Patent Publication Number: US-7900745-B1

Title: Motor driven lifting assembly

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     Not Applicable. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable. 
     REFERENCE TO A MICROFICHE APPENDIX 
     Not Applicable. 
     BACKGROUND OF THE INVENTION 
     1. Technical Field 
     This invention relates to platforms for ladders and, more particularly, to a motor driven lifting assembly for assisting a user to raise and lower an air-conditioning unit above a ground surface and along a ladder. 
     2. Prior Art 
     It is well known that ladders have been commonly used in order to carry a person or worker up or down along a building wall or other structure. Extreme difficulty is often encountered in lifting heavy objects to the top of a house, to a second or higher story of a building, or onto a roof. This can be accomplished with a crane, but the expense of using a crane is often prohibitive. For example, it would be too expensive to rent a crane in order to install a typical air conditioning unit on the roof of a private residence. In addition, it would be difficult and expensive to transport a large piece of equipment such as a crane to a job site. 
     One prior art example shows a lift apparatus for use with a ladder having two spaced upright stringers and a plurality of horizontal rungs bridging the stringers and spaced at a distance vertically from one another. The lift apparatus comprises a frame, a prime mover mounted on the frame and having a rotatable output shaft, and at least one pair of sprocket systems spaced apart vertically from each other. At least one of the sprocket systems is operatively coupled with the output shaft of the prime mover. An endless driving force transmitting mechanism can pass around the pair of the sprocket systems. A controller is provided for controlling the prime mover in such a manner as to selectively rotate the output shaft thereof in forward or reverse direction, or stop it. An operating mechanism is provided for causing a plurality of gripper elements to be engaged with the corresponding rungs of the ladder. The gripper elements are carried by the endless driving force transmitting mechanism and spaced at a distance substantially corresponding to that of the rungs of the ladder. A platform is secured to the frame for supporting an operator thereon. Unfortunately, this prior art example is designed to raise and lower an individual and is not designed for use in raising and lowering heavy inanimate objects. 
     Another prior art example shows a raising system for scaffolding, elevator cabins and the like which comprises a vertical stationary beam member from one side of which equally vertically spaced similar rack teeth are protruding, each tooth having a substantially horizontal straight top face, a framework guided for up and down movement along said beam member, a power driven shaft rotatably supported by said framework and carrying spaced radial supports, a series of parallel idle shafts extending between the supports, equally spaced from one another and radially equally spaced from the driven shaft, and a plurality of cylindrical roller assemblies rotatably mounted on the idle shafts and adapted to come in successive rolling engagement with the top faces of said teeth during upward or downward movement of said framework. Each roller assembly includes several rollers disposed side by side. Unfortunately, this prior art example is not designed for use in raising and lowering a heavy inanimate object in cooperation with a ladder. 
     Accordingly, a need remains for a system in order to overcome the above-noted shortcomings. The present invention satisfies such a need by providing an assembly that is convenient and easy to use, is lightweight yet durable in design, and assists a user to raise and lower an air-conditioning unit above a ground surface. The assembly allows a user to securely place a ladder upon a ground surface and safely raise an air-conditioning unit to a desired height easily and efficiently. Such an assembly allows an individual user to do the work of two or more people. The assembly greatly reduces the possibility of injury to an individual, or damage to an object, and allows a user to accomplish a task more quickly. The present invention is simple to use, inexpensive, and designed for many years of repeated use. 
     BRIEF SUMMARY OF THE INVENTION 
     In view of the foregoing background, it is therefore an object of the present invention to provide an assembly for a motor driven lifting assembly. These and other objects, features, and advantages of the invention are provided by a motor driven lifting assembly for assisting a user to raise and lower an air-conditioning unit above a ground surface. 
     The assembly includes a ladder, a plurality of fastening members removably and pivotally attached to an upper end of the ladder, and a plurality of wheels advantageously attached to a lower end of the ladder. Such fastening members adjustably attach the upper end of the ladder to a support surface during operating conditions such that a more stable environment is effectively created while raising and lowering the platform (herein described below). 
     The assembly further includes a platform for supporting the air-conditioning unit, and the platform is conveniently located at an anterior face of the ladder. Such a platform includes a planar support member that has top and bottom surfaces. Such a support member is pivotally adjustable such that an air-conditioning unit placed upon the top surface is effectively maintained along a substantially horizontal plane above a ground surface while traveling along the longitudinal length of the ladder. The support member further has top and bottom ends oriented perpendicular to the longitudinal length of the ladder, and pivots about the top end. 
