Dynamic geometry of a rotating paper machine roll

The research is initiated by the findings that the coating variation of light weight coated LWC paper is in synchronization with a backing roll of a coating station. The frequency of the largest peak in the quality variation spectrum corresponds to the oval geometry of a backing roll, but no such a phenomenon has been measured in a roll shop. It was realized that the cause of the research problem could be the change in the geometry as a function of running speed. First, a theoretical study is carried out by building a single cross-section FE model of a backing roll using measured wall thickness data with systematic thickness variation. Only the main variation component is used. The analysis shows the wall thickness variation is a possible cause for a dynamic geometry change. For a more detailed study, a cylindrical FE model is created, and the measured wall thickness matrix is used to define the inner surface nodes. The analysis shows dynamic geometry change although the geometry is more complex due to fixed end plates. A device is developed to measure the dynamic geometry change. The device is calibrated using a multi-lobe disc as a reference standard. Using the principles of GUM (Guide to the expression of uncertainty in measurements) it is shown that the expanded uncertainty U of the device is less than two micrometers in a typical measuring case. A full scale backing roll is used as a test roll in laboratory experiments. The measurements of the roll show similar behavior that was observed in the FE analyses. The magnitude of the change in simulation was about 20 % lower than in the measurements. It is concluded that enhancements in the FE model and the processing of the measuring data would lead to more consistent results. The significance of the phenomenon is studied using a quite large sample of backing rolls at paper mills in Finland and abroad. The measured rolls are classified according to different generations of manufacturing processes and materials used. The study showed that dynamic geometry change is a common phenomenon in industry. A correlation analysis between machine direction quality variations of LWC paper and the dynamic behavior of a backing roll shows a very strong correlation. In many cases, the dynamic geometry change is the most significant single component causing run-out, and thus the main single cause for the harmonic paper quality variation. During the research, some methods were used for maintenance purposes to improve the dynamic behavior of backing rolls. According to the measurements, the used maintenance methods proved to be useful.