Abstract:
A hand truck includes an automatic height adjusting load platform, or alternatively, a kit of components provides for adding an adjustable load platform to an existing hand truck. The load platform position is held in a raised position while under no load conditions. A user places an article including a package on the raised platform, which is configured to rest at approximately waist level. In response to the weight of the article, the platform lowers, but the downward motion of the platform is inhibited by a platform arrestor. Preferably, the platform arrestor is configured such that the distance the platform will drop due to the weight of the load is proportional to the height of the article placed upon the platform. Thus the top of the article first placed upon the load platform is now substantially at waist level. This process can continue until the platform is lowered to the bottom of the shaft that the platform is slidably mounted to. Further, as articles are removed, under normal conditions, or where the load becomes unstable due to tipping or the like, the platform arrestor will control the rate at which the platform returns to its raised position. The load platform is further detachable for applications where having the platform in a raised position is undesirable.

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field of the Invention 
     The present invention relates in general to hand truck, and more particularly to a hand truck with an automatic height adjusting load platform and a kit for modifying a hand truck to include an automatic height adjusting load platform. 
     2. Description of the Related Art 
     Hand trucks and other hand operated material moving devices have been widely used for numerous years. A hand truck is usually comprised of a substantially vertical frame, a load platform or shelf secured to the bottom portion of the frame that extends outward and perpendicular to the frame, and wheels generally placed near the load platform and on the opposite side of the frame as the load platform. A user slides the load platform under an object to be moved, or alternatively sets an object upon the load platform, then rotates the vertical frame such that the load platform is raised into the air and only the wheels contact the surface immediately below the hand truck. By balancing the hand truck on its wheels, an operator can easily move an otherwise heavy object from one position to another. 
     While the hand truck enjoys popular success today, it has been recognized by the present inventor that a user must bend or stoop to place objects or remove objects from the load platform. The bending or stooping motion can be eliminated or reduced with an automatic mechanism incorporated into the hand truck such that the height of the load platform automatically adjusts with reference to; the weight and height of the material loaded thereupon. The present inventor has further recognized that a load placed on a hand truck can be of significant weight. Should a load become unstable either due to tipping or the like, a sudden reduction in weight on the load platform could cause the platform to raise too quickly. 
     The present inventor has additionally recognized that an adjustable shelf in a raised position may be awkward for certain transportation needs. For example, it is not uncommon to secure hand trucks to the exterior of a vehicle by wedging the load platform into a predetermined position. This is difficult or impossible to accomplish with a raised platform. 
     Accordingly, there is a need for a hand truck with an automatic height adjusting load platform that incorporates a braking mechanism to prevent the load platform from transitioning too quickly due to sudden changes in load. Preferably, the load platform should be removable and repositionable. Further, the adjustable load platform should optionally be in kit form for retrofitting currently existing hand trucks. 
     SUMMARY OF THE INVENTION 
     This need is met by the present invention wherein a hand truck includes a automatic height adjusting load platform. A shelf guide secured between the frame upper and lower portions provides a substantially vertical shaft positioned between the vertical frame members of the hand truck. The load platform is slidably mounted to the shaft and interconnected to a platform arrestor for controlling the variable shelf position. The load platform position is held in a raised position while under no load conditions by the platform arrestor. For example, one of many possible options is to configure the load platform to rest at approximately waist level. A user places an article such as a package or carton on the raised platform. In response to the weight of the article, the platform lowers, but the downward motion of the platform is inhibited by the platform arrestor. Preferably, the platform arrestor is configured such that the distance the platform will drop due to the weight of the load is proportional to the height of the article placed upon the platform. Of course this presupposes that the hand truck will be used to haul items of consistent dimension and weight, but in industries such as beverage distribution, this is often the case. Thus the top of the article first placed upon the load platform is now substantially at the unloaded, load platform level. This process can continue until the platform is lowered to the bottom of the shaft that the platform is slidably mounted to. If the load platform was initially at waist level, and the proper size and weight items were being loaded, the user is never required to bend or stoop. As, articles are removed, under normal conditions, the platform arrestor will control the rate at which the platform returns to its raised position. Further, where the load becomes unstable due to tipping or the like, the platform arrestor prevents the load platform from transitioning upward too quickly. The load platform is further detachable for applications where having the platform in a raised position is undesirable. 
