Background art In a flexographic printing process, colored ink is applied to a printing medium by means of three printing rollers or cylinders which rotate synchronously and in contact, with parallel rotation axes.

The three printing rollers comprise an impression roller, designed to support the material to be printed, a printing plate cylinder, which supports the pattern to be printed (printing plate), and an anilox or inking roller which applies ink to the printing plate. Printing occurs when the ink is deposited by the printing plate onto the medium, in an amount and in a manner which depend on the characteristics of the printing plate and of the anilox roller.

The color station is constituted by all the systems that move the three rollers, the ink and the material for printing a single color.

In current printing machines, the rotary motion of the three printing rollers is obtained by way of a mechanical transmission of the motion from a single electric motor; the electric motor transmits the motion by means of a reduction stage to the impression roller, which in turn transmits the motion to the printing plate cylinder by way of two gears which are axially aligned with the respective rollers and mesh together. A similar transmission system is adopted between the printing plate cylinder and the anilox roller.

In so-called"central-drum"printing machines, the impression roller is a single roller (central drum) for all the color stations and so is the ring gear that transmits the movement to the gears in axial alignment with the respective printing plate cylinders; the toothed ring has a peripheral diameter being equal to the diameter of the drum.

In order to eliminate the problems of gear-based transmission, systems

have recently been studied and produced in which each printing roller is driven by its own electric motor, thus eliminating the mechanical connection of the gears between the printing rollers.

All the currently provided solutions are characterized by one or more transmission couplings and/or a motion reduction stage between the motor and the respective printing roller (the motion reduction stage designed to bring the rotation rate of the motor down to the rotation rate that corresponds to the speeds that the rollers must have for a correct printing process).

The reduction stage is generally constituted by a reduction unit or by gears being combined with a belt drive.

These technical solutions, which have transmission couplings and/or a motion reduction stage, have several disadvantages, the main ones being: --the plays of the gears of the reduction stage (by means of reduction units or a belt) set a limit to the precision of the printing registration of the colors; --the limited mechanical rigidity of a transmission system with transmission couplings and/or a reduction stage can cause vibration at low frequencies and can therefore facilitate the onset of resonance; --the manufacturing systems are mechanically complicated, wear easily and are also particularly expensive; --the transmission, in particular, is complicated and bulky, requires long assembly times and entails the use of precision mechanical components, which are expensive and delicate and require frequent maintenance.

Disclosure of the Invention The aim of the present invention is to eliminate or substantially reduce the above noted drawbacks, by providing a rotary printing machine with a central drum in which motion is provided to the central drum and to the printing plate cylinders without transmission couplings and/or motion reduction stages.

An object of the present invention is to provide a rotary printing machine which ensures a substantial increase in the torsional and flexural mechanical rigidity of the rotation shaft.

Another object of the present invention is to provide a rotary printing machine which raises the resonance frequency of the printing system.

Another object of the present invention is to provide a rotary printing machine which allows greater precision in printing registration, a structural simplification and a reduction in the mechanical parts that are subject to wear, so as to increase its reliability and at the same time reduce operating and maintenance costs, making them significantly more favorable than those of a conventional printing machine.

This aim and these and other objects which will become better apparent hereinafter are achieved by a rotary printing machine comprising two supporting shoulders, a central drum or cylinder being rotatably mounted on said supporting shoulders, at least one printing unit being arranged around said drum and comprising a printing plate cylinder and an anilox roller, which are rotatably mounted on a respective pair of supporting elements and whose rotation axis is parallel to the axis of said drum, characterized in that at least said central drum is actuated by a source of motion which directly engages an axial shank of said drum.

Conveniently, the source of motion for the drum has a stator being rigidly fixed to one of said supporting shoulders and a rotor being rigidly fixed at the end of an axial shank of the drum.

Advantageously, the rotary printing machine comprises a source of motion for the or each printing plate cylinder and for the or each anilox roller.

