Image forming apparatus including gear trains for transmitting driving force of motor to each of a plurality of developing units

This is a by-pass continuation application of International Application No. PCT/JP2018/045629 filed Dec. 12, 2018 claiming priority from Japanese Patent Application No. 2018-067790 filed Mar. 30, 2018. The entire contents of the International Application and the priority application are incorporated herein by reference.

The present disclosure relates to an image forming apparatus.

A conventional image forming apparatus includes a first developing roller, a second developing roller, a third developing roller, a fourth developing roller, a first clutch, a second clutch, a third clutch, and a fourth clutch. The first clutch is configured to engage transmission of a driving force to the first developing roller, and to disengage the transmission of the driving force to the first developing roller. The second clutch is configured to engage transmission of a driving force to the second developing roller, and to disengage the transmission of the driving force to the second developing roller. The third clutch is configured to engage transmission of a driving force to the third developing roller, and to disengage the transmission of the driving force to the third developing roller. The fourth clutch is configured to engage transmission of a driving force to the fourth developing roller, and to disengage the transmission of the driving force to the fourth developing roller (see Japanese Patent Application Publication No. 2003-208024).

In the conventional image forming apparatus described in Japanese Patent Application Publication No. 2003-208024, there is a demand of driving each of the first developing roller, the second developing roller, the third developing roller and the fourth developing roller using a single motor, and of halting the developing roller(s) that has been completed transfer of a toner image onto a printing sheet while continuing rotation of the developing roller(s) that has not yet completed transfer of a toner image onto the printing sheet during printing operation.

In this case, there is a possibility that torque fluctuation generated by switching operation of the clutches affect rotation of the developing roller that has not yet completed transfer of a toner image onto a printing sheet, which leads to disturbance of an image to be printed.

In view of the foregoing, it is an object of the present disclosure to provide an image forming apparatus capable of restraining occurrence of disturbance of an image to be printed when a first developing roller, a second developing roller, a third developing roller, and a fourth developing roller are individually halted during printing operation.

In order to attain the above and other objects, according to one aspect, the disclosure provides an image forming apparatus including: a first gear train; a second gear train; a third gear train; a fourth gear train; a motor; an output gear; a fifth gear train; and a sixth gear train. The first gear train includes: a first joint; and a first clutch. The first joint is connected to a first developing unit including a first developing roller. The first joint includes a first joint gear. The first clutch is configured to switch power transmission status between a first engaging state where the first clutch engages transmission of a driving force to the first joint gear and a first disengaging state where the first clutch disengages the transmission of the driving force to the first joint gear. The second gear train includes: a second joint; and a second clutch. The second joint is connected to a second developing unit including a second developing roller. The second joint includes a second joint gear. The second clutch is configured to switch power transmission status between a second engaging state where the second clutch engages transmission of a driving force to the second joint gear and a second disengaging state where the second clutch disengages the transmission of the driving force to the second joint gear. The third gear train includes: a third joint; and a third clutch. The third joint is connected to a third developing unit including a third developing roller. The third joint includes a third joint gear. The third clutch is configured to switch power transmission state between a third engaging state where the third clutch engages transmission of a driving force to the third joint gear and a third disengaging state where the third clutch disengages the transmission of the driving force to the third joint gear. The fourth gear train includes: a fourth joint; and a fourth clutch. The fourth joint is connected to a fourth developing unit including a fourth developing roller. The fourth joint includes a fourth joint gear. The fourth clutch is configured to switch power transmission state between a fourth engaging state where the fourth clutch engages transmission of a driving force to the fourth joint gear and a fourth disengaging state where the fourth clutch disengages the transmission of the driving force to the fourth joint gear. The motor includes an output shaft. The output gear is rotatable together with the output shaft. The fifth gear train is connected to the output gear. The fifth gear train is connected to the first clutch and the second clutch. The sixth gear train is connected to the output gear independently from the fifth gear train. The sixth gear train is connected to the third clutch and the fourth clutch.

