The invention provides a unit for printing a web of paper including a frame and at least a first and a second printing group, each printing group including a blanket cylinder and a plate cylinder, the printing unit also including a throw off mechanism for supporting and moving the cylinders, into at least one throw-on configuration, in which the blanket cylinders are pressed against each other and against the plate cylinders, and a throw-off configuration, in which the blanket cylinders are spaced apart from each other and from the plate cylinders. The throw-off mechanism includes at least a first system for returning the mass of at least a first cylinder of the first printing group towards a first cylinder of the second printing group and vice-versa so that the first cylinders form mutual counterweights when the printing unit moves between the throw-on and throw-off configurations thereof. |