Abstract:
An inline stacker receives a shingled stream of printed sheet products from a printing press, or finishing equipment, at line speed. A sheet counter at an infeed conveyor section controls the operation of the inline stacker. The infeed conveyor can pivot to a divert position where it feeds the stream to a bi-directional conveyor. A following gap generator section of the stacker has two conveyors with opposed movable ends that are physically separated in the conveying direction by a small gap. The infeed conveyor and gap generator conveyor sections carry the stream at a substantially constant speed, which can be the line speed. To produce a gap in the shingled stream that defines a bundle size, the movable conveyor ends defining the gap between the conveyor gap generator sections move downstream in unison at the speed of the upstream conveyor. At the same time, the speed of the downstream conveyor is briefly increased. The movable ends and the increased conveyor speed then return to their initial status. Stacks of a predetermined size are collected on a forked elevator with mirror movement side walls and other product movement and position controls. Ball-screw-driven and cam-driven side joggers (at the infeed) align the stack and infeed stream laterally. The ball screw jogger, under control of a central controller and an associated servo motor rotates continuously in one direction to open the side walls in response to a sensed jam. The stacker is portable and mechanized for side lay adjustment to align with the press or finishing equipment.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims priority under §35 USC 119 of prior U.S. provisional application Ser. No. 60/476,498 filed Jun. 5, 2003, the disclosure of which is incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     This invention relates in general to apparatus and methods for forming stacks of sheet materials. More specifically, it relates to an in-line stacker that receives a shingled stream of printed sheets, e.g. from a printing press, and/or in-line or off-line finishing equipment, and reliably forms them into a series of stacks of a predetermined number at press speed.  
         [0003]     In many applications, it is desired to form a shingled stream of sheet products into a succession of bundles each containing a known number of sheet or folded pieces. A major application is the stacking and bundling of printed material as it leaves a printing press, and/or in-line or off-line finishing equipment such as rotary cutters, folders, perforators, and die-cutters. Ideally the stacking apparatus is fed directly from the output of the press or finishing equipment and operates at the press speed so as not to limit use of the full productive capability of the press. Also, the stacking is ideally highly reliable, which usually means resistant to jams or other interruptions in the operation of the stacker. Other objectives include the ability to handle material of varying size, weight and flexibility, to form bundles of a wide variety of counts, including as few as ten, to reduce both make-ready time and the manpower needed to collect and package the material, and to control the waste of good printed material.  
         [0004]     A large number of stackers are known. U.S. Pat. Nos. 4,130,207 and 4,867,435 describe, respectively, a top stacker with an elevator and pusher operating on a shingled stream of booklets, and a belt stacker that forms a reverse shingled stream of signatures that stack on end on a conveyor. The &#39;207 machine collects a vertical stack of booklets on a set of laterally spaced “finger ramps”. The booklets are aligned by oscillating side plates and an oscillating rear “finger”. The formed stack is pushed from the collection site by a pneumatically driven “finger”.  
         [0005]     U.S. Pat. No. 4,161,092 to Buday et al. describes a system that receives a shingled stream of flat articles, periodically creates a gap in the shingled stream by accelerating a fixed downstream conveyor with respect to a fixed upstream conveyor, and deposits the flat articles in a series of moving containers each holding one stack.  
         [0006]     U.S. Pat. No. 3,768,382 to Zernov et al. describes an apparatus that creates a gap in a shingled stream of carton blanks by momentarily interrupting the blanks upstream of a stop point, while increasing the infeed conveyor speed. The stream of blanks thus gapped collects on an elevator. A pusher plate pushes a completed stack off the elevator onto a table or conveyor. Retraction of the pusher plate triggers the elevator to lift to a start forming another stack.  
         [0007]     U.S. Pat. No. 6,295,922 to Salamone et al. describes another stacker operating on a shingled stream of goods. A plate or nip roll interrupts the shingled steam carried on a fixed upstream conveyor while an adjacent, fixed, downstream conveyor carries a downstream set of the shingled goods, thus creating a gap. The shingled, gapped goods are top fed to a collection site on a reciprocating table or elevator. When a stack is completed, the elevator table lowers to a point where a pusher removes the stack from the table to a gripper apparatus that carries each stack to a bander or other processing or storage site. Extendible fingers collect the goods while the elevator table is lowered. The table is laterally split to allow it to begin to return to a raised, collecting position while the pusher is still extended, or at least partially extended. A side jogger aligns the stack by driving stacked goods against a fixed member.  
