Abstract: A sheet stacker, particularly for corrugated paperboard container blanks, has a downwardly movable elevator for supporting stacked sheets, firing rollers for feeding successive sheets in a path above the elevator, and a stop for stopping each successively fed sheet above the elevator and enabling each stopped sheet to drop onto a stack being formed. A flexible cam, preferably a resiliently flexible loop, is rotatably mounted above the elevator. The flexible cam is intermittently rotated to move the cam out of the path of a sheet being fed to allow a leading portion of this fed sheet to pass under the flexible cam, then to bring the cam into contact with a rear portion of the fed sheet to urge this rear portion towards the stack, and then to bring the flexible cam to rest in kissing contact with the fed sheet when resting on top of the stack being formed. Tolerance variations in the sheet thickness can be accommodated by flexing of the cam.