Abstract:
A push back storage rack system for storing multiple loads in a single inclined cart lane. Each lane contains at least two wheeled carts, each cart being capable of receiving and storing multiple pallet loads. The carts are vertically spaced so that they can freely slide underneath each other when unloaded. Beginning with the first or lowest level cart in the system, each successively higher cart is also wider and longer than the cart immediately beneath it. The carts are positioned on at least one but potentially two pairs of rectangular tracks or tubes, each tube being capable of supporting two or four individual carts, depending on how the carts are constructed and installed on the tubes. The tubes are mounted on an incline away from a loading end of each lane so that when loads are placed on or are removed from a lane, the carts are biased toward the loading end of the lane by the force of gravity. Each tube has a single, planar upper support surface which has inside and outside edges. The wheels of each cart ride only on either the inside or outside edges of the tubes on which they are mounted, allowing more than one vertically spaced cart to occupy the same tube.

Description:
[0001]    This application claims the benefit of U.S. Provisional Application No. 60/237,797 filed on Oct. 4, 2000, which is incorporated herein by reference. 
     
    
     
       BACKGROUND  
         [0002]    The present invention is directed to the field of storage rack systems. More particularly, the present invention is directed to a push back type of storage rack system for storing a plurality of loads in which multiple loads may be stored in a single storage lane.  
           [0003]    Push back storage racks normally comprise an assembly of shelves and vertical uprights for supporting loads placed on tracks or other base members in one or more storage lanes. Each storage lane has one loading position capable of storing one load. One or more vertically spaced push back carts are positioned in the loading position. Each cart is capable of receiving one load, being pushed toward the back of the lane by the next load, and sliding over the top of one another when unloaded. Such systems normally have their tracks in each lane tilted toward the loading position so that the force of gravity causes the next cart in line to return to the loading position when a load occupying the position is removed.  
           [0004]    When adding a load to a particular lane, the operator pushes the added load against a previously stored load occupying the lane&#39;s loading position. This forces the cart under the previously stored load further up the lane and out of the loading position, thereby making room for the load being added. If additional carts are in the loading position, the operator then positions the load on the next available cart. If all the carts have been pushed out of the loading position, the added load fills the lane to capacity, and the operator places the added load directly onto the tracks or base member of the lane itself.  
           [0005]    Previous push back storage systems have also included designs which permit unloaded carts to automatically slide into the loading position of their respective storage lanes to receive loads. Most designs allow the empty carts to simultaneously occupy the same loading position by incorporating either a nesting or telescoping cart arrangement.  
           [0006]    In previous nesting designs, higher level carts retract or nest within the horizontal dimensions of each next lower level cart. Such designs have been limited in both the number of carts that can be included in a single system and in the relative strength of each cart since the designs typically require the use of a single pair of track members and since the required horizontal clearance for successive carts prevents the inclusion of structural cross members. Due to the resulting limitations on the amounts of available space in such designs, these characteristics have also severely limited the number of carts that can be used and thus the number of loads that can be stored in a single lane. Additionally, smaller and weaker components may be used which substantially reduce the load-bearing capabilities of the system. In addition to substantially limiting the system&#39;s load-bearing capacity, smaller components, such as cart wheels, also tend to increase the amount of external force necessary to operate such systems. This ultimately leads to the need for more steeply sloped track inclines, which are undesirable, and normally increases the amount of wear and potential damage to the system, loading equipment, and stored loads.  
           [0007]    In previous telescoping designs, individual carts have been vertically spaced so that each higher level cart merely slides over the top of the next adjacent lower level cart. Previous telescoping designs have been severely limited in the number of carts that can be incorporated in a single lane due to the vertical space needed to include a rigid support piece across the width of each cart. Such cross pieces tend to make the additional vertical height required for each cart too great to incorporate many carts into a single lane. In contrast, eliminating such pieces tends to severely reduce the load capacity of each individual cart.  
           [0008]    Previous telescoping designs have also been limited by the fact that most use only a single pair of track members with one or more support surfaces upon which the wheels of the various carts ride. As with nesting designs, this characteristic of most telescoping designs has severely limited the number of carts and thus the number of loads which can be included in a single lane, while posing the same problems of wear, potential damage to the system, equipment, and loads. In the few instances where multiple pairs of tracks have been incorporated, some portions of the various support surfaces have been left unused. As a result, both space and load-bearing capacities have been wasted in such previous systems, reducing their cost-effectiveness and versatility.  
           [0009]    In some previous designs, push plates have been positioned at the trailing edge of the lowest or last-loaded cart to assure that an operator maintains proper pallet clearance during loading and to indicate, when it is not visible to the operator, that a particular lane is filled to capacity. It has been observed from time to time that pallets on which loads are stored drag against an adjacent surface of the push plate, causing damage to the pallets during loading and unloading.  