     The platform further includes first, second, third, and fourth planar panels. Such first and second panels have top ends attached to the top end of the support member, and further have bottom surfaces advantageously spaced from the anterior face of the ladder when the platform is attached to the ladder. Such third and fourth panels have top surfaces advantageously spaced from a posterior face of the ladder when the platform is attached to the ladder and spaced from the bottom surfaces of the first and second panels. The first, second, third, and fourth panels are substantially similar in shape and size and have outside edges effectively extending beyond a lateral width of the ladder. 
     The platform further includes a plurality of rectilinear spacers having axially opposed top and bottom ends. Such top ends of the spacers are statically attached to the outside edges of the first and second panels respectively, while such bottom ends of the spacers are statically attached to the outside edges of the third and fourth panels respectively. The spacers effectively maintain the first, second, third, and fourth panels rigidly affixed and equidistantly offset from the ladder during operating conditions. 
     The platform further includes a plurality of wheels directly attached to the spacers respectively, and further advantageously attached to the bottom surface of the first and second panels and the top surface of the third and fourth panels respectively. Such wheels are conveniently located at opposed outside corners of the first, second, third, and fourth panels, and directly contact the anterior and posterior faces of the ladder respectively when the platform is attached thereto. 
     The platform includes a plurality of telescopically adjustable shafts that have opposed ends pivotally attached to the bottom surface of the support member and the top surfaces of the first and second panels respectively. Such shafts are conveniently located at the opposed outside corners of the bottom ends of the support member and the first and second panels respectively. The shafts have a plurality of apertures formed along a length thereof such that the apertures effectively form a continuous passageway through a diameter of the shafts. A plurality of cotter pins is removably interfitted within the apertures. 
     The assembly further includes a mechanism for raising and lowering the platform such that the platform is linearly adjustable along an entire longitudinal length of the ladder. Such a raising and lowering mechanism is advantageously anchored in close proximity to the lower end of the ladder such that the raising and lowering mechanism is easily operable by the user. The platform raising and lowering mechanism includes a housing that has a user interface conveniently formed within a top surface thereof, and a motor enclosed within the housing. 
     The platform raising and lowering mechanism further includes a driveshaft that has a first end directly connected to the motor and a second end extending outwardly from the housing and disposed exterior thereof. Such a driveshaft has a longitudinal axis oriented parallel with the bottom end of the support member, and further has a spool monolithically formed with the second end of the driveshaft. A remote device effectively operates the motor from a distance, and a power cord is included. 
     The platform raising and lowering mechanism further includes a rectilinear rod effectively spanning along an entire interior width of the ladder. Such a rod is advantageously oriented perpendicular to a longitudinal length of the ladder and further is conveniently anchored in close proximity to the upper end of the ladder. A grooved wheel is attached to a center portion of the rod and is freely rotatable thereabout. 
     The platform raising and lowering mechanism further includes a flexible cord nested within the grooved wheel. Such a cord has a first end adjustably attached directly to the spool and further has a second end statically attached to the platform. The cord travels along a first linear path defined along the anterior face of the ladder and further travels along a second linear path defined along a posterior face of the ladder. Such first and second linear paths are advantageously registered parallel to each other. 
     The wheels of the assembly have a diameter greater than a distance between the anterior and posterior faces of the ladder such that the lower end of the ladder effectively remains elevated above the ground surface after the upper end of the ladder is adapted towards the ground surface. 
     A method for raising and lowering an air-conditioning unit above a ground surface includes the steps of providing a ladder, removably and pivotally attaching a plurality of fastening members to an upper end of the ladder, attaching a plurality of wheels to a lower end of the ladder, supporting the air-conditioning unit on a platform located at an anterior face of the ladder; and raising and lowering the platform such that the platform is linearly adjustable along an entire longitudinal length of the ladder. The raising and lowering mechanism is anchored in close proximity to the lower portion of the ladder such that the raising and lowering mechanism is easily operable by the user. 
