Transfer unit and image forming apparatus including the same

A transfer unit including a transfer roller having a core metal and an elastic layer laid on an outer circumferential surface of the core metal, the transfer roller making the elastic layer pressed contact with an image carrying member to form a transfer nip portion, the transfer unit transferring a toner image formed on the image carrying member to a recording medium passing through the transfer nip portion, the transfer unit comprising:

• the transfer roller including a first roller and a second roller, the second roller being disposed above the first roller and different from the first roller;
• a first bearing member that rotatably supports the first roller;
• a second bearing member that rotatably supports the second roller;
• a roller holder including a first bearing holding portion and a second bearing holding portion that hold the first and second bearing members respectively such that the first and second bearing members are slidable in directions toward and away from the image carrying member;
• a first urging member disposed between the first bearing holding portion and the first bearing member, the first urging member urging the first bearing member in a direction toward the image carrying member;
• a second urging member disposed between the second bearing holding portion and the second bearing member, the second urging member urging the second bearing member in a direction toward the image carrying member;
• a switching cam having a first guide hole with which a first engaging portion and a second engaging portion formed in the first and second bearing members, respectively, engage;
• a driving mechanism that drives the roller holder and the switching cam to rotate;
• a unit frame that rotatably supports the roller holder and the switching cam;
• and
• a fixed cam fixed to the unit frame,
• wherein
• the roller holder is rotated such that either the first or second roller is arranged opposite the image carrying member and
• the switching cam is rotated to change an engagement position of the first or second engaging portion with the first guide hole to thereby arrange the first or second roller arranged opposite the image carrying member selectively between a reference position where the first or second roller is kept in pressed contact with the image carrying member to form the transfer nip portion and a separation position where the first or second roller is kept away from the image carrying member, and
• the fixed cam includes:
• a second guide hole formed so as to overlap the first guide hole, the second guide hole being engaged with the first and second engaging portions;
• a positioning groove formed in an outer peripheral part of the second guide hole along a radial direction, the positioning groove being engaged with, when the first roller is arranged opposite the image carrying member, the first engaging portion and, when the second roller is arranged opposite the image carrying member, the second engaging portion;
• and
• a cam positioning recessed portion formed in an outer peripheral part of the second guide hole along the radial direction, the cam positioning recessed portion being engaged with the second engaging portion when the first engaging portion engages with the positioning groove.