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CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     Not applicable.  
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
       [0002]     Not applicable.  
       BACKGROUND OF THE INVENTION  
       [0003]     The present invention relates to a step ladder and, more particularly, to a spreader that facilitates the folding and unfolding of a step ladder.  
         [0004]     Step ladders are widely used for tasks such as painting, wiring, and grounds maintenance. Often the surface on which the ladder is to be supported is sloping in, at least, one plane. For example, one common problem encountered by ladder users is reaching a ceiling over a stair. While each step of the stair is horizontal and flat, the slope between successive steps makes the use of a step ladder, with its four spaced apart points of support, impractical or unsafe.  
         [0005]     McCrystal, U.S. Pat. No. 6,073,726 discloses a step ladder with adjustable stiles and back legs to facilitate use of the ladder on sloping surfaces. Each of the stiles and back legs of the step ladder can be independently adjusted to facilitate use of the ladder on surfaces that slope in more than one direction. Since the lengths of the stiles or step supporting rails are adjustable, the positions on the stiles of several steps can be adjusted to make the ladder easier to use. While step ladders with individually adjustable legs are very flexible and can be used on uneven surfaces, such ladders are relatively complicated and difficult to use. To set the ladder up on a sloping surface, the ladder must be balanced on one or more points of support while the user independently adjusts the lengths of two or more legs. A latching mechanism is required for each of the legs and for each end of each of the independently adjustable steps or treads, increasing the complexity, cost, and weight of the ladder.  
         [0006]     What is desired, therefore, is a step ladder that is of uncomplicated construction and that is easy to set up and use on stairs or other surfaces that slope in a single direction.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]      FIG. 1  is a perspective view of an adjustable step ladder.  
         [0008]      FIG. 2A  is a perspective view of a back leg lock for the adjustable step ladder of  FIG. 1  illustrating a locked condition.  
         [0009]      FIG. 2B  is a perspective view of the back leg lock of  FIG. 2A  illustrating an unlocked condition.  
         [0010]      FIG. 3  is an elevation view of an adjustable step ladder on a surface sloping upward toward the back legs of the ladder.  
         [0011]      FIG. 4  is an elevation view of an adjustable step ladder on a surface sloping downward toward the back legs of the ladder. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0012]     Referring in detail to the drawings wherein similar parts of the invention are identified by like reference numerals and referring specifically to  FIG. 1 , the adjustable step ladder  24  comprises generally a top cap  22 , a ladder element  24 , a back leg assembly  26 , and a spreader  28 . The ladder element  24  includes a pair of spaced apart step supporting rails or stiles  30  that are fixed to the top cap  22  at one end. Feet  32 , attached to the other end of the stiles  30 , engage the surface supporting the stiles. A plurality of steps or treads  34  are spaced at substantially even increments along the stiles  30  and fixed to the stiles at each end of the tread. The back leg assembly  26  is attached to the top cap  22  by a top cap hinge  36  that permits the back leg assembly to be pivoted toward the ladder element  24  to a position substantially coextensive with the ladder element to minimize the size of the step ladder  20  for moving and storage. To use the step ladder  20 , the back leg assembly  26  is pivoted away from the ladder element  24  to a bracing position as illustrated in  FIG. 1 . During use, the back leg assembly  26  braces the ladder element  24  at a ladder angle  38  to form a self supporting triangular structure in conjunction with the surface supporting the ladder. The spreader  28  prevents the back leg assembly  26  from pivoting toward the ladder element  24  while the ladder is in use.  
         [0013]     The spreader  28  is typically an over-center linkage connecting the ladder element  24  and the back leg assembly  26 . The spreader  28  comprises a pair of spreader beams  40 , each comprising a pair of spreader bars  42  and  44  centrally hinged to each other by a spreader hinge pin  46 . The second end of one spreader bar  44  of each spreader beam  40  is pivotally connected  48  to one of the stiles  30  of the ladder element  24 . The second end of the second spreader bar  42  of each spreader beam  40  is pivotally connected to the back leg assembly  26  by a hinge bar  50 .  