     The method further includes the steps of providing a housing that has a user interface formed within a top surface thereof, providing a motor enclosed within the housing, providing a driveshaft that has a first end directly connected to the motor and a second end extending outwardly from the housing and disposed exterior thereof, providing a remote device for operating the motor from a distance, and providing a power cord. The driveshaft has a longitudinal axis oriented parallel with the bottom edge of the support member, and further has a spool monolithically formed with the second end of the driveshaft. 
     The method further includes the steps of spanning a rectilinear rod along an entire interior width of the ladder, attaching a grooved wheel to a center portion of the rod and freely rotatable thereabout, and nesting a flexible cord within the grooved wheel. The rod is oriented perpendicular to a longitudinal length of the ladder and further is anchored in close proximity to the upper end of the ladder. The cord has a first end adjustably attached directly to the spool and further has a second end statically attached to the platform. The cord travels along a first linear path defined along the anterior face of the ladder and further travels along a second linear path defined along a posterior face of the ladder. The first and second linear paths are registered parallel to each other. 
     There has thus been outlined, rather broadly, the more important features of the invention in order that the detailed description thereof that follows may be better understood, and in order that the present contribution to the art may be better appreciated. There are additional features of the invention that will be described hereinafter and which will form the subject matter of the claims appended hereto. 
     It is noted the purpose of the foregoing abstract is to enable the U.S. Patent and Trademark Office and the public generally, especially the scientists, engineers and practitioners in the art who are not familiar with patent or legal terms or phraseology, to determine quickly from a cursory inspection the nature and essence of the technical disclosure of the application. The abstract is neither intended to define the invention of the application, which is measured by the claims, nor is it intended to be limiting as to the scope of the invention in any way. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
       The novel features believed to be characteristic of this invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and method of operation, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawings in which: 
         FIG. 1  is a perspective view of the motor driven lifting assembly, in accordance with the present invention; 
         FIG. 2  is a cross sectional view of the platform shown in  FIG. 1 , taken along  2 - 2 ; 
         FIG. 3  is a perspective view of the raising and lowering mechanism and the wheels respectively; 
         FIG. 4  is a perspective view of the shafts; 
         FIG. 5  is a perspective view of the rod and the grooved wheel respectively; 
         FIG. 6  is an exploded perspective view of the assembly showing the raising and lowering mechanism, the wheels, the platform, the ladder, and the fastening mechanism respectively; 
         FIG. 7  is a cross sectional view of a track and a wheel, taken along line  7 - 7 ; 
         FIG. 8  is a perspective view of the assembly shown attached to a support surface and with an air-conditioning unit placed upon the platform; and 
         FIG. 9  is a schematic block diagram of the raising and lowering mechanism. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which a preferred embodiment of the invention is shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiment set forth herein. Rather, this embodiment is provided so that this application will be thorough and complete, and will fully convey the true scope of the invention to those skilled in the art. Like numbers refer to like elements throughout the figures. 
     The assembly of this invention is referred to generally in  FIGS. 1-9  by the reference numeral  10  and is intended to provide a motor driven lifting assembly. It should be understood that the assembly  10  may be used to raise and lower many different types of objects and should not be limited in use to raising and lowering only those types of objects described herein. 
     Referring to  FIGS. 1 ,  3 ,  5 ,  6 , and  8 , the assembly  10  includes a ladder  20 , a plurality of fastening members  21  removably and pivotally attached to an upper end  22  of the ladder  20 , and a plurality of wheels  23  attached to a lower end  24  of the ladder  20 . The ladder  20  is of a type commercially available to the public and well known within the industry. Such fastening members  21  adjustably attach the upper end  22  of the ladder  20  to a support surface  11  during operating conditions, which is essential such that a more stable environment is created while raising and lowering the platform  30  (herein described below). For example, by attaching the fastening members  21  to a support surface  11  while an air-conditioning unit  12  is being raised and lowered, a user reduces the possibility of the upper end  22  of the ladder  20  prematurely and undesirably disengaging from the support surface  11  and thereby causing injury to an individual, or damage to the air-conditioning unit  12  or support surface  11 . 
