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CROSS-REFERENCE TO RELATED APPLICATIONS 
   This claims the benefit of U.S. Provisional Patent Application No. 60/828,876 filed Oct. 10, 2006. 

   STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
   Not applicable. 
   FIELD OF THE INVENTION 
   The invention relates generally to movable step assemblies for recreational vehicles and in particular to an apparatus for extending and retracting a movable step assembly. 
   BACKGROUND 
   Automatic step systems for recreational vehicles, motor homes, and the like are well known in the art. These systems are typically electrically-controlled and electrically-actuated to extend and retract an entryway step in response to a signal provided by an individual wishing to enter or exit the vehicle. One common system extends the step when the vehicle door is opened, and then retracts the step when the vehicle door is closed. Other systems offer a switch located just inside the vehicle door which controls the extension and retraction of the step. These systems also include a master power switch which can be used to lock the step in a given position. 
   Alternative systems incorporate a motor assembly for automatically-extending and retracting the step assembly. The motor rotates a pivot rod through a gear assembly which is coupled to the rod. The pivot rod moves a linkage assembly to extend and retract the steps. However, these systems often fail over time, giving the step a “spongy” or unstable feel. Therefore, an improved mechanism for extending and retracting collapsible steps in recreational vehicles is needed. 
   BRIEF DESCRIPTION OF THE INVENTION 
   The present invention provides an improved collapsible step assembly for recreational vehicles. In one aspect, the invention provides a movable step apparatus including a mounting frame, at least one step mounted to the frame through a linkage assembly and a pivot member having a longitudinal axis of rotation. 
   In use, the pivot member is rotatably mounted to the frame and rotates the linkage assembly and the at least one step between an extended position and a retracted position. Rotating the pivot member in a first direction moves the step to the extended position, and rotating the pivot member in an opposite direction moves the step to the retracted position. In a preferred version the pivot member is a rod. 
   Opposing ends of the pivot member are attached to a two-part linkage, each linkage mounted directly to the frame. The two-part linkage comprises a first link or crank mounted to rotate with the rod and pivotally attached to a second link. When the at least one step is in the extended position, the crank is substantially aligned and parallel with the straight link. In use, when the at least one step is in the extended position, the crank rotates around the pivot member, and engages against the frame to stop in a position in which the crank is nearly aligned with the link thereby preventing excessive forces from being transferred to the drive or transmission system when weight is placed on the step. 
   The linkage assembly comprises a plurality of linkages pivotally connecting the frame to the at least one step. 
   In a second aspect, the invention provides a movable step apparatus comprising a mounting frame, a motor mounted to the frame, at least one step, a pivot member mounted to the frame, a linkage assembly and a transmission assembly. The pivot member rotates about its longitudinal axis of rotation and moves the linkage assembly connecting the at least one step, the frame and the pivot member to the transmission assembly. The transmission assembly moves the pivot member (preferably a rod), which moves the at least one step between an extended position and a retracted position. 
   The linkage assembly may comprise a first linkage pivotally connecting the frame to a first at least one step, a second linkage pivotally connecting the frame to a second at least one step, and a third linkage pivotally connecting the first and second steps together. 
   In a third aspect, the invention also provides a mounting frame and a motor drive unit. The motor, mounted to the frame, has an output shaft, and is engaged to a drive gear through the motor&#39;s output gear, which is preferably a worm gear drive train. The drive gear is rotatable about an axis of rotation. In use, rotating the motor output gear, which may be rotated by a worm gear on the motor shaft, rotates the drive gear, which in turn rotates the pivot rod through a link arm to move the at least one step into an extended position. When in the extended position, the crank is pivoted around the pivot rod, engaging against the frame in a stopped position in which the crank is nearly aligned with the link of the two-part linkage system. 
   By utilizing the two-part linkage system on opposing ends of the pivot rod, the motor is nearly isolated from the forces exerted on the steps. This configuration improves the durability and longevity of motors for collapsible step assemblies while reducing servicing requirements, while also preventing the “spongy” or unstable feel conventional step assemblies can develop over time and avoiding “popping” type impacts that can occur if the step changes direction while being driven. 

