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LIQUID TRANSFER APPARATUS

The present invention relates to a liquid transfer apparatus for transferring liquid to at least one roller of a sheet printing or coating apparatus . The invention relates particularly, but not exclusively, to an apparatus for transferring ink in a lithographic sheet printing apparatus . Ink application apparatus are known in which ink is transferred from an ink reservoir via alternating rubber ink transfer rollers and copper ink transfer rollers to rubber inker rollers in contact with the surface of a copper plate cylinder for transferring an image for printing via a rubber blanket cylinder to a metal sheet. The plate cylinder is connected via gears to the other copper rollers, and the copper rollers drive the rubber rollers with which they are in contact by friction. In particular, the plate cylinder and some of the copper ink transfer rollers drive inker rollers in contact with the surface of the plate cylinder by friction such that the rotational velocities of the plate cylinder and contacting inker rollers are matched to minimise relative motion between the mutually contacting parts of the inker rollers and the plate cylinder.

This arrangement suffers from the drawback that under certain circumstances, for example as a result of some ink media where solvent action is aggressive to rubber parts of rollers, some swelling or ' shrinkage can occur as a result of which changes in roller diameter can occur. As a result, relative movement of the touching points of the rollers occurs, which in turn causes degraded print performance and wasted power consumption. This known arrangement also suffers from the disadvantage that adjustments to plate cylinder dressings (underpackings) for adjusting the length of the printed image will directly affect surface velocity matching. Furthermore, in order to wash the inker, or run the inker for printing preparation, but without running the press, it is necessary to provide an auxiliary motor and clutched transmission, which significantly increases the cost of the apparatus.

Preferred embodiments of the present invention seek to overcome one or more of the above disadvantages of the prior art .

According to an aspect of the present invention, there is provided a liquid transfer apparatus for transferring liquid to at least one roller of a sheet printing or sheet coating apparatus having a liquid application roller for applying liquid to a sheet of material, the liquid transfer apparatus comprising:

at least one first roller for transferring liquid from a liquid inlet or from at least one second roller to at least one third roller;

at least one first motor for causing rotation of at least one said first roller; and

control means for (i) receiving at least one first input signal dependent on at least one first torque of a respective said first motor and at least one second input signal dependent on at least one second torque of a respective second motor for causing rotation of a liquid application roller for applying liquid to a sheet of material, and for (ii) controlling at least one said first motor and at least one said second motor in dependence on at least one said first input signal and at least one said second input signal to reduce relative movement between mutually engaging surfaces of at least one said first roller and said third roller,

wherein said control means is adapted to determine at least one first torque generated by a respective said first motor and at least one second torque generated by a respective said second motor. By providing control means for (i) receiving at least one first input signal dependent on at least one operating parameter of a respective said first motor and at least one second input signal dependent on at least one operating parameter of a respective second motor for causing rotation of a liquid application roller for applying liquid to a sheet of material, and for (ii) controlling at least one said first motor and at least one said second motor in dependence on at least one said first input signal and at least one said second input signal to reduce relative movement between a location on a surface of at least one said first roller touching a said third roller and a location on a surface of said third roller touching said first roller, this provides the advantage of enabling relative movement between the contact points on the surfaces of the first roller and the corresponding third roller to be minimised, thereby minimising shear forces applied to the liquid, as a result of which print quality and efficiency of energy consumption can be improved. The present invention also has the advantage that the gears driving the rollers can be subjected to a slight preload, which reduces rattling during operation of the apparatus, thereby further improving print quality. The further advantage is provided that an inker of the apparatus can be driven independently of the press, thereby avoiding the need for a clutched transmission and therefore reducing the cost of the apparatus . The control means may be adapted to generate at least one first output signal for controlling a respective said first motor and at least one second output signal for controlling a respective said second motor and to adjust at least one said first and/or second output signal in response to changes in at least one said first and/or second input signal.

