Patent ID: 7400852

Claim:
An image forming apparatus comprising: photosensitive drums; an exposing unit which exposes the photosensitive drums, and forms latent images on the surfaces of the photosensitive drums; developing units which develop the latent images formed on the surfaces of the photosensitive drums; a transfer belt which moves while contacting or separating from the surfaces of the photosensitive drums; primary transfer rollers which are provided at the positions opposite to the photosensitive drums, moved to the transfer belt to make the transfer belt contact with the photosensitive drums, and transfers the images on the photosensitive drums to the transfer belt; a secondary transfer roller which transfers the images transferred to the transfer belt to a paper sheet; a transfer roller drive unit which has a contact mode to move all or some of the primary transfer rollers to the transfer belt and make the transfer belt contact with all or some of the photosensitive drums, and an all-separate mode to move all of the primary transfer rollers to the opposite side of the transfer belt, and separate the transfer belt from all of the photosensitive drums, and sets selectively the contact mode and all-separate mode; and a controller which starts the rotation of the photosensitive drums and sets the contact mode by the transfer roller drive unit for the exposing by the exposing unit, and sets the all-separate mode by the transfer roller drive unit and stops the rotation of the photosensitive drums after the paper sheet passes through the secondary transfer roller, wherein the transfer roller drive unit has springs which give the primary transfer roller a deviating force toward the transfer belt; a motor; a shaft to transmit the power of the motor; a first cam and a second cam provided in the shaft; at least one first lever which moves forward and backward according to the rotation of the first cam; at least one second lever which moves forward and backward according to the rotation of the second cam; at least one first hook which engages with at least one of the primary transfer rollers and moves the primary transfer roller to the opposite side of the transfer belt against the deviating force of the spring by interlocking with the forward movement of the first lever, and releases the engagement with the primary transfer roller and moves the primary transfer roller to the transfer belt by the deviating force of the spring by interlocking with the backward movement of the first lever; and at least one second hook which engages with one of the other primary transfer rollers not engaged with the first hook and moves the primary transfer roller to the opposite side of the transfer belt against the deviating force of the spring by interlocking with the forward movement of the second lever, and releases the engagement with the other primary transfer roller and moves the primary transfer roller to the transfer belt by the deviating force of the spring by interlocking with the backward movement of the second lever.