         [0008]     In known in-line stackers, e.g. those operating with a printing press, there is typically an electrical connection between the press and the stack such as an output signal from an encoder on a rotary cutter that is used to set the speed of the stacker and control its operation. In practice, however, additional wiring at each location is necessary, the encoder signal has proven to be not sufficiently reliable, and this control is insensitive to downstream problems of the stacker such as a paper jam. Further, jams are facilitated by interruptions in a continuously moving shingled stream. A “finger” or nip roll that blocks the advance of the steam causes the lead sheet in the stream to accumulate and thicken the height of the stream. This “fat” leading end of the stream, once released by lifting the interrupting finger or nip roll, is more susceptible to jamming than a stream of uniform height. Jams in known machines are often difficult and time-consuming to clear. Until they are cleared, either the apparatus feeding the product must be stopped, or its output directed to a waste collection or hold system. There is further time and product lost on restart.  
       SUMMARY OF THE INVENTION  
       [0009]     An inline stacker that solves the various problems of prior art stacking systems noted above can receive a shingled stream of printed sheet products from a printing press or finishing equipment at line speed. A sheet counter at an infeed conveyor section controls the operation of the inline stacker. The infeed conveyor can pivot to a divert position where it feeds the stream to a bi-directional conveyor. A following gap generator section of the stacker has two conveyors with opposed movable ends that are physically separated in the conveying direction by a small gap. The infeed conveyor and gap generator conveyor sections carry the stream at a substantially constant speed, which can be the line speed. To produce a gap in the shingled stream that defines a bundle size, the movable conveyor ends defining the gap between the conveyor gap generator sections move downstream in unison at generally at the speed of the upstream conveyor. At the same time, the speed of the downstream conveyor is briefly increased to a value that is greater than the upstream conveyor speed. Once the gap in the product stream is created, the movable conveyor section ends and the increased conveyor speed return to their initial, steady state values.  
         [0010]     Stacks of a predetermined size are collected in a “well” on a forked elevator with mirror movement side walls and other product movement and position controls. A ball-screw-driven side jogger aligns the stack laterally. The ball-screw jogger, under control of a central controller and an associated servo motor rotates continuously in one direction to open the side walls in response to a sensed jam.  
         [0011]     Continuous rotation of the ball screw also changes the lateral dimension of the collection well to accommodate products of different width. One positive control is an idler wheel mounted over the collection well that deflects products leaving the gap generator to the collection well without overshoot.  
         [0012]     At the infeed, a pair of jogger side walls oscillate laterally in mirror fashion to align the shingle products in the stream. This oscillation is preferably cam and spring driven off a rotating shaft. The infeed conveyor is preferably a set of parallel belts, with a wide central belt that opposes a set of driven nipper wheels and plural narrow belts spaced to accommodate the oscillating jogger plates at different settings for products with different widths.  
         [0013]     The stacker is portable and mechanized for side lay adjustment to align with the press or finishing equipment.  
         [0014]     In one form of the invention an online stacker that receives a stream of shingled sheet products moving at a first speed and forms the shingled sheet products into a succession of stacks each containing a predetermined number of the sheet products, has 1) an infeed conveyor section that receives and conveys said the product stream at generally the first speed, 2) a counter that counts the number of products thus received at said infeed section, and generates an output control signal when the predetermined number is reached, and 3) a controller for the stacker that coordinates the operation of the online stacker at least in part in response to the counter output control signals.  
         [0015]     In another form, the inline stacker of this invention has a gap generator section and a stacker section, the gap generator section receiving said the product stream from the infeed section and conveying it to the stacker section at generally the first speed with a gap in said stream between successive adjacent groups of the predetermined number of sheet products. In another preferred form of the invention, the gap generator has upstream and downstream conveyor sections with adjacent, movable ends that are 1) physically separated in the direction of movement of the stream, and, 2) can move in unison in the direction of said the product conveying at the speed of the upstream conveyor. While the movable ends move downstream, the speed of the downstream conveyor section is increased to a second speed in excess of said first speed to create the gap in the shingled stream.  
