Field of the invention

The invention relates to a method according to the preamble of the appended claim 1 for cutting a cellulose web. The invention also relates to a device for implementing the aforementioned method in accordance with the preamble of the appended claim 8, and to a sheet cutter according to the appended claim 16.

Background of the invention

In the production of cellulose, the cellulose web is cut into sheets after drying, the sheets are baled and transported forward, for example to a paper mill to be used as raw material for paper or paperboard pulp. The cutting of the sheets takes place in a sheet cutter, in which a 4 to 7 meters wide full-width cellulose web is first cut in the longitudinal direction into partial webs of suitable width, whereafter they are cut into sheets in a cross-cutting device. The size of a sheet may be for example 80O x 800 mm. The cutter is also used for cutting of wrappings that are for example 1400 mm long. The speed of the cellulose web in the cross-cutting device is 100 to 250 m/min.

There are several different cross-cutting devices for a cellulose web. Typically they all contain at least one rotating rotor, for example a roll on the periphery of which at least one cutting blade extending over the entire length of the roll has been mounted in the axial direction of the roll. In a cross-cutting device of a pulp drying machine commonly in use at present, the cutting of the cellulose web takes place by means of such a roll equipped with at least one blade and a stationary counter blade. The length of the piece to be cut is changed by controlling the peripheral speed of the roll equipped with a cutting blade.

In another cross-cutting device for a cellulose web, which is called a twin rotary cutter, the cellulose web is guided between two rolls rotating against each other and having a diameter of equal size, of which on the

periphery of at least one roll is arranged at least one cutting blade extending over the axial length of the roll. The other roll functioning as a counter roll comprises such a number of counter elements, either cutting blades or grooves that correspond to the number of cutting blades. If the counter element is a cutting blade, the cross-cutting takes place by cutting. If the counter element is a groove, the cross-cutting of the cellulose web takes place by punching. The rotating speeds of the rolls are controlled so that they are equal, thus ensuring that the cutting blade and the counter element meet at the cutting point. The length of the piece to be cut is changed by controlling the speed of the rolls.

WO publication 99/58306 discloses a cross-cutting device of the above kind for a cellulose web, in which the cutting of the cellulose web takes place by means of two rolls rotating against each other. At least one cutting blade is mounted on the periphery of both rolls, and the position of the blade attached to one of the rolls can be controlled to control the clearance between the blades. Similar cross-cutting device is also disclosed in the publication WO 00/13862 in which two cutting blades are mounted in both rolls rotating against each other, said cutting blades cutting the web into sheets. The speed of the rolls is synchronized by means of an endless belt loop that encircles the end flanges of the rolls and is in contact with them.

The drawback of the aforementioned cross-cutting devices of prior art is that the cellulose web is clamped between the blades at the cutting point before it breaks. The clamping causes compression of fibers in the edge areas, as well as hard bundles of fibers, so-called fish eyes, which cause problems in later steps of utilizing the pulp.

The running of the cellulose web in cutters of the above kind is also uneven. This results from the fact that in the cutters the cutting is performed so slowly that the blades in both rolls are in contact with the web for a significantly long time before the blades meet and the web is cut. Consequently, when sheets are cut, and the peripheral speed of the rolls is greater than the speed of the cellulose web, the web stretches every time the blades approach the cutting point. When

wrappings are cut, wherein the peripheral speed of the rolls is considerably smaller than when cutting sheets, i.e. smaller than the speed of the cellulose web, the blades damp the web. Due to these irregularities, the running of the web in the cutter is jerky. The jerking of the web generates vibration in the web, which produces dust and noise problems.

It is also difficult to control the clearance between the blades in cutters of prior art. The size of the clearance is affected by the temperature of the blades and their surroundings, as well as wearing of the blades. To control the clearance, the cross-cutting device must contain separate means to compensate for the temperature and the wearing.

Brief description of the invention

The purpose of the present invention is thus to produce such a method and a device for cutting a cellulose web as well as a sheet cutter that avoid the above-mentioned problems and enable steady run of a cellulose web in a cross-cutting device.