     In accordance with one aspect of the present invention, a hand truck having a substantially vertical frame of spaced parallel bars includes a toe platform extending substantially outward from the frame, and wheels connected to the frame lower portion opposite the toe platform. A shelf guide is secured to the frame to provide a shaft. A load platform slidably mounts to the shaft to provide a surface for placing articles. A platform arrestor is secured to the frame lower portion, for example along the shaft axle, and provides a control to adjust the position of the adjustable load platform. The platform arrestor is connect further, to a pulley assembly at its upper portion. A cable connected between the load platform and the shelf guide upper portion extends over the shelf guide upper portion, downward and through the pulley, and upward to the shelf guide upper portion where it is secured. At least one pulley is further included on the shelf guide upper portion to guide the cable. 
     The shelf guide of the hand truck includes top mounting bracket secured to the frame between the spaced parallel bars and includes at least one bracket pulley. The shelf guide lower portion is secured to the frame lower portion, for example, by a bracket mounted to the toe plate. A substantially vertical shaft is secured between the shelf guide upper portion via the top mounting bracket, and the shelf guide lower portion for engagement with the slidable load platform. It is preferable that the hand truck includes a pair of stabilizing bars placed one on either side of the shaft to aid in the alignment of larger sized loads. The load platform further includes a shelf plate slidably mounted to the shelf guide and a platform releasably secured to the shelf plate. 
     Preferably, the platform arrestor includes one or more springs connected between the frame lower portion and the first pulley, and additionally, one or more spring brakes connected between the frame lower portion and the first pulley. The springs may be optionally releasably connected so as to be able to adjust for various loading conditions. The brake can comprise a hydraulic cylinder for example positioned inside the spring. 
     In accordance with another aspect of the present invention, a kit of components is provided for adding an adjustable load platform to a host hand truck. The host hand truck has in its original configuration, a substantially vertical frame having spaced parallel bars, a frame upper portion and a frame lower portion. The frame lower portion includes a toe platform extending substantially outward from the frame and one or more wheels connected to said frame lower portion opposite said toe platform. The kit of components comprises a shelf guide having a shelf guide upper portion and a shelf guide lower portion. The shelf guide upper portion is secured to the frame upper portion of the host hand truck, while the shelf guide lower portion is secured to the frame lower portion of the host hand truck. Further, a load platform is slidably mounted to the shelf guide. A platform arrestor having a platform arrestor first end and a platform arrestor second end is included for controlling the shelf height. The platform arrestor is connected to the host hand truck frame lower portion. A first pulley is connected to the platform arrestor first end. To attach the load platform to the platform arrestor on the host hand truck, a cable is connected between the load platform and the shelf guide upper portion. The cable extends over the shelf guide upper portion, downward and through the first pulley which is connected to the upper portion of the platform arrestor, and upward terminating by attachment to the shelf guide upper portion. The kit further includes a pair of stabilizing bars connected between the frame upper portion and the frame lower portion for balancing loads held in a raised position. 
     It is an object of the present invention to provide a hand truck having a height adjustable load platform that is initially in a raised position, and automatically adjusts to the weight of the objects placed upon it. 
     It is another object of the present invention to provide a hand truck that includes a braking device to prevent sudden oscillations or velocity changes in the load platform due to sudden changes in loading. 
     It is yet another object of the present invention to provide a hand truck that allows the height adjustable load platform to be repositionable or removable. 
     It is yet another object of the present invention to provide a hand truck kit that allows a height adjustable load platform to be retrofitted to conventional hand trucks. 