           [0010]    Many of the previous designs of push back rack systems have also been plagued by the problem of outward bowing of the beam adjacent each lane&#39;s loading position. The problem is associated with the repeated forces exerted by a system&#39;s carts as they automatically return to their respective loading positions. As each cart repeatedly returns to this position, stopping forces are exerted upon the adjacent beam member which, over time, tends to bend or warp outwardly and away from the storage lane in which it is mounted This is an additional problem which previous push back storage systems have yet been unsuccessful in resolving.  
         SUMMARY  
         [0011]    The present invention is a push back storage rack system for storing a plurality of pallet loads in which multiple loads may be stored in a single storage lane. Each lane contains at least two wheeled carts, each cart being capable of receiving and storing multiple pallet loads. The carts are vertically spaced so that they can freely slide underneath each other when unloaded. Beginning with the first or lowest level cart in the system, each successively higher cart is also wider and longer than the cart immediately beneath it. The carts are positioned on at least one but potentially two pairs of rectangular tracks or tubes, each tube being capable of supporting two or four individual carts, depending on how the carts are constructed and installed on the tubes. The tubes are mounted on an incline away from a loading end of each lane so that when loads are placed on and removed from a lane, the carts are biased toward the loading end of the lane by the force of gravity. Each tube has a single, planar upper support surface which has inside and outside edges. The wheels of each cart ride only on either the inside or outside edges of the tubes on which they are mounted, allowing more than one vertically spaced cart to occupy the same tube. A support beam is located at the loading end of each lane.  
           [0012]    The end of each cart closest to the loading end of the lane in which the cart is mounted is the trailing end of the cart and the opposite end of each cart is the leading end. When the carts are unloaded and positioned in a loading position at the loading end, the trailing end of the lowest cart rests flush against the adjacent support beam. The carts are constructed so that each successively higher cart is slightly longer than the next cart below. Following the lowest level cart of the system, a structural member at about the leading end of each successively higher cart contacts a structural member at about the leading end of the cart immediately below it when returning to the loading position. This transfers the impact force of each load to the trailing end of the lowest cart, thereby minimizing curling and excessive warping of the beam. Additionally, tensionbars positioned between approximately the middle of the structural beam and other structural members of the rack system also serve to significantly reduce the bowing or warping effect to a minimum. A push plate mounted on the trailing end of the lowest cart is offset from vertical, away from this cart, to prevent dragging on the end of pallets and subsequent damage to individual pallets and loads.  
           [0013]    In the preferred embodiment of this invention the structural member at the leading end of each cart is an angle plate having two sections at a 90° angle to one another. The fact that each successively higher cart is slightly longer than the next lower cart enables the end of each cart&#39;s horizontal section to contact the vertical section of the cart beneath it so that the carts stack above one another without greatly adding to the vertical height needed for each additional cart. Additionally, the vertical section of each leading angle piece provides rigidity for supporting heavy loads.  
           [0014]    At the trailing edge of each cart, relatively thin loading plates can also stack without adding a great deal of vertical height to the system. To maintain rigidity, vertical stiffeners extend downward below each loading plate, substantially along the width of the cart. For each successively higher cart, the stiffener is placed slightly further away from the cart&#39;s trailing edge than the stiffener on the cart immediately beneath it, allowing the stiffeners to remain clear of each other when the carts are stacked in the loading position.  
           [0015]    In an optional embodiment of the design, up to four additional carts may be included in the system. In this embodiment, the wheels of every second adjacently spaced pair of carts travel along the same edges of a particular pair of tubes, the two adjacent carts being interlocked by having the trailing wheels of one cart positioned between the leading and trailing wheels of the other. Thus, it becomes possible to position twice the number of carts on the same number of tubes without consuming substantial additional space.  
           [0016]    The narrow stacking characteristics inherent in this novel design enable as many as nine loads to be positioned in a single storage lane. Each individual cart, up to a maximum of eight, can store one load. A ninth load can then be positioned directly on the storage lane&#39;s tubes after all of the carts are loaded. For applications requiring the storage of five or fewer loads per lane, minor modifications to the design enable the system to be even more compact. The use of multiple rails and interlocked carts enable heavier components, such as wider and higher capacity wheels, to be incorporated into the design. Such components require less external force for operation, allowing for a gentler inclined slope for the inclined tubes and a higher load capacity for the individual carts, thereby reducing the amount of wear and potential damage experienced by the system, stored loads, and loading equipment.  
