Abstract:
The present invention provides a method for creating uniform pressure at a nip comprising the acts of providing a stationary inner core, pivotally mounting a plurality of shoes to the stationary inner core, creating a plurality of annular chambers wherein each of the shoes occupies one of the chambers. In addition, the method further provides filling each of the chambers with a magneto-rheological fluid, surrounding the plurality of chambers, shoes, and stationary inner core with a thin, metal, rotatable shell; detecting a deformation in the shell and changing a magnetic field in at least one of the chambers to change a viscosity of the magneto-rheological fluid to correct the deformation.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This is a Divisional of application Ser. No. 10/795,010, filed Mar. 5, 2004, now U.S. Pat. No. 7,258,654. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates in general to pressure rollers, and in particular, to a compliant pressure roller which adjusts to deformation in a thin outer shell. 
     BACKGROUND OF THE INVENTION 
     In preparing certain substrates it is important that at least one surface have a smooth finish. Often this smooth finish requires very high tolerances. Prior art solutions to the problem of creating a smooth finish have not been completely satisfactory. One prior art solution has been to use elastomeric rollers. Elastomeric rollers, however, cannot be ground to the same high tolerance as metallic rollers. 
     Another prior art solution has used carrier webs having a smooth surface finish as a backing material for the substrate as it passes through the nip. A problem with this solution is that the carrier web usually must be discarded or recycled adding to the cost of the manufacturing process. Another problem is that the carrier web often wrinkles. 
     Using a roller having a metal sleeve with a smooth surface finish over an elastomeric backing is a possibility. This solution, however, also has drawbacks. The thin metal sleeve is subject to cracking after repeated use, especially along the margins. 
     One prior art solution is discussed in U.S. Pat. No. 5,146,664 (Biondetti). The solution proposed is a series of hydraulic pistons. These hydraulic pistons, however, operate on a thick outer shell to correct for beam deflections in the roller. An apparatus as disclosed would be expensive and not suitable for all applications. 
     For a patterned roller, a rapid pressure rise at a nip is important to force material to the pattern. Metallic shells of small diameter inherently will create a small nip footprint, for a given nip load, which results in a higher pressure. In the formation of extrusion cast web materials the nip action improves the replication of the polished or patterned roller surfaces. 
     In an ideal situation, rollers would be perfectly cylindrical and the molten resin would be uniformly distributed. In practice, neither condition is achieved. An ability to locally adjust roller shape is desirable to improve cross width nip loading. Consequently the web material will have more consistent thickness and surface uniformity. Self adjusting rollers are utilized in paper manufacturing or web calendaring operations which require high nip loads, but they have minimal heat transfer capabilities. 
     It is desirable to have a roller capable of creating a smooth finish, with a small nip footprint, having an improved heat transfer capability, and capable of adjustment. 
     SUMMARY OF THE INVENTION 
     Briefly, according to one aspect of the present invention a compliant pressure roller for creating uniform pressure at a nip has a stationary inner core. A plurality of shoes is pivotally mounted to the stationary inner core. Non-magnetic dividers create a plurality of annular chambers and one shoe occupies one of the chambers. A magneto-rheological fluid fills each of the chambers and a plurality of magnetic field generators changes a magnetic field in each of the chambers. A thin, rotatable shell surrounds the plurality of chambers, shoes, and stationary inner core. Changes in the magnetic field generators causes changes in the resistance of the magneto-rheological fluid, which in turn exerts force on the thin, rotatable shell, thus compensating for small deformities in the shell. 
     A pressure roller, according to the present invention has a thin walled metallic shell. The profile of the shell under load can be adjusted at various points along the rotation axis to provide a uniform contact nip. Regions are created along the axis of this roller in which the force exerted on the inner surface of the shell can be varied to compensate for forces that cause local shell deformation. 
     In an embodiment, a hydrodynamic force is created on the inner shell by the shearing action of a magneto-restrictive fluid contained within the chambers created between the inner shell and the outer shell. Changes to magnetic field affect the fluid viscosity. This generates a larger hydrodynamic force for a given rotation speed. The main components consist of an internal cylindrical core, spring loaded pivot shoes, zone separation rings, magnetic field source, and thin walled external shell. 
