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CROSS-REFERENCES TO RELATED APPLICATIONS 
     Not Applicable 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable 
     MICROFICHE APPENDIX 
     Not Applicable 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to the field of ladders. More specifically, the invention comprises a modular ladder having removable steps. 
     2. Description of the Related Art 
     The incorporation of removable rungs into various structures is well known in the prior art. As one example, U.S. Pat. No. 6,247,553 to Jones (2001) discloses a removable rung (or step) designed to be applied to a steel T-post of the type commonly used for cattle fencing. It allows a user to step over a modern fence line without having to put weight on the strands of barbed wire. The Jones device is not easily removable, however. It is designed to be installed and left in place. Thus, it is ill suited for the type of application encompassed by the present invention. 
     U.S. Pat. No. 3,833,090 to Georgianna (1974) discloses a removable step designed to be locked into a steel support column. This invention is directed to warehouse racks, whose vertical columns already include slots for the mounting of shelves and the like. The step design in the &#39;090 patent takes advantage of these pre-existing slots. The step is quite large and cumbersome, however. It would be difficult to store a set of such steps on the user&#39;s person, and quite cumbersome to carry them over long distances. 
     A similar device is disclosed in U.S. Pat. No. 4,450,936 to Strom (1984). The Strom device also takes advantage of the pre-existing slots found in warehouse columns. Unlike the Georgianna device, though, it is formed of simple bar stock components. The &#39;936 device should therefore be easier to fabricate. It is still quite bulky, however. In addition, it requires a support column having a large interior cavity so that the step can be “snaked” into position. While such a large cavity is often found in warehouse columns, it is rarely found elsewhere. 
     Accordingly, the prior art devices are limited in that they: 
     1. Are difficult to apply and remove; 
     2. Are heavy; 
     3. Are bulky; and 
     4. Require a vertical column having a large interior cavity. Require the deployment 
     BRIEF SUMMARY OF THE INVENTION 
     A ladder is composed of a vertical support structure and a set of rungs. In the present invention, the vertical support structure is formed by stacking a series of interlocking sticks. The sticks incorporate features allowing the removable installation of a set of rungs. The rungs are small and light, so that a set of such rungs may be easily carried by a user. The vertical sticks generally remain attached to the object to be climbed—such as a tree or pole. When the user wishes to climb the object, the user installs the rungs as he or she climbs. The rungs are then removed upon descent. In this manner, unauthorized climbing of the object is inhibited. 
     Several different embodiments of the removable rungs are disclosed. The common feature of all these embodiments is the fact that the rungs cannot come loose from the vertical stick while they are under load. The application of the invention to different types of hunting tree stands is also disclosed in detail, although the application of the invention extends far beyond hunting products. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     FIG. 1 is an isometric view, showing a prior art tree stand and ladder. 
     FIG. 2 is an isometric view, showing a prior art climbing stick in greater detail. 
     FIG. 3 is an isometric view, showing the vertical support element of the proposed invention. 
     FIG. 4 is an isometric view, showing the vertical support element from a different angle. 
     FIG. 5 is a detail view, showing the features designed to engage and hold the removable rung. 
     FIG. 6 is an isometric view, showing the removable rung. 
     FIG. 7 is an isometric view, showing the installation of the removable rung. 
     FIG. 8 is an isometric view, showing the installation of the removable rung. 
     FIG. 9 is an isometric view, showing the installation of the removable rung. 
     FIG. 10 is an isometric view, showing the installation of the removable rung. 
     FIG. 11 is an elevation view, showing the removable rung locked in place. 
     FIG. 12 is an elevation view, showing the camming action which holds the removable rung in place. 
     FIG. 13 is an isometric view, showing the installation of the removable rung in the opposite side of the slotted stick. 
     FIG. 14 is an isometric view, showing a completed assembly with removable rungs in place. 
     FIG. 14B is an isometric view, showing a completed assembly with two vertical support elements stacked together. 
     FIG. 15 is an isometric view, showing a prior art ladder stand. 
     FIG. 16 is an isometric view, showing the application of the present invention to a prior art ladder stand. 
     FIG. 17 is an isometric view, showing the detail of the vertical column used in a ladder stand. 
     FIG. 18 is an isometric view, showing an alternate embodiment of the removable rung. 
     FIG. 19 is an isometric view, showing an alternate embodiment of the removable rung. 
     FIG. 19B is an isometric view, showing the alternate rung installed. 
     FIG. 20 is an isometric view, showing an additional locking device. 
     FIG. 21 is a detail view, showing the operation of the locking device. 
     FIG. 22 is a detail view, showing the operation of the locking device. 
    