           [0017]    Other embodiments of the invention include mechanisms for preventing accidental lifting or disengagement from the tubes without increasing the sizes of successive carts. In one embodiment, wheels from the one or two carts positioned on the inside edges of the outer tubes are positioned to extend toward the carts&#39; middle portions in order to provide clearance for vertical anti-lift extensions that reach downwardly from the carts on the outside edges of the inside tubes. The inner carts, in turn, have horizontal flanges which prevent vertical movement of the outer carts. In another embodiment, the system&#39;s lowest and highest carts dispose vertical extensions which lock the carts to stop flanges located below the inside edges of the inside tubes and below the outside edges of the outside tubes, respectively. A series of interlocked flanges positioned between the individual carts then works as an integrated mechanism to prevent vertical movement of the carts. Both of these embodiments save additional space and enable the incremental spacing of successive carts to remain substantially constant regardless of the particular tube or edge on which the cart&#39;s wheels are positioned.  
           [0018]    Various other features, advantages, and characteristics of the present invention will become apparent to one of ordinary skill in the art while reading the following specification. This invention does not reside in any one of the features of the push back rack system disclosed below. Rather, this invention is distinguished from the prior art by its particular combination of features which are disclosed. Important features of this invention have been described below and shown in the drawings to illustrate the best mode contemplated to date for carrying out this invention.  
           [0019]    Those skilled in the art will realize that this invention is capable of embodiments which are different from those shown and described below and that the details of the structure of this push back rack system can be changed in various manners without departing from the scope of this invention. Accordingly, the drawings and description below are to be regarded as illustrative in nature and are not to restrict the scope of this invention. The claims are to be regarded as including such equivalent push back rack systems as do not depart from the spirit and scope of the invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]    For a more complete understanding and appreciation of this invention and many of its advantages, reference should be made to the following, detailed description taken in conjunction with the accompanying drawings wherein:  
         [0021]    [0021]FIG. 1 is a perspective view of a push back storage rack system according to the invention illustrating the relative positioning of carts and loads in multiple independently operating lanes positioned throughout the system;  
         [0022]    [0022]FIG. 2 is a side view of the push back storage rack system of FIG. 1, further demonstrating the relative inclines of the individual vertically spaced storage lanes;  
         [0023]    [0023]FIG. 3 is a perspective view of an independent push back storage system as would typically occupy one cart lane having four individual and unloaded carts as if positioned at the loading end of a particular lane;  
         [0024]    [0024]FIG. 4 is a front view of two adjacent tubes from the push back storage system of FIG. 3 depicting the respective wheels, angle plates, and hold-down cart components positioned adjacent to the depicted tubes;  
         [0025]    [0025]FIG. 5 is an exploded view of the push back storage system of FIG. 3 depicting the structural details of the individual carts of the system;  
         [0026]    [0026]FIG. 6 is perspective view of an independent push back storage system as would typically occupy one cart lane having five individual and unloaded carts as if positioned at the loading end of a particular lane;  
         [0027]    [0027]FIG. 7 is a front view of two adjacent tubes from the push back storage system of FIG. 6 depicting the respective wheels, angle plates, and hold-down cart components positioned adjacent to the depicted tubes;  
         [0028]    [0028]FIG. 8 is an exploded view of the push back storage system of FIG. 6 depicting the structural details of the individual carts of the system;  
         [0029]    [0029]FIG. 9 is a side view of the trailing edges of the carts of the push back storage system of FIG. 3 depicting the loading plates and stiffeners of the individual carts.  
         [0030]    [0030]FIG. 10 is a side view of the leading edges of the carts of the push back storage system of FIG. 3 depicting the leading angle plates of the individual carts; and  
         [0031]    [0031]FIG. 11 is a side view of an alternate embodiment of the leading edges of the carts of the push back storage system that is the subject of the invention depicting the leading angle plates of the individual carts.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0032]    Referring to the drawings, identical reference numerals and letters designate the same or corresponding parts throughout the several figures shown in the drawings.  
         [0033]    [0033]FIG. 1 shows a push back storage rack system of the type that is the subject matter of the invention. The system is based on a storage rack assembly  20  comprising a number of interconnected, vertical uprights  22  and horizontal beams  24 . Side horizontals  26  and diagonal cross pieces  28  may also extend between the vertical uprights  22  to make up rack cells  30  along the depth of the rack system. Two separate cart lanes  34 , each having a load end  32 , are positioned along the lengths of the beams  24 . To prevent the beams  24  from bowing outwardly and away from the rest of the system due to the stresses exerted on the beams  24  during operation, one or more diagonal tension bars  36  may also be positioned between about the center of one or more of the beams  24  and one or more vertical uprights  22  under the cart lanes  34 . Alternatively, diagonal tension bars may be fastened between about the center of a beam  24  and a beam at the end of the first rack cell  30  that is connected to the uprights  22  at the end of that rack cell. As shown in FIG. 1, a separate push back assembly occupies each individual lane  34 .  
