Abstract:
A variable torque device for maintaining constant tension in a web of material as the web is unwound from a supply roll. The variable torque device includes a lead screw attached to the shaft of the supply roll via an appropriate gear ratio. A coil spring is located between a hub on the lead screw and a set of rings and friction pads. As the lead screw rotates, the hub translates, thus changing the displacement of the coil spring, which in turn changes the loading of the rings and friction pads. With a full supply roll the coil spring is compressed so as to create an amount of torque to create the desired web tension. As the supply roll unwinds, the rotation of the lead screw translates the hub to expand the coil spring, reducing the torque and maintaining the web tension constant.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   Reference is made to and priority claimed from U.S. Provisional Application Ser. No. 60/531,351, filed Dec. 19, 2003, entitled VARIABLE TORQUE DEVICE FOR MAINTAINING CONSTANT WEB TENSION. 

   FIELD OF THE INVENTION 
   This invention relates in general to a variable torque apparatus, and more particularly to a variable torque device for maintaining constant tension in a web of material, as it is unwound from a roll. 
   BACKGROUND OF THE INVENTION 
   Many applications and/or processes involve the unwinding of material, in the form of a web, from a roll as the material is subsequently used in the application or process. Subsequent to unwinding, the material may be used in the process and then wound onto a take-up roll, or it may be cut or slit into parts to be consumed in the process. Materials used in these processes may have widely varying physical properties, such as flexibility, breaking strength, elasticity, etc. Depending upon the physical properties of the material and/or the nature of the subsequent process, it may be important to maintain the tension in the material constant or at least within a narrow range, as the material is being unwound from the roll. As anyone skilled in the art will recognize, if the torque exerted on the roll is constant, the tension in the material will increase as it is unwound and the diameter of the roll decreases. 
   An example of such a process is a web type release fluid applicator for fusers in electrostatographic reproduction apparatus such as copier/duplicators, printers, or the like. In electrostatographic reproduction apparatus a latent image charge pattern is formed on a uniformly charged charge-retentive or photoconductive member having dielectric characteristics. Pigmented marking particles are attracted to the latent image charge pattern to develop such image on the photoconductive member. A receiver member, such as a sheet of paper, transparency, or other medium, is then brought into contact with the photoconductive member, and an electric field applied to transfer the marking particle developed image to the receiver member from the photoconductive member. After transfer, the receiver member bearing the transferred image is transported away from the photoconductive member, and the image is fixed (fused) to the receiver member by heat and pressure to form a permanent reproduction thereon. 
   One type of fuser assembly for typical electrostatographic reproduction apparatus includes at least one heated roller, having an aluminum core and an elastomeric cover layer, and at least one pressure roller in nip relation with the heated roller. The fuser assembly rollers are rotated to transport a receiver member, bearing a marking particle image, through the nip between the rollers. The pigmented marking particles of the transferred image on the surface of the receiver member soften and become tacky in the heat. Under the pressure, the softened tacky marking particles attach to each other and are partially imbibed into the interstices of the fibers at the surface of the receiver member. Accordingly, upon cooling, the marking particle image is permanently fixed to the receiver member. 
   With roller fuser assemblies, it is common practice to use release fluids, such as silicone oil for example, applied to the fuser roller surface to improve the release of image-carrying receiver members from the fuser roller. The most common types of release fluid applicators or oilers are a rotating wick roller, a donor/metering roller, an oil impregnated pad or roller, an oil impregnated web, or variations or combinations of the above. In a release oil applicator utilizing an oil-impregnated web, the web is, for example, formed as a porous membrane capable of retaining release oil. The oil-impregnated web extends from a supply roll to a take-up roll. The portion of the oil impregnated web between the supply roll and the take-up roll is directed about intermediate rollers, at least one of which is a back-up roller urging the oil-impregnated web into contact with the heated fuser roller. Another of the intermediate rollers and/or the take-up roller is driven for pulling the oil-impregnated web from the supply roll onto the take-up roll. 
   As explained above, if the torque applied to the supply roll is maintained constant, the tension in the oil impregnated web will increase as the diameter of the supply roll decreases due to unwinding of the oil impregnated web. As the tension increases the oil impregnated web can become distorted, thus affecting the uniformity and rate of oil delivery to the heated fuser roller. Non-uniform and variable rate of oil delivery to the heated fuser roller can cause unacceptable image quality defects in the fused marking particle image on the receiver member. 
   SUMMARY OF THE INVENTION 
   In view of the foregoing discussion, an object of this invention is to provide a variable torque device so as to maintain constant tension in a web of material, as the web is unwound from a supply roll. The variable torque device of this invention includes a lead screw attached to the shaft of the supply roll via an appropriate gear ratio. A coil spring is located between a hub on the lead screw and a set of rings and friction pads. The spring constant is selected to meet the torque requirement corresponding to the desired web tension. As the lead screw rotates, the hub translates, thus changing the displacement of the coil spring, which in turn changes the loading of the rings and friction pads. With a full supply roll, the coil spring is compressed so as to create an amount of torque to create the desired web tension. As the supply roll unwinds, the rotation of the lead screw translates the hub to expand the coil spring, reducing the torque and maintaining the web tension constant. 
