Image forming apparatus

Abstract

To bring first color, second color, and third color developing rollers into pressure contact or separate the developing rollers at pieces of predetermined timing shifted from each other.SOLUTION: An image forming apparatus comprises: a color interlocking gear train 110 that mechanically interlocks to rotate a first cam 90Y, a second cam 90M, and a third cam 90C, the color interlocking gear train 110 interlocking the cams 90 while maintaining a situation in which phases of the first cam 90Y, second cam 90M, and third cam 90C are shifted from each other; a monochrome transmission gear train 120K that transmits driving force of a motor M to a fourth cam 90K; a monochrome clutch 150K that switches whether to cause the monochrome transmission gear train 120K to transmit the driving force from the motor M to the fourth cam 90K; a color transmission gear train 120A that transmits the driving force of the motor M to the color interlocking gear train 110; and a color clutch 150A that switches whether to cause the color transmission gear train 120A to transmit the driving force from the motor M to the color interlocking gear train 110.SELECTED DRAWING: Figure 4

Description

Technical Field

[0001] The present invention relates to an image forming apparatus capable of moving a developing roller between a pressure contact position where the developing roller is in pressure contact with a photosensitive drum and a separation position where the developing roller is separated from the photosensitive drum.

Background Art

[0002] Conventionally, an image forming apparatus capable of moving a developing roller between a pressure contact position where the developing roller is in pressure contact with a photosensitive drum and a separation position where the developing roller is separated from the photosensitive drum is known (Patent Document 1). In this invention, developing rollers corresponding to three colors of yellow, magenta, and cyan are arranged in this order from the upstream side in the moving direction of the belt. And each of the three-color developing rollers is configured to be simultaneously in pressure contact with or separated from the corresponding photosensitive drum.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] By the way, in order to minimize the deterioration of the toner used for image formation, it is desirable to press the developing roller against the photosensitive drum for only the minimum time required for transferring from the photosensitive drum to the belt. However, in the invention of Patent Document 1, since each developing roller is simultaneously in pressure contact with or separated from the corresponding photosensitive drum, the developing rollers of some of the three colors may be in pressure contact with the photosensitive drum earlier or separated later than the timing required for development.

[0005] Therefore, an object of the present invention is to provide an image forming apparatus capable of sequentially pressing or separating a developing roller against a photosensitive drum.

Means for Solving the Problem

[0006] The image forming apparatus for achieving the above-described object includes a plurality of process units corresponding to each of a first color, a second color, a third color, and a fourth color. Each process unit includes a developing unit having a developing roller and a drum unit having a photosensitive drum. The image forming apparatus further includes a transfer unit having a belt disposed facing each photosensitive drum of the plurality of process units, a motor, a first cam for moving the developing roller of the first color between a pressure contact position and a separation position with respect to the corresponding photosensitive drum, a second cam for moving the developing roller of the second color between a pressure contact position and a separation position with respect to the corresponding photosensitive drum, a third cam for moving the developing roller of the third color between a pressure contact position and a separation position with respect to the corresponding photosensitive drum, a fourth cam for moving the developing roller of the fourth color between a pressure contact position and a separation position with respect to the corresponding photosensitive drum, a color interlocking gear train for mechanically interlocking and rotating the first cam, the second cam, and the third cam, the color interlocking gear train maintaining a state in which the phases of the first cam, the second cam, and the third cam are shifted and interlocking each cam, a monochrome transmission gear train for transmitting the driving force of the motor to the fourth cam, a monochrome clutch for switching whether to transmit the driving force from the motor to the fourth cam by the monochrome transmission gear train, a color transmission gear train for transmitting the driving force of the motor to the color interlocking gear train, and a color clutch for switching whether to transmit the driving force from the motor to the color interlocking gear train by the color transmission gear train.

[0007] According to such a configuration, the driving force of the motor drives the color interlocking gear train via the color transmission gear train and the color clutch. The color interlocking gear train mechanically interlocks and rotates the first cam, the second cam, and the third cam. At this time, since the color interlocking gear train maintains a state in which the phases of the first cam, the second cam, and the third cam are shifted and interlocks each cam, the developing rollers of the first color, the second color, and the third color can be brought into pressure contact with or separated from each other at a predetermined shifted timing.

[0008] The process unit may have a hinge that connects the developing unit to the drum unit so that the developing roller can swing between a pressure contact position where the developing roller is in pressure contact with the corresponding photosensitive drum and a separation position where the developing roller is separated from the corresponding photosensitive drum.

[0009] The color interlocking gear train can have a first cam gear that rotates integrally with the first cam, a second cam gear that rotates integrally with the second cam, a third cam gear that rotates integrally with the third cam, a 1-2 idle gear that is an idle gear meshing with the first cam gear and the second cam gear, and a 2-3 idle gear that is an idle gear meshing with the second cam gear and the third cam gear. In this case, the first cam, the second cam, the third cam, and the fourth cam are arranged in the order of the first cam, the second cam, the third cam, and the fourth cam from upstream to downstream or from downstream to upstream in the moving direction of the portion of the belt facing the photosensitive drum, and the color transmission gear train may be configured to engage with the third cam gear.

