The present invention relates to a method, according to the preamble of Claim 1, for compensating nip-loading variations caused by shape errors in a roll in a calender. A calender of this type comprises a first roll and a second roll, which form a nip, through which the web to be calendered is threaded, as well as loading devices for altering the loading of the nip.

Calendering is intended to increase the smoothness and gloss of paper or board and to improve other properties of the printing surface. In calendering, the paper or board web is pressed in a nip formed by two rolls arranged to rotate opposite to each other. Extremely tight tolerances are set for the accuracy of the shape of the shells of the calender rolls, because errors in shape will cause variations in the nip loading. Variation in the nip loading will, in turn, cause detrimental variations in the calendered web. For this reason, efforts are made to ensure that the shells are as cylindrical as possible. However, it is difficult to manufacture ideal, perfectly cylindrical roll shells, instead their circularity is always slightly irregular. In addition, shape errors arise in the roll shells during the operation of the calender, for example, as a result of wear.

The invention is intended to create a solution, by means of which variations in the nip loading arising from shape errors in the roll shell can be compensated.

The invention is based on determining the shape profile of the roll shell forming the nip and controlling the nip-loading devices according to the shape profile and the rotational angle of the roll, in order to compensate from variations in the nip loading arising from shape errors.

More specifically, the method according to the invention is characterized by what is stated in the characterizing portion of Claim 1.

Considerable advantages are gained with the aid of the invention.

Variations in the nip loading, i.e. changes in the pressure acting on the web in the nip, arising from the non-circularity of the rolls, are reduced, thus reducing thickness variation in the web, and improving the quality of the end product. By means of the solution according to the invention, it is possible to compensate particularly for changing in loading with a long wavelength. In addition, premature wear of the rolls due to loading variations is reduced.

Li the following, the invention is examined in greater details with reference to the accompanying drawings.

Figure 1 shows a side view of a twin-roll calender.

Figure 2 shows a schematic diagram of one embodiment of the invention in the calender of Figure 1.

Figure 3 shows a side view of a multi-roll calender, the carrier devices of the intermediate rolls of which are equipped with load-relief cylinders.

Figure 1 shows a soft calender 1 , which comprises a lower roll 3 and an upper roll 2, which are arranged to be in nip contact with each other while the calender is running. There then remains between the rotating rolls 2, 3 a compression point, i.e. a nip N, of the same length as the roll, through which the web 9 being calendered is threaded and in which a compressive force is directed to the web 9.

The type of the upper roll 2 is a deflection-compensated roll, by means of which the compression force acting in the nip N on the web 9 is adjusted, in the nip loading in the cross-direction of the web 9. Using a deflection-compensated roll 3, it is possible to adjust the web's 9 cross-direction thickness and gloss profiles. The deflection- compensated roll 3 comprises a stationary shaft, around which a tubular rotating roll shell is arranged. The roll shell is mounted in bearings at both ends of the shaft. The shell is usually of cast iron and is surfaced with a cover, preferably a flexible polymer cover. The shaft is installed at both ends in bearing housings 4. The bearing housing 4 is, in turn, permanently attached to the frame 5 of the calender.

Hydraulic loading elements 11, which act in the nip plane N on the inner surface of the roll shell, are arranged between the roll shell of the deflection-compensated upper roll 2 and the shaft 19. The loading elements 11 are supported on the shaft 19 of the roll 11. The loading element 11 comprises a sliding shoe 13, arranged against the inner surface of the roll shell, and a pressure chamber 13, in which there is a pressure medium. The pressure of the pressure medium loads the sliding shoe 12 against the inner surface of the roll shell. By regulating the pressure of the pressure medium, the deflection of the roll shell and thus the nip loading can be profiled in the longitudinal direction of the shaft 19 of the roll. The pressure of the pressure medium is regulated with the aid of the control system. Hydraulic oil, for example, is used as the pressure medium.

