Image forming apparatus

An image forming apparatus, comprising:

• a plurality of process units, each of which corresponds to each one of a first color, a second color, a third color, and a fourth color, each of the plurality of process units having:
• a developing unit including a developing roller;
• and
• a drum unit including a photosensitive drum;
• a transfer unit having a belt arranged to partly face the photosensitive drums in the plurality of process units;
• a motor;
• a first cam configured to move the developing roller with respect to the photosensitive drum in a first one of the plurality of process units corresponding to the first color between a pressed position and a separated position corresponding to the first color;
• a second cam configured to move the developing roller with respect to the photosensitive drum in a second one of the plurality of process units corresponding to the second color between a pressed position and a separated position corresponding to the second color;
• a third cam configured to move the developing roller with respect to the photosensitive drum in a third one of the plurality of process units corresponding to the third color between a pressed position and a separated position corresponding to the third color;
• a fourth cam configured to move the developing roller with respect to the photosensitive drum in a fourth one of the plurality of process units corresponding to the fourth color between a pressed position and a separated position corresponding to the fourth color;
• a color-linkage gear train configured to rotate the first cam, the second cam, and the third cam mechanically in conjunction with one another in phases shifted from one another, the color-linkage gear train maintaining the phases of the first cam, the second cam, and the third cam shifted to operate the first cam, the second cam, and the third cam in conjunction with one another;
• a monochrome-transmission gear train configured to transmit a driving force from the motor to a monochrome clutch, the monochrome clutch being configured to engage and disengage transmission of the driving force from the monochrome-transmission gear train to the fourth cam;
• and
• a color-transmission gear train configured to transmit the driving force from the motor to a color clutch, the color clutch being configured to engage and disengage transmission of the driving force from the color-transmission gear train to the color-linkage gear train.