Roll Press Comprising Two Dissimilar Rolls With Equal Deflection Characteristics

Abstract

In a pair of rolls for pressing a running web, one roll has a shell supported at its ends and the other roll has a shell supported at the middle of its length by being connected to a through shaft, whereas its end portions are free of said shaft. The wall thicknesses of the shells vary along their lengths in a definite mutual relation in order to make the rolls equally flexible. Wen the first-mentioned roll has a shell of uniform thickness, the thickness of the shell of the other roll first decreases and then increases when following the shell from the middle towards the ends.

Description

The invention relates to a roll press comprising two parallel rolls adapted for being pressed from opposite sides against a running web of e.g. paper and to provide a pressure evenly distributed over the length of the line of contact with the web.

When the rolls of a pair of rolls are being pressed against each other by a pressure upon their bearing journals, the rolls normally will bend away from each other. In order nevertheless to obtain a line pressure equally distributed over the pressure nip, it is customary to design one roll or both with a crown. However, in this case the pressure distribution varies with the deflection of the rolls, and the desired even distribution is obtained only at a predetermined pressure value. That is true also when so-called anti-deflection rolls are used. Only by the use of a controllable hydraulic pressure exerted upon the inside of the roll shell has it been possible hitherto to fulfil the desideratum of maintaining the same uniform pressure distribution when the total pressure varies over a wide range.

The object of the present invention is to provide a roll press of a simple mechanical design, in which the uniform distribution of the pressure over the press nip is maintained in spite of variations of the total pressure. While previously suggested solutions usually have aimed at keeping the nip line as straight as possible and independent of the load, the present invention is based upon the principle of letting the rolls bend and making them equal in respect of resistance to deformation in all points along the nip, so that the rolls have an inherent tendency of assuming the same curvature when bent. With such a design of the rolls the pressure distribution over the nip will be independent of the total pressure.

According to the invention, said object is reached in a roll press of the above-mentioned kind, in which a first roll has its ends united with bearing journals and the second roll consists of a tubular shell surrounding a through shaft having bearing journals at its ends, to which shaft said shell is connected merely at its center part, whereas the rest of shell is free of the shaft and distant therefrom, and in which press an important feature consists in that the wall thickness of one roll or both rolls varies along the length in such a manner that, starting from the center and progressing towards the ends, the quotient of the wall thickness of the second roll shell and the wall thickness of the first roll shell, both measured in the same transverse plane, first decreases and then becomes constant or increases.

In such a roll press it can be realized that the two roll shells are deformed in full congruence, so that the press can be used for total nip pressures varying within wide limits, and the pressure variations will not change the distribution of the pressure over the length of the nip. It should be observed that said uniform line pressure at varying loads is obtained without the use of hydraulics, cantilevering of the rolls (i.e. loading of extensions of the roll journals) or similar complicated measures.

In order to obtain the above-mentioned effects the rolls should be given truly equal deformation qualities, with respect of the total deformation of the nip line, in each transverse section through the rolls. The two rolls being differently built, their deformation curves would differ if the wall thickness of the roll shell were constant along the length. However, mathematical analysis forming the basis of the invention, has shown that it is possible by a correct mutual adaptation of the wall thicknesses of the roll shells to obtain the desired equality of the rolls in respect of deformation in the various cross-sections. Said analysis has also resulted in certain calculation formulas which confirm the above-mentioned general rule guiding the thickness variation of the roll shells and which enable an accurate dimensioning in each particular case, whereby the desired pressure distribution is reached to its fullest extent.

