A sheet stacking device includes a stacking tray, an aligning mechanism, a pair of alignment members, and a pair of height detectors. The pair of alignment members can move independently in a sheet width direction and in an up and down direction, and contact side edges in the sheet width direction of sheets stacked on the stacking tray, from both sides in the sheet width direction, so as to align the sheets in the sheet width direction. The pair of height detectors allow the pair of alignment members to contact an upper surface of the sheets near side edges of the sheets, so that sheet heights on both sides in the sheet width direction are detected. The sheet stacking device can individually correct heights of the pair of alignment members when aligning the sheets, based on the sheet heights detected by the pair of height detectors.

There are provided a sheet feeding apparatus and an image forming system which can horizontally maintain a posture of a topmost envelope in stored envelopes, by a simple configuration. The sheet feeding apparatus includes: a stacking surface portion 321; a sheet feeding roller 313; a pressing surface portion 362; and a curve forming portion 363. A length of the stacking surface portion 321 in a width direction is shorter than a length of each of envelopes P in a width direction. The pressing surface portion 362 is pressed by the sheet feeding roller 313 via the envelopes P. The curve forming portion 363 is arranged outside the sheet feeding roller 313 in a width direction, projects more upward in a vertical direction than the pressing surface portion 362, and supports the plurality of stacked envelopes P with a predetermined size.

A method for reducing the effects of variations in an unwinding, convolutely wound roll of web material is disclosed. The method utilizes the steps of: a. selecting a reference objective relating to a downstream operation, b. choosing at least one feedback device correlated to the reference objective, c. collecting process data from the at least one feedback device at different positions within a time-varying operation cycle for at least one operation cycle at a learning speed, d. calculating an error as the difference between the collected process data from step (c) and a reference signal related to the selected reference objective, e. generating a correction signal based upon the calculated error from step (d) and, f. applying the correction signal to the actuator during a succeeding time-varying operation cycle.