Couple of gear wheels for driving printing drum with means for mutual phase restoration

A couple of gear wheels for driving a printing drum of a printing machine having a machine body, comprising a first gear wheel mounted to the printing drum, and a second gear wheel rotatably supported by a part of the machine body, the first and second gear wheels meshing with one another when the printing drum is set at a printing position located inside the machine body so as to drive the printing drum by a drive source provided in the machine body via the meshing engagement between the first and second gear wheels, and to cancel the meshing engagement when the first gear wheel is axially moved together with the printing drum toward outside of the machine body, the first and second gear wheels being either of a same diameter as one another or different in diameter from one another such that one gear wheel is integer times larger than the other gear wheel, the first and second gear wheels having each first and second axial portions, the first axial portions being closer to the printing drum than the second axial portions, while the second axial portions being closer to the part of the machine body than the first axial portions, the first and second axial portions being each formed as either a spur gear or a helical gear with a shifting of phase between the first and second axial portions of each of the first and second gear wheels, the first and second gear wheels having each a particular circumferential portion which are due to be in a meshing engagement with one another when the first and second gear wheels are at a predetermined rotational relationship relative to one another, wherein the second portions of the first and second gear wheels are each one of a tooth shape axially aligned with that of the corresponding first portion and a lack of the teeth exceptionally at the particular circumferential portions thereof, so that the first and second gear wheels can axially move into and out of the meshing engagement for allowing the printing drum to move into and out of the printing position only when the couple of gear wheels are in the meshing engagement with the respective second axial portions thereof aligned with one another at the particular circumferential portions.