To attain this purpose, the method according to the invention is primarily characterized in what will be presented in the characterizing part of the independent claim 1.

The device according to the invention, in turn, is primarily characterized in what will be presented in the characterizing part of the independent claim 8.

The sheet cutter according to the invention, in turn, is primarily charac- terized in what will be presented in the characterizing part of the independent claim 16.

The other, dependent claims will present some preferred embodiments of the invention.

The invention is based on the idea, that the peripheral speed of the rolls in a cross-cutting device is controlled in such a manner that a steady run of the cellulose web in the cross-cutting device is attained and the cutting process does not cause changes in the run of the web. This is achieved by controlling the peripheral speed of one of the rolls rotating against another roll in the cross-cutting device so that it is higher than the speed of the other roll rotating against it. The peripheral speed of the roll that rotates faster is equal or higher than the speed of the cellulose web run in the cutter. The peripheral speed of the roll that rotates more slowly determines whether sheets or wrappings are cut from the cellulose web, and also their length. The peripheral speed of the roll that rotates more slowly is higher or lower than the speed of the cellulose web.

The two rolls in the cross-cutting device, i.e. the first and the second roll that rotate against each other, are equipped with at least one cutting blade extending in the axial direction of the roll. In the cutting, the web is cut in the direction of the radiuses of the rotating rolls. Thus, the blades cut the web like scissors. As the rolls rotate, the cellulose web is cut already before the line segment connecting the central points of the cross-section of the rolls, on which line segment the trajectories of the blades are within each other.

When the peripheral speeds of the rolls are controlled to differ from each other in accordance with the invention, fast cutting of the web in its thickness direction is attained that does not interfere with the run of the web. Fast, scissor-like cut also reduces the production of fish eyes and reduces the noise caused by the cross-cutting device. Another advantage of the invention is that by means of the invention it is possible to cut pieces of different lengths, i.e. sheets and wrappings without interfering with the run of the cellulose web.

Brief description of the drawings

In the following, the invention will be described in more detail with reference to the appended drawings, in which

Fig. 1 shows a schematical side view of a sheet cutter according to the invention, used in the cutting of a cellulose web,

Fig. 2 shows a schematical side view of a cross-cutting device according to the invention, and

Figs 3a to 3d show the different stages of cutting the cellulose web at the cutting point of the cross-cutting device.

Detailed description of the invention

Fig. 1 shows a sheet cutter 1 used in the cutting of the cellulose web. The cellulose web coming from a drying device, which is not shown in the figure, is fed in the direction of the arrow A to a winder 3 by means of a tail threading device 2. The tail threading device 2 is only required in a tail threading situation and in normal use the cellulose web travels past the tail threading device 2. In the winder 3 the full-width cellulose web is cut into several partial webs of desired width. The partial webs are passed through a draw press 4 to a cross-cutting device 5. The cut sheets S are directed forward on a bale table 8 by means of a pair of turning rolls 6 and pairs of turning rolls 7 effecting the interlacing of the sheets.

The cross-cutting device 5 is composed of two rolls rotating against each other, a first roll 5a and a second roll 5b, on the periphery of both of which at least one cutting blade extending over the entire length of the roll has been mounted in the axial direction of the roll. The peri- pheral speeds of the rolls are controlled in such a manner that the peripheral speed U ₁ of the first roll 5a is higher than the peripheral speed U ₂ of the second roll 5b. The controlling of the peripheral speed of the rolls 5a and 5b is conducted in accordance with the length of the sheets or wrappings cut at a given time as well as the running speed of the web by means of a control apparatus 9 connected to the rolls. The controlling of the peripheral speed of the rolls can take place for example in

such a manner that sensors (not shown in the figure) are attached to the ends of the rolls to measure the speed of rotation of said roll. The measured speed of rotation of the roll is transmitted to the control unit 9, which calculates the peripheral speed of the rolls, compares it to a set value calculated for each running situation and transmits an control message to the driving means of the rolls 5a and 5b, if necessary. The peripheral speed of the roll can be calculated as follows:

