Transfer unit with a non-selected transfer roller retracted behind a guide and image forming apparatus therewith

A transfer unit that transfers a toner image formed on an image carrying member, to a recording medium as the recording medium passes through a transfer nip, the transfer unit comprising:

• a transfer roller including a metal shaft and an elastic layer laid around an outer circumferential face of the metal shaft the transfer roller forming the transfer nip by keeping the elastic layer in pressed contact with the image carrying member, the transfer roller including a first roller and a second roller, the second roller being arranged downstream of the first roller in a conveyance direction of the recording medium and being different from the first roller in a length of the elastic layer in an axial direction;
• a roller switching mechanism that arranges the first or second roller selectively at a reference position where the first or second roller forms the transfer nip by being kept in pressed contact with the image carrying member;
• an upstream-side guide arranged upstream of the transfer nip in the conveyance direction of the recording member;
• and
• a downstream-side guide arranged downstream of the transfer nip in the conveyance direction of the recording member,
• wherein
• when the first roller moves to the reference position by the roller switching mechanism, the second roller moves to a first retracted position behind the downstream-side guide, and
• when the second roller moves to the reference position b the roller switching mechanism, the first roller moves to a second retracted position behind the upstream-side guide and opposite the image carrying member across the upstream-guide, wherein
• the roller switching mechanism comprises:
• a first bearing member that rotatably supports the first roller;
• a second bearing member that rotatably supports the second roller;
• a roller holder that has a first bearing holding portion and a second bearing holding portion that respectively hold the first and second bearing members slidably in directions toward and away from the image carrying member;
• a first urging member arranged between the first bearing holding portion and the first bearing member, the first urging member urging the first bearing member in the direction toward the image carrying member;
• a second urging member arranged between the second bearing holding portion and the second bearing member, the second urging member urging the second bearing member in the direction toward the image carrying member;
• a switching cam that has a guide hole that engages with a first engaging portion formed on the first bearing member and a second engaging portion formed on the second bearing member;
• and
• a driving mechanism that drives the roller holder and the switching cam to rotate,
• wherein
• by rotating the roller holder, one of the first and second rollers is arranged opposite the image carrying member, and
• by rotating the switching cam to change positions at which the first and second engaging portions engage with the guide hole, the first or second roller that is arranged opposite the image carrying member is arranged selectively either at the reference position at which the first or second roller is kept in pressed contact with the image carrying member to form the transfer nip or at a released position at which the first or second roller stays away from the image carrying member.