System of rolls in a supercalender

A supercalender comprises a stationary variable-crown upper roll (1), a floating variable-crown lower roll (2), several intermediate rolls (3,4) between the upper roll (1) and the lower roll (2), and a hydraulic control system. The upper roll (1) has a roll mantle (12) that is supported on the roll shaft (11) by hydrostatic loading elements (13) acting in the direction of compression and by end bearings at the ends of the roll mantle, which bearings prevent displacement of the mantle (12) ends in the radial direction relative to the shaft (11) of the upper roll. The lower roll (2) has a roll mantle (22) that is supported on the roll shaft (21) adjustably in the direction of compression by means of hydrostatic loading elements (23). Position detectors (23,31) are provided at the ends of the shaft of at least one roll. The control system is arranged to regulate the positions of the rolls (1 to 4) in the system of rolls and to balance the forces effective in the supercalender in response to measurement signals received from the position detectors (27,31) and from the hydrostatic loading elements (23) of at least the lower roll.

BACKGROUND OF THE INVENTION 
The invention concern a system of rolls in a supercalender, comprising a 
variable-crown upper roll adjustable in zones, a variable-crown lower roll 
adjustable in zones, several intermediate rolls placed between the upper 
roll and the lower roll, as well as a hydraulic control system for the 
system of rolls. 
During operation, the system of rolls in a supercalender is deflected, 
which results mainly from the journalling of the intermediate rolls in the 
system of rolls. This is why, at present, in supercalenders, 
variable-crown rolls, in particular rolls adjustable in zones, are used 
commonly as the upper and lower rolls. In principle, the variable-crown 
rolls and the rolls adjustable in zones may be either stationary, i.e. 
rolls in which the roll mantle is mounted at its ends relative the roll 
shaft by means of bearings so that the positions of the ends of the mantle 
cannot be changed relative the shaft, but in the area between the ends, 
the roll mantle can be deflected relative the roll shaft, or then, the 
rolls may be floating, i.e. rolls in which the roll mantle can also move 
at its ends relative the roll shaft in the direction of compression. 
In a supercalender, a solution is known in prior art wherein a stationary 
variable-crown roll is used as the upper roll and as the lower roll in the 
system of rolls. In such a system of rolls, however, a considerable 
drawback consists of vibrations of the system of rolls, which are seen as 
inferior paper quality. The properties of attenuation of vibrations of a 
system of rolls are quite poor if both the upper roll and the lower roll 
are stationary variable-crown rolls. 
Moreover, in prior art, solutions are known in which both the upper roll 
and the lower roll are floating variable-crown rolls, or in which the 
upper roll is floating and the lower roll is stationary. A floating upper 
roll, however, involves the considerable drawback that, when the roll 
shaft is loaded by means of external loading means, the upper roll cannot 
be made straight, because at the ends of the upper roll there are no 
bearing loads. In such a case, the distribution of the linear load at the 
upper roll cannot be made straight, and the profile of the linear load 
cannot even be made sufficiently accurate. Moreover, some paper qualities 
require relatively low levels of linear load, in which case the mantle of 
a floating upper roll cannot be placed in the correct position within the 
zones, but often the nip between the upper roll and the topmost 
intermediate roll may even remain open in the lateral areas. In connection 
with a floating upper roll, attempts have been made to correct the 
deflection by means of counter-zones provided in the area of the ends of 
the roll mantle, the direction of effect of said counter-zones being 
opposite to the direction of compression. However, the deflection cannot 
be corrected by means of the counter-zones either, because the roll mantle 
is very stiff. 
A floating upper roll further involves the drawback that the upper roll 
must be positioned from time to time, e.g., owing to leakages in the 
hydraulic pressure cylinders acting as external loading means. Moreover, 
positioning must, of course, be carried out always when the size of the 
intermediate rolls is to be changed. Moreover, the requirements of 
accuracy of the positioning are very high, and since the positioning is 
carried out by means of hydraulic cylinders, these form a critical point 
in view of the hydraulic system of the set of rolls, because the pressures 
effective in the hydraulic cylinders are very high. 
