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: 
FIELD OF THE INVENTION 
     The present invention relates to a step unit, which includes a step member adjacent to a vehicle sliding door. 
     BACKGROUND OF THE INVENTION 
     Conventionally, a step unit is provided on a vehicle main body to be adjacent to a vehicle sliding door thereof. For example, referring to Non-Patent Document 1, a step unit includes a step member (step) and a rail plate member. A lower rail extending in the opening-closing direction of a vehicle sliding door is provided on the lower surface of the step member. The lower rail supports rollers coupled to the sliding door, so that the rollers and the sliding door are guided along the lower rail. Such a step unit has a cutout portion formed in a part of the lower rail. With the rail plate member (sliding door lower rail plate) removed, the cutout portion allows rollers to be supported by the lower rail or to be removed from the lower rail. 
     PRIOR ART DOCUMENT 
     Non-Patent Document 
     
         
         Non-Patent Document 1 
         Repair Manual for TOYOTA ALPHARD VELLFIRE, volume F, May 2008 (DH-282 through DH-285, DH-246, DH-247 and other pages) 
       
    
     SUMMARY OF THE INVENTION 
     In the above described step unit, the step member and the fastening piece formed by bending the rail plate member each have a fastening hole, and the step member and the rail plate member are assembled together by a bolt passed through the fastening holes. However, to arrange these members of the step unit such that the fastening holes match each other, the rail plate member needs to be held underneath the step member in by touch. This complicates the assembling process. 
     Further, in the above described step unit, the rail plate member receives a great load from the rollers, and the lower rail has a low rigidity because of its discontinuous structure on the ends of the cutout portion. The rail plate therefore has a complicated shape. That is, a typical rail plate member is formed by welding two metal sheets together, such that one of the sheets protrudes to be flush with the inner surface of the lower rail. A typical rail plate member also has a structure for reinforcing the ends of the cutout portion of the lower rail. The rail plate member has such a complicated structure. 
     Accordingly, it is an objective of the present invention to provide a step unit that facilitates the assembly and simplifies the shape of a rail plate member. 
     To achieve the foregoing objective and in accordance with one aspect of the present invention, a step unit including a step member and rail plate is provided. The step member is provided on a vehicle main body to be adjacent to a vehicle sliding door. The step member has, on a lower surface thereof, a pair of lower rails extending in an opening-closing direction of the sliding door. The rail plate member is arranged at a cutout portion, which is formed in a part of one of the lower rails. The rail plate member makes the lower rail continuous in the opening-closing direction. Rollers are coupled to the sliding door and arranged between the pair of lower rails. The rollers, together with the sliding door, are guided by the lower rails in the opening-closing direction. The step member is molded of a plastic material and has an insertion slit and a support extension, the insertion slit extending through the step member in the vertical direction at a position that corresponds to the cutout portion. The support extension extends from an end of the cutout portion of the lower rail to support the rail plate member against load applied to the rail plate member by the rollers. The rail plate member is assembled to the step member by being inserted through the insertion slit from above the step member. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view from above illustrating a step unit according to one embodiment of the present invention; 
         FIG. 2  is a perspective view from below illustrating the step unit shown in  FIG. 1 ; 
         FIG. 3  is a partial plan view illustrating the step unit shown in  FIG. 1 ; 
         FIG. 4A  is a cross-sectional view taken along line  4 A- 4 A of  FIG. 3 ; 
         FIG. 4B  is a cross-sectional view taken along line  4 B- 4 B of  FIG. 3 ; 
         FIG. 5  is an exploded perspective view illustrating the pulley and the structure for supporting the pulley shown in  FIG. 2 ; 
         FIG. 6  is a partial bottom view illustrating the pulley and the structure for supporting the pulley shown in  FIG. 2 ; 
         FIG. 7  is an explanatory exploded perspective view from below illustrating the support extensions and the rail plate member in the step unit shown in  FIG. 1 ; 
         FIG. 8  is a partial bottom view showing the step unit shown in  FIG. 1 ; and 
         FIG. 9  is a cross-sectional view taken along line  9 - 9  of  FIG. 8 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     One embodiment of the present invention will now be described with reference to  FIGS. 1 to 9 . 
