Belt transfer device

A belt transfer device for transferring a monochromatic toner image on a monochromatic image carrier and color toner images on a plurality of color image carriers to a sheet of paper, the monochromatic and color image carriers being arranged on a line, the transfer device comprising:

• a main frame;
• a plurality of rollers supported by the main frame;
• an intermediate transfer belt made of elastic material;
• the intermediate transfer belt running over the rollers;
• a monochromatic image transfer member for primarily transferring the monochromatic toner image on the monochromatic image carrier to the intermediate transfer belt;
• color image transfer members each for primarily transferring the color toner image on one of the color image carriers to the intermediate transfer belt;
• shifters reciprocatable in specified opposite directions so as to each shift one of the monochromatic and color image transfer members between a transfer position where the transfer member is in compressive contact with an inner surface of the intermediate transfer belt and a home position where the transfer member is away from the belt surface;
• a detector for outputting a signal representing the position of one of the shifters in the opposite directions;
• and
• a controller for determining whether the tensile force of the intermediate transfer belt is proper based on the state of the signal output from the detector while the associated shifter is reciprocating each time in the opposite directions so as to determine whether a length of the intermediate transfer belt running over the rollers is proper;
• the belt transfer device being adapted to secondarily transfer the monochromatic and color toner images on the intermediate transfer belt to the sheet of paper.