Abstract:
A paper transport system includes an apparatus for controlling movement of the sheets along the predetermined course to provide sheet buffering. The apparatus includes first and second friction surfaces mounted for movement in orbital paths on opposite sides of the predetermined course in opposed relationship to define a nip through which the paper sheets pass. A selectively operable drive is provided for independently controlling the movement of the first and second friction surfaces, and the first friction surface has a coefficient of friction with the paper sheets which is greater than the coefficient of friction of the second surface with the paper sheets which is in turn greater than the coefficient of friction between the paper sheets.

Description:
BACKGROUND OF THE INVENTION 
     Cross-reference and citation is made to two copending applications addressing some of the same problems and technology by the same assignee: U.S. application Ser. No. 08/057,941, filed May 7, 1993 by Richard S. Smith, entitled &#34;Print Skip Avoidance for On-Line Compiling&#34;, and U.S. application Ser. No. 08/065,099, filed May 19, 1993, by Barry P. Mandel, et al., entitled &#34;Trail Edge Buckling Sheet Buffering System&#34;. 
     The subject invention is directed to the art of paper sheet handling and, more particularly, to a sheet buffering system. 
     The invention is especially suited for use in the paper handling and transport system of an electrophotographic printing machine and will be described with reference thereto; however, as will become apparent, the invention could be used in many types of paper sheet handling systems in a variety of different machines. 
     In electrophotographic printing machines, it is sometimes necessary or desirable to temporarily hold or delay the transport of individual paper sheets at various times in the processor to provide additional time for downstream processing to be performed. Such temporary holding or delaying of sheets is generally referred to as &#34;buffering&#34; and has been accomplished in many different ways. 
     One prior art method of buffering has been to temporarily slow or stop a roll nip or other paper transport for a period of time equal to the inter-copy-gap between successive sheets. Of course, this yields only a very short buffering time. If longer times are required, other systems must be used. For example, multiple path systems and systems which run sheets against stalled roll pairs or stop gates have sometimes been used. 
     BRIEF SUMMARY OF THE INVENTION 
     The subject invention provides a simple and effective buffering system which uses only one paper path and maintains positive drive on the sheets at all times. 
     Generally, the subject invention comprises a paper transport system wherein paper sheets are moved seriatim along a predetermined course with the system including an improved apparatus for controlling movement of the sheets along the course. The improved apparatus comprises first and second friction surfaces mounted for movement in orbital paths on opposite sides of the predetermined course in opposed relationship to define a nip through which the sheets must pass. Selectively operable drive means are provided for independently controlling the movement of the first and second friction surfaces. The first friction surface has a coefficient of friction with the paper sheets which is greater than the coefficient of friction of the second surface with the paper sheets which is, in turn, greater than the coefficient of friction between the paper sheets. 
     Because of the noted relationship between the coefficients of friction of the first and second friction surfaces, the apparatus allows two sheets to be stopped and held in the same nip and then fed out either independently or simultaneously. Note that when a first sheet is in the nip, the first friction surface can be stopped while the second friction surface continues to be driven. The first sheet will, however, be stopped and frictionally held by the first friction surface. A second sheet can then be fed between the stopped first sheet and the second friction surface. By then stopping movement of the second friction surface, both the first and second sheets are held in the nip. Alternatively, by selectively driving both or a selected one of the friction surfaces, both or a selected one of the sheets can be driven from between the nip. 
     In accordance with a further aspect of the invention, there is provided a method of controlling the movement of paper sheets in a sheet transport system wherein the sheets are moved seriatim along a predetermined course of movement. The method comprises providing along the predetermined course of movement a nip through which the paper sheets are passed, with the nip being defined by opposed first and second friction surfaces mounted for movement in orbital paths. The first friction surface is selected to have a coefficient of friction with the paper sheets which is significantly greater than the coefficient of friction of the second friction surface with the paper sheets which is, in turn, selected to be significantly greater than the coefficient of friction of the paper sheets with each other. The method further comprises driving the first and second friction surfaces in their orbital paths to cause the nip to impel sheets therethrough, and when it is desired to stop a first sheet traveling through the nip, stopping orbital movement of the first friction surface and maintaining it stopped at least until a second sheet enters the nip. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a better understanding of the invention as well as other objects and further features thereof, reference is made to the following drawing and descriptions. 