Conveniently, said source of motion for the or each printing plate cylinder comprises an electric motor in which the stator is rigidly fixed to one of said supporting elements and is monolithic therewith, and a rotor which is rigidly fixed to a motor shaft being coaxial to the respective

printing plate cylinder and being slideable transversely to said stator and rigidly with the respective printing plate cylinder.

Brief description of the drawings Further features and advantages of the present invention will become better apparent from the following detailed description of a preferred embodiment thereof, given by way of non-limitative example with reference to the accompanying drawings, wherein: Figure 1 is a schematic side elevation view of a central-drum flexographic machine according to the present invention; Figure 2 is an enlarged-scale schematic top view of the central drum, with some parts shown in cross-section, of the flexographic machine of Figure 1; Figure 3 is a schematic and partial side view of the motor of the central drum of Figure 2; Figure 4 is a partial sectional view, taken along the line IV-IV of Figure 3; and Figure 5 is a sectional view, taken along a longitudinal plane, of a color unit of the machine of Figure 1.

Ways of carrying out the Invention With reference to the drawings, a multiple-color flexographic rotary machine 1 provided with a central drum 2, according to the present invention, is constituted by two supporting shoulders 3 and 4, on which the drum 2 is rotatably mounted, and by an electric motor 5 for the actuation of the drum 2.

As shown in Figure 2, the drum 2 has an axial shank (front part 9a and rear part 9b), which is rotatably supported by the shoulders 3 and 4, for example by means of adjustable roller bearings, designated by the reference numerals 10a and 10b respectively. Preferably, the outer part 9a of the shank is cropped just after the respective shoulder 3. An inwardly flanged end of the rotor 13 of the electric motor 5 is bolted at the end of the shank 9a (by

means of bolts 12); the stator 14 of the motor is flanged externally and bolted, by means of bolts 15, to the outer face of the shoulder 3.

In this manner, the shank 9a, and therefore the drum 2, is directly coupled to the electric motor 5, which in turn is directly axially connected to the drum 2. The bearings 10a and 10b in fact, besides supporting the weight of the central drum 2, ensure a perfectly coaxial arrangement of an annular rotor 13 and of a stator 14 of the motor 5. Accordingly, the drum 2 does not need to have a ring gear for its rotary actuation.

Multiple printing units 11 (eight in the example shown in Figure 1) are provided around the central drum 2, and each one (usually comprising a printing plate cylinder 30 and an anilox roller 8) has its own source of motion. This means that in addition to a servomotor (i. e., the motor 5) for the central drum 2, there are eight servomotors 6 for the printing plate cylinders 30 and eight servomotors 7 for the anilox rollers 8; all the servomotors are driven by means of an electronic controller 16 (shown in Figure 1).

More particularly, on the shank 9a (or 9b) there is a transducer 17, for example an encoder, which is mounted coaxially to the shank 9a inside the annular rotor 13, as shown in Figure 2, and is designed to transmit to the controller 16 data related to the rotation rate of the central drum 2 in order to control the operation of the electric motor 5 of said drum and synchronize it with the electric motors 6 and 7 of each printing unit 11.

Preferably, the electric motor 5 is cooled by way of a cooling system of any suitable type, for example with water fed by an appropriately provided pump (not shown in the drawings) and designed to flow through a labyrinth, represented schematically by a system of channels 18 arranged around the stator 14 starting from an inlet 19 up to an outlet 20, as shown in Figure 4.

In order to prevent dust, which in the long run might compromise the correct operation of the motor 5, from entering said motor 5, there is a pressurization system formed by channels 21 between the stator 14 and the

rotor 13 which are connected to an inlet 22 for slightly pressurized air which originates from a blower, not shown and being of any suitable type, through a pressure reduction unit 23.

A disc 25 is fixed, for example bolted by means of bolts 24, on the other side of the drum 2, i. e., on the shank 9b (Figure 2); one or more caliper brakes 26 can act on the disc and are driven by the controller 16 of Figure 1 in order to control the deceleration of the drum 2 during emergency braking and keep the drum 2 locked in a precise angular position when necessary.