    
     REFERENCE NUMERALS IN THE DRAWINGS 
     
       
         
               
               
             
           
               
                   
               
             
             
               
                 10 
                 tree 
               
               
                 12 
                 tree stand 
               
               
                 14 
                 climbing stick 
               
               
                 16 
                 fixed rung 
               
               
                 18 
                 securing strap 
               
               
                 20 
                 standoff 
               
               
                 22 
                 slotted stick 
               
               
                 24 
                 slot 
               
               
                 26 
                 inclined hole 
               
               
                 28 
                 joining pin 
               
               
                 30 
                 strap lock 
               
               
                 32 
                 relief notch 
               
               
                 34 
                 joining hole 
               
               
                 36 
                 insertion slot 
               
               
                 38 
                 removable rung 
               
               
                 40 
                 step 
               
               
                 42 
                 insertion cylinder 
               
               
                 44 
                 key 
               
               
                 46 
                 camming wall 
               
               
                 48 
                 free wall 
               
               
                 50 
                 camming surface 
               
               
                 52 
                 ladder stand 
               
               
                 54 
                 slotted column 
               
               
                 56 
                 base 
               
               
                 58 
                 alternate stick 
               
               
                 60 
                 transverse hole 
               
               
                 62 
                 first alternate step 
               
               
                 64 
                 first alternate key 
               
               
                 66 
                 front face 
               
               
                 68 
                 right side face 
               
               
                 70 
                 left side face 
               
               
                 72 
                 rear face 
               
               
                 74 
                 vertical support column 
               
               
                 76 
                 alternate insertion slot 
               
               
                 78 
                 Z step 
               
               
                 80 
                 second alternate stick 
               
               
                 82 
                 lock slide 
               
               
                 84 
                 retaining pin 
               
               
                 86 
                 access cut 
               
               
                 88 
                 pin channel 
               
               
                 90 
                 lock hole 
               
               
                 92 
                 open channel 
               
               
                 94 
                 handle 
               
               
                   
               
             
          
         
       
     