   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 
     In the detailed description of the preferred embodiments of the invention presented below, reference is made to the accompanying drawings, in which: 
       FIG. 1  is a schematic side elevational view of an electrostatographic reproduction apparatus fusing assembly, including an oil impregnated web oiler mechanism, the tension of which may be maintained by this invention; 
       FIG. 2  is a side elevational, cross-section view of the variable torque device according to this invention; 
       FIG. 3A  is a radial section view of one component, hub  64 , of the variable torque device of  FIG. 1 ; 
       FIG. 3B  is a radial view of a second component, non-rotating disk  66 , of the variable torque device of  FIG. 1 ; 
       FIG. 3C  is a radial view of a third component, rotating disk  68 , of the variable torque device of  FIG. 1 ; 
       FIG. 3D  is a radial view of a fourth component, friction disk  70 , of the variable torque device of  FIG. 1 ; 
       FIG. 4  is a perspective view of the sub-assembly of lead screw, hub, coil spring, and disks of this invention; and 
       FIG. 5  is a perspective view of the housing containing the sub-assembly of  FIG. 4 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to  FIG. 1 , there is schematically shown various components of an electrostatographic reproduction apparatus fuser assembly, including an oil impregnated web oiler mechanism, the tension of which may be maintained by the variable torque device of this invention. The fuser assembly, designated generally by the numeral  10 , has a fusing member  12  in the form of a roller, although a belt, sleeve, or any other variation thereof would be applicable. The fusing member  12  is heated, and is located in nip relation with a pressure roller  14 . The fusing nip between the rollers  12  and  14  is associated with the receiver member transport path of the reproduction apparatus. That is, as a receiver member R bearing a marking particle image travels along the transport path, the marking particle image is fixed to the receiver member R by application of heat and pressure in the fusing nip before the receiver member R is delivered from the transport path to an output device or a duplex reproduction re-circulation path. 
   The oil impregnated web oiler mechanism is designated generally by the numeral  20 . The oil impregnated web oiler mechanism  20  includes an elongated web  22  extending from a supply roll  24  to a take-up roll  26 . The elongated web  22  is, for example, formed as a porous membrane capable of retaining release oil. Illustrative examples of such porous membrane would be micro-porous PTFE web materials, or non-woven polyester web materials. The elongated web  22  is impregnated with any well known release oil, for example silicone oils with functional groups such as amino or mercaptu groups. That portion of the web  22  between the supply roll  24  and take-up roll  26  is directed about intermediate rollers  28  and  30 . The intermediate roller  28 , is a drive roller, driven by a motor  32   a,  for removing the web from the supply roll  24 . The intermediate roller  30  is a back-up roller urging the web into intimate contact with the fusing member  12  to apply impregnated oil from the web to the fusing member surface. The take-up roll  26  is coupled to a drive motor  32   b  through a slip clutch  34  for winding the web on the take-up reel. The supply roll  24  is associated with variable torque device  36  for maintaining constant tension in web  22  as it is fed from the supply roll and wound on the take-up roll. As shown the direction of movement of the web  22  is opposite to the direction of movement of the surface of the fusing member  12 . 
   Motors  32   a,    32   b  (may be a single motor with plural output drives) are operatively associated with a logic and control unit  40  to receive appropriate activation signals therefrom to turn on the motors for a predetermined period of time. The logic and control unit  40  includes, for example, a microprocessor receiving appropriate input signals. Based on such signals and a suitable program for the microprocessor, the unit  40  produces signals to control operation of the reproduction apparatus and carrying out of the reproduction process. The production of the program for a number of commercially available microprocessors is a conventional skill well understood in the art. The particular details of any such program would, of course, depend upon the architecture of the designated microprocessor. 
   As noted above, the logic and control unit  40  may be located in the main reproduction apparatus logic and control or in the separate logic and control for the fuser assembly  10 . A look-up-table  40   a  is incorporated in the logic and control unit  40 . The look-up-table interrelates the drive for the motors  32   a,    32   b  with the receiver member type and the image content. Accordingly, the movement of the web  22  relative to the fuser member  12  effects a controlled lay down of release oil per receiver member. Therefore, excess oil on the receiver member is substantially avoided, particularly on coated and transparent media, and oil-related image quality defects in the image on the receiver member are prevented. Moreover, less oil is carried out by the fused receiver members, and thus there is potential for less contamination of other reproduction apparatus systems by the release oil during duplex printing runs. 