[0010] According to such a configuration, by engaging the color transmission gear train with the third cam gear close to the fourth cam gear, the phase error between the third cam gear and the fourth cam gear can be suppressed, and the accuracy of the timing of pressure contact and separation of the four-color developing rollers can be improved.

[0011] The color interlocking gear train can have a first cam gear that rotates integrally with the first cam, a second cam gear that rotates integrally with the second cam, a third cam gear that rotates integrally with the third cam, a 1-2 idle gear that is an idle gear meshing with the first cam gear and the second cam gear, and a 2-3 idle gear that is an idle gear meshing with the second cam gear and the third cam gear. In this case, the first cam, the second cam, the third cam, and the fourth cam are arranged in the order of the first cam, the second cam, the third cam, and the fourth cam from upstream to downstream or from downstream to upstream in the moving direction of the portion of the belt facing the photosensitive drum, and the color transmission gear train may be configured to engage with the second cam gear.

[0012] According to such a configuration, since the color transmission gear train engages with the second cam gear located at the center of the color interlocking gear train, it is possible to minimize the error from the designed value of the phase, which is caused by backlash or the like between the first cam gear and the third cam gear.

[0013] The color interlocking gear train can include a first cam gear that rotates integrally with the first cam, a second cam gear that rotates integrally with the second cam, a third cam gear that rotates integrally with the third cam, a 1-2 idle gear that is an idle gear meshing with the first cam gear and the second cam gear, and a 2-3 idle gear that is an idle gear meshing with the second cam gear and the third cam gear. In this case, the first cam, the second cam, the third cam, and the fourth cam are arranged in the order of the first cam, the second cam, the third cam, and the fourth cam from upstream to downstream or from downstream to upstream in the moving direction of the portion of the belt facing the photosensitive drum, and the color transmission gear train may be configured to engage with the first cam gear.

[0014] According to such a configuration, since the color transmission gear train engages with the first cam gear that is the farthest from the fourth cam gear, it is easy to layout the color transmission gear train without interfering with the monochrome transmission gear train, and the size of the image forming apparatus can be reduced. Further, when the color transmission gear train engages with the first cam gear located at the most upstream in the moving direction of the belt, the timing of the start of rotation of the first cam is important for the pressing operation at the start of printing. Therefore, in this case, by shortening the gear train that engages with the first cam, the error from the designed value of the phase of the first cam gear can be minimized.

[0015] The color interlocking gear train can include a first cam gear that rotates integrally with a first cam, a second cam gear that rotates integrally with a second cam, a third cam gear that rotates integrally with a third cam, a 1-2 idle gear that is an idle gear meshing with the first cam gear and the second cam gear, and a 2-3 idle gear that is an idle gear meshing with the second cam gear and the third cam gear. In this case, the first cam, the second cam, the third cam, and the fourth cam are arranged in the order of the first cam, the second cam, the third cam, and the fourth cam from upstream to downstream or from downstream to upstream in the moving direction of the portion of the belt facing the photosensitive drum, and the color transmission gear train may be configured to engage with the 1-2 idle gear.

[0016] According to such a configuration, since the color transmission gear train engages with the 1-2 idle gear, an error from the design value of the phase between the first cam gear and the third cam gear can be suppressed. Also, since the 1-2 idle gear is relatively far from the fourth cam gear, it is easy to layout the color transmission gear train without interfering with the monochrome transmission gear train, and the size of the image forming apparatus can be reduced.

[0017] The color interlocking gear train can include a first cam gear that rotates integrally with a first cam, a second cam gear that rotates integrally with a second cam, a third cam gear that rotates integrally with a third cam, a 1-2 idle gear that is an idle gear meshing with the first cam gear and the second cam gear, and a 2-3 idle gear that is an idle gear meshing with the second cam gear and the third cam gear. In this case, the first cam, the second cam, the third cam, and the fourth cam are arranged in the order of the first cam, the second cam, the third cam, and the fourth cam from upstream to downstream or from downstream to upstream in the moving direction of the portion of the belt facing the photosensitive drum, and the color transmission gear train may be configured to engage with the 2-3 idle gear.

[0018] According to such a configuration, since the color transfer gear train engages with the 2-3 idle gears, the phase error between the cam gears of the second and third colors and the fourth cam gear can be relatively suppressed. Further, since the color transfer gear train engages with the 2-3 idle gears, the error of the phase of the first cam gear and the third cam gear from the design value can be suppressed.

[0019] In the above-described image forming apparatus, the first color may be yellow, the second color may be magenta, the third color may be cyan, and the fourth color may be black.

[0020] The belt is transferred with a toner image from the photosensitive drum, and the image forming apparatus may further include a secondary transfer roller that transfers the toner image on the belt to a sheet.

Effect of the Invention

[0021] According to the present invention, the developing rollers of the first, second, and third colors can be brought into pressure contact with, or separated from, each other at predetermined shifted timings.

