Patent ID: 7201370

Claim:
A sheet processing apparatus comprising: a switchback portion configured to receive conveyed sheets and send them in an opposite direction, thereby invert a conveying direction of the sheets; a first detector to detect lengths of the sheets in the conveying direction before being received by the switchback portion; a second detector to detect lengths of the sheets in the conveying direction after being sent from the switchback portion; and a first overlapped sheets detector to detect overlapping of the sheets when detection results of the first and second detectors are different, wherein the switchback portion includes a drive roller, driven to rotate in both forward and backward directions, on one side of the sheets, and a driven roller, following the sheets, in a state that the sheets are held between the drive roller and the driven roller, and wherein the switchback portion is so constructed that: two overlapping sheets are shifted with respect to each other when the rotation of the drive roller is decelerated in the forward direction because a sheet contacting the drive roller slows down according to the deceleration of the drive roller, while the other sheet contacting the driven roller is maintained at a constant speed by inertial rotation of the driven roller; and the two overlapping sheets are more shifted with respect to each other when the drive roller is rotated in the backward direction because the sheet contacting the drive roller moves in the backward direction according to the backward rotation of the drive roller, while the other sheet contacting the driven roller is stopped because the driven roller is maintained at rest by inertia.