Abstract:
A web cut-off device including a frame defining a piston chamber, a piston movable in the piston chamber, a cutter blade attached to the piston such that upon movement of the piston in a first direction, the cutter blade is adapted to engage and cut a web, at least one pressure chamber and lower connecting ducts for connecting the pressure chamber(s) to a lower part of the piston chamber. The ducts opening directly into opposed side walls defining the piston chamber such that the piston operates as a valve between the piston chamber and the pressure chamber(s). The device also includes at least one return pressure duct in communication with an upper part of the piston chamber such that upon introduction of pressure into the return pressure duct(s), the piston is moved in a second direction opposite to the first direction, and at least one bottom duct in communication with a bottom of the piston chamber such that by introducing pressure into the bottom of the piston chamber via the bottom duct(s), movement of the piston in the first direction is initiated and by evacuating pressure from the bottom of the piston chamber via the bottom duct(s), the piston is maintained stationary in the piston chamber. A method for cutting a web using the device is also disclosed.

Description:
FIELD OF THE INVENTION 
     The invention concerns a web cut-off device for cutting a web in a web-handing machine such as a paper machine and a coating machine. 
     BACKGROUND OF THE INVENTION 
     In devices for processing of a paper web, such as, for example, paper machines and coating machines, cut-off devices are used, whose function is to cut off the web at a number of points when some disturbance of operation takes place in the machine, such as, for example, an uncontrolled web break. The purpose of the cutting off is to protect components that might be damaged when the paper web proceeds as wrinkled and having lost its tension inside the machine, for example, through a coating station or through a calender with soft rolls. 
     Further, a cut-off device is employed in reelers as a part of the machine reel change equipment. At present, for cutting off a paper or board web, among other things, the following methods are used: 
     Board and paper grades whose basis weight is high, typically &gt;150 g/m 2  (grams per square meter), are cut off by using a so-called bag change or a cutting band or a cutting string. In bag change, the machine reel that is being completed is slowed down while, at the same time, pressurized air is blown to below the web. The web rises and forms a bag and enters into the nip. A sudden jerk cuts off the web. In change by means of a band, a separate band or string is fed to which an adhesion face has been fitted, for example, by means of an adhesive tape at the inlet side of the nip so that the adhesive tape affixes the band to the face of the reel spool outside the web, and the web tears the web apart in the nip while winding the web in spiral shape around the reel spool. In band change devices, the band that cuts off the board or paper web remains in the interior of the machine reel that is being completed on the face of the reel spool, where it produces bottom broke. The board/paper is glazed at the band over a thickness of several tens of windings. Nor is the reliability of operation of band change devices good, and their operation requires an abundance of manual work and great care. 
     With medium-weight paper grades, from about 80 to about 150 g/m 2 , a so-called swan-neck change is employed, in which a little cut is cut into the web before the nip, and after the nip pressurized air is blown to the cut area, in which connection the web is torn up to the edges. 
     With thin paper grades, about &lt;100 g/m 2 , in reelers of the Optireel™ type marketed by the current assignee&#39;s, blowing from below the web or from the side is employed in order to pierce and to cut off the web. In particular with thin paper grades and at high running speeds, cutting off of a paper web by blowing produces detrimental paper chips, because the process of cutting is not controlled. With thicker paper grades, when blowing exclusively by means of pressurized air is employed, problems arise because the pressurized air cannot pierce the web or does the piercing untidily. 
     Further, there are applications in which a high-pressure water jet/jets is/are employed as the cutting element, which jets move across the web at a high speed. A problem of water-jet cutting is the necessity of very quick linear movements in the cross direction of the web and long cutting tails. In water-jet cutting, uncontrolled web breaks may also arise, in which case the web is torn by itself further from the cut that has been made by the jet. 
     There are also applications in which various mechanically striking blades are employed, which cut off the web as of full width. 
     The current assignee&#39;s FI Patent No. 97,339 constitutes the prior art most closely related to the present invention. In said patent, a cut-off device based on a mechanical blade is described. Said cut-off device includes a cut-off blade and an actuator that is fitted to act upon the cut-off blade to produce a cut-off stroke. The actuator consists of a number of cylinders fitted inside a chamber, each of which cylinders includes a piston and a piston rod. The piston rods have been attached with a certain spacing to a cut-off blade extending across the whole web width. A rapid stroke of the cut-off device is produced by means of rapidly opening pressure-controlled control valves of large area for the cylinders attached to the cut-off blade and by placing the cylinders in the interior of a pressure chamber so that the path of the compressed air from the pressure chamber into the cylinders is as short as possible. 