According to another aspect, the disclosure provides an image forming apparatus including: a motor; an output shaft; a first gear train; and a second gear train. The motor includes an output shaft. The output gear is rotatable together with the output shaft. The first gear train connects the output gear to a first developing unit including a first developing roller. The first gear train includes a first clutch configured to switch power transmission state between a first engaging state where the first clutch engages transmission of the driving force from the motor to the first developing unit and a first disengaging state where the first clutch disengages the transmission of the driving force. The second gear train is independent from the first gear train and connects the output gear to a second developing unit including a second developing roller. The second gear train includes a second clutch configured to switch power transmission state between a second engaging state where the second clutch engages transmission of the driving force from the motor to the second developing unit and a second disengaging state where the second clutch disengages the transmission of the driving force.

The particular features and advantages of the embodiment(s) as well as other objects will become apparent from the following description taken in connection with the accompanying drawings, in which:

FIG. 1 is a schematic diagram illustrating a configuration of an image forming apparatus;

FIG. 2 illustrates gear trains for transmitting a driving force of a motor to each of four developing units illustrated in FIG. 1 ; and

FIG. 3 is a perspective view of the gear trains illustrated in FIG. 2 .

<1. Overview of Image Forming Apparatus>

An overview of an image forming apparatus will be described with reference to FIG. 1 .

An image forming apparatus 1 includes a main body casing 2 , a sheet supply tray 3 , four photosensitive drums 4 Y, 4 M, 4 C and 4 K, four chargers 5 Y, 5 M, 5 C and 5 K, an exposure unit 6 , four developing units 7 Y, 7 M, 7 C and 7 K, a transfer unit 8 , and a fixing unit 9 .

<1.1 Main Body Casing>

The main body casing 2 constitutes an outer shell of the image forming apparatus 1 . The main body casing 2 accommodates therein the sheet supply tray 3 , the four photosensitive drums 4 Y, 4 M, 4 C and 4 K, the four chargers 5 Y, 5 M, 5 C and 5 K, the exposure unit 6 , the four developing units 7 Y, 7 M, 7 C and 7 K, the transfer unit 8 , and the fixing unit 9 .

<1.2 Sheet Supply Tray>

The sheet supply tray 3 accommodates therein printing mediums S. The printing medium S in the sheet supply tray 3 is conveyed toward the photosensitive drum 4 Y. The printing medium S is for example, a printing sheet.

<1.3 Four Photosensitive Drums>

The four photosensitive drums 4 Y, 4 M, 4 C and 4 K are arrayed in a conveying direction in which the printing medium S is conveyed by a conveyer belt 11 . The conveyer belt 11 will be described later. The photosensitive drum 4 M is positioned downstream of the photosensitive drum 4 Y in the conveying direction. The photosensitive drum 4 C is positioned downstream of the photosensitive drum 4 M in the conveying direction. The photosensitive drum 4 K is positioned downstream of the photosensitive drum 4 C in the conveying direction. That is, the four photosensitive drums 4 Y, 4 M, 4 C and 4 K are arrayed in the order of the photosensitive drum 4 Y, the photosensitive drum 4 M, the photosensitive drum 4 C and the photosensitive drum 4 K in the conveying direction.

The photosensitive drum 4 Y is rotatable about a drum axis extending in an axial direction. The axial direction is a direction crossing the conveying direction. Preferably, the axial direction is perpendicular to the conveying direction. The photosensitive drum 4 Y extends in the axial direction and has a hollow cylindrical shape. Note that each of the photosensitive drums 4 M, 4 C and 4 K has a configuration the same as the configuration of the photosensitive drum 4 Y. Accordingly, description as to the configuration of each of the photosensitive drums 4 M, 4 C and 4 K will be omitted.

<1.4 Four Chargers>

The charger 5 Y is configured to charge a circumferential surface of the photosensitive drum 4 Y. The charger 5 M is configured to charge a circumferential surface of the photosensitive drum 4 M. The charger 5 C is configured to charge a circumferential surface of the photosensitive drum 4 C. The charger 5 K is configured to charge a circumferential surface of the photosensitive drum 4 K. Specifically, each of these four chargers 5 Y, 5 M, 5 C and 5 K is a scorotron charger. Note that each of these four chargers 5 Y, 5 M, 5 C and 5 K may be a charge roller.