     Referring to  FIGS. 1 ,  2 ,  6  and  8 , the assembly  10  further includes a platform  30 , for supporting the air-conditioning unit  12 , located at an anterior face  25  of the ladder  20 . Such a platform  30  includes a planar support member  26  that has top  27  and bottom  28  surfaces. The platform  30  is suitably sized and shaped to accommodate a wide range of commercially produced air-conditioning units  12  readily available to the public. Such a support member  26  is pivotally adjustable, which is critical such that an air-conditioning unit  12  placed upon the top surface  27  of the support member  26  is maintained along a substantially horizontal plane above a ground surface while traveling along the longitudinal length of the ladder  20 . Such adjustability allows a user to maintain the air-conditioning unit  12  at a stable horizontal position during operating conditions regardless of the angle of incline of the ladder  20 . The support member  26  has top  29  and bottom  35  ends oriented perpendicular to the longitudinal length of the ladder  20 , and pivots about the top end  29 . 
     Referring to  FIGS. 2 and 6 , the platform  30  further includes first  31 , second  32 , third  33 , and fourth  34  planar panels. Such first and second panels  31 ,  32  have top ends  36  attached to the top end  29  of the support member  26 , and further have top surfaces  65  facing the bottom surface  28  of the support member  26  and bottom surfaces  37  spaced from the anterior face  25  of the ladder  20  when the platform  30  is attached to the ladder  20 . Such third and fourth panels  33 ,  34  have top surfaces  38  spaced from a posterior face  39  of the ladder  20  when the platform  30  is attached to the ladder  20  and spaced from the bottom surfaces  37  of the first and second panels  31 ,  32 . The first, second, third, and fourth panels  31 ,  32 ,  33 ,  34  are substantially similar in shape and size and have outside edges  40  extending beyond a lateral width of the ladder  20 , and have bottom ends  66  oriented parallel to the bottom end  35  of the support member  26 . 
     Referring to  FIGS. 1 and 8 , the platform  30  further includes a plurality of rectilinear spacers  41  having axially opposed top  42  and bottom  43  ends. Such top ends  42  of the spacers  41  are statically attached to the outside edges  40  of the first and second panels  31 ,  32  respectively, while such bottom ends  43  of the spacers  41  are statically attached to the outside edges  40  of the third and fourth panels  33 ,  34  respectively. Placement of the spacers  41  on the outside edges  40  of the first, second, third, and fourth panels  31 ,  32 ,  33 ,  34  respectively prevents the spacers  41  from impeding the linear motion of the platform  30  along the longitudinal length of the ladder  20  during raising and lowering of the air-conditioning unit  12 . The spacers  41  maintain the first, second, third, and fourth panels  31 ,  32 ,  33 ,  34  rigidly affixed and equidistantly offset from the ladder  20  during operating conditions. Further, the spacers  41  maintain the first, second, third, and fourth panels  31 ,  32 ,  33 ,  34  in a rigid spatial relationship, which is crucial for providing a sufficiently strong construction that allows safe and secure placement of an air-conditioning unit  12  on the support member  26 . 
     Referring to  FIGS. 1 ,  2  and  7  the platform  30  further includes a plurality of wheels  44  directly attached to the spacers  41  respectively, without the use of intervening elements, and further attached to the bottom surface  37  of the first and second panels  31 ,  32  and the top surface  38  of the third and fourth panels  33 ,  34  respectively. Such wheels  44  are located at opposed outside corners  45  of the first, second, third, and fourth panels  31 ,  32 ,  33 ,  34 , and directly contact the anterior and posterior faces  25 ,  39  of the ladder  20  respectively, without the use of intervening elements, when the platform  30  is attached thereto. The wheels  44  allow the platform  30  to be raised and lowered with a minimum of effort, as well as a minimum of friction, thereby prolonging the usable life of the wheels  44  and reducing the damage that would occur to the first, second, third, and fourth panels  31 ,  32 ,  33 ,  34  and the anterior and posterior faces  25 ,  39  of the ladder  20  respectively if the wheels  44  were not present. 
     Again referring to  FIGS. 2 and 7 , the platform  30  further includes a plurality of linear tracks  70  directly attached to the bottom surface  37  of the first and second panels  31 ,  32  respectively, and the top surface  38  of the third and fourth panels  33 ,  34  respectively. Such tracks  70  are oriented parallel to the longitudinal length of the ladder  20  and adjacent to the outside edges  40  of the first, second, third, and fourth panels  31 ,  32 ,  33 ,  34  respectively. The wheels  44  interfit within the tracks  70  and freely rotate therein while the platform  30  is being raised and lowered. The tracks  70  provide a guide for the wheels  44  during operating conditions, which is crucial for maintaining the wheels  44  in a travel path oriented along the longitudinal length of the ladder  20 . 
