing rollers; a process motor that drives the first to fourth developing rollers and the first to fourth photoconductor drums; a first output gear 781 that is arranged coaxially with the process motor and outputs driving force from the process motor; a first gear train 791; and a second gear train 792. The first gear train 791 includes a first intermediate gear 79A meshing with the first output gear 781, and transmits the driving force from the process motor to the first to fourth developing rollers. The second gear train 792 includes a second intermediate gear meshing with the first output gear, and transmits the driving force from the process motor to the first to fourth photoconductor drums.SELECTED DRAWING: Figure 4

Description

[Technical field]

[0001] The present invention relates to an image forming apparatus. [Background technology]

[0002] As an electrophotographic image forming apparatus, for example, a tandem printer having a plurality of developing units corresponding to the colors of yellow, magenta, cyan, and black is known. In order to support color printing and monochrome printing, a configuration has been proposed for such a tandem printer in which the yellow, magenta, and cyan developing units and the black developing unit are driven by separate motors (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2015-87585 A Summary of the Invention [Problem to be solved by the invention]

[0004] In the configuration of Patent Document 1, the motor that drives the black developing unit also drives the fixing unit, etc. Specifically, a gear train is provided that branches off from the gear train between the motor and the fixing unit and transmits driving force to the black developing unit. However, because a high load is applied to drive the fixing unit, rotational irregularities that occur in the fixing unit may be transmitted to the black developing unit, resulting in banding.

[0005] The present invention aims to suppress the occurrence of banding in an image forming device capable of both color and monochrome printing by driving all developing rollers and photosensitive drums with the same motor and by branching the gear train that drives the developing rollers and the gear train that drives the photosensitive drums directly from the motor. [Means for solving the problem]

[0006] The image forming apparatus of the present invention includes a first developing roller, a second developing roller, and a third developing roller each carrying a color developer, a fourth developing roller each carrying a black developer, a first photosensitive drum corresponding to the first developing roller, a second photosensitive drum corresponding to the second developing roller, a third photosensitive drum corresponding to the third developing roller, and a fourth photosensitive drum corresponding to the fourth developing roller. The image forming apparatus also includes a motor for driving the first developing roller to the fourth developing roller and the first photosensitive drum to the fourth photosensitive drum, and a first output gear arranged coaxially with the motor and outputting a driving force from the motor. The image forming apparatus also includes a first intermediate gear meshed with the first output gear, and a first gear train for transmitting the driving force of the motor from the first developing roller to the fourth developing roller. The image forming apparatus also includes a second intermediate gear meshed with the first output gear, and a second gear train for transmitting the driving force of the motor from the first photosensitive drum to the fourth photosensitive drum.

[0007] According to the above configuration, all of the developing rollers and photosensitive drums are driven by the same motor, and the first gear train that drives each developing roller and the second gear train that drives each photosensitive drum branch off directly from the first output gear of the motor, so that rotational irregularities of each gear train are less likely to be transmitted to the other gear train, thereby suppressing the occurrence of banding.

[0008] In the above image forming apparatus, the first gear train may include a third gear train that transmits a driving force to the first, second and third developing rollers, and a fourth gear train that transmits a driving force to the fourth developing roller. The third gear train is disposed upstream of the first, second and third developing rollers, and has a pendulum gear that can swing between a first position that transmits a driving force and a second position that cuts off the driving force. The fourth gear train is disposed upstream of the fourth developing roller, and has an electromagnetic clutch that can be displaced between a first state that transmits a driving force and a second state that cuts off the driving force.

[0009] According to the above-mentioned configuration, the use of the pendulum gear allows for cost reduction, and the use of the electromagnetic clutch allows for space saving.

[0010] In the image forming apparatus described above, the third gear train may include the first intermediate gear, and the fourth gear train may branch off from the third gear train upstream of the pendulum gear.

[0011] In the above image forming apparatus, the third gear train may include a third intermediate gear that meshes with the first intermediate gear, and the fourth gear train may branch off from the third gear train at the third intermediate gear.

[0012] According to the above configuration, by branching off at a gear as close as possible to the first output gear of the motor, rotational irregularities in either the third or fourth gear train are less likely to be transmitted to the other gear train, and each gear is driven stably. Also, the fourth gear train can be made shorter than in a configuration where it branches off directly from the first output gear.

[0013] In the image forming apparatus described above, some of the gears in the third gear train and some of the gears in the fourth gear train may be disposed coaxially.

[0014] According to the above configuration, the first gear train can be made compact.

[0015] In the above-mentioned image forming device, the third developing roller may be arranged adjacent to the fourth developing roller, and the part of the gears in the third gear train may be a second output gear arranged coaxially with the third developing roller and outputting a driving force to the third developing roller.

[0016] In the above-mentioned image forming apparatus, the fourth gear train may have a clutch input gear that inputs a driving force to the electromagnetic clutch, and a clutch output gear that has a different number of teeth than the clutch input gear and outputs the driving force from the electromagnetic clutch.

[0017] According to the above-mentioned configuration, the rotation of the clutch output gear is stabilized because the rotation speed is increased or decreased before and after the electromagnetic clutch, thereby improving image quality.

[0018] In the above-mentioned image forming apparatus, the fourth gear train may have a third output gear arranged coaxially with the fourth developing roller and outputting a driving force to the fourth developing roller, and the clutch output gear may be configured to mesh with the third output gear.

