Patent ID: 8387974

Claim:
A sheet stacking apparatus, comprising: a lifting and lowering tray on which discharged sheets are stacked, and which lowers with an increase in a number of the sheets stacked on the tray; and an aligning portion pair configured to align the sheets stacked on the tray in a width direction intersecting with a sheet discharging direction, and to align the sheets constituting a next sheet bundle stacked on a sheet bundle previously stacked in a position displaced in the width direction from a stacking position of the sheets bundle previously stacked, each of the aligning pair including: a lifting and lowering first aligning member which is moved in the width direction to be brought into contact with a side edge in the width direction of discharged sheets on the tray; a second aligning member which is moved in the width direction to be brought into contact with a side edge in the width direction of discharged sheets on the tray; and a support unit which supports the first aligning member so that when the tray lowers within a predetermined distance, the first aligning member is laid on the sheet bundle previously stacked on the tray, and when the tray lowers beyond the predetermined distance, the first aligning member is separated from the sheet bundle previously stacked, the sheet stacking apparatus further comprising: a driving unit which drives the aligning portion pair; and a controller configured to control the driving unit in such a manner that when the aligning portion pair aligns sheets, the first aligning member of one of the aligning portion pair is lifted to a position separated from the sheets to be aligned, and the second aligning member of the one of the aligning portion pair and the first aligning member of the other of the aligning portion pair align the sheets, and when the aligning portion pair performs an aligning operation of aligning sheets constituting the next sheet bundle, the first aligning member of the other of the aligning portion pair is lifted to a position separated from the sheets to be aligned, and the second aligning member of the other of the aligning portion pair and the first aligning member of the one of the aligning portion pair align the sheets, so that the aligning portion pair performs the aligning operation every time a first predetermined number of the sheets constituting the next sheet bundle are discharged while the first aligning member of the one of the aligning portion pair is laid on the sheet bundle previously stacked, and then the aligning portion pair performs the aligning operation every time a second predetermined number less than the first predetermined number of the sheets constituting the next sheet bundle are discharged after the first aligning member of the one of the aligning portion pair is separated from the sheet bundle previously stacked by a descent of the tray.