u = 2πnR (1 )

in which n = speed of rotation of the roll, and R = radius of the roll

The controlling of the peripheral speed of the rolls is required especially when the length of the pieces cut by means of the cutter changes, for example after cutting of sheets, wrappings are cut or vice versa. For determining the control message information on the running speed of the cellulose web is also required, which is determined for example by measuring it by means of a separate measuring device 16 placed be- fore the cross-cutting device 5. The measuring device 16 can be for example a speed measuring device that measures the web optically and produces a signal that is transmitted to the control apparatus 9 via a line 16a. The running speed of the cellulose web can also be determined on the basis of the speed of rotation of a roll measured from a roll that is located before the cross-cutting device 5 and is in nip contact with the web, for example from a roll 4a of a draw press 4a, whose rotating speed is transmitted to the control unit 9 via a line 17. This signal is illustrated with a broken line in Fig. 1.

The control apparatus also follows the peripheral speed of the rolls during cutting and adjusts it, if necessary. The measuring signals and control messages of the speed of rotation of the rolls are transmitted between the rolls 5a and 5b and the control unit via lines 9a and 9b. The control and calculation algorithms used by the control apparatus are known as such by a person skilled in the art, and therefore they are not described in more detail in this context.

The above-presented embodiment of the invention is suitable for a cross-cutting device in which the first roll 5a and the second roll 5b comprise separate driving means. It is also possible that only one of the rolls 5a or 5b comprises driving means, for example a driving motor, to rotate the roll. In this embodiment the second roll, which does not have a drive is rotated for example by means of cogged wheels installed at the ends of both rolls and arranged to rotate against each other, or by means of belt transmission arranged between the shafts of the rolls. Thus, the control of the peripheral speed of the rolls is conducted by controlling the peripheral speed of the roll that contains a drive. The diameters of the rolls used in this embodiment and the number of the cutting blades mounted thereto is, of course selected in such a manner that by controlling the peripheral speed of the rolls it is pos- sible to attain sheets or wrappings of desired length. When the length of the pieces to be cut by means of the cutter changes, the peripheral speed of the roll containing the drive is controlled on the basis of the measured/determined running speed of the cellulose web and the desired length of the sheets or wrappings. Typically, the drive means is arranged in a roll 5b having a larger diameter.

As Figs 1 and 2 show, the rolls forming the cross-cutting device 5 have diameters of different sizes. The diameter d-i of the first roll 5a is smaller than the diameter d ₂ of the second roll 5b. When two rolls having diameters of different sizes are used in the cross-cutting device, it is necessary to increase the peripheral speed of the roll having smaller diameter higher than the peripheral speed of the roll having larger diameter, so that the cutting blades can be made to meet each other at the cutting point. When a cross-cutting device is used in which the drive is arranged only in the other roll, it is necessary to select the diameters of the rolls to be used and the devices transmitting the rotating force to the roll not containing a drive, i.e. the cogged wheels and the belt transmission means and their size in such a manner that the peripheral speed of said roll is set in a suitable value so that the cutting blades in the first roll 5a and in the second roll 5b meet each other. The rolls used in the cross-cutting device are selected in such a manner

that the second roll 5b has a 10 to 20 % larger diameter than the first roll 5a.

By using two rolls having diameters of different sizes and rotating at different peripheral speeds, it is possible to attain rapid cutting, and less fish eyes in the edge areas of the cut sheets/wrappings than in known cross-cutting solutions. The peripheral speed of the roll having smaller diameter is advantageously approximately 20% higher than the speed of the cellulose web, or equal thereto. For example, when sheets whose length is 800 mm are cut, the peripheral speed of the roll having smaller diameter can be 420 m/min, and the peripheral speed of the roll having larger diameter can be 350 m/min. Furthermore, when wrappings whose length is 1400 mm are cut, the peripheral speed of the roll having smaller diameter can be 240 m/min, and the peripheral speed of the roll having larger diameter can be 200 m/min. Is should be noted that the peripheral speeds of the rolls are dependent on the running speed of the cellulose web and the diameters of the rolls. The diameter of the smaller roll is slightly over half of the diameter of the larger roll, the diameter of the larger roll can be for example 2 m, wherein the suitable diameter of the smaller roll is for example 1.2 m.

