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FIELD OF THE INVENTION  
         [0001]    The present invention relates generally to elongated working platforms and in particular to an elevating platform assembly of the type having a rack and pinion drive mechanism.  
         BACKGROUND OF THE INVENTION  
         [0002]    Elongated working platforms are well known in the art and are commonly used during construction to support workers and equipment at desired elevations. Platforms of this nature include stationary scaffolding as well as moveable elevating platform assemblies. Although stationary scaffolding is useful, in many instances it is desired to change quickly the elevation of workers and equipment and thus, elevating platform assemblies are advantageous.  
           [0003]    One known type of elevating platform assembly is manufactured by Hydro Mobile of L&#39;Assomption, Quebec. This elevating platform assembly includes an elevating platform that is supported at one end by a mast. A drive mechanism acts between the elevating platform and the mast. The drive mechanism includes a trolley moveable along the mast to which the platform is secured. A motor is mounted on the trolley and drives pinions that cooperate with a rack secured to the mast. In this manner, the elevating platform can be moved upwardly and downwardly along the mast.  
           [0004]    Although this elevating platform assembly works satisfactorily, when heavy loads are placed on the elevating platform near its end furthest from the mast, significant torque can be applied to the trolley by the elevating platform. The torque applied to the trolley acts to pull the trolley away from the mast. If the trolley moves relative to the mast under the influence of the torque, misalignment between the teeth of the pinions and the teeth of the rack results. This of course can result in stripping of the pinions as the trolley is advanced along the mast. As will be appreciated improvements to elevating platform assemblies of this nature are desired.  
           [0005]    It is therefore an object of the present invention to provide a novel elevating platform assembly of the type having a rack and pinion drive mechanism.  
         SUMMARY OF THE INVENTION  
         [0006]    According to one aspect of the present invention there is provided an elevating platform assembly comprising:  
           [0007]    a generally vertical mast having a generally vertical rack mounted thereon;  
           [0008]    a first trolley coupled to said mast and being moveable therealong;  
           [0009]    a drive mechanism carried by said first trolley, said drive mechanism including at least one pinion in mating engagement with said rack; and  
           [0010]    an elongated elevating platform extending from said mast and having a second trolley mounted adjacent one end thereof, said second trolley coupling said elevating platform to said mast and being moveable along said mast, wherein said first trolley is coupled to at least one of said second trolley and said elevating platform in a manner to maintain alignment of said at least one pinion and rack when loads are placed on said elevating platform that create moments at said mast.  
           [0011]    In one embodiment, the first trolley is coupled to the elevating platform via a shock absorbing arrangement that includes an elastomeric element. The shock absorbing arrangement permits the elevating platform to pivot relative to the first trolley without significant forces being applied to the first trolley that act to pull the first trolley away from the mast.  
           [0012]    In another embodiment, the first trolley is positioned on the mast below the second trolley and is coupled to the second trolley through shock absorbing elements carried by at least one of the first and second trolleys.  
           [0013]    According to another aspect of the present invention there is provided an elevating platform assembly comprising:  
           [0014]    an upright mast having a vertical rack extending along at least one side thereof;  
           [0015]    a motor trolley coupled to said mast, said motor trolley carrying a drive mechanism including at least one rotatable pinion in mating engagement with said rack, rotation of said at least one pinion advancing said motor trolley vertically along said mast;  
           [0016]    an elongated elevating platform extending from said at least one side of said mast, said elevating platform including a generally horizontal work surface; and  
           [0017]    a main trolley acting between said elevating platform and said mast and being moveable vertically along said mast, wherein said motor trolley is coupled to one of said main trolley and said elevating platform in a manner so as to maintain alignment of said at least one pinion and said rack when loads are placed on said elevating platform.  
