A device attached to the infeed system of a printing press (especially one used in sheetfed offset lithography) that compensates for the effect of fan-out, an undesirable expansion of a sheet of paper caused by pressure between the impression cylinder and the blanket cylinder. This increased pressure causes the sheet to expand its tail end just prior to printing. When the sheet leaves the printing unit, it relaxes nearly to its original dimensions, but the image that is printed will consequently have a narrower tail end. When the sheet passes into a second printing unit, the sheet fans out again, but this second image will have a narrower tail than the first, and will thus be out of alignment. A gripper-bowing device reduces this problem by altering the alignment of the grippers (typically on the rotating drum feeding the sheet from the feedboard to the impression cylinder). The gripper bar (the bar containing the set of grippers) is bowed, setting the center gripper forward by as much as 0.008 inch, so that the sheet encounters a convex surface, and distorts slightly. This exaggerates the fanning out of the sheet, and causes the first image to print with a narrower tail end than would be printed with a normal gripper bar. As a result, the narrow images printed by successive printing units (whose grippers are not bowed) will fit the first printed image. The key to this system is ensuring that the exaggerated fanning out of the sheet in the first printing unit produces a narrowed image equal to that produced by normal degrees of fanning out in successive printing units.
A gripper-bowing device can also be used to bow the grippers in, which has the effect of widening an image at its tail end, which may be desirable in some printing conditions.