     In the cut-off device described in the FI Patent 97,339, variation may occur in the cutting speed. This comes at least from wear of the seals of the retainers and/or from problems related to the two-part structure of the instant-discharge principle used for the discharge. 
     However, when it operates correctly, cutting off taking place by means of a blade involves a number of advantages, such as controlled cutting off and tidy cutting result also at high speeds, as well as easy regulation of the cutting capacity by means of regulation of pressure in compliance with the paper grade to be cut off. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     The object of the present invention is to provide a very rapid and precise cutting off of a web based on a mechanical cut-off blade extending across the whole web width. 
     In order to achieve this object and others, a web cut-off device in accordance with the invention comprises means defining a piston chamber having a pair of opposed side walls, a piston movable in the piston chamber, a cutter blade attached to the piston such that upon movement of the piston in a first direction, the cutter blade is adapted to engage and cut a web, means defining at least one pressure chamber and lower connecting ducts for connecting the pressure chamber(s) to a lower part of the piston chamber. The ducts open directly into the side walls of the piston chamber and at least at level of a bottom face of the piston chamber such that the piston operates as a valve between the piston chamber and the pressure chamber(s). The device also includes at least one return pressure duct in communication with an upper part of the piston chamber such that upon introduction of pressure into the return pressure duct(s), the piston is moved in a second direction opposite to the first direction, and at least one bottom duct in communication with a bottom of the piston chamber such that by introducing pressure into the bottom of the piston chamber via the bottom duct(s), movement of the piston in the first direction is initiated and by evacuating pressure from the bottom of the piston chamber via the bottom duct(s), the piston is maintained stationary in the piston chamber. Stop cushions may also be arranged in a top portion of the piston chamber for stopping movement of the piston in the first direction. 
     A cut-off device in accordance with the present invention can be used for cutting off of all paper and board grades. 
     The device is easy to manufacture, because its construction is very simple and it comprises just one mobile part. The tightness of the device also remains stable, because the backup face of sealing consists of the lower face of the piston, which retains its shape very well. 
     With a cut-off device in accordance with the invention, within normal pressure ranges of compressed-air systems, max. about 7 bars, very high rates of accelerations of up to 1000×g and very high speeds of 30 meters per second can be achieved. High speed is an essential property required in cutting off of a paper and board web. A particularly high speed is also required when the web is cut off in the unwind stand of a coating machine or when running at a high speed. 
     The solution in accordance with the invention permits wider dimensioning of flow ducts and uniform supply of air to below the piston. This is why, behind the piston, full pressure is obtained immediately, which pressure results in an acceleration as efficient as possible. On the other hand, owing to the efficient acceleration, it is possible to achieve high speeds even with a piston of relatively low weight. As a result of this, it is possible to use a short acceleration distance, in which case the blade and the piston assembly become of lower height. This again results in a reduced mass and in lower forces of deceleration, which has the consequence of better mechanical strength in particular at high speeds. 
     Since the acceleration is efficient, full system pressures do not have to be used in the cut-off device either. This is why an adequate allowance for regulation of the stroke power is obtained so that the cut-off device can be used for cutting off of paper and board grades of highly different thicknesses. 
     Owing to its speed reserve and to the simultaneous timing in the cross direction of the web, the cut-off device in accordance with the invention is also highly insensitive to different variables in a papermaking process, which are usually detrimental to the carrying out of changes, and to the maximal values of said variables, to ranges of variations, cross-direction profiles, etc. Such variables include, among other things, the web speed, web tension, basis weight/grade, tension profiles, moisture, moisture profiles, web strengths, strength proportions. 
     The cut-off device can also be placed very freely in a machine, because the cut-off device is compact, and the cut-off device also operates when turned upside down because of the pneumatic retainer. Since the retaining takes place by means of a vacuum, the top side of the piston can be kept free from pressure, which improves the accelerations and speeds that are achieved. 
     As regards the discharge, the cut-off device operates highly reliably, for it operates even if a significant proportion of the discharge valves were not in operating condition. Since there are several discharge valves and since their operation is very quick, being directly electrically controlled, it is possible to achieve a highly simultaneous start of movement (typically &lt;0.5 ms) across the entire length of the device. This is why the web can be cut off highly simultaneously across its entire width, in which case the cutting off is controlled and the risk of web break and/or of an uncontrolled tear is very little. 