           [0018]    The present invention provides advantages in that the coupling between the first trolley and either the second trolley or the elevating platform inhibits the at least one pinion from becoming misaligned with the rack when loads are placed on the elevating platform that create moments at the mast. By maintaining the at least one pinion and rack in alignment regardless of loads placed on the elevating platform, the likelihood of stripping of the teeth on the at least one pinion is reduced. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]    Embodiments of the present invention will now be described more fully with reference to the accompanying drawings in which:  
         [0020]    [0020]FIG. 1 is an isometric view of an elevating platform assembly in accordance with the present invention;  
         [0021]    [0021]FIG. 2 is an isometric view of a portion of the elevating platform assembly of FIG. 1 showing a motor trolley, main trolley and elevating platform arrangement;  
         [0022]    [0022]FIG. 3 is an isometric view of a portion of FIG. 2 showing the coupling between the motor trolley and the elevating platform;  
         [0023]    [0023]FIG. 4 is a side elevational view of FIG. 2; and  
         [0024]    [0024]FIG. 5 is an isometric view of another embodiment of a motor trolley, main trolley and elevating platform arrangement. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0025]    Referring now to FIG. 1, an elevating platform assembly is shown and is generally identified by reference numeral  10 . As can be seen, elevating platform assembly  10  includes a generally vertical mast  12  that is supported by a base assembly  14  resting on a ground surface. An elongate elevating platform  16  extends from one side of the mast  12  generally at a right angle. The elevating platform  16  includes a generally planar work surface  20  secured to an underlying supporting framework  22 . Guard rails  24  surround the work surface  20 . The elevating platform  16  is coupled to the mast  12  in a manner that permits the elevating platform to move vertically along the mast  12  thereby to allow the work surface  20  to be positioned at desired elevations as will be described.  
         [0026]    The mast  12  is formed from a series of stacked, box-type mast sections  30 , one of which is shown in FIG. 2. As can be seen, mast  12  includes four vertical corner rails  32  joined by horizontal crossbars  34  at vertically spaced locations. A plurality of diagonal cross-members  36  extends between the rails  32  and the horizontal crossbars  34  to provide additional support to the mast  12 . A vertical rack  40  is secured to the horizontal crossbars  34  on one side of the mast  12  by suitable fasteners  42 .  
         [0027]    A main trolley  50  is coupled to the mast  12  and runs along the rails  32  that are on opposite sides of the rack  40 . The main trolley  50  includes a generally rectangular frame structure  52 . Each side of the frame structure  52  is constituted by a pair of vertical side members  54  and  56  joined together by a series of steps  58 . Upper and lower cross members  60  and  62  span the sides of the frame structure  52 . A roller set support  64  is positioned at each corner of the frame structure  52  and extends inwardly towards the mast  12 . Three sets of rollers  68  are mounted on each support  64 . The rollers  68  on the supports  64  surround and engage the rails  32 .  
         [0028]    The main trolley  50  is secured to the framework  22  of the elevating platform  16  by upper and lower angles  72  and  74  respectively on opposite sides of the main trolley  50 . Specifically, the upper angles  72  secure the main trolley  50  to a main upper beam  76  that supports the work surface  20 . The lower angles  74  secure the main trolley  50  to a main lower beam  78 . Since the elevating platform  16  is fixed to the main trolley  50 , the elevating platform and the main trolley  50  move as a unit.  
         [0029]    Nested within the main trolley  50  is a motor trolley  100  (best illustrated in FIG. 3). As can be seen, the motor trolley  100  includes a generally rectangular frame structure  102  including a pair of vertical side members  104  joined at their upper and lower ends by supporting plates  106 . A horizontal member  108  spans the side members  104  intermediate the supporting plates  106 . A roller set support  109  is positioned at each corner of the frame structure  102  and extends inwardly towards the mast  12 . A set of rollers  111  is mounted on each support  109 . The rollers  111  on the supports  109  surround and engage the rails  32 .  
         [0030]    A drive mechanism  110  is mounted on each supporting plate  106 . Each drive mechanism  110  includes a motor  112  having an output shaft  114 . Shaft  114  extends through a bushing on the supporting plate  106  and has a gear  116  keyed to its other end. Gear  106  engages a pair of vertically spaced pinions  118  that are in mating engagement with the rack  40 . Rotation of the shafts  114  by the motors  112  imparts rotation of the pinions  118  via the gears  116 . This of course allows the motor trolley  100  to advance along the rack  40  and hence, along the mast  12 .  