         [0034]    Referring briefly to FIG. 2 along with FIG. 1, the storage rack assembly  20  is capable of storing multiple loads  38  in each cart lane  34 . Each load  38  is placed on a separate cart that rides on a set of tracks which are mounted at a slight incline away from the loading end  32  of the cart lane  34 . First, second, third, and fourth carts  41 ,  42 ,  43 , and  44  are shown in an extended position in three vertically stacked cart lanes  34  on tracks which comprise a pair of tubes  50  that extend along their respective cart lane  34 .  
         [0035]    The four-cart configuration of the carts  41 - 44  is depicted in its unloaded position in FIGS. 3 and 4. The pair of tubes  50  includes a pair of parallel inside tubes  52  and a pair of parallel outside tubes  54 . Each of the inside tubes  52  has a single upper support surface  56  having an inside edge  58  and an outside edge  60 . In this embodiment, each outside tube  54  is substantially similar to each inside tube  52 , with the outside tubes  54  also each having a single upper support surface  55 , an inside edge  57  and an outside edge  59 .  
         [0036]    As shown in FIG. 5, the first cart  41  has a pair of parallel side angle plates  62  with each angle plate  62  having a substantially horizontally planar surface  64  and a substantially vertically planar surface  66 . The horizontal surface  64  extends outwardly from the vertical surface  66  and away from the middle of the first cart  41 . Each vertical surface  66  has at least two cart wheels  68  mounted on it which are positioned to ride directly on the inside edge  58  of the upper support surface  56  of the inside tube  52  on which they are mounted. The first cart&#39;s wheels  68  also extend outwardly and away from the middle of the first cart  41 . Each wheel  68  has a flanged edge  70  for engaging the inside edges  58  of the tubes  52  and for properly positioning the wheels  68 .  
         [0037]    Each cart has a leading end which is the end that is farthest from the loading end  32  of the cart lane  34  in which it is located and has a trailing end which is farthest from the loading end  32 . Referring again to FIG. 5, a structural member shown as a leading angle plate  72  extends between the side angle plates  62  at the leading end of the first cart  41 . Like the side angle plates  62 , the leading angle plate  72  has a substantially vertically planar surface  74  and a substantially horizontally planar surface  76  forming the cart&#39;s leading end  78 . A trailing connecting tube  80  also extends between the side angle plates  62  forming a trailing end  82  of the cart  41 .  
         [0038]    A push plate  81  is mounted at the cart&#39;s trailing end  82 . Though having a substantially vertically planar lower surface  85 , the push plate  81  has an upper portion  83  that is bent slightly, for example five degrees away from vertical, leaning away from the first cart  41 . The functional advantages of this feature are examined more closely below.  
         [0039]    Referring back to FIG. 4, the second cart  42  rides on the outside edges  60  of the upper support surfaces  56  of the inside tubes  52 . The second cart  42  has a pair of parallel side angle plates  84  with each angle plate  84  having a substantially vertically planar surface  86  and a substantially horizontally planar surface  88 . The horizontal surface  88  extends inwardly from the vertical surface  86  and toward the middle of the second cart  42 . Each vertical surface  86  also has at least two cart wheels  90  mounted on it which are positioned to ride directly on the outside edge  60  of the inside tube  52  on which they are mounted. The second cart&#39;s wheels  90  also extend inwardly and toward the middle of the second cart  42 . Each wheel  90  has a flanged edge  92  for properly positioning the wheels  90  and for engaging the outside edges  60  of the tubes  52 .  
         [0040]    As best shown in FIG. 5, a structural member shown as a leading angle plate  94  extends between the side angle plates  84  of the second cart  42 . The leading angle plate  94  also has a substantially vertically planar surface  96  and a substantially horizontally planar surface  98  forming the cart&#39;s leading end  99 . A trailing loading plate  100  extends between the side angle plates  84  forming the trailing end of the second cart  102 .  
         [0041]    Referring again to FIG. 4, the third cart  43  rides on the inside edges  57  of the upper support surfaces  55  of the outside tubes  54 . The third cart  43  has a pair of parallel side angle plates  104  with each angle plate  104  having a substantially vertically planar surface  106  and a substantially horizontally planar surface  108 . The horizontal surface  108  extends inwardly from the vertical surface  106  and toward the middle of the third cart  43 . Each vertical surface  106  also has at least two cart wheels  110  mounted on it which are positioned to ride directly on the inside edge  52  of their respective outside tube  54 . The third cart&#39;s wheels  110  also extend inwardly and toward the middle of the third cart  43 . Each wheel  110  has a flanged edge  112  for engaging the inside edges  57  of the tubes  54  and for properly positioning the wheels  110  on these tubes.  