<1.5 Exposure Unit>

The exposure unit 6 is configured to expose the photosensitive drum 4 Y to light. After the circumferential surface of photosensitive drum 4 Y is electrically charged by the charger 5 Y, the exposure unit 6 irradiates the circumferential surface of the photosensitive drum 4 Y that has been electrically charged with light for exposure, thereby forming an electrostatic latent image on the circumferential surface of the photosensitive drum 4 Y. Specifically, the exposure unit 6 is a laser scanner unit that scans the circumferential surface of the photosensitive drum 4 Y with laser beam. Alternatively, an LED unit including an LED array can be employed as the exposure unit 6 . The exposure unit 6 is also configured to expose the photosensitive drums 4 M, 4 C and 4 K to lights.

<1.6 Four Developing Units>

The developing unit 7 Y is configured to accommodate therein toner. The developing unit 7 Y may be a developing cartridge attachable to the image forming apparatus 1 . The developing unit 7 Y includes a developing roller 10 Y.

The developing roller 10 Y is rotatable about a developing roller axis extending in the axial direction. The developing roller 10 Y extends in the axial direction and has a solid cylindrical shape. The developing roller 10 Y has a portion accommodated inside the developing unit 7 Y. The developing roller 10 Y contacts the circumferential surface of the photosensitive drum 4 Y. With this configuration, the developing roller 10 Y can supply toner in the developing unit 7 Y to the circumferential surface of the photosensitive drum 4 Y. As the toner in the developing unit 7 Y to the circumferential surface of the photosensitive drum 4 Y through the developing roller 10 Y, the electrostatic latent image is developed, whereby a toner image is formed on the circumferential surface of the photosensitive drum 4 Y.

Note that the developing unit 7 M includes a developing roller 10 M. The developing unit 7 C includes a developing roller 10 C. The developing unit 7 K includes a developing roller 10 K. The developing roller 10 M contacts the circumferential surface of the photosensitive drum 4 M. Hence, the developing roller 10 M can supply toner in the developing unit 7 M to the circumferential surface of the photosensitive drum 4 M. The developing roller 10 C contacts the circumferential surface of the photosensitive drum 4 C. Therefore, the developing roller 10 C can supply toner in the developing unit 7 C to the circumferential surface of the photosensitive drum 4 C. The developing roller 10 K contacts the circumferential surface of the photosensitive drum 4 K. Accordingly, the developing roller 10 K can supply toner in the developing unit 7 K to the circumferential surface of the photosensitive drum 4 K.

<1.7 Transfer Unit>

The transfer unit 8 includes the conveyer belt 11 . That is, the image forming apparatus 1 includes the conveyer belt 11 . The conveyer belt 11 is configured to convey the printing medium S supplied from the sheet supply tray 3 toward the fixing unit 9 . The printing medium S conveyed by the conveyer belt 11 passes through a portion between the conveyer belt 11 and the four photosensitive drums 4 Y, 4 M, 4 C and 4 K. At this time, the transfer unit 8 transfers the toner images formed on the respective four photosensitive drums 4 Y, 4 M, 4 C and 4 K onto the printing medium S.

<1.8 Fixing Unit>

The fixing unit 9 is configured to apply heat and pressure to the printing medium S onto which the toner images have been transferred to fix the toner images to the printing medium S. The printing medium S moved past the fixing unit 9 is discharged on an upper surface of the main body casing 2 .

<2. Details of Image Forming Apparatus>

Next, details of the image forming apparatus 1 will be described with reference to FIGS. 2 and 3 .

As illustrated in FIG. 2 , the image forming apparatus 1 includes a motor 21 , and gear trains 20 configured to transmit a driving force of the motor 21 to each of the four developing units 7 Y, 7 M, 7 C and 7 K (see FIG. 1 ).

<2.1 Motor>

Specifically, the motor 21 is a brushless DC motor. The motor 21 includes an output shaft 21 A. The output shaft 21 A extends in the axial direction. The output shaft 21 A is rotatable about an axis A extending in the axial direction.

<2.2 Gear Trains>

The gear trains 20 include an output gear 22 , a first gear train 23 Y, a second gear train 23 M, a third gear train 23 C, a fourth gear train 23 K, a fifth gear train 24 , and a sixth gear train 25 . That is, the image forming apparatus 1 includes the output gear 22 , the first gear train 23 Y, the second gear train 23 M, the third gear train 23 C, the fourth gear train 23 K, the fifth gear train 24 , and the sixth gear train 25 .