         [0014]     To fold the step ladder  20  for moving or storage, the user lifts the spreader hinge pin  46  and thereby the central hinged ends of the spreader bars  42  and  44 . As the center of the spreader  28  is lifted, the spreader beams  40  fold at the central hinge pin  46  and the spreader bars  42 ,  44  rotate about the pivotal connections at the stiles  30  and back leg assembly  26 , respectively. The back leg assembly  26  pivots about the top cap hinge  36  to fold to a position substantially coextensive with the stiles  30  of the ladder element  24 . To use the step ladder  20 , the user pulls the back leg assembly  26  away from the ladder element  24 . As the back leg assembly  26  pivots about the top cap hinge  36 , the folded spreader beams  40  straighten and extend. When the spreader beams  40  are fully extended, the user pushes the spreader hinge pin  46  down to an over-center position, below a line connecting the spreader beam connections  48  and  50  at the stiles  30  and the back leg assembly  26  where a stop  43  prevents further movement of the hinge pin. The weight of the ladder and the user will urge the ladder element  24  to pivot toward the back leg assembly  26  which, in turn, will urge folding of the spreader beams  40 . However, the spreader beams  40  are prevented from folding downward by the stop and are prevented from folding upward because the spreader hinge pin  46  is in the over-center position. The back leg assembly  26  is secured in the bracing position at a bracing angle  59  to the ladder element  24 .  
         [0015]     The step ladder  20  engages the supporting surface at four points; the two ladder element feet  32  at the ends of the stiles  30  and a pair of back feet  52  attached to the back leg assembly  26 . When the step ladder  20  is set up for use, the ladder element  24 , and the back leg assembly  26 , in conjunction with the supporting surface, form a self-supporting triangle with the ladder element inclined at a ladder angle  38  to the supporting surface. When the surface is level, the feet  32  at the ends of the stiles  30  are at the same elevation as the back feet  52  and the center of the combined mass of the step ladder  20  and the user will be located approximately midway between the ladder element feet  32  and the back feet  52 . In this condition, the step ladder  20  is stable. However, on a stair or other surface that slopes toward or away from the back feet  52 , the ladder element  24  will pivot about the ends of the stiles  30  and the ladder angle  38  will decrease (the ladder element becomes more horizontal) or increase (the ladder element becomes more vertical), respectively. As a result, the combined center of mass of the step ladder  20  and the user will be shifted nearer the points of support for the ladder, either feet  32  or feet  52 , and the ladder will become less stable. As the user climbs the ladder or moves while performing work, the combined center of mass may move outside the points supporting the step ladder causing the ladder to topple. The present inventor realized that a step ladder could be used on stairs or other sloping surfaces if the length of the back leg assembly  26  could be adjusted, as appropriate, to maintain the ladder angle  38  within a prescribed, useable range. Further, the inventor realized for many commonly encountered situations, such as a stair, the supporting surface is level in one direction and that use of the ladder can be facilitated by limiting the number of adjustments to those necessary to accommodate surfaces sloping in a single direction.  
         [0016]     Referring to  FIGS. 3 and 4 , to permit the step ladder  20  to be used when the surfaces supporting the back leg  26  and the ladder element  24  are at differing elevations, the length of the back leg assembly of the step ladder  20  is adjustable. The back leg assembly  26  comprises an upper back leg assembly  54  and a lower back leg assembly  56  that is affixed to the back feet  52 . The upper back leg assembly  54  comprises a pair of channels  58  arranged generally parallel to each other and pivotally connected to the top cap  22  by the top cap hinge  36 . The spreader beams  40  controlling the angular position of the back leg assembly  26  and the ladder element  24  are pivotally connected to the channels  58  of the upper back leg  54  and, when extended, maintain a bracing angle  59  appropriate for ladder stability between the back leg assembly  26  and the ladder element  24 . As illustrated in  FIGS. 2A and 2B , the channels  58  of the upper back leg assembly  54  have a generally C-shaped cross-section with a pair of spaced apart channel legs  60  fixed to a channel base  62  forming the throat  64  of the channel. The channels  58  are attached to the top cap  22  such that the channel bases  62  are toward the outer edges of the ladder and the throats  64  face each other. A plurality of upper channel crossmembers  60  connects the two upper back leg channels  58  to maintain the substantially parallel alignment. One leg  60  of each of the upper channels  58  includes portions defining a plurality of spaced apart apertures  66 .  