[0019] The above-mentioned image forming apparatus may further comprise a sheet metal member having a positioning hole and supporting the fourth gear train, a first cover having a first engagement portion and fixed to one side of the sheet metal member and supporting the electromagnetic clutch, and a second cover having a second engagement portion that engages with the positioning hole and the first engagement portion, fixed to the other side of the sheet metal member and supporting one end of the clutch output gear.

[0020] According to the above configuration, the second cover is positioned on the sheet metal member, and the first cover is positioned on the second cover, so that the electromagnetic clutch and the clutch output gear can be positioned with high accuracy.

[0021] The above image forming apparatus may further include a belt conveying section that conveys a sheet from the first photosensitive drum toward the fourth photosensitive drum, and a fifth gear train that branches off from the second gear train at the second intermediate gear and transmits a driving force to the belt conveying section.

[0022] According to the above configuration, by branching off the fifth gear train from the second intermediate gear, which is the most upstream of the second gear train, it is possible to suppress transmission of rotational irregularities between the fifth gear train and the second gear train.

[0023] In the image forming apparatus, the second gear train and the fifth gear train may have a reverse rotation prevention mechanism that prevents the gears from rotating in the reverse direction when the motor rotates in the reverse direction. The reverse rotation prevention mechanism has a protruding portion of one of two coaxially arranged gears, and a first contact portion and a second contact portion of the other gear with which the protruding portion can come into contact. When the motor rotates in the forward direction, the protruding portion comes into contact with the first contact portion to rotate the two gears, and when the motor switches from the forward rotation to the reverse rotation, the two gears rotate idly until the protruding portion moves away from the first contact portion and comes into contact with the second contact portion.

[0024] According to the above configuration, abnormal noise generated from the pendulum gear when the pendulum gear is separated is suppressed, so that when the motor is temporarily rotated in reverse, the photosensitive drum and the belt transport unit can be prevented from rotating in reverse. Effect of the Invention

[0025] According to the present invention, in an image forming device capable of both color and monochrome printing, all of the developing rollers and photosensitive drums are driven by the same motor, and the first gear train that drives the developing rollers and the second gear train that drives the photosensitive drums are branched off directly from the motor, thereby suppressing the occurrence of banding. [Brief description of the drawings]

[0026] [Figure 1] 1 is a central cross-sectional view of an image forming apparatus according to an embodiment. [Diagram 2] FIG. [Diagram 3] FIG. [Figure 4] FIG. 4 is a diagram showing a state in which the main motor and covers are removed from FIG. [Diagram 5] FIG. 5 is a diagram showing a state in which the metal plate members and the photosensitive drums in FIG. 4 have been removed. [Figure 6] FIG. 4 is a front view of a second intermediate gear. [Figure 7] 7 is a cross-sectional view of FIG. 6 taken along line B-B. [Figure 8] 3 is a cross-sectional view taken along line AA in FIG. 2. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0027] 1 are defined as the front and rear sides, respectively, of the image forming apparatus 1, and the front and rear sides, respectively, of the paper in Fig. 1 are defined as the right and left sides, respectively, of the image forming apparatus 1. In addition, the upper and lower sides, respectively, of the image forming apparatus 1 are defined as the upper and lower sides, respectively.

[0028] [Overall configuration of image forming device] FIG. 1 is a central cross-sectional view of an image forming apparatus 1 according to an embodiment. The exterior of the image forming apparatus 1 is formed by an apparatus main body 2, which is a substantially rectangular parallelepiped housing. The apparatus main body 2 has a main body frame (not shown) that supports each component, and a main body cover 21 that covers the main body frame and forms the exterior. The main body frame has a left frame 20 (see FIG. 2) located on the left side, and a right frame (not shown) located on the right side. The main body cover 21 has a discharge tray 211 on the upper surface.

[0029] The image forming apparatus 1 is a color laser printer that forms a multi-color image on a sheet S such as paper or an OHP sheet by electrophotography. A sheet tray 3, a conveying section 4, an image forming section 5, a re-conveying section 6, a driving section 7, and an MP tray 8 are housed in the apparatus main body 2. Roughly speaking, within the apparatus main body 2, the image forming section 5 is located in the upper and central portion, the conveying section 4 is located below and behind the image forming section 5, the re-conveying section 6 is located across the front and rear of the apparatus main body 2 and below the conveying section 4, the sheet tray 3 is located below the re-conveying section 6, the driving section 7 is located to the left of the sheet tray 3 and the re-conveying section 6, and the MP tray 8 is located in front of the image forming section 5.

[0030] An opening 22 is formed in the lower front portion of the device body 2, which serves as an insertion port for the sheet tray 3. The sheet tray 3 includes a box-shaped sheet storage section 31 that is open at the top, and a pressure plate 32 that supports the sheets S so that they can be raised and lowered. The sheet tray 3 can be inserted into and removed from the device body 2 by moving it in the front-rear direction through the opening 22.

[0031] An opening 23 that serves as a storage port for the MP tray 8 is formed above the opening 22 on the front surface of the device body 2. The MP tray 8 includes a tray 81 that can be opened and closed with the lower end of the opening 23 as a fulcrum, and a pressure plate 82 that supports the sheet S so that it can be raised and lowered. The MP tray 8 can swing between a closed position (position shown by a two-dot chain line in FIG. 1) that covers the opening 23, and an open position (position shown by a solid line in FIG. 1) that opens the opening 23. When in the open position, the MP tray 8 can support a plurality of stacked sheets S.

