PFFC - March 2009 - (Page 44) TECHNICAL REPORT ROLLER CROWNING The views and opinions expressed in Technical Reports are those of the author(s), not those of the editors of PFFC. Please address comments to the author(s). Crowning Benefits The proper crowning of industrial rollers can bring important advantages to converters. By Matthew Menges, Menges Roller Co. M shape/profile is compromised, safest methods that can be it will cause more problems performed (see Figure 5). It than not crowning at all. can be used to interpret results The amount of crown need- from the actual impression ed usually is very small and taken at the nip. Additionally, within a magnitude of a few mils (0.001 in.). The amount Figure 1. A roller crown of crown (barrel shape) is is the shape that calculated from beam defleccauses uniform nip tion formulas. The shape of a pressure distribution. Compensating for Deflection crown is nominally the shape Roll crowning ensures accurate of a beam deflected under a Crown/2 web alignment and counters the uniformly distributed load as effects of deflection. In a simple shown in Figure 3. nip system, roller deflection is The latest computerized one of the main reasons for nip grinding machines, along with variations across the width of the the basic beam principle, roller roll face. Deflection is evident in specifications, and the user’s end-loaded rollers. The roll will operating load requirements, Figure 2. Crowning deflect or bend away from the are the factors leading to precicompensates for effects nip, which causes pinching at sion crowned rollers. In general of deflection when rolls the ends as seen in Figure 2. terms, a program takes the bend away from the nip. Roller deflection means there deflection formula or tabulated is bending due to its dimensions, values to produce the correct material, and the load being ap- shape. There are a couple of plied to the roll in its particular commonly used crown angles application. Therefore, roll for many converting operacrowning is one common meth- tions and for heavily loaded od of reducing nip variation rollers with great lengths or caused by roller deflection. unique header designs. Another benefit of crowning a roll is improved product Checking & Testing Figure 3. Calculation of quality. And more imporA number of methods are used a crown is the shape of a tantly, crowning helps the roll to assure correct crowning beam deflected under a covering last longer, increases prior to shipment (see Figure uniformly distributed load. the rubber life, and helps re4 ). They include measurement duce machine downtime. by a laser micrometer to check targeted crown shape and Assuring Results magnitude across the width of Deflection Specifying a crown on a roller the roll. Another check is done Uniform Load must be done with precito look at the nip profile with sion. This is especially true nip impression paper and other when grinding or cutting an similar techniques. Deflection accurate crown. It is such a The static nip impression difficult process that if the test is one of the easiest and Good Good 44 | MARCH 2009 Crown Too Low For Given Load Crown Too Low For Given Load any industrial manufacturers rely on crowned rollers. A crown is a shape or diameter profile necessary to compensate for deflection and obtain uniform nip pressure distribution (see Figure 1). a dynamic nip impression test is a tool used to crosscheck quality of the working crown. There are other methods of checking nip impressions. One is using embossed foil, which is evaluated by reading the embossing elements flattened in the closed nip. This method is helpful when testing heated nip rolls. The embossed foil eliminates worrying about ink running, bleeding, or blotting that results in using carbon paper. A popular form of crown evaluation is an electronic nip reader. This device converts the nip loading and variances into real time data. Its benefit is in quickly reading and adjusting roll alignment or loading at the startup or at maintenance shut down. This electronic device has no real advantage, except speed, over a static or dynamic nip impression test in terms of crown adjustment. Why? Crown correction is done outside the production machine or more precisely at a grinding shop. Results from testing confirm the following items: u The need for a crown (on uncrowned rolls); u Crown accuracy (for crowned rolls); u Roll alignment quality; u Actual nip width. Nip Load Versus Crown Roll crowning is the first aspect, but another very important factor must be noted in WWW.PFFC-ONLINE.COM http://WWW.PFFC-ONLINE.COM
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