Abstract:
A long nip press for machines transporting fibre webs, which press drains water from the fibre web and comprises two hydrostatic bearings opposite each other, both comprising a pressure shoe comprising a pressure chamber, the pressure shoe having a sealing element in the form of a flexible pressure-balancing diaphragm, and a press belt to press the fibre web against a fabric, the pressure-balancing diaphragm protruding on its edge-zone without support from the inner rim of the pressure shoe towards the pressure chamber. So as to make the long nip press very simple in structure and to enable self-adjusting, even and efficient lubrication in the nip, the protruding edge-zones comprise pinholes for transporting hydraulic fluid. The invention also relates to a pressure-balancing diaphragm.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The invention relates to a long nip press for machines transporting fibre webs, such as paper and board machines, which press drains water from the fibre web and comprises a hydrostatic bearing.  
           [0002]    A long nip press of the kind described above is known from Finnish patent publication 94368. An advantage of the press is that it provides a good and even drying result in a web being dried, and it endures long-time use, thus eliminating the very expensive shutdowns caused by the production breaks of a paper machine. Due to its free and flexible edge-zone, the pressure balancing diaphragm of the pressure shoe is self-adjusting in such a manner that if there is leakage causing locally, close to the upper part of the pressure shoe, a decrease in hydraulic pressure, the free edge of the diaphragm bends towards the press roll (or an opposing pressure shoe) until the same pressure prevails on both sides of the diaphragm edge. This way, the fibre web is pressed in the nip at an even pressure and the drying result is even. In addition, this known press is relatively simple in structure and when necessary can easily be installed afterwards in paper and board machines without requiring any major modification work. The lubrication of the press is implemented by lubricating ducts on the outer rim of the pressure shoe, from which the lubricant flows towards the press belt through ducts built on the outer rim of the sealing list covering the pressure shoe.  
           [0003]    Even though the above long nip press functions well, the present invention discloses improvements to the known long nip press that aim to further simplify the structure of the press and to provide an efficient, self-adjusting lubrication in the area of the pressure shoe.  
         SUMMARY OF THE INVENTION  
         [0004]    To achieve said aim, the present invention provides according to one aspect thereof a long nip press for machines transporting fibre webs, such as paper and board machines, which press drains water from the fibre web and comprises a hydrostatic bearing and a press belt sliding on the bearing to press the fibre web against a press roll in such a manner that the fibre web when moving presses against at least one fabric, the bearing comprising a pressure shoe comprising at least one pressure chamber, and on a surface of the pressure shoe that is towards the press belt, there is a sealing element in the form of a flexible pressure balancing diaphragm and comprising an outer rim, an inner rim and an opening defined by the inner rim, the pressure balancing diaphragm being supported by the pressure shoe, but protruding from an inner rim of the pressure shoe towards the pressure chamber in such a manner that on a protruding edge-zone thereof, it is at least mainly unsupported, whereby an inner edge thereof is free, and the pressure balancing diaphragm being made of a material, the rigidity of which enables it to bend during use according to hydraulic pressure changes occurring in an area of the pressure shoe away from the press roll and correspondingly towards the press roll in such a manner that hydraulic fluid can flow away from the nip from the area between the press roll and the pressure shoe, and when the hydraulic fluid flow exceeds a certain value, the pressure balancing diaphragm is adapted to bend towards the press roll to reduce the hydraulic fluid flow, wherein the protruding edge-zone of the pressure balancing diaphragm comprises pinholes for transporting hydraulic fluid in the pressure chamber from the pressure chamber to the press belt to lubricate the press belt.  