     Other objects of the present invention will be apparent in light of the description of the invention embodied herein. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     The following detailed description of the preferred embodiments of the present invention can be best understood when read in conjunction with the following drawings, where like structure is indicated with like reference numerals and in which: 
     FIG. 1 is an illustration of key component parts laid out for clarity and includes the components comprising the hand truck kit; 
     FIG. 2 is an illustration of a side view of the hand truck with the platform in a raised position; 
     FIG. 3 is an illustration of a front view of the hand truck with the platform in a raised position; 
     FIG. 4 is an illustration of a rear view of the hand truck with the platform in a raised position; 
     FIG. 5 is an illustration of a side view of the hand truck with the platform loaded with a first item; 
     FIG. 6 is an illustration of a side view of the hand truck with the platform loaded with a second item; 
     FIG. 7 is an illustration of a typical platform arrestor; 
     FIG. 8 is an illustration of the shelf plate assembly; and 
     FIG. 9 is an illustration of the load platform from a top view, the dashed lines indicating various positions of the platform engaged with the shelf plate. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The Hand Truck 
     Referring to FIG.  2  through FIG. 4, a typical hand truck  100  with a adjustable load platform is illustrated. The typical hand truck  100  is formed from a substantially vertical frame consisting of one or more substantially horizontal support bars  126  spanning between a pair of substantially vertical supports  128 . The hand truck usually includes a handle  130  connected to the vertical frame along the top portion to provide a convenient position for a user to grasp the hand truck. A toe platform  108  usually extends from, and is considered part of the lower portion of hand truck  100 . Toe platform  108  consists of a plate, typically rectangular in dimension that extends outward from the hand truck  100  substantially horizontally, and as close to the ground surface as possible. With the garden variety of hand truck, a user would place articles to be transported upon toe platform  108 . In addition to providing a shelf to place articles, the toe platform  108  provides a convenient way to balance the hand truck  100  while in a stationary position. The present invention however, includes a load platform or load platform assembly  103  that traverses vertically so that a user need not bend or stoop to load hand truck  100 . As can be seen in FIG.  5  and FIG. 6, the load platform assembly  103  is capable of suspending articles in a raised position. Depending upon the intended load for the hand truck  100 , it may be desirable to adjust toe plate  108  such that the hand truck  100  maintains balance equally well while loaded and unloaded. Whereas toe plate  108  is typically perpendicular to the frame of hand truck  100 , adjusting the angle may be necessary. Adjusting the toe plate by 5 to 10 degrees may better balance the hand truck when loaded, while still maintaining a good balance unloaded, however this is dependant upon the desired load. Additionally, wheels  132  are included, usually secured to an axle extending horizontally through the frame near ground level. Finally, while not required, support bars  134 , often referred to as “stair climbers” are optionally included on the back side of the frame. Support bars  134  provide a support member to aid in navigating stepped surfaces such as stair steps and the like. 
     The load platform assembly  103  engages the hand truck  100  so as to be freely  10 slidable. It can be seen that the load platform assembly  103  could be assembled so as to slide up and down a pair of vertically spaced bars, for example the vertical frame bars of hand truck  100 . However, if the load is not evenly distributed, the load platform assembly  103  may bind on one side making the load unstable. To avoid this occurrence, the load platform assembly  103  traverses along a single shaft  120 . Shaft  120  is positioned approximately midway between the substantially vertical support bars  128 . Support bars  128  form the frame of hand truck  100 . Shaft  120  is secured at its lower end to the lower portion of the frame of hand truck  100 . For example, shaft  120  can secure directly to toe platform  108  using any suitable connection including welding, bracketing, screwing or bolting. An example of a suitable bracket would include toe bracket  107  as illustrated in FIG. 1 mounted to toe plate  108 . Toe bracket  107  includes a bore extending therethrough for receiving the lower end of shaft  120 . Toe bracket  107  is secured to toe plate  108  using any suitable means including bolting, screwing or welding. Shaft  120  is secured at its top end using top bracket assembly  102  and can be secured using means similar to those used to secure the bottom of the shaft. The length of shaft  120  is chosen to provide the desired range of travel of the load platform assembly  103 . A typical dimension for the shaft  120  would be around 39½ inches, but any length can be chosen. Further, the circumference of the shaft  120  and the material of construction thereof, can vary depending on the weight requirements of a particular application. Preferably, for light to medium duty applications involving loads of up to 200 pounds, the shaft  120  should be a ¾ inch to ⅝ inch hardened steel bar. One factor to consider when selecting the material for shaft  120  is the weight of the load. If the load is excessive for the shaft  120 , it is possible for deflection of the shaft  120  to occur. Deflection should be kept as minimal as possible. While the load platform assembly  103  of the present invention avoids binding and the like due to shaft  120  deflection, excessive deflection of shaft  120  could lead to early wear. 
     The load platform assembly  103  traverses along shaft  120  according to the load placed upon the hand truck  100 . Referring to FIG. 8, an example of a suitable method for connecting the load platform assembly  103  to shaft  120  is illustrated. The load platform assembly  103  consists of a shelf plate  104  which engages bar  120  using linear pillow block bearings  136 . Linear pillow block bearings  136  slip over shaft  120  prior to securing either end of shaft  120  to hand truck  100 . Preferably, two linear pillow block bearings  136  are bolted onto shelf plate  104  vertically bisecting shelf plate  104  and spaced apart such that a first linear pillow block bearing  136  is positioned near the top, center of shelf plate  104 , while a second linear pillow block bearing  136  is positioned near the bottom center of shelf plate  104 . Optionally, pillow block mount adaptor  134  can be inserted between shelf plate  104  and linear pillow block bearings  136 . Pillow block mount adaptor  134  is used to adjust the tilt of the platform or shelf  106  relative to bar  120 . The angle of the pillow block mount adaptor  134  can be adjusted to any suitable angle depending upon the intended load. A typical example would be in the range of 3 to 6 degrees. An example of a suitable assembly would include dowel pinning linear pillow block bearings  136  to pillow block mount adaptor  134  which in turn is bolted to shelf plate  104 . 
