Patent ID: 8571450
Filing Date: 2013-10-29
Classification: G03G

Abstract:
1. An image forming apparatus, comprising: a plurality of N image carriers, including first through Nth image carriers, to carry toner images formed thereon, satisfying a relation of N≧2; a transfer unit including an endless transfer belt an outer surface of which contacts the plurality of N image carriers, to successively transfer the toner images formed on the plurality of N image carriers onto either the outer surface of the endless transfer belt or a surface of a recording medium carried on the endless transfer belt; a plurality of M belt supporting members disposed in contact with an inner surface of the endless transfer belt to support the endless transfer belt, satisfying a relation of M>N, the plurality of M belt supporting members including a plurality of first through Nth nip opposing members contacting the inner surface of the endless transfer belt at positions corresponding to where the plurality of N image carriers contacts the outer surface of the endless transfer belt to form N transfer nips thereat, the first nip opposing member defining a first supported area of the endless transfer belt in a circumferential direction thereof where the first nip opposing member contacts and supports the endless transfer belt; a contact and separation mechanism including a retaining unit that supports at least the first nip opposing member, the contact and separation mechanism moving the retaining unit to move the first supported area of the endless transfer belt into and out of contact with the first image carrier; and a mark detector provided in proximity to the endless transfer belt, fixedly mounted on the retaining unit, and moved by the contact and separation mechanism together with the retaining unit that supports at least the first nip opposing member, the mark detector detecting a plurality of adjacent marks formed at a predetermined pitch on the endless transfer belt in the circumferential direction of the endless transfer belt, the image forming apparatus further comprising a controller operatively connected to the mark detector and to a motor that drives the endless transfer belt, the controller controlling a motor speed to control a drive speed of the endless transfer belt based on detection results obtained by the mark detector, wherein the first nip opposing member and the first image carrier together form an extreme downstream nip of the N transfer nips in a direction of rotation of the endless transfer belt with the endless transfer belt interposed therebetween, the retaining unit including a first sub-retainer that retains the first nip opposing member and a second sub-retainer that retains the plurality of N nip opposing members other than the first nip opposing member, the contact and separation mechanism comprising: the mark detector being fixedly mounted on the first sub-retainer.