           [0005]    The present invention provides according to a second aspect thereof a long nip press for machines transporting fibre webs, such as paper and board machines, which press drains water from the fibre web and comprises two hydrostatic bearings located opposite each other, both comprising a pressure shoe comprising at least one pressure chamber, the pressure shoe having. on an surface thereof a sealing element in the form of a flexible pressure balancing diaphragm and comprising an outer rim, an inner rim and an opening defined by the inner rim, and a press belt sliding on a bearing to press the fibre web, when it moves, against at least one fabric, the pressure balancing diaphragm being supported by the pressure shoe, but protruding from the inner rim of the pressure shoe towards the pressure chamber in such a manner that on an protruding edge-zone thereof, it is at least mainly unsupported, whereby an inner edge thereof is free, and the pressure balancing diaphragm being made of a material, the rigidity of which enables it to bend during use according to hydraulic pressure changes occurring in an area of the pressure shoe towards the opposing pressure shoe and correspondingly away from it so that hydraulic fluid can flow away from the nip from the area between the pressure shoe and an surface of the pressure shoe, and when hydraulic fluid flow exceeds a certain value, the pressure balancing diaphragm is adapted to bend towards the pressure shoe to reduce the hydraulic fluid flow, wherein the protruding edge-zone of the pressure balancing diaphragm of both pressure shoes comprises pinholes for transporting hydraulic fluid in the pressure chamber from the pressure chamber to the press belt to lubricate the press belt.  
           [0006]    In this context, the term ‘pinhole’ refers to a hole having a very small diameter. Alternatively, said pinholes can be called microholes. The diameter of the hole is in the range of 0.01 to 50 micrometers.  
           [0007]    The invention is based on the idea of implementing lubrication through the free edge-zone of the pressure-balancing diaphragm, which means that the structure of the pressure shoe can be considerably simplified and the sealing list left out. By moving the lubrication away from the outer rim of the pressure shoe, the size of the pressure shoe can be reduced, in which case upper surface of the pressure shoe is preferably bevelled towards the outer rim of the pressure shoe in such a manner that the bevelled section forms an angle of 5 to 10 degrees with respect to the top level of the pressure shoe. This provides a better drying process, because the web being dried detaches faster from the fabric and there can be no significant rewetting from the fabric towards the web when the web moves on from the pressure shoe.  
           [0008]    Preferred embodiments of a long nip press of the invention are disclosed in the attached claims 3 to 10.  
           [0009]    The greatest advantages of the long nip press of the invention are that it is very simple in structure and it enables a self-adjusting, even and efficient lubrication in the nip. No lubrication channels are needed inside the body of the pressure shoe. When pressure increases in the nip, the seeping of the lubricant through the pinholes in the pressure-balancing diaphragm also increases, because the lubricant then passes through the pinholes at a higher pressure. In addition, the invention makes it possible to use a small pressure shoe, in which rewetting seldom occurs. Further, the invention is easy to install in paper and board machines (or other machines transporting webs) even afterwards without requiring any major modification work.  
           [0010]    The present invention also relates to a pressure-balancing diaphragm comprising an outer rim, an inner rim and an opening defined by the inner rim. The pressure-balancing diaphragm of the invention is characterized in that it comprises pinholes in an edge-zone close to the inner rim.  
           [0011]    The main advantages of the pressure-balancing diaphragm of the invention are that it enables the advantages mentioned above.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    In the following, the press of the invention will be described in more detail by means of two preferred embodiments and with reference to the attached drawing, in which  
         [0013]    [0013]FIG. 1 is a schematic side view of the press,  
         [0014]    [0014]FIG. 2 is an end view of the press of FIG. 1,  
         [0015]    [0015]FIG. 3 is a cutaway view along the line III-III of FIG. 1,  
         [0016]    [0016]FIG. 4 is a schematic representation illustrating a cross-profile at the long nip,  
         [0017]    [0017]FIG. 5 is a top view of the pressure shoe of FIG. 4,  
         [0018]    [0018]FIG. 6 is a more detailed end view of the structure of the pressure shoe, and  
         [0019]    [0019]FIG. 7 shows an alternative embodiment at the long nip. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0020]    The long nip press of FIG. 1 for drying a fibre web comprises a body and on it, a press roll  1 , long nip  2  and a hydrostatic bearing in association thereto and comprising a pressure shoe  3 , and guide rolls  4 ,  5  for supporting and rotating a belt  6 , i.e. a fabric-reinforced press belt. In addition, the press comprises guide rolls  7 ,  9  for guiding press fabrics  13 ,  14  and the fibre web  8  being dried to the nip  2 , in which water drains from the web  8 . The reference number  9  indicates a guide roll of the fibre web  8 .  
