JP2008090107A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To reduce the degradation of image quality due to gears transmitting driving force to a developing roll. <P>SOLUTION: A bevel gear 68 is connected to a driving gear 76 driven by a motor gear 74 rotated by a motor 72, via a coupling 78. Bevel gears 70Y, 70M and 70C are rotatably supported by support sections 84Y, 84M and 84C slidably supporting a shaft 86, respectively. A bevel gear 88 is arranged in the top end portion of the shaft 86 and bevel gears 90C, 90M and 90Y are arranged with a predetermined interval in order below the bevel gear 88. The bottom end of the shaft 86 is rotatably supported by a cam mechanism 92 and the shaft 86 is supported nearly perpendicularly by the support sections 84Y, 84M and 84C. The cam mechanism 92 converts the rotational motion of a stepping motor 94 arranged below to a reciprocating motion in a nearly vertical direction and moves up and down the shaft 86 in the nearly vertical direction. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an image forming apparatus such as a printer, a copying machine, or a facsimile.

The latent images formed on the four photoconductors are respectively developed with Y (yellow), M (magenta), C (cyan), and K (black) toners, and the developed toner images are superimposed to form a color image. An image forming apparatus is known.
In this type of image forming apparatus, it is known that all developing rolls are driven to rotate when a full-color image is formed, and the driving rotation of the developing roll of the color developer unit is stopped or decelerated when a monochrome image is formed (see Patent Document 1). .

JP 2002-006579 A

  However, if the driving force of one drive transmission gear is sequentially transmitted to the three developing rolls (C, M, Y), the number of gears for transmitting the driving force increases, resulting in banding and color misregistration. There was a problem.

  SUMMARY An advantage of some aspects of the invention is that it provides an image forming apparatus capable of reducing deterioration in image quality due to a gear that transmits driving force to a developing roll.

  In order to achieve the above object, the present invention is characterized in that only the first developing roll, the second developing roll, and the first developing roll are driven, or the first developing roll and Switching means for driving or switching the second developing roll, and the switching means is provided on a shaft portion and the shaft portion, and is provided on each of the first developing roll and the second developing roll. An image forming apparatus having a second gear meshing with the formed first gear.

  Preferably, the switching means includes a motor that rotates the shaft portion forward and backward, and a clutch that transmits only rotation in one direction of the shaft portion to each of the first gear and the second gear. Have.

  Preferably, the switching unit includes a displacement driving unit that displaces the shaft portion in the axial direction.

  Preferably, the switching unit has a coupling for driving the first developing roll.

  Preferably, the first gear and the second gear are bevel gears.

  Preferably, the second gear is provided with a tracking roll for determining a meshing position with respect to the first gear.

  Preferably, the second gear is provided with play in the axial direction with respect to the shaft portion, and is biased toward the first gear.

  Preferably, the second developing roll is provided with a positioning portion for positioning with respect to a support portion that slidably supports the shaft portion.

  Preferably, the apparatus further includes a third developing roll and a fourth developing roll, and the switching unit is provided in the shaft portion, and includes the third developing roll and the fourth developing roll. It further has a second gear meshing with the first gear formed on each of them, and drives only the first developing roll, or the first developing roll, the second developing roll, the third The developing roll and the fourth developing roll are driven or switched.

  According to the present invention, it is possible to reduce the deterioration of the image quality due to the gear that transmits the driving force to the developing roll.

Next, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 is a cross-sectional view showing an outline of an image forming apparatus 10 according to an embodiment of the present invention. The image forming apparatus 10 includes an image forming apparatus main body 12, an image forming unit 14 in the image forming apparatus main body 12, a recording medium supply device 54 that supplies a recording medium such as paper to the image forming unit 14, A power supply unit 16 and a control unit 66 used as control means for controlling each unit constituting the image forming apparatus 10 are provided. In addition, a recording medium discharge unit 15 for discharging a recording medium on which an image has been formed is provided on the upper part of the image forming apparatus main body 12.

  The image forming unit 14 is of an electrophotographic system for forming a color image. The drum-shaped photoconductors 22Y, 22M, 22C, and 22B as image carriers for carrying a developer image, and the photoconductors 22Y and 22M. , 22C, 22B as charging means provided with a charging roll for uniformly charging, and latent images for writing an electrostatic latent image by light on each of the photoreceptors 22Y, 22M, 22C, 22B. Optical writing devices 26Y, 26M, 26C, and 26B as image forming means, and developing rollers 28Y, 28M, and 28C that develop the latent images written on the photosensitive members 22Y, 22M, 22C, and 22B with a developer (toner) 28B, a transfer unit 42 used as transfer means for transferring the developer images formed on the respective photoconductors 22Y, 22M, 22C, and 22B to a recording medium, and a transfer unit 42. A photoreceptor 22Y after transferring the developer image by the unit 42 is made, 22M, 22C, a cleaning device 30Y for cleaning the waste developer example scraped by remaining in 22B, 30M, 30C, and 30B.