As regards the motor drive of the printing plate cylinder 30 and the support and rotation system, Figure 5 illustrates a currently preferred example of embodiment.

The printing plate cylinder 30 has a front shank 31a being supported by two bearings 34 and 36 which are necessary in order to give flexural rigidity to the cylinder.

The bearings 34 and 36 can be inserted in a sleeve 38 which allows the axial sliding of the printing plate cylinder 30 (arrow L) in order to allow the movement, for example by +/-6 mm, required for transverse registration of the print.

Again in order to increase the flexural rigidity of the printing plate cylinder 30, the cylinder has a rear shank 31b which is supported by the supporting shoulder 4 by way of two bearings: the roller bearing 37 and the double ball bearing 35. Moreover, the bearing 37 is arranged as close as possible to the sleeve or printing plate 60 in order to limit the deflection related to the flexural deformation of the roller 30 and limit the hunting oscillations (as shown by the arrow S in Figure 5) during the operations for changing the sleeve 60.

The rear shank 31b of the printing plate cylinder 30 is further keyed to an electric motor 6 whose partially hollow shaft 45 is locked on the rear shank 31b by way of a conical keying element 46.

Advantageously, the outer or stator part 52 of the electric motor 6 is

rigidly flanged to the slider 32 by means of a cast-iron support 47 and is thus rigidly coupled to the slider 32. The rotor 44 is fixed to the motor shaft 45, being supported by two roller bearings 48a and 48b which allow the axial sliding, for example by +/-6 mm, of the motor shaft 45 and therefore of the rotor 44. The bearings 48a and 48b withstand very well the radial loads generated by the flexing of the printing plate cylinder 30, ensuring high flexural rigidity.

The system for joining one another the printing plate cylinder 30 and the electric motor 6 is preferably provided by the insertion of the rear shank 31b inside the partially hollow end of the motor shaft 45. There is also a conical keying element 46 for rigidly closing the rear shank 31b on the motor shaft 45; this coupling system ensures the transmission of very high moments and perfect mating between the shank 31b and the motor shaft 45.

In practice, the above described system for supporting and mechanically connecting the printing plate cylinders 30, the motor shaft 45, the rotor 44 and the stator 52 ensures a mechanical rigidity which is greatly increased with respect to the solutions currently used to motorize the printing plate cylinders 30. In particular, the body constituted by the rigid coupling between the printing plate cylinder 30, the motor shaft 45 and the rotor 44 combines very high flexural and torsional rigidity with the ability to perform a translation movement along the rotation axis to the extent required for transversely registering the print.

An encoder 49 or other suitable transducer system is fixed to the stator 52 of the motor 6 by means of a coupling 50 which is torsionally very rigid but axially very flexible in order to allow the movement of transverse registration by +/-6 mm.

The axial coupling for transverse registration is provided by means of a double ball bearing 35, whose inner ring (not shown in the figures) is locked by means of an annular element 53 approximately halfway along the rear shank 31b, while the outer ring (not shown in the figures) is rigidly coupled

to an oval flange 43 to which the trapezoidal screw 42 is fixed in the upper part; said screw, turned by the transverse registration device 54, generates the axial movement of the printing plate cylinder 30.

With a rotary or flexographic printing machine structured as described above, a substantial simplification of the mechanical components with respect to conventional-type machines is achieved. In particular, the direct coupling between each source of motion (electric motors 5 and 6) and, respectively, the central drum 2 and the printing plate cylinders 30 allows to achieve high mechanical rigidity, accordingly achieving a considerable increase in the value of the resonance frequency of the motor-cylinder- supporting structure system, so as to be able to increase the speed of response to the dynamics of said system so as to maintain an unchanged (constant) print quality.

The above described invention is susceptible of numerous modifications and variations within the protective scope defined by the content of the appended claims.

The materials and the dimensions may be various according to requirements.

The disclosures in Italian Patent Application No. VR2000A000013 from which this application claims priority are incorporated herein by reference.