Brief Description of the Drawings

[0022]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Figure 11

Mode for Carrying Out the Invention

[0023] [First Embodiment] As shown in FIG. 1, the image forming apparatus 1 according to the first embodiment is a color laser printer using an electrophotographic process, and forms a color image on a sheet S. The image forming apparatus 1 includes a housing 10, a sheet supply unit 20, an image forming unit 30, a control unit 2, and a motor M. In this embodiment, the right side of FIG. 1 is the front, the left side is the rear, the up and down directions are the same as the up and down directions, the near side of the paper surface of FIG. 1 is the left, and the far side of the paper surface is the right.

[0024] The housing 10 has a front cover 11 and a discharge tray 13. The front cover 11 is rotatable about its lower end and can open and close the opening 12 on the front side of the housing 10. The discharge tray 13 is formed on the upper surface of the housing 10.

[0025] The sheet supply unit 20 includes a sheet tray 21 for accommodating the sheet S and a paper feed roller 22. The sheet S is a medium on which an image can be formed by the image forming apparatus 1, and includes plain paper, envelopes, postcards, thin paper, thick paper, glossy paper, resin sheets, seals, etc.

[0026] The image forming unit 30 includes a plurality of process units P, a cartridge tray TR, an exposure device 40, a transfer unit 70, a secondary transfer roller 78, a fixing device 80, and a discharge roller 15.

[0027] The plurality of process units P includes four process units PY, PM, PC, and PK corresponding to the colors yellow, magenta, cyan, and black. In the present embodiment, yellow is the first color, magenta is the second color, cyan is the third color, and black is the fourth color. Hereinafter, in this specification and the drawings, for members provided corresponding to each color, when distinguishing the colors, Y, M, C, and K are attached to the reference signs.

[0028] As shown in FIG. 2, each process unit P includes a developing unit 60 having a developing roller 61 and a drum unit 50 having a photosensitive drum 51. The plurality of developing units 60 includes a developing unit 60Y having a developing roller 61Y corresponding to yellow, a developing unit 60M having a developing roller 61M corresponding to magenta, a developing unit 60C having a developing roller 61C corresponding to cyan, and a developing unit 60K having a developing roller 61K corresponding to black. The plurality of drum units 50 includes a drum unit 50Y having a photosensitive drum 51Y corresponding to yellow, a drum unit 50M having a photosensitive drum 51M corresponding to magenta, a drum unit 50C having a photosensitive drum 51C corresponding to cyan, and a drum unit 50K having a photosensitive drum 51K corresponding to black. Although illustration is omitted, each drum unit 50 has a charger for charging the photosensitive drum 51.

[0029] Each developing unit 60 has a developing frame 62 for accommodating toner. Each developing roller 61 is rotatably supported by the developing frame 62. Each drum unit 50 has a drum frame 52. Each photosensitive drum 51 is rotatably supported by the drum frame 52.

[0030] Each process unit P has a hinge 65 that connects the developing unit 60 to the drum unit 50 so that the developing roller 61 can swing between a pressing position where the developing roller 61 presses against the corresponding photosensitive drum 51 and a separating position where the developing roller 61 is separated from the corresponding photosensitive drum 51. Specifically, each developing frame 62 is swingably connected to the drum frame 52 by a hinge 65. With this configuration, the developing unit 60 can swing between a position where the developing roller 61 presses against the corresponding photosensitive drum 51 as shown in FIG. 2 and a position where the developing roller 61 is separated from the corresponding photosensitive drum 51 as shown in FIG. 3. Also, since the developing frame 62 and the drum frame 52 are connected by the hinge 65, the developing unit 60 cannot be separated from the drum unit 50. Each developing frame 62 has a separating lever 62A for swinging the developing frame 62 by being pushed by a cam 90 described later. The separating lever 62A extends downward for a long distance. Although not shown, the developing frame 62 is biased in the direction in which the developing roller 61 presses against the photosensitive drum 51 by a spring (not shown) in the rotational direction around the hinge 65.

[0031] Returning to FIG. 1, the cartridge tray TR is a tray on which a plurality of process units P can be mounted. The cartridge tray TR is slidable with respect to the housing 10. The cartridge tray TR can be pulled out from the opening 12 of the housing 10 and taken out, or mounted through the opening 12 when the front cover 11 is opened.

[0032] The exposure device 40 includes a laser diode, a deflector, a lens, and a mirror (not shown). The exposure device 40 is configured to emit a plurality of light beams indicated by a two-dot chain line for exposing a plurality of photosensitive drums 51 and expose the surfaces of the respective photosensitive drums 51.

[0033] The transfer unit 70 includes an endless belt 71, a driving roller 72, a driven roller 73, a tension roller 74, and a plurality of transfer rollers 75. The belt 71 is wound around the driving roller 72, the driven roller 73, and the tension roller 74. The outer surface of the belt 71 is disposed to face each photosensitive drum 51 of the plurality of process units P. Each transfer roller 75 is disposed inside the belt 71 so as to sandwich the belt 71 between each transfer roller 75 and the photosensitive drum 51.