The first gear train 23 Y is configured to transmit a driving force of the motor 21 to the developing unit 7 Y (see FIG. 1 ). The second gear train 23 M is configured to transmit the driving force of the motor 21 to the developing unit 7 M (see FIG. 1 ). The third gear train 23 C is configured to transmit the driving force of the motor 21 to the developing unit 7 C (see FIG. 1 ). The fourth gear train 23 K is configured to transmit the driving force of the motor 21 to the developing unit 7 K (see FIG. 1 ). Each of the first gear train 23 Y, the second gear train 23 M, the third gear train 23 C, and the fourth gear train 23 K includes a clutch. Specifically, the first gear train 23 Y includes a first clutch 32 Y, the second gear train 23 M includes a second clutch 32 M, the third gear train 23 C includes a third clutch 32 C, and the fourth gear train 23 K includes a fourth clutch 32 K. Each of the first clutch 32 Y, the second clutch 32 M, the third clutch 32 C, and the fourth clutch 32 K is configured to be switched individually between a state in which transmission of the driving force is engaged and a state in which the transmission of the driving force is disengaged.

The fifth gear train 24 is connected to the first clutch 32 Y and the second clutch 32 M. The sixth gear train 25 is connected to the third clutch 32 C and the fourth clutch 32 K. The fifth gear train 24 and the sixth gear train 25 are connected to the output gear 22 independently from each other.

The above configuration can restrain transmission of torque fluctuation occurring by switching operation of each of the first clutch 32 Y and the second clutch 32 M to the third gear train 23 C and the fourth gear train 23 K.

Also, the above configuration can restrain transmission of torque fluctuation occurring by switching operation of each of the third clutch 32 C and the fourth clutch 32 K to the first gear train 23 Y and the second gear train 23 M.

Next, the configuration of the gear trains 20 will be described in detail.

<2.2.1 Output Gear>

The output gear 22 is coupled to the output shaft 21 A. The output gear 22 is rotatable together with the output shaft 21 A.

<2.2.2 First Gear Train>

The first gear train 23 Y is connected to the developing unit 7 Y. As illustrated in FIGS. 2 and 3 , the first gear train 23 Y includes a first joint 31 Y, the first clutch 32 Y, and a plurality of idle gears 33 Y, 34 Y, 35 Y and 36 Y.

The first joint 31 Y is connected to the developing unit 7 Y. The first joint 31 Y is rotatable about an axis A 1 . The axis A 1 extends in the axial direction. The first joint 31 Y includes a first joint gear 41 Y and a coupling 42 Y. The first joint gear 41 Y is rotatable about the axis A 1 . The coupling 42 Y is rotatable together with the first joint gear 41 Y about the axis A 1 . The coupling 42 Y extends in the axial direction.

The coupling 42 Y is coupled to a developing coupling of the developing unit 7 Y. The developing coupling is not illustrated. The coupling 42 Y is rotatable together with the developing coupling about the axis A 1 in a state where the coupling 42 Y is coupled to the developing coupling. Hence, the first joint 31 Y can transmit a driving force to the developing unit 7 Y while the coupling 42 Y is coupled to the developing coupling.

The first clutch 32 Y is positioned in the middle of the first gear train 23 Y. Specifically, the first clutch 32 Y is disposed between the idle gear 35 Y and the idle gear 36 Y. The first clutch 32 Y is configured to switch power transmission state between a first engaging state and a first disengaging state. The first clutch 32 Y engages transmission of a driving force to the first joint gear 41 Y when the first clutch 32 Y is in the first engaging state. The first clutch 32 Y disengages the transmission of the driving force to the first joint gear 41 Y when the first clutch 32 Y is in the first disengaging state.

Specifically, the first clutch 32 Y is an electromagnetic clutch. The first clutch 32 Y is configure to be switched between an ON state and an OFF state. The first clutch 32 Y becomes the first engaging state when the first clutch 32 Y is in the ON state. The first clutch 32 Y becomes the first disengaging state when the first clutch 32 Y is in the OFF state.

The idle gear 33 Y is in meshing engagement with the first joint gear 41 Y. The idle gear 34 Y is rotatable together with the idle gear 33 Y. The idle gear 35 Y is in meshing engagement with the idle gear 34 Y. The idle gear 36 Y is rotatable together with the idle gear 35 Y when the first clutch 32 Y is in the ON state.

