Abstract:
An extended nip press has a shoe driven by two pistons to engage a blanket-supported web against a crown controlled roll. Each ENP shoe piston is offset from the line of force application of the crown control piston by a moment arm distance. A balancing of the hydraulic pressures in the ENP shoe cylinders and the crown control cylinder is achieved by two equalizer valves. Each valve has a slidable spool with faces of a selected cross-sectional area to respond to hydraulic fluid from the various cylinders to retain the correct proportion between the forces applied by the ENP shoe pistons and the crown control piston.

Description:
FIELD OF THE INVENTION 
     The present invention relates to papermaking machinery in general, and to extended nip presses for paper manufacturing applications in particular. 
     BACKGROUND OF THE INVENTION 
     As the manufacturing of paper has improved over time, a major factor in increasing the cost effectiveness with which paper is manufactured has been the increase in machine speed. A critical problem with increased machine speed is the difficulty in increasing drying speed without increasing the number of dryers proportional to the higher machine speeds. A major advance in increasing the dryness of the web as it leaves the pressing section of the papermaking machine was achieved through the introduction of extended nip presses. 
     An extended nip press utilizes a shoe which is forced against a backing roll. A conventional press roll will have a nip of no more than about one to two inches in width, while an extended nip will have a width of about ten inches. 
     Extended nip presses can increase the dryness of the web with fewer nips, thus resulting in a shorter papermaking machine. A shorter papermaking machine occupies less space and generally has fewer components thereby contributing to lower costs. 
     Extended nip presses can also contribute to enhancing the bulk properties of the paper and the surface finish of the final web. To get maximum control over the affect which the extended nip press has on the web, it is desirable to be able to control the shape of the pressure profile the web is subjected to as it moves through the nip formed between the shoe and the backing roll. In order to gain better contact over the extended nip the shape of the shoe may be varied and the shoe can be supported on two spaced apart pistons. But with greater controllability comes greater complexity and the possibility of instabilities in the control system. 
     What is needed is a control system for controlling the forces generating the pressure profile of an extended nip press which has inherent simplicity and reliability. 
     SUMMARY OF THE INVENTION 
     An extended nip press has a shoe driven by two pistons to engage a blanket-supported web against a crown controlled roll. Each ENP shoe piston is offset from the line of force application of the crown control piston by a moment arm distance. A balancing of a resultant force of the pistons produced by the hydraulic pressures in the ENP shoe cylinders and the resultant force produced by the piston in the crown control cylinder is achieved by two equalizer valves. Each valve has a slidable spool with faces of a selected cross-sectional area to respond to hydraulic fluid from the various cylinders to retain the correct proportion between the forces applied by the ENP shoe pistons and the crown control piston. 
     One equalizer valve has a spool with surface areas to insure that the product of the force applied and the moment arm of each one of the ENP shoe pistons is a fixed ratio. A second equalizer valve has a spool with surface areas to insure that the forces applied by the two ENP shoe pistons are equivalent to the force applied by the controlled crown shoe piston. This hydraulic control system makes it possible to adjust the force level in a single ENP shoe cylinder, with the remaining ENP shoe cylinder and the crown control cylinder automatically following. 
     It is a feature of the present invention to provide an extended nip press apparatus in which the multiple hydraulic pistons operating on the ENP shoe are automatically controlled to remain in balance with a crown controlled roll piston. 