  Each of the optical writing devices 26Y, 26M, 26C, and 26B includes a laser exposure device. The optical writing device 26Y corresponds to a laser beam corresponding to a yellow image on the photosensitive member 22Y, and the optical writing device 26M corresponds to a magenta image on the photosensitive member 22M. The optical writing device 26C emits a laser beam corresponding to a cyan image to the photoconductor 22C, and the optical writing device 26B emits a laser beam corresponding to a black image to the photoconductor 22B, and the photoconductors 22Y, 22M, and 22C. , 22B are written with electrostatic latent images, respectively.

  Among the members included in the image forming unit 14, the photosensitive members 22Y, 22M, 22C, and 22B, the charging devices 24Y, 24M, 24C, and 24B, the developing rollers 28Y, 28M, 28C, and 28B, and the cleaning devices 30Y, 30M, 30C, and 30B include It is integrated as a process cartridge 32 used as an exchange unit, and is detachably attached to the image forming apparatus main body 12 from the front side (left side in FIG. 1).

Further, in the process cartridge 32, developer cartridges 34Y, 34M, 34C, and 34B as replacement units that respectively store the developer supplied to the developing rolls 28Y, 28M, 28C, and 28B are provided to the image forming apparatus main body 12. It is detachably mounted from the side.
In the developer cartridges 34Y, 34M, 34C, 34B, developer accommodating chambers 36Y, 36M, 36C, 36B for accommodating a developer such as toner and waste developer accommodating chambers 38Y, 38M, 38C, 38B are respectively formed inside. Has been.

  For example, the developer cartridge 34Y is filled (contained) with yellow toner, the developer cartridge 34M with magenta toner, the developer cartridge 34C with cyan toner, and the toner cartridge 34B with black toner.

  The transfer unit 42 is disposed so as to contact the photoreceptors 22Y, 22M, 22C, and 22B of the process cartridge 32. The transfer unit 42 is integrated as a unit, and includes two support rolls 44a and 44b, a conveyance belt 46 as a conveyance unit for conveying a recording medium or an image, and an adsorption unit that adsorbs the recording medium to the conveyance belt 46. And transfer rolls 50Y, 50M, 50C, and 50B for transferring the developer images formed on the photoreceptors 22Y, 22M, 22C, and 22B to a recording medium that is being conveyed by the conveyance belt 46, respectively. It is attached.

  The suction roll 48 is provided in pressure contact with the support roll 44 a via the transport belt 46, and a voltage is applied from the power supply unit 16 to electrostatically attract the recording medium to the transport belt 46.

  The transfer rolls 50Y, 50M, 50C, and 50B are each applied with a transfer bias, and the developer images formed on the photoconductors 22Y, 22M, 22C, and 22B are sequentially transferred onto the recording medium being transported by the transport belt 46. Then, a color developer image is formed by superimposing developer images of four colors of yellow, magenta, cyan, and black on the recording medium.

  In addition, a fixing device 52 that fixes the developer image transferred to the recording medium by the transfer unit 42 to the recording medium is provided in the upper part of the image forming apparatus main body 12. The fixing device 52 includes a heating roll 52a and a pressure roll 52b. The recording medium passing between the heating roll 52a and the pressure roll 52b is heated and pressed to fix the developer image on the recording medium. It is like that.

  Further, the image forming apparatus main body 12 is provided with a conveyance path 60 for conveying the recording medium supplied from the recording medium supply device 54 to the recording medium discharge unit 15, and along the conveyance path 60, the recording medium is provided. A resist roll 62, a transfer unit 42, a fixing device 52, and a discharge roll 64 are arranged in this order from the upstream side in the transport direction. The discharge roll 64 discharges the recording medium conveyed from the fixing device 52 to the recording medium discharge unit 15.