SUMMARY OF THE INVENTION 
The object of the present invention is to provide an improvement over the 
prior-art solutions and, moreover, to avoid the drawbacks of said 
prior-art solutions. In view of achieving this, the invention is 
characterized in that the upper roll is a stationary variable-crown roll, 
whose roll mantle is supported on the roll shaft by means of hydrostatic 
loading elements acting in the direction of compression and by means of 
end bearings placed at the ends of the roll mantle, said bearings 
preventing displacement of the mantle ends in the radial direction 
relative the shaft of the upper roll, that the lower roll is a floating 
variable-crown roll, whose roll mantle is supported on the roll shaft 
adjustably in the direction of compression by means of hydrostatic loading 
elements, and that the system of rolls is provided with position detectors 
provided at the ends of the shaft of at least one roll, whereat the 
control system of the system of rolls is arranged, on the basis of 
measurement impulses received by it from the position detectors and from 
the hydrostatic loading elements of at least the lower roll, to regulate 
the positions of the rolls in the system of rolls and to balance the 
forces effective in the system of rolls based on the measurement of 
position. 
As compared with the prior-art solutions, by means of the invention a 
number of advantages are obtained, whereof, e.g., the following should be 
mentioned. As compared with the case in which both the upper roll and the 
lower roll are stationary variable-crown rolls, substantially better 
properties of attenuation of vibration are achieved by means of the 
invention, because in the solution in accordance with the invention, the 
pistons in the zones of the floating lower roll act as elements that 
attenuate vibration. Moreover, the vibrating mass in the lower roll is 
smaller than in the case of a stationary lower roll. 
On the other hand, compared with a floating upper roll, the following 
advantages are obtained by means of the invention. The diameter of a 
stationary roll becomes smaller than that of a floating roll, with the 
consequence that the stationary upper roll can be deflected into the 
correct position more readily. In such a case, the distribution of the 
linear load in the uppermost nip becomes better. The end bearings of the 
stationary roll form "one more zone" at the ends of the roll, whereby, 
when the roll shaft is loaded by means of external loading means, the roll 
can be deflected down at the ends and up at the middle more readily. The 
level of the linear load in the system of rolls is regulated by means of 
the external loading means; only correction of the level of linear load is 
carried out by means of the zones in the variable-crown roll. Moreover, by 
means of the external loading means, it is possible to reduce the weight 
of the roll to a level lower than its own weight, which gives a 
significant advantage in the case of certain paper qualities, for certain 
paper qualities require very low levels of linear load. By means of the 
equipment in accordance with the invention, good properties of profilation 
are obtained with low linear loads. Since, in the invention, a stationary 
variable-crown upper roll is used, the shaft of the upper roll does not 
have to be positioned equally accurately as in the case of a floating 
upper roll.

DETAILED DESCRIPTION 
In the FIGURE in the drawing, the upper roll in the system of rolls of a 
supercalender is denoted with the reference numeral 1. The upper roll 1 is 
a stationary variable-crown roll, which comprises a roll shaft 11 and a 
roll mantle 12, the mantle being supported on the shaft 11 by means of end 
bearings 19 as well as by means of hydrostatic loading elements 13, which 
have been divided into zones in the direction of width of the roll 1. 
The shaft 11 of the upper roll is supported on the frame of the 
supercalender by means of external loading means 14, i.e. upper cylinders, 
which are used for regulating the level of the linear load in the system 
of rolls. The upper cylinders 14 are dual-action cylinders, so that by 
their means it is possible on the one hand to increase the level of linear 
load in the system of rolls in the desired way, and, on the other hand, to 
reduce the load caused by the upper roll 1 to a level lower than the 
roll's own weight. As the upper cylinders 14, it is advantageously 
possible to use differential cylinders, in which case a leakage in the 
cylinders has no essential effect on the position of the roll shaft 11 or 
on the level of linear load in the set of rolls. 