     DETAILED DESCRIPTION OF THE INVENTION 
     The proposed invention allows a user to climb many types of vertical objects. One particular application of the invention is in the field of hunting, where tree stands are often employed to provide the hunter an elevated and stationary position. FIG. 1 shows a prior art tree stand  12  attached to a tree  10 . As installing a tree stand requires considerable effort, hunters often wish to leave them in place. This requires a device for climbing the tree. FIG. 1 also discloses a prior art tree-climbing device. A series of climbing sticks  14  are linked together and attached to tree  10 . Each climbing stick  14  has a series of fixed rungs  16 . 
     FIG. 2 shows an individual climbing stick  14  in more detail. The reader should be aware that the square vertical member is hollow, so that a second climbing stick  14  can be placed on top of the one shown and linked together using the smaller square section located on top of the climbing stick  14 . Two stand offs  20  are provided to separate climbing stick  14  from the tree  10 , thereby allowing clearance for the user&#39;s boots on fixed rungs  16 . A pair of securing straps  18  are passed tightly around the tree  10  and locked in position by toggle clamps or other prior art means. 
     The prior art devices shown in FIGS. 1 and 2 are effective in allowing access to the tree stand  12 . Unfortunately, when the user leaves the tree stand  12  unattended, other persons may use the ready access to steal the tree stand. In addition, many landowners are concerned about the safety of unsecured ladders in position on their property. If children or persons unfamiliar with tree climbing climb the unsecured ladders, they may be injured. Thus, it is desirable to create a ladder which cannot be climbed without specialized equipment. 
     FIG. 3 discloses one embodiment of the present invention. Slotted stick  22  is similar in its general configuration to the prior art climbing stick  14 , except that it includes no fixed rungs  16 . Two stand offs  20  are provided, along with securing straps  18 . These straps  18  are locked in place by actuating strap locks  30 . 
     The vertical support column  74  of slotted stick  22  is substantially modified over the prior art. Its front face opens into a series of slots  24 . It is also transected by a series of inclined holes  26 , the details of which will be described subsequently. 
     The top of vertical support column  74  is formed into joining pin  28 . Turning now to FIG. 4, the reader will observe that the lower portion of vertical support column  74  opens into joining hole  34 . Those skilled in the art will therefore appreciate that a series of slotted sticks  22  can be linked together by inserting the joining pin  28  on the top of one vertical support column  74  into the joining hole  34  in a second vertical support column  74 . 
     FIG. 5 shows the intersection of slot  24  with inclined hole  26  in greater detail. Vertical support column  74  of slotted stick  22  has a square cross section—as shown. Inclined hole  26  passes completely through the square section, from its right side all the way to its left side. Slot  24  is cut into the front surface of the square section. Thus, slot  24  and inclined hole  26  intersect as shown. The front and right surfaces of the square section also open into insertion slot  36 . Insertion slot  36  runs parallel to the center axis of inclined hole  26 . The resulting geometry allows the insertion and removal of removable rung  38 , shown in FIG.  6 . 
     Removable rung  38  comprises insertion cylinder  42 , step  40 , and key. Insertion cylinder  42  and step  40  are joined at an angle. Those skilled in the art will realize that these two elements could be formed by bending a single piece of round stock. It is not necessary for the invention to include a sharply defined joint between the two. The reader will observe that key  44  also includes relief notch  32 , the purpose of which will be explained shortly. 
     FIG. 7 shows the first step in inserting removable rung  38  into vertical support column  74 . Insertion cylinder  42  is aligned with inclined hole  26  and key  44  is aligned with insertion slot  36 . In FIG. 8, insertion cylinder  42  is placed within inclined hole  26  and key  44  is shown sliding through insertion slot  36 . In FIG. 9, key  44  has been pushed all the way through insertion slot  36  and is resting completely within slot  24 . Key  44  is stopped from sliding further to the left because it has come up against the left wall of slot  24 . At this point, the user rotates insertion cylinder  42  in the direction indicated (by grasping step  40 ). Key  44  the begins rotating down into slot  24 . Relief notch  32  is provided so that key  44  does not hit the right wall of slot  24 . 
     In FIG. 10, key  44  has been rotated into its final position. The side of key  44  which is facing away from the viewer in FIG. 10 is now resting against the back of slot  24 . The depth of slot  24  is set equal to the depth of the centerline of inclined hole  26 , plus one half the thickness of key  44 . 