     The invention will be described in the following with reference to the figures in the accompanying drawings, in which a preferred cut-off device in accordance with the invention is illustrated, the invention being, however not supposed to be confined to the details of said embodiment alone. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic vertical sectional view of a preferred web cut-off device in accordance with the invention. 
     In FIG. 2 the web cut-off device is illustrated in its home position. 
     In FIG. 3 the web cut-off device has been shifted from its home position into contact with the web. 
     In FIG. 4 the web cut-off device has been discharged and the web has been cut off. 
     FIG. 5 is an enlarged view of the cut-off device in the situation shown in FIG.  3 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The frame part  100  of the web cut-off device as shown in FIG. 1 comprises a bottom piece  101  and a first  102  and a second  103  upper part to be attached to the bottom piece. Said first  102  and second  103  upper part are, in the direction of width X—X of the frame part  100 , preferably mirror images of one another in relation to a vertical plane Y—Y fitted in the middle. In its longitudinal direction, the frame part  100  extends preferably across the entire width of the web to be cut off. The bottom piece  101  of the frame part  100  and the first upper part  102  are interconnected preferably by means of bolts  110 , and the bottom part  101  and the second upper part  103  are, similarly, also interconnected by means of bolts  111 . In the bottom piece  101  of the frame part  100 , there are bores fitted for the bolts  110 , 111 , and in the upper parts  102 , 103  of the frame part, similarly, there are threaded bores adapted to the threads on the bolts  110 , 111 . The bolts  110 , 111  are placed with a suitable spacing across the length of the frame part  100  so that an adequate sealing can be achieved between the bottom piece  101  and the upper parts  102 , 103 . 
     In the frame part  100  of the cut-off device, there is a cutter blade  11 , which is attached directly to the piston  12 , e.g., by means of spring cotters. On the other hand, the piston  12  is fitted in a piston chamber  13 , in which it can move in the directions of the arrow S. The cutter blade  11 , the piston  12  and the piston chamber  13  extend preferably substantially across the entire width of the web to be cut off. The piston chamber  13  has been shaped so that the first half of the piston chamber  13  has been formed into the first upper part  102  and the second half has been formed into the second upper part  103 . The horizontal section of the piston chamber  13  is substantially shaped as an oblong rectangle. The first long side wall of the piston chamber  13  is denoted with the reference numeral  14  and the second long side wall with the reference numeral  15 . 
     Further, the frame part  100  includes a first pressure chamber  30  in direct vicinity of the first long side wall  14  of the piston chamber  13  and a second pressure chamber  31  in direct vicinity of the second long side wall  15 . From the first pressure chamber  30  a first short lower connecting duct  32  passes into the lower part of the first long side wall  14  of the piston chamber  13  to the level of the bottom of the piston chamber  13 , and from the second pressure chamber  31 , similarly, a second short lower connecting duct  33  passes to the lower part of the second long side wall  15  of the piston chamber  13  to the level of the bottom of the piston chamber  13 . The pressure chambers  30 , 31  extend substantially across the entire length of the frame part  100 . Moreover, the pressure chambers  30 ,  31  communicate with an outside source of pressure, which is not shown in the figure. The lower connecting ducts  32 ,  33  that pass from the pressure chambers  30 , 31  into the piston chamber  13  preferably consist of a number of separate ducts parallel to one another and fitted in the longitudinal direction of the frame part  100 , i.e. in the cross direction of the web. It is also possible to imagine that, in stead of two pressure chambers  30 , 31 , just one pressure chamber is employed, from which a number of separate lower connecting ducts pass into the piston chamber  13 . 
     In the bottom piece  101  of the frame part  100  of the cut-off device, in said vertical plane Y—Y, there are bottom connecting ducts  16  that pass into the bottom of the piston chamber  13  to below the piston  12 , which ducts can be connected through discharge valves  40  to a discharge pressure duct  50  and/or to a vacuum duct  51 . The discharge pressure duct  50  communicates with an outside source of pressure, which is not shown in the figure, and the vacuum duct  51  communicates with an outside source of vacuum, which is not shown in the figure either. Discharge valves  40  and connected bottom connecting ducts  16  as well as connecting ducts  41 , 42  passing from the discharge valves  40  to the discharge pressure duct  50  and to the vacuum duct  51  are fitted with a suitable spacing, for example 5 to 10 per meter, across the entire length of the frame part  100 . 
