A printing unit has a printing form cylinder. A plate changer is arranged in a manner which is allocated to the printing form cylinder. The plate changer has a bearing surface on which a printing form, which is arranged or is to be arranged on the printing form cylinder, can be laid. That bearing surface is arranged such that it can be moved to and fro between at least two defined positions longitudinally with respect to the rotational axis of the printing form cylinder. The printing form is arranged and registered on the bearing surface of the plate changer. At least two edges of a carrier for the relevant printing form, which are arranged at a right angle with respect to each other, are brought into physical contact with register pins which are arranged on the bearing surface of the plate changer. A first edge of the carrier of the relevant printing form is arranged to bear against a first register pin and a second edge, orthogonal with respect to the first edge, of the carrier of the relevant printing form, is arranged to bear against a second register pin. The printing unit is preferably arranged as a device for printing hollow bodies.

A printing unit has a printing forme cylinder. A plate changer is arranged in a manner which is allocated to the printing forme cylinder. The plate changer has a bearing surface on which a printing forme, which is arranged or is to be arranged on the printing forme cylinder, can be laid. That bearing surface is arranged such that it can be moved to and fro between at least two defined positions longitudinally with respect to the rotational axis of the printing forme cylinder. The printing forme is arranged and registered on the bearing surface of the plate changer. At least two edges of a carrier for the relevant printing forme, which are arranged at a right angle with respect to each other, are brought into physical contact with register pins which are arranged on the bearing surface of the plate changer. A first edge of the carrier of the relevant printing forme is arranged to bear against a first register pin and a second edge, orthogonal with respect to the first edge, of the carrier of the relevant printing forme, is arranged to bear against a second register pin. The printing unit is preferably arranged as a device for printing hollow bodies.

An apparatus is comprised of a plurality of printing units for printing hollow elements. A conveying device conveys the hollow elements to be printed into a printing zone of at least one of the printing units. At least one of the printing units includes a plate cylinder to which a plate replacing mechanism is assigned.

A printing cylinder assembly for a printing machine includes a rotating shaft pivotally supported at one axial end shaft section thereof in a cantilever structure, and a sleeve cylinder removably mounted on the rotating shaft so that the cylinder can be fitted on and extracted from the rotating shaft from the other axial end side of the rotating shaft. The rotating shaft is formed at one axial end side thereof with a sleeve cylinder supporting tapered surface, and the sleeve cylinder is formed at one axial end side of an inner diameter portion thereof with a tapered surface, whereby the tapered surface of the sleeve cylinder can be fitted on and in close contact with the sleeve cylinder supporting tapered surface of the rotating shaft and thereby detachably coupling one axial end portion of the sleeve cylinder with one axial end portion of the rotating shaft.

A stopper for assuredly preventing shifting of a cylindrical printing plate mounted on a plate cylinder is provided. A plate cylinder 1 has a plate-mounting surface 12 on which a cylindrical printing plate 3 is fitted from the front end side and mounted. A front stopper 4 is provided on the end portion of the front end side. The front stopper 4 is configured so as to be able to switch between a protruding orientation in which at least a portion of the stopper is positioned so as to protrude to the outside of the plate-mounting surface 12 in the radial direction and a retracted orientation in which the entire stopper is positioned to the inside of the plate-mounting surface 12 in the radial direction.