[0032] The conveying section 4 is composed of each member arranged along the conveying path P1, and conveys the sheet S along the conveying path P1. The conveying path P1 is a path that connects the sheet tray 3 or the MP tray 8 to the discharge tray 211 via the image forming section 5. The conveying section 4 is composed of a supply section 41, a belt conveying section 42, and a discharge section 43, in that order from the upstream side of the conveying direction of the conveying path P1.

[0033] The supply unit 41 includes a supply mechanism 41A that separates, picks up, and conveys the sheets S stored in the sheet tray 3 one by one, a supply mechanism 41B that separates, picks up, and conveys the sheets S stacked on the MP tray 8 one by one, and a registration roller pair 415. The supply mechanism 41A includes a supply roller 411, a separation roller 412, a separation pad 413, and a conveying roller pair 414.

[0034] The sheets S stored in the sheet tray 3 are sent one by one to a conveying path P1 by a supply roller 411, a separation roller 412, and a separation pad 413. The sheets S sent to the conveying path P1 are conveyed toward the image forming unit 5 by a conveying roller pair 414 and a registration roller pair 415.

[0035] The supply mechanism 41B includes a supply roller 416, a separation roller 417, and a separation pad 418. The sheets S stored in the sheet tray 3 are sent out one by one to a conveying path P1 by the supply roller 416, the separation roller 417, and the separation pad 418. The sheets S sent out to the conveying path P1 are conveyed toward the image forming unit 5 by a pair of registration rollers 415.

[0036] The belt conveying section 42 includes a driving roller 421 that rotates in conjunction with the image forming section 5, a driven roller 422 that is rotatably disposed at a position spaced apart from the driving roller 421, and a conveying belt 423 that is wound between the driving roller 421 and the driven roller 422. The conveying belt 423 rotates with the sheet S placed thereon, thereby conveying the sheet S along a conveying path P1 opposite to the lower portions of the photosensitive drums 55A to 55D of the image forming section 5, and then sending the sheet S to the fixing section 53.

[0037] The discharge section 43 includes an intermediate discharge roller pair 431 and a discharge roller pair 432 that are disposed between the branch point P11 of the conveying path P1 and the discharge tray 211. The intermediate discharge roller pair 431 and the discharge roller pair 432 are switchback rollers that are configured to be able to switch between rotating in a forward direction and a reverse direction. The intermediate discharge roller pair 431 and the discharge roller pair 432 rotate forward when the sheet S conveyed from the fixing section 53 is to be discharged onto the discharge tray 211, and rotate reversely when the sheet S is to be inverted and conveyed to the re-conveying path P2.

[0038] The image forming unit 5 is of a so-called direct tandem type, capable of both color printing and monochrome printing, and includes a first drum unit 51A, a second drum unit 51B, a third drum unit 51C, and a fourth drum unit 51D arranged side by side in the front-to-rear direction, a first exposure LED head 52A, a second exposure LED head 52B, a third exposure LED head 52C, and a fourth exposure LED head 52D, a fixing unit 53, and a belt cleaner 54.

[0039] The first drum unit 51A corresponds to yellow toner (developer), the second drum unit 51B corresponds to magenta toner, the third drum unit 51C corresponds to cyan toner, and the fourth drum unit 51D corresponds to black toner (developer). The first drum unit 51A includes a first photosensitive drum 55A, a first developing roller 56A, and a first transfer roller 57A that faces the first photosensitive drum 55A with the conveyor belt 423 interposed therebetween.

[0040] Similarly, the second drum unit 51B includes a second photosensitive drum 55B, a second developing roller 56B, and a second transfer roller 57B that faces the second photosensitive drum 55B with the conveyor belt 423 in between. The third drum unit 51C includes a third photosensitive drum 55C, a third developing roller 56C, and a third transfer roller 57C that faces the third photosensitive drum 55C with the conveyor belt 423 in between. The fourth drum unit 51D includes a fourth photosensitive drum 55D, a fourth developing roller 56D, and a fourth transfer roller 57D that faces the fourth photosensitive drum 55D with the conveyor belt 423 in between.

[0041] In the image forming unit 5, each of the photosensitive drums 55A to 55D uniformly charged by the charger is selectively exposed by each of the exposure LED heads 52A to 52D. Through this exposure, charges are selectively removed from the surface of each of the photosensitive drums 55A to 55D, and an electrostatic latent image is formed on the surface of each of the photosensitive drums 55A to 55D.

[0042] A developing bias is applied to each of the developing rollers 56A to 56D, and when the electrostatic latent image formed on each of the photosensitive drums 55A to 55D faces each of the developing rollers 56A to 56D, toner is supplied from each of the developing rollers 56A to 56D to the electrostatic latent image due to the potential difference between the electrostatic latent image and each of the developing rollers 56A to 56D. As a result, a toner image is formed on the surface of each of the photosensitive drums 55A to 55D.

[0043] The sheet S transported toward the image forming unit 5 is transported by the transport belt 423 and passes sequentially between the transport belt 423 and each of the photosensitive drums 55A to 55D. Then, when facing the sheet S, the toner images on the surfaces of the photosensitive drums 55A to 55D are transferred onto the sheet S by the transfer bias applied to each of the transfer rollers 57A to 57D.

[0044] The fixing unit 53 is disposed downstream of the fourth photosensitive drum 55D in the transport direction on the transport path P1. The fixing unit 53 includes a heating roller 531 and a pressure roller 532 that is in pressure contact with the heating roller 531. The heating roller 531 is disposed on the image forming surface side of the sheet S and is driven to rotate in synchronization with the transport belt 423 to apply a transport force to the sheet S while heating the toner.