         [0042]    Again as best shown in FIG. 5, a structural member shown as a leading angle plate  114  extends between the side angle plates  104  of the third cart  43 . The leading angle plate  114  also has a substantially vertically planar first surface  116  and a substantially horizontally planar second surface  118  forming the cart&#39;s leading end  119 . Like the second cart  42  and unlike the first cart  41 , a trailing loading plate  120  extends between the side angle plates  104  forming the trailing end of the third cart  122 .  
         [0043]    Referring once again to FIG. 4, the fourth cart  44  rides on the outside edges  59  of the upper support surfaces  55  of the outside tubes  54 . The fourth cart  44  has a pair of parallel side angle plates  124  with each angle plate  124  having a substantially vertically planar surface  126  and a substantially horizontally planar surface  128 . The horizontal surface  128  extends inwardly from the vertical surface  126  and toward the middle of the fourth cart  44 . Each vertical surface  126  also has at least two cart wheels  130  mounted on it which are positioned to ride directly on the outside edge  59  of their respective outside tube  54 . The fourth cart&#39;s wheels  130  also extend inwardly and toward the middle of the fourth cart  44 . Each wheel  130  has a flanged edge  132  for engaging the outside edges  59  of the tubes  54  and for properly positioning the wheels  130  on these tubes.  
         [0044]    Again as best shown in FIG. 5, a structural member shown as a leading angle plate  134  extends between side angle plates  136  of the fourth cart  44 . The leading angle plate  134  also has a substantially vertically planar surface  138  and a substantially horizontally planar surface  140  forming the cart&#39;s leading end  142 . Like the second and third carts  42  and  43 , and unlike the first cart  41 , a trailing loading plate  144  extends between the side angle plates  136  forming the trailing end  146  of the fourth cart  44 .  
         [0045]    In operation, carts  41 - 44 , being unloaded, remain positioned over top of one another at the loading end  32  of their lane as shown in FIG. 3. The fourth cart  44 , being the highest cart in the system, stands available to receive a load. Referring to FIG. 1, the operator, using appropriate lifting equipment, lifts and carries a load  38  over the beam  24  at the loading end  32  of the selected lane  34 . The load  38  is then lowered into position on the fourth cart  44 .  
         [0046]    While positioning the load  38 , the operator carefully raises the load  38  to a sufficient height so that the load  38  has adequate vertical clearance to avoid contact with the push plate  81 . As noted above, the push plate  81  has an upper portion  83  that is bent slightly away from the first cart  42 . As the operator lowers the load  38  into position, the slight bend of the push plate&#39;s upper portion  83  allows a pallet carrying the load  38  to contact the inside planar surface of the upper portion  83  rather than contact the push plate  81  along its top edge  148 . This reduces the likelihood of damage to the pallet or to the load  38  that could potentially result from the heavy downward force of the load  38  being exerted against the top edge  148 . Additionally, when a load  38  is removed, the slight bend of the upper portion  83  of the push plate  81  reduces friction between the pallet and the push plate, reducing the potential for damage to the pallet.  
         [0047]    After positioning a load  38  on the fourth cart  44 , the operator positions an additional load  38  by lifting the additional load with appropriate lifting equipment and carrying the load  38  over the selected lane&#39;s beam  24 . As it is carried forward, the load  38  contacts the previously positioned load  38 , pushing the load  38  and its supporting fourth cart  44  further up the inclined outer tubes  54 . The fourth cart  44  slides away from the loading end  32  of the cart lane  34 , making the third cart  43  available to receive the next load  38 . This load is then lowered into the cart lane  34  at the lane&#39;s loading end  32 .  
         [0048]    Subsequent loads  38  are added to the cart lane  34  in a similar manner. The operator uses each subsequent load  38  to push the previous load  38  and its respective cart further up the inclined tubes  52  and  54 , thereby making the next lower cart available to receive and store the subsequent load  38 . In an embodiment of the design using four carts, up to five loads may be positioned at one time in the same cart lane  34 . After the first cart  41  has been loaded with a fourth load, the operator can add a fifth load by pushing the fifth load against the load  38  previously positioned on the fourth cart  41 . Thus the first cart  41  and the load on it slide further up the inclined tubes  52 . The operator then lowers the fifth load directly on to the tubes  52  and  54 , filling the cart lane  34  to capacity. The push plate  81 , mounted on the first cart  41 , then moves along with the first cart  41  away from the loading end  32  of the cart lane  34  where it is blocked from view by the fifth load. Since the push plate  81  is not visible, the operator knows the cart lane  34  is fully loaded.  