At least one cutting blade is mounted on the outer periphery of the rolls 5a and 5b used in the cross-cutting device, said blades meeting each other when the rolls rotate, thus cutting the cellulose web travelling therebetween. Advantageously, one or two cutting blades are mounted on both rolls. There may be a similar number of blades in both rolls. Such an alternative is shown schematically in Fig. 1 in which two blades 11 are mounted on the first roll 5a and two blades 10 are also mounted on the second roll 5b. It is also possible that the roll having larger diameter has two blades, and the roll having smaller diameter has one blade, which alternative can be seen in Fig. 2, in which the first roll 5a contains one blade 11 and the second roll 5b contains two blades 10. By controlling the peripheral speed of the rolls 5a and 5b it is possible to align the blades 10 and 11 when cutting the sheets.

RECTIFIED SHEET (RULE 91) ISA/EP

It should be noted that although in Figs 1 and 2 the positioning of the rolls with respect to the cellulose web to be cut is shown in such a manner that the diameter of the roll 5a below the web is smaller than the diameter of the roll 5b above the web, the positioning of the rolls with respect to the cellulose web can be arranged otherwise as well, depending on the dimensions of the sheet cutter and the positioning of the cross-cutting device. The pair of rolls 5a, 5b can also be arranged in an inclined position with respect to the vertical plane.

In the cross-cutting device according to the invention the cutting blades cut the cellulose web passed between them substantially in the direction of the radius of the rotating rolls, i.e. substantially in the thickness direction of the web. Figs 3a to 3d show the cutting process of the cross-cutting device according to the invention in phases. The outer surface of the first roll 5a is shown by means of an unbroken line 12 and the outer surface of the second roll 5b is shown by means of an unbroken line 13.

Fig. 3a shows the situation before the cutting action. The blade 11 in the first roll 5a and the blade 10 in the second roll 5b come closer to each other when the rolls 5a and 5b rotate. The cellulose web W is fed between the rolls in the direction of the arrow A, and it is cut substantially in the direction of the radius B of the rotating rolls 5a and

5b (shown in Fig. 3b) while the rolls are rotating and the blades pene- trate into the cellulose web W.

In Fig. 3b the rolls 5a and 5b have rotated and the blades 10 and 11 meet each other and intermesh. In other words, the blades cut the web like scissors. At this stage the cellulose web W is cut entirely in two. Thus, the cellulose web is cross-cut before the blades 10 and 11 reach the connecting line segment C between the diagonal line 16 parallel to the shaft of the first roll 5a and the diagonal line 17 parallel to the shaft of the second roll 5b. The line segment C is also shown in Fig. 2.

In Fig. 3c the cellulose web W is cross-cut and the cut sheets or wrappings travel forward to a bale table. The cellulose web W continues its

run through the nip formed by the first roll 5a and the second roll 5b. A new sheet or wrapping is cut from the cellulose web again, when the blades 10 and 11 mounted on the rolls meet each other.

Fig. 3d shows a stage in which the first roll 5a and the second roll 5b have moved and the blades 10 and 11 mounted thereon have moved along with them. Fig. 3d shows the paths of motion of the points of the blades 10 and 11 during the cutting process. The trajectory of the blade 11 mounted on the first roll 5a is marked with a broken line 14 and the path of motion of the blade 10 mounted on the second roll 5d is marked with a broken line 15. As can be seen in the figure, trajectories of the blades 10 and 11 extend inside each other during the cutting process.

The invention is not intended to be limited to the embodiments pre- sented as examples above, but the invention is intended to be applied widely within the scope of the inventive idea as defined in the appended claims.