Rolls deflection control

With the introduction of deflection control rolls this general flaw can be overcome. The principle of all such rolls is that the bending deflection is taken by an axial beam which supports the shell by means of a kind of hydraulic cushion , be it just one or several cushions across the vtidth of the roll. This cushion presses the shell to the counteracting roll. 

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Deflection control rolls (Section 5.2.3) overcome the disadvantages of deflection of the conventional rolls resulting from internal and external forces. 

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. 

The basic version of the softcalender is the two-roll softcalender. Its main components are the soft covered deflection control roll and the heating roll. The linear pressure of a softcalender ranges from approximately 10-350 N mm" and the... 

The thickness of the calendered product must be uniform in both the machine and cross-machine directions. Any variation in gap size due to roll dimensions, setting, thermal effects, and roll distortion due to high pressures developing in the gap, will result in product nonuniformity in the cross-machine direction. Eccentricity of the roll with respect to the roll shaft, as well as roll vibration and feed uniformity, must be tightly controlled to avoid nonuniformity in the machine direction. A uniform empty gap size will be distorted in operation because of hydrodynamic forces, developed in the nip, which deflect the rolls. The resulting product from such a condition will be thick in the middle and thin at the edges, as shown in Fig. 15.2. 

As this unit is a pinch-type device, there is potential for the pinch to damage very light-gauge soft material if the roll surface is not perfect. Care must be given to maintain the roll surface in a smooth, clean condition at alt times. It also has proven to be beneficial to have the roll position controlled by positive stops in the chocks or cylinders. Merely pinching the rolls with pressure control is not adequate and may cause steering problems. Also, it will introduce an undefined tension in the strip, resulting from the work required to deflect the roll surface. 

Calendering can achieve surprising accuracy on the thickness of a sheet. Typically the tolerance is 0.005 mm but to achieve this it is essential to have very close control over roll temperatures, speeds and proximity. In addition, the dimensions of the rolls must be very precise. The production of the rolls is akin to the manufacture of an injection moulding tool in the sense that very high machining skills are required. The particular features of a calender roll are a uniform specified surface finish, minimal eccentricity and a special barrel profile ( crown ) to compensate for roll deflection under the very high presurres developed between the rolls. 

Generally, this machine has much smaller rolls, much closer together, so that the material can be bent progressively, up and down, over a significantly smaller bend radius (Fig. 17). Flights of backup rolls, supported by rigid upper and lower frames, control the deflection of these small, flexible work rolls. Depending on machine capacity, they could have from 9 rolls for thick material and up to 17-23 rolls for thin material. 

Introduced by Beloit in 1960, the shell of this crown control (CCj roU is supported by one shoe with hydrodynamic lubrication. The shoe can move in the nip direction in the fixed axial beam which takes the line load forces and deflects. Loading is external as with the swimming roll. 

Machine calenders consist of two or more hard rolls and are practically always installed on-line. They are primarily used for paper that only requires moderate finishing or for pre-calendering grades that need further finishing treatment in order to obtain higher gloss and/or smoothness. Machine calenders are equipped with overall deflection or zone-controlled deflection rolls (Fig. 6.69). 

A third method of thickness control is achieved by roll bending . Extra bearings set on extensions to the roll ends in order to provide leverage are forced apart by hydraulic rams situated between them. By using the main journal as a fulcrum (see Fig. 7) it is possible to bend the roll in opposition to the deflection caused by the material and thus neutralise it. If a roll bending system is fitted it will do the pull back operation as well. 

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