Next, an outline of a mechanism for driving the developing rolls 28Y, 28M, 28C, and 28B will be described.
FIG. 2 is a front view showing an example of a mechanism for driving the developing rolls 28Y, 28M, 28C, and 28B (a state in which only the developing roll 28B is driven). One end of the developing roll 28B is provided with a bevel gear (bevel gear) 68 as a developing roll gear, and the other end is rotatably supported by the image forming apparatus main body 12 (not shown). The developing rolls 28Y, 28M, and 28C are provided with bevel gears 70Y, 70M, and 70C at one end, and the other end is rotatably supported by the image forming apparatus main body 12 (not shown). Yes. The bevel gear 68 may be formed integrally with the developing roll 28B, and the bevel gears 70Y, 70M, and 70C may be formed integrally with the developing rolls 28Y, 28M, and 28C, respectively.

  The bevel gear 68 is connected via a coupling 78 to a drive gear 76 that is driven by a motor gear 74 that is rotated by a motor 72. The coupling 78 includes a drive shaft side component 80 fixed to the drive gear 76 and a driven shaft component 82 fixed to the bevel gear 68, and transmits the rotation of the drive gear 76 to the bevel gear 68. .

  The bevel gears 70Y, 70M, and 70C are rotatably supported by support portions 84Y, 84M, and 84C that support a shaft 86, which will be described later, so as to slide and rotate. The support portions 84Y, 84M, and 84C are respectively fixed to the image forming apparatus main body 12 (not shown).

  A bevel gear 88 is disposed at the upper end of the shaft (shaft portion) 86, and bevel gears 90 </ b> C, 90 </ b> M, and 90 </ b> Y are sequentially disposed below the bevel gear 88 at predetermined intervals. Further, the lower end of the shaft 86 is rotatably supported by the cam mechanism 92, and is supported substantially vertically by the support portions 84Y, 84M, and 84C. The cam mechanism 92 converts the rotational motion of the stepping motor 94 disposed below into a reciprocating motion in a substantially vertical direction, and moves the shaft 86 up and down in a substantially vertical direction.

  The stepping motor 94 is fixed to the image forming apparatus main body 12 (not shown), and the bevel gears 88, 90C, 90M, and 90Y are changed to the bevel gear 68, the bevel gears 70C, 70M, and 70Y according to the control of the control unit 66. Are rotated so as to switch between a full-color operation mode (see FIG. 8) that meshes with each other and a monochrome operation mode (state shown in FIG. 2) that is separated from the bevel gear 68 and the bevel gears 70C, 70M, and 70Y.

Next, the bevel gears 90Y, 90M, 90C, and 88 will be described in detail.
The bevel gears 90Y, 90M, 90C, and 88 differ only in the arrangement with respect to the shaft 86, respectively, and are configured (similarly shaped) in substantially the same manner. Therefore, the bevel gear 90Y will be described below.

In FIG. 3, the details of the bevel gear 90Y and its periphery are shown.
The bevel gear 90 </ b> Y is provided with a through-hole 95 in the axial direction, and the shaft 86 passes through the through-hole 95 so that the teeth are positioned above, and is slidable with respect to the shaft 86. . The bevel gear 90Y is provided with a tracking roll 96 at the upper part (one axial end) and a receiving hole 98 at the lower part (the other axial end).

  The tracking roll 96 determines the axial position of the bevel gear 90Y with respect to the bevel gear 70Y by making sliding contact with a tracking roll 110 described later. The receiving hole 98 is an axial hole formed so that a biasing member 108 to be described later can stably receive a biasing force against the bevel gear 90Y. The receiving hole 98 is formed with a pin receiving hole 100 with which a pin 106 described later is engaged.

  The shaft 86 passing through the bevel gear 90 </ b> Y has an E ring 102 attached below the bevel gear 90 </ b> Y, an E ring 104 attached above the bevel gear 90 </ b> Y, and a pin 106 between the E ring 102 and the E ring 104. Is attached. As shown in FIG. 4, for example, the pin 106 protrudes in a substantially horizontal direction so as to be orthogonal to the shaft 86 and engages with the pin receiving hole 100 with play in a substantially vertical direction. A biasing member 108 such as a spring is provided between the E ring 102 and the bevel gear 90Y. The urging member 108 surrounds the shaft 86 and has a lower end in contact with the E-ring 102 and an upper end in contact with the receiving hole 98.