     Preferably, platform  106  is separable from the hand truck  100 . It is further preferable to allow the platform  106  to be adjustable such that it can be extended or retracted to meet various user requirements. An acceptable approach is to allow the platform  106  to slide substantially horizontally through shelf plate  104 . The platform  106  retracts by sliding it in through the shelf plate  104  such that only the front portion extends outward. The forked portion of platform  106  extends through the back of the hand truck  100 . This is useful for example when a user wants to stow the hand truck  100 . 
     Shelf plate  104  contains a pair of T-shaped slots positioned to receive the forked end of the shelf or platform  106 . The slots are dimensioned to allow the forks of platform  106  to reciprocate through the slots smoothly. Rectangular torquing blocks  138  are bolted to the back side of shelf plate  104  directly above the slots such that the upper horizontal slot edge is adjacent to the lower horizontal edge of a rectangular torquing block  138 . Platform guiding blocks  140  are connected to shelf plate  104  directly below the slots on both the front side and the back side of shelf plate  104  such that the lower horizontal slot edge is adjacent to the upper horizontal edge of platform guiding blocks  140 . The Platform guiding blocks  140  are a substantially rectangular block notched in the upper portion forming a trough extending therethrough and aligned with the shelf plate  104  such that the trough portion of platform guiding blocks  140  aligns with the lower T portion of the T-shaped slots of shelf plate  104 . 
     Referring to FIG. 9, the platform  106  contains two T-shaped forked portions positioned to engage the slots of shelf plate  104 . The T-shape of the forks in platform  106  allow it to reciprocate through the complementary T-shaped slots provided in shelf plate  104 . Platform  106  is secured to shelf plate  104  by inserting the platform  106  into the T-shaped slotted portion of shelf plate  104  such that the forks of platform  106  protrude from the back of shelf plate  104 . Optional spring clips  142  provide pressure against the forked portion of platform  106  to reduce platform vibration during transportation and the like. Spring clips  142  also assist in maintaining the shelf in a retracted position. As can be seen in FIG. 9, divots  144  are placed along the forked portion of platform  106  for receiving spring clips  142 . By pulling or pushing the platform  106  through shelf plate  104  until the spring pins  142  click into the desired divots  144 , vibration between the platform  106  and shelf plate  104  is eliminated, or at least greatly reduced. Further, pin blocks  158  are provided on the front portion of shelf plate  104  positioned adjacent to, and to the inside of the slots. Pin blocks  158  serve to maintain the platform  106  engaged with the shelf plate  104  and include a spring biased pin which provides interference with the flanged corners  156  of the previously inserted platform  106 . In operation, the pin portion of pin blocks  158  are retracted and platform  106  is slipped through the slots in shelf plate  104 . When the pins of pin blocks  158  are retracted, sufficient clearance exists for the flanged portion  156  of the platform  106  to slip through the slots of the shelf plate  104 . Once the platform  106  is inserted, the pins of pin blocks  158  are released. This approach allows platform  106  to be completely removable from the hand truck  100 , or adjustable between a fully extended and fully retracted position. A rubber bumper may optionally be glued to the front outer edge of platform  106 . 
     Top bracket assembly  102  is formed of a support plate that extends horizontally between vertical support bars  128 . The top bracket assembly  102  is positioned preferably near the top of the frame of hand truck  100  to provide maximum travel for platform  106 . However, the exact positioning of the top bracket assembly  102  can be adjusted depending upon the size of the shaft  120  selected. The top bracket assembly  102  will preferably contain a pulley assembly  101 . The pulley assembly includes at least one pulley for transitioning the cable around the upper portion of hand truck  100 . While FIG. 2 illustrates two pulleys extending over the top of bracket assembly  102 , it is possible to use a single pulley. Referring to FIGS. 4-5, a bracket (not shown) allows the cable to extend from the shelf plate  104  behind shaft  120 . By running the cable behind the shaft  120 , a single pulley can be used. Further, an optional shroud (not shown) is placed over the pulley assembly  101  to guard potential pinch points. The pulley assembly  101  will be positioned preferably along the top portion of bracket assembly  102 , but can be equally positioned along the bottom portion(not shown). 