     FIG. 11 shows the same assembly in an elevation view. The reader will note that insertion cylinder  42  rests within inclined hole  26 . Slot  24  is bounded on its right side by free wall  48 , and on its left side by camming wall  46 . The left facing surface of key  44  is designated as camming surface  50 . The geometry of the device tends to hold removable rung  38  in place because of the following sequence: If insertion cylinder  42  is rotated so that key  44  moves toward the viewer in FIG. 11 (the only way it can be rotated), then camming surface  50  will bear against camming wall  46  and push removable rung  38  to the right. 
     FIG. 12 shows the assembly after this rotation has started. The reader will observe that camming surface  50  has rotated against camming wall  46  and forced insertion cylinder  42  to slide to the right as indicated. This results in step  40  moving up and to the right, as well as rotating as shown. As a practical matter, this motion cannot occur when the user&#39;s weight is placed upon step  40 . In other words, in order for key  44  to move out of its locked position, step  40  must overcome the user&#39;s weight and actually lift the user. Stated in reverse—the user&#39;s weight upon step  40  locks removable rung  38  securely in place. However, once the user&#39;s weight is removed, then the user can grasp removable rung  38 , rotate it to the position where key  44  aligns with insertion slot  36 , and remove it. 
     Of course, it is of little use to have rungs on only one side of vertical support column  74 . Removable rung  38  must therefore be capable of insertion in either side of vertical support column  74 . FIG. 13 shows the insertion of removable rung  38  in the left side of vertical support column  74 . Referring briefly back to FIG. 4, the reader will observe that successive inclined holes  26  are inclined in opposite directions. FIG. 13 illustrates an inclined hole  26  configured to accept an insertion from the left. Likewise, insertion slot  36  is shown opening to the left. Removable rung  38  is absolutely identical to the one shown in FIGS.  6  through  12 —it has simply been reoriented. Those skilled in the art will realize that removable rung  38  can be installed from the left by inserting insertion cylinder  42  into inclined hole  26  (with key  44  going through insertion slot  36 ) and thereafter rotating key  44  down into slot  24 . It will then be locked in place under the same principles as described above. 
     FIG. 14 shows slotted stick  22  with four removable rungs  38  in place. As noted above, inclined holes  26  alternate in orientation (along with the insertion slots  36 ) to allow the rungs to alternate. FIG. 14B shows two slotted sticks  22  stacked together. The reader will observe that the alternating pattern of removable rungs  38  continues through the stack of two or more slotted sticks  22 . 
     In actual use, a stack of four or more slotted sticks  22  would be placed on the tree  10  or other object to be climbed. The sticks  22  are attached to the tree  10  using the securing straps  18  which are well known in the prior art. All removable rungs  38  would be removed before leaving the device unattended. A user wishing to climb the device would need to bring along a set of removable rungs  38 . The user would then progressively install removable rungs  38  as he or she ascends the ladder. When the user later descends the ladder, removable rungs  38  would be progressively removed. 
     A rung to rung spacing of 9 inches in typical for this type of device. Accordingly, in order to ascend a twelve foot object, the user would need to bring approximately sixteen removable rungs  38 . These rungs  38  can be made of aluminum alloy, resulting in a modest weight for a set of sixteen. 
     Material selection is important for slotted sticks  22 , as it must withstand significant mechanical forces. It must also withstand prolonged exposure to sunlight and temperature extremes. Many metals could be used, but since the device must often be carried into the woods on foot, weight is a factor. Accordingly, glass reinforced ABS has been found to be particularly effective. 
     The embodiment disclosed in FIGS. 3 through 14B is well suited for use with roughly cylindrical objects. It should be noted, however, that the invention can also be applied to other devices. FIG. 15 illustrates one such device. Ladder stand  52  is a common prior art device. Being rigid, it is affixed to tree  10  by one or two securing devices up near its top. The bottom portion is simply placed on the ground. While quite effective, it faces the same security problems as many other prior art devices. Once left unattended, anyone can climb up and remove the device. Likewise, persons may be injured while climbing the device. 
     FIG. 16 illustrates the application of the present invention to ladder stand  52 . Slotted column  54  is substituted for the prior art ladder. A set of removable rungs  38  are then placed in slotted column  54 . Base  56  is provided to distribute the weight of the device and prevent slotted column  54  from sinking into the ground. 
     FIG. 17 shows a detail view of the junction of slotted column  54  with base  56 . The reader will observe that slotted column  54  incorporates a series of slots  24 , inclined holes  26 , and insertion slots  36 . These are configured to allow the installation of removable rungs  38  on alternating sides, as shown in FIG.  16 . Thus, the application of the present invention to the ladder stand  52  allows the user to leave the ladder stand in place without any rungs  38  being present. 