     Referring to  FIG. 4 , the platform  30  further includes a plurality of telescopically adjustable shafts  46  that have opposed ends  47  pivotally attached to the bottom surface  28  of the support member  26  and the top surfaces  65  of the first and second panels  31 ,  32  respectively. Such shafts  46  are located at the opposed outside corners  45  of the bottom ends  35 ,  66  of the support member  26  and the first and second panels  31 ,  32  respectively. The shafts  46  are adjustable, which is vital such that the support member  26  can be maintained at a horizontal position regardless of the angle of incline of the ladder  20 . In addition, the shafts  46  stabilize the support member  26  while the platform  30  is being raised and lowered. 
     Again referring to  FIG. 4 , the shafts  46  have a plurality of apertures  48  formed along a length thereof, which is necessary such that the apertures  48  form a continuous passageway  49  through a diameter of the shafts  46 . A plurality of cotter pins  50  is removably interfitted within the apertures  48 . Such cotter pins  50  are inserted into the apertures  48  for maintaining the shafts  46  at a substantially stable position and preventing the support member  26  from prematurely biasing toward the first and second panels  31 ,  32  during operating conditions. The placement of the apertures  48  allows a user to extend and secure the shafts  46  along the entire longitudinal length thereof. 
     Referring to  FIGS. 1 ,  3 ,  6 ,  8  and  9 , the assembly  10  further includes a mechanism  51  for raising and lowering the platform  30 , which is essential such that the platform  30  is linearly adjustable along an entire longitudinal length of the ladder  20 . Such a raising and lowering mechanism  51  allows a user to efficiently move an air-conditioning unit  12  from the lower end  24  of the ladder  20  to the upper end  22  of the ladder  20 , or from the upper end  22  of the ladder  20  to the lower end  24  of the ladder  20  as desired. The raising and lowering mechanism  51  is anchored in close proximity to the lower end  24  of the ladder  20 , which is critical such that the raising and lowering mechanism  51  is easily operable by the user while the user is standing on a ground surface  13  and in close proximity to the lower end  24  of the ladder  20 . The platform raising and lowering mechanism  51  includes a housing  52  that has a user interface  53  formed within a top surface thereof, a motor  54  enclosed within the housing  52 , and a receiver  75  connected to the motor  54 . 
     Again referring to  FIGS. 1 ,  3 ,  6 ,  8  and  9 , the platform raising and lowering mechanism  51  further includes a driveshaft  55  that has a first end directly connected to the motor  54 , without the use of intervening elements, and a second end extending outwardly from the housing  52  and disposed exterior thereof. Such a driveshaft  55  has a longitudinal axis oriented parallel with the bottom end  35  of the support member  26 , and further has a spool  58  monolithically formed with the second end of the driveshaft  55 . A remote device  59  operates the motor  54  from a distance when a user is not in close proximity to the lower end  24  of the ladder  20 , and a power cord  60  is included. 
     Referring to  FIGS. 1 ,  5 ,  6  and  8 , the platform raising and lowering mechanism  51  further includes a rectilinear rod  61  spanning along an entire interior width of the ladder  20 . Such a rod  61  is oriented perpendicular to a longitudinal length of the ladder  20  and further is anchored in close proximity to the upper end  22  of the ladder  20 . Such placement of the rod  61  near the upper end  22  of the ladder  20  allows a user to raise the platform  30  as high as possible during operating conditions. A grooved wheel  62  is attached to a center portion of the rod  61  and is freely rotatable thereabout. 
     Again referring to  FIGS. 1 ,  5 ,  6  and  8 , the platform raising and lowering mechanism  51  further includes a flexible cord  63  nested within the grooved wheel  62 . Such a cord  63  has a first end adjustably attached directly to the spool  58 , without the use of intervening elements, and further has a second end statically attached to the platform  30 . The flexibility of the cord  63  allows the cord  63  to be rotationally stored on the spool  58 , and looped about the grooved wheel  62 , without premature breakage of the cord  63  during raising and lowering of the platform  30 . The cord  63  travels along a first linear path defined along the anterior face  25  of the ladder  20  and further travels along a second linear path defined along a posterior face  39  of the ladder  20 . Such first and second linear paths are registered parallel to each other, and further are registered in parallel with the longitudinal length of the ladder  20 , which is critical for preventing the platform  30  from shifting laterally during operating procedures and impeding the linear movement of the platform  30  during operating conditions. 
