Technical background Paper and paperboard are manufactured by compressing a pulp suspension continu- ously as a planar web. The web is transported, for example, with the help of rolls or wires. In this case, there may occur a difference in the drawing tension of the track in the transverse direction. In the tightest point, the friction force is respectively higher so that the track tends to move to the tighter side. In some stages it is also preferable to spread the web in the transverse direction, for example, for compensat- ing shrinkage. Curved rolls have been used for the spreading. The lateral transition caused by variations in tightness causes problems especially with these kinds of spreading rolls.

The use of curved rolls for the transverse control is difficult. Especially the adjust- ment is hard as it has to be made by turning the curve of the roll.

Summary of the invention In accordance with the independent claims, a method and apparatus for the trans- verse control of the track have now been invented. The dependent claims present some advantageous ways for the realisation of the invention.

In accordance with the invention, an electrostatic charge is formed to the track at least to a part of its width, and the charged track is provided with a laterally influ- encing force with the help of an electric field.

The force can be directed to the track as it is on a base, especially a base moving along with the track, such as a roll or wire. When the surface against the track is charged, the track tends to adhere to the base or detach from it, depending on the charge of the base surface. In a preferable arrangement the electric field goes through the track to the base so that there is an electrode establishing a charge to the

track on the other side of the track. As the adhesion increases, for example, the trac- tion force directed to the track by the drawing roll increases respectively at this place. The increase in adhesion also reduces, for example, the amount of air remain- ing between the track and the base at this point.

The track to be charged can be the web to be manufactured or the wire used for transporting it. The system can be made to be adjustable.

The charge can be established to the track especially in an adjustable manner. The track can also be provided with a charge intermittently to different places in the transverse direction, especially in a way advancing in the transverse direction.

According to the invention, the track can be controlled by a simple apparatus, the power demand of which is low. The system can be made so that it does not contact the track. Adjustability is good. Reduction in noise and dust is achieved as a further advantage.

One object of the invention is to eliminate the transverse transitions of the track.

A second object of the invention is the spreading of the track in the transverse direc- tion.

A third object of the invention is the discharge of air from between the web and the layer below. In this case, an electrostatic charge advancing intermittently in the transverse direction, especially from the centre to the side, is directed to the web.

A fourth object of the invention is the control of grip on the output roll (or beam) in a calender.

Drawings The enclosed drawings are part of the written description of the invention, and they present some ways for realising the invention.

Figure 1 is a top view of the spreading roll device of the web.

Figure 2 is a side view of the apparatus in Figure 1.

Figure 3 presents the air discharge device of the reeler.

Figure 4 presents a second air discharge device of the reeler.

Figure 5 presents a third air discharge device of the reeler.

Figures 6a and 6b present a fourth air discharge device of the reeler, seen from the side.

Figure 7 is a side view of a calender with an aoutput beam.

Detailed description of the invention An electrostatic charge can be directed to the track by using a suitable charging de- vice, such as a rubbing device, by contacting with a charged electrode, or by induc- ing or by using an ion blast electrode. A method not contacting the track is most preferably used, such as ion blast. A sufficient charge can be achieved with very low power, for example, a current of about under 5 mA. Charging devices can also be located at different sides of the plane of the track. Suitable commercially available electrodes can be used in the system; for example, ion blast electrodes.

According to one embodiment, a charge is established to the track at the same time as an electric field achieving a laterally effecting force is directed to it. In this case it is especially possible to use an arrangement, in which the track is on the base, and on the other side of the track, there is an electrode providing the track with a charge.

When the track and the base are in a different potential, the track tends to adhere to the base. In practice, this is best achieved by using a conducting earthed base. A still bigger adhesion force is achieved by keeping the base in a potential, which is oppo- site to that of the electrode.

According to a second arrangement, a charge is first established to the track, after which it is directed to the electric field.

The system has at least one charging device, with which a charge can be directed to at least a part of the width of the track, especially asymmetrically. Most preferably the system has several charging devices in the transverse direction. In this case, there can be one charging device, for example, on both edges of the track, or several charging devices placed at different places in the transverse direction. The charging devices can be located at the same place or at different places in the longitudinal di- rection of the track.

The charging devices are most preferably adjustable so that the charge to be directed can be altered.