According to a preferred embodiment of the roll press, the thickness of the end-supported roll shell is made uniform over the entire length used for pressing the web. Then the thickness of the center-supported roll shell will first decrease from the center towards the ends and then increase again at the shell ends. In said embodiment the mathematical calculations of the truly correct shell thicknesses are simplified. Involved in these calculations is the knowledge that the total deformation is composed of the deformation caused by the bending of the roll shells as a beam, and of the deformation consisting of the depression of the shell at the nip, i.e. the deviation of the cross-section of the shell from the circular shape. The calculations may be regarded as taking place in two steps, the first one being determination of the wall thickness of the center-supported roll shell in each cross-section with regard taken merely to bending moments and shearing stresses. This step results in a stuff distribution having a maximum thickness value at the center of the roll and gradually diminishing thicknesses towards the shell ends. In a second step of the calculations, regard is taken also to the localized shell impression at the nip, which increases in the direction towards the shell ends and which must be compensated by a thickness increment. When said increment is added to the thickness obtained by the first calculation step, the result is a wall thickness which at a certain distance from the shell center stops decreasing and instead takes an approximately constant value or increases again; the latter usually being the case. Of course, the shape of the curve representing the thickness of the shell will vary in dependence upon the length and diameters of the rolls, the elastic properties of the materials used, the permissible maximum pressure etc. Calculations carried out for a number of different roll presses of the sizes common in great modern paper making machines have given a collection of values form which certain rough rules or limits for the dimensioning can be found. Thus it has shown that the minimum value of the above-mentioned shell thickness quotient, or of the shell thickness of the center-supported roll when the other roll shell is of uniform thickness, appears at a point situated at a distance from the center of the rolls equal to 50 to 90 percent of the distance from the center to either shell end, in most cases equal to 60 to 80 percent thereof. Under the same presumption, i.e. uniform thickness of the shell of the end-supported roll, the shell of the center-supported roll will at the above-mentioned point generally be of a minimum thickness amounting to 30 to 40 percent of the maximum thickness at the center of the roll shell, and the thickness of said shell at its end will usually lie between 50 and 65 percent of said maximum thickness. These normal figures should not be construed as restricting the scope of the invention.

In the embodiment of the invention where also the end-supported roll shell is of a varying thickness along its length, it is more difficult to find the truly correct thickness values by calculation, but one has to resort essentially to experience and trails. However, also for those rolls the above-mentioned general rule concerning the shell thickness quotient holds true.

An embodiment of the roll press according to the invention will be more closely described herein below with reference to the accompanying drawings, in which

FIG. 1 is an axial sectional view of the press, and

FIG. 2 is a diagram accurately showing the variation of the shell thickness. The abscissae represent the distances from the transverse center plane, and the ordinates represent the corresponding thicknesses.

In FIG. 1 the press is shown as consisting of two parallel rotary rolls A and B. The first roll A comprises a cast iron shell 11 having outer and inner cylindrical faces, thus being of a uniform thickness over its entire length. Disk-shaped end closures 13, 15 are attached to, preferably screwed into the shell ends, and bearing journals 17, 19 are integral with said end closures. The second roll B likewise comprises a tubular shell 21 with a cylindrical outer face. Said shell is symmetrical about a transverse plane 23 extending through the center of the length of the shell. At the middle part of the shell where the thickness takes a maximum value, the inner face is cylindrical and when following the shell from said cylindrical part 25 toward either shell end, thickness values will be encountered which first decrease and then increase again, a minimum value occuring at a point 29 distant from the end, and the thickness of the shell at the extreme end 27 taking a medium value. The generatrix of the inner shell face forms a curve which on either side of the cylindrical shell part 25 is concave over its entire length and is continuously and smoothly bent without abrupt changes. As clearly shown in FIG. 2, the point 29 of minimum wall thickness is situated at a distance from the center plane 23 equal to 75 percent of the total distance to the shell end 27.

The shell 21 is supported by a through steel shaft 31 having bearing journals 33, 35 at its ends. The center part 37 of the shaft is cylindrical and connected to the cylindrical part 25 of the shell, preferably by a shrinkage fit. The rest of the shell encloses the shaft at a distance thereto and without being supported thereby. The shaft is tapered towards the ends so that the shell cannot contact the shaft at maximum deflection.

The journals 17, 19, 33, 35 are supported by bearings (not shown) by means of which, as indicated by the arrows, the rolls can be pressed against each other, or in operation against opposite sides of a paper web running between them. Moreover, at least one journal is connected to a driving means.

The press rolls are shown in the bent shape corresponding to the application of the maximum permissible pressure, which in this case has been assumed to amount to 100 kg per cm length of the nip. The axis 41 of shaft 31 is bent in the opposite direction to the axis 43 of shell 21 and to the axis 39 of shell 11. Due to the shell impression at the nip, the line 45 of contact of the rolls shows a curvature slightly different from that of the axes 43, 39. In the nip the pressure between the rolls, measured per unit of length, is the same over the whole effective nip length as a result of the application of the invention, and this holds true also when the pressure is partly relieved and the roll shells are more and less straightened out.

In order to provide the required flexibility also of the middle supported portion of the shell 21, according to a further feature of the invention a recess 47 is turned out of the shell, whereby the shrinkage fit upon the shaft is divided up into two zones lying on opposite sides of and distant from the center plane 23. According to the drawing, said zones extend to a distance from the center amounting to about 10 to 15 percent of the total distance to the shell ends, but in other cases said figure can be raised up to about 25 percent at the most.