   
     BRIEF DESCRIPTION OF THE FIGURES 
       FIG. 1  is a front, top perspective view of a step assembly of the invention in the extended position; 
       FIG. 2  is a rear, bottom perspective view of the assembly in the extended position; 
       FIG. 3  is a side plan view of the assembly in the retracted position; 
       FIG. 4  is a side plan view of the assembly in the extended position; 
       FIG. 5  is a bottom plan view of the assembly in the extended position; 
       FIG. 6  is a detail view of the drive portion of the circumscribed by line assembly  6 - 6  of  FIG. 5  in the extended position; 
       FIG. 7  is a top view of the assembly of  FIG. 6  in the retracted position; 
       FIG. 8  is a detail cross-sectional view of the assembly along the line  8 - 8  of  FIG. 5  in the extended position; 
       FIG. 9  is a view like  FIG. 8  but in a retracted position; 
       FIG. 10  is a side cross-sectional view of the assembly along the line  10 - 10  of  FIG. 5  in the extended position; and 
       FIG. 11  is a side view like  FIG. 10  but in the retracted position. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The invention comprises a collapsible step assembly  10  for use with recreational vehicles. Referring to  FIGS. 1-4 , the assembly  10  comprises a generally rectangular and planar upper step  12 , a lower step  14  and a frame  16 . The steps  12 ,  14  move between an extended position ( FIGS. 1 ,  2  and  4 ) and a retracted position ( FIG. 3 ), wherein the steps  12 ,  14  and frame  16  remain substantially parallel to one another at all times. 
   Each step  12 ,  14  may be covered with a non-skid material (not shown) to increase the friction of their respective surfaces. The lengths of the steps  12 ,  14  are approximately one-half of their respective widths. 
   Each step  12 ,  14  also has arms  18  ( 18 A of step  12  and  18 B of step  14 ) which extend in a rearward direction from their outer edges. Arms  18  are approximately equal in length to the steps  12 ,  14  and may be reinforced by pieces of angle bar sock welded to them as illustrated. 
   The frame  16  is generally box-like in shape and has open front, rear and bottom sides, and retains the steps  12 ,  14  when the assembly  10  is in the retracted position. The frame  16  may also includes amounting assembly to a vehicle (not shown), or may be bolted, welded, or otherwise fixed to the vehicle. 
   The steps  12 ,  14  and the frame  16  are interconnected by a linkage assembly including three pivotable links; a rearward link  20 , a medial link  22 , and a forward link  24  ( FIG. 2 ). The links  20 ,  22 ,  24  comprise straight, flat metal strips having two opposing lower and upper ends symmetrically arrayed along each side of the assembly  10 . The links  20 ,  22 ,  24  pivot around each point of attachment between the extended and retracted positions. Rectangular support brackets  64 ,  66  secured to the respective medial and forward links  22 , 24  perpendicular to the steps  12 ,  14  help support the steps  12 ,  14  and reinforce the assembly during use. 
   The rearward link  20  connects the upper step  12  to the frame  16 . The upper ends of each rearward link  20  are pivotally mounted near the upper rearward corners of the frame  16 . The lower ends of each rearward link  20  are pivotally mounted near the rearward ends of the upper step arms  18 A. 
   In use, when the assembly  10  is in the extended and retracted positions, the rearward links  20  are skewed downward towards the ground at approximately 45 degrees forward and rearward, respectively, relative to the frame  16 . 
   The medial link  22  has a dogleg shape and pivotally connects the lower step  14  to the frame  16  and has its approximate midpoint pivotally connected to the upper step  12  near the point where the step  12  meets the upper step arm  18 A. The medial link  22  is approximately twice as long as and slightly wider than the rearward link  20 . The upper ends of the medial link  22  are pivotally mounted to the upper forward corners of the frame  16 . The lower ends of the medial links  22  are pivotally mounted near the ends of the lower step arms  18 A. 