  On the other hand, the bevel gear 70Y meshing with the bevel gear 90Y is provided with a tracking roll 110 on the shaft 86 side, and further with a hemispherical protrusion 112 on the shaft 86 side of the tracking roll 110. The protrusion 112 determines the position of the bevel gear 70 </ b> Y with respect to the support 84 </ b> Y by engaging the hole 114 having a V-shaped cross section formed in the support 84 </ b> Y. That is, the protruding portion 112 is a positioning portion for positioning the developing roll 28Y with respect to the support portion 84Y. The tracking roll 110 determines the distance between the teeth of the bevel gear 70 </ b> Y and the support portion 84 </ b> Y together with the protruding portion 112. Further, the tracking roll 110 determines the distance between the teeth of the bevel gear 70Y and the teeth of the bevel gear 90Y when the tracking roll 96 comes into sliding contact with the operation of the stepping motor 94 described later.

Next, the cam mechanism 92 and its periphery will be described in detail.
5 to 7, the details of the cam mechanism 92 and its periphery are shown.
The cam mechanism 92 includes a rotating part (driving node) 116 that is rotated by a stepping motor 94, and an elevating part (driven node) 118 that moves up and down in accordance with the rotation of the rotating part 116. The rotating part 116 has a lower end fixed to a rotating part of the stepping motor 94, a rotating shaft 120 that extends substantially vertically, and an upper part of the rotating shaft 120, and is arranged in a substantially horizontal direction so as to be orthogonal to the rotating shaft 120. And a protruding rotating pin 122.

  The elevating part 118 has a cylindrical elevating part main body 123, and two opposed slots 124 are formed on the side surface of the elevating part main body 123. The slots 124 are staggered with respect to the longitudinal direction of the lifting / lowering unit main body 123 so that the ends of the rotating pins 122 are in sliding contact with each other and the rotational movement of the rotating pins 122 is converted into the lifting / lowering movement (linear movement) of the lifting / lowering section 118. It is a long hole having a portion inclined to the surface. Further, the slot 124 is configured such that the upper end portion and the lower end portion thereof are substantially horizontal, and the locking torque of the stepping motor 94 is reduced when the elevating part 118 is displaced to a predetermined position. Furthermore, as for the raising / lowering part 118, the two rotation stoppers 126 protrude from the side surface of the raising / lowering part main body 123, for example. The rotation stopper 126 slides in the vertical direction along a guide (not shown) provided in the image forming apparatus main body 12, and prevents the elevating unit 118 from rotating with the rotation of the rotation pin 122. At the same time, the direction in which the elevating unit 118 is displaced is stabilized.

  On the upper surface of the elevating / lowering portion 118, a hole portion 130 is provided for rotatably supporting a through portion 128, which is a portion in which the shaft diameter near the lower end of the shaft 86 is reduced. Since the diameter of the part above and below the penetrating part 128 is larger than the diameter of the hole part 130, the shaft 86 moves up and down as the lifting part 118 moves up and down. Further, the shaft 86 may be provided with play in the axial direction with respect to the hole 130.

  Therefore, when the elevating unit 118 is raised by the rotation of the rotating unit 116, the bevel gears 88, 90C, 90M, and 90Y are positioned and meshed with the bevel gears 68, 70C, 70M, and 70Y, as shown in FIG. The bevel gear 68 transmits the driving force of the motor 72 transmitted through the drive gear 76 to the bevel gear 88. Here, the bevel gear 88 may be positioned by the sliding roll 96 in sliding contact with the tracking roll provided on the coupling 78 side of the bevel gear 68, or may be positioned by the coupling 78.

  The bevel gear 88 transmits the driving force transmitted from the bevel gear 68 via the shaft 86 to the bevel gears 90C, 90M, and 90Y. That is, when the elevating unit 118 is raised by rotating the rotating unit 116, the driving force of the motor 72 can be transmitted to drive the bevel gears 68, 70C, 70M, and 70Y, and the developing rolls 28B, 28C, 28M, 28Y can be rotated.

Next, a modified example of the mechanism for driving the developing rolls 28Y, 28M, 28C, 28B will be described.
FIG. 9 is a front view showing a first modification of the mechanism for driving the developing rolls 28Y, 28M, 28C, and 28B (a state in which only the developing roll 28B is driven).
In the first modified example of the mechanism for driving the developing rolls 28Y, 28M, 28C, and 28B, substantially the same parts as those constituting the embodiment shown in FIG. It is.

The drive gear 132 has a spur tooth portion 134 at one end and a bevel tooth portion 136 at the other end, and is rotatably supported by the image forming apparatus main body 12 (not shown). The spur tooth portion 134 is driven by a motor gear 74 that is rotated by a motor 72. The bevel tooth portion 136 transmits the driving force received by the spur tooth portion 134 to the bevel gear 138 or the bevel gear 140 described later.
The developing roll 28B is provided with a bevel gear 68 as a developing roll gear at one end, and both ends are rotatably supported by the image forming apparatus main body 12 (not shown). Further, like the bevel gear 70Y and the like, the developing roll 28B may be rotatably supported at one end on the shaft 137 side by a support portion fixed to the image forming apparatus main body 12.