In the deflection-compensated roll 2, there are typically 20 - 60 loading elements 11 arranged next to each other in the longitudinal direction of the shaft 19. The number of loading elements 11 depends on the width of the deflection-compensated roll. The distance between the centre-points of the adjacent loading elements in the longitudinal direction of the shaft is typically 100 - 250 mm. The pressure medium is led to pressure chambers 13 of the loading elements through feed channels in the shaft 19. The feed channels are arranged in such a way that the loading elements can be controlled individually, or in zones comprising two or more loading elements. There are typically 6 - 8 independently controlled zones.

The lower roll 3 of the calender is of a type that is a hard-surface thermo-roll. The shaft of the lower roll 3 is mounted in bearing housings 6 at both ends. A lever arm 7 is attached to both bearing housings 6, and is mounted rotatably in bearings in the frame 5. At each lever arm 7, there is a loading cylinder 8, attached to the frame 5 of the calender, the piston of which acts of the lever arm 7. With the aid of the loading cylinders 8, the lower roll 3 is pressed against the upper roll 2 and in this way the nip load is adjusted over the entire width of the roll. The shell of the thermo-roll 3 is usually made from cast iron or from steel. The thermo-roll 3 can be heated with the aid of a heat-exchanging liquid circulating inside it, or by heating the roll inductively internally or externally.

The shape errors in the shell of the thermo-roll 3, i.e. the points at which non-circularity

appears in the shell of the roll, cause variations in the nip loading when the roll rotates. The changes in the nip loading in turn cause variations in the thickness of the web 9 being calendered. In the solution according to the invention, the points in the shell of the thermo-roll 3 at which the shape errors occur, and the magnitude of the shape errors are first of all determined. On the basis of them, the shape profile of the roll shell is defined.

The shape errors in the roll shell can be determined indirectly, for example, by measuring the thickness variations appearing in the calendered web 9 after the nip. On the basis of the measurement results, the shape profile of the roll shell can be defined. The thickness variations of the web 9 are measured, for example, by using an electromechanical device suitable for this purpose, which is in contact with one, or preferably both sides of the web 9, or which is not in contact with the web 9.

The shape errors in the roll shell can also be determined indirectly, by measuring the hydraulic-fluid pressure variations appearing in the hydraulic circuits 13 of the loading elements 12 of the deflection-compensated upper roll 2. On the basis of the pressure variation and the rotation angle of the thermo-roll, the shape profile of the thermo-roll can in turn be defined.

The shape errors in the roll shell can be determined directly by measuring the shell's shape, for example, in connection with grinding. The shape profile of the roll shell can be defined on the basis of the measurement results.

Once the shape profile of the thermo-roll' s shell has been defined, the profile data is stored in a database 17.

Connected to the hydraulic circuits of the loading elements 11 of the deflection- compensated roll 2 are valves 14, 15, with the aid of which the pressure prevailing in the hydraulic circuits, and thus the loading imposed on the roll shell 18 by the loading elements 12 can be adjusted. If the loading elements 11 are controlled individually, two valves are connected to the hydraulic circuit of each loading element 11. The first valve 14 is connected to the hydraulic-fluid feed channel, and by opening the valve the pressure of he hydraulic fluid in the pressure chamber 13 of the loading element 11 can

be increased, in which case the loading imposed on the roll shell 18 by the loading element 11 will increase. At the same time, the nip loading will increase in the area of the effect of the loading element 11. The second valve 15 is connected to the hydraulic- fluid exit channel, and by opening it the pressure of the hydraulic fluid in the pressure chamber 13 of the loading element 11 can be reduced, in which case the loading imposed on the roll shell 18 by the loading element 11 will diminish. At the same time, the nip loading will diminish in the area of effect of the loading element.

If the loading elements 11 are controlled as zones, corresponding valves 14, 15, by means of which the loading imposed by the zone's loading elements on the roll shell 18 can be altered, are connected to the hydraulic circuit of each zone. At the same time, the nip loading changes in the area of effect of the zone's loading elements.

