Sheet discharge device and image forming apparatus

A sheet discharge device comprising:

• a sheet conveying portion configured to convey sheets one by one;
• a discharge port through which the sheets are discharged;
• a sheet storage portion configured to store in a stack a plurality of sheets that have been conveyed by the sheet conveying portion, in a state where front ends of the plurality of sheets protrude from the discharge port;
• a sheet discharge portion configured to discharge a plurality of sheet bundles from the sheet storage portion through the discharge port, each sheet bundle including a plurality of sheets;
• a discharge tray on which sheet bundles discharged through the discharge port are stacked;
• a tray lifting and lowering mechanism supporting the discharge tray in such a way as to lift and lower the discharge tray;
• an upper-surface detection sensor configured to detect whether or not an upper surface of the sheet bundles stacked on the discharge tray has reached a predetermined height;
• a lowering control portion configured to, upon a discharge of a sheet bundle onto the discharge tray, control the tray lifting and lowering mechanism to lower the discharge tray by a predetermined lowering distance;
• a lifting control portion configured to, after the discharge tray is lowered by the lowering control portion, wait until a front end of an initial sheet of a next sheet bundle becomes in a state of protruding from the discharge port as detected by one or more sheet sensors, and then lift the discharge tray until the upper-surface detection sensor detects that the upper surface of the sheet bundles has reached the predetermined height;
• a shift detection processing portion configured to detect whether or not the upper surface of the sheet bundles is shifted from a detectable range of the upper-surface detection sensor,
• wherein in a case where the shift detection processing portion has detected that the upper surface of the sheet bundles is shifted, the lifting control portion waits until the front end of the initial sheet of the next sheet bundle becomes in the state of protruding from the discharge port as detected by the one or more sheet sensors and then lifts the discharge tray;
• and
• wherein the shift detection processing portion detects whether or not the upper surface of the sheet bundles is shifted from the detectable range of the upper-surface detection sensor, based on a result of comparison between a lowering distance by which the discharge tray was lowered by the lowering control portion and a lifting distance by which the discharge tray was lifted by the lifting control portion.