The shaft 137 has a bevel gear 138 disposed at the upper end, and bevel gears 140, 90C, 90M, and 90Y are sequentially disposed below the bevel gear 138 at a predetermined interval. The bevel gear 138 and the bevel gear 140 have substantially the same configuration as the bevel gear 90Y and the like, and the pin receiving hole 100 and the pin 106 are engaged like the bevel gear 90Y. However, the bevel gear 138 is provided such that teeth are located below the shaft 137, and the bevel gear 140 is provided so that teeth are located above the shaft 137.
The bevel gear 138 may be provided so as to be rotatable with respect to the shaft 137 without the pin 106 engaging with the pin receiving hole 100.

Further, the lower end of the shaft 137 is lifted and lowered in a substantially vertical direction by a solenoid 142 and is rotatably supported. The shaft 137 is supported by the support portions 84Y, 84M, and 84C so as to slide and rotate substantially vertically. The solenoid 142 is operated in the monochrome operation mode (the state shown in FIG. 9) in which the bevel gear 138 meshes with the bevel gear 68 according to the control of the control unit 66, and the bevel gears 140, 90C, 90M, and 90Y are the bevel gear 68, the umbrella. The shaft 137 is moved up and down in a substantially vertical direction so as to switch between the full-color operation mode (the state shown in FIG. 10) that meshes with the gears 70C, 70M, and 70Y.
Therefore, in the monochrome operation mode, the driving force of the motor 72 is transmitted to the developing roll 28B via the bevel teeth 136, 138, and 68. In the full color operation mode, the driving force of the motor 72 is transmitted to the developing roll 28B via 136, 140, 68, and is transmitted to the developing rolls 28C, 28M, 28Y via the bevel teeth 136, 140 and the shaft 137. The
The bevel gear 138 and the bevel gear 140 are positioned by the tracking roll 96 being in sliding contact with the bevel tooth portion 136 and the tracking roll 110 of the bevel gear 68, respectively.

FIG. 11 is a front view showing a second modification of the mechanism for driving the developing rolls 28Y, 28M, 28C, and 28B.
In the second modified example of the mechanism for driving the developing rolls 28Y, 28M, 28C, and 28B, substantially the same parts as those constituting the embodiment shown in FIG. It is.

  The motor 144 rotates forward and backward according to the control of the control unit 66, and rotates the shaft (shaft part) 150 forward and backward via the motor gear 146 and the drive gear 148. The shaft 150 is rotatably supported by the support portions 84B, 84Y, 84M, and 84C, and the counter drive gear 148 side is rotatably supported by the bearing 152. The support portion 84B has substantially the same configuration as the support portions 84Y, 84M, and 84C, and rotatably supports the bevel gear 68 provided at one end of the developing roll 28B.

  Further, the shaft 150 passes through the bevel gears 154, 156B, 156C, 156M, and 156Y arranged at predetermined intervals in order from above. The bevel gear 154 has teeth positioned below and meshes with the bevel gear 68, and transmits the driving force of the shaft 150 received via the clutch 158 to the bevel gear 68. The clutch 158 is a one-way clutch. For example, when the motor 144 rotates forward, the rotational driving force of the shaft 150 is transmitted to the bevel gear 154, and when the motor 144 rotates reversely, the bevel gear 154 is transmitted to the shaft 150. I will make it idle.

  The bevel gears 156B, 156C, 156M, and 156Y are arranged so that their teeth are located above and mesh with the bevel gears 68, 70C, 70M, and 70Y, respectively, and the driving force of the shaft 150 received through the clutch 160 is received. Communicate each. The clutch 160 is a one-way clutch. For example, when the motor 144 rotates in the reverse direction, the rotational driving force of the shaft 150 is transmitted to the bevel gears 156B, 156C, 156M, and 156Y, respectively, and when the motor 144 rotates forward. The bevel gears 156B, 156C, 156M, and 156Y are each idled with respect to the shaft 150.

  That is, in the monochrome operation mode, the driving force of the motor 144 is transmitted to the developing roll 28 </ b> B via the bevel tooth portions 154 and 68, and the bevel gear 156 </ b> B rotates around the shaft 150. In the full color operation mode, the driving force of the motor 144 is transmitted to the developing rolls 28B, 28C, 28M, and 28Y via the bevel gears 156B, 156C, 156M, and 156Y, respectively, and the bevel gear 154 is idle with respect to the shaft 150. To do.