The valves 14, 15 of the hydraulic circuits of the deflection-compensated roll 2 are controlled on the basis of the angle of rotation of the thermo-roll and the shape-profile data of the roll shell in a database 17. The valves are controlled with the aid of a control circuit 16. How the nip loading changes as the roll 3 rotates, can be determined on the basis of the angle of rotation of the thermo-roll 3 and the corresponding shape-profile data of the roll shell. The speed and angle of rotation of the thermo-roll 3 are measured using a measuring device suitable for this purpose.

The controller 16 controls the valves 14, 15 in such a way as to use the loading elements 11 to alter the nip loading in a direction opposite to that of the change in the nip loading caused by the shape error in the shell of the thermo-roll 3. If the nip loading increases as a result of a shape error, the nip loading is decreased by the loading elements 11.

Correspondingly, if the nip loading decreases as a result of a shape error, the loading elements 11 are used to increase the nip loading. For example, if there is a bump in the surface of the shell of the thermo-roll 3, the loading of the loading elements 11 is altered in such a way that the nip loading is reduced at the bump when it is in the nip plane N. If there is a cavity in the surface of the shell of the thermo-roll 3, the loading of the loading elements 11 is altered in such a way that the nip loading increases at the cavity when it is in the nip plane N. The change in the nip loading created by the loading elements 11 is attempted to be kept to the same magnitude as, but in the opposite direction to the nip-

loading change caused by the shape error in the roll shell.

In a second embodiment of the invention, according to Figure 3, the changes in the nip loading caused by shape errors in the roll are compensated in a multi-roll calender, in which the nip loading arising from the mass of the 25 - 32 idle rolls and their related auxiliary devices is lightened using load-relief devices 20 arranged in connection with the calender. A multi-roll calender comprises a roll stack installed in the frame of the calender, in which there is a deflection-compensated top roll 23 and a deflection- compensated bottom roll 24, as well as idle rolls 25 - 32 arranged between the top and the bottom rolls. There are two or more idle rolls. The rolls in the roll stack are arranged in such a way that they form nips, through which the web 21 being calendered is threaded. The web 21 is guided first of all to the nip Nj formed from the top roll 23 and the highest idle roll 25, and from there through the other nips to the bottom nip N ₉ formed by the bottom roll 24 and the lowest idle roll 32. Between the nips N ₁ - N ₀ , the web 21 is taken off the surface of the rolls with the aid of take-off rolls 22. The construction of the deflection-compensated top 23 and bottom 24 rolls is the same as that of the deflection-compensated roll 2 described in the embodiment in Figures 1 and 2.

The shaft of the top roll 23 is mounted at both ends in bearing housings 33, which are in turn secured, in a manner permitting them to slide, in vertical guides 35 attached to the frame 34 of the calender. At each bearing housing 33 of the top roll 23, there is a loading cylinder 36, which is attached to the frame of the calender and the piston of which acts on the bearing housing 33 of the top roll. The bearing housing 33 of the top roll 23 can be moved vertically along the guides 35, with the aid of the loading cylinders 36.

The shaft 24 of the bottom roll 24 is mounted at both ends in bearing housings 37, which are in turn secured, in a manner permitting them to slide, in vertical guides attached to the frame 34 of the calender. At each bearing housing 37, there is a loading cylinder 36, which is attached to the frame 34 of the calender and with the aid of which the bottom roll 24 can be moved vertically.

The idle rolls 25 - 32 are either hard-surface thermo-rolls or rolls equipped with a soft cover, preferably a polymer cover. The idle rolls 25 32 are arranged in the roll stack in

such a way that at least one of the rolls of each nip is equipped with a soft cover. In the reversing nip N ₅ , both of the rolls 28, 29 forming the nip are equipped with a soft cover. The shafts of the idle rolls 25 - 32 are fitted at both ends rotatably in bearing housings 39. Carrier components 40, which are mounted rotatably in bearings 41 in the frame 34 of the calender. The carrier components 40 are equipped with load-relief devices 20, by means of which the nip loading arising from the masses of the idle rolls can be lightened. In the embodiment of Figure 3, pressure-medium operated relief cylinders 20 are used as the load-relied devices. The first end of the relief cylinder 20 is attached to the carrier component and the other end to the frame 34 of the calender. Hydraulic fluid or compressed air, for example, is used as the pressure medium of the relief cylinders 20.