[0034] The secondary transfer roller 78 is disposed behind the driven roller 73 so as to sandwich the belt 71 between the secondary transfer roller 78 and the driven roller 73. The secondary transfer roller 78 transfers the toner image on the belt 71 to the sheet S conveyed between the secondary transfer roller 78 and the driven roller 73.

[0035] The fixing device 80 is provided behind the plurality of process units P. The fixing device 80 includes a heating roller 81 and a pressure roller 82 disposed opposite to the heating roller 81.

[0036] The discharge roller 15 is disposed above the fixing device 80 so as to discharge the sheet S discharged from the fixing device 80 onto the discharge tray 13.

[0037] In the image forming unit 30, after the surface of the photosensitive drum 51 is uniformly charged by the charger, it is exposed by the light beam irradiated from the exposure device 40. As a result, an electrostatic latent image based on the image data is formed on the photosensitive drum 51. Further, the toner accommodated in the developing unit 60 is carried on the surface of the developing roller 61 and supplied from the developing roller 61 located at the contact position to the electrostatic latent image formed on the photosensitive drum 51. Thereby, a toner image is formed on the photosensitive drum 51. Next, the toner image on the photosensitive drum 51 is transferred to the belt 71 by passing between the transfer roller 75. Then, the toner image on the belt 71 is transferred to the surface of the sheet S conveyed between the belt 71 and the secondary transfer roller 78. Further, the sheet S onto which the toner image is transferred passes between the heating roller 81 and the pressure roller 82, and the toner image is thermally fixed to the sheet S. Thereafter, the sheet S is discharged onto the discharge tray 13 by the discharge roller 15.

[0038] Next, a configuration for pressing the developing roller 61 against the photosensitive drum 51 or separating them will be described. As shown in FIG. 2, the image forming apparatus 1 includes a plurality of cams 90, a color interlocking gear train 110, and a fourth cam gear 118 as mechanisms for separating the developing roller 61 from the photosensitive drum 51.

[0039] The plurality of cams 90 includes a first cam 90Y that moves the yellow developing roller 61Y between a pressure contact position and a separation position with respect to the corresponding photosensitive drum 51Y, a second cam 90M that moves the magenta developing roller 61M between a pressure contact position and a separation position with respect to the corresponding photosensitive drum 51M, a third cam 90C that moves the cyan developing roller 61C between a pressure contact position and a separation position with respect to the corresponding photosensitive drum 51C, and a fourth cam 90K that moves the black developing roller 61K between a pressure contact position and a separation position with respect to the corresponding photosensitive drum 51K. The first cam 90Y, the second cam 90M, the third cam 90C, and the fourth cam 90K are arranged in the order of the first cam 90Y, the second cam 90M, the third cam 90C, and the fourth cam 90K from upstream to downstream in the moving direction of the portion of the belt 71 facing the photosensitive drum 51.

[0040] The color-linked gear train 110 is a gear train that mechanically interlocks and rotates the first cam 90Y, the second cam 90M, and the third cam 90C. The color-linked gear train 110 includes a first cam gear 111, a second cam gear 112, a third cam gear 113, a 1-2 idle gear 114, and a 1-3 idle gear 115. The diameters of the respective cam gears 111, 112, 113 of the color-linked gear train 110 are the same. The first cam gear 111 is a gear integrally formed with the first cam 90Y and rotates integrally with the first cam 90Y. The second cam gear 112 is a gear integrally formed with the second cam 90M and rotates integrally with the second cam 90M. The third cam gear 113 is a gear integrally formed with the third cam 90C and rotates integrally with the third cam 90C. The 1-2 idle gear 114 is an idle gear that meshes with the first cam gear 111 and the second cam gear 112. The 2-3 idle gear 115 is an idle gear that meshes with the second cam gear 112 and the third cam gear 113. Note that the black fourth cam 90K is integrally formed with the fourth cam gear 118. The diameter of the fourth cam gear 118 is the same as the diameter of each gear of the color-linked gear train 110. The gears constituting the color-linked gear train 110 mesh with each other, so that when one gear rotates, the other gears also rotate mechanically in conjunction.

[0041] Each cam 90 has a cam crest 91. As shown in FIG. 2, the cam crests 91 of the first cam 90Y, the second cam 90M, and the third cam 90C are phase-shifted by a predetermined angle. The color-linked gear train 110 interlocks the respective cams 90 while maintaining this state where the phases are shifted by a predetermined amount.

[0042] Each cam 90 rotates by receiving a driving force from the motor M. Then, when the cam crest 91 is not in contact with the separation lever 62A as shown in FIG. 2, the developing roller 61 is pressed against the photosensitive drum 51 by the biasing force of a spring (not shown). On the other hand, as shown in FIG. 3, when the cam crest 91 is in contact with and pushed by the separation lever 62A, the developing roller 61 separates from the photosensitive drum 51.