     Further, in the frame part  100 , there is a first return pressure duct  60  in direct vicinity of the first long side wall  14  of the piston chamber  13 , and a second return pressure duct  61  in direct vicinity of the second long side wall  15 . From the first return pressure duct  60  a first short upper connecting duct  62  passes to the top portion of the first long side wall  14  of the piston chamber  13 , and from the second return pressure duct  61  a second short upper connecting duct  63  passes to the top portion of the second long side wall  15  of the piston chamber  13 . The return pressure ducts  60 , 61  extend substantially across the entire length of the frame part  100 . Further, the return pressure ducts  60 , 61  communicate with an outside source of pressure, which is not shown in the figure. The upper connecting ducts  62 , 63  passing from the return pressure ducts  60 , 61  into the piston chamber  13  preferably consist of a number of separate ducts parallel to one another and fitted in the longitudinal direction of the frame part  100 , i.e. in the cross direction of the web. It is also possible to imagine that, in stead of two return pressure ducts  60 , 61 , just one return pressure duct is employed, from which a number of separate upper connecting ducts pass into the piston chamber  13 . 
     Further, in the top portion of the piston chamber  13 , there are piston  12  stop cushions  17 , 18 . Between the stop cushions  17 , 18 , a first slot  21  has been formed which guides the blade  11 . Between the upper parts  102 , 103  of the frame part  100 , at the vertical plane Y—Y, above the piston chamber  13 , a second slot  22  has been formed. This second slot  22  is slightly wider than the first slot  21 , and the blade  11  can move freely in this second slot  22  in the direction of the arrow S. 
     At the bottom of the piston chamber  13 , at the long side walls  14 , 15  of the piston chamber  13 , into the bottom piece  101  of the frame part, grooves have been machined for a seal band  19 , 20 . When the piston  12  is in its lower position pressed against said seal bands  19 , 20 , the pressure present in the pressure chambers  30 , 31  at the sides of the piston chamber  13  cannot act upon the bottom face of the piston  12 . Below the piston  12 , into the bottom of the piston chamber  13 , into the space defined by the seal bands  19 , 20  in the lateral direction X—X, further, a shallow recess  23  has been formed, into which the connecting bottom ducts  16  are opened. This recess  23  may also be provided on the piston  12 , in which case the bottom of the piston chamber  13  is smooth. 
     The upper parts  102 , 103  of the frame part  100  can also be made asymmetric if it is desirable, for example, to provide the apparatus with a blowing that facilitates the adhering of the cut-off web end to the face of a reel spool above the return pressure ducts  60 , 61 . 
     The bottom piece  101 , the upper parts  102 , 103  and the piston  12  of the device can be manufactured, for example, out of extruded profiles of light metal. The piston  12  and the blade  11  attached to the piston constitute the only mobile part in the frame part  100 . 
     The size of the pressure chambers  30 , 31  and the cross-sectional flow areas of the lower connecting ducts  32 , 33  have been measured by means of computing so that the accelerating working pressure behind the piston  12  remains high over the whole duration of the stroke and that no throttle occurs in the lower connecting ducts  32 , 33 , i.e. the losses of flow have been minimized. The return pressure ducts  60 , 61  have been measured so that the back pressure formed above the piston  12  during a stroke does not become very high. The return pressure ducts  60 , 61  also include instant discharge valves (not shown in the figures), by whose means the return pressure ducts  60 , 61  can be emptied quickly during a stroke, or, by means of the instant discharge valves, at least part of the air is allowed to pass out of the return pressure ducts  60 , 61  into the open air during the stroke. 
     In the following, the operation of the cut-off device will be described with reference to FIG.  1 . 
     First, a return pressure is passed into the return pressure ducts  60 , 61  placed in the upper parts  102 , 103  of the frame part  100  at both sides of the piston chamber  13 . This return pressure has access from the return pressure ducts  60 , 61  through the upper connecting ducts  62 , 63  passing into the piston chamber  13  to act upon the top part of the piston  12 . In this connection, the piston  12  moves into the lower part of the piston chamber  13  and is pressed against the seal bands  19 , 20  placed there. At the same time, the piston  12  closes the lower connecting ducts  32 , 33  passing from the piston chamber  13  into the pressure chambers  30 , 31 . When the piston  12  is pressed against the seal bands  19 , 20  provided at the bottom of the piston chamber  13 , the pressure passed into the pressure chambers  30 ,  31  cannot pass to the bottom side of the piston  12 , but it acts upon the side faces of the piston  12  only. 
