Offset printing apparatus operating in tandem

An offset printing apparatus operating in tandem including a frame having an upper portion on which are mounted an inking device and a moistening device for respectively applying a film of ink and a film of moistening liquid on a touching cylinder having a horizontal axis and rotatively mounted on the frame and having a lower portion on which are rotatively and tangently mounted a blanket cylinder and counter-cylinder, with a web to be printed passing therebetween, and having axes which extend horizontally and transversely and therefore perpendicularly to the web to be printed, the axis of the blanket cylinder and the axis of the touching cylinder being placed in a same vertical plane, and wherein it further includes two plate cylinders having horizontal and transverse axes placed respectively on either side of a vertical plane passing through the axes of the touching cylinder and blanket cylinder, a support mounted at a level intermediate between the touching cylinder and blanket cylinder, said support being provided for the two plate cylinders and being tiltably mounted about the axis of the blanket cylinder, two starting cylinders having horizontal and transverse axes disposed respectively slightly below and outside with respect to each of the two plate cylinders, and control means for controlling a tilting motion of the support of the two plate cylinders for setting under pressure each of said two plate cylinders and for rotatively driving the starting cylinders in order to bring alternatively each of the two plate cylinders either in a printing position in which the plate cylinder is in contact under pressure both with the upper touching cylinder and with the lower blanket cylinder after having previously started the plate cylinder in rotation at a same peripheral speed as that of the blanket cylinder and having keyed the plate cylinder in an angular position, or in a stopped waiting position in which the pate cylinder is retracted from the touching cylinder and blanket cylinder and is bearing against a corresponding starting cylinder for enabling changing of a printing plate thereof.