Complicated calculations are needed to define the optimum oil pressure in the different zones in order to give the best approximation to the desired CD line load profiles. These have to take into account all elastic bending characteristics not only of the deflection control roll itself but also the whole roll system involved. A control system based on these models assists the operator or closes the control loop in automatic operation.
188.8.131.52 Objectives and Basic Design Criteria
Depending on their application, roll covers or coatings in the paper machine have to fulfill various functions. The main objective of roll coverings or coatings may be:
. • to protect the roll body against corrosion in a corrosive environment
• to protect the roll against wear from doctors or wires or felts
• to generate a soft – and thus wide – nip compared to a hard – and thus narrow – nip
. • to reduce the hydraulic pressure in a press nip
. • to ensure good release of the paper web
. • to transfer coating or sizing to the paper
. • to provide elastic support for the paper in calendering
. • to avoid the agglomeration of deposits
. • to support the paper in winding operations.
The various applications in the paper industry require a considerable variety in the design and materials of roll covers and coatings. The materials used range from very hard metallic coatings to very soft elastomeric covers, the surface geometry from plain polished to profiled with a highly open surface. In the following the term cover is used for cover thicknesses of approximately 10 to 30 mm, the term coating is used for coating thicknesses of less than 1 mm. This is not a standard¬ized nomenclature, but is established in the industry.
The design of a cover or coating is either defined by the application or by the design needs of the cover material used. Covers and coatings for the paper in¬dustry normally have a multilayer design (see Fig. 6.6) where the outermost layer provides the function of the cover for the papermaking process and the innermost layer ensures the bonding to the metal roll body. The use of additional intermedi¬ate layers is often required in order to withstand the shear stresses within the cover in loaded positions. A gradual adjustment of material properties like hardness, Young’s modulus, thermal expansion etc. in the radial direction is also desirable in order to avoid residual stresses in the cover. These residual stresses are caused by the differences in material properties of the metallic roll shell and the cover/ coating.
Covers/coatings have to reach extremely high load cycles during their opera¬tional lifetime. In double nip installations, where the cover is loaded twice per revolution of the roll, the number of load cycles goes up to 109. This dynamic load, in combination with temperature and the, eventually wet, chemical environment, requires high fatigue resistance of the cover/coating. Covers are usually ground several times before recovering is required. Depending on the application, opera¬tion lives of a cover of between 18 months and several years are standard.