         [0016]     In yet another form of the invention, servo motors drive the rolls of the infeed, gap generation, and stacker sections as well as the oscillating side walls of the stacking section and a carriage that carries the movable, gap-defining end rolls of the two sections of the gap generator. The controller controls the operation of all the servo motors as well as the operation of the diverter and a transfer mechanism for formed stacks in response to the counter and at least one jam sensor. The diverter and the stack removal can be effected by pneumatic actuators, also controlled by a central controller. The at least one jam sensor is located over the infeed to the stacker section.  
         [0017]     These and other features and objects of the invention will be more fully understood from the following detailed description which should be read in light of the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]      FIG. 1  is a top plan view of a stacker constructed according to the present invention;  
         [0019]      FIG. 2  is a view in side elevation of the stacker shown in  FIG. 1 ;  
         [0020]      FIG. 3  is a top plan view of an alternative embodiment of a stacker according to the present invention;  
         [0021]      FIG. 4  is a view in side elevation of the stacker shown in  FIG. 3 ;  
         [0022]      FIG. 5  is a top plan view of the sidelay base of the stacker shown in  FIGS. 3 and 4 ;  
         [0023]      FIG. 6  is a detailed view in side elevation of the gap generator section and stream diverter of the stacker shown in  FIGS. 3 and 4 ;  
         [0024]      FIG. 7  is a top plan detailed view of the output and stack-forming end of the stacker shown in  FIGS. 3 and 4 ;  
         [0025]      FIG. 8  is a detailed end view in side elevation of the bucket “paddle” or reciprocating side jogger shown in  FIGS. 3, 4 , and  8 ;  
         [0026]      FIG. 9  is an end view in side elevation of the stacker shown in  FIGS. 3, 4 , and  7 ;  
         [0027]      FIG. 10  is a view in perspective of the infeed section of the stacker shown in  FIGS. 3-11  with the diverter in the normal operating position directing product from an upstream press to the stacker;  
         [0028]      FIG. 11  is a detailed to plan view of the side jog mechanism at the infeed of the stacker  10  shown in  FIGS. 3, 4 , and  9 ;  
         [0029]      FIG. 12  is a perspective view of the infeed section and beginning of the gap generator section looking in the direction of the travel of the shingled stream with the input conveyor lowered to divert the product to a transverse divert conveyor;  
         [0030]      FIG. 13  is a view in perspective of the stacker section shown in  FIGS. 3-11  showing the collection bucket with the elevator lowered and pusher extended;  
         [0031]      FIG. 14  is a view in perspective showing the head roll and variable angular position idler rolls;  
         [0032]      FIG. 15  is a view in perspective of the stacker shown in  FIGS. 3-14  showing the ball screw drive with the side jog plates and portions of the sidelay assembly; and  
         [0033]      FIG. 16  is a view in perspective of the output from the gap generator section and the stacking section looking downstream and showing a shingled stream of product carried on conveyor sections. 
     
    
     DESCRIPTION OF THE INVENTION  
       [0034]     With reference to  FIGS. 1-16 , the stacker  10  of the present invention has various features that alone, or in combination, solve various problems, such as those discussed above, that have plagued the known stackers. A principal feature is that the control of the stacker speed and stacking operations originates with a sheet counter  12  located at a stacker infeed section  14 . The counter  12  sets the speed of the stacker to match the output of the equipment  16  which feeds the stacker. For the purposes of this description, this in-line equipment operating a succession of flat sheet products to be stacked will be described as a printing press and the products as printed paper sheets, including folder sheets such as signatures. The counter  12  also controls the operation of a gap generator  18  of the stacker  10  to produce a gap in an infeed shingled stream  20  of products  22 . The gap in the shingled stream is created after a predetermined number of the products has been detected by the counter  12 .  
         [0035]     The counter  12  input to a controller  24  generates output control signals to a set of servo motors M that each drive a conveyor roll  26  and side jogger plates  28 .  