     It is an additional feature of the present invention to provide an extended nip press apparatus in which the levels of fluid pressure in multiple hydraulic pistons applied to the ENP shoe and the crown controlled roll may be retained in balance at various selected overall force levels. 
     It is another feature of this invention to provide a system of hydraulic equalizer valves for an ENP and crown controlled roll apparatus which automatically maintain desired force relationships. 
     Further objects, features and advantages of the invention will be apparent from the following detailed description when taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic view of an extended nip press and crown-controlled roll apparatus of this invention. 
     FIG. 2 is a cross-sectional view of an equalizer valve assembly of the apparatus of FIG. 1. 
     FIG. 3 is a cross-sectional view of another equalizer valve assembly of the apparatus of FIG. 1. 
     FIG. 4 is a schematic hydraulic diagram of the apparatus of FIG. 1. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring more particularly to FIGS. 1-4, wherein like numbers refer to similar parts, an extended nip press apparatus 20 with hydraulic control is shown in FIG. 1, The apparatus 20 has an extended nip press (ENP) 22 with a crown controlled roll 24 which is opposed to an ENP shoe 26. The shoe has a concave surface 28 which conforms to the outer cylindrical surface 30 of the crown controlled roll 24 and forms a nip 25 between the roll 24 and the ENP shoe 26. A continuous looped blanket 32 extends through the nip 25 between the roll 24 and the shoe 26. A press felt 34 passes over the blanket 32, and a paper web 33 is supported on the felt as the blanket 32, felt 34, and web 33 pass through the nip 25. The ENP shoe 26 is supported and urged against the surface 30 of the roll 24 by a first hydraulic piston 38 and a second hydraulic piston 36 which move in piston cavities 42, 40. The piston cavities 42, 40 are formed in a non-rotating support beam, not shown. Extended nip presses are well known in the papermaking art, and are typically utilized in the pressing and drying of a paper web in the pressing or drying sections of a papermaking machine. 
     The crown controlled roll 24 has an outer shell 44 which is supported on a plurality of hydraulic cylinders 46 with support pistons 48. The support pistons 48 are positioned in piston cavities 50 in a lateral support beam 52. Each piston 48 has a support surface or shoe 54 which engages the inner surface 21 of the outer shell 44 and controls the position of the roll 24 at the nip 25 where it meets the ENP shoe 26 to press the web therebetween in a consistent manner. 
     Because two pistons 36, 38 are used to advance the ENP shoe 26 against the roll 24, it is necessary to keep the total force of the ENP shoe 26 against the roll equal to the force of the crown controlled roll against the shoe. This maintenance of forces is obtained by a first equalizer valve 56 shown in FIG. 2, and a second equalizer valve 58 shown in FIG. 3. 
     The pressure level to the piston 38 at the wet end of the ENP shoe 26 is set at a selected level to suit the particular application of the ENP press apparatus 20. For example, different qualities of paper may require different levels of pressure on the ENP shoe 26. While the pressure to the wet end piston 38 is selected, the pressure to the piston 36 at the dry end of the shoe is controlled by the equalizer valve 56. 
     A simple force diagram, shown in FIG. 1, indicates the desired relationship between the pressures on the wet end piston 38 and the dry end piston 36. For consistent performance, a fixed ratio is established between the moments applied by the two pistons 36, 38 with respect to the central line 60 of the resultant force of the controlled crown piston 48, which is to say the force F2 applied by the wet end piston 38 multiplied by the moment arm distance X between the point of application of the force F2 and the central line 60 will have a fixed ration with respect to the force F3 applied by the dry end piston 36 multiplied by the moment arm distance Y between the point of application of the force F3 and the central line 60: 
     
