Printing assembly

The present invention provides a printing assembly which includes a printing drum such as an impression drum, a pair of eccentric bearings between and on which the drum is mounted for rotation, a pair of self aligning bearings on which the eccentric bearings are supported, and a pair of frameless motors directly connected to the eccentric bearings in order to effect the rotation thereof through closed loop feedback control, in order to facilitate the automatic and accurate control of pressure level and uniformity along the nip of a printing press.

FIELD OF THE INVENTION

The present invention is concerned with a printing assembly, and in particular a printing assembly for use in a printing press and which enables closed loop control of the pressure along a nip defined between an intermediate transfer medium (ITM) drum and an impression (IMP) drum.

BACKGROUND OF THE INVENTION

The image transfer mechanism in many printing presses is based on hot and pressurized thermal transfer. Such printing presses generally incorporate an intermediate transfer medium (ITM) drum and an impression (IMP) drum between which is defined a nip though which paper or other printing medium is drawn, and onto which paper an image is transferred in known manner. In order to achieve optimal performance in terms of PQ, transferability and durability of the image, the pressure level and uniformity along the nip, and therefore from side to side of the ITM drum, must be strictly controlled. Currently, the dimensions of the nip rather than the pressure along the nip is monitored.

If a strain gauge is incorporated, then an open loop control is achieved on the pressure along the nip. However, such open loop control still requires manual adjustments and suffers from an inability to make the required corrections in high speed machines, especially in cases when the printing is to be carried out on several kinds of paper with different grammage. In such situations, several null cycles are required in the switching of the paper.

European patent application EP 1594017 discloses a fuser system of a xerographic device having a fuser drum and a pressure drum which in use bear against one another such as to form a nip, and which system utilises closed loop feedback to control the nip pressure.

The system of the present invention has been developed to overcome the above mentioned problems. In addition the present invention is adapted to eliminate or reduce printing artefacts e.g. banding which are formed when the blanket drum rolls into the region of the gripper's gap in the impression drum. At the start of the gap the force between the blanket drum and the impression drum goes abruptly to zero causing the printing unit to vibrate and also accelerate the blanket drum. When the drums come into contact again at the end of the gap the pressure abruptly builds up again. These fluctuations of the contact pressure and circumferential speed cause printing artefacts. The printing assembly of the present invention provides a high bandwidth along with utilization of advanced control options such as: feed-forward and gain scheduling in order to allow the printing assembly to adequately withstand the abrupt torque disturbance keeping the IMP drum position and speed.

SUMMARY OF THE INVENTION

There is provided a printing assembly comprising a drum; a pair of eccentric bearings between and on which said drum is mounted for rotation; a pair of drives to effect independent rotation of the eccentric bearings; and a closed loop control circuit actuating the drives in response to torque based feedback from the drives.

The assembly may optionally comprise a pair of self-aligning bearings, each eccentric bearing being mounted within one of said self-aligning bearings, said pair of drives effecting rotation of said eccentric bearings on said self-aligning bearings.

Optionally each drive is rigidly coupled to the respective eccentric bearing.

Optionally each drive is positioned concentrically with the respective self-aligning bearing.

Optionally, said torque based feedback is derived from the electric current drawn by the drives.

Optionally, each each drive comprises a frameless motor.

Optionally, each eccentric bearing comprises an outer casing and an inner bearing mounted eccentrically within said outer casing.

Optionally, each drive is rigidly coupled to said outer casing of the respective eccentric bearing.

Optionally, each eccentric bearing comprises an outer casing and an inner bearing mounted eccentrically within said outer casing, said outer casing of each eccentric bearing being mounted concentrically within the respective self-aligning bearing.

Optionally, each self-aligning bearing comprises a spherical roller bearing.

Optionally, each inner bearing comprises a cylindrical roller bearing.

Optionally, the assembly comprises a pair of encoders, each of said drives having one of said encoders in operative association therewith and operable to indicate the rotational position of said drive.

Optionally, each drive comprises a frameless motor having a rotor directly coupled to said outer casing of the respective eccentric bearing.

