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FLEXO Magazine : August 2013
rial are indications that the raw materials used to produce the rolls are not homogenous. Variation in the distribution of mass, due to manufacturing tolerances, which must be allowed on all machine surfaces, is a major contributing factor. Any manufacturing tolerances that permit eccentricity---lack of squareness of machine surfaces with respect to the rotational axis---are sources of imbalance. Non-symmetrical distortion of a roller, while running at its operating speed, can also produce excessive vibration. This distortion is generally the result of poor design, such as too small a diameter in relation to face length, or of variations in wall thickness of the material used to manufac- ture the roller. BEARINGS & RESTRAINTS The need for balancing a flexographic press roll is evident when we consider that the center of mass of a roll will not nec- essarily coincide with the rotational axis, as determined by its supporting bearings. A roll that is not restrained by bearings will naturally rotate about its center of mass. When the same roll is restrained by bearings, it is forced to rotate about an axis other than its center of mass due to the run-out of the bearings. Consequently, centrifugal forces are introduced, which cause vibrations. To state it very simply, balancing is merely an operation that eliminates vibration by redistributing the mass of the cylinder to cause its center of rotation to lie on the same axis as that determined by the supporting bearings. Although there is varying terminology used with respect to balancing, we are concerned primarily with two types: static imbalance and dynamic imbalance. Static Imbalance---In static imbalance, only gravity, or weight force, is involved, and balancing can be accomplished without rotation. This type of balancing is generally not satis- factory for flexographic rolls, due to the rotational speeds at which they operate and the ratio between their diameter and axial length. Dynamic Imbalance---Rotating parts ordinarily have both static and dynamic imbalance. This combined static and dynamic imbalance is corrected by weights placed in two dif- ferent planes perpendicular to the axis of rotation (Image 1). The weight compensates for the static imbalance at rest and for the dynamic imbalance caused by centrifugal force when the roll is rotating. This two-plane balancing, which corrects both static and dynamic unbalance, is generally referred to by engineers as dynamic balancing. Due to the physical and rotational char- acteristics of flexographic rolls, they should be in dynamic balance when high press speeds are used. CENTRIFUGAL FORCE The centrifugal force exerted on the restraining bearings of an unbalanced roll is proportional to both the weight of the www.flexography.org AUGUST 2013 FLEXO 37