         [0017]     The lower back leg assembly  56  comprises a pair of spaced apart; lower back legs  68  and a back leg lock  70 . The spaced apart relationship of the lower back legs  68  is maintained by a latch plate  72  and a crosshead  74  affixed at the upper ends of the lower back legs  68  and one or more crossmembers  76  that are affixed between the lower back legs along their length. The lower back legs  68  comprise a first section  78  and a second section  80 . The first sections  78  of the lower back legs  68  have a generally C-shaped cross-section with a pair of spaced apart channel legs  82  affixed to a channel base  84  and forming the throat  86  for the channel. The lower back legs  68  are attached such that the channel bases  84  of the first sections  78  face the outer edges of the ladder  20  and the throats  88  face each other. The first sections  78  of the lower back legs  68  are arranged substantially parallel and are spaced apart so that they slidably engage the channels  58  of the upper back legs  54 . Portions of the first sections of lower back legs  78  define a plurality of spaced apertures  88 . The second sections  82  of the lower back legs  68  project at an angle from the first sections  78  to spread the points of support for the ladder and increase its lateral stability. The second section  82  of each of the interconnected lower back legs  68  terminates in a back foot  52  that contacts the surface against which the back leg assembly  26  braces the ladder element  24 .  
         [0018]     Referring to  FIG. 3 , when the back leg supporting surface  122  is at a higher elevation than the ladder element supporting surface  124  the length of the back leg assembly  26  can be shortened to maintain the ladder element  24  at a ladder angle  38  within a range that provides good stability. On the other hand, if the back leg supporting surface  126  is at a lower elevation than the ladder element supporting surface  128 , as illustrated in  FIG. 4 , the length of the back leg assembly  26  can be extended to maintain the ladder angle  38  within the useable range.  
         [0019]     To adjust the length of the back leg assembly  26 , the user unlocks the upper  54  and lower  56  back legs and slides the upper back leg in the lower back leg until the ladder element  24  is at the correct ladder angle  38 . The upper  54  and lower  56  back legs are locked together to maintain the correct length of the back leg assembly  26  during use of the step ladder  20 . The back leg assembly  26  includes a lock to selectively permit or prevent the upper back leg channels  58  from sliding in the lower back leg channels  78 . A lock plate  90  is hinged  92  to the crosshead  74  that secures the upper ends of the lower back legs  56 . A plurality of pins  94  are affixed to the lock plate  90  and arranged to align with the plurality of apertures  88  in a leg  82  of each of the lower back leg channels  78 . A handle  96  attached to the lock plate  90  facilitates pivoting of the spring loaded, hinged lock plate  90  by the user. When setting up the ladder on a sloping surface, the user grasps the handle  96  and pivots the lock plate  90  to disengage the lock pins  94  from apertures  66  in the upper back legs  58  that are co-located with the apertures  88  in the lower back legs  78 . The upper back leg channels  58  can then be slid in or out of the lower back leg channels  78  until the ladder element  24  is at an appropriate ladder angle  38  and apertures  66  in the upper back leg channels  58  are in alignment with the apertures  88  in the lower back leg channels  78 . The user then pivots the lock plate  90  toward the lower back leg channels  78  to engage the pins  94 , with the co-located apertures  88 ,  66  and lock the back leg assembly  26  against further change in length. A spring in the hinge  98  urges the lock plate  90  to pivot to an engaged position and maintains engagement of the pins  94  with the apertures. A bar  100  slidably engaging the lock plate  90  selectively engages the latch plate  72  to prevent the lock plate from pivoting to a disengaged position while the ladder is in use.  
         [0020]     The distance between the top cap  22  and the back feet  52  of the step ladder  20  can be quickly and easily adjusted to promote stable use of the step ladder on stairs and other surfaces sloping toward or away from the ladder.  
         [0021]     The detailed description, above, sets forth numerous specific details to provide a thorough understanding of the present invention. However, those skilled in the art will appreciate that the present invention may be practiced without these specific details. In other instances, well known methods, procedures, components, and circuitry have not been described in detail to avoid obscuring the present invention.  
         [0022]     All the references cited herein are incorporated by reference.  
         [0023]     The terms and expressions that have been employed in the foregoing specification are used as terms of description and not of limitation, and there is no intention, in the use of such terms and expressions, of excluding equivalents of the features shown and described or portions thereof, it being recognized that the scope of the invention is defined and limited only by the claims that follow.

Summary:
A spreader assembly having spreader beams respectively bendable about a common central pivot provides a step ladder of simple construction for convenient use.