The lower roll in the system of rolls is denoted with the reference numeral 
2, and said lower roll is a floating variable-crown roll, which comprises 
a roll shaft 21 as well as a roll mantle 22 that is rotably supported on 
the roll shaft 21 exclusively by the intermediate of hydrostatic loading 
elements 23. Thus, the mantle 22 of the lower roll is not supported on the 
shaft 21 by means of stationary end bearings. Moreover, in the embodiment 
of the FIGURE, the lower roll 2 is provided with external loading means 
24, i.e. lower cylinders, by means of which the position of the lower roll 
2 can be altered. Between the upper roll 1 and the lower roll 2, in the 
conventional way, a number of intermediate rolls are provided, of which 
said intermediate rolls the lowermost and the uppermost intermediate rolls 
3 and 4 are shown in the FIGURE in the drawing. Further, in the FIGURE, 
the upper nip, i.e. the nip between the upper roll 1 and the uppermost 
intermediate roll 4, is denoted with the reference symbol N.sub.1 and, 
correspondingly, the lower nip, i.e. the nip between the lower roll 2 and 
the lowermost intermediate roll 3, is denoted with the reference symbol 
N.sub.2. 
The hydraulic system of the system of rolls of a supercalender in 
accordance with the invention is provided with a hydraulic pump 6, which 
supplies the pressure medium, e.g. hydraulic fluid, to the upper cylinders 
14, to the lower cylinders 24, and to the hydrostatic loading elements 13 
and 23 of the upper and lower roll. The pressure medium is supplied to the 
loading means 14 and 24 of the upper and lower roll through the regulating 
valves 15 and 25 and, correspondingly, the pressure medium is passed to 
the hydrostatic loading elements 13 and 23 of the upper and lower roll 
through separate regulating valves 16 and 26. As was already stated above, 
both in the upper roll 1 and in the lower roll 2, the hydrostatic loading 
elements 13 and 23, respectively, have been divided into zones in the 
direction of width of the rolls, whereby there is a regulating valve 16 
and 26, respectively, of its own for each zone in the hydraulic system. At 
both ends of the shaft 21 of the lower roll, position detectors 27 are 
installed, which measure the position of the ends of the lower roll 
relative to the supercalender frame 28 and also measure the straightness 
of the lower roll 2. Moreover, at each end of the lowermost intermediate 
roll 3, corresponding position detectors 31 are provided, which measure 
the position of the ends of the lowermost intermediate roll relative to 
the supercalender frame 28 and also measure the straightness of said 
intermediate roll 3. The position detectors 31 may be provided at the ends 
of any of the intermediate rolls 3,4, but the most advantageous effect is 
obtained by placing the position detectors exactly at the ends of the 
lowermost intermediate roll 3. 
The system of rolls of a supercalender in accordance with the invention is 
further provided with a control computer 5 that controls the hydraulic 
system of the set of rolls. The control computer 5 regulates the 
regulation valves 15 and 25 for the external loading means of the upper 
and lower roll and the regulation valves 16 and 26 for the hydrostatic 
loading elements in the upper and lower roll in accordance with the 
starting data given and with the control impulses received. The necessary 
regulation control impulses are received by the control computer 5, on one 
hand, from the position detectors 27 and 31 of the lower roll 2 and the 
intermediate roll 3, the control computer 5 constantly adjusting the 
position of the lower roll 2 and the straightness of the system of rolls 
correctly in accordance with the impulses given by said position detectors 
27 and 31. On the other hand, the control computer 5 receives, control 
signals representative of the zone pressures in the hydraulic loading 
elements 13 and 23 of the upper roll and of the lower roll 2. From the 
zone pressures of the loading elements 23 of the lower roll 2, the control 
computer calculates the weight of the system of rolls, and from the weight 
of the system of rolls it further calculates the loading of the system of 
rolls. Thus, in accordance with the control signals received from the 
hydrostatic loading elements 13 and 23, the control computer 5, on one 
hand, adjusts the upper cylinders 14 so as to provide the correct level of 
linear load. On the other hand, the control computer 5 regulates the 
regulation valves 16 and 26 of the hydrostatic loading elements 13 and 23 
of the upper and lower roll so as to adjust the linear load to the correct 
values across the width of the system of rolls. Thus, the forces effective 
in the system of rolls can be balanced on the basis of measurement of the 
position. 
The invention is not confined to the embodiment shown in the accompanying 
schematical drawing alone, but the various embodiments of the invention 
may show variation within the scope of the inventive idea defined in the 
accompanying claims.