         [0049]    During the unloading of the cart lane  34 , individual loads  38  are removed from the tubes or from their respective carts, allowing the carts  41 - 44  to slide back down along the tubes  52  and  54  toward the loading end  32  under the force of gravity. In a four-cart system, during removal of the fifth load  38  from the tubes  52  and  54 , the fourth cart  41  begins to slide back down the tubes and return to the loading end  32  of the cart lane  34 . Once the fourth cart  41  reaches the loading end  32 , the lower surface  85  of the push plate  81 , being mounted at the first cart&#39;s trailing end  82 , comes into contact with the beam  24  extending across the lane&#39;s loading end  32 . When the operator removes the next load  38  positioned on the first cart  41 , the second cart  42  begins to slide back down the tubes and over the top of the first cart  41  into the loading end  32  of the cart lane  34 . However, unlike the first cart  41 , the trailing edge  102  of the second cart  42  does not make contact with the beam  24 .  
         [0050]    [0050]FIG. 10 is a side sectional view of the relative positioning of the leading angle plates  72 ,  94 ,  114 , and  134  of the carts  41 ,  42 ,  43  and  44 , respectively when all the carts are at the loading end  32  of their respective lane  34 . Consider the second cart  42  returning to the loading position  32  after the unloading of the first cart  41 . According to this invention, the vertical surface  96  of the leading angle plate  94  of the second cart  42  contacts the end of the horizontal surface  76  of the leading angle plate  72  of the first cart  41 . Referring to FIGS. 3 and 4 along with FIG. 10, the contact between these two surfaces of the structural members at the leading ends of the carts  41  and  42  restricts subsequent movement of the second cart  42  down the inclined tubes  52  on which it is mounted. Thus, loading end impact between the various carts occurs at the carts&#39; leading ends rather than at the carts&#39; trailing ends. This impact relationship is repeated between the angle plates  114  of cart  43  and  44  of cart  42  and between angle plates  134  of cart  44  and  114  of cart  43 .  
         [0051]    [0051]FIG. 9 is a side view of the trailing ends of the various carts in a typical 5-deep system made in accordance with this invention when the trailing connecting tube  80  of the first cart  41  and the loading plates  100 ,  120 , and  144  of the second, third and fourth carts  42 - 44  are all positioned at the loading end of the cart lane  32 . That is, all of the leading ends of the carts  41 - 44  have contacted and restricted the movement of each adjacently higher cart as shown in FIG. 10, and the loading plates of the carts  41 - 44  do not contact one another and do not transmit force between loads.  
         [0052]    In FIG. 9 according to another aspect of this invention, stiffener angle plates  150  and  152  have been added to the bottoms of the loading plates  120  and  144 , respectively, and a stiffener flange plate  153  has been added to the bottom of the loading plate  100 , respectively. These added stiffener plates  150 ,  152  and  153  serve to give additional cross member strength to their respective loading plates  100 ,  120 , and  144 , thereby adding additional cross strength to each plate&#39;s respective carts  42 - 44 , enabling each cart to bear heavier loads. These stiffener plates  150 ,  152  and  153  are staggered beneath the carrying surfaces of the carts to enable the carts to have a low profile, while increasing their load capacity.  
         [0053]    In accordance with the above, FIG. 9 shows the gap  154  between stiffener plate  152  and loading plate  120 , gap  156  between stiffener plate  150  and loading plate  100  and gap  158  between stiffener  153  and connecting tube  80  after all the carts  41 - 44  have made contact through their leading angle plates  72 ,  94 ,  114  and  134 , respectively, as shown in FIG. 10. Moreover, contact with the first cart&#39;s push plate  81  is not made by any of the second, third, or fourth carts  42 - 44  due to the tilt of the upper portion  83  of the push plate  81 . As a result, contact of every higher cart  42 - 44  with the beam  24  of the cart lane  32  is made only indirectly through the first cart  41 . Consequently, all impact forces exerted on the beam  24  are exerted indirectly only through the lower surface  85  of the push plate  81 .  
         [0054]    Referring now to FIGS. 3 and 4, the side angle plates  62 ,  84 , and  124  of the first, second, and fourth carts  41 ,  42 , and  44  each have a number of downward reaching retaining hooks  160 ,  162 , and  164 , with each safety hook  160 ,  162 , and  164  having a horizontal locking surface  166 ,  168 , and  170 , all respectively positioned. The horizontal locking surfaces  166 ,  168 , and  170  each extend under flanges  172 ,  174 , and  176  that are adjacent their respective carts. Each of the flanges  172 ,  174 , and  176  are positioned below the respective tube edges  58 ,  60 , and  59  under which their respective carts&#39; wheels  68 ,  90 , and  130  roll and extend along the length of their respective tubes  52  and  56 . In the event of a vertical movement, such as an accidental lifting by the operator&#39;s loading equipment of one or more of the first, second, or fourth carts  41 ,  42 , and  44 , the resulting upward movement of the hooks  160 ,  162 , and  164  causes the horizontal locking surfaces  166 ,  168 , and  170  to contact with each hook&#39;s respective flange  172 ,  174 , and  176 , restricting further cart movement and preventing disengagement of the carts  41 ,  42 , and  44  from their proper positioning.  