Optionally, the assembly comprises a housing on either side of said drum, each housing enclosing the respective eccentric bearing and self-aligning bearing; and in which assembly each drive comprises a frameless motor having a stator housing rigidly connected to the respective housing.

In another aspect there is provided a printing assembly comprising a drum; a pair of bearings on which said drum is mounted for rotation and which facilitate the radial displacement of a respective end of said drum; a pair of drives operable to effect the independent displacement of each end of said drum; wherein each drive is directly connected to the respective bearing.

In a further aspect there is provided a printing assembly comprising a first drum and a second drum between which is defined a nip; a pair of eccentric bearings between and on which said first drum is mounted for rotation; a pair of self-aligning bearings, each eccentric bearing being mounted within one of said self-aligning bearings; a pair of drives to effect independent rotation of the eccentric bearings on the self-aligning bearings in order to substantially equalize the pressure along the nip by controlling the gap between the first and second drums; and a closed loop control circuit actuating the drives in response to torque based feedback from the drives.

As used herein, the term “directly” is intended to mean that a connection between two components does not involve intermediate couplings or linkages or the like, which would introduce a flexibility between the two components, and which when concerned with the transfer of torque from one to the other component, would thus result in a lag in the rotational displacement between the two components, or in other words a lack in synchronisation between the two components when undergoing rotation displacement.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring now to the accompanying drawings, there is illustrated a printing assembly, generally indicated as10, which comprises a drum12which in the embodiment illustrated is an impression (IMP) drum, but which could of course be any other drum, and which is intended to form part of a larger printing press (not shown). In particular, the assembly10is intended to be located adjacent an intermediate transfer medium (ITM) drum (not shown) such as to define a nip between the drums, as is well known in the art. In use, an image receiving material, such as for example a sheet of paper (not shown) having a toner image applied, is passed through the nip. As it passes through the nip, pressure and heat are applied to the toner image on the paper, which is thus fixed to the paper or other printing material.

As will be described in detail hereinafter, the printing assembly10of the invention is adapted to automatically adjust the position of the drum12relative to the ITM drum (not shown) or other surface, in order to vary the pressure along the nip. The printing assembly10can thus maintain the pressure within desired operating specifications. This will, for example, allow the assembly10to automatically adjust the pressure along the nip to suit different weight papers, or to effect corrections to the pressure in order to compensate for any number of internal or external factors affecting the pressure or uniformity thereof. The assembly10is adapted to measure and monitor the pressure along the nip and to use that information to provide feedback for the automatic adjustment of the pressure along the nip. Thus, as will be described, the assembly10provides closed-loop feedback control of the pressure level and uniformity along the nip.

The assembly10comprises a first bearing assembly14on one side of the drum12, and a second bearing assembly16on an opposed side of the drum12. The drum12is mounted for rotation between the bearing assemblies14,16via a shaft18of the drum12.

Referring now in particular toFIGS. 3 and 4, and as mentioned above the printing assembly10is adapted to automatically effective the displacement of the drum12towards and away from, in use, an ITM drum or the like, in order to accurately control the pressure along the nip. The bearing assemblies14,16, in terms of implementing this functionality, are substantially identical in configuration and operation.

Referring first toFIG. 3, the first bearing assembly14is illustrated in section. The bearing assembly14comprises an eccentric bearing20on which the shaft18is supported for rotation. The eccentric bearing20is itself mounted for rotation within a self-aligning bearing22, which is optionally a spherical roller bearing (SRB) but may of course be any other suitable functional alternative. The self-aligning bearing22, and therefore the eccentric bearing20, are mounted rigidly within a cylindrical housing24which in use is suitably and rigidly secured in position within a printing press (not shown), in the embodiment illustrated by means of an annular flange26.

The eccentric bearing20comprises a cylindrical outer casing28, which is concentrically mounted within the self-aligning bearing22, the casing28having an eccentrically positioned aperture30in the front thereof, and an inner bearing32on which the shaft18is supported. In the embodiment illustrated the inner-bearing32comprises a cylindrical roller bearing (CRB) although any other suitable alternatively may be employed. However, as will become apparent from the following description, the use of a cylindrical roller bearing provides significant stiffness in supporting the drum12.