     A thin shell on the roller provides finer axial profile adjustment and superior heat transfer capability, which is important for extrusion casting systems. Improved heat transfer may allow for a smaller roller diameter which improves die to nip geometry and enables shorter melt curtain lengths. This can improve melt curtain thickness non-uniformity due to reduced neck-in. The roller can also be used on non-nip applications in which the surface of the roller with respect to another object needs to be adjustable. 
     The invention and its objects and advantages will become more apparent in the detailed description of the preferred embodiment presented below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an isometric representation of an axially compliant pressure roller according to the present invention with a partial cross section taken through the outer shell to expose internal components. 
         FIG. 2A  is a cross section of the axially compliant pressure roller shown in  FIG. 1  taken at the center of one of the internal flow chambers. 
         FIG. 2B  is an enlarged view of the interface of the shoe and the outer shell as noted in  FIG. 2A . 
         FIG. 3  is a graph of radial deformation (UX) of the outer shell normal to the nip point for various internal pressure conditions. 
         FIG. 4  is a cross section of a nip formed between two rollers 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention will be directed in particular to elements forming part of, or in cooperation more directly with the apparatus in accordance with the present invention. It is to be understood that elements not specifically shown or described may take various forms well known to those skilled in the art. 
     Referring now to  FIG. 1  a compliant pressure roller is referred to in general by numeral  10 . Compliant pressure roller  10  is comprised in general of a stationary inner core  12  and a plurality of shoes  14  which are pivotally mounted to the stationary inner core  12 . A series of non-magnetic dividers  16  create a plurality of annular chambers  18  and each of the shoes  14  occupies one of the annular chambers  18 . 
     Referring to  FIGS. 1 ,  2 A, and  2 B, shoe  14 , which is eccentrically mounted, is shown. One surface of shoe  14  is curved. A pivot point  15  and spring loading assembly  17  are attached to shoe  14 . A non-magnetic, metallic material is used in the construction of the shoe  14 , but the present invention is not limited to this embodiment. The curvature of the face of the shoe  14  is slightly smaller than the curvature of the inner surface of the thin walled outer shell  24 . This creates a converging cross section at the interface between these components. 
     The compliant pressure roller  10  comprises of a non-rotating inner core  12 , which is the main support structure for the compliant pressure roller  10 . A non-magnetic, metallic material is used in the construction of the inner core, but the present invention is not limited to this embodiment. The inner core  12  has a cylindrical form in which axial holes  19  have been provided. At least one of these holes is used to house the magnetic field generator  13 . In the preferred embodiment one magnetic field generator  13  is associated with each of the plurality of shoes  14 . This allows for local adjustments to the thin walled outer shell  24 . In an alternate embodiment a magnetic field generator  13  may be located in each of the plurality of shoes  14  as shown in  FIG. 2B . 
     Axial holes  19  are used for the circulation of heat transfer media within the core. A series of pockets  26  are created in a radial direction to serve as supports for the shoes  14 . Seats on the inner core  12  enable mounting of bearings  20  and fluid seals  22 . 
     In operation, the hydrodynamic effect of a viscous fluid subject to the shear stress created by the relative velocity of the thin walled shell with respect to the shoe, develops a pressure profile within the converging section  11 . This pressure acts on the thin walled shell curved surface  25  and the curved surface  27  of the shoe. The pressure acting on the shoe results in a force normal to the curvature at the center of pressure. This force is resisted by the spring preloading force acting on the shoe  14 . The pressure acting on the rotating thin walled outer shell  24  creates an internal force on the shell. The net difference in force acting on the shell from the internal hydrodynamic action and the external nip force will result in a localized deformation of the thin walled shell in this region. 
     A thin walled shell of small shell diameter is possible with this embodiment because the structural design of the shell is not dictated by beam bending criteria or shell crushing criteria. The wall thickness of the shell can be significantly thinner because the surface of the shell subjected to the external nip force is directly supported internally by the pressure created by the interaction of the magneto-rheological fluid  28  and the shoe  14 . 