[0045] On the other hand, the pressure roller 532 is disposed on the opposite side of the heating roller 531 with the sheet S sandwiched therebetween, and presses the sheet S towards the heating roller 531. As a result, the pressure roller 532 receives a rotational force from the heating roller 531 via the sheet S in contact with the heating roller 531, and is rotated by the pressure roller 532.

[0046] The fixing section 53 further includes a pair of fixing and discharging rollers 533 that are disposed downstream of the heating roller 531 and the pressure roller 532 in the conveying direction of the sheet S.

[0047] In the image forming unit 5, the sheet S onto which the toner image has been transferred is thermally fixed to the sheet S while passing between a heating roller 531 and a pressure roller 532. The sheet S onto which the toner image has been thermally fixed is conveyed to a pair of intermediate discharge rollers 431 while being sandwiched between a pair of fixing and discharge rollers 533.

[0048] The belt cleaner 54 is disposed between the belt conveying section 42 and the re-conveying section 6, and is a member for collecting waste toner from the conveying belt 423.

[0049] The belt cleaner 54 includes a backup roller 541 , a cleaning roller 542 , a collection roller 543 , a scraping blade 544 , a collection chamber 545 , and a waste toner box 546 .

[0050] The backup roller 541 is disposed inside the conveyor belt 423 and contacts the conveyor belt 423 from above. The cleaning roller 542 is disposed at a position facing the backup roller 541 across the conveyor belt 423 and contacts the conveyor belt 423 from below.

[0051] In the belt cleaner 54, the waste toner adhering to the conveyor belt 423 is collected by a cleaning roller 542, a collection roller 543, a scraping blade 544, and a collection chamber 545, and the collected waste toner is stored in a waste toner box 546.

[0052] The cleaning operation is started after the image formation by the image forming unit 5 is completed. In the cleaning operation, the waste toner adhering to the surface of the conveyor belt 423 is electrostatically held on the surface of the cleaning roller 542 by a bias applied to the cleaning roller 542, and then electrostatically transferred to the collection roller 543. The waste toner transferred to the collection roller 543 is scraped off by a scraping blade 544 and falls into the collection chamber 545. The waste toner that has fallen into the collection chamber 545 is transported backward by a transport means (not shown) and stored in the waste toner box 546.

[0053] The re-conveying section 6 is composed of each member arranged along the re-conveying path P2, and conveys the sheet S along the re-conveying path P2. The re-conveying path P2 branches off from a branching point P11 between the fixing section 53 and the pair of intermediate discharge rollers 431 in the conveying path P1, passes above the sheet tray 3, and merges with a junction point P12 between the sheet tray 3 and the image forming section 5. With this configuration, the re-conveying path P2 can be made shorter than in a configuration in which the re-conveying path P2 is arranged below the sheet tray 3.

[0054] The re-conveying section 6 includes a detachable re-conveying unit 61 above the sheet tray 3 and below the image forming section 5. The re-conveying unit 61 includes a pair of oblique feed rollers 611 and a re-conveying roller 612 for guiding the sheet S along the re-conveying path P2. A driven roller 613 that forms a nip with the re-conveying roller 612 is disposed in the device body 2 above the re-conveying roller 612.

[0055] An opening 24 is formed on the rear surface of the device body 2 as an insertion port for the re-feeding unit 61. The re-feeding unit 61 can be inserted into or removed from the device body 2 by moving it in the front-rear direction through the opening 24. The location of the re-feeding unit 61 is not particularly limited, and it may be located below the sheet tray 3, for example.

[0056] The re-conveying section 6 conveys the sheet S, which has an image formed on one side and is conveyed from the intermediate discharge roller pair 431, to the junction P12. The sheet S, which is re-conveyed from the junction P12 to the image forming section 5, has an image formed on the other side by the image forming section 5, and is then discharged by the discharge section 43 onto the discharge tray 211. If a jam occurs in the re-conveying section 6, the jam can be cleared by pulling out the re-conveying unit 61.

[0057] In this way, in the image forming apparatus 1, a sheet S, on one side of which an image has been formed by the image forming unit 5, can be re-transported to the image forming unit 5 via the re-transport path P2, and an image can be formed on the other side of the sheet S, thereby performing so-called double-sided printing.

[0058] The drive unit 7 is composed of an electric motor and a gear train disposed on the left surface of the left frame 20, and drives the sheet tray 3, the conveying unit 4, the image forming unit 5, the re-conveying unit 6, and the MP tray 8. Below, the drive unit 7 will be described in detail, along with a configuration for suppressing the occurrence of banding.

[0059] [Drive unit] Fig. 2 is a left side view of the drive unit 7, Fig. 3 is a right side view of the drive unit 7, Fig. 4 is a view of Fig. 3 with the main motor 76 and covers removed, and Fig. 5 is a view of Fig. 4 with the sheet metal member 75 and the photosensitive drums 55A to 55D removed. As shown in Figs. 2 to 5, the drive unit 7 includes a first drive unit 71, a second drive unit 72, a first cover 73, a second cover 74, and a sheet metal member 75.