     FIG. 1 is a schematic elevational view of a portion of a paper transport system incorporating the invention; 
     FIGS. 2 through 5 are partial schematic elevational views similar to FIG. 1 but showing a possible sequence of steps in using the apparatus of FIG. 1; 
     FIG. 6 is a schematic showing of a modified form of sheet buffering system; and, 
     FIG. 7 is a side elevational view of the FIG. 6 embodiment. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     While the present invention will be described hereinafter in connection with the preferred embodiment thereof, it will be understood that it is not intended to limit the invention to that embodiment or embodiments. On the contrary, it is intended to cover all alternatives, modifications, and equivalents as may be included in the spirit and scope of the invention as defined by the appended claims. 
     For a general understanding of the features of the invention, the preferred embodiment is shown in FIG. 1 of the drawings. In the drawings, like reference numerals have been used throughout to designate identical elements. FIG. 1 schematically depicts a portion of a sheet transport system having a sheet buffering arrangement according to the invention incorporated therein. The system shown in FIG. 1 is specifically intended for use in an electrophotographic printing machine; however, the apparatus and system could clearly be used in a variety of other types of equipment incorporating sheet handling and transportation systems. Broadly, as illustrated in FIG. 1, the apparatus generally comprises guide means which define a predetermined course of paper movement or path indicated generally by the dash dot line P. In the preferred embodiment, the guide means comprise a spaced pairs of respective upper and lower guide panels 12 and 14, respectively, which direct sheets to a first pair of horizontally positioned driven rolls 16 and 18, respectively. The rolls 16 and 18 are positioned in opposed relationship and driven in the direction of the arrows to define a first drive nip 20. 
     The buffering station 24 is located immediately downstream of the drive rolls 16, 18 and includes upper and lower sheet guides 26 and 28 which are positioned in spaced opposed relationship and arranged to direct sheets coming from the drive nip 20 downwardly into the nip 30 of a second pair of spaced rolls 32 and 34, respectively. 
     Sheets passing through the nip 30 are received and directed along the predetermined path of paper movement to subsequent use or processing equipment (not shown) by suitable guide means in the form of guide plates or panels 36 and 38. 
     For reasons which will subsequently become apparent, the rolls 32 and 34 are each provided with separate drive means capable of independent operation. Also, roll 32 is equipped with a braking means. In the FIG. 1 showing, the drive means are depicted schematically and could comprise any standard type of drive motor. As illustrated, roll 32 is provided with a first independent drive means 40 and a brake 41. Roll 34 is driven in a similar manner from an independent drive means 42. The drive means 40 and 42 are controlled in a manner subsequently to be described from a main controller unit 44. Suitable sheet sensors 46 and 48 are positioned immediately downstream of the rolls 16 and 18 to detect the presence of sheets entering the buffering station 24. 
     The system and apparatus shown in FIG.1 allows two sheets to be stopped in the buffer station 24 and held in nip 30 and then fed out either independently or at the same time. While this function is being carried out, the system maintains a positive drive on the sheets at all times. To accomplish this function and result, the system is arranged so that the rolls 32 and 34 have a particular relationship in their coefficient of friction relative to the paper being handled. Specifically, the roll 32 is a high friction roll and has a coefficient of friction relative to the paper being transferred which is higher than the coefficient of friction of roll 34 relative to the paper being transferred, and both rolls have their coefficient of friction selected so that they are higher than the coefficient of friction of the paper to paper. 
     Although many different materials could be used to form the rolls 32 and 34 to have the required relative coefficient of friction. In the subject embodiment, the low friction roll 34 is made of microcellular urethane with a coefficient of friction to paper of approximately 1.2, and the high friction roll 32 has a surface coating of an RTV silicone with a coefficient of friction to paper of approximately 2.5. The soft, compressible nature of the microcellular urethane yields a wide nip zone. This distributes the nip force over a large area resulting in low pressures that minimize the forces that can cause image smear. 
