Sheet stacker and image forming apparatus

A sheet stacker, comprising:

• a load table to stack a bundle of sheets;
• a conveyor to convey a sheet to the load table in a conveyance direction at a conveyance speed;
• a guide including a holder to:
• hold a leading end of the sheet conveyed by the conveyor by the holder at a standby position;
• and
• move the holder to guide the sheet along a movement path in the conveyance direction at a moving speed while holding the sheet by the holder;
• a leading-end aligner including a partition extending to the movement path of the holder, the leading-end aligner to align, in the conveyance direction, a leading-end position of the sheet on the load table;
• and
• a shifter to move the leading-end aligner in the conveyance direction;
• and
• a shift motor to move the shifter;
• circuitry configured to differentiate the conveyance speed of the conveyor and the moving speed of the holder to detach and load the sheet, held by the holder, on the load table at a detachment position,
• wherein:
• the shift motor steps out under a step-out load,
• the leading-end aligner has a first withstand load,
• the guide has a second withstand load, and
• the step-out load of the shift motor is smaller than each of the first withstand load of the leading-end aligner and the second withstand load of the guide.

An image forming apparatus, comprising:

• a sheet stacker;
• a guide including a holder a load abnormality detector to detect a stop of the holder as an occurrence of an abnormality,
• the sheet stacker including:
• a load table to stack a bundle of sheets;
• a conveyor to convey a sheet to the load table in a convevance direction at a conveyance speed:
• the holder to:
• hold a leading end of the sheet conveyed by the conveyor by the holder at a standby position;
• and
• move the holder to guide the sheet along a movement path in the conveyance direction at a moving speed while holding the sheet by the holder;
• a leading-end aligner including a partition extending to the movement path of the holder, the leading-end aligner to align, in the conveyance direction, a leading-end position of the sheet on the load table:
• circuitry configured to differentiate the conveyance speed of the conveyor and the moving speed of the holder to detach and load the sheet, held by the holder, on the load table at a detachment position;
• and
• a plurality of sheet ejection destinations different from the sheet stacker,
• wherein the circuitry causes the conveyor to convey the sheet to any of the plurality of sheet ejection destinations in response to a detection of the abnormality by the load abnormality detector.