   As discussed above, if the tension in web  22  increases, as web  22  is unwound from supply roll  24 , web  22  may become distorted, resulting in non-uniform and variable rate oil delivery to fusing member  12 . In order to maintain the tension in web  22  constant, as it is unwound from supply roll  24 , the torque on supply  24 , is controlled by the variable torque device  36  of this invention. Referring to  FIG. 2 , there is shown a cross-section view of variable torque device  36  of this invention. A lead screw  52  is terminated at one end with a hub  64  and at the other end with a connecting shaft  46 . The radial cross-section of hub  64  is circular with two flats as shown in  FIG. 3A . Lead screw  52  is rotatably mounted axially within a cylindrical housing  58 , by bearing  48 , in transverse end plate  50 , at the connecting shaft end, and by bearing  76 , in transverse end plate  72 , at the hub end. 
   Transversely interleaved on hub  64  are rings  66  and  68  and friction pads  70 . Rings  66 , shown in  FIG. 3B , have ears  67  protruding from the outer periphery and a circular inner periphery. Ears  67  extend respectively into a series of axial grooves  58   a  (only two shown in  FIG. 2 ) in the inner wall of cylindrical housing  58 . Rings  66  are thus constrained from rotating with hub  64 , but are free to move axially. Rings  68 , shown in  FIG. 3C , have a circular outer periphery and a circular inner periphery with two flats. Rings  68  thus rotate with hub. 64  and are also free to move axially. Friction pads  70 , shown in  FIG. 3D , have circular outer and inner peripheries. Friction pads  70  are thus free to move axially and may or may not rotate depending upon their frictional relationship to rings  66  and  68 . In the embodiment shown in  FIG. 2  rings and friction pads are positioned on hub  64  in the sequence  66 - 70 - 68 - 70 - 66 - 70 - 68 - 70 - 66 - 70 - 68 - 70 - 66 . The number of rings and friction pads on hub  64  may vary, depending upon the torque requirements, but the minimum will be the sequence  66 - 70 - 68 - 70 - 66 . Thus if n rings  66  are used, (n- 1 ) rings  68  and 2(n- 1 ) friction pads  70  will be used. Collar  74  is placed on hub  64 , before the first ring  66 , to space the first ring  66  from transverse end plate  76 . 
   Threaded hub  54  rides on lead screw  52  and is constrained from rotating by pins  56  protruding from threaded hub  54  into axial grooves  60  in cylindrical housing  58 . Thus, as lead screw  52  rotates, threaded hub  54  translates axially. A coil spring  62  is positioned between threaded hub  54  and the last of rings  66  on hub  64 . As lead screw  52  rotates and threaded hub  54  translates axially, the displacement of coil spring  62  changes, which changes the compression loading on the rings  66  and  68  and friction pads  70  on hub  64 . As the compression loading on the rings  66  and  68  and friction pads  70  on hub  64  changes, the torque exerted on lead screw  52  changes. 
   The variable torque device of this invention, described above, maintains the tension in web  22  constant as web  22  is unwound from supply roll  24  as follows: Connecting shaft  46  is connected to the shaft  24   a  of supply roll  24  through a gear reduction coupling  24   b,  of any well known design. The combination of the pitch of lead screw  52  and the reduction ratio of the coupling between the shaft  24   a  of supply roll  24  and connecting shaft  46  is selected so that threaded hub  54  travels a predetermined distance in the number of revolutions that are required to completely unwind a new supply roll  24 . When a new supply roll  24  is installed, threaded hub  54  is positioned for maximum compression of coil spring  62 . The combination of the spring constant of coil spring  62  and the number and frictional characteristics of rings  66 ,  68 , and friction pads  70  are selected so that the torque on lead screw  52 , at the maximum compression of lead screw  52 , is less than will result in enough tension in web  22  to cause distortion in web  22 . As web  22  is unwound from supply roll  24 , the rotation of lead screw  52  translates threaded hub in the direction to expand coil spring  62  (to the right in  FIG. 2 ), thus reducing the compression loading on rings  66 ,  68 , and friction pads  70 , thereby reducing the torque on lead screw  52 . The reduction of the torque on lead screw  52  is such as to maintain the tension in web  22  as the diameter of supply roll  24  decreases as web  22  is unwound from supply roll  24 . 
   Rings  66  and  68  and friction pads  70 , are of the type typically used in slip clutches, such as those manufactured by Custom Products Corporation. Rings  66  and  68  are typically metal, for example, brass, or steel. The material of friction pads  70  is typically non-metallic and chosen for its frictional properties with respect to rings  66  and  68 . In the embodiment described above, friction pads  70  are separate from rings  66  and  68 . However, an alternative is to bond friction pads  70  to one of rings  66  or  68 . For example a friction pad  70  may be bonded to each side of each ring  68  or alternatively, to each side of each ring  66 . 
   The invention has been described in detail with particular reference to preferred embodiments thereof, but it should be understood that variations and modifications can be effected within the spirit and scope of the invention.