[0043] Next, a configuration for rotating or stopping the rotation of the color interlocking gear train 110 and the fourth cam gear 118 will be described. As shown in FIG. 4, the image forming apparatus 1 includes a monochrome transmission gear train 120K for transmitting the driving force of the motor M to the fourth cam 90K, a monochrome clutch 150K, a color transmission gear train 120A for transmitting the driving force of the motor M to the color interlocking gear train 110, and a color clutch 150A as a configuration for transmitting the driving force of the motor M to the color interlocking gear train 110 and the fourth cam gear 118.

[0044] The motor M is provided with an output gear M1 on the output shaft. The motor M is controlled and driven by the control unit 2.

[0045] The monochrome transmission gear train 120K includes a first idle gear 121, a second idle gear 122, a third idle gear 123, and a fourth idle gear 124. The first idle gear meshes with the output gear M1. The second idle gear 122 meshes with the first idle gear 121. The third idle gear 123 meshes with the second idle gear 122. The fourth idle gear 124 meshes with the third idle gear 123.

[0046] The monochrome clutch 150K is a clutch that switches whether to transmit the driving force from the motor M to the fourth cam 90K by the monochrome transmission gear train 120K. As shown in FIGS. 5(a) and 5(b), the monochrome clutch 150K includes a clutch gear 151, a lever 155, and a solenoid actuator 157. The clutch gear 151 has a sector gear portion 152A provided with gear teeth on the outer periphery and a toothless portion 152B not provided with gear teeth. Although not shown in detail, the clutch gear 151 has an output gear 152C having the same diameter as the sector gear portion 152A, not meshing with the fourth idle gear 124, and meshing with the fourth cam gear 118 at a position axially displaced from the sector gear portion 152A. Further, the clutch gear 151 has a notch 153 in a part of the outer periphery of the portion axially displaced from the sector gear portion 152A.

[0047] The lever 155 is swingable about a swing axis 155X. The lever 155 has a protrusion 155A engageable with the notch 153. The lever 155 is biased by a spring (not shown) in a direction in which the protrusion 155A engages with the notch 153. In a state where the protrusion 155A of the lever 155 is engaged with the notch 153, the toothless portion 152B faces the fourth idle gear 124, so that the driving force is not transmitted from the fourth idle gear 124 to the clutch gear 151, resulting in a disengaged state. Further, the monochrome clutch 150K has a spring (not shown) that biases the clutch gear 151 clockwise in the figure in the disengaged state of FIG. 5(a).

[0048] The solenoid actuator 157 has an operating part 157A connected to the lever 155. When a current is passed through the solenoid actuator 157 by the control unit 2, the operating part 157A is drawn in for a predetermined time. When the clutch gear 151 rotates clockwise from the disengaged state shown in Fig. 5(a) and the sector gear part 152A meshes with the fourth idle gear 124, as shown in Fig. 5(b), a transmission state is established in which the fourth idle gear 124 rotates the clutch gear 151 clockwise. After the sector gear part 152A meshes with the fourth idle gear 124, the current to the solenoid actuator 157 is stopped. When the clutch gear 151 has made one full rotation from the disengaged state, the missing tooth part 152B again faces the fourth idle gear 124, and the protrusion 155A engages with the notch 153, causing the clutch gear 151 to stop.

[0049] Since the diameter of the output gear 152C of the clutch gear 151 is half the diameter of the fourth cam gear 118, when the clutch gear 151 rotates one full turn and stops, the fourth cam gear 118 rotates half a turn and stops. In this way, each time the solenoid actuator 157 is actuated once, the fourth cam gear 118 rotates half a turn, and accordingly, the fourth cam 90K rotates 180 degrees.

[0050] The color transmission gear train 120A has the first idle gear 121, the second idle gear 122, the third idle gear 123, and the fourth idle gear 124, similar to the monochrome transmission gear train 120K. The first idle gear 121 meshes with the output gear M1. The second idle gear 122 meshes with the first idle gear 121. The third idle gear 123 meshes with the second idle gear 122. The fourth idle gear 124 meshes with the third idle gear 123

[0051] The color clutch 150A is a clutch that switches whether to transmit the driving force from the motor M to the color interlocking gear train 110 by the color transmission gear train 120A. Since the color clutch 150A has the same configuration as the monochrome clutch 150K, a detailed description thereof is omitted. The sector gear portion 152A of the color clutch 150A can mesh with the fourth idle gear 124 of the color transmission gear train 120A.

[0052] In the standby state where image formation is not being performed, as shown in FIG. 2, the control unit 2 positions all the developing rollers 61 at the separated positions. Then, when a print job is received, the transfer unit 70 is driven to move the belt 71, and the photosensitive drum 51 is sequentially exposed by the exposure device 40 in the order of yellow, magenta, cyan, and black. Then, at a predetermined timing, the solenoid actuators 157 of the color clutch 150A and the monochrome clutch 150K are moved to rotate the first cam 90Y, the second cam 90M, the third cam 90C, and the fourth cam 90K.