         [0055]    In order to maintain approximate incremental sizing of the carts  41 - 44 , it is necessary to omit the positioning of hooks and flange assemblies to restrict vertical movement of the third cart  43 . As described above, the second horizontal surfaces  88  of the third cart&#39;s side angle plates  84  extend inwardly from the angle plates&#39; vertical surfaces  86  and toward the middle of the third cart  43 . This permits extension flanges  180  to extend from the vertical surfaces  86  of the second cart&#39;s side angle plates  84  over the adjacently positioned wheels  110  of the third cart  43  to guard against the possibility of accidental disengagement. This also permits incremental cart spacing without further widening the distance between each adjacent inside tube  52  and outside tube  54 . The extension flanges  180  are substantially horizontally planar in shape and extend approximately three-quarters of the outside length of the second cart  42 , being centered lengthwise in this dimension on the angle plates  84 . In the event of vertical movement of the third cart  42 , the third cart&#39;s wheels  110  contact the extension flanges  180 , which, being connected to the second cart  42 , are restricted in upward movement by the second cart&#39;s hooks  162  and flanges  174 . This arrangement thus prevents accidental disengagement of the third cart  43  without requiring the added space of separate hooks proximate to the directional line of travel of the third cart&#39;s wheels  110 .  
         [0056]    In an optional embodiment of the design, up to four additional carts may be placed in a single storage lane without increasing the number of tubes needed for the storage system. Referring to FIGS. 6 and 7, this embodiment incorporates an inside and an outside pair of inclined tubes  182  and  184 . The inside pair of tubes  182  has an upper support surface  186  having an inside edge  188  and an outside edge  190 . The outside pair of tubes  184  also each include an upper support surface  192  divided into inside and outside edges  194  and  196 . Unlike the previously-described embodiment for systems of up to four carts, the upper support surfaces  192  of the outside tubes  184  are vertically spaced above the upper support surfaces  192  of the inside tubes  182  rather than all support surfaces being at an approximately even level.  
         [0057]    As is best understood comparing FIG. 6 to FIG. 7, the first cart  210  includes side angle plates  212  having vertically planar surfaces  214 , each vertically planar surface  214  disposing leading and trailing wheels  213  that extend outwardly and away from the middle of the first cart  210 . The first cart&#39;s wheels  213  are positioned to roll on the inside surfaces  188  of the inside tubes  182 . The second cart  220  also includes angle plates  222  having vertically planar surfaces  224 , each vertically planar surface  224  disposing leading and trailing wheels  223  that extend inwardly and toward the middle of the second cart  220 . The second cart&#39;s wheels  223  are also positioned to ride on the inside surfaces  188  of the inside tubes  182 . In order to allow both the wheels  213  and  223  of the first and second carts  210  and  220  to run on the same inside edges  188  of the inside tubes  182 , the trailing wheels  213  of the first cart  210  are positioned between the leading and trailing wheels  223  of the second cart  220 , thereby interlocking the wheels  213  and  223  and allowing for relative movement along the same directional line defined by the inside tubes&#39; inside edges  188 .  
         [0058]    Again, as is best understood comparing FIG. 6 to FIG. 7, the third cart  230  includes side angle plates  232  having vertically planar surfaces  234 , each vertically planar surface  234  disposing leading and trailing wheels  233  that extend inward and toward the middle of the third cart  230 . The third cart&#39;s wheels  233  are positioned to roll on the outside surfaces  190  of the inside tubes  182 . The fourth cart  240  also includes angle plates  242  having vertically planar surfaces  244 , each vertically planar surface  244  disposing leading and trailing wheels  243  that extend inwardly and toward the middle of the fourth cart  240 . The fourth cart&#39;s wheels  243  are also positioned to ride on the outside surfaces  190  of the inside tubes  182 . In order to allow both the wheels  233  and  243  of the third and fourth carts  230  and  240  to run on the same outside edges  190  of the inside tubes  182 , the trailing wheels  233  of the third cart  230  are positioned between the leading and trailing wheels  243  of the fourth cart  240 , thereby interlocking the wheels  233  and  243  and allowing for relative movement along the same directional line defined by the inside tubes&#39; outside edges  190 .  