<2.3 Second Gear Train>

The second gear train 23 M is connected to the developing unit 7 M. The second gear train 23 M includes a second joint 31 M, the second clutch 32 M, and a plurality of idle gears 33 M, 34 M, 35 M and 36 M.

The second joint 31 M is connected to the developing unit 7 M. The second joint 31 M is rotatable about an axis A 2 . The axis A 2 extends in the axial direction. The second joint 31 M includes a second joint gear 41 M rotatable about the axis A 2 . The second joint 31 M has a configuration the same as that of the first joint 31 Y, and the above description as to the first joint 31 Y can be applied to the second joint 31 M.

Therefore, description as to the configuration of the second joint 31 M will be omitted.

The second clutch 32 M is positioned in the middle of the second gear train 23 M. Specifically, the second clutch 32 M is disposed between the idle gear 35 M and the idle gear 36 M. The second clutch 32 M is configured to switch power transmission status between a second engaging state and a second disengaging state. The second clutch 32 M engages transmission of a driving force to the second joint gear 41 M when the second clutch 32 M is in the second engaging state. The second clutch 32 M disengages the transmission of the driving force to the second joint gear 41 M when the second clutch 32 M is in the second disengaging state.

Specifically, the second clutch 32 M is an electromagnetic clutch. The second clutch 32 M is configured to be switched between an ON state and an OFF state. The second clutch 32 M becomes the second engaging state when the second clutch 32 M is in the ON state. The second clutch 32 M becomes the second disengaging state when the second clutch 32 M is in the OFF state.

The idle gear 33 M is in meshing engagement with the second joint gear 41 M. The idle gear 34 M is rotatable together with the idle gear 33 M. The idle gear 35 M is in meshing engagement with the idle gear 34 M. The idle gear 36 M is rotatable together with the idle gear 35 M when the second clutch 32 M is in the ON state.

<2.4 Third Gear Train>

The third gear train 23 C is connected to the developing unit 7 C. The third gear train 23 C includes a third joint 31 C, the third clutch 32 C, and a plurality of idle gears 33 C, 34 C, 35 C and 36 C.

The third joint 31 C is connected to the developing unit 7 C. The third joint 31 C is rotatable about an axis A 3 . The axis A 3 extends in the axial direction. The third joint 31 C includes a third joint gear 41 C rotatable about the axis A 3 . The third joint 31 C has a configuration the same as that of the first joint 31 Y, and the above description as to the first joint 31 Y is applicable.

Hence, description as to the configuration of the third joint 31 C will be omitted.

The third clutch 32 C is positioned in the middle of the third gear train 23 C. Specifically, the third clutch 32 C is disposed between the idle gear 35 C and the idle gear 36 C. The third clutch 32 C is configured switch power transmission status between a third engaging state and a third disengaging state. The third clutch 32 C engages transmission of a driving force to the third joint gear 41 C when the third clutch 32 C is in the third engaging state. The third clutch 32 C disengages the transmission of the driving force to the third joint gear 41 C when the third clutch 32 C is in the third disengaging state.

Specifically, the third clutch 32 C is an electromagnetic clutch. The third clutch 32 C is configured to be switched between an ON state and an OFF state. The third clutch 32 C becomes the third engaging state when the third clutch 32 C is in the ON state. The third clutch 32 C becomes the third disengaging state when the third clutch 32 C is in the OFF state.

The idle gear 33 C is in meshing engagement with the third joint gear 41 C. The idle gear 34 C is rotatable together with the idle gear 33 C. The idle gear 35 C is in meshing engagement with the idle gear 34 C. The idle gear 36 C is rotatable together with the idle gear 35 C when the third clutch 32 C is in the ON state.

<2.5 Fourth Gear Train>

The fourth gear train 23 K is connected to the developing unit 7 K. The fourth gear train 23 K includes a fourth joint 31 K, the fourth clutch 32 K, and a plurality of idle gears 33 K, 34 K, 35 K and 36 K.

The fourth joint 31 K is connected to the developing unit 7 K. The fourth joint 31 K is rotatable about an axis A 4 . The axis A 4 extends in the axial direction. The fourth joint 31 K includes a fourth joint gear 41 K rotatable about the axis A 4 . The fourth joint 31 K has a configuration the same as that of the first joint 31 Y, and the above description as to the first joint 31 Y is applicable.

