For several reasons, there is an ever greater need for well-aligned rolls in paper mills. On one hand, precisely aligned rolls are a vital prerequisite for producing paper of consistently high quality. On the other hand, consumables such as felts or roller coatings are used up much faster when rolls are misaligned, resulting in tremendous cost for more frequent replacement.
Roll Alignment so far
Up to now, alignment of rolls has relied almost exclusively on optical equipment. When measuring with devices such as theodolites, a line of sight between the center line outside the machine and the rolls to be aligned is necessary. When using sticks for the alignment of rolls, a line of sight between the rolls is necessary as well. If a paper mill is already in operation, encased areas of the machine are particularly difficult to access, thus further compounding this problem. Not least, with all traditional ways of alignment a great amount of time is required to perform the measurements.
New approach by Prüftechnik Alignment Systems
In contrast to all conventional system, PARALIGN® is based on a completely new technology for the alignment of rolls. PARALIGN® contains highly accurate ringlaser gyroscopes, which have a so-called 'inertia'. With three gyroscopes in the same housing, it is possible to make this device "capable of learning". If a point in Cartesian space is specified as a reference point (i.e. as the ideal point) with the device, the device can then specify every other point in the same space with respect to its deviation from the horizontal and vertical planes. In other words: an "ideal" roller within the machine is identified and used as a reference. PARALIGN® stores and 'remembers' its relative position and compares it to each subsequent roller that is being measured. Their relative angular position is stored, displayed and automatically processed in the PARALIGN® Explorer software.
For the measurement, PARALIGN® is put directly on the roll and subsequently a sweep is being performed. This procedure is shown in picture below.
PARALIGN® -service-measurement of offset printing machines
The following report refers to a PARALIGN® -measurement in a rotogravure machine containing six printing modules. The printing-machine had been in operation for several years already. The aim of the measurement was to achieve a complete documentation of this machine for the first time since the machine was built.
The picture above gives a qualitative and numerical overview of (among others) the printing cylinders and the cooling rolls, which the material to print on is passing through. The spectator's point of view (the operator side) is shown in green, the machine side in red. The printing cylinder of printing module 5 was chosen as the reference, and is thus standing perpendicular on the drawing plane. On the diagram this means that the green point of this reference cylinder is completely hiding the red one. Altogether it stands out that almost all cooling cylinders have a horizontal offset of about 1,5mm to the printing cylinders. The consequence of such misalignments can be an asymmetric pull on the material to print on or a run of the material to one side of the machine, which in turn results in a deterioration of printing quality.
The rotogravure machine had been aligned with a theodolite in combination with using sticks. The rolls on the ground floor and on the first floor had separately been aligned by sticks. Aligning the machine using these conventional means requires well-experienced staff in addition to an ample supply of time. But even with these prerequisites, the offset of the cooling rolls compared to the printing cylinders cannot be eliminated. This misalignment results from the fact that while measuring one of the cooling cylinders by theodolite, which gives the starting point for the alignment of the rest of the cooling cylinders by sticks, theodolite and roll are evidently not in-plane. This problem does not occur when measuring with PARALIGN® as the spatial distance of rolls spread over different floors does not affect the results of PARALIGN® 's inertial measurement technique.
By measuring with PARALIGN® , complete insight in the plant could by gained for the first time since the printing machine was built. Previously this had been close to impossible as the machine was encased in some areas and nearly impossible to measure with optical means. In the same way it is possible to document sheet-fed presses, web presses and flexo printing machines with PARALIGN®. PARALIGN® -measurements are offered by PRÜFTECHNIK as a service. In addition to the accuracy and flexibility the short measuring time is one of the major advantages of PARALIGN® . The time required for the PARALIGN® measurement was about three hours, whereas a measurement with conventional means would need a multiple of time. In the case described here the customer had provided a schematic view of the rolls in advance, so a template in the PARALIGN® -PC-Software could be set up. Because of this preparation no time is lost at the day of the measurement. The measurement data are automatically generated in PARALIGN® and are online during the measurement being stored on a notebook computer. This procedure enables a high ease of use and eliminates a number of sources of error: Thus it is neither necessary, to read off data of an ocular, like this is true for an optical measurement system, nor is there a need to take down data by hand and - risking further transmission errors - document them electronically later.