  As described above, by reducing the number of gears (number of meshing) for rotating the developing rolls 28Y, 28M, 28C, and 28B, banding and color misregistration generated in the image due to the meshing frequency and rotation difference of each gear. The noise generated by the meshing of the gears can also be reduced.

1 is a cross-sectional view illustrating an outline of an image forming apparatus according to an embodiment of the present invention. It is a front view which shows the Example (state which drives only one image development roll) of the mechanism which drives a image development roll. It is sectional drawing which shows the detail of the bevel gear provided in the shaft, and its periphery. It is the top view which looked at the bevel gears provided in the shaft from the lower part. It is a perspective view which shows the detail of a cam mechanism and its periphery, Comprising: (A) is a perspective view which shows the external appearance of a cam mechanism and its periphery, (B) is a perspective view which shows the state which removed the raising / lowering part. It is a figure which shows the detail of a cam mechanism and its periphery, Comprising: (A) is a front view of the state which the raising / lowering part descend | falls, (B) is sectional drawing of the state where the raising / lowering part descend | falls. It is a figure which shows the detail of a cam mechanism and its periphery, Comprising: (A) is a front view of the state which the raising / lowering part raised, (B) is sectional drawing of the state which the raising / lowering part raised. It is a front view which shows the Example (state which drives four developing rolls) of the mechanism which drives a developing roll. It is a front view showing the 1st modification (state which drives only one development roll) of the mechanism which drives a development roll. It is a front view which shows the 1st modification (state which drives four developing rolls) of the mechanism which drives a developing roll. It is a front view which shows the 2nd modification of the mechanism which drives a developing roll.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Image forming apparatus 12 Image forming apparatus main body 14 Image forming part 28Y, 28M, 28C, 28B Developing roll 66 Control part 68, 70Y, 70M, 70C Bevel gear 72, 144 Motor 74, 146 Motor gear 76, 148 Drive gear 78 Cup Ring 84Y, 84M, 84C Support portion 86, 150 Shaft 88, 90C, 90M, 90Y Bevel gear 92 Cam mechanism 94 Stepping motor 95 Through hole 96, 110 Tracking roll 98 Receiving hole 100 Pin receiving hole 102, 104 E ring 106 Pin 108 Energizing member 112 Protruding part 114 Hole part 116 Rotating part 118 Elevating part 120 Rotating shaft 122 Rotating pin 123 Elevating part body 124 Groove hole 126 Rotating stop 128 Penetration part 130 Hole part 132 Drive gear 134 Spur tooth part 136 Bevel tooth part 137 138, 140 Umbrella Car 142 solenoid 152 bearing 154,156B, 156C, 156M, 156Y bevel gears 158, 160 Clutch

Claims (9)

  A first developing roll, a second developing roll, and a switching unit that switches between driving only the first developing roll or driving the first developing roll and the second developing roll. The switching means includes a shaft portion and a second gear provided on the shaft portion and meshed with a first gear formed on each of the first developing roll and the second developing roll. Image forming apparatus.   The switching unit includes a motor that rotates the shaft part forward and backward, and a clutch that transmits only rotation in one direction of the shaft part to each of the first gear and the second gear. The image forming apparatus described.   The image forming apparatus according to claim 1, wherein the switching unit includes a displacement driving unit that displaces the shaft portion in an axial direction.   The image forming apparatus according to claim 3, wherein the switching unit includes a coupling that drives the first developing roll.   The image forming apparatus according to claim 3, wherein the first gear and the second gear are bevel gears.   The image forming apparatus according to claim 3, wherein the second gear is provided with a tracking roll that determines a meshing position with respect to the first gear.   The image forming apparatus according to claim 6, wherein the second gear is provided with play in the axial direction with respect to the shaft portion and is biased toward the first gear.   The image forming apparatus according to claim 3, wherein the second developing roll is provided with a positioning portion for positioning with respect to a support portion that slidably supports the shaft portion.   The image forming apparatus further includes a third developing roll and a fourth developing roll, and the switching unit is provided in the shaft portion, and is formed on each of the third developing roll and the fourth developing roll. A second gear meshing with the first gear, and driving only the first developing roll, or the first developing roll, the second developing roll, the third developing roll, and the second developing roll; The image forming apparatus according to claim 1, wherein the four developing rolls are driven or switched.

Images