[0053] As a result, from the state where all the developing rollers 61 of all colors shown in FIG. 2 are at the separated positions, it is possible to transition to the state where the yellow developing roller 61Y is at the pressure contact position shown in FIG. 6(a), the state where the yellow and magenta developing rollers 61Y, 61M are at the pressure contact positions shown in FIG. 6(b), the state where the yellow, magenta, and cyan developing rollers 61Y, 61M, 61C are at the pressure contact positions shown in FIG. 6(c), and the state where all the developing rollers 61 are at the pressure contact positions shown in FIG. 6(d).

[0054] Then, at the timing after the transfer of the yellow toner image to the belt 71 is completed, the solenoid actuators 157 of the color clutch 150A and the monochrome clutch 150K are moved for a predetermined time. As a result, from the state where all the developing rollers 61 of all colors shown in FIG. 6(d) are at the pressure contact positions, it is possible to transition to the state where the yellow developing roller 61Y is at the separated position shown in FIG. 7(a), the state where the yellow and magenta developing rollers 61Y, 61M are at the separated positions shown in FIG. 7(b), the state where the yellow, magenta, and cyan developing rollers 61Y, 61M, 61C are at the separated positions shown in FIG. 7(c), and the state where all the developing rollers 61 are at the separated positions shown in FIG. 7(d).

[0055] As described above, according to the image forming apparatus 1 of the present embodiment, the driving force of the motor M drives the color interlocking gear train 110 via the color transmission gear train 120A and the color clutch 150A. Then, the color interlocking gear train 110 mechanically interlocks and rotates the first cam 90Y, the second cam 90M, and the third cam 90C. At this time, since the color interlocking gear train 110 maintains the state where the phases of the first cam 90Y, the second cam 90M, and the third cam 90C are shifted and interlocks each cam, the yellow, magenta, and cyan developing rollers 61Y, 61M, 61C can be brought into pressure contact or separated at a predetermined shifted timing.

[0056] And in the present embodiment, the color transmission gear train 120A engages with the third cam gear 113 close to the fourth cam gear 118, thereby suppressing the phase error between the third cam gear 113 and the fourth cam gear 118 and improving the accuracy of the pressure contact and separation timing of the four-color developing rollers 61.

[0057] [Second Embodiment] Next, the second embodiment will be described. As shown in FIG. 8, in the image forming apparatus 1 of the second embodiment, the color transmission gear train 120A engages not with the third cam gear 113 but with the second cam gear 112 among the gears of the color interlocking gear train 110 as compared with the first embodiment. Specifically, the output gear 152C of the color clutch 150A meshes with the second cam gear 112. Also, in order to cope with the difference in the positions of the third cam gear 113 and the second cam gear 112 as compared with the first embodiment, the arrangement of the gears of the color transmission gear train 120A is made different.

[0058] Also according to such a second embodiment, since the color interlocking gear train 110 maintains the state where the phases of the first cam 90Y, the second cam 90M, and the third cam 90C are shifted and interlocks each cam, the yellow, magenta, and cyan developing rollers 61Y, 61M, 61C can be brought into pressure contact or separated at a predetermined shifted timing.

[0059] And in the second embodiment, since the color transmission gear train 120A engages with the second cam gear 112 located at the center of the color interlocking gear train 110, the errors of the phases of the first cam gear 111 and the third cam gear 113 from the design values can be minimized.

[0060] [Third Embodiment] Next, the third embodiment will be described. As shown in FIG. 9, in the image forming apparatus 1 according to the third embodiment, the color transmission gear train 120A engages with the first cam gear 111 instead of engaging with the second cam gear 112 among the gears of the color interlocking gear train 110, as compared with the first embodiment. Specifically, the output gear 152C of the color clutch 150A meshes with the first cam gear 111. Further, in order to cope with the difference in the positions of the third cam gear 113 and the first cam gear 111 as compared with the first embodiment, the arrangement of each gear of the color transmission gear train 120A is made different.

[0061] Also in such a third embodiment, the color interlocking gear train 110 maintains the state where the phases of the first cam 90Y, the second cam 90M, and the third cam 90C are shifted and interlocks each cam, so that the yellow, magenta, and cyan developing rollers 61Y, 61M, 61C can be brought into pressure contact with or separated from each other at a predetermined shifted timing.

[0062] And in the third embodiment, since the color transmission gear train 120A engages with the first cam gear 111 that is the farthest from the fourth cam gear 118, it is easy to layout the color transmission gear train 120A without interfering with the monochrome transmission gear train 120K, and the image forming apparatus 1 can be downsized. Also, for the pressure contact operation at the start of printing, the timing of the start of rotation of the first cam 90Y is important. In the third embodiment, since the color transmission gear train 120A engages with the first cam gear 111 located at the most upstream in the moving direction of the belt 71, by shortening the gear train that engages with the first cam 90Y, the error of the phase of the first cam gear 111 from the design value can be minimized.