   In use, when the assembly  10  is in the extended position, the medial links  22  are skewed forward and downward at approximately 65 degrees relative to the frame  16  and roughly straight down from step  12 . In the retracted position, the medial links  22  are skewed rearward and downward at approximately 25 degrees relative to the frame  16 . 
   The forward link  24  connects the lower step  14  to the upper step  12 . The forward links  24  are slightly longer than the rearward links  20  and shorter than the medial links  22 . The upper ends of the forward links  24  are pivotally mounted near the forward corners of the lower step  14 . The lower ends of the forward links  24  are pivotally mounted to the lower step  14  near the point where the lower step arm  18 B extends from the lower step  14 . 
   In use, when the assembly  10  is in the extended position, the forward links  24  are skewed downward and slightly forward at approximately 80 degrees relative to the lower step  14 . In the retracted position, the forward links  24  are skewed rearward and downward at approximately 15 degrees relative to the lower step  14 . 
   Referring now to  FIGS. 5-11 , the assembly  10  also includes a pivot rod  26  extending transversely through the frame  16 . The pivot rod  26  includes a two-part linkage  28  at opposing ends of the rod  26  perpendicularly fastened to the frame  16  ( FIGS. 5-7 ). The two-part linkage  28  comprises a first, short, bulbous crank  30  pivotally attached to a second, longer, straight link  32  ( FIGS. 8-9 ). The far end of link  32  is pivotally connected to link  22  at pivot  63 . There is a linkage  28  at each end of rod  26  to drive the step in and out. 
   The crank  30  comprises a straight link fixed to rotate with rod  26  having two opposing ends  56 ,  58 . A first end  56  is pivotally attached to the straight drive link  32  at pivot  31 . A second end  58  is fixed to the pivot rod  26  and has two lobes  60 ,  62  on each side of the longitudinal axis of the crank  30 . Lobe  62  is provided so that the crank  30  is symmetrical, i.e., it can be assembled and fixed to rod  26  with either side out. 
   In use, when the assembly  10  is in the extended position, the crank  30  is nearly straight with the link  32 . That is to say that the centerline of the crank  30 , which is the line from the center of pivot  26  to the center of pivot  31 , is almost aligned with the centerline of link  32 , which is the line from the center of pivot  63  to the center of pivot  31 . It is almost aligned but still approaching the aligned position, and preferably not beyond the aligned position. The lobe  60  stopping against the bottom of the frame  16  prevents the crank  30  from going beyond the aligned position. If it were to go beyond the aligned position, then the step would undergo a change of direction (between extending and retracting) when the crank passed the aligned position, resulting in a “pop” impact on the step drive system that is caused by backlash in the system. This is undesirable and can result in premature failure of the system. The lobe  60  stopping against the bottom of the frame  16  stops the crank short of but preferably within less than 20 degrees of the aligned position. 
   The pivot rod  26  also includes a short finger assembly  36  rigidly mounted to the rod  26 . The finger assembly  36  extends radially and perpendicularly away from the rod  26 . A link arm  38  with a fixed length is connected to the finger assembly  36  with a universal joint  40 . The universal joint  40  allows the finger assembly  36  and link arm  38  to pivot about generally vertical (about pivot  41 ) and horizontal (about the axis of the pin  35  that goes through the two arms  36 ) axes relative to the fingers  36 . 
   The link arm  38  is swivelly-mounted to a horizontal drive gear  42  by a ball joint  39  at the end of crank arm  44  which is fixed to gear  42 . The gear has teeth (not shown) which extend circumferentially along an arcuate edge portion of the gear  42 . The gear  42  is centrally- and pivotally-mounted with a second pivot pin  48  to a motor mounting plate  50 . The motor mounting plate  50  is in turn mounted to the frame  16 . The gear teeth (not shown) engage a second drive gear within housing  52  which extends from a lower side of a motor  54 . 
   The motor  54  is also mounted to the motor mounting plate  50 . The finger assembly  36  extends forward and downward relative to the pivot rod  26 , and the link arm  38  is horizontally rotated to a forward end of the gear  42  when the assembly  10  is in the extended position. While in the extended position, the link arm  38  is rotated toward the motor  54  relative to the rod  26  ( FIG. 10 ). 