         [0021]    In FIG. 2, the press of FIG. 1 is shown as seen from the right end, and for the sake of simplicity, the guide roll  9  and structures closely connected to it are left out. The figure shows that the width of the nip  2  corresponds substantially to the length of the press roll  1 .  
         [0022]    [0022]FIG. 3 is a cutaway view along the line III-III of FIG. 1. The figure shows that the pressure shoe  3  comprises only one pressure chamber  10  that is defined by the inner rim of the pressure shoe (cf. reference number  17  in FIG. 6). Because there is only one pressure chamber, the web  8  remains unmarked, i.e. without unwanted tracks, by any unevenly affecting hydraulic pressures in the pressure shoe  3 . The hydraulic fluid enters the chamber  10  though openings  11 ,  12 . The hydraulic fluid is preferably water preferably with additives improving lubricating properties and reducing surface tension.  
         [0023]    [0023]FIG. 4 illustrates how the fibre web  8  goes into the long nip  2 . The press belt  6  rotates (moves) on top of a fluid bed of the pressure shoe  3  with a very small friction, and the fibre web  8  goes into the long nip in such a manner that it is between the press fabrics  13  and  14 . When the web  8  is pressed between the press fabrics  13 ,  14  at the long nip  2 , the fabrics suck water from it. Two fabrics  13 ,  14  are not necessarily needed; it is possible that there is a fabric on only one side of the web  8 . Using two fabrics  13 ,  14  is, however, recommended, because this way the drying is more efficient.  
         [0024]    [0024]FIGS. 4 and 6 show that there are bevels  15  on the edges of the top surface  19  of the pressure shoe  3  towards the outer rim  28  that forms an angle α with respect to the top level of the pressure shoe. FIGS. 5 and 6 show that the pressure-balancing diaphragm  20  is a rectangular structure comprising an outer rim  26 , inner rim  22  and an opening  27  defined by the inner rim. Recesses  29  are made in the bevels  15  for receiving the outer rim  26  of the pressure balancing diaphragm  20  in such a manner that the outer rim of the diaphragm  20  supports itself against the shoulder or notch  19   a  of the recess and the top surface of the diaphragm  20  is level with the top levels of the bevels in the edge-most zones of the pressure shoe. The angle α is 5 to 10°. FIG. 4 shows that the press belt  6  is not transported in such a manner that it rests against said bevels  15 , but the press belt forms an angle β with respect to the top level of the bevel  15  with the angle β being smaller than the angle α. The angle β is 2 to 5 degrees smaller than the angle α, and its absolute value is preferably in the range of 3 to 7 degrees. The reference number  23  marks the top point defined by the bevel on the top surface of the pressure shoe.  
         [0025]    [0025]FIG. 5 shows the pressure shoe  3  from the top, thus showing its rectangular shape, and FIG. 6 is an enlarged and more detailed cutaway view of the pressure shoe along the line VI-VI of FIG. 5. For the sake of simplicity, FIG. 6 does not show the fibre web  8 , press fabrics  14 ,  13  or press belt  6  that are shown in FIG. 4.  
         [0026]    The pressure-balancing diaphragm  20  is a plate-like diaphragm made of stainless steel and welded to the bevelled area  15  of the pressure shoe  3  with a weld joint forming a rim, shown by a dashed line  16 . There may be several weld joints  16 . The pressure-balancing diaphragm  20  covers the pressure shoe  3  in such a manner that it can be supported by the top surface of the recess  29 . The notch  19   a  is next to the weld joint  16  and closer to the outer rim  28  of the shoe  3  than the weld joint. Instead of a weld joint, the pressure-balancing diaphragm  20  can preferably be fastened to the top surface of the recess  29  by a glue joint. An advantage of the glue joint is that it does not cause a point of discontinuity in the material of the pressure-balancing diaphragm  20 . Below the edge-zone  21 , there is a slot  30  that makes the pressure balancing diaphragm  20  protrude in its edge-zone  21  towards the pressure chamber  10  without or at least mainly without support for a length, i.e. distance, of S=20−40 mm. The slot  30  allows hydraulic fluid to enter below the edge-zone  21 . The length L of the slot  30 , i.e. its distance from the inner rim  17  of the pressure shoe to the point where the diaphragm  20  is supported, is for instance 10 to 40 mm and its thickness is for instance 1 mm. The inner edge  22  of the edge-zone  21  of the pressure-balancing diaphragm  20  is free. The purpose is that the pressure balancing diaphragm  20  is flexible in such a manner that its edge-zone  21  bends at the top part of the pressure shoe  3  according to the prevailing pressure changes towards the roll  1  and correspondingly away from it.  