[0063] [Fourth Embodiment] Next, the fourth embodiment will be described. As shown in FIG. 10, in the image forming apparatus 1 of the fourth embodiment, compared with the first embodiment, the color transmission gear train 120A engages with the 1-2 idle gear 114 instead of engaging with the second cam gear 112 among the gears of the color interlocking gear train 110. Specifically, the color transmission gear train 120A further has a fifth idle gear 125, and the output gear 152C of the color clutch 150A engages with the 1-2 idle gear 114 via the fifth idle gear 125. Also, in order to correspond to the difference in the positions of the third cam gear 113 and the 1-2 idle gear 114 compared with the first embodiment, the arrangement of the gears of the color transmission gear train 120A is made different.

[0064] Also in such a fourth embodiment, since the color interlocking gear train 110 maintains the state where the phases of the first cam 90Y, the second cam 90M, and the third cam 90C are shifted and interlocks the cams, the yellow, magenta, and cyan developing rollers 61Y, 61M, 61C can be pressed or separated at a predetermined shifted timing.

[0065] And in the fourth embodiment, since the color transmission gear train 120A engages with the 1-2 idle gear 114, the error from the design value of the phase between the first cam gear 111 and the third cam gear 113 can be suppressed. Also, since the 1-2 idle gear 114 is relatively far from the fourth cam gear 118, it is easy to layout the color transmission gear train 120A without interfering with the monochrome transmission gear train 120K, and the image forming apparatus 1 can be downsized.

[0066] [Fifth Embodiment] Next, the fifth embodiment will be described. As shown in FIG. 11, in the image forming apparatus 1 according to the fifth embodiment, compared with the first embodiment, the color transmission gear train 120A engages with the 2-3 idle gear 115 instead of engaging with the third cam gear 113 among the gears of the color interlocking gear train 110. Specifically, the color transmission gear train 120A further has a fifth idle gear 125, and the output gear 152C of the color clutch 150A engages with the 1-2 idle gear 114 via the fifth idle gear 125. Since the first idle gear 121 is close to the 2-3 idle gear, the third idle gear 123 and the fourth idle gear 124 are omitted, and the second idle gear 122 is arranged so as to be engageable with the color clutch 150A.

[0067] Also in the fourth embodiment like this, since the color interlocking gear train 110 maintains the state where the phases of the first cam 90Y, the second cam 90M, and the third cam 90C are shifted and interlocks the cams, the yellow, magenta, and cyan developing rollers 61Y, 61M, 61C can be pressed or separated at a predetermined shifted timing.

[0068] And in the fifth embodiment, since the color transmission gear train 120A engages with the 2-3 idle gear 115, the phase error between the second cam gear 112 for magenta and the third cam gear 113 for cyan and the fourth cam gear 118 can be relatively suppressed. Also, by the color transmission gear train 120A engaging with the 2-3 idle gear 115, the error from the design value of the phase between the first cam gear 111 and the third cam gear 113 can be suppressed.

[0069] Although the embodiments of the invention have been described above, the present invention is not limited to the above embodiments and can be appropriately modified and implemented as exemplified below.

[0070] For example, in the above embodiment, the color clutch 150A was disposed between the color transmission gear train 120A and the color interlocking gear train 110, but it may be disposed between the output gear M1 of the motor M and the color transmission gear train 120A, or may be disposed between any two gears of the color transmission gear train 120A. Similarly, the monochrome clutch 150K was disposed between the monochrome transmission gear train 120K and the fourth cam gear 118, but it may be disposed between the output gear M1 of the motor M and the monochrome transmission gear train 120K, or may be disposed between two gears of the monochrome transmission gear train 120K.

[0071] In the above embodiment, the first cam, the second cam, the third cam, and the fourth cam were arranged in the order of the first cam, the second cam, the third cam, and the fourth cam from upstream to downstream in the moving direction of the portion facing the photosensitive drum of the belt, but they may be arranged in the order of the first cam, the second cam, the third cam, and the fourth cam from downstream to upstream.

[0072] In the above embodiment, an image forming apparatus of an intermediate transfer type that transfers the toner image to the sheet S after transferring the toner image to the belt 71 was exemplified, but the present invention can also be applied to an image forming apparatus of a type that directly transfers from the photosensitive drum 51 to the sheet S.

[0073] Further, the image forming apparatus is not limited to a printer, and may be a multifunction machine, a copying machine, or the like.

[0074] Each element described in the above embodiment and the modification examples may be implemented in any combination.

Explanation of Reference Numerals

[0075] 1 Image forming apparatus 50 Drum unit 51 Photosensitive drum 52 Drum frame 60 Developing unit 61 Developing roller 65 Hinge 70 Transfer unit 71 Belt 90 Cam 90Y First Cam 90M Second Cam 90C Third Cam 90K Fourth Cam 110 Color Interlocking Gear Train 111 First Cam Gear 112 Second Cam Gear 113 Third Cam Gear 118 Fourth Cam Gear 120A Color Transmission Gear Train 120K Monochrome Transmission Gear Train 150A Color Clutch 150K Monochrome Clutch M Motor