     A vehicle has a step unit  1  shown in  FIG. 1 , which is located adjacent to a vehicle sliding door (not shown). 
     As shown in  FIG. 1 , the step unit  1  of the present embodiment is formed mainly by a drive device (motor unit)  2  for opening and closing the sliding door and a substantially plate-like step member (step)  3 . The step unit  1  is fixed to the main body (not shown) of the vehicle. The entire upper surface of the step unit  1  is covered with an unillustrated thin scuff plate (decorative member). The upper surface of the drive device  2  (the lower surface of the thin plate-like scuff plate) is covered with an unillustrated rigid plate (cover). 
     The step member  3  is formed by molding a plastic material. The step member  3  is located in the passenger compartment at a position adjacent to the sliding door in the closed state, and includes a flat plate portion  4 , on which an occupant places a foot when getting in or out of the vehicle, and an accommodation portion  5 . The accommodation portion  5  is formed continuous to the flat plate portion  4  and is located on a side of the flat plate portion  4  in the opening direction of the sliding door, or rearward of the flat plate portion  4 . A part of the drive device  2  is fixed and accommodated in the accommodation portion  5 . 
     Specifically, when the step unit  1  is installed in the vehicle, a bottom  5   a  of the accommodation portion  5  is located at a lower position than an upper surface  4   a  of the flat plate portion  4  as shown in  FIGS. 3 ,  4 A and  4 B. The height (depth) of the bottom  5   a  of the accommodation portion  5  is determined based on the shape of the drive device  2 . That is, as shown in  FIGS. 1 and 3 , the drive device  2  of the present embodiment includes a motor  2   a,  which is a drive source, an output portion  2   b,  which is attached to the motor  2   a  and has a gear and an electromagnetic clutch, a control circuit portion  2   c,  which is installed in the output portion  2   b . As shown in  FIGS. 4A and 4B , the accommodation portion  5  includes a motor accommodating section  5   b,  an output portion accommodating section  5   c,  and a circuit accommodating section  5   d,  which correspond to the motor  2   a,  the output portion  2   b,  and the control circuit portion  2   c,  respectively. The drive device  2  is fixed by screws such that it partly contacts the bottom  5   a  of the accommodation portion  5 . That is, a part of the drive device  2  is located lower than the upper surface  4   a  of the flat plate portion  4 . More specifically, one third or more of the entire thickness of the drive device  2  in the vertical direction is located below the upper surface  4   a  of the flat plate portion  4 . In the example shown in  FIG. 4B , substantially half the entire thickness is located below the upper surface  4   a . Also, a peripheral wall  6  is molded integrally with the step member  3  to encompass (almost the entire circumference of) the accommodation portion  5  as shown in  FIGS. 1 ,  3 , and  4 . The peripheral wall  6  extends to a position above the upper surface  4   a  of the flat plate portion  4 . 
     As shown in  FIG. 2 , pulleys  11 ,  12  are located on the lower surface of the step member  3 . A loop belt  13  is rotationally supported by the pulleys  11 ,  12  and substantially extends in the opening-closing direction of the sliding door. The drive device  2  is configured to cause the belt  13  to rotate. That is, as shown in  FIG. 4A , an output shaft  2   d  of the output portion  2   b  of the drive device  2  extends through a through hole  5   e  formed in the bottom  5   a  of the accommodation portion  5  and protrudes from the lower surface of the step member  3 . The output shaft  2   d  transmits power that is output by the drive device  2  to the belt  13  via a power transmitting portion  14  (see  FIG. 2 ), which is provided on the lower surface of the step member  3 , thereby rotating the belt  13 . In the present embodiment, the opening-closing direction of the sliding door substantially corresponds to the front-rear direction of the vehicle. The position of the belt  13  is regulated by the pulleys  11 ,  12 , which are located at ends of the step member  3  in the vehicle front-rear direction, and a regulation portion  15 , which is located between the pulleys  11  and  12  and extends from the lower surface of the step member  3 . The regulation portion  15  is molded integrally with the lower surface of the step member  3 . 