[0060] <First drive unit> The first drive unit 71 is a unit that drives the pressure plate 32 of the sheet tray 3, the pressure plate 82 of the MP tray 8, the supply section 41, the fixing section 53, the discharge section 43, and the re-conveyance section 6. The first drive unit 71 includes a main motor 76 (see FIG. 3) and a first drive transmission mechanism 77 (see FIGS. 4 and 5) connected to the main motor 76. The first drive transmission mechanism 77 has a plurality of gear trains 771-773 that transmit drive force to each of the above-mentioned sections.

[0061] <Second drive unit> The second drive unit 72 is a unit that drives the belt conveyor 42, the drum units 51A-51D, and the belt cleaner 54. The second drive unit 72 is separate from the first drive unit 71 that drives members that are subject to high loads, such as the fixing unit 53, and drives members that directly affect image quality, such as the drum units 51A-51D. Therefore, rotation irregularities of the first drive unit 71 are not transmitted to the second drive unit 72, and the occurrence of banding can be suppressed.

[0062] 4 and 5, the second drive unit 72 includes a process motor 78 as an example of a motor, a first output gear 781, and a second drive transmission mechanism 79. The first output gear 781 is disposed coaxially with the process motor 78 and is a gear that outputs a drive force from the process motor 78. In this embodiment, the first output gear 781 is a gear fixed to a rotating shaft portion of the process motor 78. The second drive transmission mechanism 79 includes a first gear train 791, a second gear train 792, and a fifth gear train 795 that transmit drive forces to the above-mentioned respective portions.

[0063] (1st gear train) The first gear train 791 is a gear train that transmits the driving force of the process motor 78 to the first developing roller 56A, the second developing roller 56B, the third developing roller 56C, and the fourth developing roller 56D. The first gear train 791 is made up of a third gear train 793 that transmits the driving force to the first developing roller 56A, the second developing roller 56B, and the third developing roller 56C, and a fourth gear train 794 that transmits the driving force to the fourth developing roller 56D.

[0064] (3rd gear train) The third gear train 793 has a first intermediate gear 79A meshing with the first output gear 781, a third intermediate gear 79B meshing with the first intermediate gear 79A, a gear 79C meshing with the third intermediate gear 79B, and a pendulum gear 79D meshing with the gear 79C. The third gear train 793 also has a gear 79E meshing with the pendulum gear 79D, a gear 79F meshing with the gear 79E, a gear 79G meshing with the gear 79E, a gear 79H meshing with the gear 79G, and a second output gear 79J meshing with the gear 79H.

[0065] The first intermediate gear 79A is a two-stage gear in which a right input gear 79K meshing with the first output gear 781 and a left output gear 79L meshing with the third intermediate gear 79B are integrally formed. The first intermediate gear 79A may be a single-stage gear meshing with the first output gear 781 and the third intermediate gear 79B. The first intermediate gear 79A also outputs a driving force to the belt cleaner 54.

[0066] The gear 79F is disposed coaxially with the first developing roller 56A and is a gear that outputs a driving force to the first developing roller 56A. The gear 79G is disposed coaxially with the second developing roller 56B and is a gear that outputs a driving force to the second developing roller 56B. The second output gear 79J is disposed coaxially with the third developing roller 56C and is a gear that outputs a driving force to the third developing roller 56C.

[0067] Gear 79E is a two-stage gear formed integrally with left input gear 79M that meshes with pendulum gear 79D and right output gear 79N that meshes with gears 79F and 79G. Gear 79E may be a single-stage gear that meshes with pendulum gear 79D, gears 79F, and gear 79G.

[0068] The pendulum gear 79D is disposed upstream of the first developing roller 56A, the second developing roller 56B, and the third developing roller 56C, and is a gear that outputs a driving force to the first developing roller 56A, the second developing roller 56B, and the third developing roller 56C. The pendulum gear 79D is capable of swinging between a first position (position shown by solid lines in FIGS. 4 and 5) where it meshes with the input gear 79M and transmits the driving force, and a second position (position shown by two-dot chain lines in FIGS. 4 and 5) where it is separated from the input gear 79M and cuts off the driving force.

[0069] As a result, color printing is possible when the pendulum gear 79D is in the first position, and monochrome printing is possible when the pendulum gear 79D is in the second position. By using the pendulum gear 79D as a means for transmitting and disconnecting the driving force, costs can be reduced.

[0070] (4th gear train) The fourth gear train 794 has a gear 79P that meshes with the third intermediate gear 79B, a gear 79Q that meshes with the gear 79P, a gear 79R that meshes with the gear 79Q, a gear 79S that meshes with the gear 79R, and an electromagnetic clutch 79U. The fourth gear train 794 also has a clutch input gear 79T that meshes with the gear 79S to input a driving force to the electromagnetic clutch 79U, a clutch output gear 79V that outputs a driving force from the electromagnetic clutch 79U, and a third output gear 79W that meshes with the clutch output gear 79V.

[0071] Gear 79S is disposed coaxially with second output gear 79J to the left of second output gear 79J. Gear 79S and second output gear 79J are separate bodies and rotate independently. Note that the gears disposed coaxially are not limited to these two gears, but may be some of the gears in third gear train 793 and some of the gears in fourth gear train 794. In this way, by disposing some of the gears in third gear train 793 and some of the gears in fourth gear train 794 coaxially, first gear train 791 can be made smaller.

[0072] The third output gear 79W is disposed coaxially with the fourth developing roller 56D, and is a gear that outputs a driving force to the fourth developing roller 56D.

