Sheet stack height control system

In a sheet stacking system for stacking plural printed flimsy print substrate sheets from a printed sheets output path into at least one sheet stacking tray, including a sheet entrance path into said sheet stacking tray by which the printed sheets are fed into said sheet stacking tray to be stacked on top of sheets previously stacked in said sheet stacking tray, and including a stack height sensing system with a stack height sensor for detecting the height of the stack of printed sheets stacked in said sheet stacking tray, and a stacking control system controlled by said stack height sensor;

• the improvement in said stack height sensor comprising:
• an elongated maximum stack height sensing bar,
• a sensing bar movement system for intermittently moving said maximum stack height sensing bar into said sheet stacking tray and placing said sensing bar into an operative measurement position on top of the stack of printed sheets in said sheet stacking tray at a position transversely of said sheet entrance path with a normal force pressing downwardly against the top of the stack of printed sheets in said sheet stacking tray for a maximum stack height measurement,
• said maximum stack height sensing bar in said operative measurement position extending at least substantially across the top of the stack in said sheet stacking tray at the height of the highest level of any portion of the stack underlying said maximum stack height sensing bar in said operative measurement position,
• and a sensing system actuated by said height of said maximum stack height sensing bar in said operative measurement position to provide signals to said stacking control system corresponding to the maximum stack height,
• said stacking control system providing a control signal when said maximum stack height in said sheet entrance path as signaled by said sensing system reaches a preset height which is preset below the height which would obstruct said sheet entrance path to said sheet stacking tray.