1. Field of the Invention
The present invention relates to an image forming apparatus which includes a transfer member for transferring toner on an image bearing member and presses the transfer member toward the image bearing member to form a transfer nip.
2. Description of the Related Art
Conventionally, as a color image forming apparatus such as a printer and copier, there has been disclosed a technique in which toner images formed on respective photosensitive drums are sequentially superimposed on an intermediate transfer belt and the toner images superimposed on an intermediate transfer belt are collectively transferred onto a recording material to form a color image. In addition, there has been disclosed a technique in which toner images formed on respective photosensitive drums are sequentially superimposed onto the recording material on a conveying transfer belt using the conveying transfer belt which conveys the recording material while the recording material is electro-statically adhered, to form a color image.
A transfer member transferring the toner images on the photosensitive drum is arranged at a position where the respective photosensitive drum faces toward the intermediate transfer belt or the conveying transfer belt. The transfer member is pressed against the photosensitive drum by a press member such as a spring to form a transfer nip. A transfer roller rotating to follow the intermediate transfer belt or the conveying transfer belt is generally used as the transfer member.
However, the transfer roller is pressed by the press member at both axial ends of the transfer roller to produce a deflection recessed in a bow shape at the axial central portion. This causes the nip pressure of the transfer nip to be lower at its both axial ends than that at its central portion, making the nip pressure uneven particularly in an image forming region (an image bearing region where the toner images are borne), which may cause a failure in transfer. In order to cope with the problem, there is known a technique in which the outer diameter of the roller is made larger at its central portion than that at its both axial ends to uniform the nip pressure of the transfer nip even if the axis is deflected.
However, the transfer roller is provided with an elastic member around its axis. An elastic force of the elastic member may vary the transfer nip secular change with time. The transfer roller is originally manufactured such that the outer diameter thereof is axially varied, complicating the production process.