     The thin walled outer shell  24  is constrained with bearings  20  to rotate about the inner core  12 . The rotation of the shell can be imparted by the friction force at the nip point  42 , shown in  FIG. 4 , or with an external drive mechanism. Along the curved surface  25  of the thin walled shell, for a given convergent interface, relative velocity, and fluid viscosity create a uniform pressure. The annular chambers  18  in conjunction with the shoes  14 , magneto-rheological fluid  28 , and axially variable magnetic field generator  13  can be subjected to variable hydrodynamic pressure forces by changing the viscosity of the fluid. The ability to exert axially variable pressure along the thin walled shell results in localized deformation changes of small magnitude and at a much higher frequency than possible by other prior art. 
       FIG. 3  shows the results of finite element calculations used to model the effect of the variable internal pressure capability of this apparatus on the radial profile of the roller surface in the nip point. The dimensions of the shell can be represented in terms of the following quantities; a flexural rigidity of approximately 1800 lb-in and a shell thickness to diameter ratio of 0.025. The flexural rigidity is defined as the quantity of the product of the material elastic modulus and the shell thickness cubed divided by the quantity of the product of a constant value  12  and the quantity of the difference of 1 and Poisson&#39;s ratio squared. An average nip pressure of 250 psi, placed on the thin walled outer shell  24  along a localized region parallel to the axis of rotation, has been used in this calculation. The variable (UX) is the radial displacement in the x-direction, which is also normal to the applied nip pressure region. 
     Curve  30  with diamond shaped markers represents the expected shell deformation under nip load but without internal support. Curve  32  with triangular shaped markers represents the effect of applying a localized pressure, on an area equivalent to the curved surface of the shoe  14  acting at the center of the shell with an average pressure of 50 psi. Curve  34  with rectangular shaped markers represents the positive effect on the radial deformation obtained by applying a gradient pressure profile along the inner surface of the shell ranging from 15 psi to 20 psi. Utilizing basic fluid dynamic principles it has been calculated that a pressure of approximately 30 psi can be developed in this region given a fluid of viscosity of approximately 10 Pa-s sheared between the outer shell and the curved surface of the shoe with an average shear rate of 250 1/s. 
       FIG. 4  shows a cross sectional view of a typical two roller nip utilized in the extrusion cast web formation. A compliant pressure roller  10  is loaded radially into the interface of the molten resin  52  and a second roller  40 . Utilizing a non-contacting deformation detector  50  such as a laser triangulation gage or an eddy current device, the resulting shell surface deformation can be measured. This measurement data can be utilized to control internal loading conditions along the axis of the roller by sending a deformation signal  54  to microprocessor  56 , which alters the strength of one or more of the magnetic field generators  13 . 
     In addition to the magneto-rheological fluid described previously, this apparatus can accommodate other fluids without magneto-rheological properties but which exhibit non-Newtonian characteristics (viscosity of fluid is dependent on shear rate imposed). Localized pressure variations can be created through adjustment of the gap between the outer shell and the curved surface of the shoe. The average shear rate in this gap is proportional to the surface velocity of the shell divided by the gap height. Non-Newtonian fluids exhibit a logarithmic relationship between viscosity and shear rate. External manipulation of the gap combined with a fluid with desirable shear sensitive properties provides an additional means of creating localized pressure differences within each chamber. 
     The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the scope of the invention. 
     Parts List 
     
         
           10  compliant pressure roller 
           11  converging section 
           12  inner core 
           13  magnetic field generator 
           14  shoe 
           15  pivot point 
           16  non-metallic dividers 
           17  spring loading assembly 
           18  annular chambers 
           19  axial holes 
           20  bearings 
           22  seals 
           24  thin walled outer shell 
           25  curved surface, shell 
           26  pockets 
           27  curved surface, shoe 
           28  magneto-rheological fluid 
           30  curve 
           32  curve 
           34  curve 
           40  second roller 
           42  nip 
           50  deformation detector 
           52  molten resin 
           54  signal 
           56  microprocessor