Image forming apparatus having curved contact surface

An image forming apparatus comprising:

• an image bearing member for bearing a toner image;
• a movable endless transfer belt for transferring the toner image from said image bearing member onto a transfer material;
• and
• a transfer device for transferring the toner image from said image bearing member toward said transfer belt, wherein said transfer device includes a transfer member for sliding on an inner surface of said transfer belt in contact with the inner surface, a supporting member for supporting said transfer member, and an urging member for urging said supporting member toward said transfer belt,
• wherein said transfer member contacts the inner surface of said transfer belt at a contact surface thereof without rotation relative to said supporting member during movement of said transfer belt,
• wherein said supporting member is rotatable about a center axis during movement of said transfer belt, and wherein when a maximum angle of rotation of said supporting member about the center axis toward a downstream side with respect to a movement direction of said transfer belt is θ, a swing radius of said supporting member corresponding to a maximum distance from a center of the center axis to the contact surface in a non-contact secondary transfer between said supporting member and said transfer belt is R, an axis-belt distance corresponding to a minimum distance from the center of the center axis to a rear surface of said transfer belt is d, a proportion of the axis-belt distance to the swing radius is m, with 0<m≦1, a radius of curvature of the contact surface is r, and a proportion of the radius of curvature to the swing radius is n, with 1≦n, the following relationship is satisfied:
• n
• 2
• -
• (
• m
• -
• b
• )
• 2
• +
• a
• n
• 2
• -
• (
• m
• +
• n
• -
• 1
• )
• 2
• ≤
• 1.1
• ⁢
• (
• a
• =
• (
• 1
• -
• n
• )
• ×
• sin
• ⁢
• ⁢
• θ
• ,
• b
• =
• (
• 1
• -
• n
• )
• ×
• cos
• ⁢
• ⁢
• θ
• )
• .