[0073] The electromagnetic clutch 79U is disposed upstream of the fourth developing roller 56D and is a member that outputs a driving force to the fourth developing roller 56D. The electromagnetic clutch 79U is displaceable between a first state in which the driving force is transmitted and a second state in which the driving force is cut off. This allows monochrome printing or color printing when the electromagnetic clutch 79U is in the first position. Using the electromagnetic clutch 79U as a means for transmitting and cutting off the driving force can save space.

[0074] Clutch input gear 79T is disposed to the left of electromagnetic clutch 79U, and clutch output gear 79V is disposed to the left of clutch input gear 79T and coaxially with clutch input gear 79T. Clutch output gear 79V preferably has a different number of teeth than clutch input gear 79T. In this embodiment, clutch output gear 79V is a gear with fewer teeth than clutch input gear 79T.

[0075] The clutch output gear 79V may have a larger number of teeth than the clutch input gear 79T. In this way, the clutch input gear 79T and the clutch output gear 79V have different numbers of teeth, so that the rotation of the clutch output gear 79V is stabilized and the image quality of the fourth drum unit 51D is improved because the rotation of the clutch output gear 79V is slowed down or accelerated before and after the electromagnetic clutch 79U.

[0076] The fourth gear train 794 branches off from the third gear train 793 at the third intermediate gear 79B because the gear 79P is meshed with the third intermediate gear 79B. In this way, by branching off the fourth gear train 794 at the third intermediate gear 79B, which is as close as possible to the first output gear 781 fixed to the rotating shaft of the process motor 78, rotational irregularities in one of the third gear train 793 and the fourth gear train 794 are unlikely to be transmitted to the other gear train, and each gear drives stably. In addition, the fourth gear train 794 can be made shorter than a configuration in which it branches off directly from the first output gear 781.

[0077] It should be noted that fourth gear train 794 may branch off upstream of the pendulum gear of the third gear train, and may branch off, for example, at first output gear 781 or gear 79C.

[0078] (2nd gear train) The second gear train 792 is a gear train that transmits the driving force of the process motor 78 to the first photosensitive drum 55A, the second photosensitive drum 55B, the third photosensitive drum 55C, and the fourth photosensitive drum 55D. The second gear train 792 has a second intermediate gear 70A that meshes with the first output gear 781, a gear 70B that meshes with the second intermediate gear 70A, a gear 70C that meshes with the gear 70B, and a gear 70D that meshes with the gear 70C. The second gear train 792 also has a gear 70E that meshes with the second intermediate gear 70A, a gear 70F that meshes with the gear 70E, and a gear 70G that meshes with the gear 70F.

[0079] The second intermediate gear 70A is a two-stage gear that includes a left input gear 70H that meshes with the first output gear 781 and a right output gear 70J that meshes with the gear 70B and the gear 70E, and that rotates together.

[0080] The gear 70D is disposed coaxially with the first photosensitive drum 55A and is a gear that outputs a driving force to the first photosensitive drum 55A. The gear 70B is disposed coaxially with the second photosensitive drum 55B and is a gear that outputs a driving force to the second photosensitive drum 55B. The gear 70E is disposed coaxially with the third photosensitive drum 55C and is a gear that outputs a driving force to the third photosensitive drum 55C. The gear 70G is disposed coaxially with the fourth photosensitive drum 55D and is a gear that outputs a driving force to the fourth photosensitive drum 55D.

[0081] According to the first gear train 791 and the second gear train 792, the first intermediate gear 79A of the first gear train 791 and the second intermediate gear 70A of the second gear train 792 are directly meshed with the first output gear 781 fixed to the rotation shaft of the process motor 78. In this way, the first gear train 791 that drives the developing rollers 56A to 56D and the second gear train 792 that drives the photosensitive drums 55A to 55D are directly branched off from the process motor 78, so that rotation irregularities of each gear train are unlikely to be transmitted to the other gear train, and the occurrence of banding can be suppressed.

[0082] (5th gear train) The fifth gear train 795 is a gear train that transmits the driving force of the process motor 78 to the belt conveying unit 42. The fifth gear train 795 has a gear 70K that meshes with the input gear 70H of the second intermediate gear 70A, a gear 70L that meshes with the gear 70K, and a gear 70M that meshes with the gear 70L.

[0083] The gear 70L is a two-stage gear that is assembled with a left input gear 70N that meshes with the gear 70K and a right output gear 70P that meshes with the gear 70M, and rotates integrally with them.

[0084] The gear 70M is a gear that outputs a driving force to the driving roller 421 of the belt conveying unit 42. The gear 70M is a two-stage gear in which a left input gear 70Q that meshes with the output gear 70P of the gear 70L and a right output gear 70R that meshes with a gear (not shown) of the driving roller 421 are integrally molded. The gear 70M may be a single-stage gear that meshes with the output gear 70P and the gear of the driving roller 421.

[0085] The fifth gear train 795 branches off from the second gear train 792 at the second intermediate gear 70A because the gear 70K is meshed with the second intermediate gear 70A. By branching off the fifth gear train 795 from the second intermediate gear 70A, which is the most upstream of the second gear train 792, in this manner, it is possible to suppress transmission of rotational irregularities between the fifth gear train 795 and the second gear train 792.

[0086] (Reverse rotation prevention mechanism) The second intermediate gear 70A and the gear 70L have a reverse rotation prevention mechanism 9. The reverse rotation prevention mechanism 9 is a mechanism that prevents the output gear 70J of the second intermediate gear 70A and the output gear 70P of the gear 70L from rotating in the reverse direction when the process motor 78 rotates in the reverse direction.

