Stack lifting apparatus and method

A stack lifting apparatus for a sheet processing machine, the stack lifting apparatus comprising, a stack lifting table for supporting a sheet stack, the sheet stack comprising a plurality of sheets, a feeding mechanism for feeding the sheets from the sheet stack into the sheet processing machine, a raising means for raising the stack lifting table and the sheet stack supported thereon, a sensor for determining the height of the sheet stack at least at maximum height level and a minimum height level and providing maximum and minimum signals corresponding thereto, means for determining the speed at which sheets are fed from the stack and providing a value corresponding thereto, and a controller, the controller including a memory for storing the distance between the predetermined maximum and minimum levels, a processor, the processor connected to the memory, to the sensor to receive the maximum and minimum signals therefrom, to the means for determining the speed at which sheets are fed, and to the raising means, the processor including a means for determining a rate at which the stack height is lessened as sheets are fed from the stack lifting table, said means operable by recalling from memory the distance between the predetermined maximum and minimum levels and combining it with the value corresponding to the speed at which sheets are fed from the stack, and a means for controlling the speed of the raising means to initially raise the stack lifting table at a rate greater than the rate at which the stack height is lessened thereby raising the top of the stack to a desired level above the minimum level as sheets are fed to the sheet processing machine, and to subsequently raise the stack lifting table at a rate corresponding to the rate at which feeding of the sheets lessens the stack height, thereby maintaining the top of the stack at substantially the desired level as sheets are fed to the sheet processing machine.