     A pair of non-annular shaft support portions  16 , which serve as a shaft support portion, are molded integrally with the step member  3 . The pulley  11 , which is located at the front end of the step member  3 , is supported by the non-annular shaft support portions  16  as shown in  FIGS. 5 and 6 . The non-annular shaft support portions  16  are formed to engage with and rotationally support the pulley  11 . Specifically, the non-annular shaft support portions  16  are separated in the vertical direction, and each have an opening  16   a,  which opens in a direction opposite to the direction of the force that is perpendicular to the axis and applied to the pulley  11  by the belt  13  in a taut state (in other words, the openings  16   a  open substantially in the forward direction of the vehicle). A shaft  11   a  of the pulley  11  is inserted in the non-annular shaft support portions  16  via the openings  16   a . Accordingly, the pulley  11  is fitted to and rotationally supported by the non-annular shaft support portions  16 . The width of the openings  16   a  is slightly smaller than the diameter of the shaft  11   a  of the pulley  11 . Therefore, the pulley  11 , which is fitted in the non-annular shaft support portions  16 , is supported by the non-annular shaft support portions  16 , so as not to fall off from the openings  16   a  unless it receives a force of a certain magnitude. The pulley  12 , which is located at the rear end of the step member  3 , is rotationally supported by the cover of the power transmitting portion  14  as shown in  FIG. 2 . 
     A pair of lower rails  21 ,  22  is molded integrally with the lower surface of the step member  3  as shown in  FIG. 2 . The lower rails  21 ,  22  extend in the opening-closing direction of the vehicle sliding door (substantially, the front-rear direction of the vehicle). In the present embodiment, the lower rails  21 ,  22  are curved inward toward the center of the passenger compartment at a front portion. The lower rails  21 ,  22  are connected to each other at both ends in the longitudinal direction. As shown in  FIGS. 7 and 8 , a cutout portion  21   a  is formed in one of the lower rails, that is, in a part of the lower rail  21 . A rail plate member  31  is arranged at the cutout portion  21   a  to make the lower rail  21  continuous in the opening-closing direction of the sliding door (substantially, in the front-rear direction of the vehicle). Rollers  32  (see  FIG. 8 ) are supported to roll between the lower rails  21 ,  22  (including the rail plate member  31 ). The rollers  32  are coupled to the sliding door, for example, via brackets (not shown). Thus, the rollers  32  and the sliding door are guided in the opening-closing direction by the lower rails  21 ,  22 . The rollers  32  are coupled to the belt  13  via brackets (not shown), so that, as the belt  13  rotates, the rollers  32  are moved in the opening-closing direction (while being guided by the lower rails  21 ,  22 ). 
     Specifically, as shown in  FIGS. 7 to 9 , the step member  3  has an insertion slit  23  extending through the step member  3  in the vertical direction at a position that corresponds to the cutout portion  21   a . Also, the step member  3  has a pair of support extensions  21   b . The support extensions  21   b  extend from the ends of the cutout portion  21   a  of the lower rail  21  to support the rail plate member  31  against the load applied to the rail plate member  31  by the rollers  32  (see  FIG. 8 ). As shown in  FIG. 8 , the pair of support extensions  21   b  extend from the ends of the cutout portion  21   a  of the lower rail  21  in the direction in which load is applied (upward as viewed in  FIG. 8 , and toward the outside with respect to the vehicle width direction), and extend toward each other without being connected to each other (so that there is a space therebetween). Also, the support extensions  21   b  are thicker than the lower rail  21  in the direction in which the load is applied (the up-side direction as viewed in  FIG. 8 ). 