Claims

1. a plurality of process units corresponding to a first color, a second color, a third color, and a fourth color, each process unit including a developing unit having a developing roller and a drum unit having a photosensitive drum; a transfer unit having a belt disposed facing each of the photosensitive drums of the plurality of process units; A motor; a first cam for moving a developing roller of a first color between a pressure-contact position and a spaced position with respect to a corresponding photosensitive drum; a second cam for moving the developing roller of a second color between a pressure-contact position and a spaced position with respect to the corresponding photosensitive drum; a third cam for moving the developing roller of a third color between a pressure-contact position and a spaced position with respect to the corresponding photosensitive drum; a fourth cam for moving the developing roller of a fourth color between a pressure-contact position and a spaced position with respect to the corresponding photosensitive drum; a color interlocking gear train that rotates the first cam, the second cam, and the third cam in a mechanically interlocking manner, the color interlocking gear train interlocking each cam while maintaining a phase shifted state of the first cam, the second cam, and the third cam; a monochrome transmission gear train that transmits the driving force of the motor to the fourth cam; a monochrome clutch that switches whether or not a driving force is transmitted from the motor to the fourth cam via the monochrome transmission gear train; a color transmission gear train that transmits the driving force of the motor to the color interlocking gear train; a color clutch that switches whether or not the driving force is transmitted from the motor to the color interlocking gear train via the color transmission gear train.

2. 2. The image forming apparatus according to claim 1, wherein the process unit has a hinge that connects the developing unit to the drum unit so that the developing unit can swing between a pressure-contact position in which the developing roller is pressed against the corresponding photosensitive drum and a spaced position in which the developing roller is spaced from the corresponding photosensitive drum.

3. The color interlocking gear train includes: a first cam gear that rotates integrally with the first cam; a second cam gear that rotates integrally with the second cam; a third cam gear that rotates integrally with the third cam; a 1-2 idle gear which is an idle gear connecting the first cam gear and the second cam gear; a 2-3 idle gear which is an idle gear connecting the second cam gear and the third cam gear; having the first cam, the second cam, the third cam, and the fourth cam are arranged in this order from upstream to downstream or from downstream to upstream in a moving direction of a portion of the belt facing the photosensitive drum, 3. The image forming apparatus according to claim 1, wherein the color transmission gear train is engaged with the third cam gear.

4. The color interlocking gear train includes: a first cam gear that rotates integrally with the first cam; a second cam gear that rotates integrally with the second cam; a third cam gear that rotates integrally with the third cam; a 1-2 idle gear which is an idle gear connecting the first cam gear and the second cam gear; a 2-3 idle gear which is an idle gear connecting the second cam gear and the third cam gear; having the first cam, the second cam, the third cam, and the fourth cam are arranged in this order from upstream to downstream or from downstream to upstream in a moving direction of a portion of the belt facing the photosensitive drum, 3. The image forming apparatus according to claim 1, wherein the color transmission gear train is engaged with the second cam gear.

5. The color interlocking gear train includes: a first cam gear that rotates integrally with the first cam; a second cam gear that rotates integrally with the second cam; a third cam gear that rotates integrally with the third cam; a 1-2 idle gear which is an idle gear connecting the first cam gear and the second cam gear; a 2-3 idle gear which is an idle gear connecting the second cam gear and the third cam gear; having the first cam, the second cam, the third cam, and the fourth cam are arranged in this order from upstream to downstream or from downstream to upstream in a moving direction of a portion of the belt facing the photosensitive drum, 3. The image forming apparatus according to claim 1, wherein the color transmission gear train is engaged with the first cam gear.

6. The color interlocking gear train includes: a first cam gear that rotates integrally with the first cam; a second cam gear that rotates integrally with the second cam; a third cam gear that rotates integrally with the third cam; a 1-2 idle gear which is an idle gear connecting the first cam gear and the second cam gear; a 2-3 idle gear which is an idle gear connecting the second cam gear and the third cam gear; having the first cam, the second cam, the third cam, and the fourth cam are arranged in this order from upstream to downstream or from downstream to upstream in a moving direction of a portion of the belt facing the photosensitive drum, 3. The image forming apparatus according to claim 1, wherein said color transmission gear train is engaged with said 1-2 idle gear.

7. The color interlocking gear train includes: a first cam gear that rotates integrally with the first cam; a second cam gear that rotates integrally with the second cam; a third cam gear that rotates integrally with the third cam; a 1-2 idle gear which is an idle gear connecting the first cam gear and the second cam gear; a 2-3 idle gear which is an idle gear connecting the second cam gear and the third cam gear; having the first cam, the second cam, the third cam, and the fourth cam are arranged in this order from upstream to downstream or from downstream to upstream in a moving direction of a portion of the belt facing the photosensitive drum, 3. The image forming apparatus according to claim 1, wherein said color transmission gear train is engaged with said 2-3 idle gear.

8. the first color is yellow; the second color is magenta; the third color is cyan; 8. The image forming apparatus according to claim 1, wherein the fourth color is black.

9. The belt has a toner image transferred from the photosensitive drum, 9. The image forming apparatus according to claim 1, further comprising a secondary transfer roller for transferring the toner image on the belt onto a sheet.

Images