The present invention relates to a method of and apparatus for feeding a sheet. More particularly, this invention concerns an adjustable feed system for picking the uppermost sheet off a stack and advancing it along a path.
In many printing and copying machines it is necessary to lift the uppermost sheet off a stack of similar sheets and advance it along a path so that a copy can be made on the sheet, it can be printed, or a similar operation can be performed on this single sheet. It is essential in such systems that the sheet arrive at the end of its path at a precisely controlled instant. Therefore, the sheet must at a precisely determined instant start at the upstream end of the path.
The sheet is normally picked off the top of the stack by means of a suction lifter such as described in the commonly assigned patent application 689,857 filed May 25, 1976 by Karl-Heinz Vollrath. Such a suction lifter delivers the sheet after lifting it from the top of the stack to the nip between a pair of rollers. One of these rollers is usually driven continuously and the other roller is moved synchronously with the suction lifter toward and away from the rotating roller. The movable roller moves toward the fixed roller as the sheet is inserted between it so as to press this sheet against the rotating fixed roller and cause it to be advanced along the displacement path for the sheet.
Since in such devices the displacement speed of the lifter is normally not the same as the peripheral speeds of the rollers, it is absolutely essential that the vacuum lifter release the sheet before it is gripped between the rollers. If the vacuum lifter releases the sheet too late, tearing of this sheet can readily result.
Furthermore, if the vacuum lifter releases the sheet too soon, that is before it is properly engaged between the juxtaposed rollers, misalignment of the sheet in the path can readily occur so that any downstream operation performed on the sheet will not be properly centered on the sheet or aligned with the edges of the sheet. It is also possible when the sheet is released too soon that this sheet not even be placed between the rollers so that a misfeed will occur that can readily lead to subsequent jamming-up of the feed apparatus.
For this reason the movable roller is normally provided with some adjustment means that allows its rest position, that it when it is closest to the fixed roller, to be varied. Thus when relatively thick sheets are being transported the movable roller is spaced further away from the fixed roller than when relatively thin or light sheets are being transported. In such an arrangement it is normally necessary to adjust each of the feed rollers, a plurality normally being provided, for each change in workpiece weight. This is a time-consuming and onerous operation. Furthermore, mal-adjustment of even one of the rollers can result in the destruction of the workpiece so that extreme care must be exercised in the adjustment operation.