     With the rolls 32 and 34 having the relative coefficient of friction in the ranges as described, the system can be operated generally in the manner best understood by reference to FIGS. 2 through 5. For example, as shown therein, a first paper sheet is driven into the buffering nip 30 by being directed thereto from rolls 16 and 18. At the time the sheet S is directed to the buffering nip 30, both rolls 32 and 34 are being driven from their respective drive means 40, 42. As the trailing edge of the sheet S passes the sensors 46, 48, the controller 44 acts to stop the driving movement of roll 32 (the roller having the highest coefficient friction relative to the paper). Also, brake 41 is simultaneously actuated to hold roll 32 in its stopped position. Roll 34 continues to be driven but the sheet S remains stationary in the position shown in FIG. 3 because of the significantly higher coefficient of friction between the sheet and the stopped roll 32. Of course, the driven roll 34 merely continues rotating and slips on the surface of sheet S. 
     It should be noted as shown in FIG. 3 that when the sheet S has moved to the stopped position, the trailing edge is preferably in an upper or raised position as permitted by the shape of upper guide plate 26 and the action of a Mylar leaf spring 50. This places the first sheet S 1  in a position such that the second sheet to enter the buffering station 24 from rolls 16, 18 will enter a position beneath sheet S 1 . This is shown in FIG. 4 wherein the second sheet S 2  is engaged between the driven roller 34 and the first sheet S 1 . Because of the relationships between the various coefficient of friction, the sheet S 2  is driven along the path and slides along on the first sheet S 1  which maintains its stationary position against the stopped roll 32. With the second sheet S 2  fed into the nip 30, both rolls can be actuated to drive both sheets S 1  and S 2  out of the buffering station simultaneously. Alternatively, it is, of course, possible to maintain roll 32 in its stopped position and merely drive the second sheet S 2  through nip 30 while maintaining sheet S 1  in its stopped position. Of course, it is also possible to stop roll 32 for a predetermined period of time to hold both sheets S 1  and S 2  in position in nip 30. Thereafter, the rolls 32, 34 can be actuated to drive either or both of sheets S 1  and S 2  from the nip 30. 
     FIGS. 6 and 7 show an alternate embodiment wherein a single set of drive rolls in combination with superposed idler rolls can hold a first sheet while a second sheet is driven into the nip between the driven and idler rolls. More particularly, as illustrated, the FIG. 6 embodiment includes a first set of opposed rolls 54 and 56 which define a feed nip and are driven in the direction shown to feed paper sheets S 1  and S 2  along the path 58 to a buffering station 60. 
     The buffering station 60 includes support and guide baffles 62 and 64 which confine the sheets moving along path 58 and direct them into the corrugation roll arrangement 66. The corrugation roll arrangement 66 comprises a lower set of rolls 68 that are driven from a suitable drive 70. A superjacent set of idler rolls 78 are positioned so that individual ones of the idler rolls 78 are axially intermediate the drive rolls 68 and slightly interleaved therewith to cause the sheets S therebetween to be corrugated and gripped. 
     In operation of the FIGS. 6 and 7 embodiment, the first sheet S 1  to enter the buffering station 60 is stopped at the location shown by stopping the corrugation drive rolls 68. These rolls have the high coefficient of friction and preferably have the construction as described relative to rolls 34 of the FIGS. 1-5 embodiment. The first sheet S 1  is thus held in the position shown, as the second sheet S 2  enters the buffering station. 
     With sheet S 1  held in position, second sheet S 2  is driven into the corrugation nip between sheet S 1  and the idler rolls 78. When the second sheet S 2  is substantially even with sheet S 1 , the controller actuates the drive rolls. Both sheets S 1  and S 2  are then driven through the corrugation nip since sheet S 2  is loaded against sheet S 1  by the idler rolls 78. This generates a positive drive force on the sheets yet little drag force. 
     The invention has been described with reference to the preferred embodiment. Obviously, modifications and alterations will occur to others upon a reading and understanding of this specification. It is intended to include all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.