Align Paper Story
Paper Mill Machine Alignment
When paper is racing through the roller maze at more than two kilometers per minute, everything has to be aligned perfectly.
A typical tissue line, depending on its width and rated capacity, is about 10 meters wide and exceeds 30 meters in length, and is as tall as a 3-storey building. Weighing a total of several hundred tons, it is an elaborate construction consisting of a steel supporting structure holding the moving parts in place.
An endless belt of running wire mesh is used to form the sheet of paper. It runs parallel to the so-called felt, an endless belt of woven cloth material that contacts the sheet as it passes through the paper machine. The felt guides, cushions and dries the wet paper sheet. And the paper sheet itself originates from the paper stock, which is a slurry of wood fibers (cellulosic material) suspended in water, with the water content of about 97 percent. When the paper sheet reaches the dryer – mounted in the large round structure – the immense steam and pressure reduce the water content to nearly zero within mere two seconds.
And then there are the rollers and dryers, probably the most recognizable parts in any paper machine. Weighing in excess of 10 tons each, rollers are metallic cylinders mounted horizontally and are used to smooth, press, dry or otherwise process the wet (or semi-wet) sheet of paper. Most rollers have a fixed diameter across their entire length, but some rollers (called “controlled crown”) are designed to be expanded or contracted in diameter to accommodate deflection at the center and achieve constant pressure across the line of contact between different rollers. The rollers in the Yankee dryer establish contact with the paper sheet, causing the water and moisture to evaporate.
The moving mechanical parts are crucial in the paper manufacturing process, and getting them aligned in respect to one another is important, not only to avoid jams and tears, but also to allow for the high operating speed which is required to achieve production yields of hundreds of tons per day. But not only moving parts need to be aligned, the framework needs alignment as well. All in all, more than 20 different axes need alignment.
Alignment used to be a much more daunting task than it is today. In the past, a combination of theodolites and levelling instruments was used to achieve roller alignment accuracies of 0.1 – 0.2mm, which is no small undertaking when one considers the sheer size and weight of the components.
Roller alignment the old way
Traditional instruments did not work on a coordinate system. Rather they merely measured angles, albeit very accurately. For decades it was the only way to align rollers. The so-called datum line, which runs along the entire machine being assembled, is used as a reference. The instrument is “bucked” onto this reference line, turned through 90°and the ends of the rollers are measured to a graduated scale held tangentially to the roll. The difference between the front and back readings being the magnitude of mis-alignment. This theodolite positioning needs to be done very precisely to the micron preferably using a translation stage (horizontal slide) and is heavily dependent on the operator’s skills and the sharpness of his vision.
A skilled operator with an eagle eye will perform the positioning well, an operator who is either less experienced or less meticulous in performing his work may produce completely useless measurement data.
After the parallelism of the roller has been checked using the theodolite, a spirit level is used to check its height. The physical positioning of the theodolite may take up to 10 minutes, depending on how much experience the operator may have under his belt. And this positioning needs to be repeated for each consecutive roller being checked; the operator needs to invest about 10 minutes of his time to reposition the theodolite for each new point needing inspection.
Distance between rollers are either measured by tape or gauge between the actual rollers or measured on the reference line between instrument set-ups.
The abovementioned method is heavily operator-dependent. During all crucial measurements, the accuracy of the results relies on the good vision and judicious work performed by the operator. Change the operator and you will change the error deviation, as no two sets of eyes are the same. Plus, the operator may get physically tired if performing several hours of measurements at one time, which also may negatively influence the integrity of the results produced. And the main restriction of this traditional method of aligning rollers is that rather than working in the three-dimensional space, it only accounts for two dimensions: parallelism and plumb of the roll.
Unlike the 3D procedure as described below this “old way” of alignment is dependent on the operator conducting measurements from the reference line, he requires a clear line of sight to his reference targets which is free of cranes, other machinery and people.
The task is onerous and more often than not very frustrating.
Roller alignment using modern 3D metrology equipment
As the art of science in survey evolves so too does the quality and precision of associating instruments evolve. Machines run faster and finer as the demand on their products increase. Error margins decrease and higher quality becomes the norm as opposed to the expected. Today’s machines simply cannot be serviced with antiquated survey methods and instruments.
High precision total stations such as the Leica TDA 5005 has revolutionized the science of mechanical measurements. The most obvious one is in the case of the instrument setup procedure itself. The Total Station no longer need to be positioned on a specific location, rather, it merely needs a good line of sight to the roller whose alignment it is measuring. Just like a laser tracker, the Total Station collects true three-dimensional information of the points simultaneously. In short, both parallelism and plumb are checked in one and the same step. And since the Total Station does not need to be meticulously positioned in front of a specific roller, several rollers can be inspected in one step without moving the Total Station to a new position, with the only prerequisite being a line of sight between the measurement point and the Total Station.
By using reflectors, all the measurements can be performed in one go. And one of the most important factors in the measurement process is automation. Using the Leica Geosystems Total Station incorporates Automatic Target Recognition (ATR), thus removing the human factor from the equation. The operator no longer has to develop a ‘gut feeling’ for theodolite placement or for aiming the optics. The Total Station automatically tracks the reflectors as it is being moved from point to point, producing documentable information about each position measured with the onboard Local Resection and Tie Distance software. All this to a large extent minimizes the dependability on skilled labour, which is a limited resource.
The onboard software and its methodology allows the operator to use the angle component of the instrument to its highest accuracy possible, thus meeting and even exceeding the accuracy requirement of 0.1mm in the roller alignment process. The reliability of the onboard software and its user-friendliness, coupled with the utility of the so-called roller alignment kit, which allows the operator to measure just 4 points on a roller to create two parallel lines to inspect the parallelism and perpendicularity, yield a quick, easy, highly reliable measurement system impervious to the influences by the operator. More simply put, it is a fail-proof system that creates repeatable measurement results of the highest integrity time after time.
|Old Way||3D Way|
|1||Reference line dependant||Set-up anywhere convenient|
|2||Two instruments required to measure horizontal and vertical displacements||Both planes are measured simultaneously|
|3||Only accessible rolls can be measured as instrument positioning relative to the ref line is limited.||Rolls can be measured from any visible position|
|4||Height limitations dependant on the height of the tripods or extension||No height restrictions|
|5||Operator fatigue and fluctuating expertise||Less physically demanding less stress and effort|
|6||Measurements between rollers very difficult and inaccurate||3D method covers all planes including roller positions relative to one another|
|7||Measurements are dependent on the accuracy the assistant holding the scale on the roll||ATR system does not need an assistant holding targets|
|8||No measurements possible “on the fly”||With ATR and laser tracking measurements are automated on moving components|
|9||Measurements only possible “here and now”||Automated monitoring system where time is not an issue|