Image forming apparatus and image forming unit with an improved phase adjustment means

An image forming apparatus comprising:

• a driving source which generates a driving force;
• first and second driving force transmission mechanisms, each driving force transmission mechanism including a train of gears driven by said driving source, one of said train of gears of each of said driving force transmission mechanisms having a reference mark thereon;
• a driving force transmission control mechanism operatively coupled to said first driving force transmission mechanism, said driving force transmission control mechanism selectively either transmitting or cutting-off the driving force of said driving source to said first driving force transmission mechanism;
• a first detector which detects the reference mark on the gear of said first driving force transmission mechanism to output a first control signal;
• a second detector which detects the reference mark on the gear of said second driving force transmission mechanism to output a second control signal;
• and
• a position adjustment controller which receives the first and second control signals from said first and second detectors, and which controls said driving force transmission control mechanism to either drive or stop said first driving force transmission mechanism while said second driving force transmission mechanism is driven, such that a rotational positioning relationship between said first and second driving force transmission mechanisms is kept constant.

An image forming apparatus which is housed in a housing in which a sheet conveying passage is provided, the image forming apparatus comprising:

• a plurality of image carrier drums arranged side by side at a plurality of predetermined positions along the sheet conveying passage in the housing, the plurality of image carrier drums being respectively provided with driven gears, each of the driven gears having the same shape;
• a first driving force transmission mechanism including a driving gear engaged with the driven gear of a predetermined one of said image carrier drums, the driving gear being provided with a reference mark including eccentricity information;
• a second driving force transmission mechanism including a plurality of driving gears engaged with the driving gears of the other image carrier drums, at least one of the driving gears being provided with a reference mark including eccentricity information, the driving gears of said first and second driving force transmission mechanism having the same shape;
• a driving source which generates a driving force and which transmits the driving force to the first and second driving force transmission mechanisms;
• a driving force shifter which has a first mode in which the driving force of said driving source is transmitted to said first driving force transmission mechanism, and a second mode in which the driving force of said driving source is transmitted to both of said first and second driving force transmission mechanisms;
• a first detector which detects the reference mark on the driving gear of said first driving force transmission mechanism and determines a home position of the driving gear of said first driving force transmission mechanism;
• a second detector which detects the reference mark on the at least one of said driving gears of said second driving force transmission mechanism and determines a home position of the at least one of said driving gears of said second driving force transmission mechanism;
• a first image forming unit which forms an image on the predetermined one of said image carrier drums;
• a second image forming unit which forms an image on at least one of the other image carrier drums;
• an image formation controller which causes said first image forming unit to form an image on the predetermined one of said image carrier drums when said driving force shifter is set in the first mode, and causes said first and second image forming units to form images on the predetermined one of the other image carrier drums and on the at least one of the other image carrier drums when said driving force shifter is set in the second mode;
• and
• a driving force transmission mechanism controller which initially brings said driving force shifter to said second mode to transmit the driving force of said driving source to said first and second driving force transmission mechanisms, secondarily brings said driving force shifter to said first mode in response to the determination of the home position of the at least one of said driving gears of said second driving force transmission mechanism by said second detector and to transmit the driving force of said driving source to said first driving force transmission mechanism, and which lastly brings said driving force shifter to said second mode in response to the determination of the home position of the driving gear of said first driving force transmission mechanism by said first detector and to transmit the driving force of said driving source to said first and second driving force transmission mechanisms,
• wherein a rotational positioning relationship between said first and second driving force transmission mechanisms becomes constant to make said first and second image forming units form an image on the predetermined one of said image carrier drums and at least one image on the at least one of the other image carrier drums such that the images can be transferred onto predetermined positions without forming a positional sift on a sheet from the predetermined one of said image carrier drums and the at least one of the other image carrier drums while the sheet is conveyed in the sheet conveying passage.