The inner bearing32is rigidly secured in position within the outer-casing28by means of a lock-ring34, which is pressed up against the inner-bearing32. As the inner-bearing32is positioned eccentrically of the outer-casing28, the shaft18and therefore the drum12is supported eccentrically of the outer-casing28. It will thus be appreciated that if the outer-casing28is rotated on the self-aligning bearing22, the drum12will be displaced, in an accurate path, towards and away from the ITM drum, or other words in an essentially radial direction with respect to the drum12. This motion is known as the engage/disengage motion, in that the drum12may be engaged or disengaged with the ITM drum (not shown) and/or the force of engagement with the ITM drum can be varied. Thus, and as is described in more detail hereinafter, by displacing each side of the drum12using the first and second bearing assemblies14,16, the pressure applied along the nip can be controlled.

In order to effect the rotational displacement of each eccentric bearing20, the printing assembly10comprise a pair of drives, in the form of frameless motors36, one on either side of the drum12. Each motor36comprises a stator housing38within which is rigidly mounted a stator40and a rotor42located concentrically therein. In the embodiment illustrated, and for reasons set out hereinafter, the stator housing38of each frameless motor36is rigidly connected to the housing24, while the rotor42directly and therefore rigidly connected to the outer casing28of the respective eccentric bearing20. In the embodiment illustrated, the rotor42is bolted to the respective outer casing28, although it will of course be appreciated that any other suitable means of securing the rotor42to the eccentric bearing20may be employed. Again however any such connection must be sufficiently rigid, as set out hereinafter. The frameless motors36are significantly stiffer than an equivalent conventional electric motor (not shown), thus adding to the overall stiffness and rigidity of the printing assembly10.

It will thus be appreciated that rotation of the rotor42within the stator40, by the application of an electric current to each motor36in a known manner, will effect rotation of the outer casing28of the eccentric bearing20. This rotation of the outer casing28will thus effect the engage/disengage motion of the drum12. The direction and magnitude of rotation of the outer casing28of each eccentric bearing20will determine the displacement of the drum12towards or away from the ITM drum, and will thus dictate the change in pressure experienced along the nip during use. In order that the exact rotational position of each eccentric bearing20, and therefore the respective frameless motor36can be determined, the assembly10is provided with an encoder44mounted concentrically on each motor36. Each encoder44, in known fashion, can generate a signal indicative of the rotational position of the respective frameless motor36.

Referring now toFIG. 4, the second bearing assembly16is illustrated. As with the first bearing assembly14, the second bearing assembly16also comprises an eccentric bearing20on which the shaft18of the drum12is supported. The second bearing assembly16further comprises a self-aligning bearing22within which the eccentric bearing20is mounted for rotation. Both are rigidly mounted within a cylindrical housing24which in use is rigidly fixed in position via a flange26. A stator housing38of the frameless motor36is bolted to the housing24, while a rotor42of the motor36is directly and rigidly fixed to the outer casing28of the eccentric bearing20.

The shaft18of the drum12passes through an eccentrically positioned opening30in the outer casing28, and is then supported on an eccentrically mounted inner bearing32, which in the embodiment illustrated is in the form of a cylindrical roller bearing (CRB). The inner bearing32is secured in position within the casing28by means of a lock-ring34. The free end of the shaft18projects beyond the inner bearing32and a lock nut46is secured thereon. To axially secure the shaft18, a bolt48is passes through the cylindrical lock nut46and is secured on a pair of bearing50which, in the embodiment illustrated, are provided as an angular contact ball-bearings. By tightening the bolt48, pressure is brought to bear against the lock-ring34, in order to rigidly clamp the inner bearing32in position within the outer casing28. Although the bolt48is locked in abutting engagement with the lock-ring34, the bearings50and the inner bearing32ensure that the shaft18is free to rotate within the second bearing assembly16.

As with the first bearing assembly14, the frameless motor36connected to the second bearing assembly16is operable to effect the engage/disengage motion of the drum12by effecting rotation of the outer casing28on the self-aligning bearing22.