[0087] Fig. 6 is a front view of second intermediate gear 70A, and Fig. 7 is a cross-sectional view taken along line BB of Fig. 6. Input gear 70H has a cylindrical portion 701 that protrudes rightward about the rotation axis. Cylindrical portion 701 has a notch that extends in the direction of the rotation axis, with one end face of the notch constituting first contact portion 91 and the other end face constituting second contact portion 92.

[0088] The output gear 70J has a rotating shaft portion 702 that is inserted into a cylindrical portion 701 and rotatably supported in a space inside the teeth, and a protruding portion 93 that protrudes radially from the rotating shaft portion 702. The protruding portion 93 has a third abutment portion 931 that can abut against the first abutment portion 91, and a fourth abutment portion 932 that can abut against the second abutment portion 92.

[0089] The reverse rotation prevention mechanism 9 is composed of the above-mentioned first contact portion 91, second contact portion 92, and protrusion 93. When the process motor 78 rotates forward, the third contact portion 931 of the protrusion 93 comes into contact with the first contact portion 91 (as shown by the solid line in FIG. 7), and the input gear 70H and the output gear 70J rotate integrally.

[0090] Then, when the process motor 78 switches from forward rotation to reverse rotation, the third contact portion 931 of the protrusion 93 moves away from the first contact portion 91, and the fourth contact portion 932 comes into contact with the second contact portion 92 (as shown by the two-dot chain line in FIG. 7). At this time, during the period until the protrusion 93 moves away from the first contact portion 91 and comes into contact with the second contact portion 92, the input gear 70H rotates idly and the output gear 70J does not rotate. Therefore, the reverse rotation of the output gear 70J is prevented only while the input gear 70H rotates idly.

[0091] The reverse rotation prevention mechanism of gear 70L has the same configuration as that described above, and therefore a detailed description thereof will be omitted.

[0092] The reverse rotation prevention mechanism 9 as described above suppresses abnormal noise generated from the pendulum gear 79D when the pendulum gear 79D is separated, and therefore, when the process motor 78 is temporarily rotated in reverse, it is possible to prevent the photosensitive drums 55A to 55D and the belt transport unit 42 from rotating in reverse. Note that the process motor 78 is rotated in reverse by one or two gear teeth.

[0093] Conversely to the above configuration, the first and second contact portions 91 and 92 may be provided on the output gear 70J, and the protrusion 93 may be provided on the input gear 70H. In other words, the reverse rotation prevention mechanism 9 may be configured with the protrusion 93 of one of the two coaxially arranged gears, and the first and second contact portions 91 and 92 of the other gear.

[0094] Further, the reverse rotation prevention mechanism 9 may be provided on gears in the second gear train 792 and the fifth gear train 795 other than the second intermediate gear 70A and the gear 70L.

[0095] <First cover> As shown in Fig. 2, the first cover 73 is a resin cover that is fixed to one side of the sheet metal member 75 and supports the electromagnetic clutch 79U. In this embodiment, the first cover 73 is screwed to the left side of the sheet metal member 75. Fig. 8 is a cross-sectional view taken along line AA in Fig. 2. As shown in Fig. 8, the first cover 73 has a first engagement portion 731 that engages with the second engagement portion 741 of the second cover 74 and a third engagement portion 732 that engages with an elongated hole 752 of the sheet metal member 75. The first engagement portion 731 and the third engagement portion 732 are cylindrical bosses that protrude toward the sheet metal member 75.

[0096] <Second cover> As shown in Fig. 3, the second cover 74 is a resin cover that is fixed to the other surface side of the sheet metal member 75 and supports one end of the clutch output gear 79V. In this embodiment, the second cover 74 is screwed to the right surface of the sheet metal member 75. As shown in Fig. 8, the second cover 74 has a second engagement portion 741 that engages with the first engagement portion 731 of the first cover 73 and the positioning hole 751 of the sheet metal member 75. The second engagement portion 741 is a cylindrical boss that protrudes toward the sheet metal member 75.

[0097] <Sheet metal parts> The sheet metal member 75 is a metal plate that supports the gear trains 791-795, the first cover 73, and the second cover 74. As shown in Fig. 8, the sheet metal member 75 has a circular positioning hole 751 that engages with the second engagement portion 741 of the first cover 73.

[0098] With such first cover 73, second cover 74, and sheet metal member 75, the outer peripheral surface of the second engagement portion 741 fits into the positioning hole 751 of the sheet metal member 75, and the inner peripheral surface of the second engagement portion 741 fits into the outer peripheral surface of the first engagement portion 731. As a result, the second cover 74 is positioned to the sheet metal member 75, and the first cover 73 is positioned to the second cover 74. Therefore, the electromagnetic clutch 79U supported by the first cover 73 and the clutch output gear 79V supported by the second cover 74 can be positioned with high accuracy.