Hence, description as to the configuration of the fourth joint 31 K will be omitted.

The fourth clutch 32 K is positioned in the middle of the fourth gear train 23 K. Specifically, the fourth clutch 32 K is disposed between the idle gear 35 K and the idle gear 36 K. The fourth clutch 32 K is configured to switch power transmission status between a fourth engaging state and a fourth disengaging state. The fourth clutch 32 K engages transmission of a driving force to the fourth joint gear 41 K when the fourth clutch 32 K is in the fourth engaging state. The fourth clutch 32 K disengages the transmission of the driving force to the fourth joint gear 41 K when the fourth clutch 32 K is in the fourth disengaging state.

Specifically, the fourth clutch 32 K is an electromagnetic clutch. The fourth clutch 32 K is configured to be switched between an ON state and an OFF state. The fourth clutch 32 K becomes the fourth engaging state when the fourth clutch 32 K is in the ON state. The fourth clutch 32 K becomes the fourth disengaging state when the fourth clutch 32 K is in the OFF state.

The idle gear 33 K is in meshing engagement with the fourth joint gear 41 K. The idle gear 34 K is rotatable along with the idle gear 33 K. The idle gear 35 K is in meshing engagement with the idle gear 34 K. The idle gear 36 K is rotatable together with the idle gear 35 K when the fourth clutch 32 K is in the ON state.

<2.6 Fifth Gear Train>

The fifth gear train 24 includes a plurality of idle gears 51 , 52 , 53 and 54 . The idle gear 51 is in meshing engagement with the output gear 22 (see FIG. 2 ). Hence, the fifth gear train 24 is connected to the output gear 22 . The idle gear 52 is rotatable together with the idle gear 51 .

The idle gear 53 is in meshing engagement with the idle gear 52 . The idle gear 54 is in meshing engagement with the idle gear 53 . The idle gear 54 is also in meshing engagement with the idle gear 36 Y and the idle gear 36 M. Accordingly, the fifth gear train 24 is connected to the first clutch 32 Y and the second clutch 32 M.

<2.7 Sixth Gear Train>

The sixth gear train 25 includes a plurality of idle gears 61 , 62 , 63 and 64 . The idle gear 61 is in meshing engagement with the output gear 22 (see FIG. 2 ). Accordingly, the sixth gear train 25 is connected to the output gear 22 . The idle gear 62 is rotatable together with the idle gear 61 .

The idle gear 63 is in meshing engagement with the idle gear 62 . The idle gear 64 is in meshing engagement with the idle gear 63 . Further, the idle gear 64 is in meshing engagement with the idle gear 36 C and the idle gear 36 K. Hence, the sixth gear train 25 is connected to the third clutch 32 C and the fourth clutch 32 K.

Here, all of the gears constituting the sixth gear train 25 are not in meshing engagement with any one the gears constituting the fifth gear train 24 . That is, the sixth gear train 25 is independent from the fifth gear train 24 .

Accordingly, transmission of torque fluctuation occurring due to switching operation performed in each of the first gear train 23 Y and the second gear train 23 M to the third gear train 23 C and the fourth gear train 23 K can be restrained.

<2.8 The Number of Gears Interposed Between Output Gear and Joints>

As illustrated in FIGS. 2 and 3 , the plurality of idle gears 51 , 52 , 53 , 54 , 36 Y, 35 Y, 34 Y and 33 Y intervenes between the output gear 22 and the first joint 31 Y. That is, the number of gears intervening between the output gear 22 and the first joint 31 Y is eight.

Further, the plurality of idle gears 51 , 52 , 53 , 54 , 36 M, 35 M, 34 M and 33 M intervenes between the output gear 22 and the second joint 31 M. That is, the number of gears intervening between the output gear 22 and the second joint 31 M is eight.

Further, the plurality of idle gears 61 , 62 , 63 , 64 , 36 C, 35 C, 34 C and 33 C intervenes between the output gear 22 and the third joint 31 C. That is, the number of gears intervening between the output gear 22 and the third joint 31 C is eight.

Further, the plurality of idle gears 61 , 62 , 63 , 64 , 36 K, 35 K, 34 K and 33 K intervenes between the output gear 22 and the fourth joint 31 K. That is, the number of gears intervening between the output gear 22 and the fourth joint 31 K is eight.