     As is illustrated in the FIGS. 2-6, the platform  106  slides along shaft  120  in response to articles being placed thereon. Depending upon the arrangement of the platform  106 , it may be possible for the load to become positioned away from the frame of the hand truck  100 . Therefore, optional guide bars  121  can be positioned on either side of shaft  120 . An example of a suitable positioning, guide bars  121  can be secured to the top bracket assembly  102  on either side of the shaft, and secured to the lower portion of the frame, the toe plate for example, in a manner similar to that of shaft  120 . The intended purpose of guide bars  121  is to provide a place for articles to rest while loaded upon the platform. Depending upon intended use, the bars may be positioned parallel to the shaft  120 , or alternatively be positioned skewed with respect to shaft  120 . For example, guide bars  121  could be positioned closer at their top portion, than at the bottom portion such that the horizontal distance between the guides  121  increases closer to the lower portion of frame  121 . Likewise, the guide bars could be positioned closer together at the bottom portion of the hand truck  100 , with the horizontal distance between the guide posts  121  increasing closer to the top of the hand truck  100 . Also, guide bars  121  need not be of the same length or construction as the shaft  120 . 
     A platform arrestor  113  is connected to the back portion of hand truck  100  and serves as the lifting mechanism to automatically adjust the position of load platform assembly  103  in response to a load being placed thereon. The platform arrestor  113  holds the load platform assembly  103  suspended in the raised vertical position under no load conditions. The platform arrestor  113  is preferably enclosed in a shroud, but it is not necessary. One example of a suitable platform arrestor would include a spring  116 . Referring to FIG.  5  and FIG. 6, the spring  116  could connect to the load platform assembly  103  using a cable. Under no load conditions, the spring  116  would be in an unstretched state, holding the load platform assembly  103  in a raised, vertical position. As articles are loaded upon the load platform, the spring  116  would stretch as the load platform assembly  103  lowered due to the weight of the item placed thereon. Preferably, the vertical distance that the load platform assembly  103  drops would be set proportional to the weight of the load. The top of the first article is now approximately at the same position as the platform  106 , prior to loading the first article. As the second article is loaded, the platform  106  again lowers in response to the weight of the article loaded and the spring  116  again stretches. When the second article is removed, the spring  116  responds to the reduction in weight upon the load platform assembly  103  by pulling the load platform assembly  103  upwards. However, should the articles loaded upon the platform  106  become unstable either by tipping or the like, the sudden reduction in weight could cause the spring  116  to react too quickly thus a braking mechanism is preferably included. An example of a suitable brake is a hydraulic cylinder  114 . The hydraulic cylinder  114  is positioned such that it works in cooperation with the spring  116  to regulate the response time of the spring  116  to changes in load conditions. The brake  114  should be selected such that a sudden change in the weight placed upon platform  106  will not cause overwhelming oscillations or high velocity transients in the platform  106  position. 
     An example of a suitable platform arrestor arrangement would include a spring  116  connected to the lower back portion of the hand truck  100 . This can be accomplished in a variety of ways, including attachment to the axle of hand truck  100  utilizing cross bracket  117  and bracket  118  as illustrated in FIG. 4. A brake  114  comprised of a hydraulic cylinder  114  is positioned inside spring  116  and attaches to bracket  118  at its lower end. The upper end of brake  114  connects to the upper end of spring  116  and arrestor pulley  112 . For example, a telescopic or two stage hydraulic cylinder, a single stage hydraulic cylinder or the like can be used. Additionally, any combination of spring  116  and brake  114  can be used. For example, the brake  114  can be placed inside spring  116 . This arrangement provides the further advantage that, should the spring break or otherwise become damaged, the spring will be held to the hand truck  100  via the cylinder  114 , and will not fly out. Platform arrestor  113  can optionally provide a brake and a plurality of springs releasably connected to the arrestor pulley  112  to provide a mechanism that can be adjusted according to the weight of the load. As illustrated in FIG. 7, a first spring  152  is placed inside a second spring  154 . The upper portion of the second spring  154  is secured to the arrestor pulley  112 . The upper portion of the first spring  152  is suspended within the second spring  154 . An engagement lever  150  is included on the arrestor pulley  112  for selecting whether to engage the first spring  152 , while the second spring  154  is always connected. The engagement lever  150  includes an arm  151  for releasably engaging the upper portion of first spring  152 . If both springs  152  and  154  are engaged, the hand truck  100  is equipped for full weight load conditions, while switching out the first spring  152  out can provide for  72  weight load conditions. 
