This invention relates to a sheet transport device for use in a printer, copying machine or like image forming apparatus.
There has been known a sheet transport device provided with feed rollers 3, 4 for dispensing sheets one by one from cassettes 1, 2 containing the sheets therein and a sheet separator 5 including a forward roller 18 which rotates in a direction along transport of a sheet and a retard roller 21 rotatable in a direction against the transport of the sheet as shown in FIG. 8. The retard roller 21 is mounted on a drive shaft which is biased toward the forward roller 18 by a spring or other biasing member, and accordingly is held in contact with the forward roller 18. Further, a torque limiter is mounted to the drive shaft of the retard roller 21 so as to control transmission of torque from a driving source to the drive shaft of the retard roller 21. The torque limiter is designed to cause the retard roller 21 to idly rotate upon receipt of the torque greater than a specified value.
The driving force of the forward roller 18 is set greater than that of the retard roller 21. In a normal sheet transport operation, the driving force of the transport roller 18 is transmitted to the retard roller 21 directly or through the sheet held between the rollers 18 and 21, activating the torque limiter to cause the retard roller 21 to idly rotate. Thereby, the retard roller 21 is caused to rotate in the sheet transport direction together with the forward roller 18 to transport the sheet.
In the case where a plurality of sheets are transported to a nip between the rollers 18 and 21 with one sheet adhered to another, i.e. in case of multiple feeding, the sheets come into sliding contact with one another, preventing transmission of the driving force of the forward roller 18 to the retard roller 21. This causes the retard roller 21 to rotate in a direction opposite to the rotating direction of the forward roller 18 and thereby the sheet in contact with the forward roller 18 and the one in contact with the retard roller 21 are subjected respectively to a transporting force and a retarding force working in opposing directions and separated from each other. As a result, transport of the sheet in contact of the retard roller 21 can be deterred.
The forward and retard rollers 18 and 21 are usually arranged as shown in FIGS. 9 and 10. With reference to FIG. 9, the forward roller 18 includes a long roller member while the retard roller 21 including a plurality of short roller members arranged at specified spacings along an axis. The number of roller members and the spacing between two adjacent roller members of the retard roller 21 are determined based on the width of copy sheets standardized for use in an image forming apparatus (hereafter referred to as standardized copy sheets). With this arrangement of the roller members of the retard roller 21, the following problems arise in the case where special sheets having the small width are to be transported. The special sheet may be material made of sheet, such as an envelope. Let it be assumed that special sheets A are being transported in line in a sheet transport device provided with a retard roller 21 including two roller members spaced along a drive shaft of the roller 21 as shown in FIG. 9. In this case, the roller members of the retard roller 21 unavoidably come into contact with the forward roller 18 at outer sides thereof. If only a portion of the retard roller 21 is in contact with the forward roller 18 when the sheet is separated, the driving force of the roller 18 is transmitted to the roller 21 and thereby the torque limiter causes the roller 21 to idly rotate. This will result in unsatisfactory sheet separation. In other words, since friction drag working on the contact portion of the rollers 18 and 21 is exceedingly large, the torque limiter disengages from the drive shaft of the roller 21 which in turn rotates together with the roller 18 in the sheet transport direction. Accordingly, in the case where two special sheets A are transported with the one sheet adhered to the other, i.e. in case of double feeding, the two special sheets A cannot be subjected to the transporting and retarding forces and therefore cannot be separated from each other properly.
The sheet transport device may be provided with a retard roller 21 having a single roller member as shown in FIG. 10 so as to properly separate special sheets A fed in line as described above. This sheet transport device suffers the disadvantage that, when the two special sheets A are transported side by side at one time, a center portion of the roller 21 comes into contact with the forward roller 18 and therefore the special sheets A cannot be separated properly.
In view of the above, a requirement is that the retard roller is not in contact with the forward roller when the special sheets having the small width such as envelopes are separated. Accordingly, it is preferable that the forward and retard rollers be away from each other in a normal state. However, since the standardized copy sheets which are most frequent in use are generally thin, it is preferable to keep the forward and retard rollers in contact with one another in the normal state in order to transport the standardized copy sheets reliably. On the contrary, the special sheets such as envelopes are generally thicker than the standardized copy sheets.