That is, the number of gears intervening between the output gear 22 and the first joint 31 Y, the number of gears intervening between the output gear 22 and the second joint 31 M, the number of gears intervening between the output gear 22 and the third joint 31 C, and the number of gears intervening between the output gear 22 and the fourth joint 31 K are equal to one another.

With the above configuration, torque transmitted to the four developing units 7 Y, 7 M, 7 C and 7 K can be equalized.

<3. Operation in Image Forming Apparatus>

Next, operation performed in the image forming apparatus 1 will be described with reference to FIGS. 1 through 3 .

As illustrated in FIG. 1 , when printing is performed on the printing medium S in the image forming apparatus 1 , the conveyer belt 11 conveys the printing medium S supplied from the sheet supply tray 3 toward the fixing unit 9 through a portion between the transfer unit 8 and the four photosensitive drums 4 Y, 4 M, 4 C and 4 K.

Since the four photosensitive drums 4 Y, 4 M, 4 C and 4 K are arrayed in the order of the photosensitive drum 4 Y, the photosensitive drum 4 M, the photosensitive drum 4 C and the photosensitive drum 4 K in the conveying direction, the printing medium S conveyed by the conveyer belt 11 contacts the photosensitive drum 4 M after contacting the photosensitive drum 4 Y, contacts the photosensitive drum 4 C after contacting the photosensitive drum 4 M, and contacts the photosensitive drum 4 K after contacting the photosensitive drum 4 C. Further, the printing medium S separates from the photosensitive drum 4 M after separating from the photosensitive drum 4 Y, separates from the photosensitive drum 4 C after separating from the photosensitive drum 4 M, and separates from the photosensitive drum 4 K after separating from the photosensitive drum 4 C.

Further, the image forming apparatus 1 successively switches the first clutch 32 Y, the second clutch 32 M, the third clutch 32 C, and the fourth clutch 32 K from their ON state to their OFF state in relation to the separation timings of the printing medium S from the respective four photosensitive drums 4 Y, 4 M, 4 C and 4 K.

Specifically, the image forming apparatus 1 switches the first clutch 32 Y from the ON state to the OFF state after the printing medium S separates from the photosensitive drum 4 Y. That is, the first clutch 32 Y is switched to the first disengaging state after the printing medium S conveyed by the conveyer belt 11 separates from the photosensitive drum 4 Y. Preferably, the image forming apparatus 1 switches the first clutch 32 Y from the ON state to the OFF state after the printing medium S separates from the photosensitive drum 4 M. Therefore, torque fluctuation generated due to switching operation of the first clutch 32 Y can be restrained from affecting image formation using the photosensitive drum 4 M.

Then, after the first clutch 32 Y becomes the OFF state and after the printing medium S separates from the photosensitive drum 4 M, the image forming apparatus 1 switches the second clutch 32 M from the ON state to the OFF state. That is, the second clutch 32 M becomes the second disengaging state after the first clutch 32 Y becomes the first disengaging state and after the printing medium S conveyed by the conveyer belt 11 separates from the photosensitive drum 4 M.

Then, after the second clutch 32 M becomes the OFF state and after the printing medium S separates from the photosensitive drum 4 C, the image forming apparatus 1 switches the third clutch 32 C from the ON state to the OFF state. That is, the third clutch 32 C becomes the third disengaging state after the second clutch 32 M becomes the second disengaging state and after the printing medium S conveyed by the conveyer belt 11 separates from the photosensitive drum 4 C. Preferably, the image forming apparatus 1 switches the third clutch 32 C from the ON state to the OFF state after the printing medium S separates from the photosensitive drum 4 K. This operation can restrain torque fluctuation generated by switching operation of the third clutch 32 C from affecting image formation using the photosensitive drum 4 K.

Then, after the third clutch 32 C becomes the OFF state and after the printing medium S separates from the photosensitive drum 4 K, the image forming apparatus 1 switches the fourth clutch 32 K from the ON state to the OFF state. That is, the fourth clutch 32 K becomes the fourth disengaging state after the third clutch 32 C becomes the third disengaging state and after the printing medium S conveyed by the conveyer belt 11 separates from the photosensitive drum 4 K.

Through this operation, degradation of the photosensitive drums and the developing rollers can be restrained by halting the operation of the developing units which has already completed transfer of a toner image onto the printing sheet S.