With the aid of the relief cylinders 20, a load-relieving force is brought to the carrier components 40 of the idle rolls 25 - 32, by means of which the nip loading arising from the weight of the idle rolls 25 - 32 and the auxiliary devices attached to them is compensated entirely or in part.

In the calender according to Figure 3, variations in the nip loading arising from shape errors in an idle roll or rolls is compensated by altering the loading of the load-relief cylinders 20. In a preferred embodiment of the invention, variations in the nip loading arising from shape errors in hard-surface thermo-rolls are compensated. In the solution according to the invention, the points on the outer surface of the shell of the desired idle roll, at which shape errors appear, and the magnitudes of the shape errors are determined first of all, on the basis of which the shape profile of the outer surface of the roll shell is defined.

Shape errors in the roll shell can be determined indirectly, for example, by measuring the thickness variations appearing in the web 21 after the nip, the loading variations of which it is desired to compensate. On the basis of the measurement results, the shape profile of the roll shell can be defined. Web thickness variations can be measured, for example, using an electromechanical measuring device suitable for the purpose, which is in contact with the web from one or both sides, or which is not in contact with the web.

Shape errors of the top idle roll 25 and the bottom idle roll 32 can be determined by

measuring the pressure variation appearing in the hydraulic circuits of the loading elements of the deflection-compensated top roll 23 and bottom roll 24. On the basis of the pressure variation and the rotational angle of the idle roll 25, 32, it is possible in turn to define the shape profile of the idle roll 25, 32.

The shape profile of the idle roll can also be defined by measuring the shape of the roll shell, for example, in connection with grinding.

Once the shape profiles of the roll shells of the desired rolls have been defined, the profile data is stored in a database. The changes in the nip-loading in the top and bottom nips, caused by the shape errors in the top idle roll 23 and the bottom idle roll 32 are compensated in the same way as in the embodiment shown in Figures 1 and 2, i.e. by altering the pressures of the hydraulic circuits of the loading elements of the deflection- compensated roll, according to the shape profile and the angle of rotation.

The variations in the nip loading in the other nips, due to the shape errors in the idle rolls, are compensated with the aid of the load-relief cylinders 20. In that case, the pressure of the hydraulic fluid in the hydraulic circuits of the relief cylinders 20 of one or other of the rolls in the nip, and thus the nip loading, is changed on the basis of the roll's shape-profile data and the angle of rotation. Connected to the hydraulic circuit of each relief cylinder are two valves, by opening which the pressure of the hydraulic fluid in the hydraulic circuit can be increased and decreased, in the same way as by using the valves 14 and 15 in the embodiment of Figures 1 and 2. The control circuit operates the valves on the basis of the shape profile in the database and of the angle of rotation. The rotational velocity and angle of rotation are measured using a measuring device suitable for the purpose.

The control circuit operates the valves of the relief cylinders 20 in a corresponding manner to that in the embodiment of Figures 1 and 2, i.e. the nip loading is altered by the relief cylinders in the opposite direction to the change in the nip loading caused by the shape error in the roll shell. If the nip loading is increased as a result of a shape error, the relief cylinders are used to decrease the nip loading. Correspondingly, if the nip loading decreases as a result of a shape error, the relief cylinders are used to increase the nip

loading. If, for example, there is a bump on the surface of the roll shell, the loading of the relief cylinders is altered in such a way that the nip loading is lightened when the bump is on the nip plane. If there is a cavity in the surface of the roll shell, the loading of the relief cylinders is altered in such a way that the nip loading increases at the cavity when it is on the nip plane. The intention is to keep the change in the nip loading created by the load-relief cylinders at essentially the same magnitude as the nip-loading change caused by the shape error in the roll shell, but in the opposite direction.