PFFC - March 2009 - (Page 46) ROLLER CROWNING regard to the nip load versus crown. In Figure 2, there is an externally applied force at the ends of one of the rolls that actually provides the nip load. Therefore, the amount of deflection, and as a result the crown amount, is determined by the nominal nip load. There is a load (x) that determines the required crown (y), and thus a given crown (y) matches only the value of load (x). Any other load than the match load above will not provide uniform nip loading across the width. Consequently the problem in a simple nip system is that the nip load cannot be changed without negatively affecting the uniformity of the nip. This poses a big challenge when there are a number of different grades or products that must be run. Fortunately, there are solutions to this problem, Figure 4. Good and bad crowning, from two perspective views. Good Crown Too Low For Given Load Crown Too High For Given Load Good Crown Too Low For Given Load Crown Too High For Given Load Unbalanced Loading Or Misalignment Banding—Roller Wear, Grinding Unbalanced Loading Or Misalignment Banding—Roller Wear, Grinding Figure 5. Static impression is the rolls closing the nip but not in rotation; the dynamic test is a closed nip and rotating the rolls through to the end of the impression paper to see if the nip impression is consistent through the roll rotation. The static impression, which is often the only one that can be taken, assumes the roll is the same dimension or concentric in the entire circumference. The dynamic impression tries to figure out if the roll is uneven throughout its circumference. Static Nip Impression Dynamic Nip Impression 1 2 and they consist of deflection compensation techniques. These techniques include using a small roller on a large roller nip system, roller skewing, and using a specialty roller like a controlled crown roller. The end result is that these techniques are weak and provide only a limited range of adjustment. As a result, we advise crowning one of the rolls to the lightest load and using deflection techniques to achieve higher operating loads. PFFC-ASAP 132 46 | MARCH 2009 WWW.PFFC-ONLINE.COM http://WWW.PFFC-ONLINE.COM
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