     The rail plate member  31  is installed by being inserted through the insertion slit  23  from above the step member  3  as shown in  FIG. 7 . The rail plate member  31  is formed by processing a metal plate. As shown in  FIGS. 1 and 7 , the rail plate member  31  has an angled portion  31   a  at the upper edge (the upper edge in a state after being installed). The angled portion  31   a  extends in a direction perpendicular to the vertical direction, or into the passenger compartment with respect to the vehicle width direction in the present embodiment. The rail plate member  31  is assembled with the step member  3  by being inserted into the insertion slit  23  such that the lower side of the angled portion  31   a  contacts the upper surface of the step member  3 . The rail plate member  31  is fixed to the step member  3  through press-fitting as shown in  FIG. 9 . Specifically, the rail plate member  31  has a pair of press-fit portions  31   b  on both sides. The press-fit portions  31   b  slightly protrude sideways and are spaced from each other in the vertical direction to be pressed against the ends of the cutout portion  21   a  of the lower rail  21 . When the rail plate member  31  is inserted through the insertion slit  23  from above the step member  3 , the press-fit portions  31   b  are pressed against the ends of the cutout portion  21   a  of the lower rail  21 . At this time, the sides of the cutout portion  21   a  may be slightly shaven or elastically deformed by the press-fit portions  31   b.    
     In the above described configuration, the rail plate member  31  can be removed to insert rollers  32  into the space between the lower rails  21 ,  22  through the cutout portion  21   a  or remove the rollers  32  from the space between the lower rails  21 ,  22 . When the drive device  2  is operated, the belt  13  is rotated. Accordingly, the rollers  32  are moved while being guided by the lower rails  21 ,  22 , and the sliding door is operated to open or close. The output shaft  2   d  of the drive device  2  or the housing of the drive device  2  has an seal ring, which is not shown, so that water is completely or almost completely prevented from entering the interior of the drive device  2  or the bottom  5   a  of the accommodation portion  5  through the through hole  5   e  formed in the bottom  5   a  of the accommodation portion  5 . 
     The present embodiment has the following advantages. 
     (1) The step member  3  has an insertion slit  23  extending through the step member  3  in the vertical direction at a position that corresponds to the cutout portion  21   a,  which is formed in a part of the lower rail  21 . The rail plate member  31  is installed by being inserted through the insertion slit  23  from above the step member  3 . This configuration facilitates the assembly process. Also, the step member  3  has a pair of support extensions  21   b , which extends from the ends of the cutout portion  21   a  of the lower rail  21  to support the rail plate member  31  against the load applied to the rail plate member  31  by the rollers  32 . Accordingly, the rail plate member  31  is prevented from being deformed by the load applied by the rollers  32 . Further, since the support extensions  21   b  extend from the ends of the cutout portion  21   a , the rigidity of the ends of the cutout portion  21   a  is increased. This eliminates the need for providing, the rail plate member  31  with a structure for reinforcing the ends of the cutout portion  21   a . Therefore, the shape of the rail plate member  31  can be simplified as in the present embodiment, in which the rail plate member  31  is formed by a single plate (metal plate), for example. Further, if a part of a rail plate member is caused to protrude such that it is flush with the inner surface of a lower rail (the surface that contacts rollers) as in the conventional art, the corners of that part will be rounded. Accordingly, steps are likely to be formed between the ends of the cutout portion of the lower rail and the rail plate member. In the present embodiment, the rail plate member  31  does not need to be molded to protrude. Therefore, it is easy to prevent such steps from being formed. This contributes to smooth movement of the rollers  32  and the sliding door. Further, unlike conventional step members that are formed through sheet-metal processing, the step member  3  is formed through molding a plastic material. This allows the step member  3  to have wide variety of shapes. Accordingly, for example, the support extensions  21   b  can be easily molded integrally with the step member  3 . 
     (2) Since the rail plate member  31  is fixed to the step member  3  through press-fitting, fasteners such as bolts and rivets are not necessary. The number of components of the step unit is thus reduced. 
     (3) Since the rail plate member  31  is press fitted in the insertion slit  23  to be pressed against the ends of the cutout portion  21   a  of the lower rail  21 , steps between the ends of the cutout portion  21   a  and the rail plate member  31  are further reduced. Therefore, it is possible to directly suppress chattering of the rail plate member  31  in the opening-closing direction of the sliding door, that is, the direction of movement of the rollers  32 . 
     (4) The angled portion  31   a , which extends in a direction perpendicular to the vertical direction, is provided at the upper end of the rail plate member  31 . The angled portion  31   a  reliably prevents the rail plate member  31  from fall off (the insertion slit  23  of) the step member  3 . Also, with the rail plate member  31  assembled with the step member  3 , the rail plate member  31  can be easily detached from the step member  3  by pushing the lower surface of the angled portion  31   a  upward, for example, with a jig. 