         [0027]    The top surface of the pressure-balancing diaphragm  20  comprises numerous very small holes  25 , so-called pinholes, having a diameter of 1 to 10 micrometers for lubricating the press belt  6 . The lubrication keeps the friction of the pressure shoe against the press belt  6  very small and due to good lubrication, the pressure-balancing diaphragm  20  endures long-time use. It is difficult to achieve good lubrication in known pressure shoes, especially in the section of the pressure shoe that is on the side of the inlet nip. This is the case despite the provision of the lubrication channels in the body of the pressure shoe. The pinholes  25  are illustrated by small dots in FIG. 5. For the sake of simplicity, FIG. 5 only shows a few dots on the left at the bottom, but the pressure-balancing diaphragm  20  comprises pinholes  25  on the entire edge-zone  21 , i.e. on the entire inner rim of the diaphragm. It can be expected that the diameter of the pinholes  25  is outside said range, being for instance in the range of 0.01 to 10 micrometers. Even the range of 10 to 20 micrometers may be possible in some applications. It is difficult to produce very small pinholes. If the diameter of the pinholes  25  is too small, the lubricant, preferably an aqueous fluid in the pressure chamber  10 , cannot even at a high pressure of 20 to 50 bar flow through them or the flow is too slow for good lubrication. Field tests determine a suitable size for the pinholes of the pressure balancing diaphragm  20  and the rigidity (thickness) of the diaphragm  20  so that water can filter through the diaphragm in such a manner that the diaphragm still serves as a sealing. The pressure of the fluid in the pressure chamber  10  is generally in the range of 20 to 100 bar when the web  8  is transported. If the diameter of the pinholes  25  is too big, the lubricant exits the pressure chamber and the necessary pressure is not maintained in the pressure shoe  3 . To prevent the pinholes  25  from being blocked, a filter (not shown) can be used to prevent small particles from entering the pinholes. The shape of the pinholes  25  can vary: they may be round, oblong, elongated, rectangular, etc. Their number and arrangement, for instance into different patterns, can also vary. The pinholes  25  are made using electron beam cutting or laser cutting. The pinholes  25  can be made using for instance the Micro-EDM (Micro Electro-Discharge Machining) system, Model 82, of Panasonic, with which it is possible to make pinholes of different shapes.  
         [0028]    A pressure p caused by the hydraulic fluid prevails in the edge-zone  21  of the pressure-balancing diaphragm  20 , the pressure “normally” corresponding to a pressure p 1  in the pressure chamber  10 . Because the geometry of the nip  2  is not completely constant all the time and because the thickness of the web  8  is not always exactly the same, moments occur when the hydraulic fluid tries to escape from the nip  2  from the area between the roll  1  and the pressure-balancing diaphragm  20 . When the fluid escapes and exits the pressure chamber  10 , the hydraulic pressure p above the edge-zone  21  of the diaphragm  20  decreases rapidly. As a result of the pressure decrease, an unbalance occurs, with a smaller pressure prevailing above the edge-zone  21  of the diaphragm than below the edge-zone  21 . A standard pressure p 1  prevails in the pressure chamber maintained by pressure supply means (not shown in the figures with the exception of pressure supply openings  11  and  12 ) that continuously supply hydraulic fluid into the pressure chamber  10 . Because the pressure balancing diaphragm  20  is flexible, its edge-zone  21  protruding from the sealing list bends due to the unbalance in such a manner that the edge-zone and the free edge  22  of the diaphragm move towards the roll  1 . Due to the bending, the pressure p increases in the edge-zone back to the value p 1  and the nip geometry returns to normal. Fluid leakage lessens and stops quickly. The pressure-balancing diaphragm  20  adjusts in the manner presented above to the prevailing pressures and provides a self-adjusting nip  2  with no or minimum fluid leakage and an even drying of the web.  