         [0059]    This relative arrangement pattern repeats itself for the carts  250 ,  260 ,  270  and  280  positioned on the outside pair of tubes. The fifth cart  250  includes side angle plates  252  having vertically planar surfaces  254 , each vertically planar surface  254  disposing leading and trailing wheels  253  that extend outward and away from the middle of the fifth cart  250 . The fifth cart&#39;s wheels  253  are positioned to roll on the inside surfaces  194  of the outside tubes  184 . The sixth cart  260  also includes angle plates  262  having vertically planar surfaces  264 , each vertically planar surface  264  disposing leading and trailing wheels  263  that extend inwardly and toward the middle of the sixth cart  260 . The sixth cart&#39;s wheels  263  are also positioned to ride on the inside surfaces  194  of the outside tubes  184 . In order to allow both the wheels  253  and  263  of the fifth and sixth carts  250  and  260  to run on the same inside edges  194  of the outside tubes  184 , the trailing wheels  253  of the fifth cart  250  are positioned between the leading and trailing wheels  263  of the sixth cart  260 , thereby interlocking the wheels  253  and  263  and allowing for relative movement along the same directional line defined by the outside tubes&#39; inside edges  194 .  
         [0060]    The two highest carts are similarly interlocked. The seventh cart  270  includes side angle plates  272  having vertically planar surfaces  274 , each vertically planar surface  274  disposing leading and trailing wheels  273  that extend outward and away from the middle of the seventh cart  270 . The seventh cart&#39;s wheels  273  are positioned to roll on the outside surfaces  196  of the outside tubes  184 . The eighth cart  280  also includes angle plates  282  having vertically planar surfaces  284 , each vertically planar surface  284  disposing leading and trailing wheels  283  that extend inwardly and toward the middle of the eighth cart  280 . The eighth cart&#39;s wheels  283  are also positioned to ride on the outside surfaces  196  of the outside tubes  184 . In order to allow both the wheels  273  and  283  of the seventh and eighth carts  270  and  280  to run on the same outside edges  196  of the outside tubes  196 , the trailing wheels  273  of the seventh cart  270  are positioned between the leading and trailing wheels  283  of the eighth cart  280 , thereby interlocking the wheels  273  and  283  and allowing for relative movement along the same directional line defined by the outside tubes&#39; outside edges  196 .  
         [0061]    Due to the greater number of carts being present in a similarly confined space, this later cart-and-tube arrangement for systems of up to eight carts is inherently more crowded than are embodiments for up to four carts only. Consequently, a different system must be incorporated to prevent accidental disengagement due to accidental cart lifting. In FIGS. 6 and 7, downward reaching retaining hooks  286  and  288  extend from the side angle plates  212  and  282  of the first cart  210  and eighth cart  280 , each disposing horizontal locking surfaces  240  and  242  which extend under adjacent flanges  244  and  246 . The flanges  244  and  246  are positioned along the lengths of the inside tubes  182  below the tubes&#39; inside edges  188  and along the outside tubes  184  below the tubes&#39; outside edges  196 . In the event of vertical movement of the first or eighth carts  210  or  280 , the horizontal locking surfaces  240  and  242  of the retaining hooks  286  and  288  lock against the flanges  244  and  246 , restricting the carts&#39; movement and preventing accidental tube disengagement.  
         [0062]    Focusing now on FIG. 7, horizontally planar top flanges  290  are positioned on the side angle plates  214  of the first cart  210  extending outwardly from the middle of the first cart  210  and over a lower angle flange  302  mounted on the side angle plate  222  of the second cart  220 . The lower angle flanges  302  are substantially horizontally planar, extending approximately three-quarters the length of their respective second cart  220  on the cart&#39;s side angle plates  220  and approximately centered on the side angle plates&#39; vertical surfaces  224  in the horizontal dimension. In the event of vertical movement of the second cart  220 , the second cart&#39;s lower angle flanges  302  collide with the planar top flanges  290 , which, as part of the first cart  210 , are restricted in movement by the retaining hook  286 . Thus, the contact between the second cart&#39;s lower angle flanges  302  and planar top flanges  290  restricts further upward movement of the second cart  220 . To restrict higher level carts, the second through seventh carts  220 ,  230 ,  240 ,  250 ,  260 , and  270  each dispose top angle flanges  300  on their respective side angle plates  222 ,  232 ,  242 ,  252 ,  262 , and  272  which extend outwardly and away from the middles of their respective carts. The top angle flanges  300  of each adjacently lower cart are positioned to contact the lower top flanges  302  and thereby restrict vertical cart movement in the event of accidental lifting of one or more carts. Thus, each second through seventh cart  222 ,  232 ,  242 ,  252 ,  262 , and  272  is ultimately restricted indirectly by the retainer hook  286  of the first cart  210  via the interlocked system of planar and angle flanges  290 ,  300 , and  302 . Vertical movement of the eighth cart  284  is restricted by the presence of its own retainer hooks  288  locking against flanges  246 .  
         [0063]    Those skilled in the art will recognize that the various features of this invention described above can be used in various combinations with other elements without departing from the scope of the invention. Thus, the appended claims are intended to be interpreted to cover such equivalent push back rack systems which do not depart from the spirit and scope of the invention.