According to another aspect of the present invention, there is provided a sheet printing or sheet coating apparatus comprising a liquid application roller for applying liquid to a sheet of material, a liquid transfer apparatus as defined above, at least one third roller for receiving liquid from at least one said first roller, and at least one second motor for causing rotation of said liquid application roller.

According to a further aspect of the present invention, there is provided a computer program product adapted to be executed by a computer to control a liquid transfer device as defined above, the computer program product comprising first computer code executable by a computer to cause said control means to (i) receive at least one first input signal dependent on at least one first torque of a respective said first motor and at least one second input signal dependent on at least one second torque of a respective second motor for causing rotation of a liquid application roller for applying liquid to a sheet of material, and (ii) control at least one said first motor and at least one said second motor in dependence on at least one said first input signal and at least one said second input signal to reduce relative movement between mutually engaging surfaces of at least one said first roller and said third roller, and second computer code executable by a computer to cause said control means to determine at least one first torque generated by a respective said first motor and at least one second torque generated by a respective said second motor.

The computer program product may further comprise third computer code executable by a computer to cause said control means to generate at least one first output signal for controlling a respective said first motor and at least one second output signal for controlling a respective said second motor and to adjust at least one said first and/or second output signal in response to changes in at least one said first and/or second input signal.

A preferred embodiment of the invention will now be described, by way of example only and not in any limitative sense, with reference to the accompanying drawings, in which : -

Figure 1 is a schematic end view of part of a sheet printing apparatus embodying the present invention. Referring to Figure 1, a sheet printing apparatus 2 of the present invention for applying ink to a sheet 4 of material such as metal receives the sheet 4 in the direction of arrow A between a copper impression cylinder 6 and a rubber blanket cylinder 8 for transferring an image to the sheet 4, in a manner which will be familiar to persons skilled in the art. Ink is transferred from an ink reservoir (not shown) via first rubber ink transfer rollers 10, 12 and first copper ink transfer rollers 11, 13, 15 to rubber second ink transfer rollers 14, 16, 18, 20 which are in rolling contact with the surface of a plate cylinder 22 and which are known to persons skilled in the art as inker rollers .

The first copper ink transfer rollers 11, 13, 15 are driven by means of a first motor 24 connected to gears (not shown) , and rotation of the first copper ink transfer rollers 11, 13, 15 drives first rubber ink transfer rollers 10, 12 and second rubber ink transfer rollers 14, 16, 18, 20 by means of friction. The first motor 24 provides first input signals via first input line 26 to a controller 28 enabling the controller 28 to determine the torque generated by the first motor 24. The controller 28 provides first output signals along first output line 30 for controlling operation of the first motor 24. The impression cylinder 6 and plate cylinder 22 are interconnected by gears (not shown) and are driven via a drive gear 32 connected via a bevel gear 34 to an output shaft of a second motor 36, and rotation of the copper impression cylinder 6 and copper plate cylinder 22 causes rotation of the rubber blanket cylinder 8 by friction. The second motor 36 provides second input signals via second input line 38 to the controller 28 enabling the controller 28 to determine the torque generated by the second motor 36. The controller 28 also provides second output signals along second output line 40 for controlling operation of the second motor 36. In operation, the controller 28 determines the torque generated by the first 24 and second 36 motors and controls operation of the first 24 and second 36 motors to minimise relative motion between the mutually touching locations on the surfaces of the second ink transfer rollers 14, 16, 18, 20 and the plate cylinder 22. This reduces the shear forces on the ink applied to the plate cylinder 22 by the second ink transfer rollers 14, 16, 18, 20, thereby improving the print quality. The controller 28 can operate by means of a suitable algorithm, or can apply incremental changes to the first and second control signals and increase, maintain or decrease those changes in response to detected changes in the first and second input signals.

It will be appreciated by persons skilled in the art that the above embodiment has been described by way of example only, and not in any limitative sense, and that various alterations and modifications are possible without departure from the scope of the invention as defined by the appended claims.