         [0029]    Said flexibility of the pressure-balancing diaphragm  20  is in practice achieved by manufacturing the diaphragm  10  of a suitably rigid material. The material and its thickness determine the rigidity of the diaphragm  20 . In this context, the length S of the edge-zone  21  of the diaphragm  20  should also be taken into consideration. The greater the length is, the smaller a force is needed to bend the edge-zone  21 . The diaphragm  20  is preferably made of stainless steel, in which case the thickness of the diaphragm is preferably approximately 0.5 to 1 mm. The thickness range of the diaphragm  20  can presumably be approximately 0.2 to 3 mm. If the diaphragm is too thick, it does not bend. The diaphragm could alternatively be made of titanium or a titanium alloy or a composite, in which case it can be made of a carbon fibre-reinforced plastic, for instance PTFE (polytetrafluoroethylene or Teflon). If the diaphragm  20  is made of a material having a lower elastic modulus than steel, for instance a titanium alloy or a composite, its thickness is greater than that of a steel diaphragm, for instance 1 to 5 mm.  
         [0030]    The length S of the free edge-zone  21  of the diaphragm  20  is preferably 10 to 40 mm. The diaphragm  20  is preferably made of a uniform rectangular steel plate that covers the pressure shoe  3  in such a manner that the front, back and side edges of the pressure shoe are covered. This way, the manufacturing of the diaphragm  20  is inexpensive.  
         [0031]    [0031]FIG. 6 shows further that the inner edge  22  of the edge-zone  21  of the pressure-balancing diaphragm  20  is bent away from the press roll  1 , thus preventing the edge  22  of the pressure-balancing diaphragm from damaging the press belt  6 .  
         [0032]    [0032]FIG. 7 shows a most preferable alternative manner of implementing the pressing of the web  8  in a long nip. In the embodiment of the figure, the press roll  1  of FIG. 1 is replaced by a pressure shoe  3   a . The structure of the pressure shoes  3   a ,  3   b  corresponds to the structure of the pressure shoe  3  with the sole exception that in the solution of FIG. 7, the pressure shoes  3   a ,  3   b  are planar and not curved as clearly shown in FIG. 6. The reference numbers of FIGS.  1  to  6  are used in FIG. 7 for the corresponding components and an ‘a’ or ‘b’ is added to the number indicating whether it is a top or a bottom shoe. The pressure balancing diaphragms  20   a ,  20   b  of the pressure shoes are planar and self-adjusting using the same principle as the diaphragm  20  in FIG. 6. In principle, it is possible to leave out either of the press fabrics  13   a  or  13   b.    
         [0033]    The solution of FIG. 7. is cheaper than the solution of FIG. 4 to implement, because the large press rolls  1  are not needed. It is also very easy and inexpensive to modernize an old machine using the solution of FIG. 7.  
         [0034]    In the above, the invention has been described using only two preferred embodiments, and therefore, it should be noted that the invention can in detail be implemented in many ways within the scope of the attached claims. Thus, it is also possible to have several pressure chambers in the pressure shoe. Because of the pressure balancing diaphragm, several pressure chamber are, however, not necessary. This is an advantage, since several pressure chambers make the structure of the pressure shoe more complex and expensive; in addition, marking often occurs in the web when using several chambers. Differing from what is shown in the figures, the pressure balancing diaphragm can cover the entire top surface of the pressure shoe and extend until the outer rim  28 ,  28   a ,  28   b  of the pressure shoe. It is also possible to fasten the pressure-balancing diaphragm to the pressure shoe in some other manner than welding and glueing. The manufacturing material of the pressure shoe can be other than stainless steel; for instance a water-permeable reinforced pressure-proof plastic or a water-permeable composite film are possible. The press belt can be a round roll. In the example of FIG. 1, the pressure shoe is located below the press roll, but it can also be above the press roll.