         [0036]     The infeed section  14  includes an infeed conveyor  30  that moves between a run position  30   a  that feeds the input shingled stream  20  of products  22  for stacking, and a divert position  30   b  where the input shingled stream  20  drops to a right angle bump turn assembly  32  operating in conjunction with a divert conveyor  34  oriented generally transversely with respect to the stacker infeed conveyor  30 . The conveyor  30  preferably pivots at its upstream end under control of a pneumatic cylinder  36  that is, in turn, actuated by a signal from the controller  24 . The transverse diverter conveyor is bi-directional. Movement in one direction carries the diverted, turned shingled stream to a waste collection bin. Movement in the opposite direction carries “good” product, in a 90°, re-directed shingled stream, to a good product collection bin for hand bundling or other disposition. Because all product input to the stacker—whether stacked, diverted to waste, or diverted to a good product collection bin—is counted by the counter  12 , it is possible to not only reduce waste of good product, but also to keep count of all dispositions of products received from the press  16  and to restart the stacking operation without a loss of count.  
         [0037]     The non-diverted input stream  20  is carried on a two-section conveyor belt  38 . The upstream section  38   a  carries the shingled stream  20  at a fixed input speed set by the counter at a value that accommodates the output of the in-line press, finishing, or like equipment  16 . The speed can vary, but for a given product run it remains substantially constant. The downstream section  38   b  is driven independently from the upstream section by a separate drive roll  26   a  and associated servo motor M. There is a slight gap  40  between the output end of the conveyor  38   a  on the input end of the downstream conveyor  38   b . This gap  40  is the point at which a gap  42  in the shingled stream is created on an output command signal from the controller  24  produced when the product cannot reach a preset number equal to the number of products desired in a stack. The cycle of operation of the stacker  10  is sufficiently fast, e.g. 2 seconds or less, that stacks and bundles of as few as ten products can be formed reliably.  
         [0038]     Whereas in the prior art it is known to create a gap by accelerating a downstream conveyor with respect to an upstream conveyor, and while interrupting movement the product at a point upstream of the gap between the conveyors, the present invention creates the gap  42  using a moving, conveyor-to-conveyor, gap  40 . Movement of the gap  40  is accompanied by a brief acceleration of the conveyor speed of the downstream conveyor  38   b,  e.g. to a speed faster than the operating speed of the upstream conveyor  38   a  (e.g. twice the press speed), and then a deceleration back to the normal operating speed of conveyor  38   a  in coordination with a movement of the conveyor gap  40  back to its steady state starting position.  
         [0039]     To move the gap  40 , the conveyor rolls  26   a  and  26   b  adjacent the gap  40 , and associated pairs of idle rolls  26   c - 26   f  are all rotationally mounted on a carriage  44  that reciprocates linearly along the direction  46  of the advancing shingled product stream  20 . Each conveyor belt is carried on its end opposite the gap on fixed rollers  26   g ,  26   j . As a result, as the carriage  44  translates in a downstream direction from its normal operating position at a speed equal to the incoming material stream, the upstream conveyor  38   a  telescopes out to create a longer upper, product-carrying belt run  38   a , and the downstream conveyor “telescopes” in to shorten the length of the upper downstream belt run  38   b ′. Because the mutual spacing of the rolls mounted on the carriage are fixed with respect to each other, while the gap  40  moves downstream at the speed of the upstream conveyor. The increased speed of the downstream conveyor draws the shingled stream forward at a like accelerating speed to create a physical separation or gap  42  in the shingled stream. It is significant that this gap  42  is made without any physical interruption or blocking of the movement of the stream  20 . The gap appears with no apparent, or real, slowdown of the flow of products  20  along the conveyors  38   a ,  38   b . After the shingled gap  42  is created, typically in a fraction of a second, and over a travel of about one foot, the carriage returns to its starting position as the speed of the downstream conveyor is decelerated to again match that of the upstream conveyor. A servo motor M drives the carriage  44  back and forth under control of the controller  24  and in coordination with an increase of the speed of the downstream conveyor  38   b , and then a return to the normal operating speed in each shingled gap-creating cycle of operation.  
         [0040]     The conveyor belts of the upstream and downstream conveyors  38   a ,  38   b  are preferably formed as a continuous band or loop of any known suitable material. The belts have a width that accommodates all anticipated product widths. At the infeed conveyor, the belt  30   c  is not a single band of material. Rather, there is a narrow central, closed-loop, band  30   d,  e.g. three inches in width, and at least one, and as shown four, circular cross-section bands  30   e  mutually spaced on either side of the wider central band  30   c.  A support plate closely underlies the central band at least at a point coincident with a set of driven trolley nip rolls  50  that bear on the shingled stream to control and advance the stream either to the diverter or to the stacker conveyors.  