<4. Function and Effect>

As illustrated in FIGS. 2 and 3 , in the image forming apparatus 1 , the fifth gear train 24 connected to the first clutch 32 Y and the second clutch 32 M, and the sixth gear train 25 connected to the third clutch 32 C and the fourth clutch 32 K are connected to the output gear 22 independently from each other.

Accordingly, torque fluctuation generated by the switching operation of the first clutch 32 Y and the second clutch 32 M is not transmitted to the sixth gear train 25 , and therefore the torque fluctuation does not affect rotations of the developing roller 10 C (see FIG. 1 ) and the developing roller 10 K (see FIG. 1 ).

Further, torque fluctuation generated by the switching operation of the third clutch 32 C and the fourth clutch 32 K is not transmitted to the fifth gear train 24 , and hence, the torque fluctuation does not affect rotations of the developing roller 10 Y (see FIG. 1 ) and the developing roller 10 M (see FIG. 1 ).

As a result, disturbance of an image to be printed can be restrained when each of the developing roller 10 Y, the developing roller 10 M, the developing roller 10 C and the developing roller 10 K is individually halted during the printing operation.

<5. Modifications>

In the above-described embodiment, the gear trains 20 configured to transmit a driving force to each of the four developing units 7 Y, 7 M, 7 C and 7 K are exemplified and a configuration where the first gear train 23 Y, the second gear train 23 M and the fifth gear train 24 for transmitting the driving force to the developing units 7 Y and 7 M, and the third gear train 23 C, the fourth gear train 23 K, and the sixth gear train 25 for transmitting the driving force to the developing units 7 C and 7 K are connected to the output gear 22 independently from each other is described.

However, the number of developing units is arbitrary. For example, as gear trains for transmitting a driving force to each of two developing units 7 M and 7 K, a first gear train for transmitting a driving force to one developing unit 7 M and a second gear train for transmitting a driving force to the remaining one developing unit 7 K may be connected to the output gear 22 independently from each other.

That is, the image forming apparatus 1 may include the first gear train connecting the output gear 22 to the developing unit 7 M, and the second gear train independent from the first gear train and connecting the output gear 22 to the developing unit 7 K. In this case, the first gear train includes a first clutch, and the second gear train includes a second clutch. Specifically, the first gear train according to this modification corresponds to the second gear train 23 M and the fifth gear train 24 according to the above-described embodiment. Further, the second gear train according to this modification corresponds to the fourth gear train 23 K and the sixth gear train 25 according to the above-described embodiment. Further, the first clutch according to the modification corresponds to the second clutch 32 M according to the above-described embodiment. Further, the second clutch according to this modification corresponds to the fourth clutch 32 K according to the above-described embodiment.

Further, in the above-described embodiment, an electromagnetic clutch is exemplified as each of the first clutch 32 Y, the second clutch 32 M, the third clutch 32 C and the fourth clutch 32 K. However, a dog clutch and a friction clutch are also available as the first clutch 32 Y, the second clutch 32 M, the third clutch 32 C and the fourth clutch 32 K.

Further, although not illustrated, the image forming apparatus 1 may include a single drum unit including the four photosensitive drums 4 Y, 4 M, 4 C and 4 K, and four developing cartridges attachable to the drum unit.

Further, although not illustrated, the image forming apparatus 1 may include a first drum cartridge including the photosensitive drum 4 Y, a first developing cartridge attachable to the first drum cartridge, a second drum cartridge including the photosensitive drum 4 M, a second developing cartridge attachable to the second drum cartridge, a third drum cartridge including the photosensitive drum 4 C, a third developing cartridge attachable to the third drum cartridge, a fourth drum cartridge including the photosensitive drum 4 K, and a fourth developing cartridge attachable to the fourth drum cartridge.

Further, although not illustrated, the image forming apparatus 1 may include a first process cartridge including the photosensitive drum 4 Y and the developing unit 7 Y, a second process cartridge including the photosensitive drum 4 M and the developing unit 7 M, a third process cartridge including the photosensitive drum 4 C and the developing unit 7 C, and a fourth process cartridge including the photosensitive drum 4 K and the developing unit 7 K.

While the description has been made in detail with reference to the embodiments thereof, it would be apparent to those skilled in the art that many modifications and variations may be made therein without departing from the spirit of the disclosure.