The ion blast device has a spike-like electrode and a large flat counter-electrode, be- tween which a strong electric field is established. The electric field then forms a conical corona extending from the tip of the electrode. There are usually several spike-like electrodes so that a desired coverage is achieved. The counter-electrode is generally in the earth potential. A corona discharge is established at the tip of the spike-like electrode, ionising the surrounding gas. The ions with opposite signs in relation to the charge of the electrode and the particles adhered to these are then transported towards the counter-electrode in a conical field. Thus, a flow is gener- ated, which is called ion blast. The voltage of the spike-like electrode is most pref- erably negative, because negative ions are naturally formed of the gas molecules in the air. A uniform and controllable ion flow is then easily achieved. The specific voltage of the spike-like electrode can be, for example, in the area of 30-1000 kV, typically in the area of 80-160 kV. The current can be, for example, 1-30 mA, typically under 10 mA. The distance of the electrode from the counter-electrode can be, for example, 2-2000 mm, typically 10-100 mm, most preferably under 50 mm. A negative voltage is most preferably used.

In the arrangement of the invention, when using ion blast, the track is transported across the field so that charged particles adhere to the track. Ion blast is controllable and adjustable.

The specification FI 104645 presents some ways for charging the track by using the ion blast technique, among others.

According to one embodiment, the counter-electrode of the ion-blowing electrode is a roll. In this case, the roll can be earthed.

According to one embodiment, the adhesion between the track and the roll is al- tered. Thus it is possible, for example, to correct the variations in tightness of the various lanes of the track and to prevent the lateral transitions caused by the varia- tions. The system can then include means for determining the lateral position of the track, on the basis of the data provided by which the track will be charged. Thus, for example, no mechanical wire controllers will be needed in the embodiment. In this way it is especially possible to correct lateral transitions occurring on the spreading rolls and coating machines.

As the grip between the web and the roll is mainly caused by the electrostatic charge, the spreading force depends only slightly on the tightness of the web. The effect of the tightness can further be reduced by decreasing the refracting angle of

the web on the roll. The refracting angle can be, for example, about under 15 de- grees, such as about 5 degrees, when also electrostatic force is used for the spread- ing.

According to one embodiment, several charging devices are used in the transverse direction, with which the charge is directed intermittently to different places in the transverse direction, especially in a way advancing in the transverse direction. Thus, it is possible to establish a charge front moving from the middle of the track towards the edges. The electrodes can be directed by an alternating voltage so that a charge sweeping towards the edges is established. This can be done, for example, so that the alternating current is directed to adjacent electrodes by phase displacement, or a purely pulse-type charge can be used. The alternating current can be, for example, sinusoidal. The phase displacement can be, for example, 1-180 degrees, especially just 180 degrees. An intermittent charge can be used, for example, for spreading the web so that the web also tightens in the transverse direction. At the same time, it is possible to adjust the profile of the transverse tightness over the entire area of the track. If the charging devices are placed only to the edges of the web, it is only pos- sible to spread the web.

Electrodes can also be placed so that a positive and negative charge is alternately di- rected to the track in the transverse direction so that an electric field spreading from the middle outwards is established. When using ion blast electrodes, one can then talk about a kind of a"corona engine".

According to one embodiment, the invention is used for profiling the web so that spreading forces of different sizes are directed to different places. The spreading can be more extensive in the thicker point, and thus, it is possible to reduce the trans- verse variation in thickness.

In accordance with the invention, the spreading of the web can be realised in a clearance, replacing the spreading rolls. The spreading can also be performed by us- ing a straight roll or wire, or against a drying cylinder or other similar roll.

According to one embodiment, the tightness of the track is measured in the trans- verse direction, and the track is spread or transferred accordingly.

The charge front advancing intermittently in the transverse direction can also be used especially for discharging air between the track and the base, such as the web to be rolled and the web layer below it. Especially at least three charging devices placed at different places in the longitudinal direction of the track can then be used,

the charging devices forming a plough so that the tip electrode of the plough meets the track first. The method is easy to control as only a charge of one sign, most pref- erably a negative charge, needs to be used. It is also suitable for high speeds. The principle may be that the electric fields are used for sort of pumping air to the sides of the web. Alternatively, a wedge-type field can be used, which spreads from the middle of the web to the edges. When a field advancing to both sides from the cen- tre is used, the transfer range of air is as short as possible.

According to one embodiment in calendering, an electrostatic charge is formed to the web and, for keeping the web free from the calender roll, the web is directed through an output element. Depending on the charge of the output element, the web either tends to come detached from the surface of the output element, due to the electrostatic forces, or to attach to it. By changing the charges the grip between the web and the output element can thus be regulated.

If the web and the output element have charges of the same sign, the web tends to come detached from the surface of the output element. Thus, less frictional forces causing a risk of breaking are directed to the paper on the output element. This is especially useful in connection of web feeding and with low running speeds so that no gas film effectively reducing friction is established between the web and the out- put element. The detachment force can be regulated by changing the magnitude of the charges. The grip can be increased with charges of different signs by reducing the gas film. This can be preferable especially with high running speeds.

The output element can have a suitably curved surface. The output element can be, for example, a roll or a fixed beam.