         [0041]     The construction of the infeed conveyor belt  30   c  accommodates a pair of side jogger plates  28 ,  28  that extend upwardly along the side of the shingled stream and oscillate toward and away from one another, in unison, to repeatedly contact and urge the shingled stream products  22  into a laterally aligned stream in a well-defined, central position.  
         [0042]     The side jogger plates  28 ,  28  each have a lower lip  28   a angled inwardly under one of the circular cross-section bands  30   c . The position of the side plates is laterally adjustable to accommodate products of varying widths. The position of the lips  28   a,    28   a  under the bands  30   e  assures that the products will not jam between the side jogger plates and the conveyor belting. The mirror-image reciprocation of the side plates  28 ,  28  is produced in the preferred form by mounting each plate on a block  54  that is axially slideable along a shaft  56  and biased by spring  58  toward the shingled stream. A common drive shaft  60  positioned between the mounting shafts carry rotary cams  62 ,  62  that engage cam followers secured to the mounting blocks  54 . Rotation of the drive shaft thereby causes the side plates to reciprocate in unison, in a mirror-like manner.  
         [0043]     The counter  12  is preferably mounted over the shingled stream at the infeed section  14 . A suitable counter is a Denex brand edge detector sold under the trade designation “Laser Copy Sensor”. It generates an infrared beam direction onto the shingled stream and detects the beam reflected off the products  22 . Changes in luminance detect product edges and yield an input product count.  
         [0044]     The gapped shingled stream carried in the downstream conveyor exits to a collection well or “bucket”  64  defined by an elevator  66  and back plate  69 , and at its sides by another set of mirror-image reciprocating jogger plates  68 ,  68 . The elevator  66  preferably has an open frame construction. The back plate  69  is positioned to intercept the leading edge of the products  22  as they exit the conveyor  38   b , causing them to top stack on a pair of rail members  66   b ,  66   b  extending in the direction  46  to form the floor of the elevator. The elevator members are laterally positioned and spaced so that there is a central clearance  66   a  for a pusher plate  70  when it is extended by pneumatic cylinder  72  to push a completed stack off the elevator to a bander  74 , conveyor, or the like.  
         [0045]     The infeed to the collection bucket  64  includes a driven head roll  76  and an associated pair of nipping wheels  78 ,  78  mounted over the drive roll to ensure that the products are driven into the bucket at an angle conducive to building a stack. The angular position of the nipping wheels  78  with respect to the head roll is adjustable to accommodate sheets of different weights, size and flexibility and different travel speeds. An air jet also assists the stack formation.  
         [0046]     The elevator reciprocates vertically under the control of a pair of laterally spaced air cylinders between a raised, “collect” position and a lowered, stack “discharge” position. In the raised position, the stack is aligned as it is formed by a rear oscillating tapper plate  80  and the side jog plates  68 ,  68 . The side plates are secured to mounting blocks  82 ,  82  that slide axially on a pair of smooth guide shafts  84 ,  84  oriented at right angles to the direction  46 . A ball screw arrangement translates the jog plates  68 ,  68  toward and away from one another. A servo motor M operating under control of the controller  24  rotates a shaft  86  that is threaded half in one direction and half in the other the opposite direction. The shaft  86  threads into the blocks  82 ,  82  which act as followers. Rotation of the shaft  86  in one direction, e.g. four revolutions of the associated motor M, translates both plates  68 ,  68  apart, to their original position. A rapid succession of rotations of four revolutions, in alternating directions, produces an oscillation of the plates that jogs the products into a laterally aligned stack. The same drive mechanism also can set the lateral, non-reciprocating position of the plates for different width products. In the event of a paper jam in the bucket, the rotation of the shaft  86  in a direction that opens the plates  68 ,  68  quickly and automatically facilitates clearance of the jam. The plates  68 ,  68  straddle, and extend below, the rails  66   b,    66   b  forming the floor of the elevator  66  and the bottom of the collection bucket  64 . As with the infeed side joggers  28 ,  28  at the infeed, this arrangement avoids jamming of the printed sheets between the floor of the elevator and the reciprocating plates.  