[0099] 2 and 4, the sheet metal member 75 has an elongated hole 752 that engages with the third engagement portion 732 of the first cover 73. By engaging the third engagement portion 732 with the elongated hole 752, it is possible to prevent the first cover 73 from rotating about the first engagement portion 731 as a fulcrum when assembling the first cover 73 to the sheet metal member 75 to which the second cover 74 is attached during the assembly work. This improves workability. [Explanation of symbols]

[0100] 1. Image forming device 42 Belt conveyor 56A First developing roller 56B Second developing roller 56C 3rd developing roller 56D 4th developing roller 55A First photosensitive drum 55B Second photosensitive drum 55C Third photosensitive drum 55D 4th photosensitive drum 70A 2nd intermediate gear 73 1st Cover 74 2nd Cover 75 Sheet metal member; 78 Process motor (motor) 79A 1st intermediate gear 79B 3rd intermediate gear 79D Pendulum Gear 79J 2nd output gear 79T clutch input gear 79U Electromagnetic clutch 79V Clutch output gear 79W 3rd output gear 91 1st contact part 92 Second contact part 93 Protrusion 731 First engagement part 741 Second engagement part 781 1st output gear 791 1st Gear Train 792 2nd Gear Train 793 3rd Gear Train 794 4th Gear Train 795 5th Gear Train S Seat

Claims

1. a first developing roller, a second developing roller, and a third developing roller each carrying a color developer; a fourth developing roller carrying a black developer; a first photosensitive drum corresponding to the first developing roller; a second photosensitive drum corresponding to the second developing roller; a third photosensitive drum corresponding to the third developing roller; a fourth photosensitive drum corresponding to the fourth developing roller; a motor that drives the first developing roller to the fourth developing roller and the first photosensitive drum to the fourth photosensitive drum; a first output gear that is disposed coaxially with the motor and outputs a driving force from the motor; a first gear train including a first intermediate gear meshing with the first output gear and transmitting a driving force of the motor from the first developing roller to the fourth developing roller; a second gear train including a second intermediate gear meshing with the first output gear and transmitting a driving force of the motor from the first photosensitive drum to the fourth photosensitive drum, The first gear train includes: a third gear train that transmits a driving force to the first developing roller, the second developing roller, and the third developing roller; a fourth gear train that transmits a driving force to the fourth developing roller, the third gear train is disposed upstream of the first developing roller, the second developing roller, and the third developing roller, and has a pendulum gear that is swingable between a first position for transmitting a driving force and a second position for disconnecting the driving force; an electro-magnetic clutch that is disposed upstream of the fourth developing roller and that is displaceable between a first state in which a driving force is transmitted and a second state in which the driving force is cut off;

2. the third gear train includes the first intermediate gear, 2. The image forming apparatus according to claim 1, wherein the fourth gear train branches off from the third gear train upstream of the pendulum gear.

3. the third gear train includes a third intermediate gear that meshes with the first intermediate gear, 3. The image forming apparatus according to claim 2, wherein the fourth gear train branches off from the third gear train at the third intermediate gear.

4. 4. The image forming apparatus according to claim 1, wherein a part of the gears in the third gear train and a part of the gears in the fourth gear train are arranged on the same axis.

5. the third developing roller is disposed adjacent to the fourth developing roller, 5. The image forming apparatus according to claim 4, wherein the one of the gears in the third gear train is a second output gear that is disposed coaxially with the third developing roller and outputs a driving force to the third developing roller.

6. The fourth gear train includes: a clutch input gear that inputs a driving force to the electromagnetic clutch; 6. The image forming apparatus according to claim 1, further comprising: a clutch output gear having a number of teeth different from that of the clutch input gear, the clutch output gear outputting a driving force from the electromagnetic clutch.

7. the fourth gear train includes a third output gear that is disposed coaxially with the fourth developing roller and outputs a driving force to the fourth developing roller, 7. The image forming apparatus according to claim 6, wherein the clutch output gear meshes with the third output gear.

8. a sheet metal member having a positioning hole and supporting the fourth gear train; a first cover having a first engaging portion, fixed to one surface side of the sheet metal member, and supporting the electromagnetic clutch; 8. The image forming apparatus according to claim 6, further comprising: a second cover having a second engagement portion that engages with the positioning hole and the first engagement portion, fixed to the other side of the sheet metal member, and supporting one end of the clutch output gear.

9. a first developing roller, a second developing roller, and a third developing roller each carrying a color developer; a fourth developing roller carrying a black developer; a first photosensitive drum corresponding to the first developing roller; a second photosensitive drum corresponding to the second developing roller; a third photosensitive drum corresponding to the third developing roller; a fourth photosensitive drum corresponding to the fourth developing roller; a motor that drives the first developing roller to the fourth developing roller and the first photosensitive drum to the fourth photosensitive drum; a first output gear that is disposed coaxially with the motor and outputs a driving force from the motor; a first gear train including a first intermediate gear meshing with the first output gear and transmitting a driving force of the motor from the first developing roller to the fourth developing roller; a second gear train including a second intermediate gear meshing with the first output gear and transmitting a driving force of the motor from the first photosensitive drum to the fourth photosensitive drum, a belt conveying section that conveys a sheet from the first photosensitive drum to the fourth photosensitive drum; a fifth gear train branching off from the second gear train at the second intermediate gear and transmitting a driving force to the belt transport unit.

10. the second gear train and the fifth gear train each have a reverse rotation prevention mechanism that prevents the gears from rotating in reverse when the motor rotates in reverse, The reverse rotation prevention mechanism includes: A protrusion provided on one of two coaxially arranged gears; the other gear has a first contact portion and a second contact portion with which the protrusion can come into contact, When the motor rotates in the forward direction, the protruding portion abuts against the first abutment portion, and the two gears rotate.

10. The image forming apparatus according to claim 9, wherein when the motor switches from forward rotation to reverse rotation, the protrusion rotates freely between the two gears until it moves away from the first contact portion and contacts the second contact portion.

Images