In the shown embodiment, the shells of rolls A and B measure 5,200 mm in length and 875 and 785 mm, respectively, in diameter. The shell 11 of roll A has a uniform wall thickness of 67.5 mm, whereas the wall thickness of the shell 21 of roll B varies in the manner shown in the diagram FIG. 2, from a maximum value of 75 mm at the center to a minimum value of 24.5 mm at a distance of 1,950 mm from the center and then rises to a value of 45 mm at the end.

It is preferred to make the outer face of the two rolls cylindrical. However, it is also possible within the scope of the invention to give the rolls slightly varying diameters. Another modification may consist in making the end-supported roll solid, in which case the radial measure is taken for the wall thickness in the above-mentioned description and the following claims.

Claims

I claim:

1. A roll press comprising two parallel rotary rolls adapted for being pressed against opposite sides of a running web, e.g. of paper, and for providing a pressure evenly distributed over the length of the line of contact with the web, the first one (A) of said rolls having its ends united with bearing journals, whereas the second roll (B) which consists of a tubular shell (21) surrounds a through shaft (21) having bearing journals at its ends and is connected to said shaft merely at a center part and for the rest is free and distant therefrom, the wall thickness of one roll or both rolls varying along the length in such a manner that, starting from the center and progressing towards the ends, the quotient of the wall thickness of the second roll shell (21) and the wall thickness of the first roll shell (11), measured in the same transverse plane, first decreases and then becomes constant or increases; characterized in that said quotient stops decreasing at a distance from the center (23) of the rolls equal to 50 to 90 percent, preferably 60 to 80 percent, of the distance from the center to either end.

2. A roll press comprising two parallel rotary rolls adapted for being pressed against opposite sides of a running web, e.g. of paper, and for providing a pressure evenly distributed over the length of the line of contact with the web, the first one (A) of said rolls having its ends united with bearing journals, whereas the second roll (B) which consists of a tubular shell (21) surrounds a through shaft (21) having bearing journals at its ends and is connected to said shaft merely at a center part and for the rest is free and distant therefrom, the wall thickness of one roll or both rolls varying along the length in such a manner that, starting from the center and progressing toward the ends, the quotient of the wall thickness of the second roll shell (21) and the wall thickness of the first roll shell (11), measured in the same transverse plane, first decreases and then becomes constant or increases; characterized in that the shell (21) of the second roll is connected with a shrinkage fit to the shaft (31) along two center zones situated symmetrically on opposite sides of the transverse center plane (23) and extending endwardly to a point situated at a distance from the center plane equal to 25 percent at the most, and preferably equal to 10 to 15 percent, of the full distance to either roll end.

3. In a roll press for pressing a paper web in a paper making machine, a first roll comprising a hollow cylindrical shell of uniform wall thickness, means connected to either end of said shell for rotatably supporting the same, a second roll comprising a shaft rotatably supported at its ends and a hollow shell having a cylindrical outer face cooperating with the first-mentioned shell to form a pressure nip extending along the lengths of the shells, a shrinkage connection between the center portions of said shaft and of said last-mentioned shell, the inner contour of said last-mentioned shell forming an inwardly concave curve extending from said center connection to either end and defining a shell wall thickness of a maximum value at the center and a minimum value at a point lying between the center and the end; characterized in that said minimum value is 30 to 40 percent of said maximum value and relates to a point of the roll situated at a distance from the center equal to 50 to 90 percent of the distance from the center to the roll end.

4. A roll press comprising two parallel rotary rolls adapted for being pressed against opposite sides of a running web, e.g. of paper, and for providing a pressure evenly distributed over the length of the line of contact with the web, the first one (A) of said rolls having its ends united with bearing journals, whereas the second roll (B) which consists of a tubular shell (21) surrounds a through shaft (21) having bearing journals at its ends and is connected to said shaft merely at a center part and for the rest is free and distant therefrom, the wall thickness of one roll or both rolls varying along the length in such a manner that, starting from the center and progressing toward the ends, the quotient of the wall thickness of the second roll shell (21) and the wall thickness of the first roll shell (11), measured in the same transverse plane, first decreases and then becomes constant or increases; characterized in that the wall thickness of the first roll shell (11) is substantially constant over the entire effective length, whereas the wall thickness of the second roll shell (21) first decreases from the center outwards and then increases closer to the ends; and in that said quotient stops decreasing at a distance from the center (23) of the rolls equal to 50 to 90 percent, preferably 60 to 80 percent, of the distance from the center to either end.