Sheet postprocessing apparatus and image forming system

A sheet postprocessing apparatus comprising:

• a processing tray on which sheets are stacked and arranged;
• a vertical registration member for registering a front edge side and a rear edge side in a transport direction of the sheets stacked and arranged on the processing tray;
• and
• a sheet pressing member for pressing the sheets stacked on the processing tray from above,
• the vertical registration member increasing a rate of increase in transport force generated for the sheets by the vertical registration member, in conjunction with a rate of increase in transport resistance generated for the sheets by the sheet pressing member with an increase in number of the sheets stacked on the processing tray,
• wherein the vertical registration member includes a rotating shaft, two or more flexible paddles provided radially with respect to the rotating shaft and rotating with the rotating shaft, and a same number of projecting portions provided radially with respect to the rotating shaft, each of the projecting portions is downstream from a respective one of the paddles with respect to a direction of rotation of the rotating shaft, and rotating with the rotating shaft,
• a distance from an edge of the each of the paddles to the center of the rotating shaft is longer than a distance from an edge of the each of the projecting portions to the center of the rotating shaft,
• the each of the projecting portions is disposed relative to the each of the paddles so as to create a space in which the each of the paddles is deflected toward the each of the projecting portions,
• distances from the edges of the projecting portions to the center of the rotating shaft are all equal.