     Observation stands using a tripodal support are also common in the field of hunting. The present invention could be applied to this type of stand by substituting slotted column  54  for one of the three support legs. As another example, a power company could use slotted column  54  to provide access to the top of a pole. Rather than leaning slotted column  54  against the pole, it would be bolted on directly. This would also allow the application of the device to non-cylindrical objects. With appropriate standoffs  20  to allow clearance for the user&#39;s foot, slotted column  54  could be bolted to a flat wall. Those skilled in the art will realize that a virtually infinite number of applications are possible. 
     Those skilled in the art will also realize that many types of locking mechanisms can be used to implement removable rung  38 . One alternative design is shown in FIG.  18 . First alternate step  62  is simply a straight rod having first alternate key  64 . Alternate stick  58  has slot  24 , transverse hole  60 , and alternate insertion slot  76 . First alternate step  62  is installed by placing it into transverse hole  60  (sliding first alternate key  64  through alternate insertion slot  76 ), and turning first alternate key  64  down within slot  24 . 
     FIG. 19 shows a second alternate embodiment. Insertion cylinder  42  and key  44  are the same as those shown in FIG.  6 . However, step  40  has been replaced by Z step  78 . FIG. 19B shows this second alternate embodiment installed in vertical support column  74 . 
     The previous embodiments provide security, but it is always possible that unauthorized persons may have the removable rungs and therefore be able to climb the unattended ladder. An additional security measure is therefore needed. FIG. 20 shows second alternate stick  80 . It is identical to slotted stick  22  except that it has a single open channel  92  in its front face instead of a series of slots  24 . Lock slide  82  rests within open channel  92 . It is free to slide up and down, but is retained by the fact that two retaining pins  84  are inserted through pin channels  88  in lock slide  82 . The sides of lock slide  82  open into a series of access cuts  86 . These access cuts  86  allow the user to insert removable rungs  38  when lock slide  82  is in its unlocked position. 
     FIG. 21 is a detail view showing lock slide  82  in its unlocked position. The reader will observe how lock slide  82  is free to move up and down by the interaction of pin channel  88  and retaining pin  84 . The reader will also observe how the alignment of access cuts  86  allows the insertion of removable rungs  38 . In FIG. 22, lock slide  82  has been pushed upward to its lock position. The solid side walls of lock slide  82  prevent the insertion of any removable rungs  38  when in this position. 
     Returning now to FIG. 20, the reader will observe that both second alternate stick  80  and lock slide  82  are pierced by a lock hole  90 . These two holes align when lock slide  82  is translated upward to its locked position. At that point, the user can insert a padlock or other locking device through the aligned lock holes  90 . When this is done, lock slide  82  will be secured in its locked position. 
     The reader will also observe that handle  94  is provided as part of lock slide  82 . When second alternate stick  80  is attached to a tree or other vertical object, handle  94  lies in a convenient position for the user to grab and manipulate lock slide  82 . While it is possible for the user to manipulate lock slide  82  without handle  94 , handle  94  does provide additional convenience. 
     Because the embodiment shown in FIGS. 20-22 provides additional security, it is the preferred embodiment. Having read the preceding descriptions, the reader will understand that this preferred embodiment: 
     1. Provides rungs which are easy to apply and remove; 
     2. Provides rungs which are light; 
     3. Provides rungs which are compact; and 
     4. Does not need a large interior cavity in its vertical support column. 
     Although the preceding description contains significant detail, it should not be construed as limiting the scope of the invention but rather as providing illustrations of the preferred embodiment of the invention. Thus, the scope of the invention should be fixed by the following claims, rather than by the examples given.

Summary:
A ladder incorporating a vertical support structure and a set of removable rungs. The vertical support structure is formed by stacking a series of interlocking sticks. The sticks incorporate features allowing the removable installation of the rungs. The rungs are small and light, so that a set of such rungs may be easily carried by a user. The vertical sticks generally remain attached to the object to be climbed—such as a tree or pole. When the user wishes to climb the object, the user installs the rungs as he or she climbs. The rungs are then removed upon descent. In this manner, unauthorized climbing of the object is inhibited. Locking features are also included to prevent unauthorized access by another person having a set of removable rungs. 
     Several different embodiments of the removable rungs are disclosed. The common feature of all these embodiments is the fact that the rungs cannot come loose from the vertical stick while they are under load. The application of the invention to different types of hunting tree stands is also disclosed in detail, although the application of the invention extends far beyond hunting products.