When using a fixed (i. e. not rotating) output beam, bearing mounting can be avoided. Also vibration problems of a rotating roll are avoided.

A different charge can be established to the web or output element at different places in the transverse direction of the web. Thus, the thickness of the gas film formed between them can respectively be regulated at different places. In this case, it is especially possible to establish the charge to different places in the transverse direction intermittently, especially in a way advancing in the transverse direction.

Especially the thickness of the gas film and through this the friction forces can thus be controlled.

The output element can be curved in the direction transverse to the web. Thus, the element also operates as the spreading element for the web. The curvature can also be controllable.

Examples of some embodiments of the invention are further explained in the follow- ing.

According to Figure 1, the paper web is directed to the spreading roll 2 through the tension roll 1. The tightness of the web in the direction of travel is presented with the arrows 3. The tightness can be different in different points so that, on the tighter side, the web adheres to the spreading roll with a higher friction and the web respec- tively moves laterally to the side of higher tightness.

For compensating the lateral transition, ion blast electrode groups 4 and 5 are placed at places of the different halves of the web, to which a desired, most preferably negative, voltage can be obtained from the voltage sources E1 and E2, respectively.

Thus, an electrostatic charge can be induced to the desired half of the web. As the spreading roll is earthed, the charged section of the web adheres more tightly to the roll, due to the influence of the electrostatic traction force. The detector 6 is further connected to the apparatus, for following the edge of the web and for controlling the voltage sources with the help of the signal obtained from it.

The ion blast electrode groups 4 and 5 have3 several spike-like electrodes in the transverse direction of the web, to which the desired voltage is directed.

According to Figure 3, the paper web 7 is rolled onto the earthed roll 6. Ion-blowing electrodes 8 are placed to the rolling point in the transverse direction. Sinusoidal al- ternating voltage Eac is directed from the generator 9 to the middlemost electrode 8.1 and to every second electrode, to both sides from it. Sinusoidal alternating volt- age is directed from the generator 10 to the other electrodes with a phase shift of 180 degrees. When the voltage is in its maximum on the electrode, the web is at this place compressed most tightly against the roll and the web layer below, due to the static electricity so that also air discharges from this point, transferring to the side in which the web is more weakly compressed against the roll. Thus, beginning from the middle and extending to the sides, a wave transporting air is generated.

Upon rolling, air can alternatively be discharged in the manner shown in Figure 4.

Here each electrode has its own generator 11. These are controlled by the multi- plexer (MP) 12 and the controller (PLC) 13. The generators are controlled so that a

voltage pulse is first given to the middlemost electrode 8.1, and after that to the other electrodes 8 in an order towards the sides. When the web is compressed more tightly against the roll always at the place of an electrode with voltage, air is pushed from the middle to the sides. When desired, it is also possible to adjust especially the frequency and duration of the pulses, possibly also the power of the generators.

According to Figure 5, the electrodes 8'are located on the reeler in a plough-like manner so that the middlemost electrode contacts the web first. The web is com- pressed more tightly first from the middle and then in order towards the sides at the place from each electrode. Air is thus transferred from the middle to the sides. Also here the electrodes can still be controlled in the manners disclosed in Figures 3 and 4, when desired.

When electrodes 8"placed to different places in the longitudinal direction of the track are used on the reeler, the electrodes can be located to the adjustable curved beam 14 in accordance with Figure 6a and 6b, the radius of the beam being in- creased as the roll grows. Thus the electrodes are always found within the same dis- tance from the web.

The two-nip calender of Figure 7 has the upper roll 14, the intermediate roll 15, and the lower roll 16.

For detaching the web from the surface of the intermediate roll 15 between the nips there is provided the output beam 17. The surface of the output beam may be suita- bly curved so that the web is spread in the transverse direction.

Before guiding the web to the output beam 17, it is charged electrostatically by the electrode 18 above the web, which obtains the voltage from the generatorl9. The counter-electrode 18'of the electrode is located below the web. The charge is led to the output beam using the generator 20. When the charges of the web and the output beam are of the same sign, an electric repulsive force is generated between the web and the output beam, tending to detach the web from the surface of the beam and to simultaneously enlarge the air film between the web and the beam. Thus, also the friction between the web and the beam is reduced. Respectively, by using charges of different signs, the friction can be increased. There may be several electrodes 18 so that the friction can also be adjusted locally.

The generators are linked with the control system 21, with which the charge is ad- justed. The charges can also be adjusted, for example, according to the tightness or running speed of the web.

The output beam 17 can be curved in the direction transverse to the web so that it also operates as a spreading beam. Also the curvature can be adjustable, for exam- ple, according to the transverse tightness.

When the web is charged, it can also be controlled at the transverse direction with the help of electrostatic forces.