         F3(Y)/F2(X)=constant 
    
     The first equalizer valve 56, shown in FIG. 2, maintains the desired relationships between the forces F3 and F2. 
     The pistons 36, 38, 48 in the illustrated embodiment are rectangular pistons which may have an exemplary width in the dry end piston 36 and the wet end piston 38 of three inches, and in the crown control piston of six inches. The pistons extend the full width of the ENP and the crown control roll, which may be a length of thirty to four hundred inches. Alternatively, a plurality of smaller pistons may be employed. 
     The equalizer valve 56 has a valve body 62 with a central cylindrical cavity 64 in which a free piston or spool 66 is slidably mounted. The spool 66 has several unbalanced movable surfaces which form portions of variable size chambers within the valve body 62. The cavity 64 defines a larger diameter cylinder. The spool 66 has a narrow diameter portion 68 connected to a larger diameter portion 70. The larger diameter portion 70 fits within the larger diameter cylinder defined by the cavity 64. A first area A1 is defined by the face 72 of the narrow diameter portion 68, and a second area A2 is defined by the face 74 of the larger diameter portion 70. 
     A sleeve 80 is fixed to the valve body 62 within the cavity 64, and defines a narrow diameter cavity 81. The equalizer valve 56 has a wet end piston port 78 extending from the narrow diameter cavity 81. The port 78 is in fluid communication with the wet end piston cylinder 42, such that the pressure in the wet end piston cylinder is exerted against the narrow diameter face 72 of the spool 66. The fluid entering the port 78 acts only on the narrow diameter face 72 of the spool 66. 
     A dry end piston port 82 extends from the valve body 62 and defines a fluid communication between the cavity 64 and the dry end piston cylinder 40, such that the pressure in the dry end piston cylinder 40 is exerted against the large diameter face 74 of the spool 66. A first drain port 84 extends from the cavity 64, such that movement of the spool toward the dry end piston port 82 will connect the first drain port 84 with the wet end piston port 78, and thereby drain hydraulic fluid from the wet end piston cylinder 42. A second drain port 86 extends from the cavity 64 such that movement of the spool toward the wet end piston port 78 will connect the second drain port with the dry end piston port 82, and thereby drain hydraulic fluid from the dry end piston cylinder 40. 
     The ratio of the areas A1 :A2 is selected to achieve the desired force ratio between forces F2:F3, hence the force applied to the small diameter face of the spool will be P1 (A1), which will be equal to the force applied to the larger diameter face of the spool, P2(A2). When this relation does not apply, one or the other of the drain ports 84, 86, will be uncovered and the over-high pressure will be reduced until the desired force relationship on the two ENP shoe pistons is attained. 
     To maintain a proper force relationship, it is also important that the sum of the forces applied by the two ENP pistons 36, 38 be equal to the force applied by the controlled crown piston 48. This relationship is controlled by the second equalizer valve 58 shown in FIG. 3. 
     The equalizer valve 58 has a valve body 88 with a central cylindrical cavity formed of a small diameter section 90 and a larger diameter section 91 in which a free piston or spool 92 is slidably mounted. The spool 92 has a narrow diameter portion 94 connected to a larger diameter portion 96. A first area A3 is defined by the face 98 of the narrow diameter portion 94, and a second area A4 is defined by the face 100 of the larger diameter portion 96. A third area A5 is defined by the annular face 102 defined where the narrow diameter portion 94 is connected to the larger diameter portion 96. 
     The equalizer valve 58 has a dry end piston port 104 extending from the cavity small diameter section 90. The port 104 is in fluid communication with the dry end piston cylinder 40, such that the pressure in the dry end piston cylinder is exerted against the narrow diameter face 98 of the spool 92. Hence the fluid entering the port 104 acts only on the narrow diameter face 98 of the spool 92. 
     A wet end piston port 106 extends from the cavity larger diameter section 91, and is in fluid communication with the wet end piston cylinder 42, such that the pressure in the wet end piston cylinder 42 is exerted against the annular face 102 of the spool 92. 
     A controlled crown port 108 extends from the cavity larger diameter section 91, and is in fluid communication with the controlled crown piston cylinder 50, such that the pressure in the controlled crown piston cylinder is exerted against the larger diameter face 100 of the spool 92. Drains 110, 112, 114 are positioned to extend from the small diameter section 90, and the larger diameter section 91 to be selectably in communication with the dry end port 104, the wet end piston port 106, and the crown control piston port 108 respectively. In an equilibrium position, none of the drain ports 110, 112, 114 are uncovered, and the forces exerted on the small diameter face and the annular surface are equal to the force exerted on the larger diameter face. The areas A3 and A4 will thus be selected so that when multiplied by the pressures in the dry end cylinder and the wet end cylinder respectively and added together, the sum will be equal to the pressure in the crown control roll cylinder multiplied by AS: 
     
         A3(Pressure in wet end piston cylinder) +A4(Pressure in dry end piston cylinder)=A5(Pressure in crown control roll cylinder) 
    
     Should this desired ratio become unbalanced, the spool will shift, and the appropriate cylinders will be drained until the desired equilibrium is reached. 
     An exemplary hydraulic installation of the system 20 is shown in FIG. 4. 
     It should be noted that alternative equalizer valve arrangements having the equivalent function may be employed. For example, a valve housing having a plurality of linked pistons of selected surface areas may be employed, rather than a single free piston. In addition, pistons of like diameter, but having varying moment arms linked mechanically to adjustment or bleed-off valves may be employed to achieve the desired relationship between pressure levels in the controlled hydraulic assembly. 
     It is understood that the invention is not limited to the particular construction and arrangement of parts herein illustrated and described, but embraces such modified forms thereof as come within the scope of the following claims.