   In the retracted position, the finger assembly  36  extends rearward and downward relative to the rod  26 , and the link arm  38  is generally in a common plane with the finger assembly  36 . The link arm  38  is also skewed longitudinally away from the motor  54  at an angle relative to the rod  26  ( FIG. 11 ). 
   The motor  54  rotates the segment gear  42  approximately 90 degrees between the extended and retracted positions. The particular drive for driving the gear within housing  52  that meshes with segment gear  42  may be a worm gear drive, which tends to hold its position when stopped and so is preferred to resist any torque on rod  26  exerted by the step tending to backdrive the drive system, although any suitable drive could be used to rotate rod  26 . In this regard, it is noted that, referring to  FIG. 6 , the pivots  39 , 41  and  48  are close to being aligned along a straight line so that any force exerted from the arms  36  of the rod  26  tending to backdrive the motor/drive unit would exert only a small torque about pivot  48 , which could be easily resisted by the motor/drive unit. 
   In use, the frame  16  of the assembly  10  is mounted to the underside of a vehicle adjacent to the doorway (not shown). Prior to use, the assembly  10  is in the retracted position so that the upper and lower steps  12 ,  14  are recessed and retained beneath the frame  16 . When the assembly  10  is actuated to move to the extended position, the motor  54  and associated drive train rotates the gear  42  clockwise approximately 90 degrees. As the gear  42  moves between these positions, the link arm  38  pulls the finger assembly or yoke  36  in a direction toward the gear  42  so that the rod  26  is rotated so as to extend the links  32 . This rotation causes the upper and lower steps  12 ,  14  to move to the extended position. 
   When in the extended position, the two-part linkage  28 , comprising the lobed crank  30  pivotally attached to the straight link  32 , is in a nearly aligned and not-over-center position. The lobe  60  of the crank  30  forced and stopping against the bottom of the frame  16  results in this position of the crank  30  relative to the link  32 . When pressure is placed on the upper and/or lower steps  12 ,  14  while in the extended position, the two-part linkage  28  may exert a torque on the rod  26 , albeit a small torque since the crank is nearly aligned with the link  32  and therefore the moment-producing component of the force on the rod  26  would be small. Any torque exerted on the rod  26 , tending to back drive it, is easily resisted by the drive train used to rotate rod  26 . 
   When retracted, as illustrated in  FIGS. 3 ,  6  and  9 , the arm  44  is stopped against a stop  47  and in that stopped position the pivots  39 , 41  and  48  are substantially aligned along a straight line. With the pivots aligned, any force exerted from the step tending to backdrive the motor/drive unit will be very small and easily resisted by the drive unit. Also in the retracted position, the step is preferably pulled up against rubber bumpers  53 , one on each side of the step. It is also noted that a second stop  49  may be provided to stop the arm  44  in the extended position, but the arm  44  would not normally stop against the stop  49  since it is stopped when lobe  60  contacts frame  16 . 
   The assembly  10  may also include a light  70 . 
   Of course, the description set out above is merely of exemplary preferred versions of the invention, and it is contemplated that numerous additions and modifications can be made. These examples should not be construed as describing the only possible versions of the invention, and the true scope of the invention will be defined by the claims.

Summary:
The present invention provides an improved collapsible step assembly for recreational vehicles. The movable step apparatus comprises a mounting frame, at least one step mounted to the frame through a linkage assembly, and a pivot assembly having a pivot rod with a longitudinal axis of rotation. In use, the pivot rod is rotatably mounted to the frame and rotates the linkage assembly and the steps between an extended position and a retracted position. Rotating the pivot rod in a first direction moves the step to the extended position, and rotating the pivot rod in the opposite direction moves the step to the retracted position. A two-part linkage is rotatably engaged between the pivot rod and the step linkage and transfers downward forces on the step when retracted and when extended to the drive unit so that the step stays in its position.