     Secondly, the vacuum present in the vacuum duct  51  is connected, by means of charge valves  40 , which are placed as densely spaced in the bottom piece  101 , which are directly electrically controlled and which operate very quickly, to the recess  23  provided on the bottom of the piston chamber  13 . From the vacuum duct  51 , a connecting duct  41  of its own passes to each discharge valve  40 , and, on the other hand, from each discharge valve  40  a connecting bottom duct  16  of its own passes to the recess  23  provided on the bottom of the piston chamber  13 . The vacuum that is passed into the space defined by the bottom face of the piston  12  and by the recess  23  provided on the bottom of the piston chamber  13  keeps the piston  12  in its place pressed against the seal bands  19 , 20 . 
     Thirdly, full working pressure is passed into the pressure chambers  30 , 31 . Since the pressure present in the pressure chambers  30 ,  31  and attempting to raise the piston  12  can act substantially upon the side faces of the piston  12  only, the vacuum acting upon the bottom face of the piston  12  in the area defined by the distance between the seal bands  19 , 20  even alone is sufficient to keep the piston  12  down in a stand-by position ready for discharge. 
     As the fourth step, the return pressure which acts upon the top face of the piston  12  through the return pressure chambers  60 , 61  and through their upper connecting ducts  62 , 63  passing into the piston chamber  13  is removed. Depending on the work pressures and vacuums that are used, a security level can be defined for the construction in order to keep it in the stand-by position now formed. The security can be increased, if necessary, by allowing a slight return pressure to remain effective until the moment of discharge. 
     As the fifth step, by means of the discharge valves  40 , the connection of the vacuum duct  51  to the connecting bottom ducts  16  is cut off, and at the same time a connection is opened from the discharge pressure duct  50  to the connecting bottom ducts  16  and through said ducts to below the piston  12 . When the air pressure is increased in a very short time below the piston  12  to a certain limit value, the piston  12  starts moving upwards, in which connection the lower connecting ducts  32 , 33  passing from the bottom portion of the piston chamber  13  into the pressure chambers  30 , 31  are opened, and the pressure present in the pressure chambers  31 , 32  can flow to underneath the piston  12 . The piston  12  is accelerated very quickly to the desired stroke speed, which can be regulated by regulating the working pressure employed in the pressure chambers  31 , 32 . When the piston  12  moves upwards, the blade  11  attached to the piston also moves upwards and strikes against the web and cuts it off at the desired point. 
     As the sixth step, the pressure is removed from the pressure chambers  30 , 31  and the return pressure is connected to the return pressure ducts  60 , 61 , in which connection the piston  12  and the connected blade  11  move into the initial position to the bottom of the piston chamber  13 . The piston  12  positions itself against the seal bands  19 ,  20  provided at the bottom of the piston chamber  13 . 
     In the following, the use of the cut-off device in a reel change situation at a reel-up will be described with reference to FIGS. 2 to  5 . 
     In FIG. 2, the cut-off device is in its home position, in which it can operate, for example, as a pulper guard. The frame part of the cut-off device has been fitted on a suitable shield box  200 , and the shield box  200  is again attached, for example, to hydraulic arms  201 , by whose means the frame part can be shifted towards the web W and apart from the web. At the same time, the shield box operates as a rigidifier of the frame part. 
     In FIG. 3, the cut-off device has been shifted from its home position against the web W. The upper part or upper parts of the frame part is/are in contact with the web W and raise(s) the web W slightly. When a thin paper grade is being cut off, the cut-off device can be placed apart from the web, because the high stroke speed is sufficient to cut off the web even if the web were not supported against the device. 
     In FIG. 4, the web W has been cut off by discharging the cut-off device. After this, the cut-off device can again be returned to its home position. 
     FIG. 5 is an enlarged view of the cut-off device in the situation shown in FIG.  3 . In this situation, the frame part  100  of the cut-off device is ready to be discharged against the web W, and it raises the web W slightly in order that the web W should be tight when the cut-off blade is discharged against the web W. 
     In the following, the patent claims will be given, and the details of the invention can show variation within the scope of the inventive idea defined in said claims and differ from what has been stated above by way of example only.