You are an expert at summarizing long articles. Proceed to summarize the following text:

You are an expert at summarizing long articles. Proceed to summarize the following text: 
TECHNICAL FIELD 
     This invention relates to a sleeve bearing pivot assembly between a pair of spaced walls which holds the walls in spaced relation to one another while permitting relative pivotal movement. 
     BACKGROUND OF THE INVENTION 
     Sleeve bearings are the preferred bearing for many applications because of their reasonable cost, ease of installation, long life and load carrying capability. Light weight structures which have thin parallel load bearing walls and which need to be pivotally connected to one another on an axis transverse to the walls, present a design problem in that deflection of the walls under the load carried by the pivot joint will unevenly load a typical sleeve bearing. 
     SUMMARY OF THE INVENTION 
     The present invention provides a pivot assembly interconnecting a pair of spaced parallel thin walls which uses a sleeve bearing whose bearing surfaces are evenly loaded even though the walls deflect under the load carried by the bearing. A component of the sleeve bearing is mounted in such a manner that it changes it orientation relative to a wall when either of the walls deflect under load. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     One embodiment of the invention is shown in the drawings, in which: 
     FIG. 1A is a side view of a stairway which may be raised and lowered; 
     FIG. 1B is a side view showing the stairway in a lowered position; 
     FIG. 2 is a section taken along the line II—II in FIG. 1; 
     FIG. 3 is top view of the step shown in FIG. 2; 
     FIG. 4 is an enlarged view of a pivot assembly; 
     FIG. 5 is a section on the line V—V in FIG.  4  and 
     FIG. 6 is an exploded view of the pivot assembly. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIGS. 1A-3, the illustrated stairway  11  includes a stationary base  12  with a pair of laterally spaced parallel support posts  13 ,  14  rigidly secured to the base  12 . A pair of parallel walls or stair joists  16 ,  17  have corresponding ends pivotally connected to the base  12  on a transverse horizontal axis by coaxial pivot pins  18 , only one of which is shown. The opposite corresponding ends of the stair joists  16 ,  17  are pivotally connected to a pair slanting uprights  21 ,  22  by a pair of coaxial pivot pins  19 , only one of which is illustrated, for pivotal movement about a transverse horizontal axis. A step module  23  has it laterally opposite ends rigidly secured, respectively, to the lower ends of the uprights  21 ,  22 . The stair Joists  16 ,  17  are fabricated from relatively thin flat sheets of metal and are reinforced by a cross bracing structure  24 . 
     A pair of upper links  26 ,  27  are pivotally connected at their opposite ends to upper ends of the posts  13 ,  14  and to the upper ends of the uprights  21 ,  22  by suitable pins  28 ,  29 . A pair of intermediate links  31 ,  32 , serving as hand rails, are pivotally connected at their opposite ends to the posts  13 ,  14  and to the uprights  21 ,  22  by transverse pivot pins  33 ,  34 . The support posts  13 ,  14  and the base  12  provide a stationary support for the stair joists  16 ,  17 . The axes of the pins  18 ,  28  and  33 , lie in a first plane. The axes of the pins  19 ,  29  and  34  lie in a second plane parallel to the first plane. The spacing of the pins is such as to create a parallel linkage which allows the step  23  and the uprights  21 ,  22  to be raised and lowered. The lowered position is shown in FIG.  1 B. 
     A pair of step modules  41 ,  42  are pivotally connected to the stair joists  16 ,  17  by pivot assemblies  43 . As shown in FIGS. 2 and 3, the step module  41  has thin flat vertical side walls or plates  46 ,  47  to which a tread  48  is welded. Step module  42  is of the same general construction. A pair of link rods  49 ,  51  are pivotally connected by pins  52 ,  53  to the side plates  46 ,  47  of the steps modules  41 ,  42  and have corresponding ends pivotally connected to the base  12  by pins  56  so that when the step module  23  is raised or lowered, the treads of the step modules  41 ,  42  will remain horizontal as they raised or lowered. 
     Referring to FIGS. 4-6, the pivot assembly  43  includes a stepped diameter pivot rod or cylindrical stud  61  which has a large exterior diameter part  62  and a small exterior diameter part  63  separated by a shoulder presenting a flat annular axially facing abutment surface  64 . The cylindrical stud  61  had a hollow interior defined by a bore  66  which has a coaxial relationship with the cylindrical exterior surfaces of the large and small diameter portions  62 ,  63 . The stud  61  is secured to the joist  16  by a tension member in the form of a round headed bolt  67  having square section portion  68  adjacent its head  69  which nonrotatably mates with a square hole  71  in the joist  16  and a nut  72  on the threaded end  73  of the bolt applying axial thrust against the reduced diameter end of the stud  61  through a washer  74 . 
     The side plate  46  of the step module  41  has an annular opening or bore  76  into which a radially outer sleeve  77  of a sleeve bearing module  78  is pressed. A radially inner sleeve  81  has a radially inward facing cylindrical bearing surface  82  in radial thrust bearing engagement with a radially outward facing cylindrical bearing surface  83  of the reduced diameter portion  63  of the stud  61 . 
     The sleeves  77 ,  81  have complimentary spherical load bearing surfaces  91 ,  92  which are textured in such a way that relative movement about axes at right angles to the axis  86  of sleeve bearing surfaces  82 ,  83  is permitted but relative rotation of the sleeves  1 l,  81  about axis  86  is prevented. The inner sleeve  81  is made of a plastic or synthetic composition known for its suitability for use in self lubricated or nonlubricated bearings. The illustrated two piece sleeve bearing is currently marketed as a SPYRAFLO bearing. The mating spherical surfaces  91 ,  92  allows relative movement between the inner sleeve  81  and its support sleeve  77  to compensate for deflection of the walls  16 ,  47 , thereby insuring even loading of the cylindrical bearing surfaces  82 ,  83 .

Summary:
A pivot assembly particularly adapted for pivotally interconnecting a pair of relatively thin parallel walls spaced an appreciable distance from one another. A stud with a pivot joint on one end is interposed between the walls.