         [0047]     Lowering of the elevator  66  is controlled by the counter  12  and controller  24 , not a limit switch, pressure switch, or other mechanical, optical, or electromechanical sensor and/or switch that reacts to the height of the stack. The formation of a complete stack is determined by the infeed count. When the desired number of products is collected and aligned in the bucket, the elevator lowers and the pusher  70  is activated to discharge the completed stack. At the same time, a set of fingers  88  are driven forward by associated air cylinders into position at the upper end of the bucket  64  to continue to receive and collect products for the following stacks while the elevator is lowered and cleared. When the empty elevator is raised by its air cylinder  92 ,  92 , it supports the products collected on the fingers  88 , which then retract clear of the well. The fingers are spaced to clear the side plates when extended, and the elevator central clearance  66   a  clears the pusher so that the elevator can start to return to its raised position before the pusher has fully retracted. This construction and mode of operation provide a cycle time of one to two seconds. With typical press operating speeds this allows the formation of stacks containing twenty-five pieces, or fewer (e.g. ten), sizes not presently obtainable with conventional equipment.  
         [0048]     Jam detection and clearance at the bucket  64  are also facilitated by a jam sensor  94  mounted over the collection bucket  64 . The sensor can take a variety of forms, but its present preferred form, a bar  96 , is pivotally mounted over the bucket  64  with a sensing arm  98  projecting radially toward the well. A spring  100  urges the bar to rotate to a position that places the free end of the arm  98  in the well, clear of the incoming product, but likely to be engaged and moved by an un-stacked accumulation of products in the well associated with a jam or other malfunction. A jam-sensing rotation of the arm and bar, against the force of the spring  100 , triggers a switch or other sensor, initiating automatic jam-clearance operations. These operations include operation of the infeed air cylinders  36 ,  36  to divert good product to the divert conveyor  34  which is driven to carry the product to a “good product” collection bin. This stops the input of products until the jam is cleared without wasting those products. As noted above, the ball screw  88  is operated to fully open the side jog plates and the cylinders  94  are actuated to lower the elevator to its discharge position. The servo motor M driving the conveyor  38  and head roll  76  can also be instructed to stop or slow. With these operations completed, the jam is readily and quickly cleared, and an operator can restart the stacker in its normal mode of operation.  
         [0049]     More generally, the use of servo motors and an integrated control of all the servo motors M and air cylinders noted above provide a self-contained, automatic control over the stacker operation, including jam clearance.  
         [0050]     The stacker  10  is supported on a base  102  that has wheels  104  that make it portable. It can be quickly rolled into position at the end of the press  16 . To stabilize the stacker, actuation of a set of air jacks  106  lifts the frame so that the wheels do not support the stacker  10 . Final lateral adjustments are then usually necessary to bring the stacker into adjustment with the output stream from the printer  16 . The present invention further includes a motorized side-lay adjustment for this lateral alignment. The components described above are secured, directly or indirectly, to a frame structure  112  organized within a pair of upright side walls  114  and various support members all in turn supported on a set of linear bearings  110  that rest on the base  102 . The bearings permit a small movement of the active stacker components with respect to the frame, e.g. up to three inches, in a direction transverse to the direction  46 . This side-lay adjustment is produced by a motor M-SL mounted on the main stacker that drives a threaded shaft  116  engaged in follower  118  secured, directly or indirectly, to the base  102 . These features greatly reduce set-up time as compared to known in-line stackers.  
         [0051]     When the elevator is lowered to the discharge position and the pusher  70  drives the stack from the elevator  66 , the stack being discharged preferably contacts and drives the preceding stack out of the bander  74  where it has been secured with a circumferential band. This arrangement avoids the need for a gripper assembly in the nature of a robotic arm, or the like, to grip and transport the stack from the stacker to a downstream processing, conveyor, or storage site.  
         [0052]     The stacker  10  provides a reliable operation with a single steam product input to the stacker at line speed. Jams are readily cleared and the product stream diverted in a way that saves and accounts for good products. The stacker is readily set up, with no electrical or other control connection from the printing press or finishing apparatus to the stacker. The stacker also handles sheet products of varying size, flexibility and weight, and avoids the “fat leading edge” problem that occurs with certain known stackers.  
         [0053]     While the invention has been described with respect to its preferred embodiments, it will be understood that many modifications and variations will occur to those skilled in the art. Such modifications and variations are intended to fall within the scope of the appended claims.