In order to control and co-ordinate the operation of the pair of frameless motors36, the printing assembly10comprises a closed loop feedback control circuit37, which utilises various signals associated with the assembly10, in particular from the pair of encoders44and the electric current driving the motors36, in order to effect operation and control of the position of the drum12. The main component of the control circuit37is a processor39operable to determine, based on signals inputted thereto, whether or not the nip pressure and/or uniformity is within operational specifications. If the pressure and/or uniformity is outside the operational specification, the processor39is adapted to effect actuation of the motors36in order to make the necessary corrections to the position of the drum12, and thus the pressure along the nip. As the motors36change the pressure along the nip, feedback is provided to the processor39in order to facilitate closed loop feedback control.

In order to allow the pressure along the nip to be measured, the electric current drawn by each of the frameless motors36, while effecting displacement of the drum12, is measured. By measuring this current, the torque of the motor can be measured, which is related to the pressure applied along the nip. The pressure at the nip is dictated by the force applied by the drum12, and this force is proportional to the torque of the motors36. In particular the torque is dependent on the distance between the axis of the motor36and the point of contact between the drum12and the ITM drum (not shown). This dimension will vary slightly as the drum12is displaced along an arcuate path by rotation of the eccentric bearings20, although by measuring the angular position of each side of the drum12using the encoders44, this distance can be instantaneously calculated by the processor39. Thus, by measuring the electrical current drawn by each of the motors36, the torque of the motor36can be calculated, and thus using the distance calculated by the processor39, the force and therefore pressure along the nip can be determined. If the pressure along the nip is outside the operational specifications, the processor39can suitably alter the electric current supplying one or both motors36until the drum12is positioned to give the correct pressure along the nip.

Thus, the printing assembly10is capable of continually monitoring and adjusting the pressure along the nip to ensure that it remains within operational specifications. The operational specifications may change if, for example, paper of a different thickness or grammage is being processed by the printing assembly10. In such a situation, the assembly10will automatically adjust the pressure along the nip to suit the grammage of the paper, and is capable of implementing this correction or adjustment without requiring a null cycle. This is particularly advantageous in high speed printing presses. For this purpose the processor39of the feedback control circuit37optionally has access to a look up table containing nip pressure values or ranges for particular paper grammage values or ranges.

In order to enable highly accurate closed loop feedback control, the printing assembly10, and in particular the mounting of the drum12, and the elements achieving the engage/disengage motion, must be extremely rigid or inflexible. Any flexibility in the assembly10would significantly deteriorate the sensitivity of the feedback control. It is for this reason that the motors36are provided as high rigidity frameless motors, and that each rotor42is directly and rigidly connected to the eccentric bearing20on either side of the drum12. The use of the cylindrical roller bearings for each of the inner bearings32ensures this rigidity is achieved at the interface between the shaft18and the bearing assemblies14,16.

Furthermore, the spherical roller bearing used for each self-aligning bearing22maintains the rigidity between the eccentric bearing20and the housing24. This overall rigidity enables closing of the control loop with the required bandwidth to enable accurate control of the nip pressure, even in high speed printing presses.

While both the first and second bearing assemblies14,16may be utilised simultaneously to effect identical displacements of either end of the drum12in order to control the nip pressure, it will be appreciated that independent adjustment of either side of the drum12is possible, and is often required in order to accurately control the nip pressure. It will thus be appreciated that the direction of the longitudinal axis of the drum12will undergo minute changes in response to displacements of differing magnitude at either end of the drum12. In the absence of the pair of self-aligning bearings22, such adjustments would not be possible, as the longitudinal axis of the drum12would be fixed in one direction. However, the pair of self-aligning bearings22allow these minute independent adjustments to be effected, while maintaining the overall stiffness or rigidity of the printing assembly10. The self-aligning bearings22will track the shifting axis of the drum12in order to ensure that the displacement of the axis does not result in any resistance which would affect the sensitivity of the feedback control of the motors36. The drum12is thus effectively “floating” between the pair of self-aligning bearings22. While allowing this displacement of the axis of the drum12, the self-aligning beatings22, by virtue of being spherical roller bearings, maintain the rigidity of the system10. Again, it is this rigidity that enables closed loop feedback control with the required bandwidth.

The invention is not limited to the embodiment described herein but can be amended or modified without departing from the scope of the present invention.