Sheet transport device, image reading device, and image forming apparatus

1. A sheet transport device comprising:

a sheet transport unit that transports a sheet by nipping the sheet in a nip between a plurality of pairs of transport rollers arranged in a sheet width direction intersecting a sheet transport direction;

a detector that detects a size of the transported sheet; and

a releasing unit that releases the nip of one or more of the pairs of transport rollers outside a transport region of the sheet based on the size of the sheet detected by the detector.

2. The sheet transport device according to claim 1 ,

wherein the pairs of transport rollers include a first pair of transport rollers disposed in a middle in the sheet width direction and a plurality of second pairs of transport rollers disposed at opposite sides of the first pair of transport rollers in the sheet width direction, the first pair of transport rollers including a driving roller and a first driven roller, the plurality of second pairs of transport rollers each including a driving roller and a second driven roller, and

wherein if the size of the sheet detected by the detector is smaller than a predetermined size, the releasing unit releases the nip of the second pairs of transport rollers.

3. The sheet transport device according to claim 2 ,

wherein each second driven roller is supported in a movable manner between a transport position and a separated position with a shaft as a center, the shaft rotatably supporting the first driven roller, the transport position being where the second driven roller forms the nip with the driving roller, the separated position being where the second driven roller is separated from the driving roller, and

wherein if the size of the sheet detected by the detector is smaller than the predetermined size, the releasing unit causes the second driven rollers to move to the separated position.

4. The sheet transport device according to claim 3 ,

wherein the second driven rollers move to the separated position by rotating downstream in the sheet transport direction.

5. The sheet transport device according to claim 3 ,

wherein an outermost second driven roller of the second driven rollers in the sheet width direction forms the nip with the driving roller every time the outermost second driven roller rotates by 360° from the transport position about the shaft that rotatably supports the first driven roller.

6. The sheet transport device according to claim 4 ,

wherein an outermost second driven roller of the second driven rollers in the sheet width direction forms the nip with the driving roller every time the outermost second driven roller rotates by 360° from the transport position about the shaft that rotatably supports the first driven roller.

7. The sheet transport device according to claim 3 ,

wherein at least one of the second driven rollers adjacent to the first driven roller in the sheet width direction forms the nip with the driving roller every time the at least one second driven roller rotates by 180° from the transport position about the shaft that rotatably supports the first driven roller.

8. The sheet transport device according to claim 4 ,

wherein at least one of the second driven rollers adjacent to the first driven roller in the sheet width direction forms the nip with the driving roller every time the at least one second driven roller rotates by 180° from the transport position about the shaft that rotatably supports the first driven roller.

9. An image reading device comprising:

an imaging unit that reads an image of a sheet; and

the sheet transport device according to claim 1 that transports the sheet to a read position where the imaging unit reads the sheet.

10. An image forming apparatus comprising:

the image reading device according to claim 9 that reads an image of a sheet; and

an image recorder that records the image read by the image reading device onto a recording medium.