     (5) The pair of support extensions  21   b  extend from the ends of the cutout portion  21   a  of the lower rail  21  in the direction in which load is applied (upward as viewed in  FIG. 8 ) and, extend toward each other. In this case, when arranging the rollers  32  in the space between the lower rails  21 ,  22  through the cutout portion  21   a , the support extensions  21   b  do not hamper the operation. In addition, since both ends of the rail plate member  31  are supported by the support extensions  21   b , the rigidity of the rail plate member  31  against load applied by the roller  32  is improved compared to a case in which only one end of the rail plate member  31  is supported. Also, the rigidity of the ends of the cutout portion  21   a  is increased. In the above described configuration, the rail plate member  31  can be easily pushed upward manually or with a jig by utilizing the space between the support extensions  21   b . The rail plate member  31  thus can be easily removed. 
     (6) Since the support extensions  21   b  are thicker than the lower rail  21  in the direction in which the load is applied (the up-down direction as viewed in  FIG. 8 ), the rail plate member  31  can be firmly supported, while achieving the advantage of the item (5). 
     The above described embodiment may be modified as follows. 
     In the above described embodiment, the rail plate member  31  is fixed to the step member  3  through press-fitting. However, the rail plate member  31  may be fixed through other configuration. For example, the rail plate member  31  may be fixed to the step member  3  by using fasteners such as bolts or rivets. In this case, for example, the rail plate member  31  may be fixed by the angled portion  31   a  and a fastener that is passed through the step member  3 . To prevent the fastener from interfering with the rollers  32 , the angled portion  31   a  is preferably fixed such that it extends in a direction away from the pair of lower rails  21 ,  22  (toward the outside of the passenger compartment with respect to the vehicle width direction). In the above described embodiment, the rail plate member  31  is press fitted to be pressed against the ends of the cutout portion  21   a  of the lower rail  21 . However, the rail plate member  31  may be press fitted to be pressed in the vehicle width direction (up-down direction as viewed in FIG.  8 ). 
     In the above described embodiment, the angled portion  31   a , which extends in a direction perpendicular to the vertical direction, is provided at the upper end of the rail plate member  31 . However, the present invention is not limited to this, and a rail plate member that does not have the angled portion  31   a  may be used. In such a case, for example, the support extensions  21   b  of the step member  3  may have a bottom for preventing the rail plate member from falling off (preferably through integral molding). 
     In the above illustrated embodiment, the pair of support extensions  21   b  extend from the ends of the cutout portion  21   a  of the lower rail  21  in the direction in which load is applied (upward as viewed in  FIG. 8 ), and extend toward each other. However, a support extension  21   b  may be formed only at one of the ends of the cutout portion  21   a.    
     In the above described embodiment, the rail plate member  31  is formed by processing a metal plate. However, the rail plate member  31  may be formed, for example, through molding plastic. 
     In the above described embodiment, the step unit  1  includes the drive device (motor unit)  2  for opening and closing a vehicle sliding door. However, the step unit  1  does not necessarily include the drive device  2 . That is, a step member that does not include the accommodation portion  5  may be used. In this case, the pulleys  11 ,  12 , the belt  13  and the power transmitting portion  14  are not necessary. 
     Description Of The Reference Numerals 
       3  . . . Step Member,  21 ,  22  . . . Lower Rails,  21   a  . . . Cutout Portion,  21   b  . . . Support Extensions,  23  . . . Insertion Slit,  31  . . . Rail Plate Member,  31   a  . . . Angled Portion,  32  . . . Rollers.

Summary:
A step unit includes: a step member which is provided with a pair of lower rails extending in an opening and closing direction of a slide door of the vehicle; and a rail plate member which is provided to a cutout formed in a part of either of the lower rails and which continuously connects the lower rails in the opening and closing direction. The step member is molded using a resin material and is provided with an insertion hole which vertically penetrates through the step member at a position corresponding to the cutout; and a support extension section which is extended from a side end of the cutout of the lower rail. The rail plate member is mounted to the step member by being inserted so as to penetrate through the insertion hole from above the step member.