.2.2.4 Dandy Roll
These rolls are driven and are used in fourdrinier wire sections, the aim being to improve formation and surface smoothness or to generate watermarks. Dandy rolls are wire covered rolls with a very open structure behind the wire, either built up of rings and bars or of a honeycomb design. They are placed between two flat suction boxes a short distance ahead of the “water line” (where there is no more free water on top of the web surface). Here the structure of the freshly formed web is still weak enough to be partly rearranged without being completely destroyed. The dandy roll dips into the wet web, dewaters it for a short moment during the contact and “rewets” the web on leaving the nip. At higher speeds more and more water is thrown out at the exit in the form of droplets. The application of these rolls is limited to machines speeds of below about 1000 m min–1 (see section 6.4.3).
Deflection Control Rolls
Rolls undergo a deflection under forces which may be due to roll dead weight, wire or felt tension, linear load in press or calender nips or low inside pressure at a certain circumference angle of the shell. However, most often a straight press nip or a uniform nip load in the cross machine direction is required with only small deviations allowed. One simple means of achieving this is to crown the roll. This is done by grinding a curve onto the originally cylindrical shell with larger diameter at the roll center than at the edges, thus compensating for the deflection and resulting in a uniform nip load across the width. One disadvantage of roll crown¬ing is that the local circumferential speed of the roll is different across the width due to the varying diameters. This may create problems in paper quality or in fabric operation.
The above nip conditions also have to be reached for different operating condi¬tions, such as varying linear loads in press nips. A given crown only really precisely fits for instance one line load. So changing operating conditions result in devia¬tions from a constant line load in the nip over the width or from a straight press nip. With increasing width of the paper machines the problem gets worse as de¬flection increases by the cube of the roll face.
With the introduction of deflection control rolls this general flaw can be over¬come. 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 width of the roll. This “cushion” presses the shell to the counteracting roll.