     Cable  110  forms the linkage between load platform assembly  103  and platform arrestor  113 . An example of an acceptable connection would include fastening cable  110  to shelf plate  104 . The cable  110  extending upward crossing over top bracket assembly  102  and through top pulley assembly  101 , dropping down and through arrestor pulley  112 , then extending vertically upward terminating in a connection to the top bracket assembly  102 . 
     The Hand Truck Kit 
     The components as described above are also suitable to be delivered in the form of a kit to a host hand truck such as those produced by Magliner. A layout of kit parts are illustrated in FIG.  1 . While the kit is adaptable to virtually all makes of hand truck, it will be described with respect to parts for installation onto an existing Magliner hand Truck. It should be understood that the kit is easily adaptable to virtually any hand truck. The kit of components bolts onto a host hand truck using approximately 8 bolts. The kit optionally includes a new toe plate  108  with the mounting bracket  107  pre-installed. Optionally, a user can attach bracket  107  to an existing toe plate using any suitable means. The provided toe plate  108  includes a bend such that the toe plate  108  is angled slightly greater that 90 degrees with respect to the vertical frame  100  to assist in balancing the hand truck while items are loaded and the platform is in a raised position as illustrated in FIGS. 4-5. The shaft  120  slips into the bracket  107 . 
     Shelf plate  104  comes pre-assembled as described above and includes a pair of linear pillow block bearings  136 . Two linear pillow block bearings  136  are bolted through pillow block mount adaptor  134  and onto shelf plate  104  vertically bisecting shelf plate  104  and spaced apart such that a first linear pillow block bearing  136  is positioned near the top, center of shelf plate  104 , while a second linear pillow block bearing  136  is positioned near the bottom center of shelf plate  104 . Shelf plate  104  contains a pair of T-shaped slots positioned to receive the forked end of shelf or platform  106 . Rectangular torquing blocks  138  are bolted to the back side of shelf plate  104  directly above the slots such that the upper horizontal slot edge is adjacent to the lower horizontal edge of a rectangular torquing block  138 . Platform guiding blocks  140  are connected to shelf plate  104  directly below the slots on both the front side and the back side of shelf plate  104  such that the lower horizontal slot edge is adjacent to the upper horizontal edge of platform guiding blocks  140 , and pin blocks  158  are provided adjacent to, and to the inside of the slots of the shelf plate  104 . The linear pillow block bearings  136  are slipped over the shaft  120 , then the top mounting bracket  102  is installed. 
     The top mounting bracket  102  is installed by removing the four bolts used to secure the handle to the vertical frame members  128 . The top mounting bracket is slipped between the vertical bars  128 , and before re-installing the shaft  120  is slipped into its mounting bracket. The bolts originally holding the handle are replaced and the top mounting bracket is tightened into place. The top mounting bracket pulley assembly  101  is pre-installed. 
     Next, the spring  116  is slipped over the axle of hand truck  100 . The hooked portion of the spring  116  is slipped through the opening in cross member mounting bracket  117 . Mounting bracket  118  is placed on the top portion of the axle, while cross member mounting bracket  117  is placed on the underside of the axle, and the brackets are bolted together. Cylinder brake  114  is slid through the spring  116  and threaded into the bracket  118 . The top portion of the brake  114  threads into the bottom portion of arrestor pulley  112 . The flanges of the arrestor pulley feed through the coils in the spring  116 , and are bent upwards. Cable  110  is secured to the top portion of shelf plate  104 , is fed upwards across top mounting bracket pulley assembly  101 , downward and through arrestor pulley  112 , upward and finally connecting to the top mounting bracket  102 . Shelf guide bars  121  are then installed between the top mounting bracket  102  and the toe plate  108 . Finally, the forked portion of platform  106  is pushed through the slots provided in the shelf plate  104  until the spring pins  142  click into the desired divots  144 . 
     Having described the invention in detail and by reference to preferred embodiments thereof, it will be apparent that modifications and variations are possible without departing from the scope of the invention defined in the appended claims.