         [0031]    A shock absorbing arrangement acts between the framework  22  of the elevating platform  16  and the motor trolley  100  to provide a floating couple therebetween. As can be seen, the shock absorbing arrangement includes a C-shaped member  120  having a web  122  and upper and lower limbs  124  and  126  defining a channel therebetween. The web  122  is welded to the main upper beam  76  of the framework  22 . An elastomeric shock absorbing element  128  is secured to the upper limb  124  and is positioned within the channel. The cross member  108  of the motor trolley  100  is accommodated within the channel and forms an interference fit with the elastomeric shock absorbing element  128  and the lower limb  126 .  
         [0032]    In operation, when the motors  112  are actuated to rotate the shafts  114  and hence the gears  116 , the rotation of the gears  116  imparts rotation of the pinions  118 . Since the pinions  118  are in mating engagement with the rack  40 , as the pinions  118  rotate, the pinions  118  advance along the rack  40  causing the motor trolley  100  to move along the mast  12 . The direction in which the motor trolley  100  advances along the mast  12  of course depends on the direction the shafts  114  are rotated. As mentioned above, the framework  22  of the elevating platform  16  is coupled to the motor trolley  100  via the shock absorbing arrangement. Therefore, the elevating platform  16  moves with the motor trolley  100  as a unit.  
         [0033]    During use, the elevating platform  16  may be heavily loaded. If the load is positioned on the elevating platform  16  away from the mast  12 , the loading on the elevating platform  16  may create a significant moment at the point of connection between the elevating platform and the mast  12 . As the elevating platform  16  pivots under the load and the lower limb  126  of the C-shaped member  120  pushes against the cross member  108 , the cross member  108  contacts the shock absorbing element  128 . The shock absorbing element  128  in turn deforms allowing the elevating platform to pivot relative to the motor trolley  100 . In this manner, significant forces that act to pull the motor trolley  100  away from the mast  12  are not imparted on the motor trolley  100  by the elevating platform. Thus, the pinions  118  and rack  140  remain in alignment despite the loads placed on the elevating platform  16 .  
         [0034]    Turning now to FIG. 5, an alternative motor trolley and main trolley arrangement for the elevating platform assembly  10  is shown. In this embodiment, the configurations of the motor trolley  100  and the main trolley  50  are the same as those described with reference to the first embodiment; however, the two trolleys are not nested. Instead, the motor trolley  100  is positioned below both the main trolley  50  and the elevating platform  16  with the main trolley  50  resting on the motor trolley  100 . Elastomeric elements  150  are mounted on the top roller set supports  109  of the motor trolley  100  and act between the motor trolley  100  and the main trolley  50 . Since the motor trolley  100  and the elevating platform  16  are not coupled directly, moments at the mast  12  that are caused by loads placed on the elevating platform  16  are not transferred to the motor trolley  100 . As a result, the pinions  118  and rack  40  remain in alignment regardless of the loads placed on the elevating platform  16 .  
         [0035]    As will be appreciated, by avoiding a fixed rigid connection between the motor trolley  100  and the elevating platform  16 , significant moments resulting from loading of the elevating platform  16  are not transferred to the motor trolley  100 . This of course maintains the rack  40  and pinions  118  in alignment reducing the likelihood of stripping of the pinions.  
         [0036]    Although the elevating platform assembly is shown having a single elevating platform extending from one side the mast, those of skill in the art will appreciate that the elevating platform assembly may include an additional elevating platform extending from the opposite side of the mast. The second elevating platform may be coupled to the first elevating platform and driven by the drive mechanism of the first elevating platform or may include its own motor trolley and drive mechanism. In this latter case, a second rack is provided on the mast  12 .  
         [0037]    Although preferred embodiments of the present invention have been described, those of skill in the art will appreciate that variations and modifications may be made without departing from the spirit and scope thereof as defined by the appended claims.

Summary:
An elevating platform assembly includes a generally vertical mast having a generally vertical rack mounted thereon. A first trolley is coupled to the mast and is moveable therealong. A drive mechanism is carried by the first trolley and includes at least one pinion in mating engagement with the rack. An elongated elevating platform extends from the mast and has a second trolley mounted adjacent one end thereof. The second trolley couples the elevating platform to the mast and is moveable along the mast. The first trolley is coupled to at least one of the second trolley and elevating platform in a manner to maintain alignment of the at least one pinion and rack when loads are placed on the elevating platform that create moments at the mast.