     Referring to  FIGS. 1 ,  3 ,  6  and  8 , the wheels  23  of the assembly  10  have a diameter greater than a distance between the anterior and posterior faces  25 ,  39  of the ladder  20 , which is necessary such that the lower end  24  of the ladder  20  remains elevated above the ground surface after the upper end  22  of the ladder  20  is adapted towards the ground surface  13 . For instance, when a user desires to move the ladder  20  from one location to another, the user lowers the upper end  22  of the ladder  20  toward the ground surface and thereby raises the lower end  24  of the ladder  20  above the ground surface such that the lower end  24  of the ladder  20  is spaced from the ground surface by the wheels  23 . The user then pushes or pulls the ladder  20  to the desired location. 
     The ability of a user to pivotally adjust the support member  26  of the platform  30  provides the unexpected benefit of allowing a user to maintain an air-conditioning unit  12  placed thereon in a substantially horizontal plane during operating conditions regardless of the angle of incline of the ladder  20 . In addition, the raising and lowering mechanism  51  allows a user to operate the assembly  10  while standing in close proximity to the ladder  20 , or alternatively, from a distance via the remote device  59 . Also, the wheels  23  allow a user to easily move the ladder  20  from one location to another without the assistance of another, thereby overcoming the shortcomings of the prior art. 
     In use, a method for raising and lowering an air-conditioning unit  12  above a ground surface includes the steps of providing a ladder  20 , removably and pivotally attaching a plurality of fastening members  21  to an upper end  22  of the ladder  20 , attaching a plurality of wheels  23  to a lower end  24  of the ladder  20 , supporting the air-conditioning unit  12  on a platform  30  located at an anterior face  25  of the ladder  20 , and raising and lowering the platform  30  such that the platform  30  is linearly adjustable along an entire longitudinal length of the ladder  20 . The raising and lowering mechanism  51  is anchored in close proximity to the lower end  24  of the ladder  20  such that the raising and lowering mechanism  51  is easily operable by the user. 
     The method further includes the steps of providing a housing  52  that has a user interface  53  formed within a top surface thereof, providing a motor  54  enclosed within the housing  52 , providing a receiver  75  connected to the motor  54 , providing a driveshaft  55  that has a first end directly connected to the motor  54 , without the use of intervening elements, and a second end extending outwardly from the housing  52  and disposed exterior thereof, providing a remote device  59  for operating the motor  54  from a distance, and providing a power cord  60 . The driveshaft  55  has a longitudinal axis oriented parallel with the bottom end  35  of the support member  26 , and further has a spool  58  monolithically formed with the second end of the driveshaft  55 . 
     The method further includes the steps of spanning a rectilinear rod  61  along an entire interior width of the ladder  20 , attaching a grooved wheel  62  to a center portion of the rod  61 , and nesting a flexible cord  63  within the grooved wheel  62 . The rod  61  is oriented perpendicular to a longitudinal length of the ladder  20  and further is anchored in close proximity to the upper end  22  of the ladder  20 . The cord  63  has a first end adjustably attached directly to the spool  58 , without the use of intervening elements, and a second end statically attached to the platform  30 , and converts a rotating force generated by the motor  54  and transferred to the spool  58  into a lifting force applied to the platform  30 . The cord  63  travels along a first linear path defined along the anterior face  25  of the ladder  20  and further travels along a second linear path defined along a posterior face  39  of the ladder  20 . The first and second linear paths are registered parallel to each other. 
     While the invention has been described with respect to a certain specific embodiment, it will be appreciated that many modifications and changes may be made by those skilled in the art without departing from the spirit of the invention. It is intended, therefore, by the appended claims to cover all such modifications and changes as fall within the true spirit and scope of the invention. 
     In particular, with respect to the above description, it is to be realized that the optimum dimensional relationships for the parts of the present invention may include variations in size, materials, shape, form, function and manner of operation. The assembly and use of the present invention are deemed readily apparent and obvious to one skilled in the art.